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Optical interferometry

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Optical interferometry is a technique of interferometrycombining light from multiple sources in an opticalinstrument in order to make various precise measurements.

The technique of optical interferometry can make use of white light,of monochromatic light (e.g., a sodium lamp) or ofcoherent monochromatic light (laser light). The main difference between thesetypes of light is their coherence length: for white light, only a few wavelengths, butfor laser light it can be decimeters or more. In order to see interference fringes at all,the optical path lengths travelled by the interfering beams or rays must differ by less thantheir correlation length.


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Astronomical optical interferometry

See main article astronomical interferometer.

One of the first astronomical interferometers was built on the Mount Wilson Observatory's reflector telescope in 1920 in order to measure the diameters of stars. The red giant star Betelgeuse was among the first to have its diameter determined in this way. This method was extended to measurements using separated telescopes by Johnson, Betz and Towns (1974) in the infrared and by Labeyrie (1975) in the visible. In the late 1970's improvements in computer processing allowed for the first "fringe-tracking" interferometer, which operates fast enough to follow the blurring effects of astronomical seeing, leading to the Mk I,II and III series of interferometers. Similar techniques have now been applied at other astronomical telescope arrays, including the Keck Interferometer and the Palomar Testbed Interferometer.

In the 1980s the aperture synthesis interferometric imaging technique was extended to visible light and infrared astronomy by the Cavendish Astrophysics Group, providing the first very high resolution images of nearby stars. In 1995 this imaging technique was demonstrated on an array of separate optical telescopes for the first time, allowing a further improvement in resolution, and allowing even higher resolution imaging of stellar surfaces. The same imaging technique has now been applied at the Navy Prototype Optical Interferometer and the IOTA array. In the near future other arrays are expected to release their first interferometric images, including the ISI, VLTI, the CHARA array and the MRO interferometers.

Projects are now beginning that will use interferometers to search for extrasolar planets, either by astrometric measurements of the reciprocal motion of the star (as used by the Palomar Testbed Interferometer and the VLTI) or through the use of nulling (as will be used by the Keck Interferometer and Darwin).

A detailed description of the development of astronomical optical interferometry can be found here. Impressive results were obtained in the 1990s, with the Mark III measuring diameters of 100s of stars and many accurate stellar positions, COAST and NPOI producing many very high resolution images, and ISI measuring stars in the mid-infrared for the first time. Additional results included direct measurements of the sizes of and distances to Cepheid variable stars, and young stellar objects. At the beginning of the 21st Century, the VLTI and Keck Interferometer large-telescope arrays came into operation, and the first interferometric measurements of the brightest few extra-galactic targets were performed.

Interferometers are mostly seen by astronomers as very specialized instruments, capable of a very limited range of observations. It is often said that an interferometer achieves the effect of a telescope the size of the distance between the apertures; this is only true in the limited sense of angular resolution. The combined effects of limited aperture area and atmospheric turbulence generally limit interferometers to observations of comparatively bright stars and active galactic nuclei. However, they have proven useful for making very high precision measurements of simple stellar parameters such as size and position (astrometry) and for imaging the nearest giant stars.

For details of individual instruments, see the list of astronomical interferometers at visible and infrared wavelengths.

Optical interferometry:Ast opt int lba Optical interferometry:Ast opt int mask
A simple two-element optical interferometer. Light from two small telescopes (shown as lenses) is combined using beam splitters at detectors 1, 2, 3 and 4. The elements creating a 1/4 wave delay in the light allow the phase and amplitude of the interference visibility to be measured, which give information about the shape of the light source. A single large telescope with an aperture mask over it (labelled Mask), only allowing light through two small holes. The optical paths to detectors 1, 2, 3 and 4 are the same as in the left-hand figure, so this setup will give identical results. By moving the holes in the aperture mask and taking repeated measurements, images can be created using aperture synthesis which would have the same quality as would have been given by the right-hand telescope without the aperture mask. In an analogous way, the same image quality can be achieved by moving the small telescopes around in the left-hand figure - this is the basis of aperture synthesis, using widely separated small telescopes to simulate a giant telescope.

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The Michelson-Morley experiment

Of the uses of optical interferometry mention should be made of theMichelson-Morley experiment, a test of theSpecial Theory of Relativity. Light travels along two alternative,mutually perpendicular paths, and meets again to form interferencefringes. If the classical theory of absolute space and time andpropagation of electromagnetic waves in a world aether are correct,rotating the whole device with respect to the direction of motion ofthe Earth should produce a shift in the interference fringes. No suchshift was seen.

Geodetic standard baseline measurements

A famous use of white light interferometry is the precise measurementof geodetic standard baselines as invented by Yrjö Väisälä.Here, the light path is split in two, and one leg is "folded" betweena mirror pair 1 m apart. The other leg bounces once off a mirror 6 maway. Only if the second path is precisely 6 times the first, willfringes be seen.

Starting from a standard quartz gauge of 1 m length, it is possible tomeasure distances up to 864 m by repeated multiplication. Baselines thus established are used tocalibrate geodetic distance measurement equipment on, leading to ametrologically traceable scale for geodetic networksmeasured by these instruments.

More modern geodetic applications of laser interferometry are in calibrating thedivisions on levelling staffs, and in monitoring the free fall of a reflectiveprism within a ballistic or absolute gravimeter, allowing determination of gravity,i.e., the acceleration of free fall, directly from the physical definition at a fewparts in a billion accuracy.

Holography

A special application of optical interferometry using coherent lightis holography, a technique for photographically recording andre-displaying three-dimensional scenes. The technique also lendsitself to monitoring small deformations.

Inertial navigation

In inertial navigation, ring laser gyroscopes are used that can detectrotation through optical interferometry of laser beams travellingaround a circumference in opposite directions (Sagnac interferometer). Theeffect is amplified by using optic fibres wound around thousands oftimes.

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