| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 105 === Ti:sapphire crystal is a widely used material. It has a wide emission spectra with center wavelength located around 780 nm and a 180-nm, 3-dB bandwidth. Due to its broadband emission spectra, there are various applications in wavelength tunable lasers and mode-locked lasers. Since its emission wavelength also sits in the region where there is low tissue absorption, there are also applications in biological imaging technologies, such as optical coherence tomography (OCT).
To solve the two drawbacks of Ti:sapphire crystal: low absorption cross section and low fluorescence lifetime, Ti3+:Al2O3 single-cladding crystal fiber was grown using the LHPG method. As the laser gain material, crystal fiber with length of 24 mm was used. The single crystal core diameter was 16 μm and the glass cladding outer diameter was 320 μm. The Ti3+ doped concentration was 0.049 wt.%, with regarding to a 0.075 dB/cm attenuation coefficient, which is the lowest value recorded for Ti:sapphire waveguide structures. After end face grinding and polishing, the crystal fiber was coating with dielectric coating using thin film E-gun deposition system to form the required optical cavity structure.
By using a 520-nm LD as the pumping source, intra-cavity and external-cavity lasers were constructed. Due to a gain guiding effect, the output mode is near fundamental mode. Under intra-cavity setup, with a 20% output coupler, the laser efficiency and laser threshold were 24.9% and 140.5 mW, respectively. The maximum output power at 1W pumping was 215 mW, achieving high slope efficiency and low threshold simultaneously. Under external-cavity setup, with high reflectance output coupler, the laser efficiency and laser threshold were 18.6% and 123.2 mW, respectively.
With wavelength tuning elements, such as inserting a birefringent filter or using a grating output coupler, tunable laser was achieved. Under birefringent filter setup, the tuning was not continuous. The wavelength tuning range was 710-860 nm, with a 150-nm width. Range of output power exceeding 50 mW was 130 nm, providing sufficient output power for various applications. Under grating setup, continuous wavelength tuning was achieved. Continuous range was 693.4- 876.5 nm, with a 183.1 nm width. With 3-dB tuning bandwidth of 143 nm, the broadband emission properties of Ti:sapphire crystal fiber were demonstrated .
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