A single-laser-based, heterodyne optoelectronic system that measures the displacements of two targets along the same lines of sight has been developed. Heretofore, it would have been necessary to construct a two-target laser metrological system as two partly or wholly independent subsystems, each containing a set of optoelectronic components (including, possibly, its own laser) that must be aligned separately. The present system contains only one laser, and with the exception of the targets themselves, all optical and electronic components function together as a single system that generates a separate measurement of the displacement of each target.
More precisely, the system (see figure) measures the displacement of a retroreflector on target A and a retroreflector on target B along an optical path between each target and a fiducial retroreflector. (The portion of the optical path below beam splitter 2 in the figure is common to both targets.) Light from the single laser is launched along a single-mode optical fiber, which is split into two arms. The beams in the arms are shifted in frequency by f1and f2, respectively, such that the difference frequency (f= f1- f2) is a convenient heterodyne radio frequency. Phase modulation at radio frequency fPM is applied to the f1 arm only. The outputs of the two arms are arranged to be orthogonally polarized and collimated, and the resulting beams are combined in polarizing beam splitter 1.
The subsequent propagation of the light is very similar to that in a standard heterodyne interferometer of prior design. A small fraction of each beam is reflected by a nonpolarizing beam splitter 1, and the radio-frequency signals of the two beam fractions are mixed in the reference photodetector. The component of the reference-photodetector output at heterodyne frequencyf serves as a phase reference, against which the phases of other signal components are measured as described below.
The light transmitted by nonpolarizing beam splitter 1 enters polarizing beam splitter 2. The p-polarized light (with frequency shift f2) is transmitted directly to the signal photodetector. The s-polarized light (with frequency shift f1 and phase modulation at frequency fPM) is reflected toward the fiducial retroreflector, from whence it is reflected back toward polarizing beam splitter 2. On its way to and from the fiducial retroreflector, this portion of the light makes a double pass through a quarter-wave plate and is thereby converted to p polarization. Now p-polarized, this portion of the light passes through polarizing beam splitter 2 and propagates to the target retroreflectors. [Although the target-B optics are shown as a combination of nonpolarizing beam splitter 2 and the target-B retroreflector, the target-B optics can also be realized as a retroreflector (only) that intercepts a fraction of the same light beam that propagates toward target A.] Like the light that goes to and from the fiducial retroreflector, the light that goes to and from the targets makes a double pass through a quarter-wave plate; thus, the returns from the targets are converted back to s polarization, so that upon arrival at polarizing beam splitter 2, they are reflected toward the signal photodetector.
For each target, the output voltage of the signal photodetector includes a component
VPD,i μ sin[2πft+ 2πΔxi/λ]
sin[2πfPM(t-τ) + Φ],
where i represents either A or B, Δxi is the difference between the length of the optical path from the laser straight through polarizing beam splitter 2 to the signal photodetector and the length of the optical path from the laser to target i to the signal photodetector, l is the laser wavelength, t is the present time, ti is time of propagation between the phase modulator and the signal photodetector via target i, and f is an adjustable constant component of the phase of the fPM signal.
The output of the signal photodetector is demodulated in two channels by mixing with two differently phase-sifted versions of a signal of frequency fPM, then filtering. The i component of the resulting waveform in the jth channel is given by
Vdemod,i μ sin[2 πft+ 2πΔxi/ λ]
cos[2πfPMτ - Φ - aj],
where aj is a constant component of the phase of the demodulating signal of frequency fPM in the jth channel (a1 = π/2, a2 = 0). The sine factor term is the heterodyne beat-frequency factor, the phase of which depends directly on the optical-path difference. The cosine factor establishes the amplitude of the heterodyne signal.
The waveforms in the two channels are fed to a phase meter that separately compares their phases with the phase of the output of the reference photodetector. This is the same phase-comparison process as that in a standard heterodyne laser metrological system of prior design. By choosing Φ= 2πfPMτB, one can make channel 1 of the phase meter generate a null response to the return signal from target B. By also choosing fPM = [4(τA- τB)]-1, one can make channel 1 of the phase meter generate a maximum response to return signal from target A. These choices further cause channel 2 to generate a null response to target A and a maximum response to target B. Thus, the responses to the two targets are separated, making it possible to monitor their displacements separately.
This work was done by Oliver Lay and Serge Dubovitsky of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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