Abstract
For a coherent laser radar system (CLR), the signal to noise ratio (SNR) from a target at a fixed distance should be a maximum when the focus setting is equal to the target distance. The theoretical shape of a curve of SNR vs focus setting for a target distance of 840 meters and a laser wavelength of 2.09 μm is illustrated in Figure 1. Atmospheric refractive turbulence will distort the phase front of the propagating laser beam. This distortion effectively reduces the heterodyne efficiency of the CLR system and the SNR is correspondingly affected. The curves in Figure 1 are for varying levels of atmospheric refractive turbulence (Cn2). This effect is more pronounced at shorter wavelengths. With the recent advances in 2 μm lasers for CLR, it is important that the prediction of the effect of refractive turbulence be verified. This paper presents the results of an attempt to fit experimental data to theoretical curves of SNR vs. focus and to estimate Cn2 from that fit.
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