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摘要
近年来，以激光大气传输为应用背景的光束相干合成技术被广泛研究，而关于该项技术在空间光通信中的应用研究却不多。事实上，基于光束相干合成的多孔径接收天线结构可有效缓解大气湍流影响，提高空间激光通信系统的性能。本论文简要介绍了中国科学院自适应光学重点实验室面向激光大气传输应用的激光组束传输与湍流校正技术研究近况；重点介绍关于多孔径接收空间光通信系统中的光纤相干合成研究进展，主要包括基于3 dB光纤耦合器的相干合成和基于光纤偏振合束器的相干偏振合成两种方法，在空间光通信系统中具有极大的潜在应用。
Abstract
In recent years, the coherent beam combining technique for laser transmission in atmosphere has been widely investigated, while the study of this technique's application in space optical communications is few. In fact, the structure of multi-aperture receiving antenna based on coherent beam combining could be employed to correct the atmospheric turbulence effect and to enhance the performance of the space optical communication system. In the paper, the recent development of coherent beam transmission and control technique for laser transmission application in atmosphere is reviewed. Then, the research progress of fiber-based coherent beam combining in multi-aperture receiving space optical communication system is reported in detail, including the coherent combining based on 3-dB fiber coupler and the coherent polarization combining based on fiber polarization beam combiner, which might have great potential in space optical communication system.
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Overview
Overview: To build an optical phased array (OPA) based on multi-channel fiber lasers, each channel equipped with a fiber optical collimator, is crucial and effective for the long-range laser transmission application, where the atmospheric turbulence will weaken the beam quality and power intensity on the target. Aberrations in such transmission systems include turbulence-induced dynamic aberrations located at the path from the fiber laser based OPA to the target, besides the inherent phase errors like phase noises and tip/tilt errors. With the help of the OPA structure, the monolithic optical wavefront is replaced by multi-subwavefronts emitting from the subapertures, where each subaperture is provided with the abilities of correcting the piston and tip/tilt type phase aberrations. So, the OPA structure can deal with atmospheric turbulence aberrations and inherent system aberrations to achieve the coherent beam combining, and even the conformal emission. Effective control bandwidth for eliminating such aberrations is limited by the optical transmission time delay and the increment of the array scale in the OPA system. Existing techniques, e.g., target-in-the-loop (TIL) and delayed stochastic parallel gradient descent (SPGD), are difficult to deal with the fast-changing turbulence-induced tip/tilt aberrations. In recent years, the fiber laser coherent beam combining technique was deeply studied in the Key Laboratory on Adaptive Optics, Chinese Academy of Sciences. The new-style fiber-based aberration correctors, e.g., adaptive fiber-optics collimator (AFOC) and piezoelectric fiber phase corrector (PZT-PC) were developed for the tip/tilt and piston phase error compensation. The indoor coherent beam combination of a three-element fiber array based on TIL technique was achieved. With the help of 500 W AFOC, the 2 kW incoherent beam combining of four fiber lasers was demonstrated. The coherent beam combining with tip-tilt control of seven-channel AFOC array was achieved by using the divergence cost-function in SPGD algorithm. In the front of this paper, research progress of multi-aperture laser transceiving control for coherent combining applications was presented, where the outdoor demonstration of TIL technique with seven-channel AFOC array was realized under 200 m horizontal atmosphere distance. The coherent beam combining technique for laser transmission in atmosphere has been widely investigated, while the study of this technique’s application in space optical communications is few. In fact, the structure of multi-aperture receiving antenna based on coherent beam combining could be employed to correct the atmospheric turbulence effect and to enhance the performance of the space optical communication system. In most part of this paper, the research progress of fiber-based coherent beam combining in multi-aperture receiving space optical communication system is reported in detail, including the coherent combining based on 3 dB fiber coupler and the coherent polarization combining based on fiber polarization beam combiner, which might have great potential in space optical communication system.
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图 1 多孔径接收方案原理图。(a)基于数字信号合成；(b)基于光束相干合成
Figure 1. Multi-aperture receiver. (a) Digital combining architecture; (b) Optical combining architecture. LO: local oscillator
图 2 7单元光纤激光阵列200 m传输及相干合成实验框图
Figure 2. Schematic of the experimental setup used for coherent beam combining of 7-channel fiber laser array over 200 m atmospheric path
图 3 7单元阵列激光传输控制实验中激光发射端和回光接收端的实物照片
Figure 3. The real photo of laser beam transmitter and receiver in the 7-channel laser-array transmission and control experiment
图 5 光斑分析仪采集到的开闭环条件下30 s长曝光图
Figure 5. Long-exposure intensity distribution of 30 s acquired by the light spot analyzer during open and closed loop
图 6 基于3 dB光纤耦合器的两路激光相干合成实验方案图
Figure 6. Experimental setup of CBC based on 3 dB coupler with two laser beams
图 7 基于3 dB光纤耦合器的两单元相干合成实验结果图
Figure 7. The experimental results of CBC based on 3 dB coupler with two laser beams
图 8 相干合成效率与输入光强比之间的关系曲线
Figure 8. Curves of combining efficiency as the function of the power ratio of the input beams
图 9 基于3 dB光纤耦合器的四路相干合成实验方案图。(a)一体式级联结构；(b)分布式级联结构
Figure 9. Experimental setup of the CBC of four laser beams with (a) integrated architecture and (b) distributed architecture
图 10 基于3 dB光纤耦合器的四路相干合成实验结果图。(a)一体式级联结构；(b)分布式级联结构
Figure 10. Experimental results of the CBC of four laser beams with (a) integrated architecture and (b) distributed architecture
图 12 基于光纤偏振合束器的相干偏振合成方法基本原理图。(a)不加控制；(b)相位控制；(c)偏振控制
Figure 12. Basic principle of the fiber-based CPBC. (a) Without control; (b) PL controlled; (c) PT controlled
图 13 两单元相干偏振合成实验方案图
Figure 13. Experimental setup of the fiber-based CPBC with two input beams
图 14 两单元相干偏振合成实验结果图。(a)基于相位控制；(b)基于偏振控制
Figure 14. Experimental results of the fiber-based CPBC. (a) PL control; (b) PT control
图 15 相干偏振合成效率与输入光强比之间的关系曲线
Figure 15. Curves of CPCE as the function of the power ratio of the input beams
图 16 基于相位-偏振混合控制的相干偏振合成方法原理示意图
Figure 16. Basic principle of the fiber-based CPBC with cas-caded PL and PT controls
图 17 基于相位-偏振混合控制的三单元相干偏振合成实验方案图
Figure 17. Basic experimental setup of the fiber-based CPBC of three laser beams with cascaded PL and PT controls
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