Interferometric measurements can be carried out in an optical arrangement in which two or more beams originate from the same source, but pass in different paths and so are made to interfere. Interferometers are grouped into two-beam and multiple beam interferometers.
One of the most common types of two-beam interferometers is the Fizeau interferometer, wherein two flat reflecting surfaces are placed facing each other. These two surfaces are separated by an air gap and formed with interference fringes of the same thickness when illuminated with a collimated beam.
Having one of the surfaces as a standard reference flat surface, the fringe pattern also provides a contour map indicating the errors of the test surface. Modified versions of the Fizeau interferometer can be used to analyze concave and convex surfaces, with the help of a converging or diverging beam.
Working Principle of Fizeau Interferometers
A typical Fizeau interferometer consists of a laser source and a pinhole, located at the focal point of a collimating lens. A beam-splitter is present between the lens and pinhole. The collimated beam hits a slightly wedged glass plate. The reference surface adjacent to the collimated lens has a good optical quality, such that a part of the collimated beam is reflected by this surface.
Light reflected from the tilted beam-splitter is made parallel using a lens, and is split into two beams. Each beam travels along a different path, and is reflected at the mirror. The two beams recombine at a detector, thereby forming an interference pattern based on the time difference of the two traveling beams. The interference pattern can be used to determine the speed of light.
Applications of Fizeau Interferometers
Fizeau interferometers are applied in the following:
- Measurement of the speed of light
- Evaluation of the quality of optical components and systems
- As a guide for manufacturing of optical components
- Validation of system performance
- Measurement of flat and spherical surfaces
Sources and Further Reading