Orthogonalizing the control of frequency combs for optical clockworks

Abstract

Frequency combs play a crucial supporting role for optical clocks by allowing coherent frequency division of their output signals into the electronic domain. This task requires stabilization of the comb's offset frequency and of an optical comb mode to the clock laser. However, the two actuators used to control these quantities often influence both degrees of freedom simultaneously. This non-orthogonality leads to artificial limits to the control bandwidth and unwanted noise in the comb. Here, we orthogonalize the two feedback loops with a linear combination of the measured signals in a field-programmable gate array. We demonstrate this idea using a fiber frequency comb stabilized to a clock laser at 259 THz, half the frequency of the ¹S₀→³P₀ Yb transition. The decrease in coupling between the loops reduces the comb's optical phase noise by 20 dB. This approach could improve the performance of any comb stabilized to any optical frequency standard.