**Abstract:** Selective area epitaxy (SAE) using metal–organic chemical vapor deposition (MOCVD) is a crucial fabrication technique for lasers and photonic integrated circuits (PICs). A low-cost, reproducible, and simple process for the mass production of semiconductor lasers with specific structures was realized by means of SAE. This paper presents a review of the applications of SAE in semiconductor lasers. Growth rate enhancement and composition variation, which are two unique characteristics of SAE, are attributed to a mask. The design of the mask geometry enables the engineering of a bandgap to achieve lasing wavelength tuning. SAE allows for the reproducible and economical fabrication of buried heterojunction lasers, quantum dot lasers, and heteroepitaxial III–V compound lasers on Si. Moreover, it enables the fabrication of compact photonic integrated devices, including electro-absorption modulated lasers and multi-wavelength array lasers. Results show that SAE is an economical and reproducible method to fabricate lasers with desired structures. The goals for SAE applications in the future are to improve the performance of lasers and PICs, including reducing the defects of the grown material introduced by the SAE mask and achieving precise control of the thickness and composition.

**Keywords:** selective area epitaxy; MOCVD; semiconductor laser; quantum dot; heteroepitaxy; EML; multi-wavelength laser arrays

#### **1. Introduction**

Semiconductor lasers have become crucial light sources because of their advantages, including small size, light weight, high reliability, high modulation speed, and easy monolithic integration with other optoelectronic devices, in the fields of modern optical communication, industry, medical, and military fields [1–3]. The fabrication of semiconductor laser epitaxial structures has been achieved due to the emergence and development of metal–organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Selective area epitaxy (SAE) is a method used to controllably grow III–V, II–VI and lead salts lasing materials with specific shapes and geometries. III–V compounds are widely used semiconductor laser materials. Some novel III–V nanostructure materials fabricated with SAE, such as nanowires (NWs), twin-free nanosheets, and low-dislocation nanomembranes at the heterogeneous interface, have been attractive for use in solar cells and lasers due to their good optical and electrical properties [4,5]. Although SAE using MBE has the

**Citation:** Wang, B.; Zeng, Y.; Song, Y.; Wang, Y.; Liang, L.; Qin, L.; Zhang, J.; Jia, P.; Lei, Y.; Qiu, C.; et al. Principles of Selective Area Epitaxy and Applications in III–V Semiconductor Lasers Using MOCVD: A Review. *Crystals* **2022**, *12*, 1011. https:// doi.org/10.3390/cryst12071011

Academic Editor: M. Ajmal Khan

Received: 18 June 2022 Accepted: 18 July 2022 Published: 21 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

advantages of providing a controllable thickness and ultra-high clean surface, the MBE process is time-consuming and the required equipment is expensive, which limits the large-scale production of epitaxial structures. In the fabrication of III–V semiconductor lasers, SAE employing MOCVD technologies can realize the desired laser structure on a large scale, such as buried heterostructure (BH) [6] and quantum dot (QD) lasers. SAE can decrease the high defect density due to a lattice being mismatched at the interface between III and V materials and Si, demonstrating the feasibility of high-performance III–V lasers on Si. Over the past few decades, the integration of semiconductor lasers and other optoelectronic devices on monolithic chips has been an inevitable trend to satisfy the demands of high-speed optical communication [7,8]. SAE is an effective method that realizes integrated lasers, such as electro-absorption modulated lasers (EML) [9], multi-wavelength laser arrays (MWLA) [10].

The improvement of laser structure and the design of advanced monolithic integrated devices require a deeper understanding of the principles and applications of SAE. SAE has proven to be a powerful tool for fabricating semiconductor lasers with desired structure and compact photonic integrated circuits (PICs). In this paper, we interpret SAE principles including growth-rate enhancement, composition variation, vapor-phase diffusion model, and bandgap engineering. The SAE applications such as BH laser, QD laser, heteroepitaxial laser on Si, EML, and MWLA are introduced in detail. The problems of defects in the growth of materials introduced by masks needs to be solved urgently. It is also crucial to achieving precise and controllable material growth. SAE can potentially be the primary technology for future optoelectronic devices. It is hoped that this paper can provide a reference for clearly understanding the mechanism of SAE, reducing material defects caused by selective epitaxial growth, and improving the performance of lasers.

#### **2. Principles of SAE**

SAE refers to the method of growing materials with different thicknesses and compositions at various locations on the same wafer. It involves the deposition of a dielectric material on the substrate as a mask, most commonly silicon oxide (SiO2). After designing the desired substrate pattern, the substrate was divided into regions covered by the mask and exposed areas using pattern techniques. The most straightforward and practical mask pattern is a pair of rectangular strips, as shown in Figure 1. The area exposed between the two stripes was the mask-opening region. The dielectric mask can inhibit the precursor deposition and nucleation. As a result, deposition and growth occur only in the opening region and hardly on the mask, realizing the selective growth of the material on the substrate.

**Figure 1.** A pair of oxide strips pattern mask for selective area epitaxy.

The mask material should have a low sticking coefficient to the precursor gas [11,12], can withstand high temperatures [13], is insensitive to precursors, and is compatible with subsequent processes, such as inductively coupled plasma (ICP) or chemical wet etching [14]. Common mask materials are amorphous SiNx and SiO2.

Pattern techniques realize the design of a patterned mask over the substrate. There are three primary patterning design methods. Deep ultraviolet (DUV) lithography realizes the mask size in the hundreds of nanometers [15]. Electron beam lithography (EBL) can enable the resolution of SAE on a scale of tens to hundreds of nanometers [16]. Nano-imprinting lithography (NIL) can realize the pattern design of wafer-level size.

The geometry and size of the mask affect the epilayer thickness and composition obtained via SAE compared to the uniform epilayer obtained by MOCVD on exposed planar substrates. The successful fabrication of lasers and integrated optoelectronic devices is based on the control of the surface morphology and the composition of selective area growth.

#### *2.1. Growth-Rate Enhancement and Composition Variation*

The growth-rate enhancement (or "growth-enhancement effect", GRE/GEE) effect is one of the essential properties of SAE. During the MOCVD process, the precursor gas-phase molecules were selectively adsorbed at the substrate opening, whereas few molecules were attached to the mask surface. This occurred because the sticking coefficients of precursors on the mask were much lower than that of the surface of the epitaxial region in the high temperature of MOCVD. The mask suppressed the nucleation of the crystals. Material growth occurred selectively in the opening region. According to the conservation of mass, the concentration of the precursor above the mask was larger than that of the opening area. The precursor gas was transported from the mask to the opening driven by the concentration gradient. The material grown on the opening area of the masked substrate was thicker than that grown on the unmasked planar substrate at the same time. The deposition rate near the edge of the stripe mask was higher than that of the opening area far from the mask. The lateral gas diffusion over the mask area to the opening area leads to this phenomenon [17–19]. The phenomenon mentioned above is the GRE. GRE is expressed as the ratio of the thickness of the film grown at the opening area on the masked substrate to the thickness of the film grown on the unmasked wafer. Figure 2 shows a schematic of the cross-section of the GRE cross-section.

**Figure 2.** Schematic of the cross-section of the growth-rate enhancement process. The area below the dashed line is the layer thickness grown on the unmasked substrate, and the excess thickness due to GRE is the area above the dashed line.

Another critical property of SAE is the compositional variation with the GRE. The MOCVD growth of III–V alloys on wafers is dominated by the absorption and vapor-phase diffusion of III species. The absorption, surface migration, and re-evaporation capabilities of III-group molecules on the mask are different. These processes affect the flux of III-species gas absorbed on the mask reaching the opening area.

A compositional variation appears in the opening area, especially in the sidewalls of the growth layers near the edges of the strip masks, as diffusion from the mask to the unmasked wafer occurred [20,21].

The typical opening width between the mask was 5–20 μm, and the stripe width was 5–50 μm; the material growth in the openings was determined by the widths of both the oxide mask and the opening [22]. The thickness and GRE of the growth layer increased as the width of the mask increased, which was the exact opposite of the case noted considering the opening area [23]. The GRE was still clearly visible in the opening area with a 100 μm width [24]. The GRE disappeared at positions very far from the edge of the mask stripes, which does not differ from the case involving growth on a planar substrate. The controllable growth of layers with different thicknesses and compositions was realized by changing the spacing and width of the mask stripes.

#### *2.2. Vapor-Phase Diffusion Model of SAE*

Several studies have proposed models for SAE-grown materials that quantify the diffusion process. The GRE and composition shift were considered functions of the mask geometry. The GRE mechanism comprises two parts: surface migration and vapor-phase diffusion [23]. Surface migration was thought to occur because the gas molecules absorbed on the dielectric mask migrated along the surface of the mask and the sidewalls of the epilayer. Vapor-phase diffusion refers to the flux to the unmasked region due to the concentration gradient of the undeposited precursor above the mask. The mask width is typically on the order of tens of micrometers. Surface migration in the masked area is ignored as it only occurs within a few microns of the mask surface [25].

The three-dimensional vapor-phase diffusion model is established by solving the state diffusion equation in an assumed region that is similar to the experimental conditions, as shown in Figure 3. The thickness and composition profiles of various compounds were predicted using vapor-phase diffusion calculations.

**Figure 3.** Schematic diagram of the 3D vapor-phase diffusion model.

The model assumed a stagnant area above the wafer. The height of the stagnant area is "δ"(~500 μm) [22]. The fluid layer above the stagnant layer provided a constant flow. In the equations below, *D* and *k* are the diffusion length and reaction rate of the reactant precursor, respectively. Wo and Wm are the widths of the opening area and masks, respectively. *C* is the concentration of the reactant gas molecules, usually group-III species. The growth direction was *z*, and the length along the mask was *y*. The horizontal direction, *x*, is perpendicular to the *z*- and *y*-axes.

Because the spatial gas concentration is constant, the substance diffusion equation in three-dimensional space is

$$
\frac{
\partial^2
\mathcal{C}
}{
\partial x^2
} + \frac{
\partial^2
\mathcal{C}
}{
\partial y^2
} + \frac{
\partial^2
\mathcal{C}
}{
\partial z^2
} = 0.
\tag{1}
$$

Above the stagnant area, the concentration of precursors is constant.

$$\mathbb{C}(x, y, z = \delta) = \mathbb{C}\_0 \tag{2}$$

The symmetry of the computational domain defines the left and right boundaries is expressed as

$$D\frac{\partial C(x,y,z)}{\partial x} = 0.\tag{3}$$

There is no growth occurring on the mask surfaces.

$$D\frac{\partial C(x,y,z)}{\partial z}|\_{z=0} = 0\tag{4}$$

The growth process in the unmasked region is given by

$$D\frac{\partial C(x,y,z)}{\partial z}|\_{z=0} = k\mathbb{C}.\tag{5}$$

In this model, *D/k* is the effective diffusion length of the species. The relationship between the effective diffusion length and mask length determines the dominant process in GRE. Surface migration plays a major role when the mask geometry is close to the surface-diffusion length. On the contrary, the vapor-phase diffusion model dominates this process because the mask size is larger than the diffusion length [26]. A decrease in the effective diffusion length increased the growth rate next to the stripe mask. The vapor-phase diffusion equations for different III-species precursors were obtained to obtain the GRE and composition changes.

The GRE factor *R* is expressed as

$$R = \frac{\mathbb{C}(x, y, z=0)}{\mathbb{C}(0)},\tag{6}$$

where *C(x, y, z* = 0) is the concentration of group-III vapor-phase molecules on the *z* = 0 plane, and *C*(0) is the analytic solution of Laplace's equation for the unmasked wafer. *C*(0) is expressed as [27]

$$C(0) = \frac{C\_0}{\frac{k}{B}\delta + 1} \tag{7}$$

Model calculations were performed without considering the interactions between the different III species [28]. Dupuis et al. [25,29] assumed that the growth-rate enhancement of ternary or quaternary alloys was linearly related to group-III precursors. They gave the expressions for the growth-rate enhancement factor of *AlxGayIn*1−*x*−*yAs* system film and III–element composition.

$$R\left(Al\_{x0}Ga\_{y0}In\_{1-x0-y0}As\right) = \mathbf{x}\_0 \cdot \mathbf{R}\_{Al} + \mathbf{y}\_0 \cdot \mathbf{R}\_{\mathbf{G}a} + (1-\mathbf{x}\_0-\mathbf{y}\_0) \cdot \mathbf{R}\_{In} \tag{8}$$

$$Al\%=\frac{\text{x}\_0 \cdot \text{R}\_{Al}}{\text{R}\left(Al\_{x0}\text{Ga}\_{y0}In\_{1-x0-y0}As\right)}\tag{9}$$

$$\text{Gat\%} = \frac{y\_0 \cdot R\_{\text{Gar}}}{R \left(A l\_{x0} G a\_{y0} I n\_{1-x0-y0} A s\right)} \tag{10}$$

$$In\% = \frac{(1 - x\_0 - y\_0) \cdot R\_{In}}{R \left( A l\_{x0} G a\_{y0} I n\_{1-x0-y0} A s \right)} \tag{11}$$

The reactivity of V precursors is very low at the typical deposition temperatures of MOCVD. The V precursors account for the majority of the gas-phase components, far exceeding the stoichiometric number in the film compound composition, such that it is therefore difficult to obtain an expression of the V composition of the semiconductor compound. Therefore, the deposition rate of III–V compounds is usually determined by the incorporation of III precursors. The GRE becomes more pronounced with the increase of the III–V ratio. At higher temperatures, the desorption of gas molecules on the surface is enhanced, resulting in weaker selective growth. Building an accurate growth-rateenhancement computational model for these equations helps predict and control the specific growth, including thickness and composition, enabling the design of strain and bandgap energy variations at different locations on a single wafer. The accuracy and efficiency of a diffusion model are vital for the applications of future SAE-integrated optical devices.

