Research Progress of Multi-aperture Laser Transceiving Control for

Beam Combining Applications in IOE, CAS

Feng Li

1,2

, Xinyang Li

1,2,*

, Chao Geng

1,2

, Guan Huang

1,2,3

and Yan Yang

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: Multi-aperture Laser Transceiving, Beam Combining, Adaptive Fiber Coupling.

Abstract: Nowadays, the development of fiber laser beam combining faces new challenges during propagating

through the real long-range atmosphere. Aberrations in such transmission systems include turbulence-

induced dynamic aberrations located at the path from the fiber laser array to the target, besides the inherent

phase errors like phase noises and tip/tilt errors. Existing techniques, e.g., target-in-the-loop and delayed

stochastic parallel gradient descent, are difficult to deal with the fast-changing turbulence-induced tip/tilt

aberrations. But correcting these aberrations is critical for obtaining combined laser beams on the target

with the best beam quality. In this paper, research progress of multi-aperture laser transceiving control for

beam combining applications in IOE, CAS is presented. These novel techniques presented here provide

efficient ways to achieve tip/tilt control for the beam coupling from space to fiber and the outgoing laser

beams in the beam combining applications.

1 INTRODUCTION

Fiber laser techniques have been under intensive

investigations for their excellent beam quality, high

efficiency and flexible structure. While, single fiber

laser beam with high power and good beam quality

simultaneously is hard to achieve, owing to physical

limits like nonlinear effect, thermally-induced modal

instability, and thermal damage (Yu et al., 2011).

Coherent beam combining (CBC) gives excellent

solutions to these hard issues and shows great

prospects in laser systems like directed energy, free

space laser communications, and laser radar. Tiled

beam array and filled-aperture are the two main

architectures of fiber laser CBC. Compared with the

later one, tiled beam array method not merely

provides a flexible method producing high power

output with excellent beam quality, but also could

act as a scalable fiber laser transceiver. Recently

developed coherent tiled fiber laser arrays composed

of numerous fiber collimators have aroused a

significant interest (Weyrauch et al., 2011). In such

systems, traditional and monolithic large-aperture

transmitters are replaced by smaller sub-aperture

array for improved adaptive optics (AO)

performance. The phase distortions in these systems

are divided into piston and/or tip/tilt errors within

the sub-apertures (Filimonov et al., 2014). Benefited

from the mature fiber-integrated phase devices,

issues about piston errors have been widely

researched and well solved. Meanwhile, novel

device named adaptive fiber-optics collimator

(AFOC) has been invented to manipulate the tip/tilt

phases in fiber laser CBC systems (Liu et al., 2007

and Geng et al., 2013). Compared to conventional

tip/tilt devices like FSM, the AFOC drives fiber tip

directly to execute tip/tilt. Such devices are with

advantages of precise control, small inertia, high

resonance frequency and convenience for

assembling and integration (Lachinova et al., 2008).

Further development of the tiled beam array (or

fiber laser array) aims at highly efficient propagation

through the atmosphere, not merely realization of

CBC indoor. Thus, correcting aberrations, especially

the turbulence-induced aberrations located at the

transmission path, is critical for obtaining the best

beam quality and the highest power intensity on the

target. Alleviating the sub-aperture averaged piston-

type aberrations of the laser source and turbulence,

which is named as phase locking, is the basic item

for CBC under atmosphere. Beyond the piston-type

aberrations, compensation of the higher order

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

Research Progress of Multi-aperture Laser Transceiving Control for Beam Combining Applications in IOE, CAS.

DOI: 10.5220/0006546200430049

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 43-49

ISBN: 978-989-758-286-8

aberrations within the sub-apertures is needed if we

want to further improve the performance of CBC

under the atmosphere. In fact, tip/tilt-type errors take

up the most part (almost 87%) of the aberrations for

Kolmogorov turbulence in each sub-aperture.

Without the tip/tilt-type errors, CBC under

atmosphere would be more efficient where higher

target hit-spot brightness could be obtained with

even less transmitted power (Vorontsov et al., 2016).

