Study on the Application of Fiber Array in Laser Ranging Systems

All-time laser ranging to space debris is a critical direction in the development of future laser ranging systems. During Earth's shadow periods， however， debris objects receive no solar illumination and are invisible to optical cameras. In these conditions， telescope pointing corrections cannot rely on imaging and must instead depend on echo signal strength. Because debris moves rapidly and orbit prediction errors remain， even targets briefly captured within the field of view quickly drift out. This prevents stable tracking and reduces ranging success rates.To address this challenge， this study introduces a novel approach by incorporating a fiber array into the laser ranging system. The array consists of a central fiber surrounded by six uniformly arranged multimode fibers. Each fiber is divided into two outputs： one reference fiber of equal length and one guiding fiber of varying lengths. The fourteen outputs are fused into a single fiber， which is then connected to a Superconducting Nanowire Single-Photon Detector （SNSPD）. By measuring the relative delays among echo signals， the incident fiber can be identified， enabling calculation of the target's offset direction in the field of view and guiding the telescope for tracking.Experiments were conducted using the 1.2 m coaxial laser ranging system at Yunnan Observatory during May-June 2025. Cooperative targets with small orbit prediction errors were first selected as observation objects to verify the effectiveness of the fiber array method. After validation， the fiber array method was applied to space debris ranging experiments during Earth's shadow periods. The results demonstrate that the fiber array can effectively enhance the tracking capability of the ranging system for invisible targets. For cooperative targets equipped with retro-reflectors， the system efficiently extracted echo signals and accurately identified the incident fiber， thereby deriving the offset direction and achieving stable and reliable ranging. For non-cooperative debris， the current system shows certain limitations. As shown in Fig. 6（d）， debris with NORAD ID 28480 produced strong echoes that allowed identification of the incident fiber as F5. In contrast， debris with NORAD IDs 22220 and 21393， shown in Figs. 6（e）-（f）， yielded more dispersed echoes， making it difficult to accurately invert the incident fiber from time-delay information. This phenomenon mainly arises from temporal overlap of signals received by adjacent fibers. Specifically， debris generally exhibits a size effect， where differences in reflection depth across the surface broaden echo signals in the time domain. At the same time， the low surface reflectivity of most debris weakens the echoes and increases their susceptibility to background noise. Together， these factors cause temporal overlap among the multichannel data， which appears in the residual plots as clearly dispersed echo patterns and prevents reliable fiber identification based on inherent time delays.To improve performance for non-cooperative targets， fiber parameters require targeted optimization. First， the delay difference between reference and guiding fibers should be increased so that the minimum channel separation exceeds three times the ranging precision， thereby reducing signal overlap. Second， the spatial arrangement of the array should be optimized to maximize the average channel delay difference， further suppressing inter-channel interference. Additionally， the current multimode fibers use a core diameter of 62.5 µm and cladding diameter of 125 µm， with the core occupying only 25% of the cross-sectional area. Under weak signals， this may limit detection efficiency. Future designs could reduce cladding thickness to increase area utilization while maintaining low crosstalk， thereby improving overall system performance. In summary， the proposed fiber array method provides a new solution for high-precision tracking of invisible targets and constitutes an important technical step toward all-time space debris laser ranging. With further optimization of fiber parameters， the method offers strong engineering potential and prospects for practical application.