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).
Read more about Frequency Scanning Interferometry: Reference Interferometer Precautions, Uses, Precision
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