But such goal is blocked by the available bandwidth

of the tip/tilt devices and the performance of existing

control algorithm. The appearance of AFOC has

solved the bandwidth issues to a certain extent. In

this case, the performance of the control algorithm

becomes particularly important.

Different from the traditional AO control, where

extra wavefront sensors (like Hartmann-Shack

sensor) are needed to measure the monolithic

wavefront actively, fiber laser arrays mostly depend

on optimization algorithm for CBC owing to their

discrete architecture. Phasing through target-in-the-

loop (TIL) with hill-climbing type techniques (like

stochastic parallel gradient descent, SPGD) is the

main existing approach for current CBC under

turbulence (Weyrauch et al., 2016). In these schemes,

the power-in-the-bucket (PIB) metric, which

represents the laser power backscattered by the size-

limited target point and collected by a local

telescope, is utilized for power-maximization. The

tip/tilt control and the phase locking control are

mixed together in such single metric optimization

techniques. The hardest problem is that the available

bandwidth of AFOC (no more than 5 kHz) is much

less than that of the fiber-integrated phasing devices

(up to GHz), while the channel number of the former

is double of the latter one (Geng et al., 2011).

Obviously, existing methods are not fit for fiber

laser array CBC systems with fast-changing tip/tilt

errors.

New techniques making the best use of the

limited AFOC bandwidth are in urgent need. Recent

progress on techniques of adaptive fiber coupling

reported by IOE, CAS gives out new ideas on such

issues (Luo et al., 2014). In these methods, the fiber

coupling efficiency of the modified AFOC could be

improved with active tip/tilt-type phase errors

control. Tip/tilt control for both the coupling from

space to fiber and the outgoing laser beam is

obtained. Such techniques achieve parallel tip/tilt

control for each sub-aperture of the array, which is

independent from the phase control in the CBC

systems. In this paper, the research progress of

multi-aperture laser transceiving control for beam

combining applications in IOE, CAS is presented.

2 ADAPTIVE FIBER COUPLING

Here, we present the recent work of extending the

control bandwidth of the AFOC for adaptive fiber

coupling. An improved SPGD algorithm named

precise-delayed SPGD (PD-SPGD) is proposed for

the AFOC-based adaptive coupling system.

Figure 1 shows the structural scheme of the

AFOC coupling system. A uniform plane wave laser

beam is focused and coupled into the single-mode-

fiber (SMF) tip in the AFOC, part of the coupled

optical energy is sent to a terminal receiver and the

other part is detected by a photodetector (PD). The

PD voltage is utilized as the performance metric of

the control algorithm. Two-channel control signals

from the controller are amplified by high-voltage

amplifiers (HVAs) and then drive the AFOC for

closed loop control.

AFOC

Fiber splitter

Coupling

lens

Fiber tip

positioner

Receiver

Laser with

plan wave

Optical path

Electrical path

Controller

Control

voltages

Performance

metrics

HVA

Figure 1: Structural scheme of adaptive coupling system

based on AFOC.

Response delay between the input and the output

is one of the key issues in almost all practical control

systems. Parameter τ

resp

is defined here as the delay

between the change of the control voltage and the

response of the metric and τ

SPGD

as the period of a

SPGD iterative cycle. Through massive experiments,

we find that τ

resp

keeps constant when the frequency

is below 10 kHz. The average value is about 0.1 ms

for both the X-direction and Y-direction of the

AFOCs. Such inherent response delay will lead to

inaccurate gradient estimation through the fast

iterative process of SPGD algorithm where τ

SPGD

becomes less than τ

resp

. For example, when τ

SPGD

less than 0.2 ms, metric J

(n)

and J

(n)

of the SPGD

algorithm is almost totally determined by the drive

voltages of U

(n)

+ΔU

(n)

and U

(n-1)

-ΔU

(n-1)

respectively

(n represents the time point). This will lead to

opposite gradient estimation results if traditional

SPGD algorithm is utilized as:

 

( 1) ( ) ( ) ( ) ( )n n n n n

U U U J J







   

(1)

where γ is the gain coefficient.

