In the past few years, high-power lasers have shown great potential in the application of industrial processing, medical treatment, national defense, and other fields. Fiber lasers have become a mainstream technical solution to high-power lasers due to their unique advantages of excellent output beam quality, high energy conversion efficiency, good heat dissipation capacity, compact structure, and high robustness. Currently, a single fiber laser can reach the output power of 10 kW. However, due to energy loss, quantum defect, and other factors, the waste heat generated in high-power fiber lasers has seriously restricted the further improvement of laser output power. The performance of the high-power fiber laser is significantly affected by the high temperature inside the gain fiber, which reduces the stability of the fiber laser, and results in mode instability effect and degradation of the output beam quality. Theoretical studies have shown that the mode instability effect is accompanied by dynamic gratings caused by the thermo-optical effect in the fiber core. The high-order mode and fundamental mode are coupled with each other under the interaction of dynamic gratings, causing degraded output beam quality and further restricting the power improvement of high-power fiber lasers. Therefore, the thermal effect in high-power fiber lasers is an important factor that determines laser operation stability. It is very important to measure and monitor the temperature of the fiber core in the laser for improving the stability of the high-power fiber laser and avoid the thermal damage of the gain fiber.

The refractive index of the fiber core is affected by the temperature of the fiber material to change the phase or spectrum of the reflected/scattered light. By measuring the phase or spectrum change of the reflected/scattered light in the optical fiber, the temperature change in the optical fiber core can be detected. Fiber Bragg grating (FBG) is a reflector composed of short fiber with refractive index period modulation, and it can be employed to measure the fiber core temperature with discrete points. To study the thermal effect and photon darkening effect in the gain fiber, Leich et al. adopted four FBGs to measure the fiber core temperature in the gain fiber. The research group at the National University of Defense Technology also measured the fiber core temperature by the π-phase shifted FBG and composite grating in 2018 and 2020, respectively, which realized the real-time temperature monitoring of the gain fiber. However, FBG can only measure the fiber temperature at discrete locations and cannot achieve continuous distributed temperature measurements. It misses lots of temperature information at key locations and cannot comprehensively characterize the fiber core temperature of the high-power fiber laser. Therefore, researchers from the University of Jena measured the distributed temperature of the fiber core by optical frequency domain reflectometry (OFDR), which is a distributed temperature sensing method with extremely high spatial resolution. The temperature of fiber lasers with single mode output, several hundred-watt output, and kilowatt output power was characterized in 2015 and 2017, respectively. However, their study did not measure the absolute temperature of the fiber core in the fiber laser. Thus, researchers from the University of Defense Technology optimized the OFDR method to measure the temperature of high-power fiber lasers by decoupling the temperature and stress changes of the fiber, calibrating the temperature coefficient of the fiber, optimizing the fiber winding, and reducing the end reflection. The temperature of fiber laser amplifiers and oscillators with kilowatt output power was measured based on the optimized OFDR method. Since there are still few studies on the temperature measurement of the fiber core in high-power fiber lasers, the corresponding applications have not been fully demonstrated. The current applications include the quality evaluation of the splice point, and thermal management and nonlinear effect suppression in high-power fiber lasers, providing references for further research on high-power fiber lasers.

Fiber sensing method is an important tool to monitor the temperature of the fiber core in high-power fiber lasers. FBG can only measure the temperature at discrete locations, and the OFDR method can measure the distributed fiber core temperature with high spatial resolution. With several applications of thermal management and nonlinear effect suppression, the fiber core temperature characterization shows great potential in the performance improvement of high-power fiber lasers.