The aerospace network connectivity market is currently experiencing a renewed period of intense activity in space, driven by numerous companies planning constellations of thousands of low Earth orbit satellites. These constellations - globe-spanning networks of interconnected satellites - promise to enhance Earth observation, facilitate the Internet of Things, and bring connectivity to the estimated half of the world's population still without internet. The consequent demand for data bandwidth that will follow from these developments can only be delivered through the implementation of optical inter-satellite links, routing user information on-demand and with minimum delay. The economic feasibility of these endeavors is dependent on the size, weight, and power requirements (SWaP) of the laser terminals that will provide the backbone connectivity of these constellations. Given that several optical terminals will be required per satellite, they need to not only establish gigabit data links over thousands of kilometers but also possess minimal SWaP and mostly be design-engineered for serial production. This paper discusses the requirements for optical terminals derived from constellation modeling. We update on the qualification status of the inter-satellite link terminal program, and introduce our roadmap for large scale serial production.
Broadband internet access has become a vertex for the future development of society and industry in the digital era. Geostationary orbit (GEO) satellite can provide global broadband coverage, becoming a complementary solution to optical fiber network. Low-earth-orbit (LEO) constellations have been proposed in the last years and they may become a reality soon, but still based on radiofrequency for the ground-to-satellite links. Optical technologies offer multiple THz of available spectrum, which can be used in the feeder link. The DLR’s Institute of Communications and Navigation has demonstrated Terabit-per-second throughput in relevant environment for GEO communications, in terms of the turbulent channel. In 2016 DLR set the world-record in freespace communications to 1.72 Tbit/s, and in 2017 to 13.16 Tbit/s. Two terminals, emulating the satellite and the ground station have been developed. Bi-directional communications link with single-mode-fiber coupling at both ends was demonstrated. Adaptive optics for the downlink and uplink (pre-distortion) improved the fiber-coupling in downlink and decreased signal fluctuations in uplink. A 80 Gbit/s QPSK system based on digital homodyne reception was also developed, demonstrating the use of coherent communications under strong turbulence conditions. These activities were performed in the frame of two internal DLR projects, THRUST and Global Connectivity Synergy project. Several measurement campaigns took place in the last years in a valley-to-mountain-top test-link. Turbulence has been monitored at both ends and the point-ahead-angle has been emulated by separating the downlink beacon from the receiving aperture. An overview of the system and the main results will be presented.
Free-space optical communications (FSOC) are rapidly becoming a key technology for terrestrial, aerial, and space communication, mainly because of its very high throughput capacity. To achieve multi-gigabit laser downstream, an efficient single-mode fiber coupling is required. However, atmospheric turbulence remains one of FSOC’s main limitations. The turbulence affects the communications performance by inducing wavefront distortions that develop into coupled power fluctuations. In regimes of very strong turbulence, the use of traditional adaptive optics systems is limited due to strong scintillation and higher number of phase singularities. These limitations could be solved by relying on systems based on the stochastic iterative maximization of the coupled power. The drawback of such systems is that a high number of iterations are required for signal optimization. We address this problem and propose a different iterative method that compensates the distorted pupil phasefront by operating directly on the focal plane. The technique works by iteratively updating the phases of individual speckles to maximize the received power coupled into a single-mode fiber. We show numerically and experimentally that the method can improve the quality of the received signal with reduced bandwidth utilization.