So, readjustment of the correspondence between

the metric and the disturbed voltage is needed, and

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

one way that might work is to set a controllable time

delay. This idea originates from the delayed-SPGD

(D-SPGD) algorithm, which has been successfully

used to compensate the time delay caused by the

optical wave propagation in CBC experiments

(Weyrauch et al., 2016). Such improved algorithm is

named as Precise-Delayed SPGD algorithm (PD-

SPGD) here. The iterative procedure of the control

voltage during each iteration cycle (n) can be

described by the following rule:

 

( 1) ( ) ( ) ( ) ( )

n n n n n n n n

i i N

U U U J J



   



   

(2)

where

0,1,...,i MN

. Here, the integer number Δn

= M-1 is the integral-delay parameter and i ≥ 0 is the

precise-delay parameter that accounts for the

response delay τ

resp

. N is the number of sampling

points of metrics within one time of iteration, which

determines the accuracy of the compensation that

can be achieved.

Normalized performance metrics

/ J

Iteration number / n

Δn=0, i=5

Δn=1,

i=15

Δn=1, i=12

Δn=1, i=8

Open

loop

Closed loop

Δn=1, i=10

Figure 2: Iteration curve of different delay parameter

settings in the experiment of static angular error

correction.

Figure 2 shows the experimental results of fiber

coupling with 367 μrad static angular error (almost

misalignment of 5.5 μm at the fiber end). The

iteration rate of the PD-SPGD algorithm is 8 kHz

and the sampling rate is 80 kHz, thus τ

SPGD

=0.125

ms, N=10. To compensate the response delay (96

μs), τ

comp

is set to 0.127 ms, the integral delay

parameter Δn=1 and the precise delay parameter

i=10 (the closest integer number to 0.127/0.125 and

10×0.127/0.125 correspondingly). As shown in

Figure 2, the iteration curve diverges when Δn=0

and i=5. When parameters Δn=1 and i=8, 10, 12 (the

corresponding actual compensation time is 0.1 ms,

0.125 ms and 0.15 ms), the algorithm starts to

converge and corresponding convergence time t

(equal to the time needed for the normalized metric

rise to 90%) is 55 ms, 2.5 ms and 137.5 ms through

statistics of 20 sets of data. Finally, when Δn=1 and

i=15 (equivalent to using the D-SPGD algorithm),

the iteration cure diverges again.

Sine angular jitter with frequency of 100 Hz and

amplitude of 166 μrad is loaded on the plane wave

and corresponding experimental results of SMF

adaptive coupling using SPGD and PD-SPGD are

shown in Figure 3. Iterative rates of these two

algorithms are 3 kHz and 8 kHz respectively.

Control parameters of both algorithms are carefully

tested and optimized. The metric is with an average

value of 0.81 and mean square error (MSE) of 0.13

under open loop. The average metric decreases to

0.79 and the MSE increases to 0.14 when SPGD

algorithm is utilized. The average metric increases to

0.97 and the MSE decreases to 0.019 when SPGD

algorithm executes. SPGD becomes invalid under

such circumstances, while PD-SPGD still works

fine.

Above results reveal that the PD-SPGD

algorithm proposed here has less convergence time

and could get better performance than conventional

SPGD algorithm. Meanwhile, this improved

algorithm competently deals with dynamic tip/tilt

errors with frequency approximating one hundred

Hz, which shows potential abilities when facing

turbulence-induced dynamic aberrations.

Normalized performance metrics

/ J

Iteration number / n

Mean=0.81

MSE=0.13

Open loop

SPGD

PD-SPGD

Mean=0.79

MSE=0.14

Mean=0.97

MSE=0.019

Figure 3: Comparison between the SPGD and PD-SPGD

algorithm when the frequency of sine angular jitter is 100

Hz.

3 CO-APERTURE

TRANSCEIVING OF TWO

COMBINED BEAMS

The amount of the coupled power that enters to the

SMF in the AFOC is inversely proportional to the

displacement of the fiber-tip center in respect to the

focal-spot centroid position of the incident beam.

