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Free space optical communication receiver
based on a spatial demultiplexer and a
photonic integrated coherent combining
circuit
VINCENT BILLAULT,1,* JEROME BOURDERIONNET,1 JEAN PAUL MAZELLIER,1
LUC LEVIANDIER,1 PATRICK FENEYROU,1 ANAELLE MAHO,2 MICHEL SOTOM,
2 XAVIER NORMANDIN, 3 HERVE LONJARET, 3 ARNAUD BRIGNON,1
1Thales Research and Technology, 1 Avenue Augustin Fresnel, 91767 Palaiseau, France.
2Thales Alenia Space, 26 Avenue J.F. Champollion – B.P. 33787, 31037 Toulouse Cedex 1, France
3Thales LAS, 2 Avenue Gay Lussac 78995 Elancourt, France
*vincent.billault@thalesgroup.com
Abstract: Atmospheric turbulences can generate scintillation or beam wandering phenomena
that impairs free space optical (FSO) communication. In this paper, we propose and
demonstrate a proof-of-concept FSO communication receiver based on a spatial demultiplexer
and a photonic integrated circuit coherent combiner. The system collects the light from several
Hermite Gauss spatial modes and coherently combine on chip the energy from the different
modes into a single output. The FSO receiver is characterized with a wavefront emulator bench
that generates arbitrary phase and intensity patterns. The multimode receiver presents a strong
resilience to wavefront distortions, compared to a monomode FSO receiver. The system is then
used to detect a modulation of the optical beam through a random wavefront profile.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical feeder links are currently considered as a promising technology to cope with the
continuous growth in high data rate communication demand [1-2]. However, two mains factors
limit the performances of optical communications with satellites: the low signal power
delivered to the receiver and the atmospheric turbulences [3]. The low signal power and the
weak signal-to-noise ratio at the receiver side practically limits the Bit Error Rate and so the
performances of the communication link. Besides, atmospheric turbulences distort the optical
wavefront degrading the optical coherence of the laser beam [4] and causing signal fading.
These effects reduce drastically the single mode fiber (SMF) coupling efficiency of the free
space optical (FSO) receiver.
To preserve a good coupling efficiency in a SMF FSO receiver, the aperture size of the
collection lens is limited by the correlation length of the atmospheric turbulences to tens of
centimeters [5]. A first route to overcome this limitation, lies in adaptive optics. The system
mitigates the atmospheric distortion with deformable mirrors. However, the complexity of the
control algorithm limits in practice the correction loop bandwidth [6]. Recently, different
realizations of multichannel FSO receivers have been proposed with fast electronic signal
recovery [7,8]. For multi-aperture (MA) receivers, the input wavefront is sampled by an
aperture array to sum the energy from different positions of the wavefront. MA receivers are
routinely used for radiofrequency systems [9], and MA FSO receivers have recently been
presented to improve the optical feeder links efficiency [7]. However this architecture requires
a coherent receiver for each aperture, which is costly, bulky and not easily scalable to
tens/hundreds of aperture. Alternatively, an original architecture of multichannel FSO receiver
with a multimode fiber has been proposed to increase the collection area [8]: the light is coupled
to a multimode fiber and a spatial demultiplexer projects the coupled light onto the multimode
fiber’s modes basis. Each projection is then converted into a Gaussian profile and coupled to a
separate SMF. Similarly to the MA FSO receiver, the collection area is wider than a SMF but
it requires a coherent receiver for each mode which limits the scalability of the system.
Instead of detecting the light out of each channel of a FSO receiver and subsequently adding
the data signal in the electrical domain after re-synchronization process, we propose to
coherently combine each optical output into one single beam before the optical to electrical
conversion on a single detector. This step theoretically enhances the signal to noise ratio by
reducing the noise floor with a factor equal to the number of channels or modes recombined. It
also facilitate the demodulation step avoiding the active resynchronization of the channels.
Therein, photonic integrated circuits (PIC) exhibit attractive features for achieving the coherent
combination of the multiple outputs of the receiver. PICs provide both optical and electronic
functions with ultimate small footprint, and a remarkable thermal and mechanical stability [10-
11]. The photonic integrated platform is also perfectly suited to scale to a high number of inputs.
