Optical interferometry
Encyclopedia
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. In 1960, when the definition of the meter was based on one of the spectral line emitted by krypton-86, interferometry was essential in setting up the standard. The development of laser
s 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.
, Mechanical stress/strain measurement, velocimetry. A few of these are introduced below:
s are used that can detect rotation through optical interferometry of laser beams travelling around a circumference in opposite directions
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.
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
such as white light (for example, LED/SLD, halogen lamp or supercontinuum sources) 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
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. Despite low resolution the absolute low coherence interferometers can achieve submicrometre accuracy,,.
Low coherence interferometry is either implemented via microscope-like (full field) instruments or fiber-based instruments. With fiber-based low coherence interferometry, optical probes are deployable directly in-process, at a distance from the profilometer enclosure. Industrial applications include in-process surface metrology
, roughness measurement, 3D surface metrology in hard-to-reach spaces and in hostile environments, profilometry of surfaces with high aspect ratio features (grooves, channels, holes), and film thickness measurement (semi-conductor and optical industries, etc).
Angle-resolved low-coherence interferometry
Angle-resolved low-coherence interferometry (a/LCI) uses Mie theory
angular predictions of scattered light to measure the sizes of subcellular objects, including cell
nuclei. This allows interferometry depth measurements to be combined with density measurements, yielding promising biomedical applications.
At present the most significant emerging application is determining the state of tissue health based on measurements of average cell nuclei size. It has been found that as tissue changes from normal to cancerous, the average cell nuclei size increases. Several recent studies have shown that via cell nuclei measurements, a/LCI can detect the presence of low- and high-grade dysplasia with 91% sensitivity and distinguish between normal and dysplastic with 97% specificity.
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.
. 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 network
s 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 optical interferometry
An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution.
. This interferometer was originally developed for applications to microdensitometry and microscopy but soon found uses in the characterization of transmission gratings and transmissive optical surfaces in general. More recently the N-slit interferometer has been applied to generate interferometric characters for secure optical communications in free space
.
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
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. In 1960, when the definition of the meter was based on one of the spectral line emitted by krypton-86, interferometry was essential in setting up the standard. The development of laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s 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.
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.Some applications of optical interferometry
Optical interferometry is used in a vast range of applications, including metrology, surface profiling, MicrofluidicsMicrofluidics
Microfluidics deals with the behavior, precise control and manipulation of fluids that are geometrically constrained to a small, typically sub-millimeter, scale.Typically, micro means one of the following features:* small volumes...
, Mechanical stress/strain measurement, velocimetry. A few of these are introduced below:
Inertial navigation
In inertial navigation, ring laser gyroscopeRing laser gyroscope
A ring laser gyroscope consists of a ring laser having two counter-propagating modes over the same path in order to detect rotation. It operates on the principle of the Sagnac effect which shifts the nulls of the internal standing wave pattern in response to angular rotation...
s are used that can detect rotation through optical interferometry of laser beams travelling around a circumference in opposite directions
Holographic interferometryHolographic interferometryHolographic interferometry is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision . These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing...
A special application of optical interferometry using coherent light is Holographic interferometryHolographic interferometry
Holographic interferometry is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision . These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing...
, a technique which uses Holography
Holography
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that when an imaging system is placed in the reconstructed beam, an image of the object will be seen even when the object is no longer present...
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.
Electronic Speckle Pattern InterferometryElectronic speckle pattern interferometryElectronic Speckle Pattern Interferometry , also known as TV Holography, is a technique which uses laser light, together with video detection, recording and processing to visualise static and dynamic displacements of components with optically rough surfaces...
Electronic Speckle Pattern InterferometryElectronic speckle pattern interferometry
Electronic Speckle Pattern Interferometry , also known as TV Holography, is a technique which uses laser light, together with video detection, recording and processing to visualise static and dynamic displacements of components with optically rough surfaces...
, 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
Holographic interferometry
Holographic interferometry is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision . These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing...
