INFRARED INTERFEROMETRIC SYSTEMS

Abstract

Infrared interferometric systems using a CO₂ laser operating at a wavelength of 10.6 μm have been investigated. The purpose of infrared interferometry is to test (1) optical components required for high energy laser systems such as infrared transmitting materials and diamond-turned metal mirrors, (2) unpolished rough surface optics during the early stages of fabrication, and (3) deep aspherics and other optics of nonconventional surface figures. The physical principles behind longer wavelength interferometry are as follows. First, the specular component of scattered light increases with increasing wavelength for randomly rough surfaces. Second, the aspheric departure from the best fit reference sphere (in units of probing wavelength) is reduced. This reduced sensitivity gives us a manageable number of fringes in the interferogram of deep aspherics. Specific systems developed in this work are the infrared laser unequal path interferometer (IRLUPI), the IRLUPI with infrared computer generated hologram (IRCGH), the infrared common path interferometers such as the infrared point diffraction interferometer (IRPDI), and the infrared scatterplate interferometer (IRSPI). The above interferometers produce interferograms of equal optical path difference (OPD). Other types of common path interferometers which provide interferograms of differential OPD (or slope) are also developed. They are the infrared lateral shearing interferometers (IRLSI); a germanium plane parallel plate, the Ronchi ruling, and the double grating lateral shearing interferometer. A pyroelectric vidicon (PEV) has been employed as an AC infrared detector with proper intensity modulation techniques. Chopping, panning, and phase variation of the interferogram modulate the interference pattern effectively for various types of interferometers. Germanium and zinc selenide optics are used for lenses and beamsplitters. A He-Ne gas laser is installed parallel to the CO₂ beam for the ease of initial alignment. Many test interferograms are shown using each interferometer. The statistical analysis and experimental verification of the relationship between fringe contrast and rms surface roughness enabled us to have noncontact measurement of surface roughness interferometrically. This result was used for a series of tests for the unpolished large diameter off-axis parabolic mirror during the preliminary fabrication stage. Some interesting topics are included for future investigation to fulfill the growing demand for versatility in interferometry.