High energy, high average power, pulsed lasers produce a lot of heat in the gain medium, and this heat is one of the main sources of degradation in output beam quality. The heat creates temperature gradients that consequently generate substantial mechanical stresses, which can cause optically isotropic material to become optically anisotropic. This effect is called thermal-stress-induced birefringence, and its parameters vary across the optical aperture, thus affecting the beam polarization non-uniformly across its cross-section. This polarization change is not always immediately visible, unless the beam propagates through a specific type of polarization-sensitive device, such as a polarizer, or is used in a polarization-sensitive process, such as harmonic frequency conversion. The amount of energy in an unwanted polarization state can reach up to 50%, significantly decreasing the overall efficiency of any conversion process.
Second harmonic frequency conversion of 1 µm wavelength light produces green light around 500 nm wavelength. Such wavelengths of light are typically used for the optical pumping of ultra-short pulse lasers, such as Ti:sapphire Chirped Pulse Amplification (CPA) systems or Optical Parametric CPA systems. Another possible application is in Laser Shock Peening (LSP), a technique for improving the durability of mechanical components. LSP typically features a thin water layer that confines the laser-generated shock wave in the component, but creating this confinement layer on components with complex shapes can prove problematic. Using 500 nm light would allow such components to be fully submerged, with LSP taking place underwater, an approach that would not be possible with 1 µm radiation that is strongly absorbed in water.
The above-mentioned applications could increase their throughput if a high average power (HAP) laser were used. However, thermal-stress-induced birefringence makes HAP second harmonic conversion very inefficient.
In a recent paper, the thermal issues connected specifically with harmonic conversion were addressed. The research results are published in High Power Laser Science and Engineering, Volume 11, Issue 5 (Martin Divoky, Jonathan Phillips, Jan Pilar, Martin Hanus, Petr Navratil, Ondrej Denk, Tomas Paliesek, Patricie Severova, Danielle Clarke, Martin Smrz, Thomas Butcher, Chris Edwards, John Collier, Tomas Mocek. Kilowatt-class high-energy frequency conversion to 95 J at 10 Hz at 515 nm[J]. High Power Laser Science and Engineering, 2023, 11(5): 05000e65). A custom polarimetric method was used to evaluate the polarization response of the end-stage laser amplifier of the Bivoj/DiPOLE laser system, which enabled the system's response to arbitrary input polarization to be evaluated numerically. The birefringent medium affects each individual polarization state differently. Using the polarimetric method, it was possible to find the input polarization state that is transformed to the most uniform output polarization state. The pair of input waveplates were adjusted to deliver the required input polarization, and thereby minimize the polarization non-uniformity acquired during amplification. At the output of the amplifier, another pair of waveplates were then adjusted to give a linear output beam polarization state.
Using this approach, polarization losses were reduced from 30% to about 3%, enabling conversion of almost 97% of available energy compared with only 70%. However, the wavelength conversion process itself is not 100% efficient. Due to the high thermal loading, the conversion crystal heats up and thermal gradients arise, causing a shift in optimal conversion conditions across the aperture of the crystal, and therefore lowering the overall conversion efficiency. Despite this, more than 81% conversion efficiency was achieved, based on the convertible energy available.
This work has paved the way for high energy, high power frequency conversion and extended the application potential of the Bivoj/DiPOLE laser. In the future, it will be possible to apply the same approach to increase the efficiency of the third harmonic conversion. This will generate wavelengths with applications in inertial fusion power plants, lowering the power consumption of the lasers used to compress the fuel, and in the semiconductor industry for annealing.
Forthcoming work will focus on generation of third harmonic frequency and optimizing the thermal gradients in the conversion crystals to further improve the conversion efficiency of both second and third harmonic conversion.
Tomas Mocek, Head of HiLASE comments:
"With this breakthrough in high-energy frequency conversion, we have opened new vistas for laser applications. From enhancing the durability of mechanical components by Laser Shock Peening underwater, to advancing ultra-short, high-intensity laser systems, the impact is profound. HiLASE will offer this cutting-edge, kilowatt green laser source to scientific users and industrial partners. We look forward to collaborating and fully exploiting the potential of Bivoj/DiPOLE laser technology!"
Figure Caption: (a) Laser system Bivoj/DiPOLE. (b) Schematic detail of the amplifier head and conversion crystal
