  We propose an all-fiber-generated dual-soliton pulses based scheme for the
background-free detection of coherent anti-Stokes Raman spectroscopy under the
spectral focusing mechanism. Due to the strong birefringence and high
nonlinearity of a polarization-maintaining photonic crystal fiber (PM-PCF), two
redshifted soliton pulses can be simultaneously generated relying on high-order
dispersion and nonlinear effects along two eigenpolarization axes in the
anomalous dispersion regime, while allowing feasible tunability of the
frequency distance and temporal interval between them. This proposed scheme,
termed as DS-CARS, exploits a unique combination of slight frequency-shift and
advisable temporal walk-off of this two soliton pulses to achieve robust and
efficient suppression of nonresonant background with compact all-fiber coherent
excitation source. Capability of the DS-CARS is experimentally demonstrated by
the background-free CARS spectroscopy and unambiguous CARS microscopy of
polymer beads in the fingerprint region.
