Absolute interferometric distance measurement using a FM-demodulation technique.

TLDR: An interferometric method for measuring absolute distances larger than the wavelength based on multiple-wavelength interferometry that uses a modulated light source using the fact that the wavelength of light emitted by the laser diode can be varied by means of the injection current.

Abstract: When applying interferometric techniques to measure the distance between two points separated by an amount larger than half the laser wavelength used the measurement become ambigous. To overcome this disadvantage, among others, two wavelength interferometry1–4 was developed. By applying two wavelengths \(\lambda _{1}\) and \(\lambda _{2}\) simultaneously to the object under test, the sensitivity is reduced to an effective wavelength \(\Lambda =\lambda _{1}\ \lambda _{2}/|\lambda _{1}-\lambda _{2}|\). Since the laser diode wavelength can easily be tuned via the laser injection current, the concept of two-wavelength interferometry can be extended to multiple-wavelength interferometry5–8. In this paper, we present a novel approach to derive the absolute distance information from the phase modulated detector signal by using a PLL (phase locked loop) based FM-demodulator. In this case, there is no need to generate a reference signal in the interferometer for the purpose of demodulation. By using a FM-demodulator, no unambiguity jump occurs if the phase deviation exceeds 2π. In this case, we are able to determine absolute distances with a noise limited resolution of about 1 μm, even if the distance is much larger than the synthetic wavelength.