# Transmission of optical communication signals through ring core fiber using
perfect vortex beams

Nelson Villalba  Departamento de Física, Universidad del Bío-Bío, Collao 1202,
5-C Concepción, Chile Cristóbal Melo  Departamento de Ingeniería Eléctrica ,
Universidad de Concepción, 160-C Concepción, Chile  Sebastián Ayala
Departamento de Física, Universidad de Concepción, 160-C Concepción, Chile
Millennium Institute for Research in Optics, Universidad de Concepción, 160-C
Concepción, Chile  Christopher Mancilla  Departamento de Ingeniería Eléctrica
, Universidad de Concepción, 160-C Concepción, Chile  Wladimir Valenzuela
Departamento de Ingeniería Eléctrica , Universidad de Concepción, 160-C
Concepción, Chile  Miguel Figueroa  Departamento de Ingeniería Eléctrica ,
Universidad de Concepción, 160-C Concepción, Chile  Erik Baradit  Departamento
de Física, Universidad del Bío-Bío, Collao 1202, 5-C Concepción, Chile Riu Lin
Department of Electrical Engineering, Chalmers University of Technology, 41296
Göteborg, Sweden  Ming Tang  Wuhan National Lab for Optoelectronics (WNLO) &
National Engineering Laboratory for Next Generation Internet Access System,
School of Optical and Electronic Information, Huazhong University of Science
and Technology, Wuhan 430074, China  Stephen P. Walborn  Departamento de
Física, Universidad de Concepción, 160-C Concepción, Chile  Millennium
Institute for Research in Optics, Universidad de Concepción, 160-C Concepción,
Chile  Gustavo Lima  Departamento de Física, Universidad de Concepción, 160-C
Concepción, Chile  Millennium Institute for Research in Optics, Universidad de
Concepción, 160-C Concepción, Chile  Gabriel Saavedra  Departamento de
Ingeniería Eléctrica , Universidad de Concepción, 160-C Concepción, Chile
Gustavo Cañas  Departamento de Física, Universidad del Bío-Bío, Collao 1202,
5-C Concepción, Chile<EMAIL_ADDRESS>
###### Abstract

Orbital angular momentum can be used to implement high capacity data
transmission systems that can be applied for classical and quantum
communications. Here we experimentally study the generation and transmission
properties of the so-called perfect vortex beams and the Laguerre-Gaussian
beams in ring-core optical fibers. Our results show that when using a single
preparation stage, the perfect vortex beams present less ring-radius variation
that allows coupling of higher optical power into a ring core fiber. These
results lead to lower power requirements to establish fiber-based
communications links using orbital angular momentum and set the stage for
future implementations of high-dimensional quantum communication over space
division multiplexing fibers.

##  1 Introduction

To increase the capacity of optical fiber communication systems, the physical
properties of light such as wavelength (frequency), polarization, amplitude,
and phase have been used to multiplex information through the years [1, 2, 3,
4, 5, 6, 7, 8]. Nevertheless, due to the nonlinear nature of silica-based
optical fibers, data capacity is reaching its current limits [9, 10]. To date,
transmission of $244.3$ Tb/s over $54$ km has been achieved using all the
aforementioned physical dimensions [11]. To further increase the information
carrying capacity of optical fiber, new degrees of freedom need to be
exploited, where the use of an extended fiber bandwidth and additional spatial
modes are some of the most promising solutions [3, 11]. Space division
multiplexing (SDM) has been introduced as a viable way to multiplex channels
in the spatial-domain, and novel fiber types have been developed to support
transmission of multiple spatial modes. Current examples of fibers that
support SDM include multi-core fibers, few- and multi-mode fibers, and ring-
core fiber (RCF) [12, 13, 14, 15].

The orbital angular Momentum (OAM) of light can be associated to optical beams
with helical phase-fronts described by $e^{il\theta}$, where $\theta$ is the
azimuthal angle in cylindrical coordinates and $l$ can take integer values and
it is usually called topological charge[16]. The intensity profile from OAM
modes is characterised by a doughnut shape, since destructive interference
along its propagation axis generates a dark zone. OAM multiplexing has been a
subject of intense research lately, with great interest in free-space and
fiber-based optical communications systems [17, 18, 19, 20]. OAM eigenmodes
with different $l$ values form an orthogonal basis in the associated Hilbert
space. Moreover, the number of OAM modes can be infinite in principle, so that
they present the potential to increase the information capacity carried per
photon [21, 22].

