The heterodyne displacement measuring interferometers using two-frequency HeNe lasers are widely used as a standard tool of displacement metrology for diverse applications. Many precision machines such as photolithography tools are preferably adopting heterodyne laser interferometers as position-feedback transducers for their servo control, requiring fine resolutions together with high measuring speed to increase production throughputs. In addition, the number of axes to be measured simultaneously is growing from three to eight and 15~20 in the future to compensate Abbe’s error.
In respond to such industrial demands, we proposed a new scheme of high-resolution heterodyne interferometer, which employs the two-longitudinal mode HeNe laser with an inter-mode beat frequency of 600~1000 MHz and two-way super-heterodyne technique. Super-heterodyne technique is used to lower the beat frequency down to 1 MHz, so that the phase change of the interferometer output is precisely measured with a displacement resolution of 0.1 nanometer. A thermal control scheme is adopted to stabilize the cavity length of HeNe plasma tube so that a frequency stability of 2 parts in $10^9$ is obtained by suppressing beat-frequency drifts caused by the phenomena of frequency pulling and polarization anisotropy. This two-longitudinal mode HeNe laser yields a high output power of 2.0 mW, which allows multiple measurements of up to 10 machine axes simultaneously.
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 modern 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 1.58 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. Test results prove that the proposed scheme is capable of measuring the position of the moving target at the velocity of 2 m/s with an accuracy of less than 2 nm.
