Frequency comb metrology with an optical parametric oscillator

We report on the first demonstration of absolute frequency comb
metrology with an optical parametric oscillator (OPO) frequency comb. The
synchronously-pumped OPO operated in the 1.5-μm spectral region and
was referenced to an H-maser atomic clock. Using different techniques, we
thoroughly characterized the frequency noise power spectral density (PSD)
of the repetition rate frep, of the carrier-envelope offset frequency fCEO, and
of an optical comb line νN. The comb mode optical linewidth at 1557 nm
was determined to be ~70 kHz for an observation time of 1 s from the
measured frequency noise PSD, and was limited by the stability of the
microwave frequency standard available for the stabilization of the comb
repetition rate. We achieved a tight lock of the carrier envelope offset
frequency with only ~300 mrad residual integrated phase noise, which
makes its contribution to the optical linewidth negligible. The OPO comb
was used to measure the absolute optical frequency of a near-infrared laser
whose second-harmonic component was locked to the F = 2→3 transition
of the 87Rb D2 line at 780 nm, leading to a measured transition frequency of
νRb = 384,228,115,346 ± 16 kHz. We performed the same measurement
with a commercial fiber-laser comb operating in the 1.5-μm region. Both
the OPO comb and the commercial fiber comb achieved similar
performance. The measurement accuracy was