The pitch between the delivery fiber tip and the focal

spot center is caused by the wavefront tip/tilt

Research Progress of Multi-aperture Laser Transceiving Control for Beam Combining Applications in IOE, CAS

aberrations. According to the principle of optical

reciprocity, when the received optical wave power is

maximized through the AFOC control, the tip/tilt

errors are equivalently corrected for the counter-

propagating outgoing and received laser beams in

the delivery fiber. These two laser beams could

share the same optical fiber path with help of

directional fiber circulators or free-space fiber

isolators. Meanwhile, for CBC with multi-AFOCs,

such tip/tilts control could work in parallel for each

single aperture. That means the tip/tilts control keeps

two dimensional regardless of the scale of the CBC

array.

Here, the current work about the co-aperture

fiber laser transceiving propagation using AFOCs

for both coupling and collimating in IOE, CAS are

presented (Li et al., 2015). Fiber laser array

composed of two-element AFOCs is built up and

indoor simulated turbulence is set to introduce

aberrations. SPGD algorithm is employed here to

promote the fiber coupling efficiency and the

combining efficiency of the fiber-array outgoing

beams.

Micro-

ojective

Far-field fiber laser

source @ 1064nm

Monitor

Local fiber laser

source@1064nm

PDPD

Fiber

splitter

AFOCs

Beam

splitter

Optical

circulators

Fiber

Electrical

CMOS

10x

HVA

Data acquisition &

control platform

AFOCs’

arrangement

Heater

Simulated

turbulence

Phase

modulators

32mm

Φ28mm

Figure 4: Experimental setup. PD: photo-detector. HVA:

high voltage amplifier. TL: transform lens.

Experimental setup of the co-aperture laser beam

transceiving propagation is illustrated in Figure 4. A

transform lens (TL) is employed to simulate the

optical far-field. A fiber laser with the fiber tip

located at the far-field is set to be as the unresolved

target and the target source. The target outgoing

beam is collimated by the TL, and heated to generate

the atmospheric turbulence aberrations. Then, the

phase-distorted collimating beam is focused and

coupled into the internal polarization-maintaining

fibers (PMFs) of the AFOC array separately. The

coupled beams are detected by PDs. At the same

time, the local collimated fiber laser beams from the

two AFOCs are also heated to generate conjugated

aberrations. The two phase-distorted sub-beams are

focused by the TL, and then detected by a high

speed CMOS camera for observation.

The iteration rate of the SPGD algorithm is about

1.15 kHz. Figure 5 (a) and Figure 5 (b) show the

evolution curves of coupling power of the AFOCs,

denoted as P

and P

, when tip/tilt control is off and

on under the influences of simulated turbulence.

Here, P

and P

are normalized by the maximum

value in respective closed loop. Duration of both the

open and closed states is all about 12-second. The

average value of P

for one AFOC increases from

0.61 in open loop to 0.85 in closed loop, and the

MSE drops from 0.16 to 0.048. For P

, the average

value increases from 0.57 to 0.85 and the MSE drops

from 0.14 to 0.04 correspondingly. The results

indicate that the efficiency of coupling laser beam

with distorted wave-front from space into PMF can

be effectively promoted through tip/tilt error control

with AFOCs.

Normalized P

0 5 10 15 20

0.2

0.4

0.6

0.8

Time (s)

(a)

Average=0.61

Average=0.85

Open loop

Closed loop

0 5 10 15 20

0.2

0.4

0.6

0.8

Normalized P

Time (s)

(b)

Average=0.57

Average=0.85

Open loop

Closed loop

Figure 5: Evolution curves of the coupled laser power

normalized by the maximum value in separately closed

loop. (a) Received by AFOC-1. (b) Received by AFOC-2.

