Wavelength selection and wide-temperature-range operation of neodymium-doped lasers
Abstract
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.