- 學位論文
石墨烯鎖模摻鉻釔鋁石榴石晶體光纖雷射
Graphene Mode-Locked Cr4+:YAG Crystal Fiber Laser
摘要
光學同調斷層掃描術擁有非侵入式、即時與高空間解析度之優點，頻寬愈寬，中心波長愈短，有越高的縱向解析度。Cr4+:YAG可產生波長為1.3 μm至1.6 μm的能量增益，其中心波長附近有一水吸收波段，作為OCT系統掃描時，可偵測人體皮膚中含水量分佈，對醫學應用有著極大的潛力。石墨烯由於其特殊的光電特性，近年來成為被動鎖模雷射中常見的飽和吸收體，涵蓋範圍從可見光至近紅外光波段，本論文將石墨烯與Cr4+:YAG晶體光纖搭配，有潛力製作超寬頻鎖模雷射，未來加入調頻元件，有機會成為掃頻式OCT鎖模雷射光源，能得到更強的光干涉訊號與更多的生物影像資訊。本實驗室使用雷射加熱基座長晶法成功生長出Cr4+:YAG DCF (雙纖衣晶體光纖)，其良好的散熱特性能承受較高的激發光源功率，並維持一定之增益。利用半球型外部共振腔架構以1064 nm單模二極體雷射幫浦下，量測連續輸出雷射斜線效率達8.98%，雷射閾值僅有38.1 mW。 我們使用的石墨烯為中研院原分所李連忠博士實驗室所生長。我們利用電子槍介電質蒸鍍技術製作1000 ~1600 nm之寬頻抗反射基板，由李博士實驗室將單層石墨烯轉移其上，作為穿透式石墨烯鎖模元件。我們在半球型外部共振腔插入自製石墨烯鎖模元件，距離晶纖輸出端約1 mm處，連續輸出雷射斜線效率為3.42%，閾值為156 mW。移動石墨烯距離光纖輸出端小於100 μm，連續雷射斜線效率為10.5%，閾值為158 mW。這是因為貼近石墨烯與晶體光纖輸出端距離，能大幅增加石墨烯上的功率密度，使石墨烯的吸收被飽和而降低損耗。雖未觀察到鎖模輸出，但藉由此實驗我們仍證實了石墨烯的飽和現象，沒有達到鎖模輸出的原因推測是石墨烯在強光照射下產生氧化。藉由MATLAB模擬連續輸出雷射，擬合雷射功率與殘餘幫浦功率，得到石墨烯光學參數，其飽和吸收損耗為1.88%，不可飽和吸收損耗為0.38%。我們也模擬了鎖模雷射，假設可避免石墨烯氧化，發現在幫浦功率500 mW且石墨烯上之模態直徑小於20μm時，可達成鎖模。貼近石墨烯與晶體光纖輸出端，基本模態直徑為12 μm之高斯光束，模擬得到連續雷射閾值為153 mW，增加幫浦功率大於250 mW時，將可產生鎖模現象。
並列摘要
Optical coherence tomography (OCT) plays an important role in medical applications due to its high longitudinal resolution and noninvasive detection. Higher longitudinal resolution can be obtained with wider bandwidth or shorter center wavelength. The Cr4+:YAG crystal has a broadband emission from 1.3μm to 1.6 μm with a water absorption band located near 1.4 μm. While applied in an OCT system, it can be used to detect the water distribution in human tissues and has great potential in medical applications. In recent years, graphene becomes a popular saturable absorber having broadband optical saturable absorption from visible to near infrared due to its unique zero-gap property. This study combines monolayer graphene with Cr4+:YAG crystal fiber in order to demonstrate an ultra-broadband mode-locked laser. In the future, we have chance to insert tuning optical element to develop mode-locked swept-source OCT. Such OCT system can get stronger interfering signal power, and thus get more information from bio-images. In our Lab., we have successfully fabricated Cr4+:YAG double-clad crystal fibers (DCF) with the co-drawing laser-heated pedestal growth method. Due to its good heat dissipation properties, Cr4+:YAG can endure high pump power and maintain its gross gain without prominent degradation. Hemispherical external-cavity laser was demonstrated by pumping by a 500 mW 1064-nm diode laser and the lasing threshold was 38 mW and the slope efficiency was 8.98%. The graphene we used was prepared by Dr. Lain-Jong Li’s Lab. of the Institute of Atomic and Molecular Sciences, Academia Sinica. A broadband AR coating substrate from 1000 to 1600 nm was fabricated by using dielectric E-gun deposition method. The monolayer graphene was transferred by Dr. Li’s Lab. on the substrate to form a transmitting graphene saturable absorber (GSA). Inserting the home-made GSA with 1-mm distance to Cr:4+:YAG output end in hemispherical external-cavity laser, the slope efficiency was 3.4% and the lasing threshold was 156 mW. Moving GSA close to Cr4+:YAG output end with less than 100-μm distance, the slope efficiency was 10.5% and the lasing threshold was 158 mW. This was due to the stronger saturation of the absorption of the GSA when it was placed closer to the DCF end face. No mode-locked output was observed, which could be due to the oxidation of the GSA under strong pump power. We use MATLAB to simulate CW laser by fitting laser power and residual pump power. The results shows saturable loss of graphene is 1.88%, and non-saturable loss of graphene is 0.38%. We also simulated mode-locked laser by assuming using a perfect GSA without oxidation. In the simulation, mode-locked laser output was obtained with 500-mW pump power when the mode diameter on graphene was less than 20 μm. If graphene moved close enough to Cr4+:YAG output end, we can get the smallest fundamental mode diameter of 12 μm. The simulation shows a CW laser threshold of 153 mW, and a mode-locked laser threshold of 250 mW.