Diffractive optics calibrator: Design and construction

Abstract. We present a new device, the diffractive optics calibrator, for measuring duty cycle and etching depth for computer-generated holograms (CGHs). The system scans the CGH with a collimated laser beam and collects the far-field diffraction pattern with a CCD array. The relative intensities of the various orders of diffraction are used to fit the phase shift from etching and the duty cycle of the binary pattern. The system is capable of measuring variations that cause 1-nm peakto- valley (PV) phase errors in the wavefront created by the CGH. The measurements will be used primarily for quality control CGHs, but the data can also be used to provide a lookup table for corrections that allow calibration of the lithography errors. Such calibrations may be necessary for us to achieve our goal of measuring freeform aspheric surfaces with 1-nm RMS accuracy.