Signal processing and testing of displacement metrology gauges with picometre-scale cyclic nonlinearity

The Space Interferometry Mission (see http://sim.jpl.nasa.gov) requires displacement metrology gauges with linearity ~10 pm rms over a distance of several metres. Displacement measuring interferometers are under development to meet these requirements, while also meeting thermal stability, robustness, size and geometry requirements. A persistent difficulty in attaining picometre-class performance with laser interferometric metrology gauges is the problem of 'cyclic error' which may be caused by small amounts of mixing of the interferometer measurement and reference signals. The mixing is caused by crosstalk, both optical and electronic. Other causes of cyclic error have been also found and will be discussed. This paper describes our approach to minimizing the cyclic error with emphasis on signal processing issues and the concurrent development of techniques to detect the error as it is gradually reduced to zero.