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Title: | 氧化鋁陶瓷包覆之摻鈦藍寶石晶體光纖材料分析 Materials Analysis of Al2O3 Ceramic-Cladded Ti:sapphire Single Crystal Fiber |
Authors: | 游雅鈞 Ya-Chun Yu |
Advisor: | 黃升龍 Sheng-Lung Huang |
Keyword: | 近紅外寬頻光源,摻鈦藍寶石,單模態晶體光纖,光學同調斷層掃描術, near-infrared broadband light source,Ti:sapphire,single mode crystalline fiber,optical coherence tomography, |
Publication Year : | 2023 |
Degree: | 碩士 |
Abstract: | 摻鈦藍寶石是作為寬頻雷射中常見的增益介質之一,由於摻入過渡元素離子,使它有寬廣的放光頻譜,其螢光頻譜之半高寬為180 nm,中心波長位於760 nm,其介於診療視窗範圍(600~1300 nm)中,且放光頻譜接近高斯,很適合應用於光學同調斷層掃描術(OCT)之雷射光源的應用上,我們將摻鈦藍寶石增益介質製作成光纖形式,解決了摻鈦藍寶石螢光生命週期短及低吸收截面積而難以達到低閥值輸出的問題,光纖之表面積與體積比大,因此擁有良好的散熱效果,再加上摻鈦藍寶石熔點高的性質,因此它能承受高功率的泵浦,而發出高強度、高亮度的光源。
單模晶體光纖可以使雷射光束和放大自發輻射的能量集中在纖芯,其雷射光束的發散度低、聚焦光斑尺寸小,可維持高功率密度的傳送,此外可降低多模干涉,使可調波長雷射的連續可調範圍增加,擁有較寬的放光頻譜,有利於增進OCT的縱向解析度。然而光纖欲達到單模態,其纖芯與纖衣之間的折射率差必須很小,纖芯直徑為16 μm時,其折射率差需小於約3.99∙10^(-4),由於玻璃纖衣與Ti:sapphire纖芯的色散曲線差異大,之間的單模輸出窗口小,因此我們將纖芯與纖衣使用相同的材料,解決不同材料中會有的色散問題,並在纖芯中摻入微量的TiO2,製造出纖芯與纖衣間微小的折射率差。 使用雷射加熱基座長晶法生長出直徑為16 μm的摻鈦藍寶石晶體纖芯,以浸鍍法側鍍上Al2O3纖衣包層後,在1650 ℃下燒結且持溫60小時後所製成的晶體光纖,其固態生長成內層的單晶纖衣厚度可以穩定達到5.34 μm,此厚度為雷射光有99%的能量皆在單晶纖芯與纖衣內傳輸時,單晶纖衣應達到的厚度,然而纖芯橢圓截面上(1 1 ̅0)方向的生長厚度為(0 0 1)方向的1.51倍;在高解析的掃描式電子顯微鏡所拍攝的影像中,其端面上纖芯與纖衣間已不殘存會造成傳輸損耗及散射的孔洞。為了證實我們製備的晶體光纖是否達到單模光纖的纖芯與纖衣之折射率差,因此採用共軛焦顯微鏡系統來量測,其折射率差量測結果為3.50∙10^(-3),由於折射率差的光學量測易產生實驗誤差,因此可嘗試使用雷射穿透光的方式來呈現其場型以精確判斷其模態。從螢光強度與螢光頻譜的量測結果,可驗證1650 ℃的高溫燒結製程已包含將Ti4+還原回Ti3+的退火效益。 Ti:sapphire is a common gain medium used in broadband lasers. Due to the incorporation of transition metal ions, it exhibits a wide emission spectrum. The fluorescence spectrum has a full width at half maximum of 180 nm, with a central wavelength at 760 nm. It falls within the therapeutic window range (600-1300 nm) and exhibits a Gaussian-like emission spectrum, making it well-suited for applications in optical coherence tomography (OCT) laser light sources. We have fabricated the Ti:sapphire gains medium in the form of an optical fiber, thereby addressing the issues of the short fluorescence lifetime and low absorption cross-section of Ti:sapphire, which hindered achieving low-threshold output. Furthermore, optical fibers have a larger surface area-to-volume ratio, that improves heat dissipation effectively. Additionally, due to Ti:sapphire’s high melting point, it can withstand high power pumping, enabling the generation of high-intensity and high-brightness light sources. Single-mode crystal fiber enables the concentration of laser beam and amplified spontaneous emission energy within the core, resulting in low beam divergence and small focused spot size. This characteristic allows for the transmission of high-power density while maintaining high beam quality. Furthermore, single-mode can reduce multimode interference, expand the continuous tunable range of wavelength-tunable lasers, and obtain a wider emission spectrum, which benefits improving the longitudinal resolution in OCT. However, achieving single-mode operation in fibers requires a small refractive index difference between the core and cladding. When the core diameter of single mode fiber is 16 μm, the refractive index difference should be less than approximately 3.99∙10^(-4). Finding glass materials with a refractive index similar to that of the crystalline core is indeed challenging. To mitigate the dispersion issue caused by the disparity in dispersion curves between glass cladding and Ti:sapphire core, we use the same material for both core and cladding. Moreover, we introduce trace amounts of TiO2 into the core to create a slight refractive index difference between them. The core of Ti:sapphire crystalline fibers with a diameter of 16 μm were grown by the laser-heated pedestal growth method, and coated with Al2O3 cladding by dip-coating method. The crystal fibers can achieve an inner layer of single crystal cladding with a thickness of 5.34 μm stably, after sintering at 1650 ℃ and holding at that temperature for 60 hours. For the laser light to transmit with 99% of its energy confined within the single crystal core and cladding of fiber, the thickness of the single crystal cladding should be achieved. However, the growth thickness in the (1 1 ̅0) direction on the elliptical cross-section of the core is 1.51 times that of the (0 0 1) direction. We haven’t found the pores between the core and cladding of fiber from the scanning electron microscopy images with high resolution, which can cause scattering and transmission loss. To verify whether the refractive index difference between the core and cladding of our prepared single-mode optical fiber meets the desired specifications, we employed a confocal microscope system for measurement. The obtained refractive index difference measurement result is 3.50∙10^(-3). Due to the experimental errors caused by the refractive index difference, a laser-based transmission method can be employed to accurately visualize the mode and precisely determine the modal characteristics. The measurement results of fluorescence intensity and fluorescence spectrum confirm that Ti3+ ions are well confined within the core of the fiber. Furthermore, these results validate the annealing benefits of the high-temperature sintering process at 1650 ℃, which effectively reduces Ti4+ back to Ti3+. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/88970 |
DOI: | 10.6342/NTU202303699 |
Fulltext Rights: | 同意授權(全球公開) |
Appears in Collections: | 光電工程學研究所 |
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