High-accuracy length metrology using multiple-stage swept-frequency interferometry with laser diodes

The optical length of a 1 m Fabry-Perot etalon has been determined by swept-frequency interferometry using laser diodes. The method involves progressively building up the measurement accuracy using frequency sweeps over increasing ranges, from 150 MHz (one optical fringe) to 19 GHz and 7 THz. The 7 THz sweep is referenced to the splitting of the rubidium D lines at 780 nm and 795 nm. The result from the 7 THz sweep is sufficiently accurate to use the known frequency of either end point of the scan to determine the length to a few parts in 10(10), without the need for any further measurement. The scope for further development of this technique to a range of interferometric systems is discussed.