#### *2.3. Bandgap Engineering of SAE*

The spatial variation of the energy band and strain for a given patterned mask can be deduced by varying the width of the mask and the opening area. For bulk materials, changes in composition cause a wavelength shift. In quantum wells (QWs), the GRE causes an additional wavelength shift in addition to the composition. The magnitude of the wavelength shifts depends on the width of the mask and the opening area [30]. The effective energy gap, Eg, is sensitive to the thickness of the QW and increases as the layer thickness decreases. The dependence of Eg on the layer thickness enables the tuning of emission wavelength.

Sasaki et al. [31] fabricated a 2 μm wide InGaAs/InGaAsP multiple quantum well (MQW) ridge structure, which was surrounded by stripe masks with various widths on both sides. They observed a 50 nm PL peak wavelength shift by varying the mask width from 4 μm to 10 μm, as shown in Figure 4. A series of studies have demonstrated the relationship between QW emission wavelength tuning and mask geometry [8,28,29].

**Figure 4.** PL peak wavelength profile for MQW structure along [011] direction. A 20 μm tapered transition region formed between the 10 μm and 4 μm wide mask. The inset is mask configuration. Reprinted with permission from ref. [31] © Elsevier. Copyright 1993 Journal of Crystal Growth.

The gradual gradient wavelength profiles were due to GRE along the edge of the mask. Due to the GRE, there is a thick bulge region at the interface between the mask and the epitaxial layer (Figure 2). Flat interfaces are essential in photonic devices. Adjusting MOCVD parameters, such as increasing the growth temperature [32] and reactor pressure [33], can eliminate convexity near the edge of the mask. The in situ etching process can improve the planarity between the opening area and the masked region [34,35].

Nonplanar growth on a substrate is also an indispensable research topic for SAE. In specific applications, such as butt-coupled waveguides and buried heterojunction lasers, wafers are etched to create grooves with well-defined inclined facets. The orientation of the inclined plane of the groove was different from that of the planar wafer. Distinction of the growth rate in different crystal orientations leads to enhanced diffusion on a nonplanar surface. Different growth rates on different crystal planes also enable the fabrication of

high-performance laser structures, such as BH, which will be further discussed in the following sections.

#### **3. Applications of SAE in Semiconductor Lasers**

*3.1. Buried Heterostructure (BH) Lasers*

The active region in a buried heterostructure (BH) laser is completely buried in a widebandgap hetero-material. The BH structure has a strong lateral refractive index confinement and current confinement due to the lateral heterostructure [36]. The BH laser has a low threshold current, stable waveguide mode, and nearly symmetrical beam distribution, making it a perfect structure for semiconductor lasers operating with a low threshold and high efficiency. However, conventional BH lasers require multiple MOCVD steps, easily introducing contamination and oxidation to the surface. The SAE can complete the BH structure in a single growth step, eliminating the related defects.

Galeuchet et al. [37] presented a GaInAs/InP heterostructure grown in one SAE step. The GaInAs active layer growth occurred on the (100) facet rather than {111}, where the InP layer growth took place. Different sticking coefficients or diffusion constants of InP and GaInAs may be responsible for the different growth in the (100) and {111} facets. The nucleation sites of the {111} A facets were occupied with stable molecules that did not react to a large extent, while the {111}B facets were active and provided reactants to the (100) facets. The width of the mesa decreased as the growth proceeded. Finally, the lateral size of the buried layers became zero, and the sides of the sloping {111}B facets merged into one. A BH laser was fabricated in one growth step because the GaInAs layers were buried in the InP. The catastrophic optical damage (COD) due to the absorption in the active region near the output facets usually occurs in high-power semiconductor lasers. This limits the high-power output of lasers. Lammert et al. [38] presented a BH laser with a nonabsorbing mirror region, where a fusiform patterned mask was used, as shown in Figure 5. The output power of the BH laser with nonabsorbing mirrors exhibited increments of 40%.

**Figure 5.** Schematic diagram of a stripe mask for two types of BH laser. For (**a**) Conventional stripe mask for a BH laser. For (**b**), a fusiform mask for nonabsorbing mirrors in a BH laser.

Epitaxial structures with different thicknesses and compositions were obtained by designing the mask geometry. Semiconductor lasers are integrated with other waveguides on the same wafer to form optoelectronic devices for various purposes. Kobayashi et al. [39] proposed an MQW BH laser integrated with a reduced-thickness waveguide layer. Both the gain and waveguide regions were simultaneously grown by SAE on a (100) InPpatterned substrate. The far-field FWHM of the device was 11.8◦ vertically and 8◦ laterally. The threshold current was 19 mA, with a high slope efficiency of 0.25 mW/mA. Takemase et al. [40] reported an AlGaInAs BH laser with a mode profile converter (MPC) for the first time. The device consisted of a constant thickness portion as the gain region and a vertically tapered thickness portion to expand the beam output. The gain region and thickness-tapered portion were grown via SAE. The width of the opening area between the two masks was 20 μm. AlGaInAs MQW was grown between the mask stripes with a width of 100 μm. The gain layer was approximately two times thicker than that of the end of the tapered region. The SiO2 mask was removed using HF after the current blocking layers were established. The characteristic temperature was 43 K at 60 ◦C. The poor temperature characteristic was attributed to the poor crystal quality of the AlGaInAs grown by SAE.

Bour et al. [41] described a self-aligned BH AlGaInAs QW laser using a micro-SAE. The general process of fabricating a self-aligned BH laser with a single growth step is shown in Figure 6. The 50 nm SiO2 mask stripes were formed along [011] on a 2 μm InP n-cladding layer. The widths of opening region and masks were 1.5 and 7 μm, respectively. The sample was returned to the MOCVD reactor for growing AlGaInAs MQW and a separate confinement heterostructure (SCH) under high temperature conditions, accompanied by the {111} B sidewall formation, as shown in Figure 6b. The InP layer nucleated on the {111} sidewall when the temperature was lowered, because of a decrease in the surface mobility, and therefore the InP p-cladding layer was developed and encapsulated in the active region (Figure 6c). The SiO2 mask was used as the current blocking layer without removal in the finished device, whose performance was improved compared to the conventional reverse-biased p–n junction.

**Figure 6.** Process for a self-aligned BH laser. (**a**) n-type InP cladding layer and SiO2 mask; (**b**) the active region and SCH under conditions wherein a {111} no-growth sidewall develops; (**c**) the upper p-type InP cladding layer. Reprinted with permission from ref. [41] © AIP Publishing. Copyright 2004 Applied Physics Letters.

Cai et al. [42] obtained an InGaAs/InGaAsP MQW BH laser in a single growth step. InP cladding layer was grown on {111} B sidewalls. The threshold current of the BH laser is 2.7 mA at room temperature. After a burn-in test of 600 h, the total degradation of the outpower was less than 6%, and the BH laser had excellent reliability. The SAE method has great potential for low-cost high-performance BH laser fabrications and associated device integrations due to its excellent performance and inherent manufacturing simplicity.

#### *3.2. Quantum Dots (QD) Lasers*

#### 3.2.1. QDs Lasers by EBL and Lithography

Semiconductor quantum dots (QDs) that achieve full three-dimensional confinement are sphere-like nanostructures with strong carrier confinement effects. However, in MOCVD and MBE, pyramidal or cylindrical QDs are obtained due to the existence of a wetting layer. They are typically embedded in another material with a large bandgap. Since QDs have a lower threshold current density, narrower gain spectrum, higher optical gain, higher temperature stability, and better dynamic characteristics than QWs, they are an ideal

gain medium for semiconductor lasers. However, it is challenging to grow QDs with uniform size and controlled position. The growth of high-quality QDs is a popular research topic.

The typical QD growth mode is Stranski–Krastanov (SK) growth, relying on straindriven self-assembly between different material layers. If the strain is too high to grow continuous layers, the later-grown layers split into nanoscale islands when they reach a thickness of several single atoms. Subsequently, the island structures were embedded in an extensive–bandgap material to form QDs. This process is known as SK growth (Figure 7). The QD obtained using this method is called self-assembled QDs (SAQDs). The precisely controlled diameter and thickness of the QDs can stabilize the energy-level distribution without broadening the laser gain spectrum. The gain increased as density of the QDs increased. The operating threshold and gain spectrum characteristics of QD lasers were better than those of QW lasers. However, SAQDs have two disadvantages: (1) the presence of a wetting layer causes incomplete three-dimensional confinement and (2) the randomness of the nucleation position and the variation in size (diameter, thickness) also limit the advantages of QDs. Both lead to a shift in the quantized energy level and the non-uniform broadening of the gain spectrum, which is broader than that of QWs.

**Figure 7.** Schematic of SK growth. (**a**) Original molecule deposition. (**b**) Wetting layer formation. (**c**) QD formation and strain relaxation. The atoms of substrate and epitaxial layer were represented with blue and yellow, respectively.

The SAE is a method used to overcome the shortcomings of SAQDs' growth. The patterned grown QDs have no wetting layers and can control their position and diameter to obtain uniform QDs. The most widely used method of fabricating QDs by SAE is to first pattern a mask on the substrate with high-resolution lithography, ensuring that the size of the opening region is small enough to grow QDs.

Elarde et al. [43] combined EBL and SAE to prepare uniformly distributed QDs. The EBL was used to define the exact location of QDs' nucleation, and the SAE realized the control of QDs' geometries by patterned SiO2 masks. They deposited 10 nm SiO2 and patterned 4, 6, and 8 μm mask stripes using optical lithography and wet etching on ntype Al0.75Ga0.25As cladding and GaAs barrier layers. Their samples were coated with polymethylmethacrylate to obtain arrays of 30 to 40 nm circular features in diameter using EBL. The arrays were centered over the SiO2 mask stripes. After the array pattern was transferred to the SiO2 masks by wet etching, 6.9 nm-thick In0.35Ga0.65As QD layers were formed and encapsulated by 10 nm GaAs. The p-type cladding and contact layer were then grown during a later stage. The density of the fabricated QDs with a center spacing of 100 nm was 1.2 × 1010 cm−2, and the diameter of QDs was an average of 80 nm. This report combined EBL and SAE to manufacture the first QD laser.

Mokkapati et al. [44] presented the InGaAs QD laser with different lasing wavelengths. When the width of the stripe was as small as 5 μm, QD intensity could provide insufficient gain for lasing. Correspondingly, an excessive thickness of InGaAs was obtained when the stripe width reached 20 μm, introducing defects and increasing losses. Akaishi et al. [45] fabricate a series of SiO2 stripe masks uniformly arrayed on the InP substrate. They utilized a SiO2 mask array consisting of a wide mask on the side and a series of relatively narrow mask stripes. By changing the width of the wide mask while maintaining the narrow

stripe masks at 3 μm widths, the sizes of the InAs QDs varied in each opening region. The double-capped layer enabled the uniform height of QDs. When the width of the wide stripe mask was 200 μm, the emission wavelength range of the QD array grown between the narrow masks exceeded 120 nm.

#### 3.2.2. QDs Lasers by Block Copolymer Lithography

Morever, patterned QDs can also be prepared by block copolymer (BCP) lithography. A typical diblock copolymer consists of alternately polymerizing two polymers, one of which is miscible in small amounts in another polymer. This immiscibility leads to the phase separation of the two materials and maintains the properties of each material. The interaction between the two polymers produces a QD pattern. The absence of a wetting layer resulted in perfect three-dimensional confinement. Figure 8 presents the process of SAE QD fabrication using diblock copolymer lithography.

**Figure 8.** Selective growth of QDs by diblock copolymer lithography. Reprinted with permission from ref. [46] © Elsevier. Copyright 2006 Journal of Crystal Growth.

Polystyrene-block-poly (PS-b-PMMA) was the diblock copolymer in this process. First, the amorphous mask materials were deposited onto the substrate using plasma chemical vapor deposition (PECVD). Then the PS-b-PMMA brush was coated above the mask. After spinning the PS-b-PMMA on wafers, the entire structure was exposed to UV such that the PS area was exposed as the pattern template upon the removal of PMMA. ICP transferred the pattern into the mask layer, followed by the removal of the PS region via etching. Finally, the selective area growth of QD was accomplished in the cylinder hole.

The uniform size distribution and spatial position of QDs fabricated by block copolymer lithography provide more significant advantages than SAQDs grown in SK mode. Li et al. [47] obtained uniform GaAs QDs with a density of approximately 1011 cm−2 by selective growth in the narrow openings of SiNx patterned masks using block copolymer lithography. Kim et al. [48] fabricated GaAs/InGaAs/GaAs compressive–strain QDs using SiNx patterns defined by block copolymer lithography. Figure 9 shows the InGaAs QD laser structure. The CBr4 in situ etching time was optimized to reduce the processing damage before growing the QDs, leading to a decrease of the nonradiative recombination centers of the QDs. Compared with the devices grown using non-optimized in situ etching times, the QD lasers grown using optimal etching times significantly reduced the threshold and transparency current. Furthermore, Kim et al. [49] fabricated an InAs QDs laser with an In0.1Ga0.9As QW carrier collection layer to increase the carrier injection into the QDs. The QD laser achieved lasing at RT.

**Figure 9.** Structure of a InGaAs QD laser. The lower QD base was grown at 700 ◦C, and the upper regrown layers were grown at 625 ◦C. Reprinted with permission from ref. [48] © Elsevier. Copyright 2017 Journal of Crystal Growth.

