1. Introduction {#sec1-nanomaterials-09-01216}
===============

Ultrafast fiber lasers have wide applications in a wide variety of areas, such as information transmission, frequency metrology, scientific research and so on. Over the past few decades, ultrashort pulses fiber lasers have received much attention due to their potential applications in industrial manufacturing, environmental monitoring, toxic gas detection, biomedical, defense, optical sensing and optical imaging. Currently, several passively mode-locked techniques have been employed for generating pulsed laser operations. In 2016, Ivanenko et al. reported a mode-locked long fiber master oscillator based on a nonlinear amplifying loop mirror (NALM) with intra-cavity power management, and achieved a record-high pulse energy exceeding 12 μJ \[[@B1-nanomaterials-09-01216]\]. In addition, various types of saturable absorbers (SAs), including semiconductor saturable absorber mirrors (SESAMs) \[[@B2-nanomaterials-09-01216],[@B3-nanomaterials-09-01216]\], single-wall carbon nanotubes (SWCNTs) \[[@B4-nanomaterials-09-01216],[@B5-nanomaterials-09-01216],[@B6-nanomaterials-09-01216]\], and graphene \[[@B7-nanomaterials-09-01216],[@B8-nanomaterials-09-01216],[@B9-nanomaterials-09-01216]\], have been considered the most promising approach to obtain mode-locked pulses. As known, SESAM has been widely employed in a majority of commercial mode-locked fiber lasers to achieve ultrashort pulses. However, SESAM shows some drawbacks, such as a narrow absorption bandwidth, low damage threshold, complex fabrication and packaging and high cost, which have restricted its further advanced development. In addition, SWCNTs have also been widely used as SAs due to their ultrafast excited state carrier dynamic, high damage threshold and low saturation threshold. However, the absorption bandwidth of SWCNTs depends highly on its diameter, which restricts its practical applications. Moreover, graphene is applicable as a broadband SA due to its Dirac-like electronic band structure. In 2009, Zhang et al. reported on a graphene-based erbium-doped mode-locked fiber laser \[[@B9-nanomaterials-09-01216]\]. Since then, a variety of atomically thin layered materials have been extensively investigated and employed as the SA to obtain mode-locked pulses. Transition metal dichalcogenides (TMDs) \[[@B10-nanomaterials-09-01216],[@B11-nanomaterials-09-01216],[@B12-nanomaterials-09-01216],[@B13-nanomaterials-09-01216],[@B14-nanomaterials-09-01216]\] emerged as a novel kind of layered nanomaterial exhibiting the unique layer-number dependent bandgap properties, which have shown a great potential in nanoelectronics, optical sensing and optoelectronics \[[@B15-nanomaterials-09-01216]\]. However, the limited carrier mobility in the monolayer restricts its advanced application. Recently, topological insulators (TIs) (Bi~2~Se~3~, Bi~2~Te~3~, Sb~2~Te~3~) \[[@B16-nanomaterials-09-01216],[@B17-nanomaterials-09-01216],[@B18-nanomaterials-09-01216],[@B19-nanomaterials-09-01216],[@B20-nanomaterials-09-01216]\] with larger modulation depths and high optical nonlinearity are attracting great interest in laser photonics. In 2012, Bernad et al. first demonstrated the saturable absorption of Bi~2~Te~3~ \[[@B20-nanomaterials-09-01216]\]. Black phosphorus (BP) \[[@B21-nanomaterials-09-01216],[@B22-nanomaterials-09-01216]\] is beneficial to its application in the field of near infrared optoelectronic materials due to its adjustable direct band gap. Regretfully, broadband SAs were still subject to a low laser damage threshold in practical applications.

