The fast measurement of a mild asphere by the vibration-modulated sub-aperture stitching interferometer

Sub-aperture testing methods are widely used in optical shops to test surface deformations of large diameter, high numerical aperture, or aspherical lens surfaces. We are proposing a novel 4 axis vibration modulated interferometer for subaperture testing. This interferometer takes advantage of the rotationally symmetric property of the optical lens and measures the lens surface against its symmetry axis rotationally. By adapting a synchronous random phase modulation measurement, interferometric data is acquired on the fly when the lens is being rotated. The vibration modulated interference phase is then calculated and stitched into a complete lens surface map by least squared fitting. This method has advantages over the prior methods in that it acquires the interferogram in a much shorter acquisition time, even with lower requirements on the optics and mechanical hardware. The stitch error is then significantly decreased by increasing both the lateral resolution of sub-aperture and the reduced position uncertainty of the stitched sub-aperture phase maps. A measurement on a mild asphere is demonstrated to prove the feasibility of the proposed interferometer.