  The nonlinear Schr\"odinger (NLS) equation is a fundamental model for the
nonlinear propagation of light pulses in optical fibers. We consider an
integrable generalization of the NLS equation which was first derived by means
of bi-Hamiltonian methods in [A. S. Fokas, {\it Phys. D} {\bf 87} (1995),
145--150]. The purpose of the present paper is threefold: (a) We show how this
generalized NLS equation arises as a model for nonlinear pulse propagation in
monomode optical fibers when certain higher-order nonlinear effects are taken
into account; (b) We show that the equation is equivalent, up to a simple
change of variables, to the first negative member of the integrable hierarchy
associated with the derivative nonlinear Schr\"odinger equation; (c) We analyze
traveling-wave solutions.
