  A nonlinear quantum-optical process is considered: emission of photon pairs
by the reflecting end of a fiber excited by a standing laser wave. Radiation
occurs due to periodic changes in the optical length of the fiber over time.
This radiation can be significantly enhanced in long fibers. Because of this
enhancement, a nonperturbative description of the process is required. Such a
description presented here predicts a strong peak of radiation, if the
amplitude of the oscillations of the optical length coincides with half of the
laser wavelength. The considered phenomenon can be used to create a source of
entangled photons.
