New design techniques and alignment methods for CGH-null testing of aspheric surface

The entire process of designing the test setup with a computer-generated hologram (CGH), fabricating the CGH with a laser direct writing system and performing the CGH-null test of an asphere will be described in detail. New considerations in designing the null corrector with calculating CGH's phase function, filtering off unwanted diffraction orders and aligning CGH to the interferometer are discussed. A laser writing machine is used to fabricate designing patterns onto plane substrate and substrate figure error, pattern distortion, etching and duty cycle variations that influence the reconstructed wavefront are quantitatively analyzed in theory and corresponding error equations are obtained. The CGH used for measuring a paraboloid mirror is designed and fabricated. Experimental results of interferometric CGH-null test are presented.