High-aperture diffraction of a scalar, off-axis Gaussian beam

A scalar treatment for Gaussian beams offset from the optic axis and then f ocused by a high-numerical-aperture lens is presented. Such a theory is req uired for describing certain types of Doppler microscopes, i.e., when the m easurement is simultaneously performed by more than a single beam axially o ffset and then focused by a lens. Analytic expressions for the intensity in the focal region of the high-aperture lens are derived. From these express ions we calculate the intensity in the focal region with parameters of beam size, beam offset, and the numerical aperture of the lens. The relative lo cation and variation of the intensity around the focal region are discussed in detail We show that for small-diameter Gaussian beams the Strehl ratio increases above unity as the beam is offset from the optic axis. This is ex plained by the increase in the effective numerical aperture of the offset b eam compared with the one collinear with the optic axis. From examining the focal distribution, we conclude that it rotates for small beam size and th at increasing beam diameter causes the focused distribution to rebate and s hear, i.e., to distort. Rie also show that the distortion of the distributi on increases with increasing numerical aperture. (C) 2000 Optical Society o f America [S0740-3232(00)00109-5] OCIS codes: 050.1220, 050.1960, 080.1010, 260.1960.