High-power vortex light generated directly from lasers will help drive their applications in material processing, optical manipulation, levitation, particle acceleration, and communications, but limited power has been achieved to date. In this work, we demonstrate record vortex average power of 31.3 W directly from a laser, to the best of our knowledge, using an interferometric mode transforming output coupler to convert a fundamental mode Nd:YVO4 laser into a LG01 vortex output. The vortex laser was Q-switched with up to 600 kHz pulse rate with a high slope efficiency of 62.5% and an excellent LG01 modal purity of 95.2%. We further demonstrate > 30W laser power in a high quality HG10 mode by simple adjustment of the output coupler. Experimental investigations of varying output coupling transmission are compared with theory. This successful implementation of the interferometric output coupler in a high power system demonstrates the suitability of the mode transforming method for robust turn-key vortex lasers with high efficiency and high modal purity, with scalable power and pulse rate.
Abstract. Excited state absorption (ESA) is a process that occurs in many laser gain media and can significantly impact their efficiencies of operation. In this work we develop a model to quantify the effect of ESA at the pump wavelength on laser efficiency, threshold and heating. In an analysis based on the common end pumped laser geometry we derive solutions and analytical expressions that model the laser behaviour. From these solutions we discuss the main parameters affecting efficiency, such as the laser cavity loss, pump ESA cross section and stimulated emission cross section. Methodologies are described to minimise the impact of pump ESA, for example by minimising cavity loss. It is also shown that altering the pumping geometry can significantly improve performance by improved distribution of the population inversion. Double end pumping can approximately halve the effect of pump ESA compared to single end pumping, and side pumping also has the potential to arbitrarily reduce its effect.
An analytical model is formulated to support understanding and underpin experimental development of laser action in the promising diode end-pumped Alexandrite system. Closed form solutions are found for output power, threshold and slope efficiency that for the first time incorporate the combined effects of laser ground state absorption (GSA) and excited state absorption (laser ESA), along with pump excited state absorption (pump ESA), in the case of an end-pumping geometry. Comparison is made between model predictions and experimental results from a fibre-delivered diode end-pumped Alexandrite laser system, showing the impact of wavelength tuning, crystal temperature, laser output coupling, and intracavity loss. The model is broadly applicable to other quasi-three-level lasers with combined laser and pump ESA. A condition for bistable operation is also formulated.
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