# Optics & Infrared Sensing

## Atmospheric Turbulence Modeling

Understanding the effects of atmospheric turbulence on laser propagation through the atmosphere is critical for light detection and ranging (LIDAR) systems as well as free space optical communication applications. Temperature fluctuations in the atmosphere generate index of refraction fluctuations that can cause a laser beam to steer off of a target and to break up. As a result, when a constant intensity laser beam is directed to a target, atmospheric turbulence generates intensity fluctuations or scintillation. This is illustrated by the following figure.

The severity of the scintillation depends on the wavelength of light, the pathlength, *L*, through the atmosphere and C_{n}^{2}, the index of refraction structure constant that quantifies the severity of the atmospheric turbulence. These three quantities can be combined together into the Rytov parameter. C_{n}^{2} is greatest during the midday and has minimums associated with sunrise and sunset. The figure below shows how C_{n}^{2} can vary throughout the day and at different geographical locations.

The effects of atmospheric turbulence can be quantified by determining the scintillation index, which is related to the mean and standard deviation of the intensity distribution. The figure below shows how the scintillation index varies as a function of the Rytov parameter.

To better understand the effects of atmospheric turbulence PNNL has developed a numerical simulation based on Fourier optics techniques and random phase screens that agrees well with experimental measurements. Once the modeling has been validated, a virtual system can be built within the simulation and used to inform the design of LIDAR systems; for example, determining the optimum receiver area, laser divergence and detector size for a particular pathlength and atmospheric turbulence conditions.

The figure below shows simulated returns of the FM DIAL system for different input conditions related to the Rytov parameter. For small Rytov parameters, atmospheric turbulence doesn't have a large effect and the return signal still maintains some of its "beam" qualities. As the Rytov parameter increases, the effects of turbulence increase and the return signal is more and more broken up until the return is essentially a random distribution. At this point the scintillation index rolls off and approaches one.