Terrestrial Free-Space Optical Communications

Free-space optical communications is an excellent example of a technology with both military and civilian relevance and also with compelling scientific timeliness and good market prospects. Based on experience of the IOSB’s employees in the area of laser beam propagation through turbulence, new laser-based telecommunications concepts are being designed, implemented and tested against atmospheric disturbances.

The Laser Communications Laboratory (LCL) at Fraunhofer IOSB in Ettlingen has two free-space paths that either originate or terminate there. The first path is an 800 m, double-pass link to a retroreflector and back to the receiver. Over this path, telecommunications experiments have been performed since 2018. Data rates have continuously increased: from 5 Gbit/s in 2020 to 10 Gbit/s as of 2023. As a reference, the top-of-the-range home internet which is delivered by fibre-optic cable to home users in Germany is currently limited at 1 Gbit/s. 

One of the approaches which are used by employees of Fraunhofer IOSB to increase data rates is so-called coherent modulation. Traditionally, only the intensity of the laser is modulated. This is the so-called on/off keying. In the approach we have taken, the entire optical field, i.e. the amplitude and the phase of the transmitted signal are independently modulated. As a result, with a single laser “pulse” several bits can be sent at once. We currently transmit 4 bits per symbol using this technique. With polarization diversity an additional data channel could be opened, increasing the data rate by a factor of two. 

At the same time, research is being undertaken to mitigate the effects of atmospheric turbulence on the quality of the transmitted data. With the techniques implemented in the LCL, we have achieved error-free transmission over 95% of the time. The goal is to increase this percentage to 99% in the coming years to meet industry requirements.

The second link to be eventually commissioned is a single-pass laser propagation path spanning 7.2 km. The path links two buildings: the transmitter is located on the roof of the Institute of Control Systems of the Karlsruhe Institute of Technology and the receiver is located in the LCL in Ettlingen. The link will be a fixed facility dedicated to research on the effects of atmospheric turbulence on propagation of Gaussian and other types of laser beams.