| Summary: | 碩士 === 國立臺北科技大學 === 光電工程系研究所 === 100 === Through nonlinear polarization rotation mechanism, we generate 365 kHz low repetition rate and high energy pulses without pulse breaking from Yb-doped fiber ring laser by adding 520 m long single mode fiber inside laser cavity. With 90% output coupler, the highest pulse energy above 83 nJ can be experimentally obtained using 258.7 mW pump power and the shortest pulsewidth about 616 ps was measured from the high speed oscilloscope. Besides, we reshape the spectrum of the ML pulses through the tunable spectrum filter outside laser cavity by using the knife and the grating pairs. The measured pulsewidth will decrease as the spectrum bandwidth shrinkage and the shortest pulsewidth was 322 ps at 2 nm spectrum bandwidth. Besides, we investigate the relationship between the central wavelength and pulsewidth by changing the center wavelength of selective spectrum band and maintaining the spectrum bandwidth about 3 nm. With central wavelength at 1031.4 nm, we measured the shortest pulsewitth about 332 ps. It demonstrates existence of nonlinear chirp in our mode-locked pulses. After amplification by the Yb-doped fiber amplifier, the highest pulse energy of mode-locked pulse was 385 nJ. In addition, multiple-pulses can be observed in our laser system by rotating the PC. When I reduced the output coupling ratio to 40%, ten pulses can be seen at higher pump power. From the relationship between pump powers and pulse numbers, we can observe the hysteresis phenomenon for multiple-pulses generation as pumping power increase or decrease.
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