A high-gain optical injection amplification system for coherent fiber links

We design a high-gain optical amplification system based on optical injection locking (OIL) technology, which has an all-fiber structure and is ideally suited for fiber-based optical frequency transfer system. The paper investigates the relationship between amplification gain and frequency detuning under different injection linewidths, showing that smaller injection linewidths result in higher gains for the same frequency detuning. Preliminary experiments show that with a 5 Hz injection linewidth and sufficiently small frequency detuning, an amplification gain exceeding 60 dB can be achieved. In contrast to previous approaches, we introduce an out-of-loop optical path with an acousto-optic modulator to counteract the additional phase noise introduced by the asymmetric optical paths, achieving a higher-performance optical injection amplification system. After effective phase noise suppression, the noise floor of 1m coherent fiber link constructed based on the optical amplification system achieves a fractional frequency instability floor of 1.6 × 10⁻²⁰ at an integration time of 10000 s. Based on this high-performance system, a long-distance coherent transmission experiment over a 200 km spooled fiber link was demonstrated, which showed a fractional frequency instability of 3.4 × 10⁻¹⁵ at 1 s, scaling down to 3.4 × 10⁻²⁰ at 10000 s in terms of modified Allan deviation (Mod-ADEV). This work presents a high-gain optical amplification method for transferring ultra-stable optical frequency standards, reducing the number of repeaters and amplifiers in optical frequency transfer, and simplifying the system complexity.