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Interferometry is the technique of superposing (Interference) two or more waves, to detect differences between them. Interferometry is applied in a wide variety of fields, including astronomy, Optical fiber, optical metrology, oceanography, seismology, quantum mechanics and plasma physics.

Interferometer An interferometer works because two waves with the same frequency that have the same phase (waves) will add to each other while two waves that have opposite phase will cancel out, assuming both have the same amplitude. Early interferometers principally used white light sources (e.g., Double-slit experiment of 1805). Modern researchers often use monochromatic light sources like lasers, and even the wave character of matter can be exploited to build interferometers (e.g. with electrons interferometer, neutron interferometer, atom interferometer, or even molecules).

Types of Interferometers There are many types of interferometers, but all work on the same basic principle.

Michelson Interferometer In a Michelson interferometer (or Michelson-Morley) type interferometer, the basic building blocks are a monochromatic source (emitting light or matter waves), a detector, two mirrors and one semitransparent mirror (often called beam splitter). These are put together as shown in the figure.

There are two paths from the (light) source to the detector. One reflects off the beam splitter, goes to the top mirror and then Reflection (physics) back, goes through the semi-transparent mirror, to the detector. The other one goes through the semi-transparent mirror, to the mirror on the right, reflects back to the semi-transparent mirror, then reflects from the semi-transparent mirror into the detector.

If these two paths differ by a whole number (including 0) of wavelengths, there is constructive interference and a strong signal at the detector. If they differ by a whole number and a half wavelengths (e.g., 0.5, 1.5, 2.5 ...) there is destructive interference and a weak signal. This might appear at first sight to violate conservation of energy. However energy is conserved, because there is a re-distribution of energy at the detector in which the energy at the destructive sites are re-distributed to the constructive sites. The effect of the interference is to alter the share of the reflected light which heads for the detector and the remainder which heads back in the direction of the source.

This type of interferometer was used in the Michelson-Morley experiment, to disprove the existence of the Luminiferous aether. Michelson interferometers are also used in astronomical interferometers (see Interferometry#Astronomical optical interferometry below) and Gravitational radiation detectors.

Mach-Zehnder interferometer

Interferometers are used in integrated optical circuits, in the form of a Mach-Zehnder interferometer, in which light interferes between two branches of a waveguide that are (typically) externally modulation to vary their relative phase. This interferometer's configuration consists of two beam splitters and two completely reflective mirrors. The source beam is split and the two resulting waves travel down separate paths. A slight tilt of one of the beam splitters will result in a path difference and a change in the interference pattern. The Mach-Zehnder interferometer can be very difficult to align, however its improved sensitivity enables a diverse number of applications.E. Hecht, Optics, 2nd Edition, Addison-Wesley Publishing Co., Reading, Mass, USA, 1987. p. 358 The Mach-Zehnder interferometer can be the basis of a wide variety of devices, from RF modulators to sensors to optical switches.

Sagnac interferometer

A Sagnac Interferometer is an interferometry configuration in which a beam of light is split and the two beams are made to follow a trajectory in opposite directions. To act as a ring the trajectory must enclose an area. On return to the point of entry the light is allowed to exit the apparatus in such a way that an interference pattern is obtained.

In the Sagnac configuration, the position of the interference fringes is dependent on angular velocity of the setup. This dependence is caused by the rotation effectively shortening the path distance of one of the beams, while lengthening the other. A Sagnac interferometer has been used by Albert Michelson and Henry Gale to determine the angular velocity of the Earth. It can be used in navigation as a ring laser gyroscope, which is commonly found on fighter planes Sagnac Interferometer on Eric Weisstein's World of Physics Accessed Aug 1, 2006.

Fabry-Perot interferometer

A Fabry-Pérot interferometer or etalon is typically made of a transparent plate with two Reflection (physics) surfaces, or two parallel highly-reflecting mirrors. (Technically the former is an etalon and the latter is an interferometer, but the terminology is often used inconsistently.) Its transmission optical spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon. It is named after Charles Fabry and Alfred Pérot.

Fabry-Pérot interferometers are widely used in telecommunications, lasers and spectroscopy for controlling and measuring the wavelength of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Pérot interferometers. Fabry-Pérot interferometers also form the most common type of optical cavity used in laser construction.

