透過光學材料像是鈮酸鋰晶體製作的波導耦合器能夠有將光/模態分開的功能，但當結構被訂定時其分光的結果就被決定了，也因此有許多的方法被發現能夠改變分光比，例如外加電場、改變材料特性或是光學調製。 在本論文中，透過設計特定的週期性極化反轉結構以及絕熱耦合器在鈮酸鋰晶體上，其入射光進入絕熱耦合器及週期性極化反轉結構時將會利用受激拉曼機制產生分光現象以及滿足準相位匹配產生倍頻現象，但當操作鈮酸鋰晶體的溫度在相位不匹配條件時，在基頻與倍頻轉換的過程中發現非線性相位改變，且透過增加輸入光強度也會增加其相位的改變，也就是轉換交互的過程變得更強。透過模擬，當外加高強度的光時波導兩個出口能夠得到分光比大於20dB的結果。這樣的光開關相比於電控開關來說，其調製的速度能夠快非常多，另外絕熱耦合器對於製程及輸入的波長也具有相當高的容忍度，是為一大優勢。 本研究中，利用鈦擴散波導製程製作40mm長的絕熱耦合器，產生倍頻的週期為17.22um，當輸入波長1550nm且尖端功率為589W的脈衝雷射時能夠得到分比從1:9改變至3:7，而進一步改善討論將會在後續探討。 週期性極化反轉鈮酸鋰產生倍頻在絕熱耦合器上的全光開關設計具有操作簡單、穩定以及高製程容忍度，這樣的設計對於未來積體光路以及光邏輯閘的實現將會是一大進步。 ;Waveguide couplers can perform the beam/mode splitting of an optical wave based on an optical material like lithium niobate crystal. However, usually the split ratio of a wave in couplers is fixed when the structure of the coupler is fabricated. Hence many methods have been discovered to enable the varying of the split ratio actively, such as applying voltages, modifying material properties, and using optical modulation. In this thesis, I design a specific periodically poled structure and an adiabatic coupler in a lithium niobate crystal. The periodically poled structure is used to quasi-phase-match a second-harmonic generation (SHG) process of the fundamental wave as a pump for exciting a three-waveguide adiabatic coupler system designed based on the stimulated Raman adiabatic passage mechanism. When operated at a proper phase mismatching condition of the SHG process via the temperature control of the lithium niobate crystal, a nonlinear phase shift between the fundamental and the second harmonic waves will occur during their power conversion process, increased with the increase of the input power according to dispersion relationship between the two interacting waves. This nonlinear phase shift effect causes the change of the propagation constant of the waveguide seen by the input wave and therefore changes the coupling condition of the adiabatic coupler. Ideally in my simulation, the switching between the two outer waveguides of the coupler can reach an extinction ratio of >20 dB when a properly high input power is used. Such an all optical switching device is attractive in contrast to other optical switching methods because it is relatively fast without the need of an external phase modulation mechanism using such as a fast voltage supplier. Besides, the device is robust as the use of the adiabatic coupler featured by high fabrication tolerance and broad bandwidth. In this study, a PPLN SHG of period 17.22 m is fabricated in the input arm of a 40-mm long adiabatic coupler comprising three Ti-diffused lithium niobate waveguides. An optical switching with a split ratio of from ~1:9 to 3:7 has been observed from such a device when pumped by a 1550-nm ps laser of a peak power 589 W. Further improvement of the performance of the device is discussed. The proposed PPLN SHG adiabatic coupler all-optical-switching device can be operationally simple, robust, and fabrication tolerant, which can be of great potential for many applications including optical logic gates in integrated optical circuits.
