Critical thermal drifts in external fiber amplifiers limit optical frequency metrology

Relying on the nonlinear optical effect of difference-frequency generation, passively carrier-envelope stabilized comb systems are currently considered the simplest option for generating combs with zero carrier-envelope offset frequency fceo, that is, without relying on electronic servo loops or additional acousto-optic shifters. However, recent measurements indicated statistically significant deviations from a perfect vanishing fCEO. Here we show that temperature-drift induced refractive index changes in subsequent distribution or booster amplifiers may give rise to an effective Doppler shift of the measured fCEO, thwarting precision frequency measurements up to the mHz level. Comparing this hypothesis with temperature-dependent refractive index data and gain spectra, we find excellent quantitative agreement with measurements. Mitigating these problems by allowing for sufficient warm-up times of the external amplifiers, the fceo mean value of a difference frequency comb is measured as –0.55±1.55 µHz, which is statistically equivalent to 0. The corresponding stability supersedes the current best optical clocks, with an overlapping Allan deviation of <1×10-17 at 1 second and 5.36×10−21 at 100,000 s. Previously often considered negligible, the temperature drift of external distribution or booster amplifiers plays a critical role in obtaining similar stabilities in frequency metrology for all kind of fCEO-stabilized combs.