Types of Interferometry Coherent interferometry Coherent interferometry uses a coherent light source (for example, a helium-neon laser), and can make interference with large difference between the interferometer path length delays. The interference is capable of very accurate (nanometer) measurement by recovering the phase.

One of the most popular methods of interferometric phase recovery is phase-shifting by piezoelectric sensor (PZT) phase-stepping. By stepping the path length by a number of known phases (minimum of three) it is possible to recover the phase of the interference signal, with 2 \pi = \lambda / 2.

Coherent interferometry suffers from a 2 \pi ambiguity problem: that is, if between any two measurements the interferometric phase jumps by more than 2 \pi the phase measurement is incorrect.However by combining interferometry results obtained using multiple wavelengths of illumination, such as in digital multi-wavelength holography, the ambiguity interval can be extended to indefinitely large dynamic ranges of measurement.

The applications of coherent interferometry are wide ranging: Nanometer surface profiling, Microfluidics, Mechanical stress/strain, Velocimetry, and high-definition metrology of large parts and assemblies in manufacturing.

Inertial navigation In inertial navigation, ring laser gyroscopes are used that can detect rotation through optical interferometry of laser beams travelling around a circumference in opposite directions

Speckle Interferometry In optical systems, a speckle pattern is a field-intensity pattern produced by the mutual interference of partially coherent beams that are subject to minute temporal and spatial fluctuations. This speckling effect is most commonly observed in the fields of fiber optics and astronomical speckle imaging.

Holography A special application of optical interferometry using coherent light is holography, a technique for photographically recording and re-displaying three-dimensional scenes. The technique also lends itself to monitoring small deformations in single wavelength implementations as well as dimensional metrology of large parts and assemblies and larger surface defect detection when used in multi-wavelength implementations..

Low-coherence interferometry Low-coherence interferometry utilizes a light source with low temporal coherence such as white light (for example, LED/SLD, halogen lamp) or high specification femtosecond lasers. Interference will only be achieved when the path length delays of the interferometer are matched within the coherence time of the light source (note: using a femtosecond source is somewhat more intricate).

The chief benefit of low-coherence interferometry is that it does not suffer from the 2 \pi ambiguity of coherent interferometry, and is therefore suited to profiling steps and rough surfaces. The axial resolution of the system is determined by the coherence length of the light source and is typically in the micrometer range.

Optical coherence tomography is a medical imaging technique based in low-coherence interferometry, where subsurface light reflections are resolved to give tomographic visualization. Recent advances have striven to combine the nanometer phase retrieval with the ranging cabability of low-coherence interferometry.

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

Starting from a standard quartz gauge of 1 m length, it is possible to measure distances up to 864 m by repeated multiplication. Baselines thus established are used to calibrate geodesy distance measurement equipment on, leading to a metrology traceable scale for geodetic networks measured by these instruments.

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

Astronomical Interferometry In astronomy interferometry is used to combine signals from two or more telescopes to obtain measurements with higher resolution than could be obtained with either telescopes individually. This technique is the basis for astronomical interferometer arrays, which can make measurements of very small astronomical objects if the telescopes are spread out over a wide area. If a large number of telescopes are used a picture can be produced which has Angular resolution similar to a single telescope with the diameter of the combined spread of telescopes. These include radio telescope arrays such as LOFAR and Square Kilometre Array, and more recently Optical interferometry#Astronomical optical interferometry such as Cambridge Optical Aperture Synthesis Telescope (COAST), Navy Prototype Optical Interferometer and Infrared Optical Telescope Array, resulting in the highest resolution optical images ever achieved in astronomy. The Very Large Telescope is expected to produce its first images using aperture synthesis soon, followed by other interferometers such as the CHARA array and the Magdalena Ridge Observatory Interferometer which may consist of up to 10 optical telescopes. If outrigger telescopes are built at the Keck Interferometer, it will also become capable of interferometric imaging.

Astronomical interferometers come in two types -- direct detection and heterodyne. These differ only in the way that the signal is transmitted. Aperture synthesis can be used to computationally simulate a large telescope aperture from either type of interferometer.

