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Optical Turbulence: What is it?
As light propagates through the atmosphere or underwater, it experiences changes to the index of refraction, which creates distortion in the light. This is called Optical Turbulence, and it is the result of small fluctuations in the temperature, pressure, density, and salinty/humidity of the environment. Though small, these fluctuations can have a dramatic impact on the resulting image of the signal at the receiver.
Though unfamiliar sounding, Optical Turbulence is actually a phenomenon that we are all familiar with. The heat "waves" you see rising off of a hot road or radiator, or coming from behind a jet engine, are the result of this phenomenon. In those cases, the Optical Turbulence is primarily influenced by dramatic changes in temperature.
Optical Turbulence as the result of a Jet Engine
Source: https://doy2mn9upadnk.cloudfront.net/uploads/default/8e689fcb52115041ea24e72c90d2505b2e406ee9
The effect of Optical Turbulence on our beams
Image of beam in no induced OT
Image of beam in weak induced OT
As can be clearly seen, the beam gets fairly distorted even in weak OT. This is important to how well the Machine Learning will be able to predict the correct class, AKA properly decode the signal.
Characterization of Optical Turbulence
In order to study the effect of optical turbulence on our beams and the accuracy of the Machine Learning network, we must be able to characterize and quantify the amount of turbulence our beam experiences at any point in time. The typical constant used to quantify optical turbulence in laser communication is the refractive-index structure constant, Cn2 . This number is determined based on the temperature difference between two points that differ in the transverse direction (we can do this since the changes in the refractive-index are modeled stochastically, but there is more information on this under the Random Phase Screen tab). This means the Cn2 for points in space in our experimental set up can be found and calculated by finding the temperature. It is important to note that we generally excluded changes to the pressure of the environment, as it is negligible. The temperature is the most significant of all the factors that influence the refractive-index fluctuations. In the set-up shown, the temperature of a tank filled with water is changed using a heater. Several temperature probes are used to measure the temperature and calculate the refractive-index structure consant. This allows us to quantify the amount of optical turbulence our beams experience!
Set up to record changes in the refractive-index [1]
However, how do we know what to set the heater's temperature to, and how far away the beam should be from the heater? We could experimentally find it, sure, but that is labor intensive. Instead, we use a Multiphysics software known as COMSOL to model the tank environment and theoretically determine the best temperature and distance. Then, this is experimentally validated! There is more information on COMSOL on the next page.
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