Photorefractive techniques for diode laser brightness enhancement
The experimental investigation of a number of techniques using photorefractive materials to increase the spectral and spatial brightness of the output from high power semiconductor laser arrays are discussed. The topics of semiconductor lasers and photorefractive materials are reviewed briefly, and the experimental and theoretical background of array transverse modes and laser array injection locking are discussed fully. Single longitudinal mode operation of a 100mW array coupled to a simple, hemispherical external cavity is reported. A side mode suppression ratio of 14dB was achieved. A spatial brightness enhancement by a factor of 10 is reported for a 1W laser array coupled to a phase conjugate external cavity. A number of techniques for the combination of multiple array outputs into a single beam were investigated. A power of 108mW (220mW, Fresnel corrected) was obtained in a diffraction limited, single mode laser beam using photorefractive two-beam coupling with an injection locked 1W laser array pump. Results on the operation of a reflection geometry phase conjugate master-oscillator, power-amplifier using a 500mW diode laser array are presented. The observation of coherent energy transfer between the outputs from the separate stripes of a laser array is discussed, and an external photorefractive beam combining element is proposed. Phase conjugate fidelity results are presented for a double pass, phase conjugate, multimode fibre amplifier geometry. The experiments suggest that phase conjugate modal unscrambling will still take place in the presence of ~6dB gain. A scheme for a high efficiency, high power multimode fibre amplifier is proposed.