液晶反射鏡應用在外腔可調波長半導體雷射系統反應時間之研究

Abstract

摘 要 本實驗是將液晶反射鏡應用在波長可調半導體雷射系統作更進一步的研究與改進。我們使用的液晶元件是採TN型光學調制的設計(LCPM-90-TN-01)，先用一般的半導體製程，在一面的玻璃基板上蝕刻出我們所要的電極圖形，再使用一般液晶盒的製造程序，自行製作完成。所量測元件的性質，反射率black為13％，white為64％；響應的時間rise time為50 ms，fall time為175 ms。雷射系統採用一種摺疊望遠鏡式的光柵，外腔結構使用的元件，包括一削角入射光柵、透鏡、與液晶反射鏡及一顆紅外光半導體雷射，並以電控的方法調制液晶反射鏡，使其在我們所要的空間位置回饋雷射光源，可自由的選擇單、雙波長乃至多波長的輸出。波長調制範圍9.24 nm，旁模抑制比35 dB，雷射系統的反應時間rise time為15 ms，fall time為50 ms。為降低元件的反應時間，有效率的應用回饋光源，降低元件各部分的吸收，我們對其他類型的液晶元件設計，做了進一步的量測。所使用的鐵電性液晶（ZLI4654-00），rise time可達80 s，fall time 可達130 s；有預傾角的配向劑，對降低驅動電壓，有明顯的成果；PSCT元件，因少一片polarizer的吸收，可使回饋量增加；同時也針對未來製作不同光學設計的液晶反射鏡，提供參考的特性。

Abstract This study focused on improving the properties of Liquid Crystal Pixel Mirror in tunable external-cavity semiconductor laser system. We designed a TN cell with ITO electrode pattern etched by semiconductor process and used common process of making LC cell in LCPM. The external cavity consists of a grazing-incidence grating, lens, liquid crystal spatial light reflector, and a semiconductor laser diode. The work can be successfully operated in single, dual or multi-wavelength laser mode when using the electronic-controlled LCPM. The tuning range of the wavelength was 9.24 nm and the SMSR (side-mode-suppression-ratio) was 35 dB. The rise time and the fall time of the system are 15ms and 50ms, respectively. While the rise time and the fall time of LC cell are 50ms and 175ms, respectively. To improve the performance of the laser system, fast response, higher reflectance and low driving voltage are required. We have done experiments on other LC design to compare their related properties. For ferroelectric liquid crystal；the rise time is 80 s and fall time is 130 s. Large pretilt angle cells can decrease operating voltage apparently. PSCT cells which do not need polarizer can increase feedback lasing power.