Thermal effects of the gradient doping gain fiber in all-fiber MOPA

Abstract

Thermal effect in the gain fiber is one of the main factors which restrict the power improvement of high power fiber amplifiers. Previous studies have shown that the thermal effect is closely related to the doping concentration in the gain fiber. In order to reduce the temperature at the fusion point and the maximum temperature of the gain fiber, we propose to use doping concentration varying along the gain fiber as a method to disperse the thermal effect of the gain fiber. A two stage single frequency all-fiber-integrated MOPA is demonstrated, the second stage of which has a hybrid gain fiber composed of high-doped and low-doped Yb fibers. The temperature of the gain fiber is measured by a thermal imaging camera. It is shown that compared with the constant doping fiber, temperature in the gradient doping fiber is greatly reduced when the output powers are approximately the same. Results indicate that the gradient doping of the gain fiber is an effective way to alleviate the thermal effect in high power fiber lasers.