Frequency comb metrology with an optical parametric oscillator
Optics Express, Optical Society of America, 2016/24/8/8370-8381
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 <i>f</i><sub>rep</sub>, of the carrier-envelope offset frequency <i>f</i><sub>CEO</sub>, and of an optical comb line ν<i><sub>N</i></sub>. 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 <i>F</i> = 2→3 transition of the <sup>87</sup>Rb D2 line at 780 nm, leading to a measured transition frequency of ν<sub>Rb</sub> = 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 limited by interferometric noise in the fibered setup of the Rb-stabilized laser.
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