Recently, indium selenide (In~2~Se~3~), a layered semiconducting chalcogenide, has attracted extensive attention for its various exciting physics properties. In~2~Se~3~ is considered to be a promising material for the applications in photo-voltaic devices \[[@B23-nanomaterials-09-01216]\], phase change memory \[[@B24-nanomaterials-09-01216]\], and especially in opto-electronics \[[@B25-nanomaterials-09-01216]\]. Previous research indicated that In~2~Se~3~ was also available in at least five different phases and crystal structures (α, β, γ, δ, and κ) existing at various temperatures \[[@B26-nanomaterials-09-01216]\]. It is composed of vertically stacked Se-In-Se-In-Se quintuple layers, held together through weak van der Waals forces \[[@B27-nanomaterials-09-01216]\]. Moreover, In~2~Se~3~ shows a tunable thickness-dependent optical bandgap ranging from 1.45 to 2.8 eV \[[@B28-nanomaterials-09-01216]\]. Meanwhile, In~2~Se~3~ as a direct band gap III−VI semiconductor, possesses ultrahigh photodetection responsivity with a fast response time \[[@B29-nanomaterials-09-01216]\]. Therefore, In~2~Se~3~ is highly suited to ptoelectronic devices and photonics applications. In the past several decades, the related properties of α-In~2~Se~3~ have been extensively studied. However, its nonlinear optical properties have rarely been investigated and are rarely used as SAs in a passively mode-locked operation.

At present, a few-layered In~2~Se~3~ flakes have been successfully prepared by several methods, mainly including mechanical exfoliation (ME) \[[@B29-nanomaterials-09-01216]\], chemical vapor deposition (CVD) \[[@B27-nanomaterials-09-01216]\], and physical vapor deposition (PVD) \[[@B30-nanomaterials-09-01216]\]. However, a few-layered In~2~Se~3~ flakes obtained with the ME method led to an uncontrollable size and random thickness, which limits the nonlinear optical response of In~2~Se~3~. Compared with the ME method, the thickness of In~2~Se~3~ flakes that prepared by CVD and PVD methods could be controlled accurately. Moreover, the PVD method could reduce defects of the In~2~Se~3~ flakes and achieve highly uniform films with large areas, which could improve nonlinear optical properties. Additionally, the In~2~Se~3~ flakes prepared by the PVD method possess a higher crystal quality, which could exhibit excellent laser damage threshold.

Recently, for optoelectronic devices, Zhou et al. demonstrated that the atomically layered In~2~Se~3~ exhibit p-type semiconducting behaviors with the mobility up to 2.5 cm^2^/Vs \[[@B31-nanomaterials-09-01216]\]. Meanwhile, for the ultrafast photonic applications, by inserting α-In~2~Se~3~ SA prepared with magnetron-sputtering deposition method into erbium-doped fiber laser (EDFL) systems, Yan et al. obtained soliton pulse with maximal average power/single pulse energy/pulse duration for EDFL of 83.2 mW/2.03 nJ/276 fs, respectively \[[@B32-nanomaterials-09-01216]\]. However, there were no reports on a few-layered In~2~Se~3~ flakes with a large-area prepared by PVD method as SA for high-power mode-locked operations. It is well known that the damage threshold of SA materials is an important parameter to decide whether the material can achieve a high-power laser output and be used in the practice of the material. In general, polymer based SAs have been widely integrated into fiber laser systems. However, high peak power in the cavity may lead to changes in the property of polymer and even damage the SAs.

In this report, a double-end pumped mode-locked EDFL based on In~2~Se~3~ SA for high-power laser output is presented. The uniform large-area atomically thin In~2~Se~3~ flakes were synthesized on fluorophlogopite mica (FM) by the PVD method. In addition, by a transfer process, a few-layered In~2~Se~3~ flakes were directly transferred on the facet of the fiber. This work studied the laser damage threshold of the PVD-In~2~Se~3~ SA, which possessed excellent damage threshold. As is known, the laser damage threshold of SESAM (BATOP, SA-1550-35-2ps-x) is 1.5 × 10^3^ μJ/cm^2^. Compared with the SESAM, the laser damage threshold of the In~2~Se~3~-FM SA reached as high as 24 mJ/cm^2^. Meanwhile, the nonlinear optical properties of In~2~Se~3~ SA were investigated. It exhibited excellent nonlinear optical performances, such as a large modulation depth (18.75%) and lower saturable intensity (6.8 MW/cm^2^). By employing the In~2~Se~3~ SA in a bidirectional pumping high-power mode-locked EDFL system, a variety of stable mode-locked pulses were obtained. The average output power was 122.4 mW, corresponding to a single pulse energy of 5.8 nJ. The experiment results fully prove that In~2~Se~3~ could be a potentially excellent SA for a high-power mode-locked fiber laser application in practice.

