半導體雷射干涉儀之光源穩頻與追溯

Abstract

由於半導體雷射發展日新月異，在干涉量測用途上已有取代氦氖雷射的潛力。但使用半導體雷射在干涉量測用途上有幾個問題需要考量，包括光回饋、頻率線寬太寬、缺少簡單可靠的穩頻方法、及適當的波長追溯管道等問題。本研究提出一個使用半導體雷射在干涉量測的方案，在方案中使用線寬約2 MHz的微透鏡半導體雷射建構干涉量測系統，在500 mm光程差以下不需要使用光隔離器。為了提供半導體雷射一個簡易的穩頻方法選用Fabry-Perot光學標準具來穩頻。為提供穩頻雷射波長追溯，建立了一個鈣蒸汽室，供發展657 nm的波長標準。

在簡易半導體雷射穩頻方法的工作中本研究提出，雷射鎖頻在Fabry-Perot 共振腔或光學標準具的共振峰側邊時，最好的鎖頻位置與共振腔反射鏡反射率相關。當反射鏡反射率高於85 %，這時短期頻率穩頻定度最好的位置大約落在共振峰頂位階的75 %左右，而非我們一般常用的50 %處。本研究並提出一種兩共振峰相減訊號的雷射穩頻技術，這種技術對於雷射光束角度偏移或者Fabry-Perot 共振腔角度飄移都有比較好的抵抗力，預期其長期穩定度可以優於傳統單光束穩頻，使用為干涉儀光源是一種有潛力的技術。

為解決半導體雷射波長追溯問題，本研究提出了一種簡易的鈣蒸汽室，用這個鈣蒸汽室實現657 nm的波長標準比用雷射致冷方法簡單。而這與美國國家標準與技術研究院類似訴求的原子束腔法比較起來，我們的方法，光譜訊號與光功率比高於平行系列熱原子束法，蒸汽室連續運轉的時間可以達10天以上不用破真空補充鈣，遠比原子束腔法的13個小時長。

Laser diodes have the potential to replace Helium-Neon lasers in interferometry because of their rapid quality improvements. However, certain problems remain when using laser diode in interferometry, including optical feedback, wide frequency line-width, lack of a convenient and robust frequency stabilizer and an effective method for laser wavelength calibration. To solve these problems, in this work we propose a method for the application of laser diodes in interferometer. Herein, a 2 MHz line-width micro-lens laser is used as light source of an interferometer. The interferometer requires no optical isolator until its optical path difference exceeds 500 mm. We also use a Fabry-Perot etalon as a frequency stabilizer for the light source of a laser interferometer and use a Calcium vapor cell for developing a 657 nm wavelength standard.

To develop a simple and robust method for diode laser frequency stabilization, we find that locking a diode laser wavelength to one side of a resonance of a Fabry-Perot cavity or etalon, the optimal wavelength stability point is at about 75% of the resonance's top level instead of 50 % of the level where we generally lock a laser if the reflectivity of the etalon's mirror is beyond 85 %. We also present a two-resonance-differential technique for stabilizing a laser to a Fabry-Perot etalon’s resonance, by which the error signal becomes insensitive to the angle drift of the etalon.

To solve the wavelength tracibility problem of a laser diode, we show a novel calcium vapor cell for laser spectroscopy. This cell enables the quicker development of 657 nm wavelength standard when compared with cooling atoms. Accordingly, based on the cell, we obtain better saturation peak signal to optical power ratio than that by the thermal beam cell technique developed by NIST. In addition, our cell can continuously work for up to 10 days, which also outlives that of the thermal beam cell technique.