Elimination of systematic errors in two-mode laser telemetry

We present a simple two-mode telemetry procedure which eliminates cyclic errors, to allow accurate absolute distance measurements. We show that phase drifts and cyclic errors are suppressed using a fast polarization switch that exchanges the roles of the reference and measurement paths. Preliminary measurements obtained using this novel design show a measurement stability better than 1 micron. Sources of residual noise and systematic errors are identified, and we expect that an improved but still simple version of the apparatus will allow accuracies in the nanometre range for absolute measurements of kilometre-scale distances.