In this paper, we propose and demonstrate a proof-of-concept FSO receiver for optical
communications based on a spatial demultiplexer and a PIC. In a first section, we describe the
wavefront emulator that we developed in order to mimic phase and amplitude distortion and
evaluate the receiver performances. The emulator is calibrated to selectively excite the
demultiplexer eigenmodes in order to generate arbitrary phase and amplitude spatial beam
profiles as a combination of the demultiplexer mode basis. In a second section we describe and
characterize the combining PIC. In the last section the performances of the complete FSO
receiver are evaluated.
2. Experimental setup.
The proposed receiver and wavefront emulator test bed are presented in Figure 1. The
wavefront emulator (Fig. 1A) generates arbitrary amplitude and phase light field. The input
profiles are decomposed on the Hermite Gauss (HG) basis by the spatial demultiplexer (Fig.
1B), and each projection is coupled to a SMF. Finally, the power of each fiber is coherently
combine with a PIC on a single output beam (Fig. 1C).
Fig.1. Experimental setup of the FSO receiver. Col : collimator. Pol : polarizer. PD : photodiode.
SLM : spatial light modulator. Focal lens : L1 = 30 cm, L2 = 7.5 cm, L3 = 30 cm, L4 = 50 cm.
The wavefront emulator is inspired from [12] and uses two nematic liquid crystal phase-
only spatial light modulators (LCOS SLMs) for intensity and phase modulation of an input
field. The light from a CW laser operating at 1.55 µm and passes through a half-wave plate
(HWP) and by a collimating lens (L1) to the SLM1. A 4f afocal system (with unit magnification)
images the plane of SLM1 to that of SLM2. An iris, a polarizer and a HWP are placed between
the two SLMs. The two SLMs modulate spatially the phase of the light for only one polarization
direction
orthogonal to the plane of Figure 1. The first HWP controls the polarization of
the light that addresses SLM1, in order to have the light linearly polarized along , oriented at
from
. With a polarizer in the same polarization direction , the SLM1 acts like an
intensity spatial modulator. The second HWP orients the light polarization direction to
so
that the SLM2 acts like a phase only spatial modulator. By applying respectively a phase pattern
and ) to SLM1 and SLM2, the electrical field at the wavefront emulator
output writes [12] :
(1)
with the amplitude of the input wavefront on the SLM1. So, by changing the phase
patterns written onto SLM1 and SLM2, we manipulate the amplitude and phase of the wavefront
independently at the same time. Figure 2 shows several profiles of HG modes generated with
the wavefront emulator imaged with a camera (not shown in Fig. 1). The camera is imaging the
plane of SLM1 through a 4f system (with a magnification of 0.7).
Fig. 2. Example of generated HG modes. Phase patterns on SLM1 (up) and image with the
camera (bottom).
The multichannel FSO receiver (CAILabs: Tilba) is a mutli-plane light conversion (MPLC)
module that takes a FSO wavefront for input and returns its projection on a 15 HG modes basis,
on 15 separated SMF. We first characterize the transmission of the MPLC module by
generating consecutively the different HG modes with the wavefront emulator. For a given
mode HGx,y, we measure the input power
and the ouput power for each fiber (the 15
) with a photodiode array. Figure 3 shows the transmission matrix of the MPLC,
and the Table 1 sums up the MPLC characteristics. For each mode, we calculate the insertion
losses with
and the cross talk with
of the MPLC in a receiver
configuration. The transmission matrix is quasi-diagonal and the mean cross talk is very low
(< -19 dB), making this architecture suitable for the reception and decomposition of distorted
wavefronts.
Fig. 3. (a) Transmission matrix of the MPLC (b) Output power distribution. Measured (red)
and theoretical (blue) combination (%), inset : corresponding phase pattern on SLM1.
