Coupling of stochastic electromagnetic beams into optical fibers

We derive a general analytic expression for the coupling efficiency when a partially coherent, partially polarized beam is coupled into a multimode optical fiber. We adopt the Gaussian-Schell model for incident electromagnetic beams and use our general result to discuss the effects of the partial coherence and partial polarization on the coupling efficiency of an optical beam focused onto a step-index, single-mode fiber with a lens. Our results should be useful for any application requiring coupling of partially coherent beams into optical fibers.