Realization of in situ fiber-core temperature measurement in a kilowatt-level fiber laser oscillator: design and optimization of the method based on OFDR

High power fiber lasers have been widely used in various industrial manufacture and military defense. During the development of fiber laser in the past decades, the thermal effect has always been one of the biggest obstacles. It is crucial to study the temperature characteristic and overcome the thermal restriction for a better output performance. With systematic design and optimization in this paper, OFDR (Optical frequency domain reflectometry) can achieve in-situ distributed temperature measurement of fiber core in high power fiber lasers. This allows a better study of temperature characteristics and thermal effects. The fiber-core distributed temperature of the kilowatt-level fiber oscillator is first demonstrated based on this method here. The splicing point between the high reflectivity fiber Bragg grating (HR-FBG) and gain fiber withstand the highest temperature reaching about 101.6 at 1.47 kW output. Besides the temperature of gain fiber gradually decreases from 92 to about 30 along the pumping direction. The internal temperature of the combiner and HR-FBG are also measured to evaluate their performances in the high power regime. Temperature distributions in the experiment agree well with the theoretical simulation.