Table 1. MPLC characteristics for HGx,y: insertion losses (IL) and mean cross talk (MCT) in dB
HGx,y
40
30
20
10
31
21
11
22
IL
-3.8
-3.5
-3.2
-3.6
-4.0
-3.9
-3.8
-4.2
MCT
-22
-23
-24
-22
-22
-21
-21
-23
HGx,y
00
12
13
01
02
03
04
IL
-3.5
-3.4
-4.0
-3.7
-3.2
-3.2
-3.6
MCT
-21
-21
-22
-19
-21
-20
-23
To verify the MPLC module linearity, we generate a wavefront corresponding to a combination
of 6 HG modes with random phase and power distribution (see Table in Fig.3b). As the insertion
losses slightly vary among the HG modes, we normalize each mode by the corresponding
eigenvalue of the MPLC transmission matrix. Figure 3 presents the output power distribution
of the 6 fibers measured with the photodiode array. The output power distribution is in excellent
agreement with the theoretical distribution. The small differences that emerge come from the
phase flicker of the SLMs and the residual cross-talk of the MPLC.
3. Coherent combining photonic integrated circuit
We designed a PIC to combine the energy from the 15 output fibers of MPLC. It has been
fabricated within IMEC’s (Belgium) Silicon On Insulator (SOI) photonics foundry through
multiprojects wafers shuttle and packaged by PHIX (Netherlands). The operating principle of
the PIC is described in Figure 4. The PIC is divided in 14 unit cells, each cell being composed
of two inputs (with a phase shifter in one of them), an interferometer between the two inputs
(with a phase shifter in one arm) and two outputs [13]. A photodiode at one of the cell output
detects the interference intensity. By sweeping the voltage applied to the cell phase shifters,
one can minimize the current on the photodiode. When it is minimum, the power at the other
output of the interferometer is maximum, i.e both amplitude and phase imbalance have been
corrected and the power from the two inputs of the cell are coherently combined. This principle
is applied on each cell simultaneously with a Nelder Mead optimization algorithm [14] that
minimizes the current of the photodiodes by adjusting the phase shifters driving voltages. The
combined beam can be routed for direct detection (either on an on-chip photodiode or coupled
outside the chip) or for phase and quadrature (I/Q) detection with an on-chip 90° coupler and
balanced detectors [15].
Fig. 4 (a) Scheme of the PIC. A unit cell is represented in yellow (b), (c) pictures of the PIC and
packaged device.
To validate the working principle of the PIC combiner, the optimization algorithm is applied
for 6 fiber inputs as a first proof-of-concept experiment. Figure 5 shows the evolution of the
combined power without and with the feedback loop. In open loop configuration, the phase
variations between the different inputs cause large variations of the optical power. As soon as
the feedback loop is switched on, the combined power reaches and stays to a maximum
corresponding to the total coherent combination of the inputs. The observed residual fast
fluctuations are due to the 2 pi phase jumps of the phase shifters (50 µs time scale).
Fig. 5 Combined optical power at the PIC output with/without phase locking.
4. Experimental results.
With the setup of Fig. 1, we generate arbitrary wavefronts as combination of HG modes,
and we detect and combine the power from the differents modes. To compare our system to
conventional FSO receivers, we add a second collection arm (cf. Fig. 1B) composed of a
collimator and a SMF. An afocal system (with a magnification of 0.25) adapts the size of the
wavefront without any spatial shaping to the collimator to optimize the coupling efficiency. We
apply different waveform profiles with the wavefront emulator and compare the coupling
efficiencies for the two receivers. For the monomode receiver, the coupling efficiency
corresponds to the ratio of the coupled power to the input power. For the multimode receiver,
it is defined as the ratio of the combined power (after being routed outside the PIC) to the input
power. The set of beam profiles (see Fig. 6) corresponds to: no spatial shaping on the SLMs
(pattern n°1), a combination of 6 HG modes with same amplitude and random phase (pattern
n°2-5), and a combination of 6 HG modes with random amplitudes and phase (pattern n°6-10).
For each pattern, we also extrapolate the coupling efficiency on chip (corresponding to the
coupling efficiency without the output coupling loss).
For no spatial shaping on the SLM, the coupling efficiency for the SMF FSO receiver is
much higher than for the spatial demultiplexer and PIC FSO receiver. This is largely due to the
propagation and coupling loss of our present PIC device (16 dB). However, for random
patterns, the coupling of the SMF FSO receiver strongly varies as the coupling of the spatial
demultiplexer and PIC FSO receiver remains almost constant. This shows that the latter is much
more resilient than classical FSO receivers to phase and amplitude perturbations. This property
is extremely valuable for telecommunications, as atmospheric perturbations rapidly change the
phase and energy distributions between the modes.
