Measurement and analysis optimization of large aperture laser Fizeau interferometer

Abstract

High-power laser systems, such as the National Ignition Facility (NIF) project at Lawrence Livermore National Laboratories (LLNL), require optics of extremely high quality. Surface errors, especially periodic surface relief structures, can lead to focal spot degradation at best and serious damage to downstream optics at worst. The optics in these systems must be characterized with a high degree of accuracy to ensure proper operation. To provide system optics of sufficient quality, the testing apparatus must measure surface structure with high fidelity and cannot introduce significant errors into the measurements. This paper deals with measurements taken on two WYKO phase-shifting laser Fizeau interferometers to optimize their ability to meet the measurement requirements for optics in high-power laser systems. Increasingly, tolerances on optics are being specified with the power spectral density function (PSD) of the surface height data, and thus the power spectrum is used to characterize the measurement system. The system transfer function, which is the ratio of the measured amplitude of frequency components to the actual, is calculated using several methods. The effects of various parameters on the calculated system transfer function are studied. First, the use of the finite Fourier transform to estimate the power spectrum from surface profile data was studied. Next, simulated measurements were analyzed to determine the effects of rotation, feature location, noise, windowing, and other variables on the calculated power spectral density. After the theoretical analysis, the interferometer transfer function was calculated using two techniques. The effects of wavefront propagation on the measurements were also studied. Measurements were first taken on a 150mm laser Fizeau system where the effect of changing various parameters was studied. Final measurements were taken on the 600mm system to verify system performance. The large aperture laser Fizeau interferometer as built had a system transfer that surpassed the system requirements with regards to transfer function and measurement noise. The system measured frequency amplitudes with 70% fidelity up to half the Nyquist frequency. In addition, the power spectrum of the noise plots was below the system specification of 0.1ν⁻¹·⁵⁵nm²mm over the spatial frequencies ν of interest,¹ more than ten times lower than the specification on the parts to be measured.