We present pulse characteristics and evolution processes of a Q-switched ytterbium-doped double-clad (YDDC) fiber laser. At differ- ent pump powers and repetition rates, the laser shows different pulse shapes, buildup times, and duration. Due to the self-mode-locking effect in the Q-switching process, the Q-switched pulses split up. These phe- nomena are analyzed and the control of Q-switched pulses is discussed. applications. 3-5 High-energy pulses, up to multimillijoules, can be generated either from a Q-switched fiber laser di- rectly, or by using a master oscillator and power amplifier ~MOPA!, where relatively low-energy pulses, derived from a master oscillator, are magnified by one or more power amplifiers separated with optical isolators. Obviously, the latter configuration offers an easier power and energy con- trol, and better temporal characteristics. In general, low- cost, high-performance Q-switched fiber lasers, with a pulse energy of tens of microjoules are preferred for these applications. We present a compact Q-switched ytterbium-doped double-clad ~YDDC! fiber laser. It can stably work at a repetition rate up to 80 kHz, with the maximum pump power of 3.2 W. The pulse characteristics, such as pulse duration, buildup time, etc., are examined. Due to the self- mode-locking effect, the Q-switched pulses exhibit a mul- tipeak structure. Some researchers have previously reported the split pulses in their Q-switched fiber lasers. 6-8 Lees, and Newson observed five pulses in a Q-switched envelope of an erbium ~Er!-doped fiber laser, in which the output
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