Dual wavelength optical system for multiple quantum communication transmitters in Cubesat platform

Abstract

Quantum Key Distribution (QKD) systems will play an important role in future networks for secure data communication. In order to provide a global coverage of a future QKD service, satellites are 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. A previous development of a 1/3 U CubeSat communication terminal for up to 100 Mbit/s downlinks is adapted to be used for quantum communications tests with multiple transmitters. Since the original laser terminal was designed for C-band wavelengths, a redesign of the optical system is needed to achieve polychromatic performance for the three sources. The optical system consists of a fiber collimator, a beam steering system and an afocal telescope. Most important requirement of the latter is a similar magnification for all wavelengths to ensure concentric beams pointing to the OGS. Scenarios in which the system is optimized either for lowest divergences or a similar magnification of the telescope are demonstrated in Figure 1. As the afocal telescope is used bidirectionally, it also needs to be optimized for the incident wave-front of the beacon from the OGS with respect to the beam shape at the four quadrant detector. These parameters are important for a correct pointing control. A fiber-based wavelength division multiplexer (WDM) is used for combining signals from three sources into one output fiber. As shown in the block diagram in Figure 2, it is based on cascaded thin film interference filters which are coupled to the fibers. Especially the propagation of 850 nm and 1550 nm signals in one single mode fiber is critical. Therefore the types of optical fibers were selected with respect to the bend loss, single mode propagation behavior, polarization integrity and optimal cladding diameter for the production.