Astronomical direct-detection interferometry over it (labelled Mask), only allowing light through two small holes.

One of the first astronomical interferometers was built on the Mount Wilson Observatory's reflector telescope in order to measure the diameters of stars. This method was extended to measurements using separated telescopes by Labeyrie (1975) to the visible. Thered giant star Betelgeuse was among the first to have its diameterdetermined in this way. 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.

Techniques from VLBI, in which aperture synthesis, were implemented at optical and infrared wavelengths in the 1980s by the Cavendish Astrophysics Group. This providing the first very high resolution images of nearby stars. In 1995 this technique was demonstrated on Cambridge Optical Aperture Synthesis Telescope as a Michelson Interferometer for the first time, allowing a further improvement in resolution, and allowing even higher resolution imaging of stellar surfaces. The same technique has now been applied at a number of other astronomical telescope arrays, including the Navy Prototype Optical Interferometer and the Infrared Optical Telescope Array array and soon the VLTI, CHARA array and Magdalena Ridge Observatory 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 (ESA)).

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 Interferometer measuring diameters of 100 stars and many accurate stellar positions, Cambridge Optical Aperture Synthesis Telescope (COAST) and Navy Prototype Optical Interferometer producing many very high resolution images, and Infrared Spatial Interferometer measuring stars in the mid-infrared for the first time. Additional results include direct measurements of the sizes of and distances to Cepheid variable stars, and young stellar objects.

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 galaxy. 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.

Astronomical heterodyne interferometry Radio wavelengths are much longer than optical wavelengths, and the observing stations in radio astronomical interferometers are correspondingly further apart. The very large distances do not always allow any usable transmission of radio waves received at the telescopes to some central interferometry point. For this reason many telescopes instead record the radio waves onto a storage medium. The recordings are then transferred to a central correlator station where the waves are interfered. Historically the recordings were analog and were made on magnetic tapes. This was quickly superseded by the current method of digitizing the radio waves, and then either storing the data onto computer hard disks for later shipping, or streaming the digital data directly over a telecommunications network e.g. over the Internet to the correlator station. Radio arrays with a very broad bandwidth, and also some older arrays, transmit the data in analogue form either electrically or through fibre-optics. A similar approach is also used at some submillimetre astronomy and infrared interferometers, such as the Infrared Spatial Interferometer. Some early radio interferometers operated as intensity interferometers, transmitting measurements of the signal intensity over electrical cables to a central correlator. A similar approach was used at optical wavelengths by the Narrabri Stellar Intensity Interferometer to make the first large-scale survey of stellar diameters in the 1970s.

At the correlator station, the actual interferometer is synthesized by processing the digital signals using correlator hardware or software. Common correlator types are the FX and XF correlators. The current trend is towards software correlators running on consumer PCs or similar commodity hardware. There also exist some radio astronomy amateur digital interferometers with correlator, such as the ALLBIN of the European Radio Astronomy Club.

As the usual radio astronomy interferometer is digital it does have a few shortcomings, some due to sampling and quantization effects, in addition to the obvious need for much more computing power, as compared to analog correlation. The output of both digital and analog correlator can be used to aperture synthesis in the same way as with direct detection interferometers (see above).

References

Notes

See also

External links

Interferometry is the technique of superposing (Interference) two or more waves, to detect differences between them. Interferometry is applied in a wide variety of fields, including astronomy, Optical fiber, optical metrology, oceanography, seismology, quantum mechanics and plasma physics.

Interferometer An interferometer works because two waves with the same frequency that have the same phase (waves) will add to each other while two waves that have opposite phase will cancel out, assuming both have the same amplitude. Early interferometers principally used white light sources (e.g., Double-slit experiment of 1805). Modern researchers often use monochromatic light sources like lasers, and even the wave character of matter can be exploited to build interferometers (e.g. with electrons interferometer, neutron interferometer, atom interferometer, or even molecules).

Types of Interferometers There are many types of interferometers, but all work on the same basic principle.

Michelson Interferometer In a Michelson interferometer (or Michelson-Morley) type interferometer, the basic building blocks are a monochromatic source (emitting light or matter waves), a detector, two mirrors and one semitransparent mirror (often called beam splitter). These are put together as shown in the figure.

