| Fiber lasers have developed rapidly in recent decades due to their high conversion efficiency,good beam quality,compact structure,and easy heat dissipation.However,in terms of power increase,limiting factors such as thermal effects dominated by transverse mode instability(TMI)effect and nonlinear effects dominated by stimulated Raman scattering(SRS)have appeared.These limiting factors are closely related to the temperature distribution in the fiber.Based on this,this article carried out theoretical and experimental research on the output characteristics of fiber lasers from the perspective of temperature.The main contents are as follows:1.A 2 kW laser platform was established for TMI effect testing.Based on this platform,the influence of cooling temperature on the TMI effect of the laser was studied.The experimental research on the influence of the cooling temperature of the gain fiber on the TMI effect of the laser under the room temperature environment and the low temperature environment were carried out respectively.It is verified that cooling gain fiber can suppress the TMI effect of the laser under the condition of sufficient refrigeration.2.The influence of the cooling of passive fiber and gain fiber on the SRS effect of fiber lasers are studied.Based on the kilowatt fiber laser platform,the experiment clarified that the cooling of passive fiber has little effect on the SRS effect of the high-power fiber laser;through cooling the gain fiber of the lasers in three different resonator structure,the influence of cooling the gain fiber on the SRS effect of the laser is explored,and it is found that when reduce the cooling temperature of the gain fiber,the output efficiency of the laser decreases and the SRS effect is suppressed,but the connection between these two phenomena need to be further studied.3.Based on the existing fiber laser steady-state rate equation model and the temperature characteristics of the fiber devices,temperature-dependent ytterbium-doped fiber laser steady-state rate equations are established.And through the model simulation,the output characteristics of the fiber laser with the center wavelength of 940 nm and 976 nm LD as the pump source were calculated in a wide temperature range respectively,the results of which pointed out that the incompatibility of 976 nm LD in variable temperature,and the stability of 940 nm LD in the environment of large temperature difference.Based on the simulation results,a kW-level fiber oscillator was built,and the indicator that the output power is always higher than 1 kW and the power fluctuation is less than 8% in the temperature range of-30 ℃ ～20 ℃ was completed,revealing the low-temperature operation characteristics of the fiber laser,providing the guidance for the high-power output and low-power fluctuation design of the fiber lasers. |
