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Absolute Distance Interferometry (ADI)

Absolute Distance Interferometry is a distance measuring method with an accurancy in the sub-micron range.

The diagram shows a schematic setup for ADI. It consists of a modehop-free tunable diode laser, a beam splitter cube, an optical reference arm and a photodetector. The light beam passes a beam splitter cube. A part of the beam is coupled to the mirror of the reference arm. The reflected light passes the beam splitter cube a second time before it is directed to a photodiode. The second beam is coupled to a sample. A part of the light is reflected back by the sample passes the beam splitter cute a second time before it also is directed to the photodiode.

At the photodiode, the laser beam reflected from the reference arm and the laser beam reflected from the sample interfere. Depending on the phase difference between reference beam and detaction beam, the light intensity at the photodiode changes from a minimum value for destructive interference and a maximum value for positive interference.

Tuning the wavelength of the diode laser changes the interference condiction at the photodiode and the result is a series of high and low signal. Finally, the measuring process consists in counting the numbers of dark series with the wavelength tuning. This method allows a distance measurment with sub-micron accurancy over a measuring distance up to 100m.

Sacher Lasertechnik investigated this measuring method in cooperation with Fraunhofer IPT, supported by BMBF within contract 13N6693. For more detailed information, please contact us.