Traditional monolithic fiber lasers can only achieve unidirectional high-power laser output. In this Letter, a novel high-power linear cavity fiber laser that can achieve bidirectional high-power output is proposed and demonstrated. In an ordinary laser resonant cavity, we replace the high-reflectivity fiber Bragg grating with a low-reflectivity fiber Bragg grating to realize bidirectional laser output. In our experiment, the laser cavity was composed of two fiber Bragg gratings with a reflectivity of about 10%. The pump power provided by the 976 nm laser diodes was injected into a double-clad Yb-doped fiber with core/cladding diameters of 20/400 µm. At the maximum pump power, the bidirectional output powers were 2025 W and 1948 W, respectively, and the output laser beam quality (M2 factor) at both ends was about 1.5. For the first time, to the best of our knowledge, the feasibility of a bidirectional output fiber laser that can achieve double high (2-kW-level) power was verified. Compared with a traditional unidirectional output laser, this type of bidirectional output laser can achieve a double high-power laser by employing a laser resonant cavity. Thus, the average cost and structure size can be further reduced in mass production.

In high power fiber lasers, the degradation of beam quality caused by Raman effect has attracted more and more attention in recent years, but its physical mechanism is still unclear. We're going to differentiate between heat effect and nonlinear effect by duty cycle operation. The evolution of beam quality at different pump duty cycles has been studied based on a quasi-continuous wave (QCW) fiber laser. It is found that even if the Stokes intensity is only -6 dB (energy proportion: 26%) lower than that of the signal light intensity, the beam quality has no obvious change with the duty cycle of 5%; on the contrary, when the duty cycle gradually approaches 100% (CW-pumped scheme), the beam quality distortion changes faster and faster with the increase of Stokes intensity. The experimental results are contrary to core-pumped Raman effect theory [IEEE Photon. Technol. Lett. 34, 215 (2022)10.1109/LPT.2022.3148999], and further analysis confirms that the heat accumulation in the process of Stokes frequency shift should be responsible for this phenomenon. That is the first time, to the best of our knowledge, for intuitive reveal of the origin of stimulated Raman scattering (SRS)-induced beam quality distortion under transverse mode instability (TMI) threshold in an experiment.

Fiber laser oscillators play a vital role in industrial processing, national defense research and other fields. Traditional high power fiber oscillator lasers can only achieve unidirectional output, which is determined by the resonator structure. We propose a new structure that is simple and compatible with the ordinary fiber laser oscillator, which can achieve double high power laser output with only one resonant cavity. Typically, the resonant cavity of all-fiber laser oscillator contains a high-reflectivity grating and a low-reflectivity grating. Briefly, by replacing the high inverse grating with a low inverse grating, the unidirectional output laser oscillator can be converted into a b-directional output laser oscillator. It is suitable for applications that do not require high power and can have multiple lasers working at the same time, because of the smaller size and lower cost. We select a double-cladding Yb-doped fiber with 25/400 μm core/cladding diameter. The length of fiber is 35 m and its absorption coefficient is 0.33 dB/m at the wavelength of 915 nm. By means of bi-directionally pumping with 981 nm laser diodes, the maximum output power at both ends is 2871W and 2666 W respectively. We believe that with the further study of stimulated Raman scattering and transverse mode instability, bidirectional output lasers can achieve higher power outputs.