Research Progress of Fiber-based Coherent Polarization Beam

Combining for Free-Space Optical Communications in IOE, CAS

Yan Yang

1,2,3

, Chao Geng

1,2,*

, Xinyang Li

1,2

, Feng Li

1,2

and Guan Huang

1,2,3

Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu, Sichuan 610209, China

Key Laboratory on Adaptive Optics, Chinese Academy of Sciences, Chengdu, Sichuan 610209, China

University of Chinese Academy of Sciences, Beijing 100049, China

Keywords: Free-Space Optical Communications, Coherent Polarization Beam Combining, Fiber Optics.

Abstract: Multi-aperture receiver with phased array is an effective approach to overcome the atmospheric turbulence

effect on the performance of the fiber-based free-space optical (FSO) communications, where how to

combine the multiple beams received by the sub-apertures efficiently is one of the key techniques. In this

paper, we report on the research progress of the fiber-based coherent polarization beam combining (CPBC)

in IOE, CAS, which is a promising beam combining solution for coherent FSO communications employing

the multi-aperture receiver. Phase-locking control and polarization-transforming control were proposed to

combine linearly polarized beams with orthogonal polarizations into one linearly polarized beam efficiently,

and three fiber-based CPBC schemes were proposed and experimentally validated.

1 INTRODUCTION

With the development of the Space-Ground

Integration Network (SGIN), the demand of a higher

link capacity is indubitable. Compared to the

conventional radio frequency (RF) links, the free-

space optical (FSO) communications offers

numerous advantages including: ultra-high data rates,

excellent security and large, unlicensed bandwidth,

relatively low power consumption, and immunity to

the electromagnetic interference (Chan, 2006). As

the receiver sensitivity can be improved by up to 20

dB compared with that of non-coherent intensity

modulation/ direct detection (IM/DD) scheme, the

coherent FSO communications has great potential to

be used in various applications (Agrawal, 2002).

Currently, research has tended to focus on fiber-

based coherent FSO communication systems, which

can take advantage of the established components of

fiber-optic communication systems (Li et al., 2016).

In such schemes, efficiently coupling the signal light

wavefront to optical fiber to achieve high received

signal-to-noise ratio is critical (Zhang et al., 2013).

The obvious means to increasing the received optical

power is to increase the aperture diameter. However,

optical telescopes with large apertures are very

expensive and difficult to build. Furthermore,

atmospheric turbulence adds to the difficulty of

coupling the signal light wavefront from large

telescopes into optical fiber (Ma et al., 2015). To

overcome the atmospheric turbulence effect,

adaptive optics (AO) technology can be applied, and

the receiver employing a monolithic aperture with

AO has been demonstrated to be available to

improve the coupling efficiency in fiber-based

coherent FSO communications (Zheng et al., 2017).

Nevertheless, when AO is used, equipment such as

tip/tilt mirror (TM), deformable mirror (DM) and

Hartmann Wavefront Sensor (WFS) is needed,

which is complicated, costly and hard to achieve. As

an alternative, the multi-aperture receiver with

phased array (Yang et al., 2017) combines signals

detected by sub-apertures to ease deep fades and

increase the received optical power, in which

adaptive fiber-optics coupler (AFOC) array (Li et al.,

2017) is employed to correct the tip/tilt aberrations

and promote the coupling efficiency of each sub-

aperture. Compared with the receiver employing a

monolithic aperture with AO, the multi-aperture

receiver with phased array has easier manufacture,

lower costs, superior reliability, smaller sub-aperture

sizes, and more flexible sub-aperture positions

(Yang et al., 2017). In the multi-aperture receiver

with phased array, it is necessary to combine the

signals from the aperture array, to merge these

signals and enhance the received signal-to-noise

Yang, Y., Geng, C., Li, X., Li, F. and Huang, G.

Research Progress of Fiber-based Coherent Polarization Beam Combining for Free-Space Optical Communications in IOE, CAS.

DOI: 10.5220/0006533200170023

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 17-23

ISBN: 978-989-758-286-8

ratio.

In this paper, we report on the research progress

of the fiber-based coherent polarization beam

combining (CPBC) in IOE, CAS, which can be

applied to combine multiple laser beams to one laser

beam in the fiber-based coherent FSO

communications employing multi-aperture receivers

with phased array.

