Shearing interferometer measurement of far field of a laser communication beam

A determination of the far-field intensity distribution of a laser communication beam which relies on measurement of the near-field amplitude and phase distribution and calculation (via Fourier transformation) of the far-field is discussed. Origins of the far-field divergence are reviewed, and the use of a shear plate to measure divergence and phase irregularities of a Gaussian-type beam is described. A photodiode detector is used to obtain a two-dimensional amplitude function of the field at a specified shear plate position. Data reduction processes applied to shear-plate fringe photograph data and the amplitude data are considered. Difficulties associated with atmospheric interference and the time-consuming nature of the data reduction processes are also mentioned.