Transient pump-probe analysis of pump-induced lensing in laser amplifiers.

Understanding of pump-induced lensing in laser amplifiers is essential for the optimized design of high-power lasers with high spatial quality, but there is usually incomplete knowledge of the interplay between thermal and population induced lensing mechanisms, lensing under lasing and non-lasing conditions, and transient lensing effects under pulsed operation. This paper provides quantitative insight of pump-induced lensing effects by using experimental transient pump-probe measurements in an alexandrite laser amplifier end-pumped by a short pulse pump beam with Gaussian spatial intensity distribution. Lensing results are presented showing a large difference in lensing under both non-lasing and lasing conditions and distinction of the population lens and thermal lens contributions from their different response time. Different pump beam sizes are used to show the variation of the relative strength of the lensing mechanisms. Comparison of experimental results with the analytical transient theory developed in this paper for the Gaussian pump beam gives excellent agreement and quantitative information on the thermal and population contributions to the amplifier lens. This paper provides a methodology for quantitative investigation of pump-induced lensing in general laser amplifier systems, and potentially other classes of optical materials with residual optical absorption.

High-energy acousto-optic Q-switched alexandrite laser with wavelength tunable fundamental and UV second harmonic generation.

We investigate high-energy mJ-class diode-pumped acousto-optic (AO) Q-switched alexandrite lasers with broad tunability at both the fundamental near-IR wavelength range and second harmonic generation (SHG) in the UV wavelength range. An AO Q-switched alexandrite laser with continuous-wave diode-pumping has been operated at up to 10 kHz and producing pulse energy of 700 µJ at repetition rate of 1 kHz. With pulsed double-pass diode-pumping, we demonstrate higher pulse energy of 2.6 mJ. With wavelength tuning, this laser system demonstrated broad fundamental tuning range from 719 to 787 nm with diffraction-limited beam quality (M2 = 1.05). By external cavity second harmonic generation in a Type-I LBO crystal, pulse energy of 0.66 mJ is generated at 375 nm and with UV tuning range from 361 to 391 nm. To our knowledge, this is the first demonstration of AO Q-switched alexandrite laser with broad wavelength tuning in the near-IR and UV wavelength ranges and shows its excellent potential as a pulsed source for future applications.

Temperature-tunable UV generation using an Alexandrite laser and PPLN waveguides.

We present a simple and novel technique for achieving ultra-violet (UV) wavelength-tunable laser operation in the continuous-wave regime. Wavelength tunable operation in the near infrared is obtained from a compact two-mirror Alexandrite laser cavity by temperature tuning of the laser crystal. Second-harmonic-generation to the UV is then achieved at 376-379 nm and 384-386 nm by temperature tuning of a periodically-poled lithium-niobate (PPLN) waveguide. A maximum UV power of 1.3 mW from 185 mW infra-red pump throughput is obtained from a third-order PPLN Λ=6.1µm grating. These results show promising potential for simple and wavelength tunable access to wavelengths at 360-400 nm.

Alexandrite lasers with blue-diode-pumping.

The availability of high-power and high-brightness blue diode lasers makes them attractive as low-cost pump sources for broadly tunable Alexandrite lasers. In this paper we investigate the performance of an Alexandrite laser pumped by a high-power fiber-delivered blue diode module. Output power 1.84 W is achieved, the highest power from blue diode pumped Alexandrite to date. Excellent pump absorption is demonstrated of scrambled pump polarization on both a-axis and b-axis of Alexandrite crystal. Wavelength tuning and dual wavelength operation is produced using the self-birefringent filtering of the Brewster-cut Alexandrite crystal. An analysis is made of laser efficiency and mode formation including the creation of higher-order Laguerre-Gaussian vortex modes (LG01 and LG02). Performance is compared to red diode pumping and prospects for further optimization and power-scaling are discussed.

Sampling a vortex from a Gaussian beam using a wedge-plate shearing interferometer.

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 (LG01) with a theoretical purity of >99% from an input fundamental Gaussian beam, with 98% LG01 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.

Pump-induced lensing effects in diode pumped Alexandrite lasers.