To date, OAM has been used to establish classical communications links and
transmission of $1.6$ Tb/s was achieved over a $1.1$ km fiber link using OAM
multiplexing [23] and $100$ Tb/s in free-space data link enabled by three-
dimensional multiplexing of orbital angular momentum, polarization, and
wavelength [22]. The main difficulties in implementing OAM-based SDM systems
is propagation and coupling of OAM modes into an optical fiber. First, to
propagate OAM modes in an optical fiber, the fiber should present a large
effective index separation between vector modes and a refractive index profile
compatible with the OAM field [24]. Both of these conditions can be achieved
using ring-core fiber (RCF) [24, 25, 26]. Second, coupling a large number of
OAM modes into a RCF remains a challenging task. For free-space transmission,
the Laguerre-Gaussian (LG) modes are a natural choice, as they maintain their
transverse structure during free-space propagation. However, the doughnut-
shaped intensity profile of these modes depends on the topological charge of
the beam. For this reason, most of the experimental demonstrations of OAM
propagation in RCFs use two modes either with the same topological charge with
values $\pm l$ [23], or different topological charge with similar beam
diameter [27]. This limitation determines the practical implementations of
large OAM-based SDM systems, and thus the capacity increases attainable by
this technique. To solve this problem, the use of perfect vortex beams (PVB)
has been proposed theoretically to efficiently couple OAM modes in RCF fiber
[28]. PVBs are OAM modes that present a nearly constant intensity radius
regardless of the topological charge [29, 30], and can be mathematically
obtained by Fourier transformation of Bessel-Gauss modes [31].

In this article, we experimentally study the coupling and transmission of OAM
modes using RCFs and evaluate their transmission properties for
telecommunication and quantum applications. We used both PVB and LG modes in
order to evaluate performances in classical and quantum communications,
benchmarked by the bit error rate in the classical case, and by the average
quantum state fidelity in the quantum case.

##  2 OAM generation and fiber coupling

###  2.1 Coupling OAM into optical fibers

To propagate OAM in an optical fiber, fiber eigenmodes with the same
propagation constant should be excited in order to guarantee that no mode
walk-off affects the OAM mode [32]. RCFs with a single high refractive index
ring can support the radially fundamental modes, making it an appropriate
platform for OAM transmission. Although the use of RCF can maintain the OAM
mode during propagation with reduced mode cross-talk, the high refractive
index ring dimensions and doping properties will determine the maximum number
of modes supported by the RCF. The OAM modes supported by an optical fiber can
be described as a linear combination of the linearly polarized (LP) modes [28,
33] or the vector modes [32] supported by the fiber.

A set of OAM modes typically used in telecommunications are the LG modes
characterised by index $p$ and $l$, these are radial index and OAM value,
respectively [16]. To couple OAM modes into optical fibers using LG modes, the
radial number $p=0$ is used, which corresponds to an intensity profile
consisting of a single ring [34, 23]. Using these LG modes, however, leads to
different coupling efficiencies for different $l$ values, since the ring
radius of LG modes scales with $\sqrt{|l|}$[28]. The PVB modes, on the other
hand, have a ring-like intensity distribution whose radius varies much less as
a function of the OAM values [31]. Therefore, using PVBs to couple OAM into
fiber should have a higher overall efficiency for a given set of $l$ values
compared to LG modes, as described in Ref. [28]. In the next section we define
these modes mathematically and describe techniques for their generation.

###  2.2 OAM generation using spatial light modulators

In order to generate a PVB, a real-time method capable of controlling every
beam parameter is required. It was shown in Ref. [31] that it is possible to
generate PVBs in free-space through a computer generated hologram (CGH)
imprinted upon a spatial light modulator (SLM) and an optical lens in a
Fourier transform configuration. Additionally, it is possible to implement a
Fourier transformation using the CGH through Fresnel domain pattern of the
hologram without needing a lens in the process[35]. Thus, PVB generation can
be totally controlled by a CGH. As in Ref. [35], creating a CGH for deploying
on phase-only SLM is possible using the correct encoded of the complex scalar
field, particularly the PVB. This was mathematically described in Ref. [31]:

$PVB_{l}(r,\theta)=i^{l-1}\left(\frac{\omega_{g}}{\omega_{0}}\right)e^{il\theta}e^{\left(-\frac{r^{2}+r_{r}^{2}}{\omega_{0}^{2}}\right)}I_{l}\left(\frac{2r_{r}r}{\omega_{0}^{2}}\right),$
(1)

where $r$ and $\theta$ are radial and azimuthal coordinates in a cylindrical
system, $l$ the OAM topological charge, $\omega_{0}$ is the beam waist of the
PVB beam, $\omega_{g}$ is the beam waist of the gaussian field incident on the
SLM, $r_{r}$ is the ring radius of the PVB and $I_{l}(\cdot)$ is the $l$-th
modified Bessel function from the first kind. Additionally, $r_{r}$ can be
defined in terms of the radial wavenumber ($k_{r}$) using
$r_{r}=\frac{k_{r}f}{k}$, where $f$ is the focus of the lens that allows the
Fourier transformation and $k$ is the wavenumber of the beam. The parameter
$k_{r}$ can be used experimentally as an adjustable parameter to keep a
constant ring radius from PVB mode[31].

