Two-wavelength laser interferometry using superheterodyne detection

TLDR

Two‑wavelength interferometry uses synthetic wavelengths to reduce sensitivity or extend the unambiguous measurement range. The authors introduce a superheterodyne detection technique that measures phase differences between optical frequencies beyond the resolution of conventional heterodyne detection. They propose an optical arrangement employing tunable lasers and superheterodyne detection for absolute range‑finding across arbitrary synthetic wavelengths. The method enables real‑time two‑wavelength interferometry with synthetic wavelengths from micrometers to meters, and preliminary experiments demonstrate its feasibility.

Abstract

In two-wavelength interferometry, synthetic wavelengths are generated in order to reduce the sensitivity or to extend the range of unambiguity for interferometric measurements. Here a novel optoelectronic technique, called superheterodyne detection, is presented, which permits measurement of the phase difference of two optical frequencies that cannot be resolved by direct optoelectronic heterodyne detection. This technique offers the possibility for operation of two-wavelength interferometry in real time with arbitrary synthetic wavelengths from micrometers to meters in length. Preliminary experimental results are reported. An optical arrangement for absolute range-finding applications using tunable-laser sources (e.g., semiconductor lasers) is proposed.