High-average-power planar waveguide lasers

Abstract

Reported in this thesis is some progress towards high-average-power diode-pumped planar waveguide lasers. As a format of the laser active medium, planar waveguides take advantage of their extreme slab geometry, which is compatible with that of high-power diode lasers, offer a great degree of versatility of the pump arrangement, have excellent thermal handling capability, and deliver high optical gains per unit pump power. Pumped with different schemes, three kinds of planar waveguides are investigated herein: direct-bonded double-clad planar waveguides, ion-exchanged tapered waveguides, and thick films fabricated by pulsed laser deposition, of which a double-clad planar waveguide produced laser output power of up to 58W.

The tapered waveguide structure allows diode pumping at its multimode broad channel end and ensures fundamental-mode laser output at the single-mode channel end, with adiabatic operation achievable through careful design of the interconnecting taper. Linear and parabolic taper shapes are compared. The two types of waveguides expanding to various widths over the same lengths were fabricated on the same Nd:BK7 substrate and characterised with Ti:sapphire pumping. The linear tapers show superior operation for larger guiding sizes up to taper widths of 250µm, and therefore are more compatible with high-average-power broad-stripe diode pumping.

Double-clad planar waveguides, fabricated by direct bonding YAG and sapphire, have features that are very attractive in this work: they are ideally suited to high-power diode bar/stack pumping owing to their high NA (0.46) slab-like geometry; and they are shown to robustly maintain single-mode operation by gain mode selection. Both diode bars and stacks were used to side-pump a 30µm double-clad Nd:YAG waveguide. For diode-bar pumping, an extended cavity was used to control the output spatial mode in the non-guided axis. Multimode output power larger than 10W was obtained from the waveguide with a slope efficiency of 56%, which was reduced to 33% when the external cavity was optimised for beam quality, obtaining 2M values of 1.1 (in the guided axis) by 2.8 (in the non-guided axis). For diode-stack pumping, 58W of output power was obtained from a monolithic cavity with a slope efficiency of 62%. With an extended cavity, 20W of output power and a minimum 2M value of ~7 in the non-guided axis were obtained, although the optimum results were not found as a result of waveguide damage. Further designs are discussed for power scaling to very high powers.

PLD planar waveguides are investigated as a flexible alternative in high-power planar waveguide laser systems. As an initial demonstration, a slope efficiency of 11.2% was obtained from a diode-stack-pumped 50µm Nd:GGG PLD waveguide. The effect of self-imaging was observed with a 27µm Nd:GGG PLD waveguide.

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Identifiers

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

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