Optimization and demonstration of a bidirectional output linear-cavity fiber laser with a record high power of 2 × 4 kW

The bidirectional output fiber laser can achieve two laser outputs from a single resonant cavity, which can reduce the volume and cost of high-power fiber laser systems and has good application prospects in the industry. In this work, we demonstrated a bidirectional output fiber laser oscillator structure, and a total output power of 8 kW at the 2 ports × 4 kW level was achieved experimentally for the first time. In order to suppress stimulated Raman scattering (SRS) and transverse mode instability (TMI), the double cladding ytterbium-doped fiber with a core/cladding diameter of 30/400 µm, a pair of matched output coupler fiber Bragg gratings (OCFBG) with a 3 dB bandwidth of 2 nm, and the pump source with wavelength-stabilized 981 nm laser diodes (LDs) were employed. The effect of the coiling method and diameter of the active fiber on the optical-to-optical (O-O) efficiency and TMI threshold were studied intensively. The comparison of laser performance with a unidirectional output oscillator was further exploited, and results indicated that the bidirectional output fiber laser oscillator possessed a higher O-O efficiency and TMI threshold. By optimizing the fiber coiling diameter and coiling method, we finally achieved a total laser output power of 8169 W (3769 W at end A and 4400 W at end B), with an efficiency of 74.6% and beam quality of M2A ∼ 2.13, M2B ∼ 2.36, while the total output power achieved by unidirectional output structure was only 5805 W with signs of TMI. To the best of our knowledge, it is the highest output power of the bidirectional output fiber laser.