A heterodyne optical interferometer of a type used to measure small displacements can be augmented to suppress a phenomenon, called "self-interference," that tends to limit the achievable resolution and working distance and can even render the interferometer inoperable. The technique for suppressing self-interference can be implemented by use of commercial off-the-shelf optoelectronic and electro-optical components, and does not degrade the fundamental operation of the interferometer.

This Heterodyne Optical Interferometer is augmented to suppress self-interference. The augmentation consists in the addition of the shaded parts.

Self-interference is caused by optical scattering, imperfections in optical surfaces, and misalignment of optical components. Like many other optical interferometers, an interferometer of this type includes a target and a reference optical path. Self-interference typically manifests itself as leakage, along the reference path, of part of the optical signal power intended to propagate solely along the target path. This leakage, in turn, manifests itself as a heterodyne signal with the incorrect phase that competes against the heterodyne signal with the correct phase.

The figure schematically depicts a heterodyne interferometer configured for measuring a target path of length L. This interferometer is augmented to suppress self-interference by using phase modulation to distinguish between the leaked signal and the signal returning from the target. The following is a summary of the self-interference-suppression technique, omitting some details for the sake of brevity:

The optical carrier wave (that is, the beam coming out of the laser) is phase modulated at an angular frequency Ω before it is sent along the two paths of the interferometer. The phase modulation, by itself, is invisible to the photodetectors at the reference and target photodetectors unless it is converted, by the phase delay of one path of the interferometer relative to the other, to intensity modulation at the modulation angular frequency Ω. The self-interference signal is associated with light that does not go to the target and thus does not undergo the differential delay that would give rise to this intensity modulation.

The "good" signal is associated with the optical beam that goes to the target and thus does undergo the differential delay that gives rise to intensity modulated at angular frequency Ω. Thus, demodulation by mixing with the oscillator signal at angular frequency Ω results in discrimination against all but the "good" signal.

This work was done by Serge Dubovitsky of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Electronics & Computers category.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
NASA Management Office–JPL (818) 354-4770.

Refer to NPO-20740.

This Brief includes a Technical Support Package (TSP).
Heterodyne Interferometer With Phase-Modulated Carrier

(reference NPO-20740) is currently available for download from the TSP library.

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This article first appeared in the February, 2001 issue of NASA Tech Briefs Magazine.

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