Preliminary performance data from a high spatial frequency resolution 24-inch interferometer

The National Ignition Facility is a next generation inertial confinement laser which will need approximately 7,000 large, square aperture optics with a clear aperture of approximately 430 mm on a side. The optics utilized in the laser required a unique set of wavefront specifications because of the extremely high fluences and nonlinear processes that affect the wavefront intensity evolution at these extreme power densities. The optical wavefront specification for these large optics is subdivided into specific spatial frequency regions, the lowest part defined in terms of a RMS wavefront gradient and the higher spatial frequencies limited to not exceed a maximum Power Spectral Density (PSD) defined in greater detail elsewhere.1 In order to verify that these spatial frequency wavefront specifications have been met, the NIF Program has contracted to Veeco Process Metrology (formerly Wyko) a 24-inch phase modulating interferometer capable of extremely high spatial frequency resolution to measure PSD over an operational range of 0.03 to 1.25 mm-1.2 This instrument is a standard Fizeau configuration interferometer with a linearly polarized 650-nm DFB diode laser source and a 1024 x 1024 pixel CCD detector. The optical imaging system has two fixed zoom settings, the lowest corresponding to a spatial viewing area of 431 x 431 mm square centered within the 24-inch diameter fused silica transmission sphere. The high spatial frequency zoom setting provides a 100 mm x 100 mm square aperture with a diffraction limited imaging lens designed to have an MTF > 0.9 at 0.4 mm-1 and MTF > 0.6 at 1.25 mm-1.