Tunable degree of polarization of an unpolarized, partially coherent beam with circular coherence in a high numerical aperture system

The polarization properties of a strongly focused, unpolarized, partially coherent beam with circular coherence are studied in this Letter. It is found that the degree of polarization (DoP) in the focal plane can be adjusted, not only by the initial coherence and the semi-aperture angle, but also through an annular pupil due to the unusual coherence of the source. Our results show that for an unpolarized, partially coherent incident beam, the field in the focal plane can go from almost an unpolarized state to almost a fully polarized state along a radial distance, although the initial coherence length is very short. Through adjusting the width of the annular pupil, the DoP on average can be significantly reduced or enhanced. Especially for the very narrow annulus, the approximation expression of the DoP is derived, from which the accurate positions of the peak values of the DoP can be easily calculated. Our findings provide an alternative way to tailor the DoP in three-dimensional optical fields.