| Summary: | 碩士 === 逢甲大學 === 光電研究所 === 100 === The hollow-core photonic crystal fiber can increase the overlapping between light and gas effectively, and make high optical power density easier. In this thesis, it is used as an absorption cell to observe saturated absorption on P(9) line of acetylene with the help of optical heterodyne detection. Laser beam, which passes absorption cell first time, is used as the pump beam. After double passing an acousto-optic modulator (AOM) and reflection back into absorption cell, the laser beam is used as the probe beam. The probe beam interferes with the beam reflected by splice between single mode fiber and hollow-core photonic crystal fiber. An optical heterodyne signal can be generated from a photodetector receiving both light beams. We use the self-homodyne technique to obtain the signal amplitude from the optical heterodyne signal, and retrieved the saturated absorption spectrum. Relative to original absorption center, the center of our saturated absorption signal has a frequency shift by one AOM’s driving frequency. While the conventional scheme of saturation absorption using AOM will have a miss alignment, our saturated dip center is aligned with the Doppler background center.
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