On the Phase of the Modulation Transfer Function of a Multimode Optical-Fiber Guide

Pulse dispersion in an optical fiber transmission line limits its information-carrying capacity by limiting the temporal spacing of input pulses that can be resolved at the output. The impulse response g(t), by which we mean the output power of a fiber excited by a unit impulse of optical power, provides the necessary information concerning the distortion of the pulse by modal and material dispersion. For a strictly monochromatic pulse source, only modal dispersion contributes to the distortion. However, with regard to the corresponding measurement problems, it is difficult to obtain sufficiently short (<0.5 nsec) and monochromatic (< 10 A) input pulses to accurately study fibers with very low modal dispersion. Another difficulty is the lack of availability of suitable sources that are tunable over a wide range of wavelengths, including wavelengths longer than 1 jum which are of interest for practical fiber systems. One can obtain g(t) from the modulation frequency transfer function (MTF) G(CO), which is the envelope response of the fiber to an incoherent optical signal sinusoidally modulated in amplitude at angular frequency Co. Personick 1 has shown that to the extent that certain reasonable approximations hold, g{t) and G(u) are a Fourier transform pair. In principle, the MTF can be determined experimentally. The method employed in Refs. 2 and 3 uses a xenon lamp and monochromator as a tunable 99