Double-Pass Propagation: Semi-Rough and Diffuse Targets

Incident light on a rough surface may be reflected specularly, diffusely, or both. The proportion of diffuse reflection to specular reflection depends on the relation between the roughness of the surface and the wavelength of the incident radiation. In Chapter 7 we considered laser beam reflection from only smooth targets like mirrors and retroreflectors. Reflection from such surfaces is called specular. However, in most laser radar systems the target is not smooth but has a certain surface roughness associated with it.^1–3 Therefore, in addition to atmospheric turbulence along the path to and from the target, the target itself can impose a certain phase distortion on the echo beam. That is, the roughness of a target plays an important role in determining the nature of the return echo beam. If the surface of a target is considered rough on the scale of an optical wavelength, this causes the reflected wave to be scattered back in all directions. Such a surface is called a diffuse surface or Lambertian surface. The echo beam will then contain many coherent components, each arising from a different microscopic element of the surface. Interference of the coherent components results in a granular pattern of irradiance at the Rx pupil plane known as speckle. The correlation area of the speckle in the pupil plane or image plane is important because it may be used to identify certain target characteristics and/or atmospheric components.