There are two paths from the (light) source to the detector. One reflects off the beam splitter, goes to the top mirror and then Reflection (physics) back, goes through the semi-transparent mirror, to the detector. The other one goes through the semi-transparent mirror, to the mirror on the right, reflects back to the semi-transparent mirror, then reflects from the semi-transparent mirror into the detector.

If these two paths differ by a whole number (including 0) of wavelengths, there is constructive interference and a strong signal at the detector. If they differ by a whole number and a half wavelengths (e.g., 0.5, 1.5, 2.5 ...) there is destructive interference and a weak signal. This might appear at first sight to violate conservation of energy. However energy is conserved, because there is a re-distribution of energy at the detector in which the energy at the destructive sites are re-distributed to the constructive sites. The effect of the interference is to alter the share of the reflected light which heads for the detector and the remainder which heads back in the direction of the source.

This type of interferometer was used in the Michelson-Morley experiment, to disprove the existence of the Luminiferous aether. Michelson interferometers are also used in astronomical interferometers (see Interferometry#Astronomical optical interferometry below) and Gravitational radiation detectors.

Mach-Zehnder interferometer

Interferometers are used in integrated optical circuits, in the form of a Mach-Zehnder interferometer, in which light interferes between two branches of a waveguide that are (typically) externally modulation to vary their relative phase. This interferometer's configuration consists of two beam splitters and two completely reflective mirrors. The source beam is split and the two resulting waves travel down separate paths. A slight tilt of one of the beam splitters will result in a path difference and a change in the interference pattern. The Mach-Zehnder interferometer can be very difficult to align, however its improved sensitivity enables a diverse number of applications.E. Hecht, Optics, 2nd Edition, Addison-Wesley Publishing Co., Reading, Mass, USA, 1987. p. 358 The Mach-Zehnder interferometer can be the basis of a wide variety of devices, from RF modulators to sensors to optical switches.

Sagnac interferometer

A Sagnac Interferometer is an interferometry configuration in which a beam of light is split and the two beams are made to follow a trajectory in opposite directions. To act as a ring the trajectory must enclose an area. On return to the point of entry the light is allowed to exit the apparatus in such a way that an interference pattern is obtained.

In the Sagnac configuration, the position of the interference fringes is dependent on angular velocity of the setup. This dependence is caused by the rotation effectively shortening the path distance of one of the beams, while lengthening the other. A Sagnac interferometer has been used by Albert Michelson and Henry Gale to determine the angular velocity of the Earth. It can be used in navigation as a ring laser gyroscope, which is commonly found on fighter planes Sagnac Interferometer on Eric Weisstein's World of Physics Accessed Aug 1, 2006.

Fabry-Perot interferometer

A Fabry-Pérot interferometer or etalon is typically made of a transparent plate with two Reflection (physics) surfaces, or two parallel highly-reflecting mirrors. (Technically the former is an etalon and the latter is an interferometer, but the terminology is often used inconsistently.) Its transmission optical spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon. It is named after Charles Fabry and Alfred Pérot.

Fabry-Pérot interferometers are widely used in telecommunications, lasers and spectroscopy for controlling and measuring the wavelength of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Pérot interferometers. Fabry-Pérot interferometers also form the most common type of optical cavity used in laser construction.

Types of Interferometry Coherent interferometry Coherent interferometry uses a coherent light source (for example, a helium-neon laser), and can make interference with large difference between the interferometer path length delays. The interference is capable of very accurate (nanometer) measurement by recovering the phase.

One of the most popular methods of interferometric phase recovery is phase-shifting by piezoelectric sensor (PZT) phase-stepping. By stepping the path length by a number of known phases (minimum of three) it is possible to recover the phase of the interference signal, with 2 \pi = \lambda / 2.

Coherent interferometry suffers from a 2 \pi ambiguity problem: that is, if between any two measurements the interferometric phase jumps by more than 2 \pi the phase measurement is incorrect.However by combining interferometry results obtained using multiple wavelengths of illumination, such as in digital multi-wavelength holography, the ambiguity interval can be extended to indefinitely large dynamic ranges of measurement.

