The rotating cavity laser

Abstract

This thesis describes a new technique for mitigating the detrimental thermal phenomena that often limit the power scaling potential of solid state lasers. The unavoidable heating effect that arisesfrom the quantum defect leads to a degradation in beam quality, reduced efficiency and, eventually catastrophic failure. However, lasing processes occur on a faster time scale than those associated with heat flow through a typical laser gain medium. This is made use of whenever a laser is operated in a QCW mode, the laser is operated within an adiabatic window then turned off whilst the gain medium cools. This adds a constraint to the maximum duty cycle of the laser and thus reduces the average power output. Alternatively to separating the two processes in time they can be separated in space.

The thermal process can be separated from lasing with the introduction of motion to the system. By passing a collinear pump beam and laser mode through a rotating periscope placed in front of a gain medium the lasing spot can be moved into cold material before heat is able to flow. We call this arrangement the Rotating Cavity Laser (RCL). Unlike previously demonstrated solid state lasers which make use of motion, the RCL keeps the gain medium stationary. This allows it to be heat sunk directly, simplifying the mechanical arrangement.

Within this thesis the first results from an RCL are presented with theoretical predictions of the influence motion has both on the lasing and thermal properties of the system. Attention is paid to the regime where stimulated emission is negligible and the losses due to motion are therefore greatest. The analysis of this regime allowed the threshold under motion to be calculated and the approach was verified experimentally.

The RCL architecture allowed 120 W of 1064 nm light to be generated from a single end pumped Nd:YAG ceramic slab. The presence of moving intracavity components was found to have consequences for the stability of the power output. When producing 72 W the output power varied with a standard deviation of 2.8%, importantly this variation was cyclic suggesting it would be straightforward to correct by modulating the pump source. Whilst excellent beam quality was found at low powers the M2 became poor as the pump power increased. At output powers less than 51 W the beam quality was found to be constant over a rotation period. It is postulated that the increase in M2 at high pump powers, as well as the increase in variation in beam quality over a rotation period, is partly due to the presence of a thermally induced wedge compromising the alignment of the resonator.

A number of experiments are also presented that demonstrate the effectiveness of introducing motion as a method to reduce the thermal load within a laser gain medium. Losses due to stress induced birefringence were reduced from 8% for the stationary case to less than 0.5% by rotating the periscope. The aberrating nature of the thermal lens present in the RCL was also investigated by passing a 1064 nm probe beam through it. When the periscope was stationary the probe beam degraded from an M2 of 1.1 to 2.0 under 16.3 W of pump power. Introducing motion and pumping the slab with 180 W resulted in the M2 increasing to 1.4, clearly demonstrated the greater resilience a system with motion has to detrimental thermal effects.

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