Coherent combining represents a groundbreaking technology in laser synthesis, which aims to break the limitations of traditional single-link laser systems and significantly improve laser brightness. The technology can be divided into active coherent combining and passive coherent combining according to whether the synthetic optical path contains active phase control elements for intervening detection and correcting phase errors. Active coherent combining systems have complex structures and the phase-locked bandwidths are limited by software algorithms and hardware circuits, so they are primarily utilized in high-energy laser applications. Conversely, passive coherent combining boasts a simple structure, wide phase-locked bandwidth, robust stability, and facile modular integration, thereby exhibiting broader development potential across diverse fields such as ranging, sensing, atmospheric wind data detection, and autonomous driving. Among them, optical feedback ring cavity combining based on fiber structure is one of the more popular passive combining schemes. However, one challenge faced by the ring cavity passive combining technique is that its output beam may contain multiple longitudinal modes or even multi-spectral lasers, which limits its application in remote coherent detection where ultra-long coherence length laser sources are required. To mitigate this issue, this study introduces a novel single-frequency fiber laser passive coherent combining technique, leveraging auxiliary cavity phase-locking. This technique successfully achieves excellent performance of the combined output laser in terms of narrow linewidth and long coherence length through a unique design. This proposed technique will offer robust support for advancing coherent laser lidar, coherent optical communication, and other allied disciplines.

This study investigates the single-frequency passive combining of single-mode erbium-doped fiber amplifiers. Initially, a 1550 nm single-frequency laser beam is split to serve as input for two single-mode optical amplifiers, facilitating rapid oscillation of the combined laser energy around 1550 nm. Subsequently, a 2×2 coupler is utilized to coupling and generate the open-loop output from the single-mode fiber amplifier system. Thereafter, the main cavity of the fiber ring is closed, and the seed laser is gradually extinguished. This setup produces a passive coherent combining beam output from the multi-longitudinal mode laser while effectively suppresses self-excited spike oscillations within the loop. Closure of the auxiliary ring cavity, aided by the Vernier effect and narrow-band filtering, separates laser longitudinal modes, enabling single longitudinal mode oscillation of the passively combined laser within the ring cavity. Subsequent measurements verify the achievement of a single longitudinal mode laser output. Furthermore, a delayed self-heterodyne optical path with 0.1 ms delay is implemented to analyze linewidth characteristics, providing insights into the synthesized laser properties.

Coherent beam combining of two single-mode amplifiers is investigated. In the open-loop stage, the combined laser power ranges between 0.59 mW and 51.01 mW, averaging 25.8 mW, limited by noise effects such as environmental vibration, amplified pump fluctuations, and thermal effects. Upon closing the main cavity, simultaneous amplification of the seed laser and loop resonance laser increases the closed-loop output power to 27.26?74.17 mW. After closing the seed, the laser power stabilizes within the cavity, forming a multi-longitudinal mode combined beam with phase-locked identical modes and different randomized modes, resulting in a power of 59.97 mW. Ultimately, upon closing the auxiliary cavity, the system achieves a single-longitudinal-mode phase-locked laser power of 66.4 mW, 2.57 times the open-loop average, with a combine efficiency of 89.6% and phase-locked closing time of 0.07 s (Fig. 3). Monochromatic spectra are observed for both amplifier outputs and the combined beam (Fig. 4). The single longitudinal mode capability is verified using a Fabry-Perot interferometer (Fig. 5). The linewidth of the combined laser is measured using a self-heterodyne system with a 0.1 ms delay, revealing a 20 dB linewidth of 20.813 kHz, corresponding to a 3 dB linewidth of 1.2 kHz (Fig. 6). This combined laser satisfies the requirements for laser sources with ultra-long coherent lengths in remote coherent detection applications.

In this study, a single-frequency fiber laser passive coherent combining system with auxiliary cavity phase-locking is designed. The system improves the traditional passive coherent combining technology by utilizing the Vernier effect. It achieves the single longitudinal mode output of the ring cavity passive coherent combining system while enhancing laser brightness. Additionally, a single longitudinal mode coherent combining model is derived based on the theory of ring cavity oscillation. At the experimental design stage, a ring cavity with a main cavity length of 2500 cm and auxiliary cavity lengths of 124.3 cm and 166.2 cm is selected for mode selection. A passive combining system with a total cavity free spectral range (FSR) of 193.56 GHz (significantly larger than the gain bandwidth of 62.4 GHz) is constructed to realize a single longitudinal mode output. At the experimental combining stage, open-loop laser power ranges from 0.59 mW to 51.01 mW, corresponding to an average power of 25.8 mW. After closing the main cavity, multi-longitudinal-mode combining power reaches 59.97 mW. After closing the auxiliary cavity, a single-longitudinal-mode laser output of 66.4 mW is achieved at a wavelength of 1550.445 nm and linewidth of 1.2 kHz with combine efficiency reaching 89%. This combine efficiency is notably higher than that achieved in open-loop conditions. The results indicate that this passive combining scheme has broad application prospects in ultra-long-range coherent lidar and coherent optical communication fields.