It is essential to understand the pump-induced lensing and aberration effects in solid-state lasers, such as Alexandrite, since these set limits on laser power scaling whilst maintaining high spatial TEM00 beam quality. In this work, we present direct wavefront measurements of pump-induced lensing and spherical aberration using a Shack-Hartmann wavefront sensor, for the first time, in a diode-pumped Alexandrite laser, and under both non-lasing and lasing conditions. The lens dioptric power is found to be weakly sub-linear with respect to the absorbed pump power, and under lasing, the lensing power is observed to decrease to 60 % of its non-lasing value. The results are inconsistent with a thermal lens model but a fuller theoretical formulation is made of a combined thermal and population lens model giving good quantitative agreement to the observed pump power dependence of the induced-lensing under non-lasing conditions and the reduced lensing under lasing conditions. The deduced value for the difference in excited to ground state polarizability is consistent with prior measurement estimates for other chromium-doped gain media. The finding of this paper provide new insight into pump-induced lensing in Alexandrite and also provides a basis for a fast saturable population lens mechanism to account for self-Q-switching observed recently in Alexandrite laser systems.

Tunable, dual wavelength and self-Q-switched Alexandrite laser using crystal birefringence control.

We present a red-diode-pumped Alexandrite laser with continuous wavelength tunability, dual wavelength and self-Q-switching in an ultra-compact resonator containing only the gain medium. Wavelength tuning is obtained by varying the geometrical path length and birefringence by tilting a Brewster-cut Alexandrite crystal. Two crystals from independent suppliers are used to demonstrate and compare the performance. Wavelength tuning between 750 and 764 nm is demonstrated in the first crystal and between 747 and 768 nm in the second crystal. Stable dual wavelength operation is also obtained in both crystals with wavelength separation determined by the crystal free spectral range. Temperature tuning was also demonstrated to provide finer wavelength tuning at a rate of -0.07 nm K -1. Over a narrow tuning range, stable self-Q-switching is observed with a pulse duration of 660 ns at 135 kHz, which we believe is the highest Q-switched pulse rate in Alexandrite to date. Theoretical modelling is performed showing good agreement with the wavelength tuning and dual wavelength results.

Temperature effects on tunable cw Alexandrite lasers under diode end-pumping.

Diode pumped Alexandrite is a promising route to high power, efficient and inexpensive lasers with a broad (701 nm to 858 nm) gain bandwidth; however, there are challenges with its complex laser dynamics. We present an analytical model applied to experimental red diode end-pumped Alexandrite lasers, which enabled a record 54 % slope efficiency with an output power of 1.2 W. A record lowest lasing wavelength (714 nm) and record tuning range (104 nm) was obtained by optimising the crystal temperature between 8 °C and 105 °C in the vibronic mode. The properties of Alexandrite and the analytical model were examined to understand and give general rules in optimising Alexandrite lasers, along with their fundamental efficiency limits. It was found that the lowest threshold laser wavelength was not necessarily the most efficient, and that higher and lower temperatures were optimal for longer and shorter laser wavelengths, respectively. The pump excited to ground state absorption ratio was measured to decrease from 0.8 to 0.7 by changing the crystal temperature from 10 °C to 90 °C.

Unidirectional single-frequency operation of a continuous-wave Alexandrite ring laser with wavelength tunability.

High resolution spectroscopy, metrology and quantum technologies (e.g. trapping and cooling) require precision laser sources with narrow linewidth and wavelength tunability. The widespread use of these lasers will be promoted if they are cost-effective, compact and efficient. Alexandrite lasers with a broad tuning band pumped efficiently by low-cost red diodes are a potential candidate, but full performance as a precision light source has not been fully achieved. We present in this work the first continuous-wave (CW) and single-frequency operation of a unidirectional diode-end-pumped Alexandrite ring laser with wavelength tunability. An ultra-compact bow-tie ring cavity is developed with astigmatic compensation and a novel 'displaced mode' design producing CW output power > 1 W in excellent TEM00 mode (M2 < 1.2) when using a low brightness pump (M2 ≥ 30). Wavelength tuning from 727 - 792 nm is demonstrated using a birefringent filter plate. This successful operation opens the prospects of precision light source applications.

Vortex laser from anti-resonant ring coupled cavities.

