掃頻式環腔摻鈦藍寶石晶體光纖雷射之研究

Abstract

掃頻雷射被廣泛應用於光學同調斷層掃描術上，其縱向解析度由光源頻寬所決定。對於直接式掃頻雷射而言，掃頻速度受限於增益介質的光增益；換言之若掃頻速度過快，對於各波長而言，訊號不足以在共振腔中形成雷射便消失。然而R. Huber及J. G. Fujimoto團隊在2006年展示了一種新型態的鎖模雷射-傅立葉域鎖模雷射(Fourier-domain mode locking; FDML)，在不需限縮頻寬的前提下成功打破了雷射掃頻速度的限制，實現超快速OCT成像技術。 因此，本論文將以傅立葉域鎖模雷射作為目標，利用摻鈦藍寶石光纖作為增益介質建構單向環腔式高速掃頻雷射，期望將來只需在此架構上導入色散補償延遲光纖並使掃頻速度與腔長相匹配即為FDML。 作為增益介質的摻鈦藍寶石其螢光頻譜之半高寬可達180 nm，且760 nm的螢光中心波長落於組織散射損耗及水的吸收較小的波段，因此很適合做為光學同調斷層掃描術之光源。本實驗室使用雷射加熱基座長晶法生長出纖心直徑為16 μm且衰減係數僅為 0.017 cm-1之玻璃纖衣光纖波導結構，不僅解決了摻鈦藍寶石因本身的短螢光生命週期及低吸收截面積特性而難達到低閥值輸出的問題，極高的表面積對體積比更有效提高了散熱能力，在1950 mW的幫浦下產生5.87 dB增益。 本論文首先利用光線轉換矩陣模擬元件在環形共振腔內的配置，找出其複數光束參數解使高斯光束能形成穩定雷射共振腔。接著分別將兩種波長可調元件配合光學隔離器在環形共振腔內架構出掃頻雷射。其功率輸出結果與本文所建構之環腔雷射數值模型相符，此模型並可用於未來腔內元件功率損耗可行性之評估。 在雙折射濾波器做為波長可調元件下之雷射輸出波長可由751 nm調至841 nm，達到90 nm的掃頻頻寬。法布里-佩羅濾波器調變之雷射則受限於濾波器本身56 nm的自由頻譜範圍而使可調頻寬為53 nm。此掃描雷射可以產生0.147 nm的瞬時線寬，對應之3-dB靈敏度滑落深度為0.96 mm。此外，當掃頻頻率由1 Hz升至20 kHz時，其雷射輸出頻寬由38 nm降至3 nm。經證實此現象並非雷射本身達到掃頻速率極限，而是濾波器結構中的壓電致動器速度過慢所致。

Wavelength-swept lasers are widely used in optical coherence tomography, in which their axial resolution is determined by the bandwidth of the light source. For direct wavelength-swept lasers, the sweep speed is limited by the optical gain value; in other words, if the sweep speed is too high, the signal disappears in the resonant cavity before even turning into a laser output. The Fourier-domain mode-locked (FDML) laser, which successfully overcame the tuning frequency limit without compromising the bandwidth, was introduced by R. Huber and J. G. Fujimoto in 2006. It achieves the development of ultra-fast OCT imaging technology by overcoming the restriction of laser sweep speed. Therefore, we set the Fourier domain mode-locked laser as an ultimate goal. Ti:sapphire crystal is used as the gain medium to construct a unidirectional ring-cavity wavelength-swept laser. It is expected that in the future, the presented arrangement only requires the addition of dispersion-managed delay fiber in order to create an FDML laser by matching the sweep speed to the cavity length. The fluorescence spectrum of Ti:sapphire has a full width at half maximum of up to 180 nm, with the central wavelength located at 760 nm, which falls into a region called therapeutic window with low tissue scattering and low water absorption, making it suitable as a light source for optical coherence tomography. To overcome the short fluorescence lifetime and low absorption cross section that causes the high threshold power of Ti:sapphire laser, a glass-clad crystal fiber (CF) with a core diameter of 16 μm and low attenuation coefficient of 0.017 cm-1 made by laser-heated pedestal growth (LHPG) method was used in this research. Its high surface area to volume ratio also improves heat dissipation effectively. The Ti:sapphire CF produces 5.87 dB gain at 1950 mW of pumps. The configuration of the ring resonator is analyzed with the ray transfer matrix. The solutions of the complex beam parameters are found so that a stable laser resonator for the Gaussian beam is constructed. Then, a birefringence filter and a Fabry-Perot filter are added to the resonant ring cavity to create wavelength-swept lasers. The power output results are consistent with the numerical model of the ring-cavity laser presented. This model can evaluate the feasibility of any intra-cavity components with different power losses. When the birefringence filter is used as a wavelength tuning element, the laser output wavelength can be adjusted from 751 nm to 841 nm, reaching a total bandwidth of 90 nm. On the other hand, the Fabry-Perot filter's free spectral range, which is 56 nm, restricts the laser performance, resulting in an adjustable bandwidth of 53 nm. The 0.147-nm instantaneous laser linewidth corresponds to a 3-dB coherence roll-off of 0.96 mm. Additionally, the laser bandwidth is reduced from 38 nm to 3 nm as the sweep frequency rises from 1 Hz to 20,000 Hz. It has been established that the slow speed of the piezoelectric actuator in the filter structure, rather than a lack of build-up time for the laser dynamic, is responsible for this phenomenon.