  It is shown in this paper that the light field distribution in a band gap
within periodic structures for one-dimensional photonic crystal fibers is
described by a decaying factor multiplied by a periodical function that has the
same period length as the one of the medium or has double the period length of
the medium, depending on the sign of the trace of eigen value matrix. This
fundamental property is applicable to any 1D planar periodic structures, no
matter how many layers a unit cell has, what the contrast of refractive indices
is, and whether the dielectric parameters in individual layers are homogeneous
or inhomogeneous; it plays a significant role in understanding of numerical
results in a number of previously published research works. It is also shown
that, similar to the refractive index guidance in conventional optical fibers,
the photonic band gap guidance is also a form of total internal reflection.
