  We consider coupled nonlinear Schrodinger equations (CNLSE) which govern the
propagation of nonlinear waves in bimodal optical fibers. The nonlinear
transform of a dual-frequency signal is used to generate an ultra-short-pulse
train. To predict the energy and width of pulses in the train, we derive three
new types of travelling periodic-wave solutions, using the Hirota bilinear
method. We also show that all the previously reported periodic wave solutions
of CNLSE can be derived in a systematic way, using the Hirota method.
