摻鈦藍寶石晶體光纖被動鎖模雷射研究

Abstract

本研究利用雷射加熱基座生長法製備的摻鈦藍寶石晶體光纖產生鎖模脈衝，受益於波導結構與優異的散熱效率，可助於提高幫浦效率、降低傳播損耗與更好的模態重合，並具有650 奈米至1100奈米的寬廣頻譜且短螢光壽命時間，使摻鈦藍寶石晶體光纖成為高效率雷射與製作鎖模的理想選擇。然而，當前製備的摻鈦藍寶石晶體光纖為多模光纖，架構鎖模雷射時，我們發現多橫模雷射對於鎖模環境具有諸多影響，因此對橫向與縱向模態進行探討並嘗試解決，並搭配步階式傅立葉轉換法，模擬鎖模脈衝於腔體內的演化。

通過調整腔體減少橫向模態，與更換較小N.A.值之非球面透鏡，達到空間濾波效果，皆於幫浦功率1.5 瓦時成功實現鎖模，雷射重複頻率達256 百萬赫茲，且於幫浦功率1.25瓦時量測到Q 開關鎖模雷射，由於多橫模會導致多縱模，進而抑制鎖模脈衝產生。因此隨後加入空間濾波用之光纖於雷射腔體中，成功實現單橫模的輸出模態，且於幫浦功率1.06 瓦時產生鎖模脈衝，此法有效地避免高階橫向模態對鎖模脈衝產生的影響，同時提高鎖模雷射的穩定，此成果為文獻中利用雷射二極體產生之最低閥值的摻鈦藍寶石鎖模雷射。

未來可通過時間拉伸脈衝，作為掃頻式光學同調斷層掃描儀之掃頻光源，除提升解析度外，利用脈衝的超高重複頻率特性，達到高成像速度，對於生物醫學影像等領域具有重要的應用價值。

This study utilizes laser-heated pedestal growth (LHPG) method to fabricate titaniumdoped sapphire crystal fiber for generating mode-locked pulses. Benefiting from its waveguide structure and excellent heat dissipation efficiency, the fiber exhibits improved pump efficiency, reduced propagation losses, better mode matching, and a wide spectrum ranging from 650 nm to 1100 nm, along with a short fluorescence lifetime. These characteristics make Ti:sapphire crystal fiber an ideal choice for high-efficiency lasers and mode-locking applications. However, the currently prepared fiber is multimode, and when constructing a mode-locked laser, we found that the presence of multi transverse modes has various impacts on the mode-locking environment. Therefore, we investigate and attempt to address the issues related to transverse and longitudinal modes, employing a split-step Fourier transform method to simulate the evolution of mode-locked pulses within the cavity.

By adjusting the cavity to reduce transverse modes and replacing the aspherical lens with a smaller numerical aperture value to achieve spatial filtering effects, successful mode-locking is achieved at a pump power of 1.5 W, with a laser repetition frequency of 256 MHz. Furthermore, a Q-switched mode-locked laser is measured at a pump power of 1.25 W. Multi transverse modes can lead to the generation of multiple longitudinal modes, thereby suppressing the formation of mode-locked pulses. Consequently, a fiber with spatial filtering capabilities is introduced into the laser cavity, resulting in the achievement of single transverse mode output, and mode-locking pulses are generated at a pump power of 1.06 W. This approach effectively avoids the influence of higher-order transverse modes on the formation of mode-locked pulses and improves the stability of the mode-locked laser. This achievement represents the lowest threshold for Ti:sapphire crystal mode-locked lasers pumped by laser diodes in the literature.

In the future, by utilizing pulse stretching techniques, the mode-locked laser can be used as a swept-source optical coherence tomography system, which not only enhances resolution but also utilizes the ultra-high repetition rate of the pulses to achieve high imaging speed. This has significant application value in areas such as biomedical imaging.