How Interferometers are used as Position Sensors

Interferometer sensors employ energy in the form of light or sound to make precise position/location measurements. There are a number of applications where accurate measurement of very small linear mechanical displacements and distances between objects in the order of nanometers is required, for instances the measurement of deflections of diaphragms and cantilevers in force, acceleration and pressure sensors, and thickness measurements in the microfabrication of LSI circuit chips and nanomachines. The measurement of small distances without contact can be made using optical interferometry.  An interferometer basically compares the phase between a reference light beam and a measurement beam by using the phenomena of constructive and destructive interference which occurs when two, coherent, light wave beams are summed on a surface, or a photodetector.

Basic Principle of Working of an Interferometer

The transmitted wave interacts with the reflected wave. If the peaks of the two waveforms coincide, the resultant waveform is twice the original. If the reflected wave is 180 degrees out of phase with the transmitted wave, the resultant combined waveform has zero amplitude. Between these two extremes, the combined waveforms result in a waveform that is still sinusoidal, but has amplitude somewhere between zero and twice that outputted, and will be phase shifted by between 0 and 180 degrees.

This type of sensor can determine distance to a reflective surface within a fraction of a wavelength. Because some light has wavelengths in the region of 0.005 mm, this leads to a very fine precision as a matter of fact.

If laser light is used, the waveforms can travel longer distances without being reduced in energy by scattering.

Examples of interferometers include:

• Michelson interferometer.
• Fabry-Perot interferometer.
• Mach-Zehnder interferometer.
• Normarsky interferometer.
• The Sagnac interferometer.

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