啁啾式多層堆疊量子點雷射之特性暨其適用於光學同調斷層掃描系統之可行性分析研究

Abstract

本篇論文主要是計對自行設計之啁啾式多層堆疊量子點寬頻雷射的同調性暨其是否適用於生醫領域上在近年來蓬勃發展的光學同調斷層掃描術(OCT)來做探討。為了定量寬頻光源的同調長度，我們架設了一套光纖式、並以程式自動控制的促動器來調整光程差的Mach-Zehnder式干涉儀，並搭配一張高速數位器卡整合成一套同調長度量測系統。為了驗證該量測系統的準確性，我們也用理論來數學模擬了頻譜形式較為簡單的雷射，並與之和實驗結果相比較。 為了提供OCT系統更高解析度的光源，我們嘗試設計了一種多層堆疊結構的量子點雷射(chirped- multilayer quantum dot laser diode, CMQD LD)，期望利用其基態與激發態同時雷射的特性得到高輸出功率的寬頻光源。我們藉有系統的量測來觀察該雷射的表現，包括其同調性，並發現當其操作於高電流時，於高解析度0.1 nm的量測條件下，單一量子態的發光頻譜寬度可達到29 nm，且當中不存在任何3dB凹陷，而該頻譜中心位在1270 nm，我們預估其同調長度可落在50 μm以內。由於縱向模態的存在，我們提出量測干涉圖將是用來評斷一個“寬頻”雷射是否合適於OCT系統應用的一個較直接且實際的方式。另外，於干涉圖中光程差為零的主峰，其半高寬可視為用來檢視雷射寬頻程度的重要依據。最後，我們也將提供CMQD LD在未來可以更適合做為OCT光源的設計思維。

This thesis will focus on discussing the coherence of chirped multilayer quantum dot (CMQD) laser and feasibility study of its application on optical coherence tomography (OCT). To acquire the coherence information of CMQD, we have set up a fiber-based and delay-tunable Mach-Zehnder interferometer. The accuracy of measurement has been confirmed by mathematical simulation on commercial lasers. To achieve an OCT light source with higher axial resolution, we have specially designed a chirped multilayer (N=10) quantum dot laser. The measurement of laser characteristics of CMQD was demonstrated, including interferograms. An extremely broad spectrum of 29 nm without any 3dB dip was achieved. It centered at 1270 nm and was expected to have a coherence length less than 50 μm. Due to the longitudinal modes, we conclude that measuring interferogram is a more direct and more practical way to decide if a “broad spectrum” laser is suitable or not for OCT application. Furthermore, the FWHM of the main peak in an interferogram could be viewed as an indicator for judging the broadness of the spectrum for a broadband laser. Finally, suggestions will be provided in order to improve the performance of CMQD.