Development and fabrication of a lightweighted, metallic telescope for aviation-related quantum communication

Quantum-based communication is a valid option for ensuring the high security requirements for the transmission of information between different institutions. The QuNET initiative aims to build a quantum-secure German government network based on quantum key distribution (QKD). Free-space optical (FSO) links can be used as flexible, temporary infrastructure for example to bridge the last few kilometres without fibre infrastructure or to enable QKD-links between ground-stations and aviation-related stations. High-throughput telescopes are an essential component of these FSO links, acting as transmitters and receivers. In case of aviation hubs, the telescopes must be compact and lightweighted.

This paper describes the development and manufacturing of a lightweighted seven-mirror metal telescope for aviation-based quantum communication. The unobscured, afocal system is operating with a clear telescope aperture of 100 mm, a magnification of 10x, and a FOV (field of view) of 4,4 mrad. The system is designed for the common QKD wavelengths 810 nm and 1550 nm. The targeted wavefront error of the entire system is λ/20.

The telescope´s operating temperature range (-40 °C up to +50 °C) requires an athermal system. This is realized by utilizing an aluminum-silicon alloy substrate material combined with a nickel-phosphorus polishing layer to achieve the required surface quality of the mirrors. By using two substrates, one with two mirrors and the second with three mirrors, the alignment of the telescope can simplify by means of a snap-together approach. The mirrors M1 to M4 are used to shape the beam, while the M5, M6 and M7 are used to fold the beam within the telescope. M6 serves as an active, fast steering mirror (FSM) for tip-tilt correction. All mirrors are integrated in a topology-optimised housing.

Following the process chain, the fabrication of the three-mirror substrate with respect to the manufacturing analyses and the ultra-precision diamond turning step is described in detail. The resulting quality of the mirror substrates after diamond turning is demonstrated.