Optimization of infrared encrypted optical telecommunications through the turbid atmosphere – OPTOPIRAT

Free space optical telecommunications in the infrared range represent an increasingly attractive alternative to the progressive saturation of channels dedicated to wireless technologies and the growing need for bandwidth. However, this promising technology, rapidly deployable, is vulnerable to weather conditions such as fog. Indeed, when the optical beam crosses a scattering medium, it undergoes absorption and scattering phenomena that will attenuate the ballistic signal and cause a temporal spread at high speed. These effects are all the more important as the transmission distance is large. Moreover, the multiple scattering of the beam means that the signal can be intercepted by an adversarial party located at an adequate distance. Securing the transmitted data and increasing the range of encrypted telecommunications systems through turbid environments such as fog is therefore a fundamental issue for defense and civil industrial applications.
The objective of the OPTOPIRAT project is to propose new telecommunication strategies to significantly increase the range and throughput of free space optical telecommunication systems in the presence of fog. Data security will be achieved through a cryptographic method exploiting the temporal chaos of quantum cascade laser sources. The proposed strategy is divided into three innovative approaches. The first approach will consist in exploring the physical and modulation properties of quantum cascade lasers in order to increase the modulation and chaos bandwidth of these sources emitting in the mid-infrared. The second proposed approach will consist in filtering ballistic or serpentile photons, i.e. photons having encountered few scattering events, by means of a temporal modulation and demodulation approach of the transmitted and detected signals respectively. The objective here is to obtain an efficient rejection of the multi-scattered photons which are the majority in the flow and which degrade the signal-to-noise ratio of the communication signal. Finally, the third axis of our strategy will consist in evaluating the contribution in terms of range of a "wavefront shaping" technique by phase conjugation to correct the effects of scattering. This approach, while benefiting from the filtering of serpentile photons which should allow to significantly reduce the number of modes to be corrected, could be all the more fruitful as the number of modes propagating in the scattering medium would decrease when the wavelength increases, shifting from the near infrared (SWIR) to the mid infrared (MWIR, LWIR). One of the objectives of OPTOPIRAT will also be to compare the performance of telecommunications at different wavelengths covering the main fog conditions encountered (advective and convective). In conclusion, the advantage of long-range terrestrial optical telecommunications is their reliability, their simple and economical deployment between the roofs of buildings in cities and on airports, or between airborne telecom terminals and the ground, when approaching urban areas or in situations of war or natural disaster. Thus, in the face of growing data exchange needs, there is a strong societal interest in proposing alternative solutions to counter these atmospheric limitations and increase the range of optical telecommunications systems in degraded atmospheric situations.