Microelectromechanical systems (MEMS) have produced high-quality, high-bandwidth, small form factor, and inexpensive fast steering mirror (FSM) devices potentially suitable for a large variety of applications, such as image stabilization and beam pointing in satellite-based and ground-based, free-space optical communication systems. However, one outstanding question for this application is power handling. The absorption of the mirror substrate is low, but non-negligible, so the question remains of whether thermal loading from laser radiation on a MEMS mirror will deform its surface and, if so, to what extent. We show experimental results of optical performance changes due to thermal loading for MEMS two-axis FSM devices from Mirrorcle Technologies, Inc. Results and reproducible behavior are reported and compared in ambient versus vacuum conditions, where the benefits of convective cooling are absent. Finite element analyses corroborate the experimental results and show that the mirror substrate can deform due to thermal expansion imbalances. The deformation changes the focusing characteristics of the mirror, with a peak to valley defocus (second-order Zernike mode) of up to 50 nm when the mirrors are tested in ambient and up to approximately 450 nm when under vacuum. Such defocusing negatively impacts the link budget for laser-based satellite communications.
Coherent, free-space optical communication technology offers near-quantum-limited receiver sensitivity and high spectral efficiency compared to conventional direct detection systems. In this paper, we will present the initial results from a bidirectional air-to-ground demonstration of a coherent optical link.
Within the scope of the FALCON project Mynaric Lasercom GmbH, in collaboration with Facebook Inc., has built two laser terminals for optical communications: One airborne terminal MLT-70-ATG and one optical ground station GS-200. Both terminals are designed to establish communications between the stratosphere and ground. The athermalized design of the MLT-70-ATG, its efficient temperature management system and an optimized dynamic behavior for high-altitude platforms qualify Mynaric’s system to be easily integrated into carriers that fly up to tens of kilometers. The GS-200 achieves and sustains fast and reliable free-space data transfer links between the airborne segment and ground. It is designed for outdoor operations and is mounted on a stationary stable platform. A flight campaign executed by both companies has demonstrated a 10 Gbps bidirectional error-free link between the airborne laser communication terminal and the optical ground station in a representative scenario. The optical link was acquired successfully in a few seconds and both terminals maintained a steady link. Limitations in the line of sight between the communication partners, due to the flight patterns followed by the aircraft, triggered reacquisitions that were handled by the terminals autonomously. Bidirectional data transmission with maximum data throughput has been achieved. Even for strong fluctuation conditions, which were experienced during ground-to-ground tests, the link was error-free thanks to the coding in the Laser Ethernet Transceiver (LET). The LET converts the user data to a proprietary format. The systems could recover successfully outages up to 10 milliseconds. The coding and synchronization schemes have been optimized for overcoming the inherent spurious effects of the free-space optical communication channel.
High speed optical backbone links between a fleet of UAVs is an integral part of the Facebook connectivity architecture. To support the architecture, the optical terminals need to provide high throughput rates (in excess of tens of Gbps) while achieving low weight and power consumption. The initial effort is to develop and demonstrate an optical terminal capable of meeting the data rate requirements and demonstrate its functions for both air-air and air-ground engagements. This paper is a summary of the effort to date.