Investigation of the size and phase randomization effects on the spatial coherence of secondary Gaussian Schell model sources

Our aim is to quantitatively demonstrate how the size and phase fluctuations in Gaussian Schell model (GSM) sources affect their spatial coherence. This is important because the spatial coherence of partially coherent light sources provides an effective way to control fundamental properties of the light beams in applications ranging from imaging systems to free-space communication. We generate secondary GSM sources of variable size and phase randomization by shining a focused Gaussian beam onto rotating diffusers with different roughnesses and rotation speeds. We employ a Fresnel biprism, as a robust and cost-effective interferometer, to directly investigate the spatial degree of coherence (SDC) of the light emerging from the test GSM sources. Our study shows that the SDC of these sources, as a function of their size, exhibits Lorentzian profiles with their maxima all associated with the smallest source size and widths that decrease as the phase fluctuations in the source plane increase. We also propose our procedure as an effective interferometric method to locate the beam waist of Gaussian laser beams, which might be advantageous compared with other more complicated methods.