Feeder links in laser and quantum communication

Free-space optical communication relies on the transmission of an optical signal from a transmitter to a receiver terminal over huge distances, such as from the ground to a geostationary satellite. Subject to atmospheric turbulence, free-space optical communication suffers from optical aberrations that enlarge laser beams and decrease the fibre coupling efficiency and bit rate at the receiver terminal. But adaptive optics is a successful tool to compensate for unwanted wavefront aberrations in such a way that fibre coupling efficiency and bit rate may be increased.

An adaptive optical system is installed after the receiving telescope and before the fibre coupling stage. Such a system should preferably have small dimensions, consume low levels of optical power and compensate faster than aberrations occur. A universal adaptive optical box (AO box), as a standalone device, could overcome the complexity and cumbersome procedure that is commonly attributed to such systems.

Applications in feeder links

Alternatively, there is optical pre-compensation, where the adaptive optical system is installed at the transmitter terminal. Here, the transmitted beam has targeted aberrations that are corrected while passing through the atmosphere. Such systems can be used in optical “feeder links.” Feeder links are used in satellite communication scenarios where optical beams are sent in both directions between a geostationary satellite and a ground-based telescope.

Deformable mirror as a key element

One of the critical elements in pre-compensating systems for optical feeder links is the deformable mirror. This holds especially true high throughput feeder link applications where laser powers in the system can exceed 1 kW. Based on our experience of high power deformable mirrors for laser machining (Zwobbel-series), we manufacture customised deformable mirrors with 1-40 actuators, response times of less than 2 milliseconds and high power capabilities.