Toward a spaceworthy picometer laser gauge

Metrology will be an enabling technology for a new generation of astronomical missions having large and distributed apertures and delivering unprecedented performance. The x-ray interferometer Black Hole Imager, the UV interferometer Stellar Imager, and other missions will require measurements of incremental distance as accurate as 0.1 picometer, and absolute distance capability. Our Tracking Frequency laser distance Gauge (TFG) was developed a decade ago for a NASA-funded study of a spaceborne astronomical interferometer. It has achieved an accuracy of 10 picometer with a 0.1 second averaging time, and 2 picometer in 1 minute. The standard approach, the heterodyne gauge, displays nanometer-scale cyclic bias, whose mitigation has been the subject of much effort. Our approach is free of that bias and can measure absolute distance with little or no additional hardware. It provides the option of operation with a resonant optical cavity, which in many applications would provide increased accuracy of both incremental and absolute distance. We plan to develop a next-generation laser gauge based on our unique and successful architecture. We will improve incremental and absolute distance accuracy, increase sample and readout rates, and establish a capability for measuring long distances. The new TFG will use solid-state lasers and fiber-connected components in place of a HeNe laser and free-space beams. We expect that the new TFG will have low replication cost and be rugged, modular, and easy to set up. It will employ components that are likely to be space qualifiable.