玻璃纖衣摻鈦藍寶石晶體光纖之發展與應用

Development and Applications of Glass-clad Ti:Al2O3 Crystal Fiber

王世昌(Shih-Chang Wang)

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(2016年)

https://doi.org/10.6342/NTU201602939

摘要

摻鈦藍寶石為常見的寬頻螢光與雷射材料。其常使用於寬頻可調波長雷射、鎖模雷射與生物量測系統，如:光學斷層掃描儀系統與受激拉曼光譜儀等。然而，摻鈦藍寶石有兩個相當大的缺陷:低吸收單位截面積與螢光生命週期短。因此摻鈦藍寶石之幫浦光飽和功率相當的高，以達到低閥值之雷射輸出相當不易。因此摻鈦藍寶石塊材無法以在低於雷射閥值下，以強寬頻光輸出，難以直接以自發放大輻射做運用。
以雷射加熱基座生長法成功的生長出直徑小於20微米的摻鈦藍寶石晶體光纖。由掃描式電子顯微鏡與穿透式電子顯微鏡影像可得知在摻鈦藍寶石波導結構中的最低傳播損耗來自於高品質晶體、表面平緩且均勻的纖心，其有效的降低萊利散射(Rayleigh scattering)。玻璃纖衣提供了一個對於環境變化不敏感的結構，且提供了對幫浦光與訊號光有良好的重合，相對於塊材結構而言，其有效的提升光學效率。
以玻璃纖衣摻鈦藍寶石晶體光纖做為寬頻且高亮度自發輻射增益光源。以一個雙向路徑的架構，其背向自發輻射增益之功率增益相當的顯著。以2顆1 W波長為520 nm之半導體雷射做為激發光源下，其自發放大輻射功率可達42 mW。其光源頻寬163.8 nm且高斯分布形貌之輸出頻譜與77.59 W∙mm-2sr-1之亮度相當適合於光學斷層掃描儀系統之運用。其系統需要提供高空間解析度與低影像干擾之光源。
以內腔式摻鈦藍寶石雷射做為運用，其高效率雷射之波導結構可在本論文中證實於波長532 nm之固態雷射以及波長520 nm之半導體雷射做為幫浦光下。因增益波導效應，其輸出模態近乎為單模輸出。以半導體雷射做為幫浦光源下，吸收雷射閥值與吸收光轉換效率分別為118.2 mW與29.6%。且在1 W幫浦下，其最高的輸出功率為215.8 mW。根據我們所知，這是第一個達到低閥值且高輸出效率之半導體雷射激發摻鈦藍寶石雷射。
以外腔式摻鈦藍寶石雷射做為使用，其雷射閥值為低於文獻最低值為37.3 mW。以17.8%輸出耦合鏡時，其雷射閥值與雷射轉換效率分別為123.2 mW與18.6%。寬頻波長可調可以雙折射率濾波片與刀鋒光柵達成。以雙折射率濾波片做為波長可調元件時，其輸出功率超過50 mW的頻寬可達125 nm。以光柵為可調元件時，光柵提供的高色散特性有助於光譜選擇，使其可達到180 nm波長連續可調。對於摻鈦藍寶石廣泛運用於寬頻光源，寬頻可調及鎖模雷射之領域，玻璃纖衣晶體光纖應可有機會在未來具有重要的地位。

關鍵字

晶體光纖 ； 摻鈦藍寶石 ； 寬頻光源 ； 光纖雷射 ； 寬頻可調雷射

並列摘要

Titanium (Ti):sapphire is one of the most commonly used materials in broadband fluorescence and lasers. This material has various applications such as in the construction of tunable lasers, mode-locked lasers and biological measurement systems (e.g., optical coherence tomography and Stimulated Raman spectroscopy). However, Ti:sapphire possesses two inherent drawbacks: a low absorption cross-section and a short lifetime. Therefore, the pump saturation intensity of Ti:sapphire is high, and lasing at a low threshold is difficult. Thus, Ti:sapphire bulks do not exhibit strong broadband emission below the laser threshold, and can hardly be directly used as an amplified spontaneous emission light source.
A Ti:sapphire CF with a core diameter of less than 20 μm was fabricated using a laser-heated pedestal growth method. Both SEM and TEM images indicate that the lowest propagation loss of Ti:sapphire waveguides, which can reduce the Rayleigh scattering loss, could be attributed to a high quality crystalline core, smooth perimeter, and uniform diameter. Glass-clad structures provide non-sensitive for environmental variations and useful overlapping of pump and signal lights, which improves the optical efficiency compared to a bulk structure.
Broadband and high-brightness ASE light sources were generated using a glass-clad Ti:sapphire CF. Utilizing a double-pass setup, backward ASE power increased substantially. Up to 42 mW of ASE power was generated using two pieces of 1-W 520-nm LDs as the pump source. The 163.8-nm near-Gaussian spectrum and 77.59-W∙mm-2sr-1 radiance rendered the light source eminently suitable for OCT applications in which high spatial resolution and low-image-pixel crosstalk are necessary.
In applications such as an intra-cavity Ti:sapphire laser, the highly efficient structure in present research was pumped by both a solid-state 532-nm laser and a 520-nm LD. Because of the gain guiding effect, the pattern of the lasing mode was close to the fundamental mode. The threshold and slope efficiency with a LD pump with respect to the absorbed pump power were 118.2 mW and 29.6%, respectively. At 1-W pump power, the maximum output power was 215.8 mW. To our knowledge, this is the first time that such a high slope efficiency has been achieved with a Ti:sapphire laser with a LD pumps at such a low threshold.
Using an external cavity Ti:sapphire CF laser, the lowest threshold achieved for this laser was 37.3 mW. With a 17.8% output coupling, the threshold power and slope efficiency were 123.2 mW and 18.6%, respectively. Broad wavelength tuning was achieved with a birefringent filter and a blazed grating. With the birefringent filter as a wavelength-tuning element, the wavelength associated with output power levels exceeding 50 mW was 125 nm. With the blazed grating as a wavelength-tuning element, the strong dispersion characteristics of the grating facilitates spectral mode selection and continuous wavelength tuning throughout the 180-nm coverage.
With widespread applications of Ti:sapphire in the area of broadband light sources, tunable and mode-locked lasers, the glass-clad crystal fibers should have a good chance to play an important role in the future.

並列關鍵字

crystal fiber ； Ti:sapphire ； broadband light source ； fiber laser ； broadband tunable laser.

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