  We demonstrate how a narrowband pump and a broadband spectrum can be
spatially multiplexed by selective coupling them in two distinct modes of a
few-mode microstructure fiber. The first mode carries most of the input pump
energy, and experiences spectral broadening. Whereas the second mode preserves
the narrow bandwidth of the remaining part of the pump. Bimodal propagation,
with a power unbalance strongly in favor of the fundamental mode, is naturally
obtained by maximizing coupling into the fundamental mode of the fiber. At the
fiber output, the nearly monochromatic beam and the supercontinuum carried by
the two different modes are combined by a microscope objective, and used as a
pump and a Stokes wave for self-referenced multiplex coherent anti-Stokes Raman
scattering micro-spectroscopy. The spectral resolution, the
signal-to-noise-ratio, and the possible amplification of the remaining pump
beam are discussed.