0 5 10 15 20

Time (s)

Absolute value of centroid's deviation (



rad)

Open loop Closed loop

(a)

Average=17.5 μrad

Average=2.35 μrad

Time (s)

Absolute value of centroid’s deviation (μrad)

0 5 10 15 20

-1

-16

-14

-12

-10

Frequency (Hz)

Power spectrum of centroid's deviation (rad

/Hz)

Open loop

Closed loop

(b)

Open loop

Closed loop

30 Hz

Frequency (Hz)

Power spectrum of centroid’s deviation (rad

/Hz)

-10

-12

-14

-16

-1

- 2/3

- 10/3

Figure 6: Combination of the two AFOC-collimating

beams in the far-field. (a) The absolute-value evolution

curve of the diffraction pattern centroid’s deviations. (b)

Frequency spectrum-density distribution of the curve in

(a).

Figure 6 (a) depicts the absolute-value evolution

curve of the diffraction pattern centroid’s deviations

of the two AFOC-collimating beams in the far-field.

The average absolute-value of the centroid’s

deviation drops from 17.5 μrad in open loop to 2.35

μrad in closed loop, the MSE value drops from 6.59

μrad to 1.35 μrad, and the peak-valley value drops

from 36.8 μrad to 8.54 μrad (the diffraction-limited

angle of a single AFOC aperture is about 92 μrad).

Figure 6 (b) describes the frequency spectrum-

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

density distributions of the open and closed loop

curves in Figure 6 (a), respectively. Deviations of

less than 30 Hz have been well corrected. The

results show that the tip/tilt errors of the outgoing

laser beams are equivalently corrected when the

received optical wave power is maximized.

4 CBC BASED ON 7-APERTURE

LASER TRANSCEIVING

Experiment in part 3 is focused on the tip/tilt

aberrations correction and the piston aberrations are

not considered in. However, phase-locking is the

basic condition for CBC. To obtain CBC of the

AFOC array, tip/tilt control with adaptive fiber

coupling is cooperated with the TIL-based phasing

techniques. Here we present the researches about

CBC based on multi-aperture laser transceiving in

IOE, CAS (Li et al., 2017).

PD with a

pinhole

PBS

AFOC×7

Transform lens

1×8

Fiber Splitter

Fiber laser source

11mW@1064nm

Fiber

circulator×7

PD×7

Hot water

Fiber laser source

2W@1064nm

Phase-locking

controller

Laser beam

coupling controller

High voltage

amplifier

Fiber tip

Beam

splitter

Mirror

10x

Micfro-

objective

CMOS

camera

Monitor

AFOC array

PZT-PC×7

Figure 7: Experimental setup of CBC based on 7-aperture

laser transceiving. PZT-PC: piezoelectric-ceramic-ring

fiber-optic phase compensator. AFOC: adaptive fiber-

optics collimator. PD: photo detector. PBS: polarized

beam splitter.

Fiber laser CBC experiment setup with AFOC

array of seven sub-apertures is illustrated in Figure 7.

Polarized and single-frequency fiber laser beams

from the 7-subaperture AFOC array are heated by

hot water to bring in simulated dynamic turbulence

aberrations and focused by a transform lens. Then

the focused beamlets are detected by a PD with a

pinhole for phase-locking control (PL) and a high-

speed CMOS camera for observation simultaneously.

Piezoelectric-ceramic-ring fiber-optic phase

compensators (PZT PC) are utilized here to correct

the piston-type errors of the array. A pigtail fiber

with its end located in the focal spot of the transform

lens is used as a simulated objective in the far-filed.

The monolithic-beam from the fiber tip is collimated

by the transform lens and cut off by the array, and

then focused and coupled into the PMFs of each

AFOC respectively. Metrics detected by each PD are

utilized for tip/tilt control (TT) for each AFOC in

parallel.

Average=0.94

Average=0.76

Average=0.18

Average=0.055

Normalized coupled

power

Mean square error of

the coupled power

Closed loop

Open loop

Closed loop

Open loop

0.9

0.8

0.7

AFOC number

1 2 3 4 5 6 7

AFOC number

0.3

0.2

0.1

(a) (b)

Figure 8: Average (a) and mean square error (b) of the

normalized coupled power for each AFOC when TT

control is in open loop and closed loop.

