Interferometry

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With interferometry all measurement methods are referred to the superposition or interference of waves use to measured quantities to be determined. All effects that influence waves are accessible to it, and the structure of the required measuring devices, the interferometer , is correspondingly diverse.

In principle, any type of wave , be it light, sound, matter or even water waves , can generate interference and thus also operate interferometry.

Interferometer

An interferometer is a technical device that uses the interference (superimposition of waves) for precision measurements. All effects that change the effective path length of the waves and thus the properties of the superimposed wave are measured. Examples of this are changes in length of one of the two superimposed light paths for length measurement, changes in the refractive index for measuring material properties or minimal changes in the distance between the test masses in gravitational wave detectors . The superimposed wave is z. B. observed with a screen or an electronic detector. Fields of application are length measurement, refractive index measurement, angle measurement and spectroscopy . In addition to interferometry with light waves, there are also atomic interferometers that utilize the wave property of particles according to the wave-particle dualism .

Interference in optical imaging

Systems that image optically use the interference of the light waves arriving at the entrance opening to generate a real or virtual image . This applies to a converging lens that projects an image, as well as to a telescope whose eyepiece shows a virtual image of distant objects. In this sense, the perceived image represents an interferogram.

The superimposition of the image information of several signals of spatially separated individual instruments by special devices is a frequently used method to increase the resolution of instruments. As a result, smaller details can be mapped better or at all. Examples of this are the superimposition of the telescope light from the Very Large Telescope in interferometer mode, the use of interferometers in radio astronomy or the worldwide, arithmetical combination of radio telescopes with Very Long Baseline Interferometry (VLBI) or the synthetic aperture in Synthetic Aperture Radar . In astronomy , the earth-based VLT can achieve resolutions that are not possible with individual instruments in space due to their limited size. With the VLBI, radio sources can be imaged with a resolution that was previously unimaginable in visible light, despite much larger wavelengths than with visible light.

The prerequisite for successful interference is that the waves are coherently superimposed. This means that the waves coming from different parts of the interferometer can only interfere if the paths (run lengths) differ by less than the coherence length . The coherence length depends on the wavelength and the spectral bandwidth (filter bandwidth) of the light used.

Unlike in optical astronomy, in radio astronomy signals from participating radio telescopes can be superimposed computationally using computers . To do this, one draws the complete wave information of each participating telescope, i. H. the amplitude of the signal depends on the time, the precision of the time measurement is of particular importance. Only with sufficiently precise time measurement is the phase position of the individual signals in relation to one another included in the data, and the interference can be calculated in the computer. In this way, even radio telescopes on different continents can be interconnected and thus deliver high-resolution images.

Interferometer types

There are different types of interferometers, the functionality of which is essentially the same. At least two light bundles are guided through separate optical paths using mirrors or semi-transparent plates (so-called beam splitters), reflected at the end of the path by additional mirrors and brought together again at the end. The result is an interference pattern (interference fringes or rings), which was described as a pattern by the light rays. This pattern is determined by the difference in the optical paths that the individual rays have covered before they merge.

Two-beam interferometer

• Bath interferometer
• Mach-Zehnder interferometer and its forerunner Jamin interferometer
• Michelson interferometer
  • Twyman-Green interferometer
• Rayleigh interferometer
• Sagnac interferometer
• White light interferometer
• Interferometric Synthetic Aperture Radar
• Atomic interferometer

Multi-beam interferometer

• Fabry-Perot interferometer
• Fizeau interferometer

Further interferometric measuring methods

• Speckle interferometry ( see also: Speckle )
• Phase shift method
• Intensity interferometer

application

Interferometers are used in physics , astronomy and measurement technology in various designs for a variety of purposes.

One of the most famous experiments in history is the Michelson-Morley experiment , which used a Michelson interferometer to prove that the speed of light is the same in every frame of reference. The result of this experiment shook the theory of aether and was also one of the basic assumptions of the special theory of relativity set up later by Albert Einstein . Using the same principle, a modern Michelson interferometer is being used in the Geo-600 project to detect gravitational waves , which was achieved in 2015 in the LIGO observatory .

Today interferometers are used in astronomy in all wavelength ranges to improve resolution. With interferometers from radio telescopes , an angular resolution that is around 500 times higher than in the optical range can be achieved because of the considerably longer base length. The “ Very Large Telescope ” uses interference to deliver high-resolution images in the visible spectral range (see also astronomical interferometry ).

Interferometers are also used as laser Doppler vibrometers , a measuring device for measuring vibrations. Laser interferometers use interference to measure distances and white light interferometers and phase-shifting interferometers as measuring devices for measuring the shape of workpieces. Another area of ​​application is the FTIR spectrometer , a measuring device for the chemical analysis of materials. Capillary wave spectroscopy is used to investigate interface processes.

literature

• Parameswaran Hariharan: Basics of interferometry. Elsevier Acad. Press, Amsterdam 2007, ISBN 978-0-12-373589-8 .
• WH Steel: Interferometry. Cambridge Univ. Pr., Cambridge 1983, ISBN 0-521-25320-9 .
• Robert D. Reasenberg: Spaceborne interferometry. SPIE, Bellingham 1993, ISBN 0-8194-1183-3 .
• C. Mattok: Targets for space-based interferometry. ESA Publ. Div., Noordwijk 1992, ISBN 92-9092-234-6 .

Web links

• Interferometry measurement with fiber optics
• Dirk Lorenzen : Four small telescopes simulate a large one. dradio Research News October 26, 2011.

Individual evidence

1. ^ LIGO Scientific Collaboration and Virgo Collaboration, B. P. Abbott, R. Abbott, T. D. Abbott, M. R. Abernathy: Observation of Gravitational Waves from a Binary Black Hole Merger . In: Physical Review Letters . tape 116 , no. 6 , February 11, 2016, p. 061102 , doi : 10.1103 / PhysRevLett.116.061102 ( aps.org [accessed February 19, 2019]).