Understanding and protecting against backscattered light in high-power laser systems

Optical components can be damaged during high-power laser experiments due to backscattering of the laser light. Without a thorough understanding of the mechanism that causes these damages, the risk of damaging optical components may unnecessarily restrict the potential of high-power laser experiments. In a new report, researchers simulate burn patterns that may emerge from stimulated Brillouin scattering (SBS) using experimental measurements made at the National Ignition Facility (NIF).

Light scattered by SBS can travel back through the final aperture of the beam line and through the phase plate and damage the optical components inside the beam line. The researchers based their calculations on a measured profile of a phase plate at the NIF and simulated the resulting laser-plasma interaction in a typical target. The damage patterns predicted by their model agreed with the experimental observations.

Their simulations also showed the burn patterns from these events are consistent and highly structured. According to the time-integrated data, the burn patterns are largely determined by the phase imprint of the phase plate on the returning SBS light that re-enters the beam line through the phase plate. They also found the SBS light radiation pattern to be highly modulated in space and time with large picosecond spikes, suggesting that picosecond or subpicosecond damage initiation processes might be of relevance to nanosecond laser pulses.

With a better understanding of the potential damages from backscattering, scientists and engineers may better utilize instruments and design experiments. The work may reveal where to optimally place light filters that can protect sensitive optical components vulnerable to these damages or how to design phase plates that reduce the modulation depth of the returning SBS light.

Source: “Investigation and modeling of optics damage in high-power laser systems caused by light backscattered in plasma at the target,” by T. Chapman, P. Michel, J.-M. Di Nicola, R. L. Berger, P. K. Whitman, J. D. Moody, K. R. Manes, M. L. Spaeth, M. A. Belyaev, C. A. Thomas, and B. J. MacGowan, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5070066 .