Impulse Response of Clad Optical Multimode Fibers

Although single-mode operation of clad optical fibers is possible and, in general, offers very good transmission characteristics, multimode fibers have two advantages: They impose less stringent requirements on the optical carrier (they transmit even the incoherent light from a luminescent diode) and their larger dimensions alleviate splicing problems or at least relax the tolerances required for connection. Typically, the core diameter is of the order of a hundred wavelengths and the fiber therefore transmits thousands of modes, even if the index difference between core and cladding is only a few percent (corresponding to a numerical aperture of 0.2 to 0.3). The usefulness of such fibers depends on their dispersion characteristics. Delay differences among the many modes 1 distort the signal and certainly produce a signal response inferior to t h a t of the single-mode fiber. For certain systems, on the other hand, overall system economy may place the desirable information rate of individual fiber channels 801