2. Preparation and Characterization of the In~2~Se~3~ SA {#sec2-nanomaterials-09-01216}
========================================================

2.1. Synthesis and Characterization of In~2~Se~3~ Flakes {#sec2dot1-nanomaterials-09-01216}
--------------------------------------------------------

In our experiment, the commonly-reported PVD method was employed for preparing high-quality In~2~Se~3~ flakes \[[@B30-nanomaterials-09-01216]\]. The growing progress is illustrated in [Figure 1](#nanomaterials-09-01216-f001){ref-type="fig"}. The In~2~Se~3~ flakes were grown on monolayer FM substrates via van der Waals epitaxy in a horizontal tube furnace (OTL1200). The In~2~Se~3~ power (99.99%, Alfa Aesar, Beijing, China) as an evaporation source was placed at the constant-temperature zone of the tube furnace heated to 750 °C for 60 min. The vapor was transported downstream by 50 sccm Ar gas with pressure controlled at 15 Pa. The In~2~Se~3~ flakes were grown on FM substrate placed 12 cm away from the heating center. Finally, the furnace was naturally cooled down to the ambient temperature in the gas flow of Ar.

[Figure 2](#nanomaterials-09-01216-f002){ref-type="fig"}a shows a typical scanning electrical microscope (SEM) (Sigma 500, ZEISS, Oberkochen, Germany) image of the prepared In~2~Se~3~ flakes on FM. It is obvious that most flakes exhibit asymmetric hexagonal and truncated trigonal morphology shapes, which are along the horizontal direction. Meanwhile, an energy dispersive spectrometer (EDS, XFlash 6130, Bruker, Germany) was employed for investigating the characteristic of the elemental composition. As shown in [Figure 2](#nanomaterials-09-01216-f002){ref-type="fig"}b, the stoichiometric ratio of Se (59.76%) and In (40.24%) is estimated to be 3:2. The structural characterization of the prepared In~2~Se~3~ flakes was also tested by a Raman spectroscopy (Horiba HR Evolution) with excitation light of 532 nm. The test result is displayed in [Figure 2](#nanomaterials-09-01216-f002){ref-type="fig"}c, where the three peaks at \~108, \~173 and \~205 cm^−1^ are attributed to A~1~ (LO + TO), A~1~ (TO), and A~1~ (LO) phonon modes of In~2~Se~3~. These Raman features unequivocally indicate the successful preparation of In~2~Se~3~ flakes \[[@B33-nanomaterials-09-01216]\]. A blue shift in lattice phonon mode of A~1~ (TO) might be due to the oxidized caused by the Raman excitation laser \[[@B30-nanomaterials-09-01216]\]. In addition, the crystal structure of the In~2~Se~3~ flakes was investigated through X-ray diffraction (XRD) (Bruker D8 ADVANCE, Billerica, MA, USA). [Figure 2](#nanomaterials-09-01216-f002){ref-type="fig"}d shows the relatively higher intensity of the (006) peak which indicates that the In~2~Se~3~ flakes exhibit a well-layered structure and high crystallinity. As shown in [Figure 2](#nanomaterials-09-01216-f002){ref-type="fig"}e, atomic force microscopy (AFM, Bruker Multimode 8, Germany) was used to determine the thickness of the In~2~Se~3~ flakes on FM. [Figure 2](#nanomaterials-09-01216-f002){ref-type="fig"}f shows the lateral height profile of the In~2~Se~3~ flakes. The thickness of the marked samples ranged from approximately 2.0 to 2.6 nm.

2.2. Preparation and Characterization of In~2~Se~3~ SA {#sec2dot2-nanomaterials-09-01216}
------------------------------------------------------

In the experiment, In~2~Se~3~ flakes based on a monolayer FM were peeled off into a few layers by pyrolysis tape. Then, the few layers In~2~Se~3~-FM was directly attached onto a fiber end-facet for preparing an all fiber SA with a high laser damage threshold.

