Free-space laser links

Laser-based standoff detection of chemical species, optical and thermal imaging, LIDAR and wireless optical communications are examples of applications that require transmission of optical power over long distances in air. In order to ensure small propagation losses of light in the atmosphere, it is important to minimize absorption losses and scattering losses, both of which are wavelength dependent. While the atmospheric absorption is relatively static and simple to model, scattering losses strongly depend on the type and size distribution of the light-scattering aerosols. In general, the scattering loss decreases with increasing wavelength, but the exact level of attenuation varies with weather conditions. For instance, different types of fog (such as advection fog and continental fog) have different water droplet size distributions and hence attenuate different wavelengths very differently.

High­lights

  • Development of a multiwavelength optical method for characterization of fog and fog dynamics.
  • Development of low-loss free-space laser links based on quantum-cascade lasers.

    Influence of fog and fog dynamics on free-space laser links

    Fogs are commonly described by visibility, which is the only meteorological quantity that is widely used to compare transmission conditions of free-space optical channels. Standard forward-scattering visibility detectors are calibrated to the maximum of human eye's sensitivity at 0.55 µm and are limited to local measurements. Their data cannot predict extinction of optical signal at near-infrared (NIR) or mid-infrared (MIR) spectral ranges at fog/haze conditions due to different scattering efficiencies at different wavelengths. Moreover, evolving fogs have dynamically changing droplet size distributions, and hence attenuation along the link pathway at infrared wavelengths is practically unpredictable using just local visibility measurements.

    We have carried out multiwavelength (VIS-to-MIR) characterization of optical attenuation caused by fog. In addition to rigorous tests in controlled laboratory conditions, we have built outdoor point-to-point laser links. We have investigated different types of fogs and, in particular, evolving fogs. Our research aims at gaining better understanding of the influence of fog and fog dynamics on free-space optical link losses. The experimental results confirm that, in most cases, free-space laser link attenuation can be significantly reduced by using a MIR laser instead of a NIR laser. As an example, the figure below shows an experimental comparison of the attenuation experienced by 1.55 µm and 9 µm laser links in the presence of salt-water fog similar to oceanic advection fog. In addition to applications in wireless optical communications etc., our multi-wavelength method gives information of droplet distribution and is potentially useful for meteorological characterization of fogs, clouds, and aerosols in general.

    FSO attenuation comparison

    Fig. 1. Left: An example of experimentally determined attenuations at NIR and MIR wavelengths as a function of visibility. Right: The respective water droplet size distribution at 535 m visibility, as deduced from the measurement data.