A similar technique can be used to generate LG beams. In this case, the
transverse spatial profile of the complex scalar field is given by following
equation in Ref. [16]:

$\text{LG}_{p}^{l}(r,\theta,z)=\sqrt{\frac{2p!}{\pi(p+|l|)!}}\frac{1}{\omega(z)}\left(\frac{r\sqrt{2}}{\omega(z)}\right)^{|l|}\text{exp}\left(\frac{-r^{2}}{\omega(z)}\right)L_{p}^{|l|}\left(\frac{2r^{2}}{\omega^{2}(z)}\right)e^{il\theta}\\\
\text{exp}\left(-ik\frac{r^{2}z}{2(z_{R}^{2}+z^{2})}\right)\text{exp}\left[-i(2p+|l|+1)\text{tan}^{-1}\left(\frac{z}{z_{R}}\right)\right],$
(2)

where $L_{p}^{|l|}$ is an associated Laguerre polynomial, with $l$ and $p$ the
azimuthal and radial index, respectively, $z_{R}$ is the Rayleigh range and
$\omega(z)$ is the beam waist (considered constant in this work). The $p$
parameter is associated with numbers of the concentric rings in transversal
profile in LG modes. In this work we study OAM modes coupling into RCF with
just one ring radius, then the radial parameter is fixed in $p=0$. Equation
(2) is described using the same coordinates as the PVB (Eq. (1)) and adds $z$
coordinate in the beam propagation direction.

##  3 Experimental setup

Figure 1: Experimental setup. Blue, orange and green backgrounds represent
preparation, transmission and measurement stages, respectively.

The experimental setup used in this work to generate, transmit and measure PVB
is illustrated in Fig. 1. Each stage is highlighted with a color, blue for
preparation, orange for transmission and green for measurement. A commercial
SFP+ transceiver was used to generate and detect an optical signal. The
transceiver was driven by a field programmable gate array (FPGA) with a
pseudorandom binary sequence (PRBS) of length $2^{37}$. The transmitter uses
an optical source centered at $1550\ nm$, which was amplified by an erbium-
doped fiber amplifier ($\text{EDFA}_{1}$) and a variable optical attenuator
(VOA) to control the signal power. Subsequently, the optical singal was
launched into free-space, collimated and passed through to half wave plate
(HWP) and polarizing beam splitter (PBS) to prepare the incident beam before
the SLM, not shown in Fig. 1 for simplicity. The OAM mode is generated when a
horizontally polarized Gaussian beam (with $\omega_{g}\approx 1140\ \mu m$) is
reflected by the phase-only SLM ($\text{SLM}_{1}$). $\text{SLM}_{1}$ deploys a
CGH with the desired OAM mode. The beam reflected by $\text{SLM}_{1}$ passes
through the $4f$ confocal optical system formed by two lenses ($\text{L}_{3}$
and $\text{L}_{4}$, with focus $f_{3}=f_{4}=150\ mm$) and a pinhole placed
between them. The pinhole filters the first diffraction order from the CGH,
this way the generated OAM mode is placed in the image plane of this optical
system. The OAM mode is coupled into an RCF, used to propagate the signal. To
couple the OAM mode a $20\text{x}$ objective lens is used. The RCF has an
internal and external radius given by $a=6\ \mu m$ and $b=9\ \mu m$,
respectively [27]. A total length of $300\ m$ of RFC was used. A RCF-based
polarization controller was used to control the OAM mode coupled into the
fiber. After the OAM modes are propagated through the RCF, a second
$20\text{x}$ objective lens was used to generate the image of the OAM-beam
exiting the fiber core. This optical system magnifies the mode radius
$33\times$ to achieve $r_{r}\approx 230\ \mu m$. A polarization filter
comprised of HWP followed by a PBS was used at the fiber output to ensure the
horizontal polarization of the OAM-beam. A second $4f$ confocal optical system
($\text{L}_{5}$ and $\text{L}_{6}$) was used to magnify the OAM-beam $2\times$
after exiting of polarization filter, and it allows the image of OAM-beam to
be produced on $\text{SLM}_{2}$. On $\text{SLM}_{2}$ we deployed a set of
forked holograms (CGH of the LG basis), one at a time, encoding a conjugated
OAM value relative to $\text{SLM}_{1}$ [36]. Finally, $\text{PBS}_{2}$ was
used to select whether the signal was sent to a charge coupled device (CCD)
camera or coupled into a SMF for detection. The SMF is placed at the center of
the Fourier transform plane of the last lens ($\text{L}_{7}$). To detect the
transmitted sequences, the signal was amplified by $\text{EDFA}_{2}$ and
filtered to remove out-of-band amplified emission noise. An optical spectrum
analyzer (OSA) was used to record the received spectrum and power. At the
optical receiver, the FPGA was used to determine the bit error rate (BER).

##  4 Results

Here we present the experimental results on the generation, fiber coupling and
measurements of OAM. Additionally, we present results on two important
application of OAM beams, fiber based communication systems and quantum
system.

