Focal shift and focal switch of partially coherent light in dual-focus systems

The system under consideration consists of a piece of thin diffractive element (the Fresnel zone plate) and a uniform, converging partially coherent light that is incident on the diffractive element. The center of curvature of the incident spherical wave and the primary focus of the zone plate are two foci of the system. We show that a focal shift is also present and the amount of the focal shift is not only dependent upon the Fresnel number of the focusing system, but also on the spatial coherence of the incident partially coherent light. A new phenomenon called focal switch occurs, i.e. the focal shift experiences a sudden transition as the Fresnel number approaches some specific values, and the influence of the spatial coherence on the focal switch is investigated in detail.