Two-wavelength infrared heterodyne transceiver with a continuous phase tracking system

Abstract

We describe the performance of an optical heterodyne sensor with a two-wavelength coherent infrared beam, which is used to detect the motion of remote targets in a real environment with an unambiguous sensing of the instantaneous direction along the line-of-sight propagation path. The initial phase condition of maximum sensitivity of the transceiver is stabilized and held by an active feedback system which drives the phase of the optical field of the local oscillator. The experimental results show that this feedback is particularly useful in reducing the noise generated by low-frequency environmental disturbances, leaving the signal unaffected when the respective frequency ranges are separated. Under certain circumstances, the signal can be recovered from the noise by the linearizing properties of the phase tracking on the interferometer response. The measurements reported show that the two-wavelength infrared heterodyne transceiver generates signals which replicate the target vibration exactly up to amplitudes at a multiwavelength level. This allows measurements of displacements on targets with submicron accuracy at distances of several meters.