Influences of ASE on the performances of Q-switched ytterbium-doped fiber lasers

The output characteristics of actively Q-switched ytterbium-doped fiber lasers (YDFL) are investigated in this paper. Our experimental results show that, the combined effect between the short switching time and the gain transient property of doped fiber causes the initial amplified spontaneous emission (ASE) power fluctuation, forming the multi-peak structure in the output pulse for either ring or linear cavity Q-switched YDFL. The pulse buildup time decreases with the rising of the pump. Moreover, since the broad-band ASE generated in the YDF is very high, it may saturate the doped fiber once the switch has been opened, making it difficult to achieve Q-switched laser oscillation for both fiber lasers. By using ASE filter to suppress the initial ASE, the gain supplied by the YDF can be greatly enhanced, which can not only decrease the threshold, but also greatly decrease the duration, and enhance the peak power of the Q-switched laser pulse as well. By using such an ASE filter, the Q-switched laser pulse with the peak power of 40.7 W and the duration of 30 ns has been achieved for the linear cavity YDFL pumped with 160 mW.