Demonstration of 100 Gbps coherent free-space optical communications at LEO tracking rates

Abstract

Free-space optical communications are poised to alleviate the data-flow bottleneck experienced by spacecraft as traditional radio frequencies reach their practical limit. While enabling orders-of-magnitude gains in data rates, optical signals impose much stricter pointing requirements and are strongly affected by atmospheric turbulence. Coherent detection methods, which capitalize fully on the available degrees of freedom to maximize data capacity, have the added complication of needing to couple the received signal into single-mode fiber. In this paper we present results from a coherent 1550 nm link across turbulent atmosphere between a deployable optical terminal and a drone-mounted retroreflector. Through 10 Hz machine vision optical tracking with nested 200 Hz tip/tilt adaptive optics stabilisation, we corrected for pointing errors and atmospheric turbulence to maintain robust single mode fiber coupling, resulting in an uninterrupted 100 Gbps optical data link while tracking at angular rates of up to 1.5 deg/s, equivalent to that of spacecraft in low earth orbit. With the greater data capacity of coherent communications and compatibility with extant fiber-based technologies being demonstrated across static links, ground-to-low earth orbit links of Terabits per second can ultimately be achieved with capable ground stations.

Optical Frequency Transfer for Geopotential Difference Measurements via a Flying Drone

“…The robustness of the phase stabilization system is currently limited by the actuation speed and range of the AOM. The performance of the terminal under more demanding drone flight patterns is covered in depth in [23].…”

“…The reflection of the laser off the CCR provides the beacon source for the spatial stabilization control [23]. The returning light reflects off the tip-tilt mirror, into the GBE and through the beam splitter.…”

“…The laser tip-tilt control system is engaged when returning laser power is detected at the QPD. The function and performance of the optical terminal is described in more detail in [23].…”

“…Here, we demonstrate phase-stabilized optical frequency transfer at 2.5×10 −18 via a retroreflector mounted to an airborne drone. A ground-based optical transceiver terminal [23] provided automatic acquisition and tracking of the drone, including tip-tilt optics for turbulence compensation, which enabled cycle-slip-free optical phase measurements for several minutes despite atmospheric turbulence causing both beam wander and random motion of the drone. We also show how the break-down of link reciprocity due to different outgoing and return optical frequencies and motion of the drone can be accounted for when using such as system.…”

Experimental demonstration of velocimetry by actively stabilised coherent optical transfer

“…VASCOT optically tracks the phase of the returned signal to allow for narrow-band photo-detection, thus transferring the requirements on the photodetector bandwidth to the phase-tracking bandwidth. The pointing challenges are handled by an active tracking terminal capable of suppressing angular pointing errors to the moving target [22]. VASCOT is experimentally demonstrated over a 584 m atmospheric link to a corner-cube retro-reflector (CCR) on an airborne drone, with cycleslip-free phase tracking achieved for the three-minute experiment.…”