Demonstration of stable, long-term operation of a nanosecond pulsed DPSSL at 10 J, 100 Hz.

We report on stable, long-term operation of a diode-pumped solid-state laser (DPSSL) amplifying 15 ns pulses at 1029.5 nm wavelength to 10 J energy at 100 Hz pulse rate, corresponding to 1 kW average power, with 25.4% optical-to-optical efficiency. The laser was operated at this level for over 45 minutes (∼3 · 105 shots) in two separate runs with a rms energy stability of 1%. The laser was also operated at 7 J, 100 Hz for 4 hours (1.44 · 106 shots) with a rms long-term energy stability of 1% and no need for user intervention. To the best of our knowledge, this is the first time that long-term reliable amplification of a kW-class high energy nanosecond pulsed DPSSL at 100 Hz has been demonstrated.

Laser shock peening of tungsten and its dependency on polarisation of light for induced compressive stresses.

We report on laser shock peening (LSP) of tungsten, a material used as a divertor in Tokamak machine for magnetic confinement fusion reactions such as the ITER facility (France) and JET facility (UK). Peak compressive stresses of -370 MPa and depths of up to 1.75 mm were recorded when 0.25 cm2 area of tungsten (99.95% pure) was irradiated by a 1030 nm Yb:YAG laser operating at 10 J, 10 ns. Furthermore, we demonstrate enhancement of compressive stresses in one direction, by application of circular polarised light in hard material like tungsten. However, no enhancement of compressive stresses with circular polarisation was observed in soft material like aluminium.

Confinement and absorption layer free nanosecond laser shock peening of tungsten and its alloy.

Traditionally, nanosecond laser shock peening (ns-LSP) of metals requires an additional application of an absorption layer (black paint) and more importantly a confinement layer (typically water or transparent material) on the workpiece for introduction of compressive stresses. In this paper, we demonstrate for the first time, to the best of our knowledge, introduction of compressive stresses in pure tungsten and its alloy TAM7525 (75% tungsten and 25% copper) without any absorption and confinement layer for ns-LSP. Peak compressive stresses of -349 MPa and -357 MPa were measured in pure tungsten and TAM7525, respectively, when a 0.25-cm2 area was irradiated by a Yb:YAG laser (1030 nm) operating at ∼5 J, ∼2 ns with circular polarization. These peak compressive stresses (without confinement layer) compare well to those with tungsten ns-LSP done with water as confinement layer at twice the energy at 10-ns pulse duration. Furthermore, compared to femtosecond laser shock peening (fs-LSP) of aluminum at atmospheric pressure, the depth of compressive stresses recorded in tungsten and its alloy (∼7 times denser than aluminum) is nearly four times more in the case of confinement layer free nanosecond laser shock peening (CLF-ns-LSP).

Second and third harmonic conversion of a kilowatt average power, 100-J-level diode pumped Yb:YAG laser in large aperture LBO.

We report on the successful demonstration of second and third harmonic conversion of a high pulse energy, high average power 1030 nm diode pumped Yb-doped yttrium aluminum garnet (Yb:YAG) nanosecond pulsed laser in a large aperture lithium triborate (LBO) crystal. We demonstrated generation of 59.7 J at 10 Hz (597 W) at 515 nm (second harmonic) and of 65.0 J at 1 Hz (65 W) at 343 nm (third harmonic), with efficiencies of 66% and 68%, respectively. These results, to the best of our knowledge, represent the highest energy and power reported for frequency conversion to green and UV-A wavelengths.

Modelling and measurement of thermal stress-induced depolarisation in high energy, high repetition rate diode-pumped Yb:YAG lasers.

In this paper, we present a model to predict thermal stress-induced birefringence in high energy, high repetition rate diode-pumped Yb:YAG lasers. The model calculates thermal depolarisation as a function of gain medium geometry, pump power, cooling parameters, and input polarisation state. We show that model predictions are in good agreement with experimental observations carried out on a DiPOLE 100 J, 10 Hz laser amplifier. We show that single-pass depolarisation strongly depends on input polarisation state and pumping parameters. In the absence of any depolarisation compensation scheme, depolarisation varies over a range between 5% and 40%. The strong dependence of thermal stress-induced depolarisation on input polarisation indicates that, in the case of multipass amplifiers, the use of waveplates after every pass can reduce depolarisation losses significantly. We expect that this study will assist in the design and optimisation of Yb:YAG lasers.