###  4.1 Mode generation

To study the OAM mode generation we generated PVB and conventional LG beams
using the SLM together with the $4f$ optical system. Using a CCD camera the
intensity of the generated beams was recorded in the image plane of the first
$4f$ optical system ($\text{L}_{3}$ and $\text{L}_{4}$) before the fiber
coupling system (see Fig. 1). Results of the OAM generated using a CGH on the
SLM are shown in Fig. 2a, where the detected intensity for each mode order $l$
is shown. The ring radius of the PVB are shown to remain constant as the order
$l$ is increased from $l=1$ to $l=6$, while the LG beam presents a large
radius variation. The measured ring radius for the PVB and LG modes is shown
Fig. 2b, where the LG modes presented ring radius variation from $77\ \mu m$
to $193\ \mu m$. An average ring radius of $147.9\ \mu m$ was observed when
the $k_{r}$ parameter was set to $k_{r}=3.06416$. A variation of $29\ \mu m$
in the ring radius was observed in this scenario, with the largest radius of
$165.9\ \mu m$ observed for $l=6$. Next, we studied the impact of the $k_{r}$
parameter on the ring radius of the generated PVB, with the results shown in
Fig. 2c. The optimization was performed based on $k_{r}=3.06416$ obtained for
$l=3$. Then, $k_{r}$ was either increased or decreased to maintain a fixed
ring radius using Eq. (1). In this scenario, an average ring radius of $140.5\
\mu m$ was obtained with a maximum variation of $4.7\ \mu m$. Note that the
differences reported with and without the $k_{r}$ optimization are
significantly less compared to the ring radius variation observed for the LG
modes. For example, the ring radius variance of LG modes was approximately
$1873.3\ \mu m^{2}$, while the ring radius variances measured for PVB modes
were of $5.4\ \mu m^{2}$ and $122.1\ \mu m^{2}$ with $k_{r}$ and without
$k_{r}$ optimization, respectively. Therefore, the LG modes have a variance
$15$ times higher than the PVB modes without adjusting $k_{r}$. In the case of
LG modes, it is well known that their radius grows with $\sqrt{|l|}$, and in
order to keep their ring radius constant a different optical system should be
used for each OAM.

Figure 2: a) Transversal intensity profiles of the generated OAM modes for LG
(second column) and PVB modes (third column). b) and c) represent two
graphical comparisons of PVB modes ring radius values with ring radius values
from LG modes (blue-star-points in both cases) as function of the $l$ OAM
values. Where b) compares LG modes with PVB non-optimized modes ($k_{r}$
fixed, green-dot-points), and c) compares LG modes with PVB optimized modes
($k_{r}$ variable, red-dot-points).

###  4.2 Fiber coupling and propagation

OAM modes in an optical fiber can be described using the linear combination of
LP modes, as in [28, 32, 33]. We have used a GRIN-RCF with inner and outer
ring radius of $6\ \mu m$ and $9\ \mu m$ [27]. The RCF supports propagation of
OAM modes up to $l=4$. Considering the fixed ring radius of the RCF, we used
$r_{r}\approx 140\ \mu m$ for the generated PVB modes in order to maintain a
constant $k_{r}$ parameter for all modes. At the fiber input a polarization
controller (PC) was used to control the linear combinations of LP modes in the
fiber. At the fiber output the following parameters were evaluated: field
intensity, interference between OAM and a Gaussian mode, and optical power.
The results are summarized in Fig. 3 and Fig. 4. Fig. 3 shows the intensity
profiles at the fiber output for the $LP_{a,b}$ modes excited by the OAM modes
at the fiber input. This was achieved using the polarization controller to
excite $LP_{a}$, $LP_{b}$, or an OAM mode from the bend perturbation to the
field transmitted through the RCF [37, 38, 39]. Note that the fiber modes
propagated are independent of the free-space beam used to excite them, the
difference arises from the coupled mode intensity. Additionally, the
transmitted OAM out of the RCF is shown together with the OAM-Gaussian beam
interference pattern that shows successful OAM propagation [40, 41, 31, 42,
43].

Figure 3: Modes propagated through RCF. First column shows $LP_{a}$ modes,
second column shows $LP_{b}$ modes, third column shows OAM modes, and fourth
column shows interference pattern for $l=1$ to $l=4$.

The coupling efficiency between the OAM modes generated with the CGH and the
RCF was characterized using the received power at the fiber output. The
optical power was measured at the image plane of the second objective lens
($20\text{x}$, see Fig. 1) using a constant coupling power into the RCF for
both OAM modes (LG and PVB, $\text{P}_{\text{in}}\approx 450\ \mu W$). An
average output power of $\text{P}_{\text{out}}^{\text{PVB}}\approx 16\ \mu W$
was measured for the PVB modes, while an average output power of
$\text{P}_{\text{out}}^{\text{LG}}\approx 3.32\ \mu W$ was observed for LG
modes. These results show the higher coupling efficiency of the PVB modes
compared to the LG modes. To verify the OAM modes propagated through the RCF
we used an interferometric technique between the OAM mode and a Gaussian beam
[40, 31, 41]. The interference patterns shown in Fig.3, verify qualitatively
that the OAM modes have effectively propagated through the RCF and have $l$
values of $l={1,2,3,4}$.

Figure 4: Output power measurements from RCF at the image plane of the second
objective lens $20\text{x}$ using different coupling modes, PVB modes (green-
dot-points) and LG modes (blue-star-points), as a function of OAM value.

####  4.2.1 Classical communication systems

Figure 5: Measured BER after transmission in the RCF using PVB (dot-points)
and LG (star-points) modes. The continuous lines, represent average of
$log(BER)$ values using PVB (cyan) and LG (orange) modes. The dashed purple
horizontal line represents HD-FEC threshold.

