GAUSSIAN BEAMS PASSING THROUGH APERTURED ABCD OPTICAL-SYSTEMS

Multimode expansion of a truncated Gaussian beam is used to know how t his beam is transformed when it passes through an apertured ABCD syste m. The width of the output beam is defined in relation with the mean s quare deviation of its intensity profile. In the cases of evolution be hind an aperture and behind a finite lens, the dependence of the width on the propagation coordinate results hyperbolic, as for Gaussian bea ms. The widths of both the truncated Gaussian and its Fourier transfor m give the characteristics of the hyperbola, i.e., the divergence and the width and location of the waist. Then, the problem of the focal sh ift is discussed.