| Summary: | 博士 === 國立交通大學 === 電子物理系所 === 103 === Mode locking is a exhibition of superposition of optical wave in temporal regime. Since my laboratory members have discovered that a short linear cavity can naturally generate self-mode-locked pulse in 2008, they have already successfully exploited linear short cavities to generate self-mode-locked lasers for variety Nd-doped vanadate crystals. For example, Nd:YVO4, Nd:GdVO4, and Nd:LuVO4. However, all of these are single-spectral-band self-mode-locked lasers. Therefore, my work is to investigate the performance of multi-spectral-band self-mode locking.
The fluorescence spectroscopy of laser gain media is crucial for designing multi-spectral-band lasers. In my studies, I adopted Nd-doped aluminate and vanadate crystals to be laser gain media. Accordingly, I gave priority to measure fluorescence spectroscopy of these two species of crystals. The measured results reveal that The fluorescence spectroscopy of Nd-doped aluminate crystals possess more emission lines than Nd-doped vanadate crystals do.
Because Nd-doped aluminate crystals possess wealth of emission lines, I adopt Nd-YAP to be laser gain medium and exploit an intracavity etalon to control gain-to-loss among these emission lines for my laser system which can be used to generate single-spectral-band or multi-spectral-band self-mode-locked lasers. I alter pulse duration of single-spectral-band and multi-spectral-band self-mode-locked lasers by considering spatial hole-burning effect. Moreover, the temporal overlapping of the multi-spectral-band pulses is experimentally found to lead to the generation of intensity fringe pattern in the autocorrelation trace with the optically beat frequency reaching several THz.
Although Nd-doped vanadate crystals possess relatively few emission lines among 4F3/2 → 4I11/2 transition, I combine two different Nd-doped vanadate crystals to form a composite gain medium that possesses two strong 1.06-μm emission lines which are generated by these two crystals respectively. In end pumping situation, I could flexibility control the intensity ratio of these two 1.06-μm lines via controlling mode matching for these two crystals. When the intensity ratio is unity, I investigate the spatial hole-burning and optical beat.
Finally, I compare the experiment results of single Nd:YAP crystal laser with dual Nd-doped vanadate crystals laser. The experimental results reveal that these two lasers could generate the temporal overlapping of the multi-spectral-band pulses with intensity fringe pattern of optically beat. Interestingly, the tendency of temporal synchronize multi-spectral-band pulse of these two lasers are something different as considering the spatial hole-burning effect. When the spatial hole-burning effect gradually increases, both of the pulse duration of these two lasers is gradually decrease; however, because the two Nd-doped vanadate crystals are suffered different degree of spatial hole-burning effect but single Nd:YAP crystal are not, tendency of their pulse shapes is different. A simple mathematical model is developed to elucidate the formation of a train of optically beat pulses.
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