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The beam propagates along the Z axis. Its intensity distribution in the plane perpendicular to Z is elliptical and the axis of the ellipse are coinciding with X and Y axis. The beam is shaped by an optical system. The beam propagation is calculated independantly in each XZ and YZ plane with the parameters which are effective in the considered propagation plane. In the present case, the waists in the XZ and YZ planes of the incident beam are considered to have the same position. The page calculates in each propagation plane the half width and the position of the emerging beam waist as functions of the wavelength, the focal length of the optical system, the M2 factor, the half width and position of the incident beam waist.