SLM4OAM

Free-space data transmission using laser radiation can be used wherever a broadband, interference- and tap-proof alternative to radio transmission is required and at the same time the use of optical fibers is ruled out, e.g. due to a lack of infrastructure or in the case of relative movement between transmitter and receiver (e.g. satellite communication). However, the achievable distances in terrestrial applications when using lasers in the visible or NIR spectral range are – depending on the visibility conditions – limited to a few kilometers due to excessive absorption and solar scattered radiation in the atmosphere. In addition, such signal sources are not eye-safe, which poses significant problems, especially in urban areas or, for example, on factory premises.

These effects can be avoided or at least significantly reduced when using mid-IR radiation, especially in the wavelength range around 2.1 µm. At this wavelength, a relative minimum of atmospheric extinction exists and interferences from solar and terrestrial radiation are minimal.

This option is also of interest because compact diode lasers have recently become commercially available in this spectral range and therefore the use of complex OPO or Ho:YAG lasers can be dispensed with. On the other hand, efficient data coding to achieve high transmission rates is particularly important in free-space communications because, in contrast to fiber transmission, the pulse spacing cannot become arbitrarily small due to ever-present propagation time variations (atmospheric turbulence, temperature fluctuations) and dispersion. In addition, when using single photons / entangled photons, the data rate is minimized by the rate at which they can be generated. In this context, encoding via vortex beams with varying orbital angular momentum is particularly interesting because the orbital angular momentum quantum number has all integers as its range of values, and therefore several bits per photon can be transmitted without difficulty. Thus, the data rate is increased accordingly for the same number of photons. Furthermore, vortices are much more robust against atmospheric disturbances than, for example, polarization states.

HOLOEYE tasks in the project

Within the scope of the project, HOLOEYE is developing an SLM for use at 2.1 µm, which will be used both for generating the vortex beams. A phase plate cannot be used for a real field application because drifts, for example due to thermal effects, laser drift and aberrations, cannot be compensated. An SLM can compensate for these drifts and thus provide pure and consistent orbital angular momentum states.