Three-dimensional focus manipulation by means of a birefringent plate

A method based on double imaging is proposed for three-dimensional focus manipulation. A thin birefringent plate placed behind a focusing lens was applied to generate two foci with a separation smaller than the original DOF. Depth of focus enlargement of 227% was achieved without serious degradation in lateral resolution. Experimental results obtained by using a confocal arrangement were in good agreement with theory. Both theoretical considerations and experimental results showed that in non-paraxial case the ordinary and extraordinary foci could not be excited independently.