The applications of coherent interferometry are wide ranging: Nanometer surface profiling, Microfluidics, Mechanical stress/strain, Velocimetry, and high-definition metrology of large parts and assemblies in manufacturing.

Inertial navigation In inertial navigation, ring laser gyroscopes are used that can detect rotation through optical interferometry of laser beams travelling around a circumference in opposite directions

Speckle Interferometry In optical systems, a speckle pattern is a field-intensity pattern produced by the mutual interference of partially coherent beams that are subject to minute temporal and spatial fluctuations. This speckling effect is most commonly observed in the fields of fiber optics and astronomical speckle imaging.

Holography A special application of optical interferometry using coherent light is holography, a technique for photographically recording and re-displaying three-dimensional scenes. The technique also lends itself to monitoring small deformations in single wavelength implementations as well as dimensional metrology of large parts and assemblies and larger surface defect detection when used in multi-wavelength implementations..

Low-coherence interferometry Low-coherence interferometry utilizes a light source with low temporal coherence such as white light (for example, LED/SLD, halogen lamp) or high specification femtosecond lasers. Interference will only be achieved when the path length delays of the interferometer are matched within the coherence time of the light source (note: using a femtosecond source is somewhat more intricate).

The chief benefit of low-coherence interferometry is that it does not suffer from the 2 \pi ambiguity of coherent interferometry, and is therefore suited to profiling steps and rough surfaces. The axial resolution of the system is determined by the coherence length of the light source and is typically in the micrometer range.

Optical coherence tomography is a medical imaging technique based in low-coherence interferometry, where subsurface light reflections are resolved to give tomographic visualization. Recent advances have striven to combine the nanometer phase retrieval with the ranging cabability of low-coherence interferometry.

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

Starting from a standard quartz gauge of 1 m length, it is possible to measure distances up to 864 m by repeated multiplication. Baselines thus established are used to calibrate geodesy distance measurement equipment on, leading to a metrology traceable scale for geodetic networks measured by these instruments.

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

Astronomical Interferometry In astronomy interferometry is used to combine signals from two or more telescopes to obtain measurements with higher resolution than could be obtained with either telescopes individually. This technique is the basis for astronomical interferometer arrays, which can make measurements of very small astronomical objects if the telescopes are spread out over a wide area. If a large number of telescopes are used a picture can be produced which has Angular resolution similar to a single telescope with the diameter of the combined spread of telescopes. These include radio telescope arrays such as LOFAR and Square Kilometre Array, and more recently Optical interferometry#Astronomical optical interferometry such as Cambridge Optical Aperture Synthesis Telescope (COAST), Navy Prototype Optical Interferometer and Infrared Optical Telescope Array, resulting in the highest resolution optical images ever achieved in astronomy. The Very Large Telescope is expected to produce its first images using aperture synthesis soon, followed by other interferometers such as the CHARA array and the Magdalena Ridge Observatory Interferometer which may consist of up to 10 optical telescopes. If outrigger telescopes are built at the Keck Interferometer, it will also become capable of interferometric imaging.

Astronomical interferometers come in two types -- direct detection and heterodyne. These differ only in the way that the signal is transmitted. Aperture synthesis can be used to computationally simulate a large telescope aperture from either type of interferometer.

Astronomical direct-detection interferometry over it (labelled Mask), only allowing light through two small holes.

One of the first astronomical interferometers was built on the Mount Wilson Observatory's reflector telescope in order to measure the diameters of stars. This method was extended to measurements using separated telescopes by Labeyrie (1975) to the visible. Thered giant star Betelgeuse was among the first to have its diameterdetermined in this way. 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.

Techniques from VLBI, in which aperture synthesis, were implemented at optical and infrared wavelengths in the 1980s by the Cavendish Astrophysics Group. This providing the first very high resolution images of nearby stars. In 1995 this technique was demonstrated on Cambridge Optical Aperture Synthesis Telescope as a Michelson Interferometer for the first time, allowing a further improvement in resolution, and allowing even higher resolution imaging of stellar surfaces. The same technique has now been applied at a number of other astronomical telescope arrays, including the Navy Prototype Optical Interferometer and the Infrared Optical Telescope Array array and soon the VLTI, CHARA array and Magdalena Ridge Observatory 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 (ESA)).

