Impulse Response of Fibers With Ring-Shaped Parabolic Index Distribution

Multimode operation of optical fibers relaxes the fabrication tolerances, allows the use of incoherent sources, and can alleviate handling and splicing problems. Modal (group) delay differences are nearly equalized 1,2 if the core index decreases as the square of the fiber radius from a maximum at the axis (Fig. 1). A distribution of this kind is realized in the Selfoc* fiber, which was indeed reported to have very low values of differential mode delay. 3,4 Since then, the question has been raised whether there are other index profiles which have similar equalizing effects, but are otherwise perhaps more amenable to certain fabrication techniques or have advantages with respect to splicing or bending. Although the latter part of this question is difficult to answer at this time, it is certainly possible to identify at least one profile that has quite effective equalizing properties. Imagine a slab with a square-law index distribution in transverse direction. The group velocities of all its modes are known to be nearly equal. 1 It is then plausible to expect that these properties are approximately preserved if the slab is warped in a way which results in a tube with the cross-sectional index distribution shown in * Registered trademark of Nippon Electric Co., Ltd. and Nippon Sheet Glass Co., Ltd. 1161