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.
Whilst many techniques exist for generation of an optical vortex, there remains a need for new devices and methods that can also provide vortex generation with higher powers, greater flexibility of wavelength, and generation beyond the lowest-order Laguerre–Gaussian L G 01 mode to address a broader range of practical applications. This work reveals how an all-mirror based interferometric mode transformation system can provide these properties including revealing, for the first time, the generation of a much richer set of vortex mode patterns than might have been thought possible previously. A new developed theoretical formulation, confirmed with excellent agreement by experimental demonstrations in an imbalanced Sagnac interferometer, shows interferometric transformation is possible for all orders of Laguerre–Gaussian L G 0 l modes into a rich set of high quality higher-order vortex and vortex superposition. The interferometric approach is shown to be configurable to increase or decrease vorticity. The new mathematical formulation provides the ability to perform a full modal power analysis of both the mode-transformed transmitted vortex and the complementary reflected beam at the Sagnac beamsplitter (BS) port. A discussion is made on the origin of the orbital angular momentum transferred to the vortex output from the Sagnac BS.
We demonstrate, for the first time to the best of our knowledge, the direct generation of higher-order Laguerre–Gaussian (LG) petal modes, formed of the coherent superposition of positive and negative LG modes with the topological charge of ± ℓ as an eigen mode, from a compact diode end-pumped P r 3 + : L i Y F 4 (YLF) laser with an intra-cavity lens configuration. The on-axis displacement of the intra-cavity lens with spherical aberration allows the selective operation of the desired higher-order LG modes with | ℓ | ≤ 31 from the laser cavity at 640 nm and 607 nm. Such visible higher-order LG modes will offer new applications in optical manipulation, quantum/optical underwater telecommunication, and microfabrication.
Many vortex-generation techniques have been developed to address a range of potential applications, exploiting their unique amplitude and phase profiles and their possession of orbital angular momentum. In this work, we present what may be the simplest method of vortex beam generation, requiring only a wedged optic: the wedge-plate shearing interferometer (WPSI). We show that the WPSI can reflect a first order Laguerre–Gaussian vortex beam ( L G 01 ) with a theoretical purity of > 99 % from an input fundamental Gaussian beam, with 98% L G 01 purity experimentally demonstrated. We demonstrate 1% power conversion with a route to 14%. The monolithic WPSI is a simple, compact, and highly stable device, which can operate at any wavelength that the material is transparent to. We anticipate that it will be useful where sampling a robust, high-purity vortex beam from a Gaussian laser beam is required, including low-cost vortex generation for metrology or education.
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