As described above, OAM was proposed as a way to implement SDM to increase the
capacity of optical fiber communication systems. Considering this, the use of
PVB to couple OAM into optical fibers can have a positive impact on the
systems, allowing to transmit longer distances compared to LG modes and
simplify the design for a OAM coupling system. Here, the quality of the
transmitted signals was evaluated using a commercial SFP+ transponder
operating at $1$ Gbps. The bit error rate (BER) was measured after fiber
propagation. Fig. 5 shows the measured BER as a function of the power coupled
into the RCF for the PVB and LG mode using $l={1,2,3,4}$. PVB are shown to
require lower coupling power to achieve a fixed BER of $4.7x10^{-3}$ (FEC
limit using a a $7\%$ FEC overhead [44]) compared to the LG modes. A power
difference of $8.6$ dB was observed for the mean BER between PVB and LG modes.
Additionally, the PVB modes are shown to have smaller performance difference
for different $l$ values at a fixed BER, with a $0.6$ dB difference observed,
compared to the $1.2$ dB measured for the LG modes.

From the Fig. 5, we can see that the best performance was obtained using PVB
with $l=2$, because its needs the lowest optical power to transmit the $1$
Gbps signal. This characterization shows that the PVBs are better candidates
to couple amplitude modulated information in fiber-based SDM systems, allowing
to implement systems with similar power requirements.

####  4.2.2 Quantum applications

In the quantum regime, it is well known that some quantum information
protocols (QIPs) can be more robust when using quantum states in higher
dimensions, these are quantum systems with d$>2$ levels (qudit) [45, 46, 47,
48, 49, 50]. Photons carrying OAM can be used to encode OAM-qudits, thus
allowing more information to be transmitted between the communicating parties
[17, 51, 52]. OAM-qudits have a myriad of applications in QIPs, both in free
space and optical fiber channels, most notably quantum key distribution (QKD)
protocols [53, 54, 55, 56, 57]. In particular, RCF has succeeded in this
context with demonstrations showing that the OAM modes supported by RCF can be
used to enhance classical and quantum communications links [23, 58, 27, 56,
55, 59, 60, 61].

As shown in Ref. [28], asymmetric coupling of OAM beams into RCF leads to
undesired state transformations performed by the RCF, which results in high
quantum bit error rates (QBER). To overcome this drawback PVBs are proposed
[28]. The QBER is related to the average fidelity of the quantum states used
for a QKD protocol, as follows $\bar{F}=1-$ QBER. Here we use PVBs to
experimentally demonstrate that 4-dimensional OAM-encoded quantum systems can
be propagated through the $300$ m of RCF with high fidelity. As reported in
Ref. [51], to guarantee the unconditional security of QKD protocols, we must
be below the upper limit of the QBER for the coherent attacks ($D^{coh}$),
which depends on the dimension of the quantum system used. In this case, for
4-dimensional OAM-encoded quantum systems, the upper limit of the coherent
attack is $D^{coh}=18.96\%$. Thus the average fidelity must be greater than
$\bar{F}=0.81$.

Fig. 6 shows the measured fidelity of the 4-dimensional OAM-encoded quantum
states after the propagation through the RCF with the average fidelity of
$92\%$. This paves the way for implementing QKD protocols using high-
dimensional OAM-encoded quantum states in SDM optical fibers.

Figure 6: Four measured fidelities after propagated through the $300$ m of
RCF, for the states a) $|l=1\rangle$, b) $|l=2\rangle$, c) $|l=3\rangle$, and
d) $|l=4\rangle$. The average fidelity the states is $92\%$.

##  5 Conclusion

Here we experimentally studied the generation and transmission properties of
two OAM carrying beams in ring core optical fibers. The beams studied were the
perfect vortex beams (PVB) and Laguerre-Gaussian (LG) beams. We have shown
that when using a single preparation stage for all of the OAM beams, the PVBs
present less ring radius variation and are able to couple higher powers into a
ring core fiber. Ring radius variations from $77\ \mu m$ to $193\ \mu m$ where
observed for LG beams, while an average ring radius of $140.5\ \mu m$ with a
maximum variation of $4.7\mu m$ was observed for the PVBs. Additionally,
higher optical powers were coupled into the RCF using PVBs. An average coupled
optical power of $15\ \mu W$ and $4\ \mu W$ were observed for PVB and LG
modes, respectively. In addition to successfully coupling optical OAM into a
RCF using PVBs, we have also demonstrated the possibility of establishing a
transmission link operating at $1$ Gbps. The results show that the high
coupling powers of PVB lead to lower power requirements to establish fiber-
based communications links. An average $8.6$ dB difference in required input
power was observed using PVBs compared to LG modes. In addition, we have shown
that the constant coupling power of PVBs as a function of the mode order $l$
can allow for the implementation of QKD protocols. Here the figure of merit
was the measured quantum state fidelity. We report an average fidelity of
$92\%$ in a 4-dimensional quantum system after transmission in the RCF, which
is larger than the minimum fidelity required for successful key distribution
under coherent attacks. Our work therefore can be seen as an initial step for
the development of PVBs as information carriers in optical fibers leading SDM
systems in telecommunications, and to implement high dimensional QKD systems.
In both scenarios, more efficient coupling of OAM into ring-core fibers is a
powerful tool to increased information capacity.

