Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest keep increasing. So does the technology readiness of satellite free-space optical communications. A satellite QKD system comprises a quantum communication subsystem and a classical communication subsystem (public channel). Both are implemented with freespace optics. Thus, in satellite QKD system design, there are strong synergies that should be exploited as much as possible and lead to an all-optical satellite QKD system. In this paper, we present a system like this locating all optical channels in ITU DWDM C-band. We focus on the overall conceptual design and the setup of the optical channels for quantum and classical signal transmission. The system description addresses the breadboards of a transmitter laser terminal (Alice terminal), a receiver laser terminal, (Bob terminal), the public channel implementation, the interfaced QKD system and the deployed encryption system. The design basis for the Alice terminal is the laser terminal development OSIRISv3. The design basis for the Bob terminal is the ground station development THRUST. The later contains an adaptive optics correction to enable single mode fiber coupling. This enables the interfacing to almost arbitrary quantum receivers such as the Bob modules used in the described experiment. The public channel is composed of a bi-directional 1 Gbps IM/DD system and a MODEM that implements a proprietary waveform optimized for free-space channels.The system was experimentally analyzed in a field test in the framework of the German initiative QuNET which addresses the use case of quantum secure communication for authorities. The results of the experiment are used to model a feasible LEO satellite-ground link. Performance indicators such as quantum bit error rate and secure key rate of a potential mission are estimated analytically.

Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest are ever increasing. Same is true for satellite free-space optical communications (FSOC). In order to enable a robust channel for transmission it is indispensable to account for static and dynamically changing misalignments between the transmitter and receiver pair. This work will focus on the transmitter terminal (Alice) and the design and verification process of the active beam steering system. The novelty is a recently developed variable reluctance fine steering mirror (FSM) including eddy current sensors (ECS) to measure its tip and tilt. A cascaded architecture was chosen in order to combine the optical stabilization objective with the dynamics of the mirror platform. The inner control loop makes use of an observer model whose estimated output is fed into a state controller allowing for an increased responsiveness. While high gains increase the closed loop bandwidth the eigenfrequency of the system introduces a pole to the plant which has to be avoided by the controller output. A digital notch filter was introduced to reject the excitation of the critical frequency band which gets obsolete in a system with high frequency sampling capabilities. The outer loop is engaged when a valid optical signal is received and a transition from a closed loop pointing to a closed loop tracking mode is performed. A proportional-integral (PI) controller keeps the received beam at the 4-quadrant-diode (4QD) whose center is used as the main reference through prior calibration with the transmit beam launching on the same path. The presented cascaded control scheme allows improvements in system performance and reliability.