#### *3.3. Heteroepitaxy Lasers on Si or SOI*

The natural characteristics of indirect bandgaps limit the wide applications of silicon photonic devices. This inherent property results in weak light emission and poor optoelectronic performance. Direct bandgap materials exhibit good luminescence properties. The direct bandgap III–V semiconductor lasers are high-performance light sources that may be utilized for silicon photonic integrated circuits. However, a high density of threading dislocations (TDs), planar defects, and anti-phase boundaries (APB) arises at the interface of III–V and silicon [50–53], owing to the mismatch of lattice constants, thermal expansion coefficient, and polarity between III–V materials and silicon. There is great research interest in finding methods for growing lattice constant-matched or low-defect-density III–V light sources directly on silicon. Defects introduced by the lattice and polarity mismatches between the III–V materials and Si have been substantially eliminated in a previous study using a template in conjunction with bonding techniques [54]. However, it is expensive and time-consuming to prepare such templates. Therefore, it is more desirable to directly epitaxially grow high-quality III–V materials on Si.

#### 3.3.1. Aspect Ratio Trapping (ART)

Bonding technology is the most common III–V/Si integration technique, combining a known good III–V epitaxial layer with a silicon platform. The main shortcomings of the bonding technique are the strict requirements for ultra-clean and highly flat surfaces, the high thermal resistance introduced by the oxide during bonding, and the wafer size mismatched between the Si substrate and the III–V (InP and GaAs) material substrate. These shortcomings limit the high-density integration of laser on silicon [55]. The local bonding process is technically challenging and expensive, not conducive to mass production [22]. SAE has become an economical and reproductive approach for relieving the defects resulting from mismatched crystal growth and realizing the local integration of III–V lasers on a silicon platform. Figure 10 presents the principle of SAE to decrease dislocation densities in the epitaxial layer at the interface of III–V materials and silicon.

**Figure 10.** Principle of ART and epitaxial lateral overgrowth (ELO). The circle is a partially enlarged view of the dislocation propagation. The dislocations, marked as dotted lines, only propagate to the lateral overgrowth layer through openings in the patterned mask.

The growth begins in narrow openings on the masked substrate. The dislocations that occur at the interface of III–V and silicon because of the heavily lattice-mismatched can only propagate along a certain angle inclined to the substrate plane from the opening area to the epitaxial layer. For Si (0 0 1) substrates, the dislocations propagate along with the <1 1 1> directions, forming a 54.7◦ angle with (0 0 1). The thickness of the materials exceeded that of the mask as the growth process proceeded. The epitaxial layer grows laterally along the mask surface, which is referred to as epitaxial lateral overgrowth (ELO or ELOG). Dislocations cannot propagate into the mask and vanish in the epitaxial layers because the interface lattice between the crystalline material and the amorphous mask is discontinuous. If the growth time is sufficiently long, the epitaxial layer grown in the opening region merges with the layer grown in the neighboring opening region to form a complete epitaxial layer above the masks. The mask effectively prevented dislocation propagation, and the dislocation density distributed in the lateral overgrowth partial of the epitaxial layer was several orders of magnitude lower than that grown on the standard unmasked substrate. Figure 11 shows the cross-sectional TEM image of the ELOG InP with SiO2 masks on silicon. The region above the mask was dislocation-free. Figure 11a shows that the dislocations from the buffer layer could penetrate the region above the mask if the openings were wider than the thickness of the mask. Figure 11b shows that the dislocations were filtered even above the openings if the opening width was smaller than or equal to the mask thickness. There were no coalescence defects above the mask, which refers to defects formed at the junction of two laterally grown InP layers from adjacent openings [56]. This approach, called the aspect ratio trap (ART), has been used in various materials grown on Si [57–59].

Moreover, III–V materials grown by ART are on the sub-micrometer scale, which is not conducive to micrometer-scale ridge waveguide fabrication. Han et al. [60] proposed a lateral ART scheme to grow a micrometer-scale InP sandwiched by oxides on the siliconon-insulator (SOI). A schematic of lateral ART is shown in Figure 12. Compared to the conventional ART grown on (001) Si, lateral ART started at the {111} Si surface, which would not form APBs. The dimensions of the III–V ridge width were limited by the trench width in conventional ART. The size of III–V materials grown in lateral ART is controlled by the thickness of the Si layer, realizing precise micrometer- or nanometer-scale control.

**Figure 11.** TEM cross-sectional view of ELOG InP on Si; (**a**) mask opening width was larger than the thickness of the mask; (**b**) mask opening width was small to filter dislocations. Reprinted with permission from ref. [56] © Elsevier. Copyright 2012 Current Opinion in Solid State & Materials Science.

**Figure 12.** (**a**) Schematic of conventional ART technique. The epitaxy proceeds along [001] direction. (**b**) Schematic of defect trapping and growth mechanism of lateral ART technique. The growth was along the [110] direction. Reprinted with permission from ref. [60] © AIP Publishing. Copyright 2019 Applied Physics Letters.

The ART method has been widely used in shallow trench isolation (STI) structures, where a thin Ge layer is typically grown in situ and covers the bottom of the trench as a buffer layer. The lattice mismatch between InP and Ge is only half that between InP and Si, leading to the easier nucleation of InP on the Ge surface [61]. By designing the Ge surface profiles and setting an aspect ratio larger than two, TDs were confined at the bottom of the trench, and APB formation was suppressed [62,63]. GaAs [64,65], InP [66], and InGaAs [67] have been grown on (001) Si with low defect density using SAE technology. After obtaining defect-less high-quality epitaxial layers, III–V materials were grown to

fabricate lasers. Staudinger et al. [68] fabricated wurtzite InP microdisks using SAE. They deposited 300 nm-thick SiO2 and patterned approximately 50 to 100 nm line openings. The wurtzite InP was nucleated in the trenches and grew vertically along the [111] A-direction. The zipper-points at the center of the line openings induced ELO after the wurtzite InP extended out of the SiO2 mask, and {1−100} or {11−20} wurtzite facets were formed. The micro-photoluminescence of this system demonstrated that the wurtzite InP microdisk enabled optically driven lasing at room temperature with a threshold of 365 μJ cm−2. Wong et al. [69] deposited 200 nm-thick SiO2 onto InP (111) A substrate using PECVD. They patterned 120 nm ring-shaped mask openings using EBL and ICP. A micro-ring InP laser was obtained via SAE using a two-stage growth process. In the first stage, the temperature and V/III were 750 ◦C and 300, respectively. The opening areas were completely filled with high-quality wurtzite InP, and ELO occurred. The growth temperature was then lowered to 730 ◦C, and the V/III content was increased to suppress vertical growth and enhance lateral growth. The micro-ring laser was operated at room temperature with a low threshold.

#### 3.3.2. Lasers on Si/SOI

Wang et al. [70] demonstrated an optically pumped InP-based DFB laser array integrated on (001) Si, operating at room temperature using the SAE technique. InP was selectively grown in the trenches of the STI structure. ART limits the defective layer to a thickness of approximately 20 nm, instead of several micrometers. The emitted light can couple with the optical waveguide defined in the same horizontal plane. Figure 13 shows the structure of the device. Each DFB laser in the array contained an InP waveguide with a λ/4-phase shift first-order grating, and a second-order grating was defined 30 μm away from the DFB cavities. The Si substrate below the InP laser was intentionally etched away to avoid pump light absorption. Silicon substrates supported suspended cavities at both ends.

**Figure 13.** Schematic diagram of the InP-on-Si DFB laser array structure. Reprinted with permission from ref. [70] © Springer Nature. Copyright 2015 Nature Photonics.

The suspended cavity exhibited thermal, electrical injection, and mechanical stability issues. These problems can be addressed if the laser structure is grown on a SOI. Megalini et al. [71] successfully fabricated InGaAsP MQWs in InP nanowires. SEM and TEM images of the InGaAsP MQWs are shown in Figure 14. The 500 nm-thick SiO2 was deposited on the Si surface and patterned into 200 nm-wide stripes with a spacing of 800 nm. The Si layer was etched by KOH to form V-grooves. The inhomogeneous spectrum shape broadening and wavelength peak emission observed in PL tests were attributed to the poor uniformity of the MQWs, as shown in Figure 14a. The MQWs of {001} were thicker than those of {111}, as shown in Figure 14c,d, which caused a spectrum of inhomogeneous broadening and decreased the lasing material gain. The growth was completed during the V-groove patterning of the SOI to trap most of the defects at the Si/SiO2 interface. The PL peak wavelength of the MQW was 1567 nm.

**Figure 14.** (**a**) SEM cross-section images of the InP nano-ridge. (**b**) TEM image of a single nano-ridge with a good symmetry. TEM images of the QWs and the barrier layers in (**c**) the (001) surface and (**d**) the {11−1} B facet. The dark-colored layer is the barrier; the light-colored is the QW. Reprinted with permission from ref. [71] © AIP Publishing. Copyright 2017 Applied Physics Letters.

The mask prevents dislocation propagation and the growth of the ~μm buffer layer using the ART method. Compared to optically pumped lasers, electrically pumped lasers are more challenging to manufacture. There are two main difficulties: acquiring highquality ternary or quaternary compounds and the fabrication of micrometer-scale ridge waveguides with the constraints of sub-micrometer selective masks. Kunert et al. [72,73] integrated an InGaAs/GaAs heterostructure into box-shaped GaAs ridges with (001) flat surfaces outside the trenches, as shown in Figure 15. An apparent PL of the QW was observed for different ridge sizes. A sufficient III–V volume to fabricate a micrometer-order ridge waveguide can be realized by extending the ridge width.

Several studies have reported on electrically pumped lasers. Shi et al. [74] utilized InGaAs/InP strained layer superlattices (SLSs) as filter layers to suppress the propagating TDs. III–V materials were grown in V-grooved (001) Si to realize ART. They fabricated an electrically pumped InP-based laser at 1550 nm on (001) Si. Figure 16 shows the laser structure. The threshold current density was 2.05 kA/cm2, and the slope efficiency was 0.07 W/A. The maximum output power was 18 mW without facet coating. The continuous-wave (CW) operation temperature reached 65 ◦C, and the pulsed lasing operation temperature was up to 105 ◦C.

**Figure 15.** SEM image of the InGaAs/GaAs MQWs in box-shaped GaAs nano ridge structure; the InGaP cap layer was easily observed by the darker contrast. The three MQWs were slightly lighter in color. Reprinted with permission from ref. [72] © IOP Publishing. Copyright 2016 ECS Transactions.

**Figure 16.** Schematic representation of an InP-based laser on (001) Si. Reprinted with permission from ref. [74] © The Optical Society. Copyright 2019 Optica.

Wei et al. [75] fabricated a 1.3 μm InAs/InAlGaAs quantum dash (QDash) laser on V-grooved (001) Si. The two sets of eight-period In0.16Ga0.84As/GaAs films were grown above the V-grooved Si as a buffer layer to trap dislocations at the GaAs/Si interface. TDs were filtered in three sets of ten-period In0.61Ga0.39As/InP SLSs. After the n-type InP contact and cladding layers were grown, the three layers of InAs/InAlGaAs QDashes were formed. Finally, the p-type InP cladding and contact layers were deposited. A schematic of the device structure is shown in Figure 17. The threshold current density was 1.05 kA/cm2, and the output power per facet was 22 mW. The laser realized CW operation at 70 ◦C.

**Figure 17.** Cross-section schematic of QDash laser on V-grooved (001) Si, including the structural parameters of each epitaxial layer. Reprinted with permission from ref. [75] © AIP Publishing. Copyright 2020 Applied Physics Letters.

Fujii et al. [76] designed an InP-on-SOI template consisting of an InP buffer layer, InGaAs etch stop layer, and InP layer on a (001) InP substrate. The template was fabricated using a bonding technique. A SiO2 mask was deposited on the template and patterned using photolithography. They successfully fabricated an electrically pumped eight-channel membrane DFB laser array with wavelengths ranging from 1272.3 to 1310.5 nm by adjusting the geometries of masks to optimize the InGaAlAs MQWs on InP-on-SOI. The active regions were buried by n-doped and p-doped InP, which also formed lateral p-i-n structures. A schematic of the membrane laser array is shown in Figure 18. The fiber-coupled output power was greater than 1.5 mW in each channel at 25 ◦C. The actual lasing wavelength deviated from the designed lasing wavelength by less than 2 nm, and the average channel spacing was 860 GHz.

**Figure 18.** Schematic diagram of eight-channel laser array and membrane laser cross–sections. The output light for each channel was coupled with single–mode fiber by butt–coupling the facets of the high–numerical–aperture fiber and spot-size converter (SSC). Reprinted with permission from ref. [76] © The Optical Society. Copyright 2020 Optica.

Several studies on the SAE growth of III–V materials on SOI or Si have been conducted. The aim is to obtain epitaxial structures with a low defect density on Si. The main method is ART, which confines most of the dislocations in a nanometer–thick buffer layer, preventing

defects from reaching the surface of the epitaxial layer, thus deteriorating the performance of the optoelectronic device. However, the problem of obtaining high-quality ternary or quaternary III–V compounds limits the epitaxial laser structure on Si.

#### *3.4. Integrated Semiconductor Laser*

Monolithic PIC has been developed to meet the increasing data flow transmission demand of the Internet. Advanced material growth methods have promoted the development of the PIC. The PIC refers to the integration of numerous optoelectronic components on the same chip [77]. The various device structures in PICs require different energy gaps for each portion of the wafer, and the device must be integrated in a reproducible and cost-effective manner. SAE enables the spatially localized epitaxial growth of the desired material structures for specific chip functions. This is a low-cost and reproducible method to achieve device integration. Recently, integrated semiconductor lasers have included EML and MWLA, which are suitable for wavelength division multiplexing (WDM).

#### 3.4.1. EML

Monolithically integrated EML arrays are a promising light source for modern WDM systems. EML have attracted extensive attention owing to their small size, low packaging cost, low driving voltage, and good stability [78]. In the long waveband of thelong-distance optical communication system, such as 1.31 μm and 1.55 μm, the chirp phenomenon will appear, which is not conducive to the high-speed transmission of information. The modulator is integrated with the laser to form the modulated optical signal source because the modulator can avoid the large-wavelength chirp observed in directly modulated lasers. Using the SAE, the EML is fabricated by combining an electro-absorption modulator (EAM) and a laser on the same chip. Compared with discrete EAM and lasers, EML without fiber coupling reduces loss and cost and improves device reliability. At present, there are two main methods to realize the integration of EAM and laser: butt-joint (BJ) coupling (the laser and EAM are grown separately) and SAE [79].

