Abstract

A new type of Twymann-Green interferometer developed here mainly consists of a collimated He-Ne laser beam, a holographic optical element (HOE), a reference flat mirror, an imaging lens, and a screen. The HOE is made using a collimated reference beam and a diverging spherical object beam. In the interferometer the HOE, illuminated from its back by a collimated beam, diffracts a spherical wavefront and transmits the collimated beam. The former beam is reflected by a spherical mirror under test, passes through the HOE, and reaches to a screen. The latter beam is reflected by a flat reference mirror, diffracted by the HOE, and interferes with the former beam on the screen. In the interferometer the inhomogeneity of the HOE is automatically compensated because the optical paths of both test and reference beams are common in the HOE. A testing of a commercial spherical mirror, whose radius of curvature was 200 mm and diameter was 35 mm, resulted in 1/50 wavelength rms error and 1/10 wavelength PV error.

© 1991 Optical Society of America