  We report the investigation of a collimated blue light generated in rubidium
vapor due to the combined action of an ultrashort pulse train and a cw diode
laser. Each step of the two-photon transition 5S - 5P$_{3/2}$ - 5D is excited
by one of the lasers, and the induced coherence between the 5S and 6P$_{3/2}$
states is responsible for generating the blue beam. Measurements of the
excitation spectrum reveal the frequency comb structure, indicating that each
individual mode is responsible for inducing a nonlinear process. The strong
signal dependency on the atomic density is characterized by a sharp growth and
rapid saturation.