Fig. 6 Coupling efficiency for different pattern. (Blue) Detection outside the PIC (solid line) and
theoretical coupling with detection on the PIC (dotted line). (Red) Detection with the
comparative FSO receiver (solid line). The six HG modes are HG22, HG31, HG00, HG10, HG13,
HG40. The different intensity profiles are represented above the graph.
Finally, to detect radiofrequency modulations applied to the electric field with the full setup
(wavefront emulator and multimode receiver), we use the on-chip coherent receiver. The
coherent receiver is composed of a 2v4 multimode interference (MMI) coupler and four
balanced photodiodes (cf. Fig. 4. a) to generate the phase and quadrature (I/Q) component of
the radiofrequency field. We characterize the power and phase imbalance between the 4 outputs
of the MMI coupler with test structure: an unbalanced Mach-Zehnder interferometer (MZI, cf.
scheme of Fig. 7.a) [16], positioned at a corner of the PIC. By sweeping the wavelength of the
MZI input, we determine the relative phase offsets and power imbalance of the 2v4 MMI
coupler at the output ports (cf. Fig. 7.a). The MMI outputs are shifted from respectively, ,
and ° with a 9 % power relative variations between the outputs.
To mimic a photonic microwave link, an acousto-optic frequency shifter at the wavefront
emulator input generates a frequency modulation of the electric field. The HG modes
combination of pattern n°2 from Fig. 6 is applied to the SLMs. Figure 7.b shows the temporal
profile of the I,Q detection at the coherent receiver output, for a frequency shift of 80 MHz.
Fig. 7 (a) Characterization of the 2v4 MMI coupler: scheme of the MZI (up) and relative
transmission of the 4 MMI outputs versus the input wavelength of the MZI (bottom) (b) Outputs
of the on-chip coherent receiver (red : I, blue Q) with a 80 MHz modulation applied to the
wavefront emulator input.
This preliminary result validates the use of the spatial demultiplexer and PIC based FSO
receiver for the balanced detection of modulated waveform. The system will then be tested with
more complex high-frequency modulation scheme (like QPSK, DQPSK, …).
5. Conclusion.
We have proposed and demonstrated a proof-of-concept FSO receiver based on a spatial
demultiplexer and a PIC coherent combiner. We have developed a wavefront emulator to test
the receiver with arbitrary amplitude and phase patterns. The FSO receiver has proven the
efficient coherent combination of 6 fibers for different random inputs. Compared with a
conventional SMF FSO receiver, we have showed that our system is much less sensitive to
phase and amplitude perturbations. In a final experiment, we validate the FSO receiver for the
detection of a modulated input wavefront.
As a conclusion, the implementation of FSO receivers based on a spatial demultiplexer with
coherent combination presents numerous advantages compared to standard SMF FSO receiver:
as the spatial demultiplexer collects a large number of spatial modes and have a high numerical
aperture, the collection area is larger. The aperture diameter of the collection lens with this
system could therefore become higher than the atmospheric turbulences correlation length
increasing the signal power. Secondly, the architecture shows a strong resilience to random
phase and intensity perturbation, as the coupling efficiency does not depend on the energy
distributions among the spatial modes. As a consequence, this systems ought to be less sensitive
to atmospheric turbulences. Finally, the system is largely scalable, as MPLCs can project and
convert wavefronts into a large number of spatial modes, and integrated photonic platforms
allow to encompass a large number of photonic devices on the same chip. The present approach
also opens up prospects for future developments including the design and realization of low-
loss hybrid photonic integrated SOI/SiN/III-V PIC devices, the increase of the recombined
channel number and tests on real atmospheric scenario. This first results pave the way to the
next generation of ultra-efficient FSO communication receiver.
Acknowledgment
We thank Arnaud Le Kernec for useful discussions. This work was partially supported by
the VERTIGO project from the European Union’s Horizon 2020 research and innovation
programme under grant agreement No. 822030.
Disclosure
The authors declare no conflicts of interest.
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