超寬頻掃頻摻鈦藍寶石晶體光纖雷射研究

Study of ultra-broadband Ti:sapphire crystal fiber based wavelength-swept laser

林裕展(Yu-Chan Lin)

指導教授 ： 黃升龍

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2022年)

https://doi.org/10.6342/NTU202203086

摘要

掃頻雷射做為光源已經被廣泛的應用在光學同調斷層掃描術上，由於其掃頻速度極限跟增益介質的輻射生命期及雷射腔體的往返時間有關，因此，多數研究團隊利用輻射生命期為奈秒等級的半導體光放大器為掃頻雷射的增益介質。但由於半導體光放大器的波長多為紅外光區，且可調帶寬為100奈米附近，其所提供的縱向解析度很難達到細胞級解析度。本論文使用可調波長從650奈米到1100奈米的摻鈦藍寶石晶體光纖為增益介質，且摻鈦藍寶石的發射光譜處於低組織散射及水吸收較小的區域，加上其放光頻譜近似高斯，因此非常適用於光學同調斷層掃描術。由於晶體光纖結構擁有高度的表面積對體積的比例，可以有效提高散熱能力，憑藉此摻鈦藍寶石晶體光纖的低信號傳播損耗及高散熱能力，本論文成功實現掃頻重複率1200赫茲且掃頻帶寬為250奈米的超寬頻掃頻雷射。並藉由分析掃頻雷射的鬆弛震盪，可以計算出雷射腔體內的損耗，其數值與理論模擬結果符合。此掃描雷射可以產生0.018奈米的瞬時線寬，其對應當能量降為一半時的同調掃描深度為7毫米。如將這掃頻雷射使用於掃頻式光學同調斷層掃描時，將可以實現1.8微米的縱向解析度。然而，由於本實驗所使用的增益介質為多橫模的摻鈦藍寶石晶體光纖，因此所實現的掃頻雷射輸出模態為多橫模雷射，此特性將嚴重地影響其應用於各種系統的可行性，為此將單模光纖導入雷射腔內以消除高階橫模，本論文成功地實現單橫模輸出的超寬頻掃頻雷射。

關鍵字

掃頻雷射 ； 摻鈦藍寶石晶體光纖 ； 單模雷射輸出 ； 光學同調斷層掃描術

並列摘要

Wavelength-swept lasers (WSLs) have been widely used as light source in optical coherence tomography (OCT). The wavelength tuning speed is mainly limited by the radiative lifetime of the gain media and the cavity round-trip time. Therefore, most researchers use the semiconductor optical amplifier (SOA) as the gain medium of WSL with nanosecond radiative lifetime. The gain bandwidth of a typical SOA lies around 100 nm in the infrared band, its axial resolution is very difficult to achieve cellular scale. Therefore, this dissertation uses Ti:sapphire crystal fiber (CF) with tuning ranges from 650 to 1100 nm as the gain medium. Ti:sapphire CF is very suitable for OCT because its emission spectrum lies in a region of low tissue scattering and low water absorption, as well as approximately Gaussian profile. Since the crystal fiber structure has a high surface area-to-volume ratio, it can effectively improve the heat dissipation capacity. With the low signal propagation loss and high heat dissipation of the CF, the WSL has a tuning bandwidth of 250 nm at a repetition rate of 1200 Hz. The steady-state and pulsed dynamics of the WSL were analyzed, the experimental result is fitted with the theoretical simulation. The 0.018-nm instantaneous linewidth corresponds to a 3-dB coherence roll-off of 7 mm. When using the laser for swept-source OCT, an estimated axial resolution of 1.8 μm can be achieved. However, the gain medium used in this experiment is a multi-transverse mode CF, the output mode of the WSL is also a multi-transverse mode. This characteristic will seriously affect the feasibility of its application. Therefore, the single-mode fiber is introduced into the laser cavity to eliminate the high-order transverse mode, the ultra-broadband WSL with single-transverse mode output is successfully realized.

並列關鍵字

wavelength-swept laser ； Ti:sapphire crystal fiber ； single-mode output laser ； optical coherence tomography

參考文獻

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