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.
Lasers beams with a specific intensity profile such as super–Gaussian, Airy or Dougnut-like are desirable in many applications such as laser materials processing, medicine and communications. We propose a new technique for laser beam shaping by amplifying a beam in an end-pumped bulk amplifier that is pumped with a beam that has a modified intensity profile. Advantages of this method are that it is relatively easy to implement, has the ability to reshape multimode beams and is naturally suited to high power/energy beams. Both three and four level gain materials can be used as amplifier media. However, a big advantage of using three level materials is their ability to attenuate of the seed beam, which enhances the contrast of the shaping. <p> </p>We first developed a numerical method to obtain the required pump intensity for an arbitrary beam transformation. This method was subsequently experimentally verified using a three level system. The output of a 2.07 μm seed laser was amplified in a Ho:YLF bulk amplifier which was being pumped by a 1.89 μm Tm:YLF laser which had roughly a TEM10 Hermit Gaussian intensity profile. The seed beam was amplified from 0.3 W to 0.55 W at the full pump power of 35 W. More importantly, the beam profile in one transverse direction was significantly shaped from Gaussian to roughly flat-top, as the model predicted. The concept has therefore been shown to be viable and can be used to optimise the beam profile for a wide range of applications.
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 M<sup>2 </sup>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 M<sup>2</sup> 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.
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.