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 Interferometer measuring diameters of 100 stars and many accurate stellar positions, Cambridge Optical Aperture Synthesis Telescope (COAST) and Navy Prototype Optical Interferometer producing many very high resolution images, and Infrared Spatial Interferometer measuring stars in the mid-infrared for the first time. Additional results include direct measurements of the sizes of and distances to Cepheid variable stars, and young stellar objects.

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 galaxy. 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.

Astronomical heterodyne interferometry Radio wavelengths are much longer than optical wavelengths, and the observing stations in radio astronomical interferometers are correspondingly further apart. The very large distances do not always allow any usable transmission of radio waves received at the telescopes to some central interferometry point. For this reason many telescopes instead record the radio waves onto a storage medium. The recordings are then transferred to a central correlator station where the waves are interfered. Historically the recordings were analog and were made on magnetic tapes. This was quickly superseded by the current method of digitizing the radio waves, and then either storing the data onto computer hard disks for later shipping, or streaming the digital data directly over a telecommunications network e.g. over the Internet to the correlator station. Radio arrays with a very broad bandwidth, and also some older arrays, transmit the data in analogue form either electrically or through fibre-optics. A similar approach is also used at some submillimetre astronomy and infrared interferometers, such as the Infrared Spatial Interferometer. Some early radio interferometers operated as intensity interferometers, transmitting measurements of the signal intensity over electrical cables to a central correlator. A similar approach was used at optical wavelengths by the Narrabri Stellar Intensity Interferometer to make the first large-scale survey of stellar diameters in the 1970s.

At the correlator station, the actual interferometer is synthesized by processing the digital signals using correlator hardware or software. Common correlator types are the FX and XF correlators. The current trend is towards software correlators running on consumer PCs or similar commodity hardware. There also exist some radio astronomy amateur digital interferometers with correlator, such as the ALLBIN of the European Radio Astronomy Club.

As the usual radio astronomy interferometer is digital it does have a few shortcomings, some due to sampling and quantization effects, in addition to the obvious need for much more computing power, as compared to analog correlation. The output of both digital and analog correlator can be used to aperture synthesis in the same way as with direct detection interferometers (see above).

References

Notes

See also

External links



Interferometry - Wikipedia, the free encyclopedia
Interferometry is the technique of using the pattern of interference created by the superposition of two or more waves to diagnose the properties of the aforementioned waves.

Speckle interferometry
Speckle interferometry. Speckle interferometry allows one to measure displacement fields from objects with rough surfaces. The only contact with the sample is through the photons ...

Optical Long Baseline Interferometry News
NASA JPL's Optical Long Baseline Interferometer project homepage. Aims to increase optical resolution of the local universe sufficient to detect extrasolar planets and other ...

Planet Quest: Missions - Keck Interferometer
Resolving the Recurrent Nova RS Ophiuchi with Infrared Interferometry ADS Link: Keck Interferometer Observations of FU Orionis Objects ADS Link

Basic Technology: Practical X-ray Interferometry
Grants on the web ... Summary: X-ray interferometry has the potential to provide VERY high angular resolution imaging in astronomy and ground based applications but a practical ...

Planet Quest: Technology
Astronomers used interferometry measured the shape of the star Altair, viewed here from the Mount Wilson Observatory.

The 17 August 1999, Izmit Earthquake: SAR Interferometry
Coseismic interferograms of the Izmit Earthquake and high resolution topographic data. ... The 17 August 1999, Izmit Earthquake Displacements and Topography from SAR Interferometry

OptInt - Optical Interferometry at Cavendish Astrophysics
These pages describe the research activities of the optical interferometry sub-group of the Cavendish Astrophysics Group. Some resources that may be useful to researchers in ...

A brief guide to SAR Interferometry (InSAR)
Contents SAR; Amplitude ; Phase ; Path difference and phase shift; Interferometry; Fringes ; Multiple fringes; Flattening interferograms; Phase unwrapping; Altitude of ambiguity ...

GMRC SAR Interferometry workshop abstract
GMRC SAR interferometry workshop. Poster presented at the SAR Interferometry workshop at GEC-Marconi, Chelmsford, November 1995.

 

Interferometry



 
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