  We report on the generation of four spatially multiplexed picosecond 40-GHz
pulse trains in a km-long 6-LP multimode optical fiber. The principle of
operation is based on the parallel nonlinear compression of initial
beat-signals into well separated pulse trains owing to intra-modal multiple
four-wave mixings. A series of four 40-GHz dual-frequency beatings at different
wavelengths are simultaneously injected into the LP01, LP11, LP02 and LP12
modes of a 1.8-km long graded-index few-mode fiber. The combined effects of
Kerr nonlinearity and anomalous chromatic dispersion lead to the simultaneous
generation of four spatially multiplexed frequency combs which correspond in
the temporal domain to the compression of these beat-signals into picosecond
pulses. The temporal profiles of the output pulse-trains demultiplexed from
each spatial mode show that well-separated picosecond pulses with negligible
pedestals are then generated.