Low-coherence interferometry
Low-coherence interferometry utilizes a light source with low temporal coherenceCoherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
such as white light (for example, LED/SLD, halogen lamp or supercontinuum sources) 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
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. Despite low resolution the absolute low coherence interferometers can achieve submicrometre accuracy,,.Low coherence interferometry is either implemented via microscope-like (full field) instruments or fiber-based instruments. With fiber-based low coherence interferometry, optical probes are deployable directly in-process, at a distance from the profilometer enclosure. Industrial applications include in-process surface metrology
Surface metrology
Surface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface waviness and surface roughness are the parameters most commonly associated with the field...
, roughness measurement, 3D surface metrology in hard-to-reach spaces and in hostile environments, profilometry of surfaces with high aspect ratio features (grooves, channels, holes), and film thickness measurement (semi-conductor and optical industries, etc).
Angle-resolved low-coherence interferometryAngle-resolved low-coherence interferometryFor the electrical device, see ALCIAngle-resolved low-coherence interferometry is an emerging biomedical imaging technology which uses the properties of scattered light to measure the average size of cell structures, including cell nuclei...
Angle-resolved low-coherence interferometry (a/LCI) uses Mie theoryMie theory
The Mie solution to Maxwell's equations describes the scattering of electromagnetic radiation by a sphere...
angular predictions of scattered light to measure the sizes of subcellular objects, including cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
nuclei. This allows interferometry depth measurements to be combined with density measurements, yielding promising biomedical applications.
At present the most significant emerging application is determining the state of tissue health based on measurements of average cell nuclei size. It has been found that as tissue changes from normal to cancerous, the average cell nuclei size increases. Several recent studies have shown that via cell nuclei measurements, a/LCI can detect the presence of low- and high-grade dysplasia with 91% sensitivity and distinguish between normal and dysplastic with 97% specificity.
Optical coherence tomographyOptical coherence tomographyOptical coherence tomography is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media . Optical coherence tomography is an interferometric technique, typically employing near-infrared light...
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.Geodetic standard baseline measurements
A famous use of white light interferometry is the precise measurement of geodetic standard baselines as invented by Yrjö VäisäläYrjö Väisälä
Yrjö Väisälä was a Finnish astronomer and physicist.His main contributions were in the field of optics, but he was also very active in geodetics, astronomy and optical metrology...
. 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
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of 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
Geodesy
Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...
distance measurement equipment on, leading to a metrologically
Metrology
Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...
traceable scale for geodetic network
Geodetic network
A geodetic network is a network of triangles which are measured exactly by techniques of terrestrial surveying or by satellite geodesy.In "classical geodesy" this is done by triangulation, based on measurements of angles and of some spare distances; the precise orientation to the geographic north...
s measured by these instruments.
More modern geodetic applications of laser interferometry are in calibrating the divisions on levelling
Levelling
Levelling or leveling is a branch of surveying, the object of which is1) To find the elevation of a given point with respect to the given or assumed Datum.2) to establish a point at a given elevation with respect to the given or assumed Datum....
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
Free fall
Free fall is any motion of a body where gravity is the only force acting upon it, at least initially. These conditions produce an inertial trajectory so long as gravity remains the only force. Since this definition does not specify velocity, it also applies to objects initially moving upward...
, directly from the physical definition at a few parts in a billion accuracy.
Astronomical optical interferometryAstronomical optical interferometryOne 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...
An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution.N-slit interferometry
N-slit interferometry is an approach to interferometry that evolved from the use of the N-slit laser interferometerN-Slit interferometer
The N-slit interferometer is an extension of the double-slit interferometer also known as Young's double-slit interferometer. One of the first known uses of N-slit arrays in optics was illustrated by Newton...
. This interferometer was originally developed for applications to microdensitometry and microscopy but soon found uses in the characterization of transmission gratings and transmissive optical surfaces in general. More recently the N-slit interferometer has been applied to generate interferometric characters for secure optical communications in free space
Free-space optical communication
Free-space optical communication is an optical communication technology that uses light propagating in free space to transmit data for telecommunications or computer networking."Free space" means air, outer space, vacuum, or something similar...
.