Figure 19 shows the QWs achieved by BJ, wherein the MQWs of the EAM and the laser are grown in separate epitaxy steps. The MQW laser was grown on the entire wafer in the first step. Then, wet or dry etching was used to selectively etch the region, where the EAM MQW would grow in the second epitaxy. Two epitaxy steps increase the manufacturing cost, and the etched interface became rough and easily formed defects, deteriorating the output performance of the EML. BJ enables the optimization of the laser and EAM.

**Figure 19.** Schematic of the EAM and laser MQWs achieved using the butt-joint technique.

The SAE technique avoids poor interfaces and multiple epitaxy, as shown in Figure 20. Dielectric oxide masks were deposited and patterned in the laser region. The mask width was changed such that the laser region between the masks had a higher growth rate than that of the EAM region without mask coverage. The lower bandgaps of the MQWs of the laser were formed in one epitaxy step because of the growth-rate enhancement, which avoided the absorption of light due to the rough interface introduced by the etching process between multiple epitaxial growth layers and increased the output power of this system [80]. The disadvantage of the SAE is that the optimal parameters of the active region for the laser and modulator cannot be obtained simultaneously [81]. The bandgap of the MQWs in the EAM region is typically designed to be larger than that of the laser MQWs. The MQWs were grown between and outside of the mask stripes via SAE. The thickness

and composition of the MQWs in the two regions differed in one-step epitaxy. The MQWs located between the mask stripes could be well controlled to obtain the desired gain region, while it was difficult to simultaneously obtain perfect EAM MQWs outside of the mask region. For example, the laser preferably exhibited few QWs at the lower threshold current, while the EAM region required more QWs to obtain a high extinction ratio.

**Figure 20.** Schematic of the EAM and laser MQWs achieved using the SAE technique.

Zhao et al. [82] integrated a high-mesa DFB laser and EAM using SAE. The absorption and active regions consisted of an InGaAsP/InGaAsP MQW structure and an optical confinement layer with different thicknesses caused by the fusiform mask, as shown in Figure 5b. The spacing of the two SiO2 mask stripes was fixed at 15 μm, and the mask width varied from 30 to 15 μm in the tapered region. Figure 21 presents a schematic of this device. The EML CW threshold current was 26 mA without the modulation bias for the uncoated laser. The output power of the modulation was 5.5 mW when the current was 100 mA at the laser, in single–mode operation with a side–mode suppression ratio (SMSR) > 40 dB at 1.552 μm. The on/off ratio was 15 dB at the biased voltage of −5 V on the EAM.

**Figure 21.** Ridge-waveguide EML consisting of a DFB laser and EAM. The enlarged portion shows the MQWs and interface of the EAM and laser regions. Reprinted with permission from ref. [82] © IOP Publishing. Copyright 2005 Semiconductor Science and Technology.

Kim et al. [83] designed and fabricated an EML consisting of a distributed Bragg reflector (DBR) laser and an EAM. The mask geometry and schematic of the device are shown in Figure 22. The threshold current was 5.7 mA. The output power was 5 mW when the active region injection current was 65 mA. The SMSR was >45 dB at a laser current of 60 mA. A static extinction ratio greater than 20 dB at a biased voltage of −3 V was applied to the EAM.

**Figure 22.** (**a**) Geometry of the SiNx mask and (**b**) schematic diagram of an EAM integrated with a tunable DBR laser. Reprinted with permission from ref. [83] © Elsevier. Copyright 2007 Journal of Crystal Growth.

There is a transition zone of a few microns between the EAM and the laser. The energy bandgap in this region changes slowly because of the bandgap engineering for the SAE (shown in Figure 4), leading to light absorption losses. The quantum well intermixing (QWI) technique enables bandgap changes within a few microns. QWI refers to the generation of defects on the surface using a special method involving QWs. Defects can become incorporated into the QW active region under external actions, such as thermal annealing, thereby changing the QW bandgaps. Combining the QWI and SAE techniques further reduce the coupling losses of the EML. After undergoing SAE, the laser gain and EAM regions were covered with a thermal silicon oxide layer. The QW bandgap of the interface between the EAM and laser was abruptly changed within several micrometers through ion implantation and rapid thermal annealing, reducing the absorption loss of light.

Liu et al. [84] integrated a sampled-grating distributed Bragg reflector (SG-DBR) laser with an EAM by combining SAE and QWI, as shown in Figure 23. The mask width was 20 μm, and the gap of SiO2 stripes was 16 μm. When the current was 200 mA, the threshold current was 62 mA and the output power was 3.6 mW. The current of the front and rear mirrors varied from 0 to 70 mA, with increments of 2 mA; the gain part current was 100 mA unchanged, and the phase current was kept at 0 mA. The lasing wavelength ranged from 1552 to 1582 nm, and all SMSRs were larger than 30 dB. The extinction ratio was 17 dB at a bias voltage of −5 V in the EAM.

In addition to the conventional SAE technology that fabricates EML, Zhu et al. [85] used a modified double-stack active layer (DSAL) SAE technology, which has the advantages of both BJ and conventional SAE. Contrary to Figure 20, SiO2 was deposited in the EAM region, followed by the growth of the EAM and laser MQWs sequentially in a singlestep epitaxy process. The SAE-DSAL technique enabled the optimization of the EAM and laser MQWs separately because the growth proceeded at different times. The energy-gap difference became larger between the upper and lower MQWs in the laser region, which reduced the absorption of the EAM. The SAE-DSAL technique can reduce the threshold current of the EML. The threshold current of the EML was 20 mA. The output power was 10 mW with an injection current of 100 mA. The lasing wavelength was 1550.5 nm with an SMSR of more than 41 dB. The extinction ratio was 12 dB when the −3 V bias voltage was applied to the EAM. Zhu et al. [86] also fabricated an EML that combined SAE and DSAL. The threshold current was further decreased to 16 mA, and the output power was larger than 10 mW when the injection current was 64 mA. The wavelength was 1552.28 nm with

an SMSR larger than 53 dB. A 30 dB static extinction ratio over 30 was obtained when the bias voltage in the EAM was −5 V.

**Figure 23.** Schematic of EML consisting of a SG-DBR laser and EAM by combining SAE and QWI. The active region consisted of eight compressively strained InGaAsP MQWs. The 1.2 Q InGaAsP was used as the SCH structure. The grating mask and contact layer were InP layers. Reprinted with permission from ref. [84] © Chinese Physical Society. Copyright 2008 Chinese Physics Letters.

Monolithically integrated EML arrays are key light sources in modern dense wavelength division multiplexing (DWDM) systems. DWDM systems require high-speed, high-coupling output power, single-mode, and low-chirp multi-wavelength light sources in long-distance optical communications [87]. SAE reduces the complexity of EML array fabrication. Cheng et al. [88] designed a four-channel EML array and completed the device using SAE. The threshold current was approximately 18 mA, and the output power at 100 mA was 9 mW. The lasing wavelength ranged from 1551.8 nm to 1554 nm, and the average channel spacing was approximately 0.8 nm. The average value of the single-mode SMSR was up to 45 dB. The extinction ratio was 15 dB with −5 V voltage applied to the modulator.

Xu et al. [89] reported a ten-channel EML array using SAE. The arrayed waveguide grating (AWG) combiner was integrated with an EML array using BJ. The Ti film heaters integrated into the device achieved thermal tuning. The fabricated device is shown in Figure 24. The emission of the ten-channel EML spacing was 1.8 nm. The PL peak wavelength of the ten-channel in the laser region ranged from 1530 nm to 1580 nm due to the increasing width of the stripe mask. The threshold currents of each laser in the array was between 30 and 60 mA. The output power ranged from 8 to 13 mW at an injected current of 200 mA injected current. All channels had high single-mode light emission with an SMSR > 40 dB. The device had a static extinction ratio greater than 11 dB and a modulation bandwidth larger than 8 GHz.

#### 3.4.2. MWLA

Multi-wavelength DFB laser arrays (MWLAs) can realize a wide range of wavelength tuning in WDM optical communications. The advantages of monolithic integrated laser arrays are miniaturization, cost-effectiveness, high reliability, and low consumption [90,91]. Lasers with different emission wavelengths and uniform channel spacing are realized in MWLAs by adjusting the grating pitch and the ridge waveguide width [92–95]. The use of EBL to fabricate MWLA is time–consuming and expensive, and the limitation of EBL resolution makes it difficult to fabricate MWLA with a channel spacing of 1 nm. In the SAE, the same channel spacing can be achieved with oxide stripe mask widths of a few micrometers. The reproducibility and processing simplicity of SAE are much better than those of EBL. In SAE, the lasing wavelength of the active region and the effective refractive index of the waveguide are modulated by the thickness and composition variation of

the materials, which is achieved through the control of the dielectric mask geometry and material growth conditions.

**Figure 24.** Optical graph of the fabricated device. The size of the integrated chip was 5800 × 5800 μm2. The lengths of the DFB laser and EAM were 500 and 150 μm, respectively. A 50 μm isolation region was located between the EAM and DFB laser. Reprinted with permission from ref. [89] © Elsevier. Copyright 2017 Optics & Laser Technology.

Darja et al. [96] reported a four–channel DFB laser array with multi-mode interference (MMI) for 1.55 μm coarse wavelength division multiplexing (CWDM) systems. The width of the opening region between the mask stripes was 15 μm. The device fabrication process included: (1) the formation of the InGaAsP DFB grating; (2) SiO2 mask pattern fabrication; and (3) the growth of the InGaAsP MQW, InP cladding layers, and InGaAs capping layer. The average threshold current of the four-channel was an average of 70 mA. The lasing wavelengths of the four-channel DFB laser arrays were 1521.2, 1541.4, 1564, and 1580.6 nm, with SMSR > 30 dB.

The lasing wavelength and channel spacing of the MWLA must be consistent with the desired wavelength. It is not easy to guarantee that the actual situation of growing materials between different elements in MWLA matches the ideal design. Zhang et al. [97] modified the SAE method as shown in Figure 25. The conventional SAE-deposited and patterned masks on the buffer layer realized the selective growth of the laser arrays. The thicknesses of the SCH and MQW layers increased due to growth-rate enhancement, as shown in Figure 25a. The lower SCH and MQW active regions were first formed in the buffer layer. Then, the patterned masks were deposited above the MQW layer, followed by the completion of the remainder of the laser structure, as shown in Figure 25b. The gap of each mask stripe was fixed at 20 μm. The widths of the stripe masks varied from 0 μm by 1.5 μm steps to adjust the thickness of the upper SCH layer to different Brag wavelengths. The threshold current of each channel was approximately 18 mA in MWLA, with a wavelength spacing of 0.42 nm. The lower SCH and MQW layers achieved excellent control of the lasing wavelength and channel spacing because they were insensitive to the growth-rate enhancement and composition variation of SAE. The wavelength matching of the device was better than that of the MWLA fabricated using EBL.

**Figure 25.** (**a**) Schematic of the MWLA structure achieved via conventional SAE. The upper and lower SCH and MQW were thicker due to the growth-rate enhancement generated by the masks. Reprinted with permission from ref. [98] © Elsevier. Copyright 2013 Optics Communications. (**b**) Modified SAE. The lower SCH and MQW were hardly affected by the growth-rate enhancement, enabling the realization of precise emission wavelengths. Reprinted with permission from ref. [95] © Springer Nature. Copyright 2018 Science China Information Sciences.

Zhang et al. [99] subsequently combined the modified SAE and bundle integrated guide (BIG) to fabricate a four-channel DFB laser array integrated with an MMI and a semiconductor optical amplifier (SOA). The output power of each channel in the array was 17 mW at a current of 200 mA, and the slope efficiency was 0.11 W/A. The optical spectra of the lasers in the array showed SMSRs larger than 42 dB, with an average channel spacing of 0.6 nm.

Zhang et al. [100] integrated a ten-channel EML array with MMI to fabricate a transmitter by SAE and BJ. The average output power of each channel was 0.25 W with an injected current of 200 mA. The threshold current of the ten channels was between 20 and 40 mA. The SMSRs of the entire channel were greater than 40 dB. The extinction ratio ranged from 15 to 27 dB at a biased voltage of −6 V.

Guo et al. [101] conducted a 1.3 μm six-channel DFB laser based on an SAE for the first time. The width between the SiO2 mask stripes was fixed at 30 μm, and the mask widths in the array were 20, 30, 40, 50, 55, and 65 μm. The threshold current was approximately 20 mA, and the output power of all channels was greater than 10 mW at an injection current of 100 mA. The lasing wavelengths of the six-channel DFB laser were 1301.8, 1305.1, 1308.1, 1311.2, 1313, and 1315.7 nm, respectively. The threshold current was approximately 20 mA, and the output power of all channels was greater than 10 mW at an injection current 100 mA.

Kwon et al. [102] developed a ten-channel DFB laser array operating at 1.55 μm for the transmission of a 100 Gbit/s Ethernet system. The average SMSR was larger than 50 dB, and the average channel spacing was 8.2 nm at an injection current of 50 mA. The average threshold current in the ten-channel DFB laser arrays was 25 mA. Subsequently, Kwon et al. [103] fabricated a planar-buried heterostructure (PBH) eight-channel DFB laser array to obtain a higher energy efficiency and better output beam quality attributed to fine current confinement than that of the ridge waveguide structure. The threshold current of the lasers ranged from 8.5 to 11 mA, which is lower than that of the ridge waveguide structure DFB laser array described in a previous study [102]. The output spectra showed that the lasing wavelength changed from 1528.4 to 1584 nm, with an interval of 8 nm. The SMSR of each channel was larger than 50 dB, and the power loss was less than 2 dB after a 2 km transmission.

