Coupling efficiency evaluation of multimode fiber devices using GRIN rod lenses.

For an efficient analysis of the coupling characteristics of a multimode fiber device using GRIN rod lenses, it is important to consider both the third-order aberration effects of a GRIN rod lens and the actual source fiber optical power distribution. In this paper the ray tracing technique has been used with a set of point sources representing the extended source fiber core. Such a new representation is carried out according to the best agreement with measurable near-field and far-field source fiber distributions. In such a way we determined a set of results that show the relationships between coupling efficiency, lens length, and fiber lateral misalignments, as a function of the radial position of the input fiber. The results obtained pointed out, on the one hand, the poor reliability of the predictions based on a perfectly parabolic profile and, on the other, the dependence of the coupling efficiency vs lateral misalignment relationships on two different excitation distributions: uniform and steady state. The results of the experimental measurements carried out on a 0.5-pitch GRIN rod lens are consistent with the suggested model. Such an approach can be advantageously used in the design of microoptic devices based on GRIN rod lenses, such as multi/demultiplexers, directional couplers, and lens connectors.