Twyman-Green interferometer

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The Twyman-Green interferometer (named after Frank Twyman and Alfred Green ) is an interferometer , i.e. an optical measuring device that is used to characterize and test optical components such as lenses or prisms .

Structure and functionality

The structure of a Twyman-Green interferometer essentially corresponds to that of the Michelson interferometer , consisting of a light source , a beam splitter , two mirrors and a screen / detector. However, there are two differences: on the one hand, the use of a monochromatic point light source and, on the other hand, lenses at the interferometer input and output.

How it works can be summarized as follows. The light from the light source hits the input lens, which generates a collimated beam (plane wave) of finite dimensions from the light from the point source . The collimated light then hits a beam splitter, where it is split into two identical partial beams that each hit one of the two mirrors. After being reflected on the two mirrors, the light hits the beam splitter again and is combined again and directed onto the interferometer output. The lens at the interferometer output then ensures that the light is focused on a screen or detector. As with all interferometers, when the two partial beams are combined, the partial waves are superimposed and thus there is interference depending on the optical path difference between the two partial beams. These differences can then be observed as a characteristic stripe or ring pattern and allow conclusions to be drawn about the surface shapes of the mirror and the sample.

Optical components are tested by placing the component in one of the beam paths. It must be ensured that when testing a prism, for example, the light hits the second reference mirror perpendicularly. Otherwise only part of the light or no light at all will reach the beam splitter again. When testing lenses, a curved reference mirror that fits the lens must be used instead of a flat reference mirror.

A similar structure is also used in the Fizeau interferometer .

variants

A slightly modified structure uses a polarizing, cube-shaped beam splitter and 45 ° linearly polarized light instead of a plane beam splitter . Due to this change in structure, the two partial beams are differently polarized after beam splitting, i.e. horizontally and vertically, linearly polarized and are converted into different rotating, circularly polarized light at the exit of the cube-shaped beam splitter by a λ / 4 retardation plate. The light returning from the reference mirror and the sample then passes through the λ / 4 retardation plate and the beam splitter again and arrives at the detector as differently linearly polarized light. It should be noted that these two partial beams do not interfere with one another due to their different polarization and that no light reaches the light source again. An interfering signal can be achieved by another linear polarization.

Areas of application

Twyman-Green interferometers are of great importance in the field of optical test methods. It is therefore used, among other things, in the manufacture of optical components such as lenses, prisms or reflective surfaces, to test the optical properties such as the homogeneity of flat surfaces.

literature

  • Daniel Malacara: Optical shop testing . John Wiley & Sons, 2007, ISBN 978-0-471-48404-2 , pp. 46-96 ( limited preview in Google Book Search - very detailed description of the structure and use).

Individual evidence

  1. ↑ CV under: AC Menzies: Frank Twyman. 1876-1959 . In: Biographical Memoirs of Fellows of the Royal Society . tape 5 , 1960, pp. 269-279 , JSTOR : 769291 .
  2. ↑ Patent GB103832 : Improvements in Finishing Prisms or Lenses or Combinations of the same and in Apparatus therefor. Published February 5, 1917 , inventors: Frank Twyman, Alfred Green. ‌
  3. ↑ Schüle Sven: Modular adaptive micro-optical systems in combination with micro-actuators: a micro-optical system based on micro-optical banks . KIT Scientific Publishing, 2010, ISBN 978-3-86644-529-1 , pp. 11-12 .
  4. ↑ Eugene Hecht: Optics . Oldenbourg Wissenschaftsverlag, 2005, ISBN 978-3-486-27359-5 , p. 700-701 .
  5. ↑ Michael Bass (Ed.): Handbook of Optics: Volume II - Design, fabrication, and testing; sources and detectors; radiometry and photometry . McGraw Hill Professional, 2009, ISBN 978-0-07-149890-6 , pp. 13.8 .