#### **4. Conclusions**

SAE is a crucial design and integration tool for fabricating semiconductor lasers. The thickness and composition of the active layer depend on the geometry of the patterned mask. Knowing the characteristics of growth-rate enhancement and composition variation in SAE is significant in realizing wavelength-tunable lasers. SAE has completed the fabrication of the BH laser with a low threshold and nearly circulated light spot, owing to its cost-effectiveness and inherent manufacturing simplicity. The SAE achieved uniform distribution and size of QDs. The main challenges in obtaining QDs using SAE are increasing the density of QDs and reducing the defects at the interfaces between the QDs and the surrounding embedded materials to improve the laser gain. Although the realization of electrically driven lasers on Si is limited by the availability of micrometer-scale materials and the large number of defects created by direct epitaxy on Si, novel growth schemes, including lateral ART, enable the material dimension to be micrometer-scale and block material defects within an appreciable size, which is beneficial for achieving electrically pumped lasers on Si.

SAE integrates lasers with other optoelectronic devices that are repeatable and costeffective, typically EML and MWLA. The EML and MWLA will continue to move towards high-speed arrays with low power consumption and cost-effectiveness. The emission intensity of the active region of the EML array channel decreases as the wavelength increases because of the accumulation of SAE mask defects, deteriorating long-distance optical transmission. The SAE should improve the precisely controllable lasing wavelength and channel spacing in the MWLA. Although remarkable progress has been made in the material growth and device fabrication in SAE, specific methods that can realize the precise control of emission wavelength and the position of materials in the active region are still unknown. Defects caused by the presence of the mask more or less deteriorate the output characteristics of lasers. The potential of SAE to reduce material defects, lower device consumption, achieve controllable lasing wavelengths, and improve modulation performance is yet to be explored and will be investigated in future studies regarding the SAE technique.

**Author Contributions:** Conceptualization, Y.Z. and Y.S.; methodology, Y.Z. and L.Q.; software, Y.W., L.L. and Y.S.; validation, B.W. and Y.Z.; formal analysis, J.Z., P.J., Y.L. and C.Q.; investigation, Y.Z., L.L., Y.S., J.Z., P.J., Y.L. and C.Q.; resources, Y.Z. and L.Q.; data curation, B.W. and Y.Z.; writing—original draft preparation, B.W.; writing—review and editing, Y.Z.; visualization, B.W.; supervision, Y.Z.; project administration, L.Q. and Y.N.; funding acquisition, L.Q. and L.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by National Science and Technology Major Project of China (2018YFB2200300); National Natural Science Foundation of China (NSFC) (62090051, 62090052, 62090054, 11874353, 62121005, 62090061, 61935009, 61934003, 61904179, 62004194); Science and Technology Development Project of Jilin Province<PHONE_NUMBER>9GX,<PHONE_NUMBER>2GX,<PHONE_NUMBER>6GX,<PHONE_NUMBER>7GX,<PHONE_NUMBER>8GX); Key R&D Program of Changchun (21ZGG13, 21ZGN23); Innovation and entrepreneurship Talent Project of Jilin Province (2021Y008); Special Scientific Research Project of Academician Innovation Platform in Hainan Province (YSPTZX202034); Lingyan Research Program of Zhejiang Province (2022C01108); Dawn Talent Training Program of CIOMP.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank Yugang Zeng, Yue Song, Li Qin, Lei Liang, Jianwei Zhang, Peng Jia, Yuxin Lei, Cheng Qiu, Yongqiang Ning, and Lijun Wang for helping with this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Review* **Research Progress of Monolithic Integrated DFB Laser Arrays for Optical Communication**

**Shen Niu 1,2, Yue Song 1,2,\*, Ligong Zhang 1,2,\*, Yongyi Chen 1,2,3,\*, Lei Liang 1,2, Ye Wang 1,2,4, Li Qin 1,2, Peng Jia 1,2, Cheng Qiu 1,2, Yuxin Lei 1,2, Yubing Wang 1,2, Yongqiang Ning 1,2 and Lijun Wang 1,2,5,6**

**Abstract:** Photonic integrated circuits (PICs) play a leading role in modern information and communications technology. Among the core devices in PICs is the distributed feedback (DFB) multiwavelength semiconductor laser array. Multi-wavelength semiconductor laser arrays can be integrated on a single chip and have the advantages of high stability, good single-mode performance, and narrow line width. The wavelength tuning range has been expanded through the design of the DFB laser array, which is an ideal light source for wavelength-division multiplexing systems. The preparation of DFB laser arrays with a large number of channels, ease of mass production, and accurate emission wavelengths has become an important field of research. The connection methods of lasers in DFB laser arrays are introduced systematically and the current methods of manufacturing multi-wavelength DFB laser arrays covering the perspective of technical principles, technical advantages and disadvantages, main research progress, and research status are summarized.

**Keywords:** DFB laser array; multi-wavelength; series and parallel; optical communication; photonic integrated circuits

#### **1. Introduction**

The rapid development of optical and wireless networks has driven the proportion of fiber broadband users in China from 56% at the end of 2015 up to 94% currently. The gigabit optical network covers more than 120 million households, and the end-to-end user experiences speeds of 51.2 Mbps. The continuous expansion of optical communication networks and the rapid development of big data, cloud computing, and other fields increase the requirement for sources of light. Semiconductor lasers are small in size, lightweight, have long lifespans, high efficiency, and integration, and have become an important light source for optical communication systems.

The speed of development of the optical communication industry relies on breakthroughs made in the optoelectronic device industry, such as semiconductor lasers and optical amplifiers. The current developmental trends in optical device technology are integration, intelligence, and tunability. Miniaturization, environmental protection, low

**Citation:** Niu, S.; Song, Y.; Zhang, L.; Chen, Y.; Liang, L.; Wang, Y.; Qin, L.; Jia, P.; Qiu, C.; Lei, Y.; et al. Research Progress of Monolithic Integrated DFB Laser Arrays for Optical Communication. *Crystals* **2022**, *12*, 1006. https://doi.org/10.3390/ cryst12071006

Academic Editor: Alessandro Chiasera

Received: 16 June 2022 Accepted: 18 July 2022 Published: 21 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

power consumption, and low cost are also important areas of research for future light sources used in communication.

In the NG-PON2 standard, a fiber capacity of 40 Gbit/s is achieved by utilizing multiple wavelengths and user terminal tunable transceiver technology at dense wavelengthdivision multiplexing (WDM) channel spacing. In terms of current actual products, the maturity of tunable receivers is lower than that of tunable transmitters. Therefore, tunable lasers are still the focus of this technology [1].

Tunable semiconductor lasers are an important component of future WDM systems, including external cavity diode lasers (ECDLs), vertical-cavity surface-emitting lasers (VCSELs), quantum cascade lasers (QCLs), sampling grating-distributed Bragg reflectors (SG-DBR), and adjustable distributed feedback multi-wavelength lasers and arrays (DFB-MLAs) [2].

The ECDL is usually based on a gain chip and an external mode-selection component and is used to extend the resonant cavity outside of the laser chip. By adjusting the external mode-selection component, ECDL realizes the wavelength-tuning function, allowing the line width to be significantly narrowed and a large tuning range to be obtained. However, this type of laser is large, the optical path is difficult to align, and the mechanical structure lags and wears out during the adjustment process, making it difficult to use in optical communication networks.

The unipolar light source of the QCL, based on the principle of electron transition between quantum-well sub-bands, is different from that of conventional semiconductor lasers. In the case of QCL, the lasing wavelength is limited by the forbidden bandwidth of the material and determined by the conduction and neutron bands. The energy level spacing between sub-bands can be changed by adjusting the thickness of the quantum well, thereby changing the lasing wavelength, and as a result, the QCL is widely used in the mid-and far-infrared bands. At present, QCL is mainly used in free-space communication, gas detection, and other fields, and the 1.3–1.5 μm band of optical communication does not need to use this scheme to change the wavelength.

Semiconductor lasers, such as VCSEL, SG-DBR, a vertical grating-assisted codirectional coupler laser with a sampled Bragg reflector (GCSR), DFB, and DFB-MLA, are available for use in photonic integrated circuits (PICs), which play a key role in overcoming the bottlenecks of transmission capacity and energy consumption in future broadband networks.

Tunable VCSEL lasers usually introduce air gaps in the laser manufacturing process and adjust their position using micro-electromechanical systems (MEMS) to change the size of the air gap, thereby changing the equivalent cavity length to achieve the desired wavelength tuning. However, the mechanical structure of wavelength-tunable VCSELs using MEMS results in wider line width, slower response, and lower output power.

The SG-DBR laser and GCSR laser have a front and rear grating, gain, and phase sections. Periodic modulation is performed on the uniform grating for the wavelengthtuning function, and the vernier effect of the sampling grating is used to expand the tuning range. Its tuning speed is very fast [3]. The mode stability difference between the SG–DBR laser and GCSR laser is its key weakness, which limits its application in optical communication.

Compared to the above tunable lasers, DFB lasers are advantageous because of their small sizes, easy monolithic integration, simple tuning schemes, high wavelength stability, and good single-mode performance. These characteristics promote their wide application in optical communication networks.

For DFB lasers, the output wavelength is related to the refractive index of the active region and Bragg grating period. For tunable DFB lasers, by changing the period of the laser Bragg grating and the injection current or temperature of the laser, the refractive index of the active region is changed accordingly to achieve the desired wavelength. The DFB laser is the most widely used as a mature semiconductor light source for optical communication networks. It has an excellent dynamic single mode and stability, avoids the influence of multi-longitudinal mode dispersion, and is suitable for multi-channel optical fiber communication systems. The tunable DFB laser can be set to any channel of the dense wavelength-division multiplexing system, and the device can be easily integrated into a single chip to reduce the power consumption of wavelength switching and simplify the system, thereby reducing the cost of the coupling, packaging, and energy consumption. In addition, in a definable optical network with a wavelength-selective switch, the tunable DFB laser provides a wavelength routing function, which effectively increases the reconfiguration capability of the optical network.

However, for a single DFB laser, the tuning wavelength range is generally below 10 nm, which does not meet the needs of large-scale tuning [4]. To overcome this problem, a series of DFB lasers can be assembled into arrays to expand the wavelength-tuning range. The first proposed scheme was by Japan's NEC Company [5].

The tuning mechanism of the DFB laser array can be divided into wavelength tuning of a single DFB laser and switching between different DFB lasers. Among them, the wavelength tuning of a single DFB laser, in principle, is to change the grating structure and refractive index of the laser by a certain amount by heating or adding current, thereby changing the emission wavelength. In addition, through the design of some special grating structures or active region and waveguide layer structures, wavelength tuning under different conditions can also be achieved, such as reconstruction equivalent chirp (REC) technology, selective area growth (SAG) technology, asymmetric periodic structure, and periodic gain coupling; switching between different DFB lasers is realized through a personal computer (PC) and a microprocessor (MCU). For the tuning of the DFB laser array, due to the limited space, only the REC technology and the SAG technology are mainly introduced in this paper.

The DFB laser array has good single-mode stability and a simple wavelength-tuning scheme that can be roughly tuned by switching different lasers and fine-tuned by adjusting the laser temperature or injection current. With careful design, DFB laser arrays for coarse and fine WDM systems (20 nm and 0.8 nm channel pitch, respectively) can be fabricated.

Compared with hybrid integrated laser array modules consisting of discrete DFB lasers, a well-designed monolithic integrated DFB laser array can potentially reduce system costs by simplifying the optical alignment and packaging process. The DFB laser array with accurate multichannel spacing can achieve an accurate wavelength spacing to align with the wavelength standard of all WDM channels. Furthermore, it can be tuned significantly faster than that more mature thermally tuned semiconductor lasers; therefore, it has a wide range of applications in WDM.

The time classification of wavelength channel tuning is also specified in the NG-PON standard. For class 1, the switching time is generally required to be less than 10 μs, which is difficult for ordinary electrically tuned and thermally tuned lasers to achieve, while multi-wavelength DFB laser arrays can be achieved at this point [1].

For DFB laser arrays used in optical communication, in addition to the conventional properties such as wavelength, linewidth, power, and side-mode suppression ratio (SMSR), we also need to pay attention to issues such as wavelength stability, wavelength accuracy, and wavelength locking [6]. For wavelength stability, it is usually defined by the maximum spectral excursion (MSE). In NG-PON2, when the channel spacing is 100 GHz, the MSE should be within the range of ±20 GHz, and when the channel spacing is 50 GHz, the MSE should be within the range of ±12.5 GHz [7]. For DFB laser arrays with channel switching, the MSE when the laser is on/off must also be compliant. Due to the use of tunable lasers in the array, its inherent wavelength accuracy and wavelength stability can be moderately relaxed to reduce cost, and the channel is aligned by tuning, but must be compensated by employing a wavelength locking mechanism, usually relying on optical line terminal feedback.

Multi-wavelength DFB laser arrays have been widely investigated and exploited by the researchers in institutes and corporations.

At present, the main difficulties facing multi-wavelength DFB laser arrays are that the emission wavelength needs to be highly accurate, the channels must not affect each other, the number of integrations must be as large as possible, and it is necessary to find a feasible solution suitable for mass production.

In the introduction, we consider the properties of the required light source from the perspective of practical application and compare various lasers. On this basis, the properties and working principles of the DFB laser array are briefly introduced.

DFB laser arrays can be divided into series, parallel, and series–parallel combination structures, depending on the connection mode; design and manufacturing methods of these arrays include electron beam lithography, nano-imprint lithography, reconstruction equivalent chirp, ridge width change, and selective area growth.

This paper reviews the principles, advantages, and disadvantages, research progress, and application status of the above structures and methods. It is hoped that this study can provide a reference for research on the monolithic integrated DFB laser array for optical communication, as well as the promotion of their development.

