| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 102 === 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.
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