Optical vortex Laguerre-Gaussian (LG0l) modes have wide-ranging applications due to their annular spatial form and orbital angular momentum. Their direct generation from a laser is attractive, due to the pure and high-power modes possible; however, previous demonstrations have had limited ranges of applicability. Here, we propose and implement direct LG0l vortex mode generation with an anti-resonant ring (ARR) coupled laser cavity geometry, where the gain medium inside the ARR is shared between two laser cavities. This generation uses standard wavelength-insensitive optical components, is suitable for high peak and average power levels, and could be applied to any bulk gain medium in pulsed or continuous wave regimes. This work demonstrates the technique with a diode end-pumped Nd:YVO4 gain medium. From 24 W of pump power, 8.9 W LG01 and 4.3 W LG02 modes were generated, all with high mode purity and pure handedness. The LG01 mode handedness was controlled with a new technique.

High efficiency >26 W diode end-pumped Alexandrite laser.

We show for the first time that multi-ten Watt operation of an Alexandrite laser can be achieved with direct red diode-pumping and with high efficiency. An investigation of diode end-pumped Alexandrite rod lasers demonstrates continuous-wave output power in excess of 26W, more than an order of magnitude higher than previous diode end-pumping systems, and slope efficiency 49%, the highest reported for a diode-pumped Alexandrite laser. Wavelength tuning from 730 to 792nm is demonstrated using self-seeding feedback from an external grating. Q-switched laser operation based on polarization-switching to a lower gain axis of Alexandrite has produced ~mJ-pulse energy at 1kHz pulse rate in fundamental TEM(00) mode.

Over 40-watt diffraction-limited Q-switched output from neodymium-doped YAG ceramic bounce amplifiers.

High power operation of high repetition Q-switched ceramic Nd:YAG laser is demonstrated using a master-oscillator power amplifier with a transversely-pumped bounce geometry. The laser system produced >40W high spatial quality output (M(2) <1.3) at a pulse repetition frequency of >100kHz. Pulse width of 35-80ns was achieved in 10-150 kHz pulse repetition frequency range of 10-150kHz.

Efficient continuous-wave generation in a self-organizing diode-pumped Nd:YVO4 laser with a reciprocal dynamic holographic cavity.

A diode-pumped Nd:YVO4 laser is demonstrated that has a reciprocal self-intersecting loop cavity with self-organization by formation of a dynamic holographic grating induced by the generating beam. Continuous-wave high spatial quality generation (M2 < 1.2) with 9.6 W of power and 32% optical efficiency is obtained. Spectral compositions and temporal dynamics are investigated as functions of diode-pumping power.

Unidirectional single-frequency operation of a Nd:YVO4 ring laser with and without a Faraday element.

We demonstrate high-performance unidirectional and single-frequency ring-laser operation based on a diode-side-pumped Nd:YVO4 bounce amplifier, obtained in a ring system both with and without a Faraday rotating element. Ring-laser operation with intracavity Faraday unidirectional element produces 15-W cw output in a TEM00 and single-longitudinal mode with beam propagation parameter M2 < 1.1 with 35-W diode pumping. A novel non-Faraday-based ring laser uses a polarization-dependent output coupler and asymmetric polarization state in the birefringent Nd:YVO4 gain medium and is demonstrated to produce highly unidirectional (1200:1) single-frequency output of 14 W in a TEM00 mode with beam propagation factor M2 < 1.2 at 30 W of diode pumping.

Efficient operation of a solid-state adaptive laser oscillator.

We present the results of a cw diode-pumped Nd:YVO4 laser oscillator based on a self-starting adaptive gain-grating resonator. Adaptive laser operation has been demonstrated with 12-W output for 37 W of diode pumping, producing a TEM00 mode that compensates for thermal aberrations. The issue of the finite aperture of the amplifier is discussed, and a design that incorporates an intracavity lens is used to improve the collection efficiency with severe thermal lensing at high pump powers. The powers of the beams involved in the resonator are compared with theory and are found to be in good agreement. Spectral and temporal behavior of the adaptive laser is investigated, and very interesting behavior is shown, including self-induced temporal modulation dynamics and a switching between a narrowband and a broad bandwidth of operation.