Optical interferometry
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Optical interferometry combines two or more light waves in an optical instrument in such a way that interference occurs between them.
Early interferometers used white light sources and also monochromatic light from atomic sources (e.g., Young's double slit experiment of 1805) . Such interferometers had a wide range of applications, for example, calibration of slip gauges and measurement of gas flow [1]. In 1986, when the definition of the metre was based on one of the spectral line emitted by krypton-86, interferometry was essential in setting up the standard. The development of lasers has made it much easier to produce optical interference and has led to the development of a wide range of measurement methods in engineering, physics and other fields.
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[edit] Optical interferometer configurations
There are many ways in which two or more light beams can be combined to give interference. Most of these can be found here.
[edit] Some applications of optical interferometry
Optical interferometry is used in a vast range of applications, including metrology, surface profiling, Microfluidics, Mechanical stress/strain measurement, velocimetry. A few of these are introduced below:
[edit] 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
[edit] Holographic interferometry
A special application of optical interferometry using coherent light is Holographic interferometry, a technique which uses Holography to monitor 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.
[edit] Electronic Speckle Pattern Interferometry
Electronic Speckle Pattern Interferometry, also known as TV holography, uses video detection and recording to produce an image of the object upon which is superimposed a fringe pattern which represents the displacement of the object between recordings. The fringes are similar to those obtained in holographic interferometry
[edit] 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 2π 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.
[edit] Optical coherence tomography
This is a medical imaging technique based on 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 capability of low-coherence interferometry.
[edit] 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 geodetic distance measurement equipment on, leading to a metrologically 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.
[edit] Astronomical optical interferometry
An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution
[edit] References
- ^ Longhurst RS, 1967, Geometrical and Physical Optics, Longmans, London