#### **2. Connection Method**

#### *2.1. Parallel DFB Laser Array*

2.1.1. Technical Principle, Advantages, and Disadvantages

A parallel DFB laser array is an array in which each laser has an independent waveguide and is coupled together by an S waveguide and a coupler, as shown in Figure 1.

**Figure 1.** Integrated parallel DFB laser array [8] © Springer link. Copyright 2018 Science China Information Sciences.

The light emitted by the parallel DFB laser array needs to be coupled to a waveguide through photosynthetic wave devices to output. Various photosynthetic wave devices can be used such as a star coupler, directional coupler, multi-mode interference coupler (MMI), array waveguide grating (AWG), and micro-electro-mechanical system (MEMS). A star coupler can be provided with a high wavelength ratio, but its output power between channels is not uniform and produces high power loss. A directional coupler meets the very low insertion loss and reverse reflection requirements; however, the gap between waveguides is sub-micron in size, resulting in a fabrication process that is difficult to control accurately. Currently, directional couplers are commonly used in MMIs and AWGs, as shown in Figure 2.

Based on the principle of self-imaging, the MMI is not sensitive to wavelength and is suitable for the combined output of the multi-wavelength laser array. The process is relatively simple, but it has a low output power and large insertion loss, which increases with the increase in the number of channels. To solve these limitations, the MMI can be etched in both deep and shallow ridges, where the deep ridge can reduce insertion loss and crosstalk, and the shallow ridge has less end reflection.

The AWG is large and has both deep and shallow etching structures. The deep etching structure is small in size but is accompanied by a high insertion loss of greater than 5 dB in general. In contrast, the shallow etching structure has a relatively low insertion loss, generally less than 2 dB, but it requires a large bending radius, and the device is large. In InP-based chips, the AWG has wavelength sensitivity and as the wavelength of the laser changes, its insertion loss further increases.

Additionally, couplers are no longer required when using MEMS. In this configuration, a micromechanical mirror is added to the DFB laser array to achieve coupling. As MEMS tilt mirror achieves precise optical-mechanical alignment electronically, the tolerance is relaxed, and any deviation in the package can be corrected to reduce the package cost [9]. This method increases the device size compared to on-chip integration.

**Figure 2.** Schematic diagram of different couplers, (**a**) AWG [10] and (**b**) MMI [11]. © (**a**) Elsevier. Copyright 2015 optics communications. (**b**) Springer link. Copyright 2013 Science China Information Sciences.

(**b**)

In terms of coupling stability, consider a system of N coupled semiconductor lasers described in dimensionless form by the following coupled-mode equations [12]:

$$\frac{dY\_j}{dt} = (1 - i\alpha)Z\_jY\_j + i\eta\tag{1}$$

$$\frac{T d Z\_j}{dt} = P - Z\_j - \left(1 + 2Z\_j\right) \left|Y\_j\right|^2 \tag{2}$$

where *Yj* and *Zj* are defined as the normalized electrical field amplitude and normalized excess carrier density in the *j*th laser in sequence, respectively. *t* is measured in units of the photon lifetime *τp*. *T* is the ratio between the carrier recombination time and photon lifetime, and is typically large. *P* is the normalized excess pumping current for a single laser above the threshold, *α* is defined as the linewidth enhancement factor, and *η* is the coupling constant between lasers, which represents the coupling difficulty and coupling strength required to obtain a stable in-phase solution.

If the lasers are coupled in parallel mode, the coupling term ∑ *Yk* in Equation (1) includes all the elements in the array except *k* = *j*. Then,

$$\sum Y\_k = \sum\_{K=1}^n Y\_{k'} k \neq j \tag{3}$$

It is convenient to reformulate Equations (1) and (2) in terms of the amplitude and phase of the electrical field. When *Yj* = *Ej* exp *iϕj* is introduced, Equations (1) and (2) become

$$\frac{dE\_j}{dt} = Z\_j E\_j - \eta \sum \left[ E\_k \sin \left( \varphi\_k - \varphi\_j \right) \right] \tag{4}$$

$$\frac{T d Z\_j}{dt} = P - Z\_j - \left(1 + 2Z\_j\right) E\_j^2 \tag{5}$$

$$\frac{d\varphi\_{\dot{j}}}{dt} = -aZ\_{\dot{j}} + \eta E\_{\dot{j}}^{-1} \sum \left[ E\_k \cos \left( \varphi\_k - \varphi\_{\dot{j}} \right) \right] \tag{6}$$

For parallel coupling, from the characteristic equation [12], use

$$
\eta > \eta\_P \equiv \frac{2aP}{2(1+2P)N} \tag{7}
$$

We will discuss this equation in detail in the next section when *η* is derived in a series array.

Technical advantage: the current DFB laser arrays are connected in parallel, meaning multiple DFB lasers are designed to be arranged in parallel and are coupled to a multimode interference combiner through a passive S-type waveguide. The advantage of this design is that its fabrication is relatively simple, only the fabrication of a single DFB laser and its couplings are considered, the mutual influence between the lasers is small, and the single-mode stability is high, and the tuning scheme is simple.

Technical disadvantage: during the coupling process, the output optical power encounters large losses, which is particularly critical for DFB lasers with an output wavelength of 1550 nm; therefore, an integrated optical amplifier (SOA) is often required. Moreover, as the number of lasers increases, the complexity and loss of parallel DFB laser arrays gradually increase when combining waves, the area occupied by the S-type passive waveguide increases, and the integrated optical amplifier will further add to the chip size, which is not conducive to the performance and integration of the chip, and also increases the total required current [13].

At present, for parallel DFB laser arrays, the main problem is that it is difficult to achieve low coupling loss and volume reduction simultaneously, which requires a higher output optical power of the laser or a coupling mode with lower loss.

#### 2.1.2. Main Research Progress and Status

A parallel DFB laser array was manufactured for the first time in 1984 [14] and has made great improvements in various aspects of performance over the years. Currently, in laboratory preparation and commercial application, DFB laser arrays are still mainly connected in parallel. This method can integrate 16 or more DFB lasers, covering C-band, L-band, or O-band, and is suitable for coarse wavelength-division multiplexing (CWDM) systems and dense wavelength-division multiplexing (DWDM) systems.

For DWDM systems, the parallel DFB laser array can achieve a channel spacing of 0.8 nm [15], which is difficult to achieve by other structures such as series and series– parallel combinations. However, at the same time, the coupling loss of a parallel DFB laser array increases when the number of integrated lasers increases. Additionally, the problem of large volume after the integration of the MMI and AWG has not been solved. One possible solution to this problem may be to manufacture smaller combination devices.

#### *2.2. Series DFB Laser Array*

#### 2.2.1. Technical Principle, Advantages and Disadvantages

To reduce the coupling difficulty and power loss of the laser array, a series DFB laser array is introduced, and its structure is shown in Figure 3. The technological process combines several DFB lasers in series such that all the DFB lasers are on the same waveguide. Thus, the laser array need not be laterally coupled, which avoids power losses during the coupling process, effectively reduces the chip size, and improves integration. However, in a tandem DFB laser array, the single-mode stability and wavelength accuracy of a single laser are easily affected by reflections from other lasers, resulting in poor beam quality and wavelength shift. When the wavelength spacing is less than 100 GHz (0.8 nm), grating crosstalk causes mode hopping. Using linear-chirped gratings to perform multiple *π*phase shifts in a full cavity is an effective method for reducing crosstalk and suppressing side modes.

**Figure 3.** Series DFB laser array [16] © Elsevier. Copyright 2015 optics communications.

In a series DFB laser array, the wavelength spacing between the lasers is generally large, which requires a large temperature or current range to be tuned to cover all the desired wavelengths. Although, an excessive temperature or current tuning range many lead to device failure. Furthermore, decreases in the output power affects the reliability of the device. To reduce the influence of reflection, the phases of Bragg gratings in different lasers can be designed, and phase-shift gratings can be introduced between lasers to realise single longitudinal mode lasing.

In terms of coupling stability, if the lasers are coupled in series, the coupling stability is mainly affected by the two neighboring lasers. Consider a loop configuration and the coupling term ∑ *Yk* in Equation (1) given by

$$
\sum Y\_k = Y\_{j+1} + Y\_{j-1} \tag{8}
$$

From the characteristic equation [12]

$$\eta > \eta\_{\varepsilon}(n) = \frac{nP}{2(1+2P)\sin^{2}\left(\frac{\pi n}{N}\right)}\tag{9}$$

The largest of all the *ηc*(*n*) corresponds to the wave number *n* = 1. Thus, it is required that

$$\eta > \eta\_s \equiv \frac{aP}{2(1+2P)\sin^2\left(\frac{\pi\mu}{N}\right)}\tag{10}$$

By Equations (8) and (11), the critical coupling strengths *ηs* and *ηp* have been verified to be identical if *N* = 3, since series coupling and parallel coupling are the same for three coupled lasers in a ring configuration. If *N* > 3, *ηs* becomes larger than *ηp*. Thus, series coupling requires a larger value of the coupling strength *q* to have stable in-phase solutions.

At present, the number of DFB lasers that can be integrated into a series array is far less than that of a parallel array, and generally only 3–4 DFB lasers can be integrated with a channel spacing generally above 2 nm. For the series DFB laser array, it is easy to achieve a small volume, but methods to integrate more lasers, and ensure the single longitudinal mode and beam quality have not yet been developed.

#### 2.2.2. Main Research Progress and Status

In a series DFB laser array, the number of lasers increases linearly as the required tuning range increases, which leads to an increasing length of the chip structure. By means of the shared grating sections of laser sections, that is, laser 1 passes through grating 1 and grating 2, laser 2 passes through grating 2 and grating 3, and laser *N* shines through grating *N* and grating *N* + 1, so that the total cavity length is significantly shortened, up to 40% in the array composed of 5 lasers [17].

In a series DFB laser array, the single-mode stability and wavelength reliability are poor due to the reflection of Bragg gratings in other lasers, especially when the wavelength spacing is very small. Lateral resonant modes can be suppressed by designing π phase shifts in the middle of each segment, and by inserting two mirrors on both ends of the laser to provide compensated reflection for both channels [18,19]. On this basis, a three-stage design was adopted to suppress the side mode by optimizing the current distribution and improving the priority of the dominant mode [20].

In 2021, Xiangfei et al. [21,22] proposed a tunable series-connected distributed feedback multi-wavelength laser array (DFB-MLA) based on reconstruction equivalent chirp (REC) technology. Its wavelength spacing was 2.4 nm and the SMSR was greater than 40 dB. The structure of the heat sink block was optimized for fast, continuous wavelength tuning.

#### *2.3. Series–Parallel Combined DFB Laser Array*

#### 2.3.1. Technical Principle, Advantages, and Disadvantages

Regardless of a series or parallel onefold scheme of the DFB laser array, an increase in the number of lasers should consequently increase the difficulty of ensuring a single-mode output and low power loss. Therefore, when the number of integrated lasers is large, the use of a series or parallel structure alone makes the chip structure long and narrow, leading to difficulties when packaging. Therefore, a series–parallel combination is required.

At present, the series–parallel combination scheme is relatively new, and difficult to implement because it experiences the disadvantages of both types of arrays simultaneously. Nevertheless, it is still expected to become a popular topic in DFB laser array research.

#### 2.3.2. Main Research Progress and Status

The series–parallel combination of a DFB laser array is not frequently implemented, but it may be the future development direction. In 2020, Xiangfei et al. [23,24] fabricated a 4 × 4 16-channel series–parallel DFB laser array that achieved 48 channels with a spacing of 100 GHz near 1550 nm and a temperature adjustment range below 20 ◦C. Compared with

the series-only configuration, this matrix configuration reduces the potential interference from adjacent lasers, which reduces the overall power loss compared with the parallelonly configuration.

#### **3. Implementation Method**

The wavelength of the DFB laser is related to the refractive index of the active region and grating spacing, as shown in Equation (11). When fabricating a DFB laser array in which each DFB laser has a different emission wavelength, the fabrication methods can be divided into two categories: changing the grating structure and index of refraction modulation, and is represented as follows:

$$
\lambda = \frac{2n\_{eff}\,\,\Lambda}{m} \tag{11}
$$

where *λ* is the lasing wavelength, *neff* is the effective index, Λ is the grating period, and *m* is the order of the grating.

#### *3.1. Changing the Grating Structure*

In the fabrication process of DFB laser arrays, various advanced fabrication methods are used to ensure the gratings of each laser have different periods. DFB laser arrays with different emission wavelengths can be fabricated based on one-time epitaxy of the same active region. Such technologies include electron-beam lithography, nano imprints, and REC technology.

#### 3.1.1. Fabrication Methods

Electron Beam Lithography

1. Technical Principle

Electron beam lithography (EBL) is a lithography technique with the current highest known resolution, which can reach less than 10 nm. Currently, direct-write electron-beam lithography is primarily used. A focused electron beam bombards the photoresist to form the required pattern and scans, point by point, by moving and switching the electron beam to obtain the required grating structure.

2. Technical Advantages and Disadvantages

Technical advantages: EBL has high precision, its direct point-by-point scanning does not require a photolithography mask, it can produce highly complex patterns, diffraction effects are negligible since the electron beam has a short wavelength, it supports dry etching rather than wet etching, it is easier to fabricate high-quality nanostructures, and can be used in high-precision processing of micro-nano electronic and optoelectronic devices.

Technical disadvantages: Owing to the high maintenance cost of EBL equipment, the need to write grating lines one by one leads to an extremely slow etching rate. Therefore, it is expensive and time consuming, and it is difficult to apply to the large-scale manufacturing of lasers and arrays. At the same time, EBL also has blanking or deflection errors and splicing errors, and only 35% of the lasers can be controlled within a range of ±0.2 nm [25]. Processes associated with EBL can generate errors of up to 3 nm [26]. Therefore, it is difficult to ensure the yield of this method when fabricating DFB laser arrays with small channel spacing.

