Wavelength selection and wide-temperature-range operation of neodymium-doped lasers
White, Andrew Lee
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This thesis presents research results in two general areas of the solid-state laser technology commonly used in defence applications. The first area uses volume Bragg gratings (VBGs) as cavity mirrors, giving wavelength selection, linewidth reduction and improved frequency doubling efficiency. The second area is in extending the operating temperature range of diode-pumped Nd-doped pulsed lasers through the use of VBGs, external-cavity absorber-locked diode lasers and enhancements of pump chamber efficiency. For a laser-diode end-pumped Nd:YVO4 solid-state laser, a VBG laser cavity, with output powers of up to 6.2W is demonstrated at 1064 nm. With a conversion efficiency of 0.4 and M2 of 1.2, the laser linewidth is reduced by a factor of 16, to 72 pm, compared to a conventional dielectric mirror cavity. Intra-cavity losses due to the VBG are shown to be 2%. The 1064nm VBG cavity is then Q-switched using an AO modulator to generate peak powers of 2.3 kW. The second harmonic generation in a MgO:PPSLT crystal produced a 20% increase in second harmonic power, compared to a cavity with a conventional dielectric mirror. The improvement is attributable to the reduced linewidth. A similar configuration, but using a VBG with 98% reflectivity at 1342 nm, generates up to 3.8W of output power. The use of two VBG mirrors in a single cavity, further narrows the linewidth, allowing for output powers up to 2.3 W, in a single longitudinal mode. In-cavity heating of the VBG is found to give a reduction in reflectivity, a shift in the centre wavelength and increased linewidth. This is both modelled and experimentally demonstrated. For enhanced pumping of high-energy Q-switched Nd:YAG zigzag-slab lasers, two techniques of externally locking the wavelength of the QCW laser diodes are demonstrated. A chirped 808nm VBG locks the wavelength of a newly developed chirped quantum-well laser-diode array, without the need for fast-axis collimation. Locking over a temperature range of 50 C is demonstrated, with a predicted locking range of 70 C. As an alternative to VBG locking, a laser diode bar locked at 885nm is also demonstrated by using a thin Nd:YVO4 absorber. Filtered feedback locks the spectrum into peaks, which are in the Nd:YAG absorption bands over a 45 C range. In a parallel effort, a pump laser array configuration that increases the overall absorption path length from 1 to 3 cm in a side-pumped zigzag slab is demonstrated. By optimising the bounce lengths of the pump light and the inclusion of multi-wavelength laser bars, operation up to 80 C is demonstrated with an absorption efficiency of 80%. By adjusting the diode drive pulse duration, a Q-switched laser using this pump head operates from -40 to +60 C with a constant energy of 60 mJ. The operational temperature range of this configuration is shown to be over 120 C.