Optical transceivers for high-speed space communications: en route for the New Space

Exail is involved in several EU projects to prove the ability of its components to address the need of space communications. One of them is the Feeder Optical Links for Constellation (FOLC¹) project, started in 2017. Funded by ESA, supported by the French CNES, and with Airbus Defence and Space as a partner, it aims at building the telecommunication devices for future satellites constellation. It includes the links between satellites in low Earth orbit (LEO <> LEO), the links between LEO satellites and a GEO satellite, and the links between LEO/GEO satellites and Earth. The complete telecommunications network has the goal to reach data transmission rates higher than 1 Tb/s.

The project involves Exail for the development of embedded optical communication and ground ModBox terminals while Airbus is dealing with the electronics components. Exail has thus developed the first generation of space grade transceivers for the shaping of the laser signal transmitted through space. The transceiver is based on one transmitter emitting the optical data-signal via external modulation, (before emission) and a receiver module (see detail synoptic of the optical communication chain above).

For the first part of the project (FOLC¹), that lasted until 2019, Exail built a Reference Transmitter and Receiver ModBox with 12 transmission (Tx) and reception (Rx) channels for an in-lab demonstration. This study was required to define the level of performance of the Tx and the Rx: the modulation schemes, sensitivity of the photo-receiver, optical power budget, etc.

The second part of the project, FOLC² (Feeder Optical Link Communication Chain), started in 2020 is to produce a Demonstrator Model Transceiver: a ready to flight model transmitter and receiver for a data transmission rate of 20 Gbit/s – a first step toward higher data rates. It delivered in 2022 and embarked on a geostationary telecom satellite in 2023. As a "hosted payload", it will be tested in real space conditions for interconnections between the satellite and Earth.

The FOLC project led to the development of a generic solution of transceiver available to any Exail customer, with a transmitter based on 1 to 6 emission channels and a receiver based on 1 to 8 reception channels. Parameters as the total weight of the device and its maximal power can be adapted according to the customer needs. Exail can provide transceivers for constellations counting between two to thousands of satellites.

The transceiver’s Tx is composed of Exail sub-components that have been fully space qualified. The Compact Optical Channel Emitter (OCE) is composed of a laser (commercial one) and proprietary LiNbO₃ modulator and a RF amplifier designed by Exail. Several OCEs can be multiplexed by a Mux. Transceiver’s Rx possesses a proprietary LNOA (Low Noise Optical Amplifier) and commercial grade high-speed photoreceivers.

An optical multiplexer/demultiplexer produced by Exail for other markets is also expected to obtain a space qualification in a near future, enabling its integration to the whole transceiver system.

Another demonstrator is already being manufactured for a LEO orbit satellite dedicated to Earth imaging and will allow the transfer of ultra-high-resolution images of our planet taken from the orbit.

Exail transceivers with proven robustness and reliability will be qualified for this new project, and our teams are already anticipating the next generation of ultra-fast space telecommunications, beyond the terabits/s range.

To conclude, Exail space-grade transceivers have several main advantages. First, two modulation downlink modes (OOK -On-Off Keying- and DPSK -Differential Phase-Shift Keying-) of 10 Gbit/s or 25 Gbit/s are available, for a maximum reachable data rate of 125 Gb/s (with adapted design). Second, an uplink analog modulation or 10 Gb/s digital modulation is available, for a maximum reachable data rate of 125 Gb/s (with adapted design). Finally, the transceiver design (size and weight) can be customized according to the customer needs, with same level of space qualification.