摻鈦藍寶石晶體光纖雷射系統應用於自混頻式干涉儀之研究

The study of Ti:sapphire crystal fiber laser system applied in self-mixing interferometer

許柏彥(Bo-Yan Hsu)

指導教授 ： 黃升龍

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2022年)

https://doi.org/10.6342/NTU202201036

摘要

雷射反饋於早期研究被視為影響雷射輸出穩定性之一大因素，然而自P. G. R. King和G. J. Steward證實可將其應用於空間長度之量測，以自混頻干涉術為名之研究如雨後春筍般出現並成功實行於諸多長度量測應用中，其中以波長可調式雷射進行距離遙測最引人目光。在距離遙測中，使用的雷射波長越短、可調頻寬越寬，越能得到更細緻之空間解析度，同時摻鈦藍寶石材料於可見光-近紅外光波段具有寬廣且連續的螢光放射頻譜，頻譜半高寬可達180 nm，常作為可調式雷射或是時域鎖模雷射之增益材料，因此本論文將嘗試以摻鈦藍寶石材料建構具有週期性波長調變之掃頻式雷射，並搭配自混頻式干涉儀架構嘗試進行距離量測應用，期望得到長距離、高解析及結構簡約之量測工具。
為解決摻鈦藍寶石難達到低閥值輸出之常態，本實驗室使用雷射加熱基座長晶法生長出纖心直徑為 16 μm且衰減係數僅為0.017 cm-1之玻璃纖衣光纖波導結構，使得雷射有著高效率且低閥值的輸出特性。於掃頻雷射架構中以閃耀光柵作為波長帶通濾波器，其具有348 mW之雷射閥值並在背向有著2.4%之斜線效率輸出，同時利用旋轉鏡進行波長調變，調變範圍為712.4 nm至876.8 nm，頻寬共164.4 nm，以此掃頻光源用於距離量測之理論計數方法量化誤差為1.95 μm。
於自混頻干涉術研究中，首先利用位移量測實驗對摻鈦藍寶石晶體光纖雷射中發生之自混頻干涉現象與性質進行瞭解，並可對於微米級的微小位移進行量測，然而於距離量測實驗中，掃頻式光源卻無法以自混頻干涉術實現30 cm之距離量測。回頭進行掃頻光源性質之探討後，得知其受限於掃頻過程中多波長之瞬時頻譜輸出，將頻譜以自定義之等效線寬擬合並與理論之數值模擬結果進行比較後，證實了將等效線寬概念用於評估光源可量測距離長度之可行性，並搭配可解釋的實驗量測結果，推論目前摻鈦藍寶石晶體光纖掃頻式雷射僅有約莫300 μm之距離量測能力，此距離已適用於生物表淺組織之檢測，然而在未來若想履行更長距離之量測，必須針對此掃頻光源之輸出性質進行改善。

關鍵字

摻鈦藍寶石晶體 ； 晶體光纖 ； 寬頻可調雷射 ； 掃頻式雷射 ； 自混頻干涉術 ； 距離量測

並列摘要

Laser feedback was regarded as one of the major factors affecting the stability of laser output in early research, but since P. G. R. King and G. J. Steward confirmed that it can be applied to the measurement of spatial length, the research known as self-mixing interferometry has sprung up and been implemented in many length measurement applications successfully. Among such applications, distance telemetry with wavelength-tunable lasers is the most attractive. In distance telemetry, the shorter wavelength of laser output and the wider tunable bandwidth can bring better spatial resolution. Meanwhile, Titanium (Ti):sapphire crystal has broad and continuous fluorescence emission from the visible to near-infrared (NIR) band, the full width at half maximum of the spectrum reaches 180 nm and it is often used as a gain medium for tunable lasers or time-domain mode-locked lasers. Therefore, this thesis plans to build a Ti:sapphire swept laser with periodic wavelength modulation in order to realize distance measurement by a self-mixing interferometer. It is expected to be a simple-measuring tool with long distance and high resolution.
In order to solve the common problem that Ti:sapphire laser is difficult to achieve a low threshold, our laboratory uses the laser-heated pedestal growth (LHPG) method to manufacture glass-clad and crystal-fiber waveguides with a 16-μm core diameter and a merely 0.017-cm-1 attenuation coefficient which allows the laser to exhibit high efficiency and low threshold output. In the swept-laser structure, the blazed grating is used as a bandpass filter which results in a laser threshold of 348 mW and backward output slope efficiency of 2.4%. Furthermore, a rotating mirror acts as a wavelength- modulation element, and the sweeping ranges from 712.4 nm to 876.8 nm with a total bandwidth of 164.4 nm. The quantization error of the theoretical counting method for distance measurement is 1.95 μm by using this swept-laser light source.
In the research of self-mixing interferometry, the displacement measurement was first used to understand the phenomenon and properties of self-mixing interference in the Ti:sapphire crystal fiber laser, and the micron-scale displacement could be measured. However, this swept laser could not achieve a 30-cm distance measurement by self-mixing interferometry in the distance measurement experiment. After investigating the properties of the light source, we found that it is limited by the instantaneous spectral output of multiple wavelengths during the sweeping process. By fitting the spectrum with the equivalent linewidth and comparing it with the theoretical numerical simulation results, the feasibility of using the equivalent linewidth concept to evaluate the measurable distance of the light source is confirmed. With the interpretable experimental results, it is inferred that our Ti:sapphire crystal fiber swept laser only has a distance measurement capability of about 300 μm, which is more suitable for the detection of superficial biological tissues. Therefore, the output properties of our swept laser must be improved if we want to perform longer-distance measurements in the future.

並列關鍵字

Ti:sapphire crystal ； crystal fiber ； broadband tunable laser ； swept laser ； self-mixing interferometry ； distance measurement

參考文獻

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