Adelaide Research and Scholarship
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|Title: ||A high power scalable diode-laser-pumped CW Nd:YAG
laser using a stable-unstable resonator|
|Author: ||Mudge, Damien Troy|
|Issue Date: ||2001|
|School/Discipline: ||Physics and Mathematical Physics|
|Abstract: ||Some modern laser applications require continuous wave (CW) high power (>100 W), and
diffraction limited performance near 1.064 um. One such laser application with these,
and additional, requirements is gravitational wave interferometry. This thesis will report
the development of a scalable high power laser for this application.
A high-power, single-transverse-mode laser might be produced by intensely pumping the
small mode volume within a stable resonator or by using a resonator that has a large
transverse mode. Intensely pumping a small volume can lead to crystal fracture and large
thermally-induced wavefront aberrations. Using a large transverse mode would also be
difficult if using a stable resonator as these are, in general, not suited to fundamental
modes that have large cross-sectional areas.
Unstable resonators, by comparison, routinely produce fundamental modes that have large
cross-sectional areas. They have been used for decades with high-power, high-gain chemical
or gas lasers and provide efficient energy extraction, good mode discrimination and beam
quality. However, the low gain of Nd:YAG in combination with the high output coupling
associated with unstable resonators would limit the efficiency of such a CW laser. One way
to utilize the properties of unstable resonators while reducing the output coupling, and
thus increase the efficiency, is to use a stable-unstable resonator. These resonators are
stable in one plane and unstable in the orthogonal plane, rather than unstable in both planes.
The required output coupling can be further reduced without degrading the beam quality by
using a Graded Reflectivity Mirror (GRM) as the output coupler. The soft aperturing of the GRM
also eliminates diffraction loss associated with scraper mirrors in hard-edged unstable
resonators, and enhances mode discrimination.
The stable-unstable resonator reported in this thesis is side-pumped by fibre-coupled diode-
lasers and side-cooled. It uses a total internal reflection (TIR) zigzag slab geometry, in
which the zigzag is co-planar with the pumping and cooling. The resonator is stable in the
plane of the zigzag (horizontal) and unstable in the plane orthogonal to the zigzag. In this
configuration the strong thermal lensing in the horizontal direction is averaged out by the
zigzag. The vertical thermal lens is controlled by Thermo-Electric Coolers (TECs) which are
used to adjust the temperature of the bottom and top surfaces of the slab.
To test the performance of the side-pumped, side-cooled laser head it was operated initially
with a stable resonator. Efficient operation was achieved and will be reported. Control of
the refractive index profile (thermal lens) using the TECs on the bottom and top surfaces
results in a vertical thermal lens that could be set to any value between 47 mm and 450 mm.
The thermal lens encountered by the zigzag mode in the plane of pumping and cooling is weak
(horizontal direction) and independent of TEC current. Thus, the thermal lensing in the horizontal
and vertical directions is de-coupled, as is necessary for scalability of the mode volume in the
A travelling-wave (for ease of injection locking) stable-unstable resonator was investigated
using a Fox-Li model, which assumed a greater pump power and mode volume than used for the laser
head presented in this thesis. A strip, n=2 super-Gaussian GRM is shown to be the optimum output
coupler for the stable-unstable laser. Furthermore, it is shown that the output coupling loss
associated with a resonator magnification of -1.3 could be sustained using pump densities below
the crystal fracture limit. Useful operation over a realistic range of thermal lens focal lengths
The validity of the Fox-Li modelling is confirmed using with a standing-wave stable-unstable
resonator. The standing-wave resonator was chosen as it suited the available crystal and pump
power used for the work in this thesis. The GRM reflectivity profile used the minimum commercially
available profile radius. The vertical thermal lens is varied by adjusting the pump power, and
then by adjusting the temperature of the bottom and top surfaces at full pump power. This
demonstrated CW operation of the standing-wave laser with M=1.3 and good beam quality. Good
qualitative agreement with the Fox-Li model of the standing-wave resonator is thus confirmed.
Finally, suppression of the multiple longitudinal modes by injection locking is reported.|
|Advisor: ||Veitch, Peter John|
Hamilton, Murray Wayne
|Dissertation Note: ||Thesis (Ph.D.)--Physics and Mathematical Physics, 2001.|
|Keywords: ||interferometry, wave mechanics|
|Appears in Collections:||Research Theses|
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