The linear transmission of In~2~Se~3~-FM from 400 to 2000 nm was measured by using a UV/vis/NIR spectrophotometer (Hitachi U-4100). As shown in [Figure 3](#nanomaterials-09-01216-f003){ref-type="fig"}a, In~2~Se~3~-FM SA exhibits a high transmission of 92.8% at 1560 nm. Here, the linear transmission shows a fringe, which was caused by a spectral interference caused by a thin film of a certain thickness. In addition, the nonlinear absorption properties of the In~2~Se~3~-FM SA were investigated by a power-dependent transmission technique. The representative result is shown in [Figure 3](#nanomaterials-09-01216-f003){ref-type="fig"}b. The data for the transmission is fitted by the following equation \[[@B34-nanomaterials-09-01216]\]:$$T\left( I \right) = 1 - \Delta T \times \exp\left( {{- I}/I_{sat}} \right) - T_{ns}$$ where *T* is the transmission, Δ*T* is the modulation depth, *I* is the input intensity of the laser, *I~sat~* is the saturation intensity, *T~ns~* is the non-saturable loss. According to the experimental results of fitting, the saturation intensity, modulation depth and non-saturable loss is approximately 6.8 MW/cm^2^, 18.75% and 18.89%, respectively.

3. Experimental Setup {#sec3-nanomaterials-09-01216}
=====================

The experimental setup of the double-end pumped fiber laser was shown in [Figure 4](#nanomaterials-09-01216-f004){ref-type="fig"}. As is shown, a piece of 62 cm high-concentration erbium-doped fiber (EDF, LIEKKI Er-80-8/125) with a group velocity dispersion (GVD) of −19.5 ps^2^/km was used as the gain medium. Two 976 nm laser diodes (LDs) were employed as the pump source. Two 980/1550 nm wavelength division multiplexers (WDMs) were used to couple the pump power into the ring laser cavity. A polarization-insensitive isolator (PI-ISO) was employed to ensure the unidirectional laser operation. Two polarization controllers (PCs) were employed to adjust the cavity polarization and intra-cavity birefringence. A 40/60 optical coupler was used to extract a 60% lasing signal for monitoring. A piece of 110 m single-mode fiber (SMF-28e) with a dispersion parameter of 17 ps/nm/km was added into the cavity for dispersion management. The total length of the cavity was approximately 120 m. Thus, the total net cavity dispersion was calculated to be −2.6 ps^2^. The performance of the output laser was record by a fastspeed photodetector (3G), a digital oscilloscope (DPO4054), an optical power meter (BAGGER D50A), an optical spectrum analyzer (AQ6317B) and a radio-frequency (RF) spectrum analyzer (R&S, PC1000, 1GHz).

4. Experimental Results and Discussions {#sec4-nanomaterials-09-01216}
=======================================

A \~120 m long laser cavity was designed for obtaining mode-locked operations. In addition, the phenomena of self-mode-locked or Q-switched operations were always recorded within a long-length ring fiber laser cavity due to the Kerr effect under high pump power. Thus, firstly, a piece of FM substrate was inserted into the ring laser cavity instead of the In~2~Se~3~ SA for testing the possibility of self-mode-locked or Q-switched operations. By adjusting the pump power and the polarization states of PCs, neither the self-mode-locked nor Q-switched pulse generations were detected, which excluded the Kerr effect and the saturable absorption effect of the FM substrate. Then, the In~2~Se~3~-FM SA was inserted into the ring cavity, and self-started mode-locked operations were achieved when the pump power reached 313 mW. The fiber laser exhibited a high lasing threshold due to a large output coupling ratio and a relatively large insert loss of the SA. In our experiment, the stable mode-locked operations can be maintained with the pump power increasing from 313 to 1324 mW. As is known, the formations of different solitons were due to the balancement between the various nonlinear optical effects, the total laser gain and loss and the net dispersion value within the laser cavity. In our work, by adjusting the pump power and the states of the PCs, the different soliton operations were recorded successfully, which is discussed in detail below.

