Theoretical model and numerical study of effect of target reflected light on high-power fiber laser
Authors: Duan Lei , Xu Run-Qin , Song Yun-Bo , Tan Shu-Dan , Liang Cheng-Bin , Xu Fan-Jiang , Liu Zhao-Hui ,   Institute of Software, Chinese Academy of Sciences, Beijing 100190, China,   Institute of Automation, Chinese Academy of Sciences, Beijing 100190, China
Published: March 2023

<sec>In a high-power fiber laser system, the reflected light generated when the laser hits the target may be recoupled to the laser and amplified by the laser, thereby damaging the laser system. This situation is particularly serious for a high-power laser system, such as spectral beam combining a high power fiber laser, which lacks effective light-return protection. In order to solve the above problems, it is necessary to integrate various physical effects in the whole system link, evaluate and analyze the influence of reflected light on the operating state of the system, so as to optimize the optical path layout and system structure in the beginning of the design of fiber laser to avoid unnecessary losses. Based on the atmospheric transmission theory, fiber rate equation and medium heat conduction equation model, the effect of reflected light on high-power fiber laser is analyzed. In this paper is conducted the numerical simulation of coupling efficiency of reflected light of high-power fiber laser. It is found that under certain atmospheric conditions, the reflected power is related to the transmission distance, the offset angle of optical axis, divergence angle, and the offset of center position of the beam, and will affect the output power, beam quality factor, thermal effect and the signal-to-noise ratio of the stimulated Raman scattering spectrum of the fiber laser. The coupling efficiency of reflected light has a maximum value at a certain transmission distance. For example, the reflected light power up to 140 W is obtained when the transmission distance is 1500 m, which will largely affect the laser system. The reflected power is affected by the offset angle of optical axis far less than by the transmission distance when transmission increases from 500 m to 2000 m. For example, a change of less than 0.1 W can be obtained when offset angle increases from 0.11° to 0.44°. It is also shown that as the divergence angle changes from 0 to 15<i>''</i>, the coupling power decays nearly exponentially with the divergence angle. The coupling efficiency is close to 1% near 12<i>''</i>, which is almost negligible. The output beam quality of the laser system is also affected by the beam quality of reflected light. Deterioration of the beam quality of reflected light will lead to the deterioration of the laser output beam quality. As the reflected light power enters the fiber laser system and increases, the forward output power will decrease and the backward signal power will increase, and the Raman power will increase rapidly near the fiber output end. When the reflected light is present, the thermal effects caused by selecting the gain fiber with different pump absorption coefficients are very important. The stimulated Raman scattering (SRS) rejection ratio decreases with the increase of pump absorption coefficient. For example, the SRS rejection ratio decreases by 5 dB when pump absorption coefficient increases from 1.5 dB/m to 4.5 dB/m, resulting in a decrease in the signal-to-noise ratio of the laser, which will reduce the reliability of the fiber laser system.</sec><sec>In the design and test of spectral beam combining systems for high-power fiber lasers, the attention should be paid to the reflected light in the entire process, which includes the outer optical path and the internal optical path of the laser. The comprehensive constraints of multiple key indicators are analyzed, and the probability of system damage or reliability degradation due to reflected light is evaluated. The research results of this paper are of certain guiding significance in selecting suitable outer optical path layout and system parameters and also in optimizing the design of high energy fiber laser system.</sec>