Study of Resolution and SNR on Cr4+:YAG Based Optical Coherence Tomography

在以單一光偵測器為量測系統的光學低同調掃描架構中，縱向解析度受光源的中心波長與頻寬所影響，而訊雜比受光源的不穩度所影響。本實驗室所原創之摻鉻雙纖衣晶體光纖，不但具有高頻寬，且為高穩定度的連續光源，因此可用以研製高縱向解析度與高訊雜比之近紅外光學低同調斷層掃描儀(OCT)。 藉由雷射加熱基座生長法，我們已生長出雙纖衣結構的摻鉻釔鋁石榴石光纖，當此光纖以波長為1064 nm的摻鐿光纖雷射作為幫浦光時，可產生中心波長為1.38 um、頻寬為240 nm、不穩度為0.03%的自發輻射光源。我們將其應用於OCT系統中，已可成功檢測出單層肺癌細胞層的干涉訊號，實驗結果顯示肺癌細胞厚度約為6 um。另外，經由色散補償的實驗與模擬，我們已使系統的縱向解析度從5.1 um提升至3.75 um，同時系統的訊雜比為68.3 dB。 此外，我們修正文獻中OCT訊雜比模型而建立適用於各光源的OCT訊雜比模擬模型，根據模擬結果，我們認為無法透過調變參考光強度而達到散粒雜訊(shot noise)極限，同時我們也於調變參考光強度的實驗結果中證明此預測。最後，根據振幅雜訊影響訊雜比的模擬結果，我們認為以單一光偵測器為量測系統的OCT架構中，Cr:YAG光源的訊雜比應已非常接近於此架構的最佳值。For optical coherence tomography (OCT), axial resolution is determined by center wavelength and bandwidth of the light source, whereas signal-to-noise ratio (SNR) is affected by instability of the light source. Our proprietary Cr4+:YAG double-clad fiber is not only broad in bandwidth but also highly stable; therefore, it is eminently suitable for the development of high axial resolution and high SNR near infrared OCT systems. By means of laser heated pedestal growth (LHPG) method, we have successfully developed a Cr4+:YAG double-clad fiber, generating broadband amplified spontaneous emission (ASE) centered at 1.38 um with a bandwidth of 265 nm and 0.03% instability by an 1064-nm Yb fiber pump laser. Using the ASE as light source of the OCT system, we have demonstrated an interference signal of 6-um-thick single-layer human pulmonary adenocarcinoma cells. With dispersion compensation, we have promoted the axial resolution from 5.1 um to 3.75 um with 68.3-dB SNR simultaneously. We have constructed a modified SNR simulation model, which can be applied to any kinds of OCT light sources. Based on experiment and simulation results, we conclude that it is not likely that shot-noise limited system can be reached through adjusting the reflectivity of the reference light. Besides, since our light source is quite stable, we also conclude that our SNR performance is close to theoretical limit in a single-detector system.中文摘要 Ibstract II錄 III目錄 V目錄 VII一章 緒論 1二章 OCT系統理論介紹 3.1 干涉 3.2 低同調干涉術 6.3 空間解析度 9.3.1 縱向解析度 9.3.2 橫向解析度 10.4 雜訊與訊雜比 13.4.1 雜訊 13.4.2 訊雜比 15三章 OCT系統實驗架構與實驗結果 17.1 Cr:YAG 晶體光纖 17.1.1 Cr:YAG 晶體特性 17.1.2 Cr:YAG 晶體光纖製備 20.2 Cr:YAG ASE寬頻光源 24.3 OCT系統實驗架構 28.4 OCT系統實驗結果 29.4.1 一維掃描 29.4.2 二維掃描 32四章 OCT之解析度分析 36.1 色散效應影響縱向解析度之模擬 36.1.1 考慮分光鏡厚度 37.1.2 考慮分光鏡厚度與色散效應 38.2 加入色散補償片的實驗結果與比較 43.3 寬頻光源影響橫向解析度之模擬 46五章 OCT之訊雜比分析 49.1 訊雜比分析模型之建立與修正 49.1.1 訊雜比模型之建立 49.1.2 訊雜比模型之修正 52.2 ASE振幅雜訊量測與分析 55.3 加入振幅雜訊的OCT訊雜比模型與實驗對照 58.4 振幅雜訊影響訊雜比之模擬 60六章 結論與未來工作 62考文獻 6