Orthogonalizing the control of frequency combs for optical clockworks.

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.

Hertz-level frequency comparisons between diverse color lasers without a frequency comb.

We present a simple yet powerful technique to measure and stabilize the relative frequency noise between two lasers emitting at vastly different wavelengths. The noise of each laser is extracted simultaneously by a frequency discriminator built around an unstabilized Mach-Zehnder fiber interferometer. Our protocol ensures that the instability of the interferometer is canceled and yields a direct measure of the relative noise between the lasers. As a demonstration, we measure the noise of a 895 nm diode laser against a reference laser located hundreds of nm away at 1561 nm. We also demonstrate the ability to stabilize the two lasers with a control bandwidth of 100 kHz using a Red Pitaya and reach a sensitivity of 1Hz2/Hz limited by detector noise. We independently verify the performance using a commercial frequency comb. This approach stands as a simple and cheap alternative to frequency combs to transport frequency stability across large spectral intervals or to characterize the noise of arbitrary color sources.

Bidirectional microwave and optical signal dissemination.

We describe a technique to disseminate highly stable microwave and optical signals from physically separated frequency standards to multiple locations. We demonstrate our technique by transferring the frequency stability performance of a microwave frequency reference to the repetition-rate stability of an optical frequency comb in a different location. The stabilized optical frequency comb becomes available in both locations for measurements of both optical and microwave signals. We show a microwave frequency stability of 4×10(-15) in both locations for integration times beyond 100 s. The control system uses only a standard Ethernet connection.