| Summary: | 碩士 === 國立清華大學 === 電機工程學系 === 93 === Electro-optic spectral tuning of asymmetric duty-cycle PPLN APE channel waveguide is investigated in this thesis. Firstly, we derived a useful formula for estimating spectral tuning of SHG process and figured out the influence of the decrease of electro-optic coefficient for wavelength tuning ability. From the theory of guided-wave QPM SHG, we further discussed the compromise between wavelength tuning rate and normalized conversion efficiency. Those analyses provide principles for designing such waveguide device with suitable ability of nonlinear conversion and spectral tuning.
In the experiment, we demonstrated the spectral tuning for SHG process by applying a DC voltage on waveguide chip. The wavelength tuning is linearly proportional to the applied DC voltage. A strange discovery is that normalized tuning rate is power dependent. When the power of guided-wave inside APE channel waveguide is high, the normalized tuning rate we measured is larger than theoretical value. Such a phenomenon seems to be never founded in bulked asymmetric duty-cycle PPLN case. Certain mechanisms may exist in our waveguide and assists the wavelength tuning when the power of guided-wave is high. Moreover, those mechanisms seem to be a threshold at 2mW fundamental power. When fundamental power exceeds the threshold, the normalized tuning rate increases linearly with fundamental power first and tends to saturate. We have tried to find the reasonable mechanism for interpreting the phenomenon in this thesis.
From the comparison between theoretical calculation and the normalized tuning rate we measured with 2mW fundamental wave at the waveguide input, we deduce the electro-optic coefficient could remain 80% of bulked LiNbO3 value after we performed the APE process. The normalized tuning rate can be increased by a factor four when the power of fundamental wave exceeds 25mW. On the channel waveguide with 200um spacing of electrodes and above 25mW fundamental wave at the waveguide input, we measured the largest wavelength tuning rate which is about 1.44nm/KV for fundamental wave. Second harmonic pulses are generated by applying voltage ramps on waveguide chip when the wavelength of fundamental wave is fixed. Charge accumulation may exist in channel waveguide and SiO2 buffer layer. By reducing the power of fundamental and second harmonic wave inside channel waveguide, 50us second harmonic pulse was produced in our experiment.
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