Multi-Watt, diode-pumped planar waveguide lasers

Abstract

This thesis reports on progress towards diode-bar pumped crystal waveguide lasers with a high-power, high-quality output.

It is shown that in order to accommodate the highly divergent, non-diffraction limited output from a diode-bar pump source, a waveguide with a relatively large numerical aperture (NA) is required. A selection of suitable waveguide fabrication techniques, including liquid phase epitaxy (LPE), pulsed laser deposition (PLD) and direct bonding, were investigated. A Nd:GGG on YAG waveguide fabricated by PLD was found to have a loss <0.5dB/cm, the lowest so far reported for a PLD layer. The waveguide lased on the four-level laser transition at around 1µm and, for the first time, on the quasi-three-level transition, around 940nm. However, the fluorescence spectra of the guide was significantly broadened with respect to bulk grown crystals. The fabrication technique of direct bonding was found to reliably produce good quality waveguides with losses <0.5dB/cm and in some cases as low as 0.2dB/cm. The waveguides had spectroscopic characteristics of the bulk material and the technique accommodates a wide range of material combinations. Both low and high NA structures can be fabricated and Nd:GGG/YAG (NA=0.75), Nd:YAG/YAG (NA=0.06), Nd:YAG/sapphire (NA=0.46) and Nd:YAG/glass (NA=0.82) direct bonded waveguide lasers were demonstrated.

Pumping the volume of a planar waveguide with the high average power of a diode-bar can lead to a high inversion density and a high thermal load. This novel pumping regime was investigated experimentally and theoretically modelled. In parallel spectroscopic measurements by Dr. Stephan Guy at the University of Lyon, the Auger upconversion rate in Nd:YAG was measured to be 7 x 10³s^-1; significantly lower than had been reported previously. Modelling studies have shown how this upconversion, and the saturation of the absorption, can affect the gain available in an intensely pumped system. The model, and the value for the Auger rate, was consistent with experimental amplifier measurements. The associated heat load for such a system was also modelled to compare the performance of a planar waveguide with bulk slab and bulk rod-shaped gain media. The model showed that the planar waveguide maintains the thermal advantages of a slab geometry over that of a rod, although slightly higher temperature rises and slightly lower thermal stress fracture limits were predicted in a planar waveguide compared to an identical sized, but uniformly-pumped, bulk slab.

Advances in coupling diode-bar pump lasers to planar waveguides have been made. A 20W fibre-lensed diode-bar, coupled with bulk cylindrical lenses, end-pumped an 80µm Nd:YAG planar waveguide and gave a maximum output power of 6.2W. The device had an optical-to-optical conversion efficiency of 31 % and M2 output beam quality of 3 x 140 in the guided and non-guided directions respectively. More compact, rod-lens focussing successfully coupled this pump source into sub-10-µm waveguides, and an output power of 3.7W was obtained from an 8µm Nd:YAG waveguide laser. A first demonstration of proximity coupling a diode-bar to a waveguide, with no intervening optics, gave a coupling efficiency of ~90% into an 8µm Nd:YAG/sapphire direct bonded waveguide.

Techniques for improving the quality of the spatial mode output from the waveguide laser have been considered. In the guided direction, a metal overlaid coating gave a slight improvement in the laser mode quality and also led to a polarised output. A better solution was found by designing the first double-clad waveguides. Fabricated by direct bonding a five-layer, Yb:YAG/YAG/sapphire waveguide structure was proximity coupled to a 22W diode-bar pump laser. It gave a maximum output power of 2.2W in a side-pumped configuration. In the guided direction this laser had an output divergence with an M2 very close to 1, consistent with fundamental laser mode operation, Spatial mode selection occurred due to the confined nature of the gain region, and not the cladding-pumping of a single mode core. In the non-guided direction initial experiments aiming to improve M2 by using an unstable resonator design gave promising results but further work is required in this area.

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Identifiers

ORCID for David Shepherd: orcid.org/0000-0002-4561-8184

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