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 vertical direction. 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 is predicted. 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. / Thesis (Ph.D.)--Physics and Mathematical Physics, 2001.
| Identifer | oai:union.ndltd.org:ADTP/263651 |
| Date | January 2001 |
| Creators | Mudge, Damien Troy |
| Source Sets | Australiasian Digital Theses Program |
| Language | en_US |
| Detected Language | English |
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