Precision length measurement using an all-digital phasemeter for heterodyne laser interferometry

We report on an all-digital radio-frequency phasemeter for application in precision length measurements using heterodyne laser interferometry [1]. Our phasemeter has a phase sensitivity of 200 nrad/√Hz at signal frequencies of 0.2 Hz and above. We demonstrate the use of our phasemeter in an optical heterodyne interferometric configuration, using an active Sagnac interferometer test-bed. In our low-noise Sagnac test-bed, we demonstrate more than 70 dB of laser frequency noise suppression to achieve an optical phase sensitivity of 5 μrad/√Hz, and a corresponding displacement sensitivity of 0.5 pm/√Hz at signal frequencies above 10 Hz. In addition, our phasemeter has the ability to follow full fringe signals accurately at 100 Hz and to track large signal excursions in excess of 10⁵ fringes without cycle slipping. Finally, we measure a cyclic error of ≤ 1 pm/√Hz.