Enhancement of Self-Organisation and Adaptivity in Laser Systems
Creators: name: Shardlow, Peter Charles
Publisher: name: Imperial College London
Year: 2010

description: Self-organisation is an inherent mechanism in all laser systems although it is often overlooked. The self formation of spatial and spectral modes, competition between modes and much spatio-temporal dynamics are driven by the intrinsic non-linearity of saturable gain in the laser amplifier medium coupled with feedback from a resonator structure. It is highly insightful to consider the growth, extinction and competition of modes in a laser system as an evolving ecosystem with modes as species growing by stimulated emission in the gain medium, decaying by intracavity and output coupling losses, and having to compete for the common, but finite, resource of gain which is supplied by an external excitation source. The outcome of species competition is a survival-of-the-fittest that determines the final steady-state output or dynamical set of modes that can continue to persist. This thesis presents investigations into the design solid-state laser systems which utilise the inherent dynamics of optical fields and gain media in order to self-organise the system to operate in a desirable manner. The Nd:YVO4 bounce geometry laser amplifier is employed throughout this thesis. A numerical investigation of the thermally induced lensing within the laser crystal is reported. Optimisation of the geometry parameters is explored as well as investigation into future developments, such as the utilisation of an additional sapphire crystal to directly cool the laser crystal pump face. This is shown to theoretically reduce the horizontal thermally induced lens strengths by a factor of 4. Single longitudinal mode single longitudinal mode (SLM) ring lasers where the unidirectionality is imposed either by an extra-cavity ‘parasitic’ pass of the gain media or by retro-reflection of one of the two outputs are investigated. SLM TEM00 output powers of up to 20W are demonstrated without the need for a Faraday isolator. A self-adaptive sensor which allows the measurement of remote surface vibrations is demonstrated. The two-wave mixing interaction within a saturable gain media is shown to allow measurement of high frequency phase modulations (&gt;10kHz) whilst adapting to cancel out low frequency perturbations. This sensor system is shown to have potential as a remote ultrasound detector as the holographic nature allows high frequency measurement of the vibrations of rough remote surfaces. Self-starting self-adaptive lasers, where a four wave mixing interaction within the saturable gain medium is utilised to generate phase conjugate and aberration corrective laser systems are experimentally investigated. This work is extended to show that the gain hologram is capable of adapting to low frequency phase modulations in order to maintain a high quality output. A demonstration of self-organised coherent beam combination of two bounce geometry laser oscillators into a single output beam is reported. A combined output beam of 35.7W was demonstrated from 94W of pump power. This coherent beam combination is extended into the technique of phase conjugate self-organised coherent beam combination (PCSOCBC) where a first demonstration of the combination of two self-starting self-adaptive modules is reported. It is shown that the adaptive modules allow efficient beam combination (94%) with a combined output of 27W. As the self-starting self-adaptive modules do not have predefined spatial or spectral modes it is believed that this system could be scaled to much higher numbers of modules than is possible with conventional self-organised coherent beam combination.