Hencharl J. Strauss

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.

Tm:YLF slab wavelength-selected laser

A Tm:YLF slab laser was wavelength selected to operate at 1890 nm. The oscillator consisted of a single 2.5% doped Tm:YLF slab, a volume Bragg Grating (VBG) mirror and a 90% output-coupler. The slab crystal was pumped with a 300 W diode stack using a pump reproducing configuration. The output power exceeded 80 W and the beam quality factors in the horizontal and vertical directions were 182 and 2.5 respectively.

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.

Volume Bragg Grating Wavelength Selected Tm:YLF Slab Laser Operating at 1890 nm

An 80 W Tm:YLF Volume Bragg Grating laser selected to operate at 1890 nm is demonstrated. It consisted of a single Tm:YLF slab pumped with a 300 W diode stack using a pump reproducing configuration.

Comparative study of thermal lensing in low-doped Nd:YVO 4 and Nd:GdVO 4 of equal doping concentration

Nd:GdVO4 is a relatively new laser material with spectral properties very similar to those of Nd:YVO4. It was initially reported that it had almost twice the thermal conductivity of Nd:YVO4, and therefore was said to reduce negative thermal effects. Sato and Taira [1] recently re-measured the thermal conductivity and its temperature dependency and found that it was significantly lower than that of Nd:YVO4. They also reported that dn/dT in the higher gain π-polarisation of Nd:YVO4 is 7.92 at 1064 nm, compared to 10.1 for Nd:GdVO4. One would therefore expect that Nd:YVO4 would have weaker thermal lensing, since it is influenced by both the thermal conductivity and dn/dT. To our knowledge, a direct comparison of the thermal lens of the two materials has never been performed with equal doping concentrations and dimensions. Therefore, we aimed to compare the two materials directly under diode-end-pumping by performing thermal lens measurements during lasing with a HeNe probe beam using identical low-doped (0.15%), 4×4×18 mm3 crystals. Our setup was such that we could separately analyse the dioptric powers in the σ and π polarisations.

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.

Beam intensity reshaping by pump modification in a laser amplifier.

We propose a new technique for laser beam shaping into a desirable beam profile by using a laser amplifier with a pump beam that has a modified intensity profile. We developed the analytical formula, which describes the transformation of the seed beam into the desired beam profile in a four level amplifiers small signal regime. We propose a numerically method to obtain the required pump intensity profile in the case where high pump power saturated the laser crystal or for three level materials. The theory was experimentally verified by one dimensionally shaping a Gaussian shaped seed into a Flat-Top beam in a Ho:YLF amplifier pumped by a Tm:YLF laser with a HG(01) intensity profile.

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.