Based on a reference very high capacity mission scenario, the main requirements for an operational feeder link were derived to feed the design phase of the different subsystems. The feeder link capacity was assessed for both digital and analog modulation formats and under different propagation conditions, preliminary SWaP features were evaluated and a preliminary eye safety analysis was also carried out. From the system assumptions, different building blocks were then designed, developed or upgraded and tested with the aim to be implemented in laboratory and outdoor demonstrations of high data rate optical links :
- Optical transmitters and receivers breadboards were developed and tested addressing several modulation formats including both digital and analog: OOK (10G and 25G), DPSK (25G) and DQPSK (50G), DP-QPSK to 16QAM coherent modulation and analog intensity modulation.
- 2 x 50W and 1 x 10W high optical power amplifiers and a high power multiplexer breadboard (3 channels up to 10W/ch) were developed. A setup able to coherently combine two beams of up to 50W was also integrated to generate 100W-class transmissions.
- Two types of optical ground station (OGS) setups were upgraded: a 35cm diameter single-aperture OGS (SA-OGS) equipped with Adaptive Optics and a 4 x 20 mm multi- apertures OGS (MA-OGS) with tip/tilt correction and digital signal combining as two optical approaches for the mitigation of atmospheric impairments. Concurrently, (de)coding/ (de)interleaving FPGA boards were developed as a digital mean to reduce the impact of residual atmospheric impairments on the communication signal.
Most of breadboards and setups developments and upgrades were preceded by simulation analysis and/or tests at components level in order to predict performance in the relevant upcoming environment (e.g. adaptive optics performance were evaluated based on topographic map on the line of sight to be encountered in the frame of the outdoor demo).
Finally, four demonstrations were deployed each addressing a combination of key technologies for the development of operational optical feeder links:
- A first demonstration took place in Switzerland between the high altitude station of Jungfraujoch in the Swiss Alps and the Zimmerwald observatory near Bern implementing a space terminal emulator in Jungfraujoch and the SA-OGS in Zimmerwald. An optical downlink over 53 km of free-space propagation was established successfully conveying telecom data based on previously mentioned modulation formats and a record transmission speed was achieved at 1 Tbit/s over a single wavelength.
- A second demonstration took place in Switzerland between Zimmerwald hosting the MA-OGS and the Castle of Thun hosting a space terminal emulator. Up- and downlink were both tested over a 18 km line-of-sight, transmitting data at 10G based on OOK modulation.
- A third demonstration was deployed in Berlin between two buildings implementing a 2x200m round-trip and successfully demonstrating the coherent combining of optical signals received from each of the four apertures and conveying telecom signals based on 10G OOK modulation.
- Transmissions at high and very high optical power were addressed in the frame of a secure laboratory demonstration set up in Palaiseau (France) for obvious laser safety reasons. OOK and DPSK transmissions up to 25 Gbit/s were successfully tested and a record transmission of telecom data at 97W optical power was achieved.
All the results achieved in the frame of the project were widely disseminated through international conferences, workshops, journal papers and press releases.