3. Main Research Progress and Status

EBL is a new lithography technology developed from the scanning electron microscope. Since the 1970s, it has been widely used in semiconductor integrated circuit manufacturing. Due to its slow processing speed, it is often used to manufacture application-specific integrated circuit (ASIC) with quick turn-around times and repeatable optical frames that require extremely high precision [27]. It has also been applied to laser grating manufacturing.

In the fabrication of the DFB laser array, a large number of studies have used electron beam lithography to fabricate grating structures [15,28–35], and the results are shown in Table 1. However, throughput capability, the most important problem of electron beam lithography in industrial applications, has not yet been solved. Hence, this technology is still mainly used in the laboratory and in the industry to build very small structures.

**Table 1.** Recent achievements in the fabrication of DFB laser arrays using EBL.

#### Nano-Imprint Lithography

#### 1. Technical Principle

Nanoimprint lithography (NIL) is a pattern transfer technology; under pressure or heat or UV assisted action, the pattern on the template is transferred to the resist, the resist on the substrate to produce thickness difference, and then by etching the pattern transferred to the substrate. NIL is a promising, high-resolution, low-cost, nanoscale replication patterning technology.

The NIL pattern transfer technology process is illustrated in Figure 4, and is as follows: a film is created by spin-coating a resist onto a substrate, and the film is heated to vitrification. Subsequently, specific pressure is applied to imprint the pattern from the resist-coated substrate to the mold. Afterward, the mold is exposed to heat, UV light, or other elements to solidify the pattern. Finally, the mold and the pattern are separated and the pattern resist is left on the substrate, completing the photonic lithography process. DFB lasers with different grating periods can be prepared by imprinting templates of different shapes using this technique.

#### 2. Technical Advantages and Disadvantages

Technical advantages: NIL technology combines the advantages of EBL and holographic lithography and has the additional advantages of ultra-high resolution, easy mass production, low cost, fast speed, and high consistency. The imprint template is generally manufactured using high-precision EBL and can be used multiple times, thus significantly reducing costs. NIL technology changes the resist by imprinting instead of holographic lithography; therefore, its resolution is not affected by light diffraction, scattering, reflection, etc. Lithography pattern defects are suitable for large-scale manufacturing processes.

Technical disadvantages: during small-scale manufacturing, the cost of NIL technology is high owing to the high production cost of the imprint template. During the production process, the imprint template and the photoresist can easily lead to pattern defects, thereby decreasing precision. The mechanical brittleness of the InP substrate and the temperature

difference between the substrate and the imprint template in hot imprinting results in nanoimprinting requiring a very refined process to ensure a good yield.

**Figure 4.** Schematic diagram of the principle of NIL technology.

3. Main Research Progress and Status

NIL technology mainly includes thermal and UV embossing.

Thermal NIL was first proposed by Chou in 1995 [37] and has been widely spread in the manufacturing process of optoelectronic devices such as gratings, microrings, and photonic crystals. At present, this technology is widely used in the fabrication of semiconductor laser gratings, which is a promising DFB laser array grating manufacturing technology. Thermal embossing is simple; however, the heating and cooling durations are long and time consuming, the thermoplastic polymer is prone to thermal expansion and contraction during the heating and cooling process, and the dimensional stability is poor, which easily leads to distortion of the copied graphics. This technique can be improved by employing laser-assisted direct imprinting [38].

There are two main challenges in laser grating thermal NIL. (1) The imprint process is very delicate because of the mechanical brittleness of the InP substrate and the thermal mismatch between the substrate and Si imprint. (2) Subsequent processing requires specific imprint compressive thicknesses after plate making, the crystallographic direction of the substrate, and arrangement of the grating for plate making [39].

Therefore, for thermal NIL, the most important physical properties of the mask are the thermal expansion coefficient and Poisson's ratio. In addition, properties such as hardness, durability, and surface roughness should also be considered. Material selection includes Si, SiO2, SiC, Si3N4, metal, and sapphire [40].

For InP substrates, current studies have shown that the maximum pressure in hot stamping technology does not exceed 3 bar [39].

UV imprinting is also known as room-temperature imprinting. In 1996, UV nanoimprint lithography was designed by introducing a low viscosity UV-curable polymer layer to improve the fluidity of the imprint material [41]. The process involves cross-linking the polymer by UV light on a transparent die after embossing on a UV-cured layer, which significantly reduces the pressure during nano embossing without the need for high temperatures. Low temperature eliminates deformation errors and graphic distortions caused by differences in the thermal expansion coefficient, and low pressure makes the process more likely to succeed on more brittle substrates such as InP and GaAs. Therefore, the technology is more suitable for PICs and manufacturing DFB laser arrays.

UV soft-printing technology is used to fabricate gratings, as shown in Figure 5 [42]. Compared with nano-imprinting directly using hard master printing, the pattern quality is better, especially in the case of surface unevenness that often occurs during multiple epitaxy processes. The specific implementation method is as follows. First, a soft imprint is prepared by imprinting a hard master composed of Si on a flexible intermediate polymer imprint (IPS Obducat AB) using thermal NIL. Next, the epitaxial wafer is carefully cleaned and coated with a layer of UV-curable resist (stuc-220, Obducat AB). Home-made mechanical tools are used to precisely align the soft-printed grating pattern with the grating segments on the wafer. The pattern is then transferred onto the epitaxial structure using an intermediate polymer imprinting (IPS) process by performing simultaneous thermal and UV imprinting processes. After cleaning the residual resist using an oxidative reactive ion etching (RIE) process, inductively coupled plasma (ICP) RIE is used to prepare the grating, and O2 plasma is used to remove the mask.

**Figure 5.** Preparation of gratings by UV soft printing (**a**–**c**) transfer the patterns onto the epitaxial structure (**d**) clean residual resist (**e**) fabricate grating (**f**) remove the mask [42]. © IOP Publishing. Copyright 2014 Journal of semiconductors.

However, as there is no heating or cooling process, polymer bubbles cannot be eliminated, which greatly affects the graphics. An improvement in this process uses a step-flash or thermal UV imprint technology that combines UV imprinting with stepping technology or thermal imprints, respectively. In this process, the corrosion inhibitor is the key problem. The selected resist must maintain low viscosity at room temperature, be able to cure under UV irradiation with minimal shrinkage to maximize pattern accuracy and be released from the mask after curing to adhere to the substrate. A modified poly-dimethyl-siloxane (PDMS) soft stamp has been used to reduce pattern deformation and residual layer thickness, and the residual layer thickness has been reduced by 50% [43].

NIL technology is also widely used in the fabrication of DFB lasers and array gratings, and there have been many recent innovations. Compared with Si substrates used in largescale integrated circuits, compound semiconductor substrates such as GaAs and InP often have large thickness fluctuations, resulting in the uneven thickness of the residual layer in the imprinting area when NIL technology is applied, ultimately resulting in the graph changes. To solve this problem, reverse NIL technology has been developed and is based on step-and-repeat imprinting named step and flash imprint lithography (SFIL), which can inhibit the uneven residual layer caused by substrate fluctuation [44].

In the NIL process, the duty cycle of gratings may change due to the deformation of the resist and the soft mode. However, when the grating angle is less than 3◦ and the duty cycle is between 0.4 and 0.6, the effect of error can be ignored, which can be easily achieved with SFIL technology [45].

In NIL, the imprint template is usually prepared using the EBL method, which provides better stability. In addition, during the pattern-transfer process using the NIL method, the resist is in direct contact with the stamp with little deformation. Therefore, NIL can smooth raster edges. This has important implications for the fabrication of high-quality lasers, which can reduce the spectral width of higher-order Fourier components owing to imperfect gratings.

#### 3.1.2. Special Grating Design Reconstruction

Reconstruction Equivalent Chirp

#### 1. Technical Principle

The REC technique is based on the reflection response. By designing a sampling grating based on the sampling function, the chirp structure of the grating period that changes with position is obtained, and the function of the complex grating is realized effectively. The REC technique achieves equivalence with complex grating structures by fabricating specially designed sampling gratings that are superposed on a uniform gratings pattern. The REC technology utilizes sampled Bragg gratings (SBGs) to design and fabricate lasers and laser arrays with complex grating structures. By changing the sampling mode, an equivalent chirp (continuous variation) or equivalent displacement (discrete variation) of the grating neutron grating (Fourier component) can be generated. The Bragg wavelengths of the sub-ratings can also vary with the sampling period. Thus, various sub-grating with complex structures can be obtained by designing the sampling patterns in advance. The sampling period is usually a few microns and can be formed by ordinary photolithography [46].

According to Fourier analysis, the index modulation of a grating section with sampling period *P* and a basically uniform (seed) grating period Λ0 can be expressed as [47]:

$$
\Delta n(z) = \frac{1}{2} \Delta n\_s \sum\_m F\_m \exp\left(j\frac{2\pi z}{\Lambda\_0} + j\frac{2m\pi z}{P} + \varphi\right) + c.c.\tag{12}
$$

where Δ*ns* is the index modulation of the uniform seed grating, *Fm* is the Fourier coefficient of the mth order sub-grating *z* is the position along the laser cavity, Λ0 is the seed grating period, *P* is the sampling period, *ϕ* is the initial phase of the seed grating, *m* is the order of Fourier component, and usually the +1*st* or −1*st* order sub-grating is used. The ±1*st* order period can be expressed as,

$$\frac{1}{\Lambda\_m} = \frac{1}{\Lambda\_0} + \frac{m}{P} \tag{13}$$

Therefore, if the sampling period *P* is carefully designed for a suitable wavelength grid and each laser section with a specific wavelength is tuned by changing the chip temperature, a wide wavelength range can be covered and the wavelength tuning can be realized simultaneously. According to Equation (11), Λ can change by changing *P* to achieve different output wavelengths in different lasers.

#### 2. Technical Advantages and Disadvantages

Technical advantages: the REC technology can achieve complex grating structures through micron-scale sampling patterns. In the grating fabrication process, only two steps are required: holographic exposure of the uniform base (seed) grating and lithography of the sampling pattern. All other processes are the same as those used for conventional DFB lasers. Therefore, the manufacturing costs are low. The most important advantage of this technique is that it provides precise fabrication of periodic structures for precise control of wavelengths, and its accuracy is 100-fold higher than that of general theoretical methods [48].

Technical disadvantages: owing to different sampling periods, the phase of the cavity surface of the laser will change, which destroys the uniformity of the wavelength spacing of the laser array. To obtain a uniform wavelength separation, minimal reflection is required on both laser faces; however, this results in a high threshold current and low slope efficiency. Without a reflection-enhancing coating, the wavelength deviation of the laser array can reach approximately 1 nm [47]. In addition, the REC technique reduces the SMSR of side-mode sampling.

#### 3. Main Research Progress and Status

REC technology was initially proposed in fiber Bragg gratings [49] and has been applied to optical filters, fiber lasers, OCDMA en/decoders, and other important optical devices. Xiangfei et al. conducted extensive and in-depth research on the fabrication of DFB laser arrays using REC technology. REC technology in 2009 to fabricate a multi-wavelength DFB laser array, which attracted wide attention due to its low cost and simple method [50]. In 2012, they then made an 8-channel parallel DFB, with an error tolerance of approximately 520-fold higher than the previous array [48].

For the DFB laser array, because it integrates multiple DFB lasers, the yield drops sharply compared with a single DFB laser. The yield is a key issue in the fabrication of DFB laser arrays. The 60-channel DFB laser array produced by REC technology can control the wavelength error of 83% of the laser within ±0.20 nm and 93.5% of the laser within ±0.30 nm [51], greatly improving the yield. REC technology can not only be used in the manufacturing of parallel DFB laser arrays but also in the manufacturing of series DFB laser arrays [22] and series–parallel combined DFB laser arrays [52].

#### *3.2. Index of Refraction Modulation*

In the fabrication of DFB laser arrays, in addition to realizing different emission wavelengths by preparing gratings with different periods in different lasers, wavelength changes can also be achieved by changing the refractive index in different lasers. Such technologies include ridge width variation technology and selective area growth.

#### 3.2.1. Ridge Width Variation Technology

#### Technical Principle

The systemic equivalent refractive index can be adjusted by adjusting the width of the ridge waveguide of the laser, according to Equation (12).

#### Technical Advantages and Disadvantages

Technical advantages: the process of changing the width of the laser waveguide is simple, and is suitable for manufacturing DFB laser arrays with narrow channel spacing.

Technical disadvantages: the scope of application is limited, the threshold value of the device is too wide and may cause the device to generate multiple transverse modes, the waveguide is too narrow, the series resistance is too high, the thermal effect is obvious, and it is difficult for the device to achieve consistent characteristics.

Main Research Progress and Status

Li et al. [53] proposed varying ridge widths to adjust the lasing wavelength of DFB laser arrays in 1996 and demonstrated that even if the ridge width was sufficient to support multiple lateral modes, single-mode lasers could be maintained. Therefore, the tuning range of the array could be increased without affecting the single-mode performance.

The method can be used to fabricate DFB laser arrays with very small channel spacing, which can reach 0.8 nm [54]. However, because the effective refractive index of the laser varies nonlinearly with the width of the ridge waveguide, and the width of the ridge waveguide cannot be too small (the machining accuracy is difficult to achieve) or too large (A high-order transverse mode is generated), the wavelength tuning range that can be achieved by this scheme is very limited.

