Multicore fiber beacon system for reducing back-reflection in satellite quantum key distribution

Abstract

Optical beaconing is an important part of the acquisition, pointing and tracking system necessary for free-space quantum key distribution (QKD). However, uplink beacon back-reflections from the receiver architecture can result in noise. Wavelength- and time-division multiplexing has been used, but neither is yet sufficient to make back-reflection negligible. The use of additional telescopes increases complexity and pointing error. Here, we propose the use of a 2-by-2 multicore fiber, to act as an optical uplink beacon source. This spatially separates the QKD channel and optical uplink beacon. Up to 50 dB improvement in noise rejection over a purely spectrally divided system was demonstrated. The route to further improvements through greater fiber core separation is described. Beacon systems designed in this way could provide a combination of reduced complexity and improved noise performance to free-space and satellite QKD and optical communications.

Beamshaping beacon light for satellite quantum key distribution: spatial separation and point ahead angle

“…We have previously demonstrated back-reflection reduction using a square multicore fibre beacon. 23 This non-optimal example showed up to 50dB of back-reflection reduction compared to a single mode beacon. This is despite being limited to narrow (50µm) beacon spacing by the multicore fibre.…”