Fizeau interferometer
A Fizeau interferometer (named after its inventor Hippolyte Fizeau ) is a special interferometer that is used, among other things, to assess the optical quality of surfaces and optics. For this purpose, a defined surface is compared with another surface using the interference of light.
Structure and functionality
A Fizeau interferometer for assessing opaque surfaces is constructed as follows. Monochromatic light (e.g. laser light ) is "filtered" through an objective lens and a pinhole . The pinhole is in the focus (focal point) of a second lens, the collimation lens . A beam splitter is located between the diaphragm and the second lens .
The collimated beam now strikes a glass plate, the surface of which facing the collimation lens is of good optical quality. The other surface is of lower quality, a so-called λ / 20 surface or better is sufficient. It serves as a reference surface through which part of the light is reflected. The transmitted portion goes on to the sample area. The portion reflected from the sample surface contains information about the aberration caused by the sample . The wave fronts of both parts interfere in the interferometer and are directed to a screen or detector via the beam splitter. The recorded image now shows a sharp image of the sample surface through which a striped pattern (the interference pattern) is drawn. A continuous strip shows areas of the same air gap thickness. Adjacent stripes, on the other hand, indicate a change in thickness that corresponds to half the wavelength of the light.
The structure of a Fizeau interferometer is comparable to that of a Fabry-Pérot interferometer , which also consists of two partially reflective surfaces. In a Fizeau interferometer, however, the two surfaces are usually less reflective ( reflectance around 4–30%), so that secondary reflections contribute less to the edge contrast.
The light fringes are easy to interpret and the differences of less than λ / 20 of a wavelength can be measured visually. The classic stripe pattern produced by a Fizeau interferometer is Newton's rings, so it is sometimes referred to as a Newton interferometer. These are generated by comparing a convex sphere with a flat sample surface.
In contrast to other interferometers, the beam splitter has no interferometric function. Furthermore, it is not rotated by 45 ° with respect to the collimated beam. This has a couple of practical advantages: The beam splitter
- has a simple and robust construction,
- can be smaller than other interferometers with the same aperture - and thus also the interferometer,
- is easier to align (insensitive to adjustment).
application
Fizeau interferometers are commonly used to measure the shape of an optical surface (e.g. parallelism). As a rule, a lens or mirror is compared with a comparison piece of the same shape or surface quality. Sometimes the comparison piece is realized by a diffractive optical element, since these can be manufactured using photolithographic methods and enable greater precision in manufacture. Fizeau interferometers are also used in fiber optic sensors to measure pressure, temperature, strain, etc.
Fizeau interferometers are also often used in interference microscopes.
A modified Fizeau interferometer can also be used to measure the influence of the movement of a medium (such as water) on the speed of light. As shown in the graphic on the right, it is reflected by a tilted beam splitter and split into two parallel beams with the help of a lens and a slit. The jets each traverse a different part of a pipe in which water moves. After the crossing, both rays are reflected by a further lens on a mirror in such a way that each ray takes the path of the other ray back. The two beams are combined at the detector and form an interference pattern that depends on the path difference on their way through the water.
literature
- Joseph M. Geary: Introduction to optical testing . SPIE Press, 1993, ISBN 978-0-8194-1377-2 ( limited preview in Google Book Search).
- P. Hariharan: Optical interferometry . Academic Press, 2003, ISBN 978-0-12-311630-7 ( limited preview in Google Book Search).
- P. Hariharan: Basics of interferometry . Academic Press, 2007, ISBN 978-0-12-373589-8 ( limited preview in Google Book Search).
- Gerd Litfin: Technical optics in practice . Springer, 2004, ISBN 978-3-540-21884-5 , pp. 52 ( limited preview in Google Book search).
Individual evidence
- ^ Peter R. Lawson, Principles of Long Baseline Stellar Interferometry. In: Course notes from the 1999 Michelson Summer School, 15. – 19. August 1999. National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 2000.
- ^ A b Bass Michael, Decusatis Casimer, Enoch Jay: Handbook of Optics. Volume I: Geometrical and Physical Optics, Polarized Light, Components and Instruments . 3. Edition. McGraw Hill Professional, ISBN 978-0-07-149889-0 .
- ↑ Michael Bass: Handbook of optics, Volume II: Design, Fabrication, and Testing; Sources and detectors; Radiometry and Photometry . 3. Edition. McGraw Hill Professional, 2009, ISBN 978-0-07-149890-6 , pp. 13.9 ( limited preview in Google Book Search).
- ↑ Reinhart Poprawe: Laser technology for production: Basics, perspectives and examples for the innovative engineer . Springer, 2004, ISBN 978-3-540-21406-9 ( limited preview in Google book search).
- ^ Robert Williams Wood: Physical Optics . The Macmillan Company, 1905, pp. 514 ( limited preview in Google Book search).