4.1. Bright Pulses {#sec4dot1-nanomaterials-09-01216}
------------------

Firstly, the wide-reported bright pulses were obtained in our work easily. The mode-locked characteristics of the bright pulse under the pump power of 1324 mW were depicted in [Figure 5](#nanomaterials-09-01216-f005){ref-type="fig"}. In detail, [Figure 5](#nanomaterials-09-01216-f005){ref-type="fig"}a shows the optical emission spectrum of the bright pulse, which centered at 1559.4 nm with a 3 dB spectral bandwidth of 0.305 nm. However, no Kelly sidebands were observed in the spectrum, which indicate the EDFL was not operated in the conventional soliton regime. The radio frequency (RF) spectrum was measured and shown in [Figure 5](#nanomaterials-09-01216-f005){ref-type="fig"}b. The signal-to-noise ratio (SNR) at the fundamental repetition rate of 1.71 MHz is approximately 42 dB. In order to avoid the photodetector and optical spectrum analyzer from being damaged at high output power, the output pulse was split by two output couplers with coupling rates of 40/60 and 50/50, respectively, which reduced the SNR. Accordingly, the SNR should be greater than 42 dB. Meanwhile, it also limited the wideband RF output. [Figure 5](#nanomaterials-09-01216-f005){ref-type="fig"}c shows an oscilloscope trace of the obtained single pulse. The inset shows a typical pulse train of the mode-locked operation with a pulse-to-pulse interval of 584.8 ns, corresponding to a fundamental repetition rate of 1.71 MHz, which matches well with the total cavity length of 120 m. As is shown, the full width at half-maximum (FWHM) of the bright pulse is 14.4 ns. It is generally known that the dispersion has an obvious effect on broadening the width of the pulse in an anomalous dispersion regime. Thus, the wide pulse width is mainly caused by the large net dispersion value. However, due to the limitation of response time of detector and oscilloscope, the actual pulse width will be less than 14.4 ns. Regrettably, due to the lack of autocorrelator, the actual pulse width was not measured. The average output power as a function of pump power is recorded in [Figure 5](#nanomaterials-09-01216-f005){ref-type="fig"}d. The maximum average output power is 122.4 mW at the pump power of 1324 mW. However, the single pulse energy is limited to 5.8 nJ due to the direct current power between the pulses. In our experiment, no material damage was found, which indicates that the laser damage threshold of the PVD-In~2~Se~3~ SA should be greater than 24 mJ/cm^2^. In addition, in further works, we hope to achieve large-energy mode-locked operations by adjusting the laser parameters and further optimizing the preparation of In~2~Se~3~ flakes.

To fully prove the advantages of the PVD-In~2~Se~3~ based EDFL, the results of a series of high-power mode-locked fiber lasers based on different SAs are summarized in [Table 1](#nanomaterials-09-01216-t001){ref-type="table"}. It is clear that the highest average output power based on mode-locked EDFL was obtained in our work due to the combination of the high pump source and the high damage-threshold SA. It is worth noting that most of the work exhibits the pulse duration of picoseconds or even sub-picoseconds, which has broad application prospects in biomedical, nonlinear optics and ultrafast optics. However, its output power is limited to tens of milliwatts \[[@B32-nanomaterials-09-01216],[@B36-nanomaterials-09-01216],[@B48-nanomaterials-09-01216]\]. When a picosecond or femtosecond pulse is amplified, the high peak power results in a strong nonlinear effect, which is not conducive to the application of high-power lasers. Compared with picosecond or femtosecond mode-locked pulses, nanosecond mode-locked pulses have advantages such as a strong chirp, large pulse width, low peak power, and small nonlinear phase shift accumulation. Therefore, the nanosecond fiber lasers with high pulse energy can be used directly as seed sources to achieve higher power output through single or multi-stage amplification systems. Our results indicate that the PVD-In~2~Se~3~-FM based mode-locked fiber laser can be used as a seed source for a chirped pulse amplification (CPA) system.

