Near the ground laser communication system: Fried parameter estimation from the WFS measurements

Near the ground laser communication systems must operate in the presence of strong atmospheric turbulence. The effects of atmospheric turbulence on the laser beam which are relevant to optical communications are a broadening of the laser footprint, random jitter of the laser beam, and high spatial frequency intensity fluctuations referred to as scintillation. The overall goal of our program is to improve the performance and extend the range of optical communications systems by exploring the use of adaptive optics and channel coding. Knowledge of the turbulence conditions and the ability to describe its properties are the key aspects to make these improvements effective. The multi-phase approach described here is directed to statistically describe atmospheric turbulence based on results derived from experimentally collected data. To model the performance of a real world system, we have developed an outdoor path partially over water, spanning 3 km turbulence measurement and monitoring system. The system described in this paper has two transmitters and two receivers. The transmitter side is equipped with a laser and a bank of 14 horizontally, in-line mounted LEDs. The laser transmitter and LED operate at 808 nm with laser power of 2 mW. The receiver side consists of two channels. The WFS channel captures the signal from the laser transmitter and is specifically designated for the wavefront sensing. The wavefront sensor channel consists of relay optics, a Hartmann WFS, and a CCD camera. The PSF channel is designed to capture the signal from LED bar and consists of relay optics and a CCD camera. The data collected from the PSF channel is used for measurements of anisoplanetic effects. In this paper we emphasize the data collection and consecutive analysis from the WFS channel only. Statistics of the Fried parameter r₀ are derived from 6 TB of data collected over 50 days under various day and night atmospheric conditions. Significant fluctuations of r_{0 are found with the values ranging from 2 mm and up to 15 cm.}