具液晶元件之外腔式半導體雷射系統之特性及應用研究

Abstract

本論文係研究腔内具有液晶元件之外腔式半導體雷射之雷射特性，並探討其可能的應用。 外腔式半導體雷射腔內置入一平行排列之向列型液晶盒，改變此向列型液晶盒之驅動電壓，使液晶分子旋轉造成折射率的改變，即等同於變化雷射腔長使其波長可調。此平行排列之向列型液晶盒具有無磁滯且重現性佳之特性，其響應時間為數十毫秒。實驗結果顯示，一個厚度為35.5 mm的液晶盒置於15公分的外腔中，驅動電壓僅需1.5 V即可使雷射輸出頻率可調範圍大於4 GHz；若同時改變液晶盒的驅動電壓和半導體雷射的驅動電流，則達到雷射輸出頻率無跳模連續可調範圍19.2 GHz，此結果與理論值相符合。以液晶像素反射鏡構成的數位波長可調整多通道外腔式半導體雷射系統，藉由改變液晶像素反射鏡的驅動電壓(2.8 V~6.36 V)其波長連續可微調140 MHz，若利用置入腔內厚度35.5 mm的液晶盒微調波長，單通道輸出頻率無跳模連續可調範圍達1.75 GHz。 我們將此雷射系統應用於高解析度光譜、穩頻、微調多通道雷射波長及液晶層厚度量測：(1) Rb原子同位素85B和87B D2-line (5S1/2 - 5P3/2, 780.245 nm)之精細光譜；(2)利用反饋控制液晶盒的方式，將雷射波長鎖在étalon的穿透頻譜上，鎖頻後的相對波長穩定度達2.46´10-8 (平均時間20 s)，和以同樣方式鎖頻的商品化外腔式半導體雷射比較，二者鎖頻後的相對波長穩定度具有同等數量級；(3)嘗試將此雷射鎖頻至飛秒光梳上，目前結果顯示，鎖頻後10分鐘內頻率擾動範圍約1.5 MHz，預期進一步改善後可應用雙波雷射系統產生穩定連續兆赫波；(4) 應用此雷射系統調波長的特性量測兩個平行排列之液晶盒厚度，量測結果顯示厚度值為9.7 mm和4.2 mm，相位延遲量分別為1.63 mm 和0.20 mm，和干涉法量測結果比較，量測誤差在±0.5 % 和±0.6 % 範圍內，證明此量測方法亦適用較小相位延遲量液晶層厚度的量測，此量測方法目前受限於波長儀解析度及雷射系統本身波長的飄移。

This dissertation mainly focused on the study of external-cavity diode lasers (ECDL) with intracavity liquid crystal elements and applications. An ECDL with an intracavity planar-aligned nematic liquid crystal (NLC) cell for wavelength tuning was developed. Varying the voltage driving the NLC cell, its extraordinary index of refraction would change due to field-induced reorientation of the LC director. This is equivalent to tuning the laser cavity length. As a result, the laser wavelength can be continuously tuned. The NLC cell behaves no apparent hysterisis, good tuning repeatability, and the response time is several tens milliseconds. With a NLC cell 35.5 mm in thickness, the output frequency of a 15-cm-cavity laser can be continuously tuned over 4 GHz. The root-mean-square (rms) driving voltage required was 1.5 volts. By varying the driving voltage of the NLC cell and laser diode bias current simultaneously, single-mode oscillation and mode-hop-free tuning over 19.2 GHz at 775 nm were achieved. The tuning range is in good agreement with the theoretical predictions. Digitally wavelength tuning of a channel-selectable laser (LCPM based ECDL) can be achieved by tuning the driving voltage of the LCPM and the intracavity NLC cell. Continuous tuning of 140 MHz of one selected channel was achieved by tuning the LCPM from 2.8 V to 6.36 V. Tuning of a 35.5-mm-thick NLC cell, continuously tuning over 1.75 GHz from 0.9 V to 2.26 V was achieved. The mode-hop-free tuning range is limited by the requirement of dedicated adjustment of the LD current. The developed ECDL was applied for spectroscopy, wavelength stabilizations, fine-tuning of a channel-selectable laser, and liquid crystal cell gap measurements. The hyperfine structures of the Rb 85B and 87B D2-line (5S1/2 - 5P3/2, 780.245 nm) was observed. The output wavelength of the ECDL was locked to an étalon by feedback control of the NLC cell. Relative wavelength stabilities of 2.46´10-8 (sampling time 20 s) were achieved. Frequency locking of the developed ECDL to femtosecond combs was implemented. The frequency fluctuation achieved was 1.5 MHz for a period of about10 minutes. We expect that it can be further improved and the scheme can be applied for locking a dual-wavelengths laser system for stable cw terahertz generation. The developed ECDL was applied for measuring the gap of the LC cell placed in the laser cavity through tuning of the wavelength of the laser. Measurement errors of ±0.5 % and ±0.6 % for 9.6-mm and 4.25-mm planar-aligned cells with phase retardations of 1.63 mm and 0.20 mm respectively were demonstrated. The accuracy of cell gap measurement is limited mainly by the resolution of the wavelength meter and the frequency drift of the ECDL. This method is particularly suitable for measurement of LC cells of small phase retardation.