#### 3.2.2. Selective Area Growth

#### Technical Principle

Selective area growth (SAG) is performed as follows: a series of mask stripes are fabricated on a substrate, and then epitaxial layers are grown in areas without mask stripes, as shown in Figure 6. In the SAG process, SiO2 mask stripes are formed on the substrate before the material is grown; thus, in the subsequent Metal–Organic Chemical Vapor Deposition (MOCVD) or Metal–Organic Vapor Phase Epitaxial Growth (MOPVE) process, the material is only grown where there are no mask stripes. In other words, SAG technology can be used to control the structural parameters of each laser active region, including the thickness and material composition, to change the refractive index of the laser active region, thereby changing the emission wavelength. The SAG technology can simultaneously control the multiple quantum well (MQWs) bandgap energy of the arrayed waveguide by changing the mask pattern's dummy stripe width and the width of the outer mask [55]. By using an asymmetric mask, an almost linear shift in emission wavelength can be obtained. For MQWs, because the emission wavelength is sensitive to the thickness of the well, extensive wavelength tuning can be obtained in addition to refractive index changes.

**Figure 6.** Selective area growth schematic (Zhang C et al. 2013 [11]). © Springer link. Copyright 2013 Science China Technological Sciences.

The thickness enhancement factor of the selectively grown layer is affected by the geometry of the mask pattern, including mask width and pitch. By designing the geometry of the mask, precise control of the thickness of the active region and the waveguide layer can be achieved, and thus control of the effective index of refraction can be achieved to achieve different wavelengths in the laser array [56].

The SAG mechanism is generally considered to be the surface diffusion component and the vapor phase diffusion component. Surface diffusion refers to the molecular diffusion of reactants from the mask surface to the exposed surface, while vapor phase diffusion refers to the vertical and lateral diffusion of reactant molecules in the gas phase and adhesion to the exposed surface.

In general SAG modeling, surface diffusion is usually ignored because it only occurs within a few micrometers of the edge of the mask.

Vapor phase diffusion can be described by the gas-phase diffusion model, and its diffusion equation is [57]:

$$\frac{\partial^2 \mathcal{C}}{\partial x^2} + \frac{\partial^2 \mathcal{C}}{\partial y^2} = 0 \tag{14}$$

where *C*(*y*, *z*) is the vapor phase concentration.

Assuming no growth of material on the mask, and that the gas concentration just above the crystal surface in the exposed region between the masks remains constant at equilibrium [57]:

$$\begin{cases} \begin{array}{c} z = d, \ \mathcal{C} = \mathcal{C}\_{\infty} \\ y = \frac{y\_0}{2} + w, z = \frac{r\_s}{\pi}, \mathcal{C} = \mathcal{C}\_{\mathcal{C}} \end{array} \\ \begin{array}{c} \text{(15)}\\ z = 0, \ \mathcal{C} = 0 \end{array} \end{cases} \tag{15}$$

where *d* is the height of the stagnant layer, *y*0 is the width of the mask, and *w* is the distance between the masks. *rs* is the radius of the hemicylindrical structure of the grown SAG structure at the window. The value of *rs* is equivalent to the window width, *w*, and is assumed to be much smaller than *y*0 to simplify the following formula.

Under this boundary condition, the solution of equation (14) is [57]:

$$C = A \ln\left[\sin^2\left(\frac{\pi}{y\_0}y\right) + \sinh\left(\frac{\pi}{y\_0}z\right)\right]^{\frac{1}{2}} + B \tag{16}$$

where *A* and *B* are [57]:

$$A = \frac{\mathbb{C}\_{\infty} - \mathbb{C}\_{\varepsilon}}{\ln\left[\sinh\left(\frac{\pi d}{y\_0}\right)\right] - \ln\left[\sinh\left(\frac{r\_s}{y\_0}\right)\right]} \stackrel{\simeq}{=} \frac{\mathbb{C}\_{\infty} - \mathbb{C}\_{\varepsilon}}{\ln\left[\frac{y\_0}{r\_s}\sinh\left(\frac{\pi d}{y\_0}\right)\right]}\tag{17}$$

$$B = \mathbb{C}\_{\infty} + A \ln \left[ \sinh \left( \frac{\pi d}{y\_0} \right) \right] \tag{18}$$

The growth rate *vg* can be expressed as [57]:

$$v\_{\mathcal{S}} = -\frac{vD\_{\mathcal{S}}}{w} \frac{\pi A}{r\_{\mathcal{s}}} = \frac{vD\_{\mathcal{S}}\pi(\mathbb{C}\_{\infty} - \mathbb{C}\_{\mathcal{c}})}{wr\_{\mathcal{s}}\left\{\ln\left[\sinh\left(\frac{\pi d}{y\_0}\right)\right] - \ln\left[\sinh\left(\frac{r\_{\mathcal{s}}}{y\_0}\right)\right]\right\}}\tag{19}$$

where *Dg* is the diffusion coefficient of molecules in the vapor phase.

A s a limiting case, *rs d* and *y*0 *d*,

$$w\_{\mathcal{S}} \cong \frac{v D\_{\mathcal{S}} (\mathbb{C}\_{\infty} - \mathbb{C}\_{\mathbb{C}})}{wr\_{s} d} y\_{0} \tag{20}$$

During the SAG growth process, the grown material generates a growth rate enhancement curve in the unmasked area, by formula (20), the growth rate is proportional to the width of the dielectric mask and inversely proportional to the square of the mask spacing. In addition, group III elements will have a certain compositional shift during growth, while group V elements will not; therefore, the SAG technique will lead to a certain degree of inhomogeneity [58].

The traditional MQW SAG technology forms a dielectric mask on the buffer substrate, and the selective region growth layer includes the lower separate confinement structure (SCH), MQW, and upper SCH layers. The difference is that in the new SAG technology, the buffer, lower SCH, and MQW layers are first grown on the substrate. Subsequently, mask strip pairs are formed on the MQW layer. In the following SAG process, only one upper SCH layer is grown, which effectively reduces the difficulty of the process by reducing the

number of selectively grown layers. At the same time, SAG technology can be combined with EBL and REC technologies to achieve improved performance.

#### Technical Advantages and Disadvantages

Technical advantages: SAG technology is used to fabricate DFB semiconductor laser arrays because of its simplicity, low cost, and suitability for mass production. While forming a laser array, SAG can also be used for the integration of the laser array with other optical components, such as electro-absorption modulators (EAMs) [59].

Technical disadvantages: The SAG process requires good control of the epitaxial growth process and advanced technology. Additionally, it has low repeatability and it is difficult to guarantee the single longitudinal modulus rate, requiring other technologies to control it.

#### Main Research Progress and Status

SAG is widely used in the epitaxial growth of various semiconductor devices and was first applied in the preparation of the multi-wavelength DFB laser array in 1994 [60]. This method can prepare a DFB laser array with a wide wavelength span of up to 155 nm [61]. SAG technology is very flexible and can be used in combination with a variety of technologies. For example, gratings can be made using EBL based on SAG [62], and the width of ridge waveguides can be changed while SAG technology is used [63] or by combining SAG technology, and EBL technology, and reverse mesa ridge waveguide LD processing technology [64]. SAG technology can simultaneously integrate MMI, SOA, MOD, and other devices on the chip [65]. SAG technology can also be combined with bundled integrated waveguide (BIG) technology to obtain uniformly spaced multi-wavelength emission and low-loss passive waveguide materials in one MOCVD step, greatly simplifying the integrated fabrication of the array [66].

In the SAG process, the material layers affected by SAG include two SCH and MQW layers, which may affect the uniformity of the laser array [8]. Therefore, an improved SAG method has been proposed [67]. First, the buffer, low SCH, and MQW layers are grown on the substrate, and then the size is formed on the MQW layer, gradually changing mask–strip pairs. In the next step, only the upper SCH layer is grown, and only the thickness of the upper SCH layer is changed through the SAG mask to obtain different Bragg wavelengths. Materials including the SCH and MQW layers are very sensitive to different growth conditions; therefore, the wavelength spacing can be precisely controlled.

As the thickness of the material can be controlled with precision, SAG technology is especially suitable for fabricating laser arrays with small channel spacing, which can achieve good wavelength spacing uniformity. However, DFB lasers with a high singlelongitudinal-mode rate are not guaranteed. Therefore, REC technology has been adopted to introduce an equivalent phase shift to ensure the single-mode laser output of the DFB laser [68].

#### **4. Conclusions**

In summary, the DFB laser array is among the most widely used and mature solutions in WDW. Compared with other lasers, its mode stability, high wavelength stability, and narrow linewidth make it stand out. The research on DFB laser arrays is also the most extensive. The multi-wavelength DFB laser array is a key component of the wavelength-division multiplexing system, and its manufacturing difficulties affect its mass production and use. This paper summarizes the structure and fabrication methods for multi-wavelength DFB laser arrays. At present, various DFB laser array structures and fabrication methods can still be improved. Finding a low-cost, high-yield, easy fabrication method, that can adopt a reasonable structure, are key factors to promote the further development of DFB laser arrays.

For DFB laser arrays, the main problems to be solved in the future are the wavelength accuracy and the wavelength spacing of different lasers. In particular for DWDM applications, the wavelength spacing of future DFB laser arrays may still be further reduced, which will put forward higher requirements for technical accuracy. On the one hand, we need to continue to improve the existing technologies such as EBL, NIL, and other processes; on the other hand, in the design of gratings, active regions, waveguides, etc., such as REC and SAG, it is also necessary to further reduce errors. In addition, the DFB laser arrays currently used in optical communications are mainly InP based, which is difficult to match with standard integrated circuit processes. In the future, emerging multi-wavelength WDM sources based on Si photonics are also very expected.

**Author Contributions:** Conceptualization, S.N. and Y.S.; methodology, C.Q. and Y.L.; validation, P.J., Y.C. and Y.W. (Ye Wang); formal analysis, Y.S. and L.Z.; investigation, Y.S., C.Q., Y.W. (Yubing Wang) and Y.L.; resources, S.N., Y.W. and Y.S.; writing—original draft preparation, S.N.; writing—review and editing, L.Z. and Y.S.; supervision, Y.C. and L.L.; project administration, Y.N.; funding acquisition, L.Q. and L.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is supported by the National Science and Technology Major Project of China (2021YFF0700500); National Natural Science Foundation of China (NSFC) (61904179, 62090051, 62090052, 62090054, 11874353, 61935009, 61934003, 62004194); Science and Technology Development Project of Jilin Province<PHONE_NUMBER>9GX,<PHONE_NUMBER>2GX,<PHONE_NUMBER>6GX,<PHONE_NUMBER>7GX,<PHONE_NUMBER>8GX); Key R&D Program of Changchun [21ZGG13, 21ZGN23]; Innovation and entrepreneurship Talent Project of Jilin Province [2021Y008]; Special Scientific Research Project of Academician Innovation Platform in Hainan Province (YSPTZX202034), and "Lingyan" Research Program of Zhejiang Province (2022C01108).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

## *Review* **Research on Narrow Linewidth External Cavity Semiconductor Lasers**

**Keke Ding, Yuhang Ma, Long Wei, Xuan Li, Junce Shi, Zaijin Li \*, Yi Qu, Lin Li, Zhongliang Qiao, Guojun Liu and Lina Zeng**

> Key Laboratory of Laser Technology and Optoelectronic Functional Materials of Hainan Province, Academician Team Innovation Center of Hainan Province, College of Physics and Electronic Engineering, Hainan Normal University, Haikou 571158, China<EMAIL_ADDRESS>(K.D<EMAIL_ADDRESS>(Y.M<EMAIL_ADDRESS>(L.W<EMAIL_ADDRESS>(X.L<EMAIL_ADDRESS>(J.S<EMAIL_ADDRESS>(Y.Q<EMAIL_ADDRESS>(L.L<EMAIL_ADDRESS>(Z.Q<EMAIL_ADDRESS>(G.L<EMAIL_ADDRESS>(L.Z.)

**\*** Correspondence<EMAIL_ADDRESS>
**Abstract:** Narrow linewidth external cavity semiconductor lasers (NLECSLs) have many important applications, such as spectroscopy, metrology, biomedicine, holography, space laser communication, laser lidar and coherent detection, etc. Due to their high coherence, low phase-frequency noise, high monochromaticity and wide wavelength tuning potential, NLECSLs have attracted much attention for their merits. In this paper, three main device structures for achieving NLECSLs are reviewed and compared in detail, such as free space bulk diffraction grating external cavity structure, waveguide external cavity structure and confocal Fabry–Perot cavity structure of NLECSLs. The Littrow structure and Littman structure of NLECSLs are introduced from the free space bulk diffraction grating external cavity structure of NLECSLs. The fiber Bragg grating external cavity structure and silicon based waveguide external cavity structure of NLECSLs are introduced from the waveguide external cavity structure of NLECSLs. The results show that the confocal Fabry–Perot cavity structure of NLECSLs is a potential way to realize a lower than tens Hz narrow linewidth laser output.

**Keywords:** narrow linewidth; external cavity; FSBDG; FBG; silicon-based waveguide; confocal F-P cavity

#### **1. Introduction**

Semiconductor lasers have been applied in many fields, such as high-resolution spectroscopy and broadband communication network systems. Semiconductor lasers need to have the characteristics of a narrow linewidth, high-frequency modulation and wide tunable range at the same time. In other fields, it is required that lasers have the characteristics of narrower output linewidth, larger coherence length, and narrower spatial coherence [1]. By using the external cavity technology, semiconductor lasers can produce stabilized output with a single longitudinal mode and narrow linewidth, and they can also be tuned in the range of tens of nanometers to hundreds of nanometers [2]. At the same time, some other properties of semiconductor lasers are also improved, including a lower threshold, higher output power and larger side mode suppression ratio (SMSR) [3,4]. These properties meet the requirements of coherent optical communication, coherent detection and other applications of NLECSLs. In 1964, J.W. Crowe et al. [5] first proposed the external cavity theory of the semiconductor laser. In 1975, Heckscher H et al. [6] reported the compact and relatively inexpensive external cavity structure of the laser with the III-V compound semiconductor.

An NLECSL includes a semiconductor laser active section and an external cavity. The active section, which typically contains a III–V semiconductor quantum wells structure, is used to provide the optical gain for the whole cavity, and thereby determines the lasing