## Funding

The authors acknowledge support from Fondo Nacional de Desarrollo Científico y
Tecnológico (ANID) (Grants No. 1190933, 1200266, 1200859, 1220960, 1231826)
and ANID – Millennium Science Initiative Program – ICN17_012. Also, this work
was supported by Vetenskapsrådet (VR) under the project “Scaling Quantum
Networks by Multi-dimensional Multiplexing,” and the National Natural Science
Foundation of China (62225110).

## Acknowledgments

The authors thank G. B. Xavier for valuable discussions.

## Disclosures

The authors declare no conflicts of interest.

## Data Availability Statement

Data underlying the results presented in this paper are not publicly available
at this time but may be obtained from the authors upon reasonable request

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# Streamlining CXL Adoption for Hyperscale Efficiency

Angelos Arelakis  ,  Nilesh Shah  ,  Yiannis Nikolakopoulos  and  Dimitrios
Palyvos-Giannas<EMAIL_ADDRESS>ZeroPoint Technologies AB,
GothenburgSweden

###### Abstract.

In our exploration of Composable Memory systems utilizing CXL, we focus on
overcoming adoption barriers at Hyperscale, underscored by economic models
demonstrating Total Cost of Ownership (TCO). While CXL addresses the pressing
memory capacity needs of emerging Hyperscale applications, the escalating
demands from evolving use cases such as AI outpace the capabilities of current
CXL solutions. Hyperscalers resort to software-based memory (de)compression
technology, alleviating memory capacity, storage, and network constraints but
incurring a notable ”Tax” on Compute CPU cycles. As a pivotal guide to the CXL
community, Hyperscalers have formulated the groundbreaking Open Compute
Project (OCP) Hyperscale CXL Tiered Memory Expander specification. If
implemented, this specification lowers TCO adoption barriers, enabling diverse
CXL deployments at both Hyperscaler and Enterprise levels. We present a CXL
integrated solution, aligning with the aforementioned specification,
introducing an energy-efficient, scalable, hardware-accelerated, Lossless
Compressed Memory CXL Tier. This solution, slated for mid-2024 production and
open for integration with Memory Expander controller manufacturers, offers
2-3X CXL memory compression in nanoseconds, delivering a 20-25% reduction in
TCO for end customers without requiring additional physical slots. In our
discussion, we pinpoint areas for collaborative innovation within the CXL
Community to expedite software/hardware advancements for CXL Tiered Memory
Expansion. Furthermore, we delve into unresolved challenges in Pooled
deployment and explore potential solutions, collectively aiming to make CXL
adoption a ”No Brainer” at Hyperscale.

Composable Memory Systems, OCP Hyperscale Tiered Memory Expander Spec, CXL,
Hardware Accelerated Lossless Memory Compression, Compressed Memory Tiers

##  1\. Introduction

###  1.1. Background

DRAM is a key driver of performance and cost in the Data Center. CXL offers a
path towards improved DRAM utilization (cost efficiency) in both Tiered and
Pooled scenarios, but the Total Cost of system Ownership (TCO) increases non
trivially due to added infrastructure components (Berger et al., 2023).
Moving/adding DRAM off the DDR interface to CXL requires significant return on
investment and increased efficiency. The ground breaking OCP specification for
the Hyperscale CXL Tiered Memory Expander offers a crucial solution to achieve
efficiency by specifying the addition of a compressed DRAM tier. Compression
increases the effective capacity of a CXL memory device, reducing the power
necessary to manufacture and operate DRAM components.

###  1.2. OCP Specification/ requirements

The OCP Specification (Chauhan et al., 2023) calls for a sustainable,
transparent and cost-efficient method to compress memory on CXL Type 3 devices
on a variety of compute platforms with a diversity of memory technologies. The
OCP Spec calls for Access to a cache line in a compressed block within 250ns,
with tail latency for accessing a cache line in an compressed block of ¡1us
including worst case lookup latency, decompression, power-state transitions.
Furthermore, decompression speeds of 46GB/s must be matched to 4 Channels at
1867MT/s compressed data, with 4kB/1kB blocks. State of the Art solutions fail
to meet these requirements.

###  1.3. State of the Art

General-purpose, software-based lossless data “(de)compression” techniques are
used widely in hyperscale systems to alleviate the memory, storage, and
network cost with significant associated compute overheads “datacenter taxes”
in warehouse-scale datacenter services (Karandikar et al., 2023; Jeong et al.,
2023). These (de)compression services consume 2.9% to 4.6% of fleet CPU cycles
and 10-50% cycles in key services. The existing (de)compression algorithm
implementations (at block size granularity) do not meet the latency, power
efficiency and preservation of software investment tenets spelled out in the
OCP Specification. Waiting for an entire block or page prior to decompress
data incurs latency measured in microseconds, which is suitable for storage
but not acceptable for memory performance. Software and Intel QAT
(De)compression solutions require the use of area and power intensive Xeon
cores (Will, 2023), falling order of magnitude short of the latency and
bandwidth requirements specified in the OCP Spec. State of the Art hardware
accelerators reported in open source (Zipline) or in other studies (CDPU)
reported consuming 1.3-5.7mm sq area while achieving 5-11GB/s but with
unspecified latency and performance scalability (Karandikar et al., 2023).
These designs are not geared /portable for integration into CXL Tiered Memory
Expander devices as a complete solution to the best of our knowledge.

