| This work focuses on spatially inhomogeneous polarization and how it can be applied to the laser beam shaping field. As an example of spatially inhomogeneous polarization, cylindrical vector beams are studied both theoretically and experimentally. Vector diffraction theory is applied to study beam focusing properties. It can be demonstrated that by changing the beam parameters such as pupil size, intensity profile and polarization state across the wave-front, beam shaping can be performed for high numerical aperture (NA) systems. A direct method to experimentally quantify the longitudinal component (z-component), which the beam shaping relies on, is proposed and carried out. The new concept of spiral polarization is proposed and verified by simulation using vector diffraction theory. This polarization has the capability of performing high NA beam shaping independent of numerical aperture. For low numerical aperture systems, the concept of a polarization plate from the design point of view is put forward to obtain the smallest flat-top shaped focus. The simulated annealing algorithm is adopted as a method to numerically design the proper polarization plate. Experiments have been carried out to demonstrate the flat-top focal spot obtained with a properly designed polarization plate which verifies the validity of the new concept. Fourier Optics theory is employed to demonstrate that the flat-top focal spot obtainable from the polarization plate has the theoretically smallest size. |
