Long distance absolute laser ranging at the nanometer precision level: the two-mode interference measurement in the Iliade rangemeter

Description

Some measurement schemes have been proposed and realized for the absolute measurement of long distances with an accuracy better than 10 nm. Published measurement setups use one or even two laser frequency combs. But significant engineering will be required to space qualify such a system. Simple methods, less technology-demanding would be valuable in the perspective of embedded instrumentation. We have designed and implemented a two-mode interference measurement scheme that allows sub-nanometer scale resolution in long distance measurement. The two-mode interference signal contains both - (sub-µm) interferometric information: the scale is the optical wavelength - (sub-15mm) modulation phase information: the scale is the "synthetic wavelength" corresponding to the frequency of the beat-note of the two modes. With the addition of a time-of-flight (ToF) measurement, the method allows to combine the three data (ToF, synthetic wavelength and interferometric) in a single, high-resolution, high accuracy length measurement, obtained every 50 ms. A measurement update rate of 100 µs is possible, but may rely on the availability of significantly higher data processing rates in the FPGA phase-meter. Implementation of this scheme has required that systematic errors on the phase and amplitude of the microwave optical signal be kept at a level well below 10^-4 cycle and 10^-4 respectively. One consequence of this requirement is the replacement of any parallel optics in the optical setup by wedged optics, so that multiple reflections do not interfere with the measurement and reference beams to better than 10^-8 in optical power. Systematic errors of electronic origin are more difficult to deal with because the amplitude-to-phase (AM-to-PM) couplings effects at 20 GHz appear to have, not only an instantaneous contribution, but also a transient contribution. This contribution, related to the heating of the photodiode junction under the dissipated Joule power, exceeds the limit of 10^-4 cycle by roughly two orders of magnitude. This thermal behaviour is not purely exponential with time and cannot be accurately corrected for. We will present the implementation of the setup, and the way we have suppressed, by 3 orders of magnitude, the AM-to-PM coupling effect by modifying the detection scheme of the 20 GHz beatnote. This last point is important, not only for the range meter presented, but also for in high accuracy and low phase noise microwave optical links.

Abstract

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