- 學位論文
以微波參考頻率為標準的雙光纖光頻梳多外差光譜法之研究
Multi-heterodyne Spectroscopy with Dual Fiber Combs Referenced to Microwave Frequency Standard
摘要
鎖模雷射光頻梳系統應用在光頻率的計量已發展了十幾年，它大大改革了光頻率量測上的精準度，光頻梳系統在頻域上由許多等間距同調的光梳(comb line)所組成，每一支光梳的頻率等於脈衝重複率的整數倍加上偏差頻率，所以穩住光頻梳系統的脈衝重複率和偏差頻率，每一支光梳即可看成一個頻率穩定的雷射光源，且其寬廣的頻譜提供了偵測分子光譜良好的工具。 雙光頻梳光譜利用兩組不同脈衝重複率的光頻梳，彼此拍頻使光譜信號降頻到微波頻率範圍，即可用簡易的傳統微波檢測方法量測出來，藉由分析微波範圍的頻譜資訊，進而得知光頻率上整個頻譜吸收和相位的資訊。此光譜方法具有下列優點：1. 一次性大範圍的頻譜量測的能力，2. 可以量測吸收以及相位的訊號，3. 快速極短的量測時間，可用來探測隨時變的待測系統等優勢，故近幾年來蓬勃發展，本論文也著重在開發這項技術以應用在分子光譜量測上。 本論文利用兩台工研院量測中心架設的摻鉺光纖(Erbium-doped fiber)光頻梳系統來進行雙光頻梳多外差光譜(Multi-heterodyne Spectroscopy)的研究，兩台光頻梳系統的脈衝重複率和偏差頻率皆成功的透過鎖相迴路穩在GPS調校的微波標準頻率上，兩台光頻梳的脈衝重複率分別控制在400.2 MHz以及400 MHz，其重複率的差為200 kHz。 目前我們已經可以解析出每一支射頻頻梳(RF comb line)的頻譜，並且成功的掃出乙炔P Branch的吸收光譜圖，其吸收譜線中心的精準度可在300 MHz以內，但這樣的精準度不是很好，可能原因是光譜上出現未知的振盪背景，以及400 MHz間隔的光譜取樣點，未來的實驗在這些方面還有許多改進的空間。
並列摘要
The optical frequency comb (OFC) as “frequency rulers” is well established in optical frequency metrology for ten years. It greatly improve the precision in optical frequency measurement. OFC can be considered as a composing of the evenly spaced “comb line” in the frequency domain. Due to the frequency of comb line is equal to a multiple of repetition rate plus the offset frequency, the good stabilization of the the repetition rate and the offset frequency can let each comb line viewed as a frequency stabilized laser. On the other hand, the extremely broad bandwidth of OFC provides the great tool for molecular spectroscopy. The dual-comb spectroscopy using the beating signal of two OFC with different repetition rate can let the optical spectrum down-converted into a microwave frequency region, where becomes accessible to digital signal processing. Then we can obtain the absorption spectrum and the phase delay information by analyzing the microwave spectrum. Compared to conventional CW spectroscopy, this spectroscopy method has some advantages as below: 1. Measuring a broadband spectrum at once. 2. Acquiring ont only the absorption spectrum but also the phase delay information. 3. The fast measuring time, which has the ability to measure the time-dependent system. For these reasons, the dual-comb spectroscopy had been developed recently. In this thesis, we use this technology to apply to molecular spectroscopy. In this work, we study the dual-comb multi-heterodyne spectroscopy by using two Industrial Technology Research Institute’s erbium-doped fiber OFC. The two OFC’s repetition rates and offset frequencies were stabilized through the phase locked loop on the microwave local frequency calibrated by GPS. The two OFC’s repetition rate controled at 400.2 MHz and 400 MHz and the difference of repetition rates was 200 kHz. At present, we can resolve individual RF comb lines and obtain the Acetylene P branch absorption spectrum. The absorption line centers in our results differ by greater than the overall ±300 MHz systematic uncertainty. The worse precision was possibly contributed from the unknown oscillation baseline on our spectrum and the roughly 400 MHz spaced spectrum sampling. In the future, we will overcome these shortcomings to achieve the sufficient precision in molecular spectroscopy.
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