Interferometry is the applied science of combining two or more input points of a particular data type, such as optical measurements, to form a greater picture based on the combination of the two sources. In astronomy (such as with the Keck telescopes), this is used to "combine" two telescopes.
This technique is the basis for radio telescope arrays which, spread out over a wide area of hundreds of miles, can together produce a picture with resolution similar or equivalent to a single telescope with the diameter of the combined spread of telescopes. It has more recently been used for astronomical optical interferometer arrays such as COAST, NPOI and IOTA, resulting in the highest resolution optical images ever achieved in astronomy. The VLT Interferometer is expected to produce its first images soon, followed by other interferometers such as the Keck Interferometer and the Magdalena Ridge Observatory Interferometer which may consist of up to 10 optical telescopes.
An interferometer works on the principle that two waves that coincide with the same phase will amplify each other while two waves that have opposite phases will cancel each other out. In the beginning, most interferometers used white light sources (e.g., Young's double slit experiment of 1805). Nowadays researchers often use monochromatic light sources like lasers, and even the wave character of matter can be exploited to build interferometers. One of the first examples of matter interferometers were electron interferometers, later on followed by neutron interferometers. Around 1990 the first atom interferometers were demonstrated, later on followed by interferometers deploying molecules. Currently it is not clear yet, what the maximum particle size for interferometry might be.
A very common example of an interferometer is the Michelson (or Michelson-Morley) type. Here 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 semi-transparent mirror, goes to the top mirror and then reflects back, goes through the semi-transparent mirror, to the detector. The other first 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.
The interferometer setup shown to the right was used in the famous Michelson-Morley experiment that provided evidence for special relativity. In Michelson's day, the interference pattern was obtained by using a gas discharge lamp, a filter, and a thin slot or pinhole. In one version of the Michelson-Morley experiment, they even ran the interferometer off starlight. Starlight is incoherent light, but since it is a point source of light it will produce an interference pattern. The Michelson interferometer finds use not only in these experiments but also for other purposes, e.g., in gravitational wave detection.
There are many other types of interferometer. They all work on the same basic principles, but the geometry is different for the different types. One familar use of the technique is in radio and optical interferometer telescopes. However, interferometers are perhaps even more widely 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 modulated to vary their relative phase. Such components are the basis of a wide variety of devices, from RF modulators to sensors to optical switches.
The highest-resolution astronomical images are produced using interferometers (at both optical and radio wavelengths). Here is a description of astronomical interferometry.
- Very Long Baseline Interferometry
- Aperture synthesis
- List of astronomical interferometers at visible and infrared wavelengths