[1]https://phys.org/news/2021-06-quantum-cascade-lasers-private-free-space.html

   "Free-space optical communication, the communication between two
   devices at a distance using light to carry information, is a highly
   promising system for achieving high-speed communication. This system of
   communication is known to be immune to electromagnetic interference
   (EMI), a disturbance generated by external sources that affects
   electrical circuits and can disrupt radio signals.le some studies have
   highlighted the possible advantages of free-space optical
   communication, this system of communication has so far come with
   certain limitations. Most notably, it is known to offer limited
   security against eavesdroppers. Researchers at Télécom Paris (member of
   Institut Polytechnique de Paris), mirSense, Technische Universität
   Darmstadt and University of California Los Angeles (UCLA) have recently
   introduced a unique system for more secure free-space optical
   communication based on a technology known as [2]quantum cascade laser,
   a specific type of semiconductor [3]laser that typically emits
   mid-[4]infrared light."

References

   1. https://phys.org/news/2021-06-quantum-cascade-lasers-private-free-space.html
   2. https://phys.org/tags/quantum+cascade+laser/
   3. https://phys.org/tags/laser/
   4. https://phys.org/tags/infrared+light/