We proposed linear and nonlinear interference comb-spectroscopy in order to study the interaction of a train of femtosecond pulses with an atomic medium experimentally and theoretically. Numerical calculations of the interaction of a train of short femtosecond pulses with atomic vapor has been made for comparison with experimental results. It is shown that interference of atomic states produces unidirectional coherent radiation on an adjacent transition (blue radiation at 420 nm) when input near IR radiation (780 nm) is focused into the cell filled with Rb atoms.