Multimode Theory of Graded-Core Fibers

Conventional optical fibers consist of a high-index core surrounded by a cladding of lower index. The index step between core and cladding contains the light inside the core and isolates it from the outer fiber surface, whose quality is usually difficult to control. In a more general way, inside guidance can be accomplished by any index profile which decreases from a maximum inside the fiber to a lower (cladding) value. The specific shape of the profile has an effect on the distribution of the guided optical power in the fiber and on the overall loss encountered, but, more importantly, the profile profoundly influences the velocities of the various propagating modes. A good example is the parabolic index distribution which was predicted to nearly equalize the group velocities of the propagating modes.1,2 The Selfoc fiber which closely approximates these conditions has indeed since exhibited an extremely narrow impulse response.3 4 These effects greatly enhance the chances of multimode fibers to be used in optical communication systems. On the other hand, a theory 1563