以掃頻式摻鈦藍寶石雷射為光源輔以頻譜塑型建立 超高解析度OCT系統

Abstract

光學同調斷層掃描術（Optical coherence tomography; OCT）因其非侵入、高解析度及即時成像特性，在眼科、皮膚及其他生醫領域中應用日益廣泛。為達成視網膜等組織的微細結構觀測，OCT系統若能實現2–3 μm的超高軸向解析度，這等級的解析度將使得先前只能透過病理切片觀察的視網膜內部與角膜內部結構，得以在活體中進行可視化成像，提升診斷與研究價值。 對於眼科成像而言，為避免在眼內介質中的吸收作用，光源具有800 nm的工作波段是必要的。實驗室先前開發掃頻範圍擴及683至933 nm的超寬頻掃頻式摻鈦藍寶石晶體光纖雷射，有望可達1.8 μm的細胞等級解析度。然而，頻譜非高斯分布的光源將會透過旁瓣 (Side-lobe)產生鬼影(Ghost)而侷限點擴散函數(Point spread function)的分辨率。 本研究致力於以此寬頻掃頻雷射為基礎，實際建立超高解析度掃頻式同調斷層掃描術(SS-OCT)。輔以數位訊號處理流程，並加入頻譜塑形(Spectral shaping)，著重於可能影響訊號分辨率的問題做處理，以減少鬼影訊號對解析度之影響。 本論文所架設的掃頻雷射可調波長範圍可達至少204 nm，模擬預期軸向解析度將會有2.69 μm的半高寬。研究中選用高斯函數作為頻譜塑形目標，並透過包絡線分析即時修正干涉條紋，使其趨近高斯分布。此方法亦進一步延伸應用於背景訊號扣除之建立。最終，本研究成功將淺層區域的畫素串音抑制約 50 dB，並達成 2.27 μm 的軸向解析度，符合 2–3 μm 超高解析度之目標。然而，因多模干涉與掃頻光源穩定性不足，深層訊號解析度仍受頻譜起伏與洩漏效應限制。 未來，若實驗室成功開發單模摻鈦藍寶石晶體光纖應用於日後的掃頻式光學同調斷層掃描。可有望改進掃頻雷射的頻譜輸出，並以本論文的頻譜塑形之為基礎做優化，將可實現全深度均一高解析的生醫光學影像系統，為後續OCT臨床應用及組織診斷提供基礎。

Optical coherence tomography (OCT), characterized by its non-invasive, high-resolution, and real-time imaging capabilities, has become increasingly utilized in ophthalmology, dermatology, and other biomedical fields. Achieving an ultrahigh axial resolution of approximately 2–3 μm enables clear visualization of delicate structures within biological tissues, such as retinal and corneal layers, which previously required histological examination. This advancement significantly enhances the diagnostic and research value through in-vivo visualization. For ophthalmic imaging, it is essential that the light source operates near the 800 nm wavelength region to avoid absorption within ocular media. Our laboratory previously developed an ultra-broadband swept-source Ti:sapphire crystal fiber laser, that came with a tuning range from 683 nm to 933 nm, which promises cellular-level resolution down to approximately 1.8 μm. However, the non-Gaussian spectral shape of the source may induce ghost through side-lobe artifacts, limiting the image resolution. This study aims to establish an ultrahigh-resolution swept-source optical coherence tomography (SS-OCT) system based on the setup of this broadband swept laser. By integrating digital signal processing and spectral shaping techniques, we specifically address the issues that influence signal resolution and mitigate ghost signals to enhance imaging quality. The swept-source developed in this thesis exhibits a wavelength tuning range of at least 204 nm, with simulated results predicting an axial resolution of 2.69 μm (FWHM). A Gaussian function was selected as the target function for spectral shaping, and a real-time fringe correction was performed using envelope detection to approximate a Gaussian spectral profile. This method was further extended for use in background signal subtraction. As the results, pixel crosstalk in shallow regions was successfully suppressed by approximately 50 dB, achieving an axial resolution of 2.27 μm and thus fulfilling the goal of 2–3 μm ultrahigh-resolution. However, due to multimode interference within laser cavity and instability in the output, the resolution of deeper signals remained limited by spectral fluctuations and leakage effects. In the future, if a single-mode Ti:sapphire crystal fiber is successfully developed for SS-OCT systems, we expect to significantly improve the stability of the spectral output. Based on the spectral shaping methodology established in this thesis, further optimization could enable uniformly high-resolution optical imaging across all depths, laying a solid foundation for refined clinical OCT applications and detailed tissue diagnostics.