4.2. Dark-Bright Pulse Pairs {#sec4dot2-nanomaterials-09-01216}
----------------------------

In the experiment, just by adjusting the states of the PCs, stable dark-bright pulse pair mode-locked generation has also been detected. Here, the optical characteristics of the dark-bright pulse pair under the pump power of 1324 mW were discussed. The typical output optical emission spectrum is recorded in [Figure 6](#nanomaterials-09-01216-f006){ref-type="fig"}a. It is noteworthy that the spectrum shows a typical M-shape profile with a dual-wavelength centered at 1559.4 and 1560.6 nm, respectively, which is similar to the previous report \[[@B18-nanomaterials-09-01216]\]. As described above, the large-area In~2~Se~3~ flakes synthesized by the PVD method exhibit a large nonlinear optical effect due to their high crystal quality and high flatness. Therefore, the high nonlinear optical effect within the laser cavity facilitates the generation of multi-wavelength mode-locked pulses. [Figure 6](#nanomaterials-09-01216-f006){ref-type="fig"}b shows the RF spectrum of the laser at a fundamental repetition rate of 1.71 MHz. The SNR exceeded 40 dB, which indicates a high stability of the dark-bright pulse pair mode-locked operation. [Figure 6](#nanomaterials-09-01216-f006){ref-type="fig"}c shows the corresponding single pulse profile of the dark-bright pulse pair. However, compared with bright pulse, the dark pulse exhibits different pulse intensity and pulse width, which is different from the previous study \[[@B50-nanomaterials-09-01216]\]. In our opinion, this is caused by the high-order nonlinear effect of the In~2~Se~3~ SA. Meanwhile, the inset shows the typical dark-bright pulse pair train with a period of 584.8 ns, corresponding to a fundamental repetition rate of 1.71 MHz, which verifies the mode-locked operation of the fiber laser. In addition, by adjusting the pump power and the PCs, the dark-bright pulse pair emissions also could be obtained. The relationship between the pump power and average output power is recorded in [Figure 6](#nanomaterials-09-01216-f006){ref-type="fig"}d. The mode-locked threshold of the dark-bright pulse pair operation was 397 mW, which was higher than the bright pulse mode-locked operation. The generation of dark-light pulse pair may be the result of the nonlinear refractive index of In~2~Se~3~ flakes interacting with the high nonlinear effects caused by the higher power in the laser cavity \[[@B51-nanomaterials-09-01216]\]. The average output power and the single pulse energy are 121.2 mW and 2.7 nJ, respectively. To the best of our knowledge, this is the highest output power of dark-bright pulse pair mode-locked operations based on EDFL.

Accordingly, the adjustment of the polarization state of the PCs also led to the formation of stable bright-dark pulse pair mode-locked operations. [Figure 7](#nanomaterials-09-01216-f007){ref-type="fig"} shows the output characteristics of the bright-dark pulse pair at the maximum pump power. It is obvious that the bright and dark pulses are separated from each other. [Figure 7](#nanomaterials-09-01216-f007){ref-type="fig"}a shows the typical emission optical spectrum with a dual-wavelength centered at 1559.5 and 1560.7 nm, respectively. For investigating the stability of the bright-dark pulse pair mode-locked operation, its RF spectrum was measured. As shown in [Figure 7](#nanomaterials-09-01216-f007){ref-type="fig"}b, the fundamental frequency is also located at 1.71 MHz with a SNR over 40 dB, indicating that the bright-dark pulse pair operates in a relatively stable regime. [Figure 7](#nanomaterials-09-01216-f007){ref-type="fig"}c shows the corresponding single pulse profile of the bright-dark pulse pair. Clearly, the bright and dark pulse can be observed simultaneously, which the pulse interval exceeds to approximately 200 ns. [Figure 7](#nanomaterials-09-01216-f007){ref-type="fig"}d depicted the relationship between the average output power versus the pump power. As is shown, when the pump power increases from 397 to 1324 mW, the average output power grows from 32.1 to 121.5 mW, corresponding to a maximum single pulse energy of 2.8 nJ. This is the first demonstration based on In~2~Se~3~ SA in a bright-dark pulse pair mode-locked EDFL. In addition, there were no bright pulse and bright-dark pulse pairs observed no matter how adjusted the cavity polarization state was when the In~2~Se~3~ SA was removed.

