A variable lateral-shearing Sagnac interferometer with high numerical aperture for measuring the complex spatial coherence function of light

We combine a telescopic imaging system with a common-path, lateral-shearing interferometer and use phase-shifting interferometry to measure the comple x spatial coherence function (or mutual intensity) of a linearly-polarized optical field. Our telescopic design increases the numerical aperture of th e system without distorting the shape of the wave front, and therefore with out changing the phase difference between lateral positions in the optical field. Our method of generating lateral-sheared images introduces no additi onal astigmatism. To demonstrate the use of the interferometer we extract t he information contained in the complex spatial coherence function to recon struct the amplitude and phase of a coherent optical field, and we also sho w how the spatial coherence function evolves from a coherent field to a par tially coherent one as light traverses a random multiple-scattering medium.