可調單頻 Nd:GdVO4雷射及其光譜應用

Abstract

本論文旨在以體積型布拉格光柵做為平凹共振腔輸出端反射鏡而建立中心波長1070 nm之單模Nd:GdVO4固態雷射。再經過週期性極化鈮酸鋰晶體產生二倍頻後，此雷射是鉈原子宇稱性不守恆實驗中的極佳光源。鉈原子宇稱性不守恆測量需有精密之原子理論計算，本論文利用此光源對鉈原子6P3/2 → 7S1/2能階躍遷進行精密頻率測量，測量結果可驗證原子結構理論計算的正確性。此光源也用於測量碘分子在535 nm 附近之超精細譜線。 我們建立的單模1070 nm Nd:GdVO4固態雷射的輸出功率可達300 mW。此雷射在輸出功率200mW時之光束傳輸因子 約為1.2，擴散角約為0.37°。在輸出功率100 mW時以共振腔調控之單模頻率範圍達5.1 GHz。我們以共焦式共振腔對此光源進行鎖頻，得到相對頻率穩定度為7.58 kHz。此穩頻光源由光纖放大器提升功率後再經過週期性極化鈮酸鋰晶體，藉由非線性效應產生二倍頻得到535 nm 光源。 Nd:GdVO4雷射倍頻光源掃描碘分子在535 nm附近之P(28) 30-0超精細光譜，並針對其中a1 , a10 及 a15各超精細譜線進行絕對頻率量測。本論文利用飽和光譜三次諧波解調技術產生頻率誤差訊號，將Nd:GdVO4光源穩頻在碘分子光譜的超精細譜線上，當雷射光源穩頻在a1譜線時之頻率穩定度在平均時間10秒時可達310-12。光頻梳測量雷射光源穩頻於超精細譜線的絕對頻率，絕對頻率測量結果續經壓力頻移量的修正後得到零壓力下的絕對頻率值。最後我們也針對a10超精細譜線測量其壓力增寬效應及功率增寬效應。 中空陰極管中之鉈原子兩同位素203Tl及205Tl 6P3/2 → 7S1/2躍遷的飽和吸收光譜由Nd:GdVO4雷射倍頻光源利用波長調制三次諧波解調技術得到超精細光譜，並由此得到頻率誤差訊號來進行雷射穩頻。分析雷射穩頻後的頻率誤差訊號，得到頻率穩定度在1秒平均時間時可達30 kHz，此穩頻光源將可做為鉈原子的雷射冷卻之用，也可用來研究鉈原子宇稱性不守恆。藉由精密波長儀測量穩頻光源之頻率得到鉈原子超精細譜線的絕對頻率，準確度達30 MHz，經壓力頻移量的修正兩同位素的躍遷頻率之頻率重心的準確度可達22 MHz，且推算出的同位素頻移量符合現知文獻資料。此鉈原子精密頻率測量結果將可做為鉈原子波動函數精確度的測試及檢驗基準。

This thesis work developed a single frequency diode-pumped Nd:GdVO4 laser at 1070 nm using a volume Bragg grating as the output coupler of a short plano-concave cavity. After second harmonic generation with a periodically-poled lithium niobate (PPLN), this laser is an excellent source for parity non-conservation (PNC) experiments using thallium atom. The accuracy of atomic theory which is needed to determine the thallium PNC motivated us to include the precise frequency measurement of the thallium 6P3/2 → 7S1/2 transition as part of this thesis. The light source was also utilized realize the absolute frequency measurements of the hyperfine components of molecular iodine at 535 nm. The developed 1070 nm single frequency Nd:GdVO4 laser can achieve an output power of 300 mW. The beam propagation parameter at 200 mW was ~1.2, and the divergence angle was ~0.37. The single frequency range with cavity length tuning was 5.1 GHz at 100 mW output power. We also locked the laser frequency to a confocal reference cavity and a relative stability of 7.58 kHz was achieved. After amplification of the fiber amplifier, the frequency stabilized 1070 nm laser passed through a PPLN to obtain a 535 nm light source by second harmonic generation. The absolute frequency of the a1, a10, and a15 hyperfine components of molecular iodine P(28) 30-0 line at 535 nm were measured with the frequency doubled Nd:GdVO4 laser. The frequency doubled 1070-nm Nd:GdVO4 laser was frequency stabilized to a hyperfine component of I2 using the saturation absorption spectroscopy and the third harmonic demodulation technique. The frequency stability of 310-12 was achieved at 10 second averaging time when its frequency was stabilized to the a1 component. An optical frequency comb was used to measure its absolute frequency. The pressure shift was investigated to obtain the absolute frequency at zero pressure. The effect of pressure and power broadening of the a10 component were also investigated. The saturated absorption spectrum of the 6P3/2 → 7S1/2 transition of 203Tl and 205Tl in a hollow cathode lamp was observed with the frequency-doubled 1070 nm Nd:GdVO4 laser. Similar to the iodine spectrum measurement, the third-derivative spectrum of the hyperfine components were obtained using the wavelength modulation spectroscopy and used to stabilize the laser frequency. Analysis of the error signal showed that the frequency stability reaches 30 kHz at 1 s averaging time. Such a frequency-stabilized light source at 535 nm can be used for laser cooling of thallium and for investigating the PNC effect in thallium. The absolute frequencies of hyperfine components were measured with an accuracy of 30 MHz using a precision wavelength meter. Including the pressure shift correction, the center of gravity of the transition frequency was determined to an accuracy of 22 MHz for both isotopes. Meanwhile, the isotope shift (IS) derived was in good agreement with earlier measurement. The precision measurements of thallium atomic structure can serve as the experimental constraints and benchmarks for the improvements of thallium wavefunction calculations.