5. Conclusions {#sec5-nanomaterials-09-01216}
==============

In conclusion, this study has demonstrated PVD-grown large-area In~2~Se~3~ flakes as SA for generating high-power and a large-energy passively mode-locked EDFL. The In~2~Se~3~-FM SA exhibited a high laser damage threshold of higher than 24 mJ/cm^2^, a large modulation depth of 18.75% and a saturable intensity of 6.8 MW/cm^2^. Based on the In~2~Se~3~-FM SA, the stable mode-locked pulse with a maximum average output power and a single pulse energy of 122.4 mW and 5.8 nJ were obtained successfully. To our knowledge, this is the highest output power achieved in a mode-locked EDFL based on two-dimensional (2D) materials. In addition, the dark-bright pulse pair operations with recorded high output powers were also observed for the first time. Thus, our experimental results with obvious enhancements in comparison with previous works fully indicate the superiority of our experiment design and is expected to provide an absolutely new reference for generating high-power mode-locked fiber lasers based on 2D materials as SAs.

Conceptualization, X.H., H.Z., and B.M.; investigation, X.H., H.Z., S.J., Q.G., and C.Z.; data curation, X.H., H.Z., and J.G.; writing---original draft preparation, X.H.; writing---review and editing, H.Z., and S.J.; funding acquisition, H.Z., B.M., C.Z., S.J., and D.L.

The authors are grateful for financial support from the National Natural Science Foundation of China (11674199, 11774208, 11804200, 61205174), Shandong Province Natural Science Foundation (ZR2017BA004, 2017GGX20120, ZR2016FP01, ZR2017BA018), Shandong Province Youth Foundation (ZR2018QF006) and the China Postdoctoral science Foundation (2016M602177).

The authors declare no conflicts of interest.

![Schematic diagram of the physical vapor deposition for In~2~Se~3~ flakes growth.](nanomaterials-09-01216-g001){#nanomaterials-09-01216-f001}

![(**a**) SEM image of the In~2~Se~3~ flakes; (**b**) EDS spectroscopy of the In~2~Se~3~ flakes; (**c**) Raman spectrum of the In~2~Se~3~ flakes; (**d**) X-ray diffraction of the In~2~Se~3~ flakes; (**e**) AFM image of In~2~Se~3~ flakes; (**f**) Height profile of In~2~Se~3~ flakes measured along the white line in (e).](nanomaterials-09-01216-g002){#nanomaterials-09-01216-f002}

![(**a**) Linear transmission of the In~2~Se~3~-FM saturable absorber (SA) versus the wavelength. (**b**) The nonlinear absorption property of the In~2~Se~3~-FM SA.](nanomaterials-09-01216-g003){#nanomaterials-09-01216-f003}

![Schematic illustration of the typical all-fiber ring cavity of the mode-locked erbium-doped fiber laser (EDFL) based on the PVD-In~2~Se~3~ SA.](nanomaterials-09-01216-g004){#nanomaterials-09-01216-f004}

![Typical optical characteristics of the bright pulse at the pump power of 1324 mW. (**a**) Optical spectrum with indicated 3dB bandwidth. Inset: The output optical spectrum; (**b**) radio-frequency (RF) spectrum at a fundamental frequency of 1.71 MHz with 300 Hz resolution; (**c**) the corresponding single pulse profile of the bright pulse and the inset shows the typical pulse train; (**d**) average output power versus the pump power.](nanomaterials-09-01216-g005){#nanomaterials-09-01216-f005}

![Typical optical characteristics of dark-bright pulse pairs at the pump power of 1324 mW. (**a**) The output optical spectrum; (**b**) RF spectrum at a fundamental frequency of 1.71 MHz with 300 Hz resolution; (**c**) the corresponding single pulse profile of the dark-bright pulse pair and the inset shows the pulse train; (**d**) average output power versus the pump power.](nanomaterials-09-01216-g006){#nanomaterials-09-01216-f006}