##  2\. Solution

###  2.1. Invention

We present our Hardware accelerated, Lossless memory (De) Compression
integrated IP solution [Figure 1] that increases the effective CXL Type 3
Device memory capacity by 2x through transparent, in-line memory
compression/decompression at 64 byte cache line granularity. It is designed as
an area and power-efficient solution composed of multiple integrated IP
blocks, portable across the latest process nodes, supporting (LP)DDR4 and
(LP)DDR5 memory technologies. This solution meets the latency and bandwidth
requirements of the OCP Hyperscale CXL Tiered Memory expander spec by
implementing a proprietary (De)compression algorithm at cache line
granularity, with dual hardware accelerator implementation of the open source
LZ4 algorithm to operate at page or block granularity for legacy
compatibility.

Figure 1. (De)Compression Solution integrated into CXL Expander Type 3 Device
Figure 2. Cache line (64 byte) compression algorithm

###  2.2. Innovation

Unlike state-of-the-art solutions that conduct compression at block/ page
granularity, our approach operates at a more refined 64-byte granularity (as
illustrated in Figure 2). This innovation seamlessly integrates into the CXL
Type 3 device System-on-Chip (SoC), supporting both AXI4 and CHI
specifications. The communication with the device occurs over CXL (2.0, 3.0,
3.1) command sets translated to AXI4, operating at a frequency of 1.2GHz (@4nm
Samsung). Our solution exposes a compressed memory region as an additional
NUMA tier in the memory hierarchy.

Achieving a 2-3x effective capacity increase with minimal impact on device
bandwidth and nanosecond-level latency, our solution dynamically manages the
compressed memory tier within the CXL Type 3 device. It implements real-time
compression/decompression with compaction, operating at main memory speed and
throughput. Additionally, an optional adaptive feature tunes performance based
on diverse workloads using low-level telemetry data.

When the host processor demotes pages from the directly-connec-ted DRAM to the
CXL device, the CXL (device) controller converts CXL.mem data to AXI/CHI
commands and calls our IP block in real time. The IP block intercepts data at
cache line granularity (with a proprietary (de)compression algorithm) or at
page/block granularity, as instructed over CXL.io. The compression algorithm
is then applied, and memory allocation for the compressed CXL tier is managed.
Compressed data is stored in the DRAM memory media over the DDR interface,
with the IP block situated between the CXL controller interface and the DDR
controller interface, communicating via AXI4 or CHI.

Our solution not only performs compression but also conducts compaction,
ensuring effective space utilization. The IP block implements capacity
reporting telemetry to the host. When a compressed page is requested, the IP
accesses the compressed cache lines or pages/blocks, decompresses the data,
performs necessary address translation on-the-fly, and transmits the data over
the AXI interconnect from DRAM media to the CXL interface. This entire process
is orchestrated by lightweight firmware running on an embedded processor in
the CXL Type 3 expander SoC, enabling real-time compression/decompression at
CXL line speed.

Figure 3. IP Solution Performance Characteristics. Exceed OCP Specification
requirements

###  2.3. Performance Summary

The IP Solution described above delivers 2-3X Compression Ratios while
compressing memory at Cache Line (64 Byte) granularity across a breadth of
representative Data Center workloads including SPEC2017INT/FP, Renaissance,
MLPerf/ Training, MonetDB+TPC-H [Figure4]. The solution has been verified to
operate at 1.2GHz and fits in an area of approximately 0.9mm2 (at 4nm Samsung)
where 75% of the IP solution area is occupied by SRAM. This solution currently
supports (LP)DDR4, (LP)DDR5 memory technologies and decompresses data in
single digit clock cycle latency. The overall CXL IP solution performance
characteristics [Figure 3] satisfy the requirements in the OCP Spec (Chauhan
et al., 2023). The key IP Solution performance characteristics are in Figure
3. The IP solution is slated to go into production mid 2024 into CXL
controller products and is expected to provide savings of 20-25% TCO [Figure
6] using previously published TCO model assumptions (Apostol, 2022), factoring
in 2X capacity expansion through an additional Compressed Memory Tier.

Figure 4. Geomean Compression Ratio across applications of each dataset Figure
5. Compressed Memory Tier introduced and managed by the IP Solution within the
CXL Memory Expander Type 3 device controller Figure 6. Reduced TCO. Addition
of Compressed Memory Tier increases effective capacity (GB) leading to reduced
$/GB

##  3\. Implementation / Evaluation

###  3.1. Proof of Concept Demonstration

To validate the practical implementation and effectiveness of the proposed CXL
solution, a Proof of Concept (PoC) demonstration has been developed [Figure
7]. The demonstration comprises two integral components: the frontend, based
on QEMU, emulating the host system and managing the migration of identified
cold pages to the CXL subsystem, and the backend, featuring an inline memory
compression/decompression and management accelerator implemented on an FPGA,
along with its accompanying firmware running on the FPGA-based platform.

