Super-heterodyning phase measurement for heterodyne interferometry

摘要

One of the ever-increasing demands on the performances of heterodyne interferometers is to improve the measurement resolution, of which current state-of-the-art reaches the region of sub-nanometers. So far, the demand has been met by increasing the clock speed that drives the electronics involved for the phase measurement of the Doppler shift, but its further advance is being hampered by the technological limit of modem electronics. To cope with the problem, in this investigation, we propose a new scheme of phase-measuring electronics that reduces the measurement resolution without further increase in clock speed. Our scheme adopts a super-heterodyne technique that lowers the original beat frequency to a level of 1 MHz by mixing it with a stable reference signal generated from a special phase-locked-loop. The technique enables us to measure the phase of Doppler shift with a resolution of 1.25 nanometer at a sampling rate of 1MHz. To avoid the undesirable decrease in the maximum measurable speed caused by the lowered beat frequency, a special technique of frequency up/down counting is combined to perform required phase-unwrapping simply by using programmable digital gates without 2π ambiguities up to the maximum velocity guaranteed by the original beat frequency.