2 BASIC SCHEMES FOR

FIBER-BASED CPBC

The fiber-based polarization beam combiner (PBC)

(Yang et al., 2017), as shown in Figure 1, is the key

device of the fiber-based CPBC, with the function of

combining two orthogonally polarized fiber beams

into one output fiber. Three polarization-maintaining

fibers (PMFs) are fused in the two sides of the

calcite prism. The slow axes of the PMFs in port-1

and port-2 are aligned to the Y axis and the X axis,

respectively. The slow axis of the PMF in port-3 is

rotated 45° clockwise by port-1.

stress road

fiber core

Calcite prism

port-1

Slow axis

port-2

Slow axis

port-3

Slow axis

port-1

port-2 port-3

Figure 1: Structural schematic diagram of the fiber-based

PBC.

The basic principle of the fiber-based CPBC is

shown in Figure 2. It is well known that two linearly

polarized beams with orthogonal polarizations can

be combined into one beam by using a fiber-based

PBC.

Theoretically, when the phase difference

between the input beams is random, the polarization

state of the combined beam is not a linearly

polarized one, and it is just a superposition of the

polarization states of the input beams, as shown in

Figure 2(a).

Two methods can be employed to control the

combined beam to be linearly polarized.

As shown in Figure 2(b), fiber-based CPBC with

phase-locking (PL) control compensates for the

phase difference between the input beams by using

the piezoelectric-ring fiber-optic phase

compensators (PCs) to control the combined beam to

be linearly polarized. Provided that the phase

difference between the input beams is controlled to

be 2nπ (where n is an integer), the combined beam is

linearly polarized, and the polarization direction is

related on the power ratio of the input beams. Owing

to that the slow axis of the PMF in port-3 is rotated

45° clockwise by port-1, the combined beam is

linearly polarized along the slow axis of the PMF

only when the power ratio of the input beams is

equal to be 1. If the power ratio of the input beams is

not equal to be 1, the polarization direction of the

combined beam deviates from the slow axis of the

PMF, which will significantly diminish the

combining efficiency and limit the expansibility of

the CPBC.

(a)

(b)

Combined

beam

Input

beam-1

PBC

Input

beam-2

Combined

beam

Input

beam-1

PCs

PBC

Control signals

Input

beam-2

Combined

beam

Input

beam-1

PBC DPC

Control signals

Input

beam-2

(c)

CPBC with PL control CPBC with PT control

Figure 2: Basic principle of the fiber-based CPBC (a)

without control, (b) with PL control, and (c) with PT

control.

The combining efficiency η

is defined to be the

ratio of the combined optical power along the slow

axis of the PMF in port-3 and the total input optical

powers. Without considering the excess loss of the

fiber-based PBC, the relationship between the

combining efficiency and the power ratio of the

input beams I

can be expressed by:

 









(1)

where the phase difference between the input beams

is controlled to be 2nπ.

As shown in Figure 2(c), fiber-based CPBC with

polarization-transforming (PT) control transforms

the combined beam to be linearly polarized directly

by using the dynamic polarization controller (DPC)

(Yang et al., 2017), which can transform arbitrary

polarization state to any desired polarization state.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

When PT controlled, the combined beam can be

linearly polarized along the slow axis of the PMF

whether the input beams are phase-unlocked or

beam-imbalanced.

The basic experimental setup of the fiber-based

CPBC with two input beams is carried out, as shown

in Figure 3. Two input beams are produced by

splitting the output of a linearly polarized single-

mode fiber laser at 1064 nm (NKT photonics). A

variable optical attenuator (VOA), used for beam

attenuating, is adopted in one of the input paths to

adjust the power ratio of the input beams. Two input

beams are combined by fiber-based CPBC. Then,

the fiber-based polarization beam splitter (PBS),

which has a similar structure to the fiber-based PBC

except that the slow axis of the PMF in port-3′ is

identical to the slow axis of the PMF inport-1′, with

two photo detectors (PDs) is employed to stabilize

the polarization state of the combined output beam

and evaluate the performance of the combination.

