Optical interferometry

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 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 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.

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, Microfluidics, Mechanical stress/strain measurement, velocimetry. A few of these are introduced below:

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

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

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 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 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. Despite low resolution the absolute low coherence interferometers can achieve submicrometre accuracy,,.^[2][3][4]

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.^[5] 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).^[6]

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.^[7] Several recent studies ^[8] 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.

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.

Astronomical optical interferometry

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 interferometer. This interferometer was originally developed for applications to microdensitometry and microscopy^[9] 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.^[10]

References

1. ^ Longhurst RS, 1967, Geometrical and Physical Optics, Longmans, London
2. ^ Wojtek J. Walecki, Kevin Lai, Vitalij Souchkov, Phuc Van, SH Lau, Ann Koo physica status solidi (c) Volume 2, Issue 3 , Pages984 - 989
3. ^ W. J. Walecki et al. "Non-contact fast wafer metrology for ultra-thin patterned wafers mounted on grinding and dicing tapes" Electronics Manufacturing Technology Symposium, 2004. IEEE/CPMT/SEMI 29th International Volume , Issue , July 14–16, 2004 Page(s): 323 - 325
4. ^ more references on semiconductor applications of low coherence interferometry are provided on http://www.zebraoptical.com/services.html
5. ^ Losert, R. (November, 2010), "So Far, Yet so Close: Optical Profilometer Systems Inspect Hard-to-Reach Surfaces", INSPECT Magazine: http://www.inspect-online.com 11: 42–43, http://www.inspect-online.com/sites/inspect-online.com/files/printausgabe/epapers/BLins0710E/blaetterkatalog/index.html, retrieved January 12, 2011 
6. ^ Examples of fiber-based low coherence interferometry applications are provided on http://novacam.com/nc_applications.php
7. ^ J. W. Pyhtila, K. J. Chalut, J. D. Boyer, J. Keener, T. D’Amico, M. Gottfried, F. Gress, and A. Wax, “In situ detection of nuclear atypia in Barrett’s esophagus by using angle-resolved low-coherence interferometry,” Gastrointestinal Endoscopy, vol. 65, no. 3, pp. 487–491, 2007.[1]
8. ^ A. Wax, J. W. Pyhtila, R. N. Graf, R. Nines, C. W. Boone, R. R. Dasari, M. S. Feld,V. E. Steele, andG.D. Stoner, “Prospective grading of neoplastic change in rat esophagus epithelium using angle-resolved low-coherence interferometry,” J. Biomed. Opt., vol. 10, no. 5, pp. 051604-1–051604-10, 2005. doi: 10.1117/1.2102767
9. ^ F. J. Duarte, Electro-optical interferometric microdensitometer system, US Patent 5255069 (1993).
10. ^ F. J. Duarte, T. S. Taylor, A. B. Clark, and W. E. Davenport, The N-slit interferometer: an extended configuration, J. Opt. 12, 015705 (2010).