Applications
Fizeau interferometers are commonly used for measuring the shape of an optical surface: Typically, a fabricated lens or mirror is compared to a reference piece having the desired shape. In Fig. 1, the Fizeau interferometer is shown as it might be set up to test an optical flat. A precisely figured reference flat is placed on top of the flat being tested, separated by narrow spacers. The reference flat is slightly beveled (only a fraction of a degree of beveling is necessary) to prevent the rear surface of the flat from producing interference fringes. A collimated beam of monochromatic light illuminates the two flats, and a beam splitter allows the fringes to be viewed on-axis.
The reference piece is sometimes realized by a diffractive optical element (computer-generated hologram or CGH), as this can be manufactured by high accuracy lithographic methods. Fig. 2 illustrates the use of CGHs in testing. Unlike the figure, actual CGHs have line spacing on the order of 1 to 10 microns. When laser light is passed through the CGH, the zero-order diffracted beam experiences no wavefront modification. The wavefront of the first-order diffracted beam, however, is modified to match the desired shape of the test surface. In the illustrated Fizeau interferometer test setup, the zero-order diffracted beam is directed towards the spherical reference surface, and the first-order diffracted beam is directed towards the test surface in such a way that the two reflected beams combine to form interference fringes.
Fizeau interferometers are also used in fiber optic sensors for measuring pressure, temperature, strain, etc.
Read more about this topic: Fizeau Interferometer