聲光可調式濾波器於環腔摻鈦藍寶石晶體光纖雷射之研究

Abstract

光學同調斷層掃描術(optical coherence tomography; OCT)為一種非侵入性且具備高解析度的光學成像技術，廣泛應用於醫療診斷與組織分析。其中，掃頻式光學同調斷層掃描術能更快速地獲取組織的深度資訊，其成像品質與掃描速度高度依賴光源的特性，尤其是掃頻雷射的掃描速率與光譜頻寬。傅立葉域鎖模(Fourier-domain mode locking; FDML)技術因能突破傳統掃頻雷射在掃描速率與頻寬之間的限制，成為實現高速且高解析度成像的重要關鍵。本研究旨在建構一套具備傅立葉域鎖模潛力的高速掃頻雷射系統，期望為未來結合光學同調斷層掃描術成像應用奠定基礎。摻鈦藍寶石材料具備中心波長780 nm、3 dB頻寬達180 nm的螢光光譜，落於生物組織吸收較低的波段，適合作為生醫量測系統之光源。本實驗室利用雷射加熱基座長晶法生長之摻鈦藍寶石晶體光纖直徑僅16 μm，衰減係數低至0.017 cm-1，具低傳輸損耗與高表面積對體積比，可有效提升散熱效率，助於達到低閥值且高效率的雷射。 本研究以此晶體光纖為增益介質，透過光學模擬分析，找出能實現穩定雷射共振腔的條件，並成功建立環形共振腔雷射系統，雷射閥值為645 mW，斜線效率為0.11%，並以數值模型進行擬合，計算得腔體總增益達8.4 dB。接著嘗試導入聲光可調式濾波器(acousto-optic tunable filter; AOTF)作為高速掃頻元件，其無機械結構作動、高速響應等優勢，理論上可實現與腔長相匹配之掃頻速率，進而達到傅立葉域鎖模雷射所需的條件。 然而，實際將聲光可調式濾波器整合至環形共振腔時，由於未對聲光可調式濾波器有足夠瞭解，導致系統無法產生雷射輸出。為深入探討其可行性，本研究進一步改以線形共振腔架構進行實驗，對聲光可調式濾波器之出光功率進行分析。同時，以氦氖雷射為光源搭配偏振態控制元件，探討其對繞射效率的影響。實驗結果顯示，若欲實現聲光可調式濾波器的高速掃頻雷射，未來需更精確控制光源偏振特性，並強化濾波器與共振腔系統之整合設計，以降低損耗並提升系統整體效能。

Optical coherence tomography (OCT) is a non-invasive, high-resolution imaging technique widely used in medical diagnostics. Swept-source optical coherence tomography enables faster depth scanning, with performance highly dependent on the light source, especially the sweep rate and spectral bandwidth of the laser. Fourier-domain mode-locking (FDML) technology addresses the trade-off between sweep speed and bandwidth in conventional lasers, making it ideal for high-speed and high-resolution imaging systems. This study aims to construct an FDML laser system, serving as a foundation for future integration with OCT. Ti:sapphire is selected as the gain medium due to its broad fluorescence spectrum centered at 780 nm, with a 3 dB bandwidth of 180 nm, falling within a biologically low absorption window. This makes it highly suitable as a light source for biomedical applications. The Ti:sapphire crystal fiber used in this study was fabricated via a laser-heated pedestal growth (LHPG) method. It features a core diameter of only 16 μm and a low attenuation coefficient of 0.017 cm⁻¹, providing low transmission loss and a high surface-area-to-volume ratio that improves thermal dissipation, thereby enabling low threshold and high-efficiency laser operation. Optical simulations were conducted to determine the conditions for stable laser resonance, and a ring cavity laser system was successfully constructed. The laser threshold was 645 mW, with a slope efficiency of 0.11%, and the total cavity gain was estimated to be 8.4 dB through numerical modeling. An acousto-optic tunable filter was introduced for high-speed wavelength sweeping, which theoretically enables synchronization of the sweep rate with the cavity round-trip time, thus meeting the requirements of FDML. However, practical integration of the acousto-optic tunable filter into the ring cavity failed to generate laser output, due to insufficient understanding of the filter’s behavior. To investigate its feasibility further, a linear cavity configuration was adopted, and the output power of the filter was analyzed. Additionally, experiments using a helium-neon laser and polarization control components revealed the critical role of polarization in achieving efficient diffraction. These findings highlight the need for precise polarization control and improved cavity-filter integration to realize a functional high-speed swept laser system for future optical coherence tomography applications.