Thermal effect of the gain fiber is one of the main factors which limit the power improvement of high power fiber lasers. Using traditional temperature measurement methods, we can only obtain the surface temperature of fiber while the core temperature cannot be detected. In this paper, the temperature of the gain fiber core in all-fiber laser is measured by optical frequency domain reflectometry(OFDR). The fiber laser is operating at 1080nm and the wavelength of detecting light emitted by OFDR is approximately 1550nm. The different wavelength of the two lasers has ensured the proper operating of fiber laser without effecting the operating lasers at 1080nm. The maximum output power of the fiber laser is 58.95W and results show that OFDR is a practical way to monitor the temperature of fiber laser system.
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.
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