Optical Phase Readout Instrument for Picometer-level Precision Heterodyne Interferometers

Abstract

A laser interferometer typically combines a number of beams that travel different optical paths to determine factors such as lengths, surface irregularities or the index of refraction of materials. Heterodyne detection is a well-established method for sensing tiny optical pathlength displacements through measurements of the phase shift between interfering signals. The ability of measuring displacements with high dynamic range and accuracy at the picometer-level has made this technique a crucial resource in many high-precision metrology applications, particularly for gravitational physics experiments in space, where one of the interfering beams is sensed at ultra-low light power. This article provides an overview of the design, construction and test facilities for an optical phase readout instrument able to extract picometer-stable displacement and nanometer-stable tilt measurements over thousands of seconds from a laser link operating at MHz heterodyne frequencies. The optical pathlength sensitivity of the instrument has been measured down to 1 pm/√Hz for frequencies above 3 mHz. The pitch and yaw pointing sensitivity is required below 5 nrad/√Hz and performed by applying the differential wavefront sensing technique. The instrument sensitivity seems to be limited above 3 mHz by ADC clock timing jitter and below 1 mHz by phase distortion caused by temperature fluctuations in the front-end electronics circuitry. Noise budgets and coupling mechanisms for both longitudinal and angular displacements are still under investigation with the design goal of an enhanced instrument performance with reasonable margins over the stringent sensitivity requirements.

Keywords

• gravitational physics
• optical metrology
• laser ranging
• phase-locked loop
• pseudoranges
• optical communication