  The modal group delays (GDs) are a key property governing the dispersion of
signals propagating in a multimode fiber (MMF). A MMF is in the strong-coupling
regime when the total length of the MMF is much greater than the correlation
length over which local principal modes can be considered constant. In this
regime, the GDs can be described as the eigenvalues of zero-trace Gaussian
unitary ensemble, and the probability density function (p.d.f.) of the GDs is
the eigenvalue distribution of the ensemble. For fibers with two to seven
modes, the marginal p.d.f. of the GDs is derived analytically. For fibers with
a large number of modes, this p.d.f. is shown to approach a semicircle
distribution. In the strong-coupling regime, the delay spread is proportional
to the square root of the number of independent sections, or the square root of
the overall fiber length.
  This revision also made clarification to the original paper, the group delay
statistics is also extended to high number of modes.
