Ultra high damage threshold optics for high power lasers

The power and energy of lasers have dramatically increased since their invention, opening new avenues in research such as fusion energy, charged particle beam acceleration, and extreme ultraviolet (EUV) sources for semiconductor manufacturing. Laser pulse output energy has risen from 100 mJ to 2 MJ over the past 50 years. However, the damage threshold of optical elements has not increased proportionally. Dielectric-coated mirrors typically have thresholds of 1–200 J/cm² for nanosecond pulses in the visible and near-infrared, while structured optics like gratings exhibit even lower thresholds (0.1–12 J/cm²). This limitation necessitates the use of meter-sized optics in high-energy laser systems. Optical damage is a concern not only for high-energy lasers but also for high-repetition-rate lasers. Even below the nominal threshold, damage risk exists, and the usable fluence is significantly reduced in high-repetition systems. The inability to easily replace damaged optics in large systems exacerbates the problem. While damage management techniques exist, they do not significantly increase usable intensity. Transient optics, such as plasma gratings and mirrors, offer a potential solution, but they suffer from high energy deposition leading to short lifetimes and large power requirements. This paper proposes an alternative approach using a neutral gas with a linear absorption process, offering a significantly higher damage threshold.

Previous research on high-power laser optics highlights the limitations of current materials and the need for improved damage thresholds. Studies on dielectric coated mirrors and diffractive optical elements show damage thresholds ranging from 1 J/cm² to 200 J/cm² for nanosecond pulses. The use of plasma optics has been explored as a solution to improve damage resistance, but these techniques often require high energy densities for operation, resulting in short lifetimes and limiting their practical application. Attempts to increase optical path length in neutral gas media have been made, but have not led to practical devices with sufficient diffraction efficiency. This work addresses these limitations by proposing a novel method for creating large density modulations in neutral gases to achieve high diffraction efficiency and damage resistance.

The proposed system uses a mixture of ozone (O₃) and oxygen (O₂) at atmospheric pressure. Ozone has high absorption in the deep ultraviolet (UV) region, making it suitable for creating a spatially modulated density profile using a UV writing beam. The UV writing beam (100 mJ, 10 ns pulse from a 248 nm KrF excimer laser) is spatially modulated using interferometry to generate a periodic intensity profile. Ozone molecules absorb UV photons and dissociate, converting the absorbed energy into thermal energy, leading to coupled sound and entropy waves that create a density modulation. The temporal evolution of the ozone density is measured using absorption spectroscopy with a 287 nm probe laser. The modulation of the refractive index is measured with a 598 nm probe laser. A diffraction grating is created by this density modulation. Diffraction efficiency is optimized by adjusting ozone density, UV writing beam intensity, medium thickness, wavelength, and incident angle. Saturable absorption ensures uniform absorption of the UV writing beam. The diffraction efficiency is measured using a 532 nm, 6 ns pulse from a Nd:YAG laser. The damage threshold is determined by observing the occurrence of ionization damage, manifested by emission from the damage point, changes in the transmitted laser beam profile, and wavefront distortion. Tests are performed using air, ozone-mixed oxygen gas, and ozone-mixed gas with UV laser illumination. Wavefront quality of the diffracted beam is assessed using self-referenced interferometry. Nonlinear optical effects in the gas are considered, including stimulated Raman scattering and Brillouin scattering. The study also includes an analysis of the angle dependence of diffraction efficiency and the impact of high energy fluences on wavefront distortion.

The study achieved a diffraction efficiency of >95% with the ozone-oxygen gas-based diffraction grating. The damage threshold for a 6 ns, 532 nm laser pulse was measured to be 1.6 kJ/cm², which is two orders of magnitude higher than that of conventional solid-state optics. The diffraction efficiency was 96% at 63 mJ/cm² of UV writing beam energy. The wavefront distortion of the diffracted beam was found to be less than λ/10. The angle tolerance for 80% diffraction efficiency was approximately 0.06°. High energy fluence experiments (>1 kJ/cm²) demonstrated the onset of wavefront distortion above the damage threshold. Nonlinear effects were determined to be negligible under nanosecond operation, but become significant at femtosecond pulse durations.

The results demonstrate the feasibility of using a neutral gas medium to create high-efficiency, high-damage-threshold diffraction gratings. The significantly higher damage threshold compared to conventional solid-state optics addresses a critical limitation in high-power laser systems. The achieved diffraction efficiency of 96% is high enough for practical applications. The relatively low energy requirements for the control beam (63 mJ/cm²) are also advantageous. The observed wavefront distortion at high energy fluences indicates a limitation, but the damage threshold remains significantly higher than conventional optics. This method offers a path towards miniaturization of high-energy laser systems by replacing multiple optical components with a single, robust gas-based device.

This research successfully demonstrated a novel approach for creating high-damage-threshold optics using a neutral gas medium. The achieved high diffraction efficiency and exceptionally high damage threshold open up possibilities for miniaturizing high-energy laser systems. Future research could focus on exploring different gas mixtures with even higher damage thresholds and extending the technique to other pulse durations. Investigating the scalability of the system to larger apertures and higher energy applications is also warranted.

The current system is limited by the aperture size (6 mm × 10 mm) due to the need for laminar gas flow. While the damage threshold is significantly higher than conventional optics, wavefront distortion occurs above the damage threshold, impacting the performance at the highest intensities. The study focused on nanosecond pulses; further investigation is needed to determine the performance with picosecond and femtosecond lasers, where nonlinear effects become more prominent.