TOmCAT is a technology development project with the end goal of developing a commercial optical ground station product. This ground station can with high data rate laser links communicate with the next generation of Very High Throughput Satellites.
One of the innovative elements of TOmCAT is its ability to pre-correct the laser light with adaptive optics. When light moves between the surface of the Earth and space, it gets distorted due to the fluctuations in the atmosphere. TOmCAT measures the distortion of the received laser light from the satellite, and by applying the inverse of this distortion to the transmitted light, a robust communication link can be established. To be able to do this, high speed adaptive optics is required and an important objective for the project is to build a demonstrator to test and prove this concept.
The key challenges within the project are to:
- Select and assess the performance of a communications architecture that enables future Terabit/s Optical Feeder Links
- Design an Optical Ground Station with adaptive optics that can:
- Sustain high laser power with a minimal performance loss over its lifetime
- Provide sufficient robustness of optical feeder links by pre-correcting atmospheric distortions
- Verify the Adaptive Optics pre-correction principle
- Generate high laser powers with high image quality
With the ever increasing amount of devices communicating in the radio frequency spectrum, the availability of allowed bands is becoming limited. This in itself is a good reason to investigate other means of communication between Earth and space, since all satellite and ground station operators wish to maximize their own throughput without interference with other users. With the introduction of 5G, this problem increases even more as 5G ground operators are claiming RF spectrum which is currently used by telecom satellites. At the same time, the use of optical laser communication dramatically increases the speed of which information can be transmitted. Speeds in the order of Terabit/s can be reached, which is significantly faster than what is possible today.
The increased data throughput and the simplified ground network as compared to RF systems results in an overall reduced cost per bit.
Building a reliable and robust Adaptive Optics based Optical Ground Terminal is a challenge. The system has to generate 650 Watt of optical power, levels that are high enough to burn any contamination on the mirrors and lenses. Since this would cause a performance degradation higher than what is allowed, an extremely clean environment is required.
The accuracy and stability of the moving mirrors needs to be in the order of sub-micro-radian in order to find and track the satellite 36 000 km away. This puts high constraints on the quality and stability of the multiple wavelength multiplexed lasers, each generating 100 Gbit/s of throughput. An opto-mechanical system consisting of a bulk multiplexer, adaptive optics system and a 60 cm telescope are developed, each providing the required stability and accuracy while handling the high optical power levels.
Based on the so called ‘digital transparent’ architecture’ selected in TOmCAT, RF user signals will be sampled and modulated onto the optical domain by means of a Digital Processor. High power optical Amplifiers (EDFAs) boost the optical signals to high power levels in the order of 50 Watts per channel. In order to achieve Terabit/s throughputs, multiple wavelengths are then multiplexed into two very high power optical free space beams. An Adaptive Optics system pre-corrects the optical wavefront to compensate for the expected atmospheric turbulences and a Telescope transmits the high power beams to the geostationary satellite. In addition, a downlink implementing a similar architecture enables a bi-directional Optical Feeder Link that will serve the next generation of Very High Throughput Communication Satellites. The satellite provides communications services to end-users through hundreds of RF spot beams, where each spot beam serves a geographic area on Earth.
Overview of communication links. Credit: TNO
In 2018, the System Requirements Definition was completed, as well as a successful Adaptive Optics pre-correction proof of principle tests with a 10 km ground to ground link. The test was performed in Germany and a picture from the test site and OFELIA, the breadboard used, are seen below.
Test site. Credit: DLR
Breadboard OFELIA. Credit: TNO
One of the objectives of the TOmCAT project is to design a commercialized Optical Ground Station. Preliminary design of this envisioned product has been completed in 2019. For reaching the goal of a functional and reliable system, extensive testing needs to be performed to satisfy the challenging demands. An Optical Ground Terminal Demonstrator has been designed for testing purposes, of which the designed has been finalized also in 2019.
Final product design. Credit: TNO.
Adaptive Optics and Bulk Multiplexer system design. Credit: TNO
The TOmCAT project has a number of technological phases and a product phase planned, of which the first technological phase is completed in 2019. After this initial phase, the project covers the building of an Optical Ground Terminal Demonstrator based on the design completed in phase one. The purpose of this demonstrator is to test the Adaptive Optics principle with communication added over long distances. These are the first steps towards an adaptive optics pre-corrected and wavelength multiplexed optical feeder link in-orbit demonstration, which will be the first ever of its kind.