Radio-frequency clock delivery via free-space frequency comb transmission

Abstract

We characterize the instability of an rf clock signal caused by free-space transmission of a frequency comb (FC) under typical laboratory conditions. The phase-noise spectra show the involvement of multiple random processes. For a 10 m transmission, the rms timing jitter integrated over 1-10(5) Hz is 95 fs, and the root Allan variance over 1 s is 4x10(-13). The measured Allan variance has a tau(-1) behavior and an excellent agreement with the phase noise measurement. These results indicate the feasibility of FC-based free-space rf clock distribution over short distances.

Atmospheric Clock Transfer Based on Femtosecond Frequency Combs

“…In addition, we have also made measurement with the AC system off and found the noise to be much lower. These facts confirm the effectiveness of our measurement system (Alatawi et al 2009). Fig.…”

Frequency comb-based microwave transfer over fiber with 7×10^−19 instability using fiber-loop optical-microwave phase detectors

“…For much longer-range (over tens to hundreds of km standard telecommunication fiber links) transfer of combs with <10 −18 -level microwave instability, we have an interest in exploring polarization control at the fiber link output [16] and developing a new schematic with polarization scrambling and a polarization-insensitive optical-microwave phase detector. We also note that the FLOM-PD-based timing stabilization might be particularly useful for free-space timing links [17,18], where the amount of timing/phase fluctuations is larger while the chromatic dispersion, polarization drift, and PMD are smaller than the fiber links.…”

Excess Phase Noise Characterization in Multifrequency Remote Clock Distribution Based on Femtosecond Frequency Combs

“…Recently, remote distribution of optical-frequency references has attracted considerable interest [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. There are in general two possible transfer media: optical fiber and free space.…”

“…Coherent optical links spanning well over 100 km have been experimentally demonstrated by a number of groups [5][6][7][8][9]. On the other hand, free-space clock distribution benefits from its flexibility and low overhead cost, and has proved to be a feasible option for short-distance, ad hoc networks or networks among mobile hubs, despite its much poorer link fidelity due primarily to air turbulence [15][16][17][18][19]. It is also the only option for space-terrestrial network synchronization [17].…”

“…An alternative approach, meanwhile, transfers multiple frequency references simultaneously in the form of an optical frequency comb (OFC) [13,14,16,18,19]. The advantage of the latter scheme rests on its ability to directly synchronize clocks operating at different wavelengths, for example between an Hg + clock and a Ca clock [20], or even in different domains, for example between an optical atomic clock and a traditional atomic clock, such as a Cs clock [1].…”