Figure 7. Proof Of Concept Implementation Demonstration

###  3.2. Frontend: QEMU-based Emulation

The QEMU frontend emulates a Linux host capable of running realistic
workloads. Currently, arbitrary pages of the workload are migrated into the
emulated CXL memory. This can be extended with several mechanisms for
detecting cold pages (e.g. LRU or MGLRU algorithms, Senpai, DAMON).

###  3.3. Backend: FPGA-based Accelerator and Firmware

The backend is centered around the FPGA, hosting an inline memory
compression/decompression and management accelerator. This FPGA-based
solution, coupled with its firmware, is responsible for compressing, storing,
and managing the data migrated from the host system. The FPGA acts as the
compressed memory subsystem of the CXL device, encapsulating both hardware and
firmware components.

###  3.4. Demonstration Workflow

  1. (1)

Migration of Pages: The QEMU frontend migrates pages to the CXL subsystem.
This process involves invoking the IP solution on the FPGA, which takes on the
responsibilities of compression, storage, and bookkeeping of the data.

  2. (2)

Requesting Compressed Pages: The QEMU frontend, representing the host system,
requests the migration of compressed pages by sending corresponding read
requests to the IP solution on the FPGA. This step simulates the real-world
scenario where the host system interacts with the CXL device to access
compressed data.

  3. (3)

Real Application Utilization: The demonstration includes a real application
running on the system, showcasing the practical advantages of an expanded
memory on the FPGA. This expanded memory acts as the compressed memory
subsystem of the CXL device, incorporating both hardware and firmware
functionalities.

  4. (4)

Live Telemetry Display: Throughout the demonstration, live telemetry data is
presented, including compression ratios, expansion factors, and other metrics
[Figure 8] aligned with the OCP requirements for Hyperscale CXL Tiered Memory
Expander. This real-time feedback provides insight into the system’s
performance and adherence to industry standards.

Figure 8. FPGA Proof of Concept. Dynamically adjusting CXL Compressed Memory
Tier

The PoC demonstration serves as a tangible representation of the proposed CXL
solution’s capabilities. By combining emulation with FPGA-based hardware
acceleration, the demonstration provides a holistic view of the solution’s
functionality, validating its potential for real-world applications in memory-
intensive scenarios.

###  3.5. Summary

CXL addresses the pressing memory capacity needs of emerging Hyperscale
applications. Hyperscalers (Chauhan et al., 2023) have formulated the
groundbreaking (OCP) Hyperscale CXL Tiered Memory Expander specification to
lower TCO and enable diverse CXL deployments. This paper presents a CXL
integrated solution, aligning with the aforementioned specification,
introducing an energy-efficient, scalable, hardware-accelerated, Lossless
Compressed Memory CXL Tier offering 2-3X CXL memory compression in nanoseconds
and delivering a 20-25% reduction in TCO for end customers without requiring
additional physical slots.

##  4\. Open Questions / Call for Community Collaboration

  1. (1)

Upstream Linux Driver Development: Collaboration within the CXL community
extends to upstream Linux driver development. A lightweight Linux driver is
under development and is set to be upstreamed to the Linux kernel. This driver
provides APIs that allow the host to utilize the oversubscribed compressed
memory region of the device, and is intended to be fully compliant with the
CXL upstream Linux driver. Collaboration in this area will ensure seamless
integration with the Linux ecosystem, providing a standardized interface for
CXL memory expansion.

  2. (2)

Integration and Testing/Benchmarking: To ensure the robustness and performance
of the proposed CXL solution, collaboration efforts could be directed towards
integration and testing/benchmarking with data center applications, operating
systems, and hypervisors. A comprehensive testing framework will be
established to validate the solution’s compatibility and efficiency across
diverse environments, addressing the unique requirements of different
workloads.

  3. (3)

Collaboration with Hyperscalers and Device Manufacturers: Critical to the
success of CXL adoption is collaboration with hyperscalers and device
manufacturers. Aligning on requirements, gathering feedback, and understanding
the practical challenges faced by these stakeholders are paramount to ensure
smooth production deployment. Collaborative efforts involve iterative
discussions, prototype testing, and refining solutions to ensure they meet the
specific needs of hyperscalers and device manufacturers.

  4. (4)

Addressing adoption challenges raised by Hyperscalers: Hyperscaler End
customers have recently argued against the value of implementing CXL memory
pools (Levis et al., 2023), their main arguments being: The cost of a CXL pool
will outweigh any savings from reducing RAM. CXL has substantially higher
latency than main memory, enough so that using it will require substantial
rewriting of network applications in complex ways. To reduce Total Cost of
Ownership (TCO) and enhance CXL adoption, the community can convert this
challenge into an opportunity to path find better together solutions with
compression, optical interconnects, native CXL controllers, and integrating
flash media into the CXL memory tier to create holistic solutions that address
diverse aspects of CXL infrastructure to maximize efficiency and performance
at hyper scale. The open-source community in particular has the opportunity to
take leadership, rally around to develop solutions that require no fundamental
programming changes, ensuring a smooth transition for existing software
stacks.

## References

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