以高對比光柵垂直共振腔面射型雷射開發之高速偏振靈敏光學同調斷層掃描系統

Abstract

偏振靈敏光學同調斷層掃描術(Polarization sensitive optical coherence tomography, PS-OCT)為一種提供高速的組織斷層甚至是三維影像的非侵入式影像技術，解析度可以達到數個微米，並且可以透過分析樣品的背向散射光的偏振態來提供影像額外的對比機制。在過去的25年間和PS-OCT有關的研究呈現逐年提升的趨勢，目前也已經廣泛的應用在生物醫學領域中，並且在眼科的應用上也已經證實具有臨床使用的價值。 隨著技術演進與發展，基於掃頻式光學同調斷層掃描術(Swept source OCT, SS-OCT)架構之SS-PS-OCT技術，由於其具有較高之成像速度以及成像深度，具有較高之未來臨床發展性。但SS-OCT系統所需使用之掃頻雷射(Wavelength-swept laser)其價格較高，也因此造成整體SS-OCT或SS-PS-OCT系統成本較高。若希望將這項技術普及到大型醫學中心以外的地方，降低掃頻光源的成本就會是相當重要的一環。作為一個高性能的掃頻光源，HCG-VCSEL (High-contrast grating vertical-cavity surface-emitting laser, HCG-VCSEL)因為製程的關係能夠有效的降低成本。HCG-VCSEL為利用高對比光柵作為共振腔頂部反射鏡的微機電系統可調垂直共振腔面射型雷射(Micro-electromechanical system tunable vertical-cavity surface-emitting laser, MEMS-VCSEL)。由於高對比光柵的特性，HCG-VCSEL在光源輸出光的偏振控制上具有一般MEMS-VCSEL所沒有的優勢，因此本篇論文中我們搭建了一套PS-OCT系統來評估此HCG-VCSEL於SS-PS-OCT技術應用上之可行性。此篇論文中所使用的HCG-VCSEL掃頻式光源(Laboratory prototype, Bandwidth10, Inc.)之掃頻速度為250 kHz，中心波長為1059 nm，頻寬為40 nm。我們同時準備了另一台商用光源(AXP50124-3, Axsun, Inc.)來作為與HCG-VCSEL之比較對照基準，此商用光源為使用分散式布拉格反射器(Distributed Bragg reflector, DBR)作為共振腔反射鏡的掃頻式光源，掃頻速度為200 kHz，中心波長為1052 nm，頻寬為88 nm。 PS-OCT根據不同的傳輸技術(光纖、自由空間)、輸入偏振態、偵測的變數數量而產生許多分支。於驗證HCG-VCSEL於SS-PS-OCT可行性所需之SS-PS-OCT架構設計上，我們採用基於光纖傳輸的單圓偏振態輸入的架構。最大的優勢在於其簡單的光機構設計，只要對實驗室原有的一般OCT系統做些許的改裝就可以量測到樣品在一般OCT中所無法取得的樣本偏振特性。在實驗中我們除了會量測與光源相關的特性作為比較的參考，包括軸向解析度、靈敏度、靈敏度滾降與偏振態均勻度。並針對手指指甲以及雞胸肉的影像來進行影像品質的分析。也因為是架設PS-OCT系統，所以可以額外計算包括相位延遲以及光軸方向來提供基於樣品雙折射特性的結構影像。

Polarization sensitive optical coherence tomography (PS-OCT) is a noninvasive imaging technique that can provide high-speed cross-sectional and three-dimensional imaging with a micrometer scale resolution. Also, PS-OCT can provide novel contrast mechanisms from the measurement of polarization state of backscattered light from sample. The number of PS-OCT publications keep on rising in past 25 years. Currently, PS-OCT has been applied in many biomedical fields, and it has been proven to have clinical value. Along with the technology evolution and development, SS-PS-OCT based on SS-OCT (Swept source OCT) has had a superior imaging speed and imaging depth, and because of that, it exhibits a higher potential future in clinical use. However, the price of wavelength-swept source used in SS-OCT has also increased the cost of SS-OCT system and SS-PS-OCT system. As a high performance swept source, HCG-VCSEL (High-contrast grating vertical-cavity surface-emitting laser) effectively reduce the cost due to its manufacturing process. HCG-VCSEL is a MEMS-VCSEL (Micro-electromechanical system tunable vertical-cavity surface-emitting laser) which uses a high-contrast grating as the top mirror of the laser cavity. Because of its high-contrast grating, HCG-VCSEL has the advantage in polarization control which MEMS-VCSEL lack. Therefore, in this thesis work, we developed a PS-OCT system to assess the feasibility of the HCG-VCSEL in technology application of SS-PS-OCT. The HCG-VCSEL used in this experiment provided an imaging speed of 250 kHz, center wavelength of 1059 nm and tuning range of 40 nm. Also, a commercial light source (AXP50124-3, Axsun, Inc.) is used as the comparison. This commercial light source use DBR (distributed Bragg reflector) as the mirror of the laser cavity and provided an imaging speed of 200 kHz, center wavelength of 1052 nm and tuning range of 88 nm. PS-OCT schemes differ in terms of optical technology (fiber optics vs. bulk optics), number of input polarization state, number of variables. As for the scheme design of SS-PS-OCT system that can verify the feasibility of HCG-VCSEL in SS-PS-OCT, we used the one that is based on single circular input state system in fiber optics. The biggest advantage of single circular input state PS-OCT lies in the simplicity of its optomechanical design, allowing us to measure the sample’s polarization property, which conventional OCT system is unable to get, with only few modifications to existing OCT system in our lab. We measured the characterization of the light source, including axial resolution, sensitivity, sensitivity roll-off and uniformity of polarization state to make a comparison between these two light sources. OCT imaging on nails and chicken breast were analyzed for imaging quality. Plus, phase retardation and optic axis orientation could be calculated and became structural images based on birefringent sample.