A Study of the Fiber Fuse in Single-mode 2-kW-class High-power Fiber Amplifiers

In this paper, we experimentally investigate the fiber fuse in single-mode 2-kW-class high-power fiber amplifiers, depending on the cooling method at the splicing point. We measured the temperature of the splicing point between the pump-signal combiner and gain fiber as a function of laser output power. The temperature of the splicing point increased from 20 to 32 degrees C with a slope of 0.01 degrees C/W, up to 1.2 kW of laser output power. At higher powers the temperature of the splicing point increased dramatically, with a slope of 0.08 degrees C/W. After that, the fiber amplifier was destroyed during operation at 1.96 kW of output power by fiber fuse. The bullet shape, a common feature of fiber fuse, was observed in the damaged passive fiber core of the pump-signal combiner. Later, we adopted an improved water-cooled cold plate to increase the cooling efficiency at the splicing point, and investigated the laser output power. The temperature at the splicing point was 35.8 degrees C with a temperature-rise slope of 0.007 degrees C/W at the maximum output power of 2.05 kW. The beam quality M-2 was measured to be less than 1.3, and the output beam's profile was a stable Gaussian shape. Finally, neither fiber fuse nor mode instability was observed in the fiber amplifier at the maximum output power of 2.05 kW.