![Typical optical characteristics of bright-dark pulse pair at the pump power of 1324 mW. (**a**) The output optical spectrum; (**b**) RF spectrum at a fundamental frequency of 1.71 MHz with 300 Hz resolution; (**c**) the corresponding single pulse profile of the bright-dark pulse pair and the inset shows the typical pulse train; (**d**) average output power versus the pump power.](nanomaterials-09-01216-g007){#nanomaterials-09-01216-f007}

nanomaterials-09-01216-t001_Table 1

###### 

Comparison of 1.5 μm high-power mode-locked fiber lasers based on different SAs ^a^.

  SA               Fabrication    *λ~c~* nm       *f~c~* MHz   *τ* ps     *P*~ave~ mW   *E*~pulse~ nJ   Ref.
  ---------------- -------------- --------------- ------------ ---------- ------------- --------------- -----------------------------------
  **Graphene**     ME             1568            16.34        0.844      30            1.84            \[[@B35-nanomaterials-09-01216]\]
  **Graphene**     RGO            1555            15.36        0.51       80            5.2             \[[@B36-nanomaterials-09-01216]\]
  **Bi~2~Se~3~**   SPE            1562.1          3.54         25.16 ns   10            2.824           \[[@B37-nanomaterials-09-01216]\]
  **Bi~2~Se~3~**   LPE            1560.5          0.5376       ---        33.8          62.87           \[[@B17-nanomaterials-09-01216]\]
  **Bi~2~Te~3~**   PLD            1564.1          2950         0.92       45.3          0.01536         \[[@B38-nanomaterials-09-01216]\]
  **Bi~2~Te~3~**   Solvothermal   1571            10.71        6.2        30            2.8             \[[@B39-nanomaterials-09-01216]\]
  **Bi~2~Te~3~**   ME             1560            1.7          12.8 ns    32.9          22.4            \[[@B40-nanomaterials-09-01216]\]
  **Sb~2~Te~3~**   PLD            1530            94           ---        12            0.127           \[[@B41-nanomaterials-09-01216]\]
  **WS~2~**        CVD            1568.3          0.487        1.49       62.5          128.3           \[[@B42-nanomaterials-09-01216]\]
  **WS~2~**        LPE            1531.5/1557.5   2.14         11         14.2          6.64            \[[@B43-nanomaterials-09-01216]\]
  **WS~2~**        PLD            1561            101.4        0.246      18            ---             \[[@B44-nanomaterials-09-01216]\]
  **WSe~2~**       CVD            1557.4          63.133       0.1635     28.5          ---             \[[@B45-nanomaterials-09-01216]\]
  **WSe~2~**       CVD            1562            58.8         0.185      30            ---             \[[@B46-nanomaterials-09-01216]\]
  **MoSe~2~**      BTS            1557.3          3270         0.751      22.8          0.0067          \[[@B47-nanomaterials-09-01216]\]
  **MoTe~2~**      MSD            1559            26.6         0.229      57            2.14            \[[@B48-nanomaterials-09-01216]\]
  **ReS~2~**       CVD            1565            318.5        ---        12            0.037           \[[@B49-nanomaterials-09-01216]\]
  **BP**           LPE            1559.5          8.77         0.94       53            ---             \[[@B22-nanomaterials-09-01216]\]
  **In~2~Se~3~**   MSD            1565            40.9         0.276      83.2          2.03            \[[@B32-nanomaterials-09-01216]\]
  **In~2~Se~3~**   PVD            1559.7          1.71         14.4 ns    122.4         5.8             This work

^a^*λ*~c~, the central wavelength; *f*~c~, the fundamental frequency; τ, the pulse duration; Pave, the output average power; *E*~pulse~, the pulse energy; ME, mechanical exfoliation; RGO, reduced graphene oxide; SPE, solution-phase exfoliation; LPE, liquid-phase exfoliation; PLD, pulsed laser deposition; CVD, chemical vapor deposition; BTS, bath-type sonication; MSD, magnetron-sputtering deposition.

[^1]: These authors contributed equally to this work.
