  We analyse the effect of post-Newtonian gravitational fields on propagation
of light in a cylindrical waveguide in both a straight configuration and a
spool configuration. We derive an equation for the dependence of the wave
vector upon the vertical location of the waveguide. It is shown that the
gravitational field produces a small shift in the wave vector, which we
determine, while the spooling creates additional modes which could perhaps be
measurable in future accurate experiments.
