Research On Multi-aperture Receiver With Fiber-based Coherent Beam Combining

The fiber-based free-space optical(FSO)communications has attracted more and more attentions as it has simpler structure,stronger expansibility,and can take advantage of the established components of fiber-optic communication systems.In such a system,one of the key technologies is to improve the optical power coupled into the fiber to enhance the received signal-to-noise.However,wavefront aberration of the propagating beam induced by turbulent atmosphere greatly degrades the fiber coupling efficiency.Multi-aperture receiver with optical combining architecture is an effective approach to overcome the turbulent atmosphere effect on the performance of the FSO communications and has great potential in the future.Nowadays,researches on the multi-aperture receiver with optical combining architecture are mainly focusing on the theory analysis and the feasibility verification.However,the detailed researches about the coherent combining approaches in the multi-aperture receivers have not been reported up to now.This paper is focusing on the demand of the optical combining in the multi-aperture receiver,and the key issue is to propose coherent beam combining approaches to combine the multiple indivial beams received by the sub-apertures into one polarization maintaining fiber efficiently.The main content of this paper is as follows:The basic model of the multi-aperture receiver with optical combining architecture is built,and the performance of the receiving system is analyzed.The simulation model of the multi-aperture receiver is built based on the theoretical analysis to analyze the relationship between the coupling efficiency of the system and the number of the apertures under the influence of the atmospheric turbulence.The all-fiber coherent beam combining(CBC)approach based on 3-dB fiber is is introduced and experimentally validated.Two different architectures,the integrated architecture and the distributed architecture,are proposed to combine more than two laser beams.The combining performances of the two architectures are analyzed in comparison,the experiments of CBC with four laser beams are carried out,and simulations on CBC of four laser beams under turbulent atmosphere are presented to analyze the influence of the fluctuation of the input beams on the combining efficiency.The CBC based on fiber coupler with specific coupling ratio is proposed to combine imbalanced input beams where the number of the input beams is not binary.Corresponding theoretical analysis and experimental verification are carried out in the case of CBC with three laser beams.The fiber-based coherent polarization beam combining(CPBC)is proposed and analyzed for the demand of the optical combining in the multi-aperture receiver.To achieve high coherent polarization combining efficiency and output linearly polarized combined beam,two combining strategies,the CPBC based on PL(phase-locking)control and the CPBC based on PT(polarization-transforming)control are proposed.Moreover,corresponding experimental verification and expansibility analysis and are carried out.Then,taking advantages of the two CPBC approaches,the CPBC with cascaded PL and PT controls is proposed to combine imbalanced input beams where the number of the input beams is not binary,and the experimental setup with three laser beams is carried out to validate the feasibility of the proposed CPBC approach.The CPBC based on fiber-based polarization beam combiner with specific rotating angle is proposed to combine imbalanced input beams where the number of the input beams is not binary.Corresponding theoretical analysis and experimental verification are carried out in the case of CPBC with three laser beams.In this paper,the theoretical model,numerical analysis,and experiments are studied around the multi-aperture receiver with fiber-based coherent beam combining architecture.Multiple innovative combining approaches are proposed to solve the key issue that the multiple received laser beams should be combined into one polarization maintaining fiber efficienctly.The results of this paper have great potential in subsequent researches.