Modified Phasemeter for a Heterodyne Laser Interferometer

An FPGA-based design could be exported to other heterodyne laser interferometers.

Modifications have been made in the design of instruments of the type described in “Digital Averaging Phasemeter for Heterodyne Interferometry” (NPO-30866), NASA Tech Briefs, Vol. 28, No. 9 (September 2004), page 6a. To recapitulate: A phasemeter of this type measures 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 displacements with accuracy of the order of 100 pm. This 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.