Estimation of optical turbulence characteristics from Shack Hartmann wavefront sensor measurements

Abstract

In the field of optical propagation through the atmosphere a knowledge of optical turbulence strength and other key statistical parameters is crucial for performance prediction and system design. This work presents techniques for reliably estimating the most essential parameters of optical turbulence, namely r₀, the Fried coherence length, f_G, the Greenwood frequency, and l₀, the inner scale of turbulence from Shack-Hartmann wavefront sensor measurements. The earliest approaches for estimating r0 were based on MTF measurements.1 The MTF approach requires accurate calibration and stability of the system MTF which is often problematic. Astronomers have used differential motion and scintillation to measure seeing conditions.^2-5 Others have used the slope structure function estimated from a Hartmann wavefront sensor principally for r0 estimation.⁶ We have shown that the slope discrepancy⁷ or rotational component of the slopes can be used effectively in turbulence estimates.⁸ The techniques we describe here can be used to estimate r₀, f_G, and l₀. The inner scale estimate is based on the assumption of the Hill spectrum for refractive index fluctuations.^9-11 A high resolution, high frame rate, mobile sensor has been developed to utilize these estimation techniques. Section 2 describes the estimation techniques. Results from field measurement campaigns will be presented in Section 3.