Spectrally controlled interferometry for high numerical aperture spherical cavity measurements

High numerical aperture optical elements are relied on for the most demanding applications in optical imaging but pose a significant challenge for conventional metrology techniques. Laser Fizeau interferometers provide a flexible measurement platform for measuring spherical optics by offering a common path configuration to test spherical optics against a convex reference surface. However, in this configuration, traditional piezoelectric transducer (PZT) based phase shifters produce non-uniform phase shifts which vary across the aperture as the spherical reference surface is translated along the optical axis. While these errors are negligible for low numerical aperture optics, the phase shift errors quickly become significant for high numerical aperture optics. The phase shift nonuniformity results in fringe print through and phase ripple artifacts which limit overall accuracy of phase shifted interferometry (PSI) measurements. Spectrally controlled interferometry (SCI) is a method which produces localized, high contrast interference fringes in non-zero optical path length cavities through tailored control of the sources spectral distribution. In addition to fringe location, fringe phase is also controlled through spectrum manipulation without mechanical motion or compensation. As a consequence, the SCI method produces uniform, full-aperture phase shifts with a high degree of linearity regardless of numerical aperture; thus, phase shift errors associated with traditional PZTs can be eliminated. Furthermore, because SCI is a source driven method, it can easily be integrated with any Fizeau interferometer. In this paper, we present the fundamental background for SCI and the advantages of the method as they apply to the measurement of high numerical aperture spherical optics. Additionally, we compare PSI measurements between a traditional laser Fizeau interferometer with PZT based phase shifters and an SCI Fizeau interferometer. Existing methods to this problem are discussed and compared with the presented SCI method, as well.