Coherence of an electromagnetic field propagating in a weakly guiding fiber

The coherence of an electromagnetic field propagating in a weakly guiding cylindrical fiber supporting many modes is examined. In particular, the complex degree of coherence is a periodic function of the traveled distance z for small z. For large z, the degree of coherence does not depend on z at equal times, while, as a function of time, it possesses a series of maxima with spacing dependent on z.