Efficient coupling of incoherent light into optical fibers and bundles.

The excitation efficiencies of a few lower order modes in a W-type fiber have been studied when the modes are excited with tilted as well as off-set Gaussian beams. The effect of beam size, angle of incidence, the radius of curvature of the incident wavefront, and the off-axis displacement on the excitation efficiencies has been obtained. It is found that in order to couple maximum power into the fundamental mode, the incident wavefront must be plane and the beam must be incident axially along the fiber with a suitable value of the beam size.