The Modulation Sideband Technology for Absolute Range (MSTAR) architecture is the basis of design of a proposed laser-based heterodyne interferometer that could measure a range (distance) as great as 100 km with a precision and resolution of the order of 1 nm. Simple optical interferometers can measure changes in range with nanometer resolution, but cannot measure range itself because interference is subject to the well-known integer-multiple-of-2p- radians phase ambiguity, which amounts to a range ambiguity of the order of 1 µm at typical laser wavelengths. Existing rangefinders have a resolution of the order of 10 µm and are therefore unable to resolve the ambiguity. The proposed MSTAR architecture bridges the gap, enabling nanometer resolution with an ambiguity range that can be extended to arbitrarily large distances.
The MSTAR architecture combines the principle of the heterodyne interferometer with the principle of extending the ambiguity range of an interferometer by using light of two wavelengths. The use of two wavelengths for this purpose is well established in optical metrology, radar, and sonar. However, unlike in traditional two-color laser interferometry, light of two wavelengths would not be generated by two lasers. Instead, multiple wavelengths would be generated as sidebands of phase modulation of the light from a single frequency-stabilized laser. The phase modulation would be effected by applying sinusoidal signals of suitable frequencies (typically tens of gigahertz) to high-speed electro-optical phase modulators. Intensity modulation can also be used.
An MSTAR system (see figure) would include a conventional laser heterodyne interferometer as a subsystem, plus two high-speed phase modulators and a second phase meter. The light from the laser of carrier frequency ? would first be split into a target beam and a local beam. The target beam would be phase modulated by a sinusoid of frequency FT, producing sidebands displaced from the laser (carrier) frequency at positive and negative integer multiples of FT. The carrier and all the sidebands would then be shifted in frequency by fT.The local beam would be processed similarly, except that it would be phase-modulated at a frequency FL and shifted in frequency by fL. The corresponding frequencies are chosen to differ from each other by convenient small amounts:
The primary innovation of the MSTAR architecture is the selection of phase-modulation and shift frequencies such that every sideband order m forms a heterodyne pair with a distinct heterodyne frequency,
ΔF = FT - FL
δf = fT - fL
The signal from each heterodyne pair can be isolated by appropriate filtering. In the case illustrated in the figure, one would choose the first upper sideband pair (m = +1) and the first lower sideband pair (m = –1). Filters would isolate heterodyne frequencies
mΔF - δf
The phase-meter outputs would be
fA = ΔF + δf
fB = ΔF - δf
where L is the distance that one seeks to measure and c is the speed of light. Each of these outputs would be characterized by the same range resolution and ambiguity range as those of a conventional heterodyne interferometer, and, as such, would constitute the fine incremental range outputs. The difference between these outputs,
ϕA - ϕB = 8πFTL/c
would constitute the gap-bridging coarse incremental range output, characterized by an ambiguity range of c/4FT. One could lower the modulation frequency, FT, to extend the ambiguity range as needed.
This work was done by Serge Dubovitsky and Oliver Lay of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.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) 354-7770.
Refer to NPO-30304.
This Brief includes a Technical Support Package (TSP).
Laser System for Precise, Unambiguous Range Measurements
(reference NPO-30304) is currently available for download from the TSP library.
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