Multiheterodyne Detection and Sampling of Periodically Filtered White Light for Correlations at 20 km of Delay

A frequency comb is used as a set of coherent local oscillators to downconvert and spectrally compress white light that has been periodically filtered by a Fabry–Perot etalon. Multiheterodyne detection allows white light spread across 100 GHz of optical spectrum to be compressed to 5 GHz of radio frequency (RF) bandwidth for electronic sampling on an oscilloscope. Correlations are observed at delays of up to 20 km with a minimum resolution of less than 1 mm. Calculations show that resolution may be easily increased by increasing etalon finesse and frequency comb bandwidth.