Cover caption：High average power laser beams in near-UV region are typically realized by efficient frequency conversion from near-IR wavelengths. Recently, a record pulse energy of 50 J was achieved at Hilase, Czech Republic, at 10 Hz repetition rate at 343 nm wavelength by subjecting 1030 nm high power laser beam to type-I harmonic conversion to 515 nm and type-II harmonic conversion to 343 nm.

The high energy lasers have made substantial progress over the last decade in the sense of their average power output. This is related to the diode pumping technology as well as the novel amplifier geometries allowing efficient heat extraction such as multi-slab, innoslab or thin-disk. The higher average power output enabled applications of the high energy lasers to industrial environments, driving their further development. One of such examples is laser shot processing of surface layers of metallic materials to enhance the strength and lifetime of the treated components. For this purpose, typically near IR wavelength is chosen as the process is sensitive to it and the needed power is already available from contemporary lasers. In other applications, such as laser induced damage testing, silicon annealing or inertial laser fusion experiments, shorter wavelengths are needed. Direct generation of such high energy radiation from a laser with high average power is not currently possible due to the non-existence of suitable laser materials. To obtain such laser output at shorter wavelengths, a non-linear crystal is typically used, in which the wavelength is converted to its harmonic frequencies (fractions of the fundamental wavelength, or multiples of the optical wave frequency). Under favorable conditions, this conversion is realized with energy efficiency in the order of higher tens of percents. However, In the case of high average power lasers there is a major drawback of this method, which is its sensitivity to polarization. The high energy lasers capable of high average power output typically suffer from serious depolarization effects (non-uniform change of polarization state across beam trace) and this in turn limits the performance of eventual parametric processes. In order to ensure efficient harmonic conversion, it is necessary to mitigate the depolarization losses within the source laser system.

At Hilase facility in Czech Republic, the depolarization losses of 'Bivoj' laser – high energy, high average power laser system – were optimized by employing a novel customized polarimetric technique and reduced below 3.4% of the total beam energy. This effectively allowed usage of the laser for harmonic conversion experiments. After successful demonstration of almost one kilowatt at second harmonic frequency (95 J, 10 Hz at 515 nm), in the recent experiment a record 500 W of average power was achieved at 343 nm wavelength (third harmonic frequency). The high average power output in UV was achieved by subjecting the laser output at 1030 nm fundamental wavelength to consequent type-I and type-II phase matching in Lithium Triborate crystals (LBO). The experimental layout is shown in the figure below.

Figure 1 – layout of the THG experiment. LAS+LBDS is the laser system providing laser beam via a laser beam distribution system, single components of the setup are denoted as: quarter waveplate (QWP), half waveplate (HWP), conversion crystals (LBO), partially reflecting sampling wedge (SW) and beam dump (BD). To separate the diagnostic beams, dichroic beam-splitters (DBS) and mirrors (M) are used.

This result presents more than doubling the state-of-the-art average power in the UV for pulsed lasers and for the specific case of high energy lasers, this presents more than 4 times increase. High energy pulses of 50 J were generated at 343 nm with 10 Hz repetition rate. The high average power nature of the laser beams inherently induces significant thermal load in the crystals, which in turn detunes the optimum phase matching and lowers the efficiency of the parametric process. In order to maintain high efficiency of the process, it was necessary to use suitable thermal and pointing stabilization systems. The overall efficiency of the Third Harmonic Generation (THG) was measured as 53%, which indicates non-negligible thermal gradients being imprinted in the LBO crystals. An efficient minimization of these gradients would possibly allow increasing the efficiency further.

The recent result is important specifically in the light of the latest laser fusion experiments at the NIF facility in California, where intense UV light is used for fuel ignition. Significant progress was made in terms of the fusion energy yield when a positive energy balance was achieved repeatedly. The device operates in a single shot regime, whereas for an envisioned laser based nuclear fusion power plant, a minimum of 10 Hz repetition rate for the laser drivers is considered necessary to make the process sustainable. This requirement puts the recent result at Hilase facility in the perspective of a feasibility demonstration of the envisioned technological path. Further development of the harmonic generation stages should focus on increasing the efficiency of the conversion as well as targeting higher energies in pulse in order to provide sufficient performance for next generation applications of UV light.

The work has been published in High Power Laser Science and Engineering, vol. 12, Issue 6 (Jan Pilar, Martin Divoky, Jonathan Phillips, Martin Hanus, Petr Navratil, Ondrej Denk, Patricie Severova, Tomas Paliesek, Danielle Clarke, Martin Smrz, Thomas Butcher, Chris Edwards, Tomas Mocek, "Half-kilowatt high-energy third-harmonic conversion to 50 J 10 Hz at 343 nm," High Power Laser Sci. Eng. 12, 06000e96 (2024)).