A variable aperture method to simultaneously estimate atmospheric extinction coefficient and refractive index structure constant

Both atmospheric extinction coefficient and refractive index structure constant (Cn2) are important parameters to describe laser beam propagation in the atmosphere. The typical measurement methods for these two parameters are separated. Recently an integrated measurement method has been developed with the aperture constraint condition. In this study, a variable aperture method is proposed to simultaneously evaluate the atmospheric extinction coefficient and Cn2 needless to consider the aperture constraint condition. The projection optics with CCD is employed to partially receive atmosphere modulated laser speckle images. Because the extinction coefficient and Cn2 are implicit in the far-field receiving power at a certain aperture, they can be estimated through at least twice measurements of long term speckle at different equivalent aperture on the CCD image. The uncertainty analysis is also carried out. The theoretical and experimental results demonstrate that this technique is feasible, which provides an effective and economical way to understand the complicated behavior of laser beam propagation in the atmosphere.