The PDs are PDA36A silicon amplifier detectors,

produced by THORLABS Corporation. The optical

power output from port-1′, denoted as P

′, is sent to

a servo PD to provide a control signal as the cost

function, and the stochastic parallel gradient descent

(SPGD) algorithm (Geng et al., 2013) is employed

to maximize the cost function. To quantitatively

evaluate the CPBC performance, the optical power

output from the port-2′ is acquired by a detected PD,

and denoted as P

′. We use the coherent polarization

combining efficiency (CPCE) η defined in Eq. (2) as

a figure-of-merit:











(2)

In the experimental setup, the fiber-based PBC is

produced by Advanced Fiver Resources (AFR)

Corporation, with the excess loss of no more than

0.8 dB at 1064 nm without connectors. The PC made

by our group has a half-wave voltage of 1.3 V and a

frequency response of about 32 kHz. The DPC is a

kind of phase retardation-control polarization

controller based on the principle of squeezing the

fiber, with a response time of 30 μs and the insertion

loss of no more than 0.05 dB without connectors,

produced by General Photonics.

First, the feasibilities of the two fiber-based

CPBC schemes are validated by combining two

input beams with identical optical powers.

The fiber-based CPBC with PL control is

adopted in the setup, and the SPGD algorithm

generates two PL control signals to control the PCs

to compensate for the phase difference between the

input beams. The iteration rate of the SPGD

algorithm is about 10 kHz, and the durations of the

open and closed states are both 6 s. The

experimental results are shown in Figure 4(a). The

average CPCE increases from 74.18% in the open

loop to 99.31% in the closed loop, and the mean

square error (MSE) decreases from 0.3041 in the

open loop to 0.0017 in the closed loop. The closed

loop of PL control is achieved after about 25

iterations, equivalent to 2.5 ms, of SPGD

optimization.

Fiber

laser

Optical

Electrical

Data acquisition &

control platform

VOA

PBS

Fiber

splitter

Detected

Servo

Port-1'

Port-3'

Port-2'

CPBC with

PL/PT

control

Figure 3: Experimental setup of the fiber-based CPBC

with two input beams.

The fiber-based CPBC with PT control is

adopted in the setup, and the SPGD algorithm

generates four PT control signals to control the DPC

to convert the polarization of the combined beam.

The iteration rate of the SPGD algorithm is about 6

kHz, and the durations of the open and closed states

are both 10 s. The experimental results are shown in

Figure 4(b). The average CPCE increases from

62.03% in the open loop to 99.61% in the closed

loop, and the MSE decreases from 0.0572 in the

open loop to0.0028 in the closed loop. The closed

loop of PT control is achieved after about 20

iterations, equivalent to 3.3 ms, of SPGD

optimization.

0 2 4 6 8 10 12

0.2

0.4

0.6

0.8

0 20 40 60 80 100

0.2

0.4

0.6

0.8

Time (s)

CPCE

Open loop

Closed loop

CPCE

Iteration number

Average= 74.18%

MSE=0.3041

Average=99.31%

MSE=0.0017

Closed loop

achieved

(a)

0 5 10 15 20

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80 100

0.4

0.6

0.8

Time (s)

CPCE

Open loop

Closed loop

CPCE

Iteration number

Average= 62.03%

MSE=0.0572

Average=99.61%

MSE=0.0028

Closed loop

achieved

(b)

Figure 4: The experimental results of in fiber-based CPBC

with (a) PL control and (b) PT control.

Research Progress of Fiber-based Coherent Polarization Beam Combining for Free-Space Optical Communications in IOE, CAS

Then, we intentionally increase the difference

between the optical powers of the two input beams

to demonstrate the influence in the two fiber-based

CPBC schemes. The results are shown in Figure 5.

The blue curve represents the theoretical results of

the CPBC with PL control calculated by using Eq.

