Quantum Key Distribution systems will play an important role in future networks for secure data communication.In order to provide a global coverage of a future Quantum Key Distribution service, satellites will be needed to bridge large distances. These satellite systems must be cost efficient to facilitate deployment since many network nodes will be needed. The CubeSat standard is frequently used for New Space projects as a versatile platform for satellite payloads. It is also chosen as a baseline for the construction of the system described in this paper. The DLR Institute of Communications and Navigation develops optical free space communication systems for scientific research in classical and quantum communications. In the OSIRIS4CubeSat (Optical Opace Infrared Downlink System for CubeSats) project a 1/3 U CubeSat laser-communication terminal for up to 100 Mbit/s downlinks was developed. 1 This developement is adapted to be used for quantum communications tests with multiple transmitters in the scope of the project QUBE . 2 Since the OSIRIS4CubeSat terminal was designed for C-and L-band wavelengths, a redesign of the optical system is needed to achieve polychromatic performance for C-and L-band and 850 nm. The optical system consists of a fiber collimator, a fine pointing assembly and an afocal telescope. Most important requirement of the latter is a similar magnification for all wavelengths to ensure coaligned beams pointing to the optical ground station (OGS). As the afocal telescope is used bidirectionally, it also needs to be optimized for the incident wave-front of the beacon laser from the OGS with respect to the beam shape at the tracking sensor which is used in the fine pointing assembly. These parameters are important for a correct pointing control. In addition to the laser terminal, a fiber-based wavelength division multiplexer (WDM) was specified for combining signals from three sources into one output fiber. It is based on cascaded thin film interference filters which are coupled to the fibers. Especially the propagation of 850 nm and C/L-band signals in one single mode fiber is critical. Therefore the types of optical fibers were selected with respect to the bend loss, to single mode propagation behaviour, polarisation integrity and optimal cladding diameter for production.
