Frequency Scanning Interferometry

Frequency Scanning Interferometry

Frequency Scanning Interferometry (FSI) is an absolute distance measurement technique, for measuring the distance between a pair of points, along a line-of-sight. The power of the FSI technique lies in its ability to make many such distance measurements, simultaneously. For each distance to be measured, a measurement interferometer is built using optical components placed at each end of a line-of-sight. The optical path of each measurement interferometer is compared to the optical path in a reference interferometer, by scanning the frequency of a laser (connected to all interferometers in the system) and counting fringe cycles produced in the return signals from each interferometer.

The length of each measurement interferometer is given in units of reference length by the ratio of measurement interferometer to reference interferometer fringes.

To give an example: A frequency scan might produce 100 fringe cycles in the measurement interferometer and 50 in the reference interferometer. The measured interferometer is therefore twice the length of reference interferometer, to first order (ignoring systematic errors - see below).

Reference Interferometer Precautions

A typical reference interferometer is held at a stable length in a controlled environment, to reduce the dominant systematic errors which arise from changes in optical path which occur during the laser frequency scan.


The great strength of the FSI technique is the ability to simultaneously compare any number of "measurement" interferometers to the same reference length. This has great benefit in a shape measurement system.

An FSI system is being used to monitor shape changes of the semiconductor tracker (SCT) on the ATLAS detector at CERN.


The measurement sensitivity depends on how rapidly the laser is tuned and how well systematic errors are controlled. Currently precisions of a few nm over a 6m path are possible in evacuated interferometers. In a system built for the ATLAS experiment a target of 1 micrometre precision over distances of 1 m is expected to be easily achieved.


* [] P A Coe et al 2004 Meas. Sci. Technol. 15 2175-2187 doi: 10.1088/0957-0233/15/11/001
* []

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • 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. The instrument used to interfere the waves together is called an interferometer.… …   Wikipedia

  • FSI — is a three letter abbreviation with multiple meanings, including: *Foreign Service Institute India in New Delhi, India, the Institute for training of Indian Foreign Service Officers * Failed States Index, A country ranking method * Federazione… …   Wikipedia

  • Optical coherence tomography — Intervention Optical Coherence Tomography (OCT) image of a sarcoma MeSH …   Wikipedia

  • optics — /op tiks/, n. (used with a sing. v.) the branch of physical science that deals with the properties and phenomena of both visible and invisible light and with vision. [1605 15; < ML optica < Gk optiká, n. use of neut. pl. of OPTIKÓS; see OPTIC,… …   Universalium

  • Atomic force microscope — The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high resolution type of scanning probe microscope, with demonstrated resolution of fractions of a nanometer, more than 1000 times better than the optical diffraction… …   Wikipedia

  • Laser Doppler Vibrometer — A Laser Doppler Vibrometer (LDV) is a scientific instrument that is used to make non contact vibration measurements of a surface. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are… …   Wikipedia

  • Synthetic aperture radar — (SAR) is a form of radar in which the highly directional (and thus very large) rotating antenna used by conventional radar is replaced with many low directivity small stationary antenna positions scattered over some area near or around the target …   Wikipedia

  • Earth Sciences — ▪ 2009 Introduction Geology and Geochemistry       The theme of the 33rd International Geological Congress, which was held in Norway in August 2008, was “Earth System Science: Foundation for Sustainable Development.” It was attended by nearly… …   Universalium

  • Radar — For other uses, see Radar (disambiguation). A long range radar antenna, known as ALTAIR, used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on Kwajalein Atoll …   Wikipedia

  • Measuring instrument — Captain Nemo and Professor Aronnax contemplating measuring instruments in Twenty Thousand Leagues Under the Sea …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”