Lightwave Propagation in the Air

Atmospheric turbulence that affects propagating light waves is described. A model of the atmosphere that distorts the wavefront of light waves is outlined, and the energy cascade theory of turbulence and the structure function of wind velocity are presented along with definitions of the outer scale and the inner scale. Structure functions are introduced to treat random quantities, where the relationship between the structure function of refractive index and the structure function of temperature is denoted. The Kolmogorov spatial power spectral density of refractive index fluctuations is expressed with the structure parameter of the refractive index. Representative models of the refractive index structure parameters as a function of altitude are shown with examples of calculations. Assuming that the atmosphere is a plane perpendicular to the direction of light wave propagation, the coherence length of the atmosphere, or Fried’s parameter, is derived from the wave structure function. The isoplanatic angle is presented as an indicator to show that two wavefronts with different propagation angles can be considered approximately equivalent, and the Greenwood frequencies at which two wavefronts measured can be considered approximately equivalent are introduced. For the phenomena observed in light waves propagating in atmospheric turbulence, theoretical expressions of intensity fluctuation, focal point variation, and movement of the irradiated area of the transmitted light waves are shown.