| Summary: | 博士 === 國立中山大學 === 光電工程研究所 === 97 === Cr4+:YAG is an attractive gain medium due to its broad 3-dB emission spectra all the way from 1253 nm to 1530 nm that just cover the low loss window of silica fiber. Such a broadband characteristic offers a potential to develop a broadband amplified spontaneous emission (ASE) light source, optical amplifier, and tunable laser. Growing the Cr4+:YAG bulk crystal into fiber form is necessary for generating larger gain by the better optical confinement of the waveguide structure. For the application of laser, it is superior to bulk crystal for reduced lasing threshold and better slope efficiency due to also the optical confinement effect and better heat dissipation.
Laser heated pedestal growth (LHPG) method has been used to grow high purity crystal fibers due to its crucible free nature. A novel cladding technique, co-drawing LHPG (CDLHPG), was developed to solve core-reduction problem and obtained a double-clad fiber (DCF) structure. But the power fluctuation of heating laser caused large core variation of Cr4+:YAG DCF, and further impaired the optical performance. An innovating method for suppressing the fluctuation of heating power, sapphire tube assisted CDLHPG technique, was developed and combined with power feedback control program. By this technique, 10-μm-core Cr4+:YAG DCFs which meet the adiabatic propagation criterion were fabricated.
By comparing with ASE and optical amplifier experimental data, cross sections of pump absorption, emission, and excited-state absorptions (ESAs) of pump and signal were determined. Pump ESA loss limited the optical performance that could be solve by using cladding pump scheme. A record-low threshold Cr4+:YAG DCF laser with two slopes with respect to absorbed pump power was achieved at room temperature. The threshold pump powers were 2.5 mW and 96 mW in the low and high absorbed pump powers with the same output coupler transmittance of 3.8%, respectively. The slope efficiencies of the fiber laser were 0.4% and 6.9%, respectively. By numerical simulation, 56% slope efficiency can be achieved with a length of 7 cm and an output reflectance of 80%. Our group also firstly used the ASE as the light source of optical coherence tomography, an axial resolution of 3.5 μm was achieved.
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