A high-power thulium-doped fiber laser with a wavelength in the range of 1900?2100 nm has broad application prospects in many fields, including absorption spectrum diagnosis, biomedical treatment, LIDAR, plastic processing, and mid-infrared laser pump source. Specifically, thulium-doped fiber laser systems have achieved a high power output in the kilowatt class using 793 nm laser diode (LD) pumps. However, the low optical conversion efficiency of 793 nm LD pumping mode causes problems such as serious thermal effects and refractive index distortion during high-power operation, which affect the beam quality of the output laser. Therefore, it is important to investigate the beam-quality variation characteristics of high-power thulium-doped fiber lasers under different conditions.

In our previous study, a beam quality prediction model was established based on the finite-difference beam propagation method. A complete simulation link, from solving the rate equation to predicting the output beam quality of a fiber laser, was realized. The model was applied to thulium-doped fiber laser systems with different power levels reported by other researchers and compared with the experimental measurement results; thus, the prediction accuracy of the model was verified. In this study, the model was used to further simulate and analyze the influence of different parameters, such as the input pump power, forward and backward pump ratio, active fiber absorption coefficient, and seed power, on the output beam quality of the laser.

the beam quality of the laser deteriorates with an increase in input pump power; the backward and forward pump ratio of 1∶1 can effectively suppress the internal thermal effect of the laser and is conducive to obtaining the laser output with high beam quality (Fig. 4 and Fig. 5); the active fiber with low absorption coefficient can reduce the temperature inside the fiber and improve the beam quality of the output laser (Fig. 6 and Fig. 7); with the increase of seed power, the beam quality of laser output deteriorates slightly (Fig. 9); and compared with increasing the seed power, changing the pumping ratio has a more obvious effect on the refractive index, and thereby, on the beam quality (Fig. 5 and Fig. 9). The variation in the beam quality predicted by the theoretical model is in good agreement with the experimental result, and the average deviation is approximately 10%. Based on the aforementioned results, a 15 W seed and 1∶1 forward-to-backward pumping ratio in the amplification stage were employed to experimentally verify the beam quality characteristics at higher laser power levels. At a wavelength of 1940 nm, under an input pump power of 1131 W, a laser output power of 513 W with a beam quality factor M² of 3.04 is experimentally achieved, and the deviation from the theoretically predicted M² value 2.8 is approximately 8.6% (Fig. 12 and Fig. 14).

The characteristics of the laser power output, refractive index change, and beam quality change under different conditions are analyzed via theoretical simulation. Theoretical simulation results show that the beam quality of the laser deteriorates with an increase in the input pump power and exhibits a nearly linear growth trend. A backward-to-forward pump ratio of 1∶1 can better suppress the internal thermal effect of the laser and is conducive to obtaining a laser output with a high beam quality. Under the condition of the same total gain, an active fiber with a low absorption coefficient can reduce the temperature inside the fiber and improve the beam quality of the output laser. With an increase in the seed power, the laser beam quality deteriorates slightly. Compared with increasing the seed power, changing the pump ratio and using an active fiber with a low absorption coefficient have more obvious effects on the refractive index and thus on the beam quality. Based on a 200-W thulium-doped fiber laser system developed by us, the variation in beam quality with the aforementioned parameters is revealed by experiments. The experimental results are in good agreement with the predicted law of beam quality, with a deviation of approximately 10%. Based on the aforementioned findings, to obtain the output of high-power and high-beam-quality thulium-doped fiber lasers, the oscillator output with higher power should be preferred as the seed light, a 1∶1 forward-to-backward pump ratio should be adopted, and an active fiber with a low absorption coefficient should be selected.

Based on the aforementioned theoretical simulations and experimental results, the beam-quality characteristics at higher laser power levels are verified. A 15 W seed light is selected, a 1∶1 forward-to-backward pump ratio is selected for the amplifier stage, and a pedestal thulium-doped fiber with a core ratio of 25/400 μm and an absorption coefficient of 4.2 dB/m is adopted. Under a total pump power of 1131 W, laser output with a wavelength of 1940 nm, average power of 513 W, and beam quality M² of 3.04 is experimentally achieved. The measured M² of 3.04 shows an approximate deviation of 8.6% from the theoretical prediction value of 2.8.