Spatial and spectral brightness improvement of single-mode laser diode arrays

Abstract

This thesis addresses the strong need for efficient and compact techniques for brightness enhancement of laser diode arrays and responds to the challenges created for high performance optics and techniques for laser characterisation. A novel optical inter-leaving method for a 7-bar stack of single-mode emitters, providing a nearly 2-fold improvement in the slow axis beam parameter product, enabling fibre-coupling, is demonstrated. A laser-written dual-axis optics approach is used to perform challenging slow axis collimation combined with fast axis correction for closely-packed 49-single-mode emitter bars, to provide low-loss collimation with high pointing accuracy of less than 3% and 10% of a beam divergence in the fast and slow axis direction, respectively. This produces excellent source for application beam-combined laser diode systems. An emitter-by-emitter simultaneous analysis is used to provide spectra and far field pointing for all emitters and evaluate the performance of various external cavity configurations with Volume Holographic Gratings (VHGs). For the ultra-collimated bars, high efficiency VHG-locking is shown to be maintained over enhanced range of temperatures (>17˚C) and large laser-VHG distances (>110 mm). Highly effective feedback enables the use of a folded cavity configuration for wavelength selection over a range of 8 nm for the full 49-emitter bar, giving a prospect for multi-wavelength single-VHG-locking of bars for cost-effective spectral combining. An innovative technique of wavelength stepping by individually-formed folded cavities for 5 and 7 sections along the bar demonstrates a potential to produce a source for high performance dense spectral beam combining. In a VHG-based Talbot cavity, eight emitters are coherently locked with a highvisibility interference pattern at 1W of output power. The results of phase-locking for full 49-emitter bar show that the slow axis pointing variation of ± 2mrad produces different supermodes, for a fixed alignment of the cavity, thus it must be additionally corrected for further improvement.