Wayne Koen

Ho:YLF & Ho:LuLF slab amplifier system delivering 200 mJ, 2 µm single-frequency pulses

A single-frequency single-pass amplifier based on Ho:YLF and Ho:LuLF in a scalable slab architecture delivering up to 210 mJ at 2064 nm is demonstrated. The amplifier was end-pumped by a 1890 nm Tm:YLF slab laser and was seeded with a 69 mJ single-frequency Ho:YLF ring laser operating at 50 Hz.

330 mJ single-frequency Ho:YLF slab amplifier

We report on a double-pass Ho:YLF slab amplifier which delivered 350 ns long single-frequency pulses of up to 330 mJ at 2064 nm, with a maximum M2 of 1.5 at 50 Hz. It was end pumped with a diode-pumped Tm:YLF slab laser and seeded with up to 50 mJ of single-frequency pulses.

Efficient Ho:YLF Laser Pumped by a Tm:fiber Laser

A thulium fiber laser pumped Ho:YLF laser delivering 45.1 W in a near diffraction-limited beam when pumped with 84.7 W is demonstrated. The optical-to-optical efficiency of 53 % compares favorably with similar Ho:YAG lasers.

60W Ho:YLF oscillator-amplifier system

We developed a compact Ho:YLF oscillator–amplifier system end-pumped by two 54 W unpolarised Tm:fibre lasers, and produced 60.2 W of output power at 2064 nm. The oscillator consisted of a flat input coupler mirror, a 50 mm long 0.5 % doped Ho:YLF crystal rod, a 45 degree folding mirror, an AOM, and a concave output coupler mirror. The oscillator operated vertically polarised on the holmium crystal’s σ–polarisation, ensuring good beam quality from the weak thermal lens. The concave output coupler had a radius of 300 mm and a reflectivity of 82 % at 2064 nm. The oscillator gave a maximum output of 24 W with an M2 of 1.06. The single-pass amplifier consisted of two 40 mm long, 0.5 % doped, Ho:YLF crystal rods and four folding mirrors. While the seed laser was pumped by a single fibre laser, the amplifier utilized the transmitted pump light from the seed laser in addition to the second fibre laser. With the first crystal amplifying on the σ–polarisation and the second crystal on the π-polarisation, the amplifier delivered 60.2 W with an M2 of 1.09, representing a gain of 2.5 while achieving an optical-to-optical efficiency of 55.5 %. When Q-switched with the AOM, the system delivered pulse lengths of between 43 and 113 ns at repetition rates from 15 to 40 kHz.

High average power Q-switched 1314 nm two-crystal Nd:YLF laser

A 1314 nm two-crystal Nd:YLF laser was designed and operated in both CW and actively Q-switched modes. Maximum CW output of 26.5 W resulted from 125 W of combined incident pump power. Active Q-switching was obtained by inserting a Brewster-cut acousto optic modulator. This setup delivered an average power of 18.6 W, with a maximum of 5.6 mJ energy per pulse with a pulse duration of 36 ns at a pulse repetition frequency of 500 Hz.

Compact fibre-laser-pumped Ho:YLF oscillator-amplifier system

Ho:YLF is an attractive laser material for 2 µm high energy sources since it has a much longer upper laser level lifetime (∼14 ms) and higher emission cross section than Ho:YAG. In addition, the very weak thermal lens on the σ-polarisation helps to deliver diffraction limited beams even under intense end-pumping. However, Ho:YLF has a somewhat stronger quasi-three-level nature, which implies that in order to reach transparency at the 2065 nm line, 22% of the Ho ions need to be pumped into the upper laser level (at room temperature), but it already reaches transparency at the 1940 nm pump wavelength with only 56% of the Ho ions in the upper laser level. In addition, the pump absorption cross section at 1940 nm is relatively low and strongly polarised. Therefore, the laser design requires a trade-off between efficient pump absorption and low laser threshold.

Demonstration of a wavelength selected optically pumped HBr laser

A tunable optically pumped HBr laser has been demonstrated for the first time. As pump source for the HBr oscillator, we developed a single-frequency Ho:YLF laser- amplifier system which was locked to the 2064 nm absorption line of HBr. Through the implementation of an intra-cavity diffraction grating, laser oscillation was demonstrated on nineteen molecular transition lines including both the R-branch (3870 nm to 4015 nm) and the P-branch (4070 nm to 4453 nm). The highest output energy for the given input energy was 2.4 mJ at 4133 nm.

Efficient fiber-laser-pumped Ho:YLF oscillator and amplifier utilizing the transmitted pump power of the oscillator

We present a novel scheme for a compact and robust pulsed fiber-laser-pumped Ho:YLF oscillator and amplifier system, where the pump power transmitted by the oscillator is utilized to pump the amplifier.

Development of high-power and high-energy 2μm bulk solid-state lasers and amplifiers

A selection of 2 μm lasers and amplifiers developed at the CSIR National Laser Centre in South Africa is presented. A diverse range of near diffraction-limited 2 μm lasers and amplifiers were developed which varied from high-energy, single-frequency oscillators and amplifiers, to compact and efficient MOPA systems delivering high average powers. This was made possible by exploiting various advantageous properties of holmium-doped YLF while mitigating its detrimental properties through the use of novel pump and laser design approaches.

An HBr oscillator-amplifier pumped by a high-energy Ho:YLF laser system

Laser output in the mid-infrared can be generated by starting from a diode-pumped Tm-doped solid-state laser or fibre laser at 1.9 µm, pumping a Ho-doped system at 2 µm to subsequently pump a non-linear device such as an optical parametric oscillator. For high-energy operation, optically pumped molecular lasers are attractive alternatives due to the potential to scale to very high pump energies without optical damage to the medium. One such example is an HBr laser for which the highest reported output energy was 2.5 mJ in a 132 ns pulse at 50 Hz repetition rate when pumped with a narrow-band Ho:YLF laser [1].