A digital averaging phasemeter has been built for measuring the difference between the phases of the unknown and reference heterodyne signals in a heterodyne laser interferometer. This phasemeter performs well enough to enable interferometric measurements of distance with accuracy of the order of 100 pm and with the ability to track distance as it changes at a speed of as much as 50 cm/s. This phasemeter is unique in that it is a single, integral system capable of performing three major functions that, heretofore, have been performed by separate systems: (1) measurement of the fractional-cycle phase difference, (2) counting of multiple cycles of phase change, and (3) averaging of phase measurements over multiple cycles for improved resolution. This phasemeter also offers the advantage of making repeated measurements at a high rate: the phase is measured on every heterodyne cycle. Thus, for example, in measuring the relative phase of two signals having a heterodyne frequency of 10 kHz, the phasemeter would accumulate 10,000 measurements per second. At this high measurement rate, an accurate average phase determination can be made more quickly than is possible at a lower rate.

Figure 1. A Heterodyne Laser Interferometer is used to measure changes in the length of the optical path between the corner-cube retroreflectors. These changes are proportional to changes in the phase difference between the reference and unknown signals, which are measured by the phasemeter.

Figure 1 schematically depicts a typical heterodyne laser interferometer in which the phasemeter is used. The goal is to measure the change in the length of the optical path between two corner cube retroreflectors. Light from a stabilized laser is split into two fiber-optic outputs, denoted P and S, respectively, that are mutually orthogonally polarized and separated by a well-defined heterodyne frequency. The two fiber-optic out-puts are fed to a beam launcher that, along with the corner-cube retroreflectors, is part of the interferometer optics. In addition to launching the beams, the beam launcher immediately diverts and mixes about 10 percent of the power from the fiber-optic feeds to obtain a reference heterodyne signal. This signal is detected, amplified, and squared to obtain a reference square-wave heterodyne signal, which is fed to the reference input terminal of the phasemeter.

The remaining 90 percent of the power from the fiber-optic feeds of the light from the two inputs is treated as follows: The P beam is launched toward one corner-cube retroreflector, while the S beam travels to the unknown photodiode. The S beam returns to the beam launcher, passes through it, and continues to the other retroreflector. The S beam then returns to the beam launcher where it mixes with the P beam, producing the "unknown" heterodyne signal, which is then detected, amplified, and squared in the same manner as that of the reference signal. The resulting square-wave is fed to the unknown input terminal of the phasemeter. The frequency of the unknown heterodyne signal is close to that of the reference signal: If the optics are motionless, the unknown frequency is exactly the reference frequency. Motion of the optics gives rise to a Doppler shift in the unknown frequency relative to the reference frequency. By tracking the relative phases of the unknown and reference signals, one tracks the change in the length of the optical path between the retroreflectors.

Figure 2. In the Phasemeter, the integer-cycle and fractional-cycle parts of the phase difference are measured separately. For greater accuracy, the phasemeter can average its measurements over many cycles of the heterodyne signals.

The phasemeter (see Figure 2) tracks the integer number of cycles and the fractional-cycle portions of the phase difference separately. The integer part of the phase difference is taken to equal the number of positive-or negative-going square-wave level transitions at the reference input minus the number of such transitions at the unknown input. The fractional part of the phase difference is taken to be proportional to the number of ticks of a clock of 128-MHz frequency during the time interval from the most recent reference transition to the next unknown transition. The integer and fractional phase-counter outputs are also fed to accumulators to compute the sum of many phase measurements over a programmed interval. The sum is then used to compute an average. The summing interval can be made to repeat at a fixed frequency, typically in the range between 1 Hz and 1 kHz, interval can be programmed to start any time after the summation-synchronizing clock signal and can continue for any time up to the next such signal. During the summation, each negative-going transition of the unknown signal causes a phase measurement to be summed into the integer and fractional phase accumulators. As a result, the number of readings in an average equals the duration of the summation interval multiplied by the unknown heterodyne frequency; for example, if the heterodyne frequency is 10 kHz and the summation interval is 0.1 second, then 1,000 measurements are accumulated.

This work was done by Donald Johnson, Robert Spero, Stuart Shaklan, Peter Halverson, and Andreas Kuhnert 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 Physical Sciences Category.NPO30866

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Digital Averaging Phasemeter for Heterodyne Interferometry

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This article first appeared in the September, 2004 issue of Photonics Tech Briefs Magazine.

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