(1). The red curve and the green curve represent the

experimental results of the CPBC with PL control

and the CPBC with PT control, respectively, in

which each point is averaged by ten experimental

results with the specific power ratio. On the one

hand, the experimental data in the fiber-based CPBC

with PL control are reasonably consistent with the

theoretical analysis, validating the conclusion that

the power imbalance will significantly diminish the

CPCE in the fiber-based CPBC with PL control. On

the other hand, the CPCE is near constant at all

power ratios with an average of 99.48% in the fiber-

based CPBC with PT control, which indicates that

the fiber-based CPBC with PT control can

accommodate the power imbalance of the input

beams and combine two input beams with arbitrary

power ratio efficiently.

1 2 3 4 5 6 7 8

0.8

0.85

0.9

0.95

Power ratio of the input beams

CPCE

Experimental results of the

CPBC with PT control

Experimental results

of the CPBC with

PL control

Theoretical results of the

CPBC with PL control

Figure 5: Curves of CPCE as the function of the power

ratio of the input beams.

Both of the two fiber-based CPBC schemes have

some limitations that will restrict the enhancement

of the combining performance. In the fiber-based

CPBC with PL control, the power imbalance of the

input beams will inevitably degrade the combining

efficiency of the CPBC and cannot be compensated.

In the fiber-based CPBC with PT control, a major

drawback is the increased complexity and decreased

convergence rate due to the required multiple control

signals.

3 IMPROVED SCHEME FOR

FIBER-BASED CPBC

To break through the limitations in previous fiber-

based CPBC schemes, the fiber-based CPBC with

cascaded PL and PT controls is proposed, as shown

in Figure 6, to combine imbalanced laser beams (the

number of the input beams is not binary). It is the

synthesis of the PL control and PT control. When

the input beams are balanced, PL control module is

adopted; when the input beams are imbalanced, PT

control module is employed. Moreover, the fiber-

based CPBC with cascaded PL and PT controls can

be scaled to combine multiple laser beams

efficiently, and it can incorporate the advantages of

the PL control and the PT control together.

Compared with the CPBC based on PL control, in

which the influence of the power imbalance on the

combining performance is unavoidable, the

proposed CPBC can combine imbalanced input

beams with lower loss and higher combining

efficiency. Compared with the CPBC based on PT

control, the proposed CPBC needs less control

signals, which will result in a lower complexity and

faster convergence rate.

PBC

PBC DPC

PCs

PL control module

PT control module

Control signals Control signals

Figure 6: Basic principle of the fiber-based CPBC with

cascaded PL and PT controls.

The basic experimental setup of fiber-based

CPBC of three laser beams with cascaded PL and PT

controls is carried out, as shown in Figure 7. Three

input beams are produced by splitting the output of a

linearly polarized single-mode fiber laser at 1064 nm,

and three VOAs are employed in the input paths to

adjust the optical powers of the input beams. The

VOAs are not the essential parts of the CPBC setup

and are used only for beam attenuating. Two of the

three input beams with identical optical powers are

firstly combined in a PL control module. To

compensate for the phase difference between the two

input beams and make the sub-combined beam to be

linearly polarized, two PCs are employed in the

input paths before fiber-based PBC-1 for PL control.

With the loop closed, the PL control module

produces a new, linearly polarized sub-combined

beam, while the polarization direction is aligned to

the slow axis of the PMF, as the two input beams are

power balanced and phase locked. By transmitting in

the PMF, the polarization state of the sub-combined

beam remains unchanged. When incident at the

fiber-based PBC-2 in the PT control module, the

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

polarization directions of the sub-combined beam

and the third input beam are orthogonal. Thus, the

two beams can be further coherently combined using

the fiber-based PBC-2. To transform the combined

beam to be linearly polarized, a DPC is employed

after the fiber-based PBC-2 for PT control. The

polarizer, with a PD is employed to stabilize the

polarization state of the combined beam and

evaluate the performance of the combination. In the

setup, the SPGD algorithm is employed to perform

the PL and PT controls. The optical power of the

output beam acquired by the PD is chosen as the

cost function. The control methods of the PL and PT

controls are similar to each other except for the

different values of the control coefficients δ and γ.

The SPGD algorithm generates two PL control

signals and four PT control signals simultaneously.

Optical

Electrical

PBC-1

Fiber

laser

Fiber

splitter

PBC-2 DPC Polarizer

Data acquisition & control platform

PC-1

VOAs

Sub-combined beam

Combined beam

Output beam

PL control module

PT control module

PC-2

Figure 7: Basic experimental setup of the fiber-based

CPBC of three laser beams with cascaded PL and PT

controls.

