Laser Terminals on CubeSats: Developments for Telecommunications and Quantum Links

Satellite-based laser communication is an emerging technology that is finding its way from research to industry. Compared to radio frequency (RF) systems, it has a more efficient size, weight, and power budget and, furthermore, is license free. The required space laser terminals can be designed in different sizes, depending on the mission needs. Data rate requirements range from CubeSats with Mb/s to large satellites with Gb/s data rates and sometimes even Tb/s. This enables, for example, the use of high-resolution imagers even in CubeSats or mega-constellation networks with high-rate intersatellite links. Space laser terminals are also necessary for satellite-based Quantum Key Distribution (QKD), which is increasingly important for the development of future quantum-safe networks. In contrast to classical optical links for data transmission, link budget constraints cannot be overcome by simply amplifying the power, but the end-to-end loss needs to be minimized. This is possible with high antenna gains defined by the transmit and receive optics size. Therefore, the optics size of the laser terminal is one of the most important parameters. Building optical terminals with large apertures for use in space is expensive and requires at least a small satellite platform, increasing the cost of development and launch. The New Space approach using a CubeSat platform is a cost-effective alternative because many components can be selected off-the-shelf. This paper reviews developments of laser communication terminals for CubeSats in space to ground and intersatellite scenarios with applications in quantum communications and telecommunications. The systems are selected with respect to clear space deployment, and their core parameters are compared. Special focus and detailed insight are given for the development OSIRIS4CubeSat (O4C).