The Fraunhofer approximation is valid if the propagation distance d between the aperture and observation planes is sufficiently large so that the Fresnel number N F ' = D 2 / λ ⋅ d << 1, where D is the largest radial distance within the aperture.
The Fraunhofer diffraction is the theory of transmission of light through apertures under the assumption that the incident wave is multiplied by the aperture function and using the Fraunhofer approximation to determine the propagation of light in the free space beyond the aperture. When the diameter D is reduced, the Fresnel diffraction gradually changes into the Fraunhofer diffraction. There diffraction patterns arise of bending on the edge, Figure 1. The diffraction phenomena are reflected only in narrow areas on the border of light and shadow. If D > z ⋅ λ, where z is a distance and λ is a wavelength, then the distribution of light intensity for a circular aperture can be derived by geometric optics. diameter of circular aperture, marked D, has a large influence on the optical intensity distribution in the plane of observation, marked σ. It occurs due to the short distance in which the diffracted waves propagate. The Fresnel diffraction is a process of diffraction that occurs when a wave passes through a slot and diffracts in the near field, causing any diffraction pattern observed to differ in size and shape, depending on the distance between the slot and the projection. There are two types of diffraction, Fresnel and Fraunhofer. The diffraction means a deviation from rectilinear propagation of light, which can not be explained by reflection or refraction. While the beam is passing through a lens, a diffraction can occur at the edge of the lens. Keywords: optical intensity distribution, Gaussian beam, diffraction The wireless optical communication is realized by optical elements which affect a beam on its path from the laser diode to the photodetector in place of reception. The optical beam passed by aperture and optical intensity distribution was observed at the screen, which was placed in the far field. We could use circular apertures instead of lenses, because we obtained the same effects of diffraction. These circular apertures represented transmitting lens. This beam was aimed at the circular apertures of different diameters. A laser transmitted optical beam whose intensity distribution was Gaussian. When an optical beam impacts optical lens, the diffraction can appear at the edge of lens. In free space communication laser transmitters, optical lenses and other components are used. article describes an intensity distribution of optical beam in the far field.