Experimental comparison of a Shack-Hartmann sensor and a phase-shifting interferometer for large-optics metrology applications.

We performed a direct side-by-side comparison of a Shack-Hartmann wave-front sensor and a phase-shifting interferometer for the purpose of characterizing large optics. An expansion telescope of our own design allowed us to measure the surface figure of a 400-mm-square mirror with both instruments simultaneously. The Shack-Hartmann sensor produced data that closely matched the interferometer data over spatial scales appropriate for the lenslet spacing, and much of the <20-nm rms systematic difference between the two measurements was due to diffraction artifacts that were present in the interferometer data but not in the Shack-Hartmann sensor data. The results suggest that Shack-Hartmann sensors could replace phase-shifting interferometers for many applications, with particular advantages for large-optic metrology.

Dielectric Mirror Damage by Laser Radiation over a Range of Pulse Durations and Beam Radii.

Laser induced damage characteristics have been studied for twelve, multiple-layer dielectric, 95% reflecting mirrors irradiated with single ruby laser pulses of 20 nsec and 20-psec duration and Gaussian beam radii ranging from 0.06 mm to 0.24 mm. During measurements to determine damage threshold the pulse energy, an oscilloscope trace of the pulse, and the beam's transverse energy density profile at the surface being damaged are recorded for each shot. In a separate set of experiments, the temporal development of coating breakup and plasma formation is investigated. The experimental results are discussed in the context of possible damage mechanisms.