Ultra-stable transfer of laser light over an 18 km open-air link: Multichannel high-density coherent optical & microwaves transmission for next-generation space communications

An advanced technique of free-space optical frequency transfer has been demonstrated with the aim of realizing next-generation optical communications in space between satellites and ground stations. Optical phase noise caused by severe atmospheric turbulence on multichannel optical carriers is corrected collectively by utilizing the frequency comb of an ultrashort fiber laser as the frequency reference stabilized to the atomic clock. This technique permits high-density multichannel coherent communications using not only laser light but also extremely high band microwaves.

Article | Spring 2020

A research team led by Professor Seung-Woo Kim and Young-Jin Kim of the Department of Mechanical Engineering at KAIST has developed an ultra-precision multichannel optical frequency transfer technology through the atmosphere, which suppresses the phase disturbance due to open-air turbulence in real time. This technology has potential applications in areas such as next-generation space-to-ground ultra-high-speed optical communications.

Phase-coherent transfer of optical frequencies over a long distance is required for diverse photonic applications, including optical clock signal dissemination and physical constants measurement. Several demonstrations have been made successfully over fiber networks, but no such work had been done yet through the open air where atmospheric turbulence prevails. The research team has transferred multiple ultra-stable lasers, extracted directly from the frequency comb of a fiber ultrashort laser, over an 18 km free-space link constructed between the KAIST main campus and a nearby mountain in Daejeon (See Figure 1.). This free-space transfer of optical frequencies over a long haul outdoor link was performed with suppression of atmospheric phase noise to -80 dBc/Hz. The concurrent transfer of multiple comb-rooted optical carriers has also enabled the delivery microwave signals by pairing two separate carriers with inter-comb-mode phase coherence, with phase noise of -145 dBc/Hz at 1 kHz offset for a 10GHz microwave signal. Furthermore, coherent optical communication has been demonstrated with the potential of multi-Tbps data transmission by multiplexing comb-rooted carriers in free space. The proposed free-space transfer of comb-rooted optical and microwave frequencies is expected to facilitate many photonic applications such as atomic clock dissemination, fundamental constants measurements, long-baseline interferometers, and coherent communications.

The lead author Dr. Hyun Jay Kang said, “We have shown that the multi-channel narrow-linewidth lasers that propagate through the ambient air can be transferred over 18 km with the same level of coherence properties, even under atmospheric disturbance. This result shows that the laser, which has to propagate through the existing optical fiber, can be utilized beyond the limitation of space.” And they said, “We expect that the distribution and synchronization of optical time/frequency standard through the ambient air will improve the performance of next-generation navigation systems and be used for space-to-ground ultra-high-speed optical communications.”

This research was published online in September 30, 2019 in the journal Nature Communications under the title, “Free-space transfer of comb-rooted optical frequencies over an 18 km open-air link”.