| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 89 === Two main topics are discussed in this thesis. The first topic is the study of active mode locking in semiconductor lasers. The second one is the study of subharmonic hybrid mode locking.
In the experiment of active mode locking, a commercial laser diode with AR coating on the facet is set up in a wavelength-tunable external cavity. By changing RF frequency, DC bias, and RF power, we observe the influences of these factors upon pulses. When different RF frequency are applied, a phenomenon of frequency detuning will appear. With negative frequency detuning, we can obtain pulses of width 10.9 ps. If RF power is changed, the widths of pulse will decrease with increasing RF power. Besides, the highest repetition rate we can demonstrate is 726.3 MHz with shortest pulse width 12.7 ps.
In the study of subharmonic hybrid mode locking, we can carefully choose proper operation conditions to shorten the pulses by utilizing the property that QW structure can be used as a gain medium or a saturable absorber at different time. With the same system and procedure in the experiment of active mode locking, we investigate the effects of bias conditions upon pulse width and chirp. First, RF frequency is a more crucial factor. We can find that the locking bandwidth is smaller and the same frequency detuning mechanism appears in subharmonic hybrid mode locking. Second, with increasing bias current, time-bandwidth product, pulse width and spectral width trend to become larger. Third, pulse width shortens with increasing RF power because of deeper modulation depth. After pulse compression, the shortest width we’ve obtained in subharmonic hybrid mode locking is 5.7 ps with time-bandwidth product 0.73. We also apply this technique to commercial high-power laser diode. The shortest width is 5.7 ps with average power 14.4 mW and peak power 6 W.
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