The average normalized coupled power for all

the seven AFOCs is about 0.76 without TT control

and then increases to 0.94 when TT control is

brought in as shown in Figure 8 (a). The average

MSE of the coupled power for all the seven AFOCs

is about 0.18 in open loop and then turns to 0.055 in

closed loop as illustrated in Figure 8 (b).

Average=0.21

MSE=0.096

Average=0.72

MSE=0.042

Average=0.96

MSE=0.016

Average=0.40

MSE=0.023

No control

PL and TT

0.95

0.9

0.85

0.8

0.75

10 10.02 10.04

9.98

4.995

5.005

5.01

0.6

0.4

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Time (s)

0 5 10 15 20

Normalized PIB metric

Figure 9: The normalized PIB metrics acquired by PD as

the function of time.

Figure 9 shows the normalized PIB metric

acquired from the PD as the function of time during

four stages, which are no control stage, PL stage, PL

and TT stage, and single TT stage. The PIB metric is

normalized via being divided by the maximum PIB

during each stage. Without any control, the mean

PIB metric is just 0.21 and the corresponding MSE is

0.096 due to the simulated turbulence and the phase

noise of the transmission fibers. When only PL

control turns on, the PIB metric is locked at a higher

value with an average of 0.72, which is 3.4 times of

that for no control stage. Meanwhile, the MSE

decreases to 0.042. Then the TT control is brought in

and the PIB metric is further optimized. The metric

is with an average of 0.96 and MSE of 0.016, which

is only 1/6 of that in the no control stage. The

convergence of the adaption process after the TT

control is switched on takes time about 30 ms, which

equals to the time that each AFOC needs to

maximize the coupled power. Such results indicate

Research Progress of Multi-aperture Laser Transceiving Control for Beam Combining Applications in IOE, CAS

that we realizes the parallel tip/tilt control for each

AFOC and such control is independent from the

phase locking, which is quite different from the

existing techniques.

(a) (b)

Cumulative gray

-scale value

Cumulative gray

-scale value

Cumulative gray

-scale value

Cumulative gray

-scale value

No control PL

PL and TT TT

Figure 10: The long-exposure far-field intensity

distributions. (a) No control. (b) PL control. (c) PL and TT

control. (d) TT control.

The long-exposure far-field intensity

distributions during the four control stages are

shown in Figures 10(a-d), respectively. The long-

exposure pattern is diffused by the simulated

turbulence and the fringe visibility is very weak

when no control is implemented. Fringe contrast

increases when PL works. Due to the simulated

turbulence and the initial static pointing errors of the

AFOCs during assembling, intensity of the side

lobes located bellow the main lobe approximates

that of the main lobe when only PL control works.

Further pre-compensation of the tip/tilt-type errors

through the TT control could resolve such issues and

results in pattern (Figure 10 (c)) comparable with the

ideal pattern. Sole TT control just benefits the

intensity of the central point, but not the fringe

contrast (Figure 10 (d)). All the results above

indicate that excellent fiber laser array CBC under

simulated turbulence has been achieved through the

cooperation of the tip/tilts correction based on multi-

aperture laser transceiving control and the active

phase locking control.

5 CONCLUSIONS

Future CBC of the tiled fiber laser array aims at

efficient transmission under atmosphere, which will

inevitably face challenges of alleviating the

turbulence-induced dynamic aberrations distributed

in the whole array aperture. Existing research results

mainly based on TIL and optimization algorithms

suffer from the defects of poor bandwidth utilization

efficiency, especially for the bandwidth-limited

tip/tilt devices represented by AFOCs. To solve

these issues, techniques of multi-aperture laser

transceiving control have been developed by IOE,

CAS. Such methods give abilities of efficient and

parallel adaptive correction to the tiled fiber laser

array. Research progresses on improved SPGD

algorithm of fiber coupling, co-aperture transceiving

of fiber laser array with adaptive tip/tilts corrections,

and CBC of tiled fiber laser array are presented.

Excellent results indicate the potential applications

of such techniques in fiber coupling, laser

communication and beam projection applications.

This work is supported by the National Natural

Science Foundation of China under grant No.

61675205, and the CAS “Light of West China”

program.

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