In the experiment of the fiber-based CPBC with

two input beams, the CPCE defined in Eq. (2) is

used to evaluate the performance of the combination.

Nevertheless, it is inaccurate to evaluate the

performance of the combination with more than two

input beams. Considering all the device losses in the

experimental setup, link budget can be derived out.

We assume that the optical powers of the three

input beams are denoted as P

, P

, and P

respectively.

The optical power of the sub-combined beam

combined in the PL control module can be expressed

by:

 

PC-1 PC-2 PBC-1

10 10 10

sub 1 2

10 10 10

IL IL EL

P P P



  

     

(3)

where IL

PC-1

is the insertion loss of the PC-1, IL

PC-2

is the insertion loss of the PC-2, and EL

PBC-1

is the

excess loss of the fiber-based PBC-1. η

can be

calculated by using Eq. (1), and here I

is equal to be:

 

PC-1 PC-2

IL IL





(4)

The optical power of the combined beam combined

in the PT control module can be expressed by:

 

PBC-2 DPC

10 10

com sub 3

EL IL

P P P



  

(5)

where IL

DPC

is the insertion loss of the DPC, and

PBC-2

is the excess loss of the fiber-based PBC-2.

The optical power of the output beam can be

expressed by:

out com





(6)

where IL

is the insertion loss of the polarizer.

In the experiment, the optical powers of the three

input beams are equal to be 0.55 mW, 0.55 mW, and

0.2 mW, respectively. The efficiency losses of the

fiber devices employed in the experiment are

measured as follows: IL

PC-1

=0.45 dB, IL

PC-2

=0.45 dB,

PBC-1

=0.93 dB, EL

PBC-2

=0.81 dB, IL

DPC

=0.55 dB,

=0.63 dB. By using Eq. (3), Eq. (4), Eq. (5), and

Eq. (6), it can be calculated that after combination,

the theoretical optical power of the output beam is

0.63 mW. In fact, the combined optical power can

be further enhanced by fiber fusion technique to

reduce the losses caused by the connectors.

To evaluate the CPBC performance, we use the

combining efficiency η′ defined in Eq. (7) as a figure

of merit:

det

out

100%







(7)

where P

det

is the detected optical power of the output

beam in the experiment.

The experimental results are shown in Figure 8.

The iteration rate of the SPGD algorithm is about 6

kHz. The durations of the open and closed states are

both 10 s. The average combining efficiency

increases from 18.23% in the open loop to 95.81%

in the closed loop, and the MSE decreases from

0.1475 in the open loop to 0.0044 in the closed loop.

The closed loop is achieved after about 20 iterations,

equivalent to 3.3 ms, of SPGD optimization.

0 5 10 15 20

0.2

0.4

0.6

0.8

0 20 40 60 80 100

0.2

0.4

0.6

0.8

Time (s)

Combining efficiency

Open loop

Closed loop

Combining efficiency

Iteration number

Average=18.23%

MSE=0.1475

Average=95.81%

MSE=0.0044

Closed loop

achieved

Figure 8: Experimental results of the fiber-based CPBC of

three laser beams.

Research Progress of Fiber-based Coherent Polarization Beam Combining for Free-Space Optical Communications in IOE, CAS

Supposing that the power imbalance of the input

beams in the second-stage combination cannot be

compensated, a small portion of the optical powers

will be lost, and the optical power of the output

beam can be expressed by:

 

PBC-2 DPC

sub

out sub 3

sub

10 10

EL IL

P P P





















   











(8)

After calculation, it can be noted that P'

out

is equal to

be 0.57 mW.

The average optical power of the output beam

detected in the experiment is 0.60 mW when loop

closed, and greater than the theoretical optical power

of the output beam under the assumption that the

power imbalance of the input beams in the second-

stage combination cannot be compensated, which

indicates that the fiber-based CPBC with cascaded

PL and PT controls can break through the limitation

in the fiber-based CPBC with PL control that the

power imbalance will restrict the combining

efficiency, and can combine three laser beams to one

linearly polarized beam efficiently.

