An optical time distributor includes: a master clock including: a master comb; a transfer comb; and a free-space optical terminal; and a remote clock in optical communication with the master clock via a free space link and including: a remote comb that produces: a remote clock coherent optical pulse train output; a remote coherent optical pulse train; a free-space optical terminal in optical communication: with the remote comb; and with the free-space optical terminal of the master clock via the free space link, and that: receives the remote coherent optical pulse train from the remote comb; receives the master optical signal from the free-space optical terminal of the master clock; produces the remote optical signal in response to receipt of the remote coherent optical pulse train; and communicates the remote optical signal to the free-space optical terminal of the master clock.
The invention is a method to compare and synchronize "clocks" (i.e. local timescales) through optical links across free space, which include open air paths through the atmosphere to other terrestrial sites or to satellites, as well as satellite-to-satellite paths. The challenge is to compare the clock timing without introducing unknown timing offsets and timing noise due to the long and varying time-of-flight between the sites. In the past, this timing comparison has been accomplished by radio frequency (rf)/microwave links using a two-way exchange of signals. In this two-way approach, one transmits the timing information both ways and calculates the difference. The time of-flight drops out of the difference, leaving only the time offset between the two clocks.
However, with these rf/microwave links, one is still limited by typical rf bandwidths so that the timing can be compared only at the picosecond level, at best. However, with the advent of optical clocks and oscillators, clocks can have timing at the femtosecond level. In other words, the clocks can perform much better than we could possibly "use" in a network since we cannot disseminate time/frequency at the femtosecond level. To solve this problem, we have taken this same basic two-way approach used in the rf/microwave domain but adopted it to the optical domain, where we are exchanging pulses from two frequency combs. Thus, we can synchronize clocks even between moving platforms, which is the main application for a network connected by free-space links (e.g. airborne or satellite clocks). Additionally, we can use this technique to generate coherent microwave signals at remote sites. This advance is relevant to future coherent radar systems. Finally, we can do 10 - 100x better by using information in the optical carrier phase of the frequency comb pulses.
Current methods for the distribution of time/frequency over free space use rf/microwave techniques. The most well-known example is the GPS system but there are also two-way rf techniques. These methods can compare times at the nanosecond to picosecond level after minutes to hours of averaging. Our method is technically superior by a factor of a thousand; it can compare times to femtosecond levels after averaging for below a second. The implementation of our method requires a single mode optical link. These free-space optical links are evolving rapidly in the context of high speed free-space optical communication.