On the other hand, in the fiber-based CPBC of

three beams with PT control, two DPCs as well as

eight control signals are required. It is obvious that

the closed loop will be achieved after more than 3.3

ms (the experimental results of fiber-based CPBC of

two input beams) of SPGD optimization in the

experiment of fiber-based CPBC of three laser

beams with PT control. It can be noted that,

compared with the fiber-based CPBC with PT

control, the fiber-based CPBC with cascaded PL and

PT controls needs less control signals, resulting in a

lower complexity and faster convergence rate.

4 CONCLUSION

In conclusion, we have reported on the research

progress of the fiber-based CPBC in IOE, CAS. Two

control strategies, PL control and PT control, are

proposed and experimentally validated. It can be

noted that the CPBC with PL control and CPBC

with PT control both can combine individual beams

efficiently. Nevertheless, both of the two fiber-based

CPBC schemes have some limitations that will

restrict the enhancement of the combining

performance. In the CPBC with PL control, the

power imbalance of the input beams will inevitably

degrade the combining efficiency of the CPBC and

cannot be compensated. In the CPBC with PT

control, a major drawback is the increased

complexity and decreased convergence rate due to

the required multiple control signals. To break

through the limitations in previous fiber-based

CPBC schemes, the CPBC with cascaded PL and PT

controls is proposed. It is the synthesis of the PL

control and PT control, and can combine the

advantages of the PL control and the PT control

together. We believe that the proposed fiber-based

CPBC in this paper has great potential in coherent

FSO communications employing the multi-aperture

receiver with phased array.

This work is supported by the National Natural

Science Foundation of China under grant No.

61675205 and the CAS “Light of West China”

program.

REFERENCES

Chan, V. W. S., 2006. Free-space optical communications.

J. Lightwave Technol. 24/12, 4750–4762.

Agrawal, G. P., 2002. Fiber-Optic Communication

Systems. A John Wiley & Sons, Inc. 3

edition.

Li, K., Ma, J., Tan, L., Yu, S., Zhai, C., 2016.

Performance analysis of fiber-based free-space optical

communications with coherent detection spatial

diversity. Appl. Opt. 55/17, 4649–4656.

Zhang, R., Wang, J., Zhao, G., Lv, J., 2013. Fiber-based

free-space optical coherent receiver with vibration

compensation mechanism. Opt. Express 21/15,

18434–18441.

Ma, J., Ma, L., Yang, Q., Ran, Q., 2015. Statistical model

of the efficiency for spatial light coupling into a

single-mode fiber in the presence of atmospheric

turbulence. Appl. Opt. 54/31, 9287–9293.

Zheng, D., Li, Y., Li, B., Li, W., Chen, E., Wu, J., 2017.

Free space to few-mode fiber coupling efficiency

improvement with adaptive optics under atmospheric

turbulence. 2017 Optical Fiber Communications

Conference and Exhibition (OFC).

Yang, Y., Geng, C., Li, F., Huang, G., Li, X., 2017. Multi-

aperture all-fiber active coherent beam combining for

free-space optical communication receivers. Opt.

Express 25/22, 27519–27532.

Li, F., Geng, C., Huang, G., Yang, Y., Li, X., Qui, Q.,

2017. Experimental demonstration of coherent

combining with tip/tilt control based on adaptive

space-to-fiber laser beam coupling. IEEE Photon. J.

9/2, 7102812.

Yang, Y., Geng, C., Li, F., Li, X., 2017. Combining

module based on coherent polarization beam

combining. Appl. Opt. 56/7, 2020-2028.

Yang, Y., Geng, C., Li, F., Huang, G., Li, X., 2017.

Coherent polarization beam combining approach

based on polarization controlling in fiber devices.

IEEE Photon. Technol. Lett. 29/12, 945-948.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

Geng, C., Luo, W., Tan, Y., Liu, H., Mu, J., Li, X., 2013.

Experimental demonstration of using divergence cost-

function in SPGD algorithm for coherent beam

combining with tip-tilt control. Opt. Express 21/21,

25045–25055.

Research Progress of Fiber-based Coherent Polarization Beam Combining for Free-Space Optical Communications in IOE, CAS