  We study coherent anti-Stokes Raman spectroscopy in air-filled anti-resonance
hollow-core photonic crystal fiber, otherwise known as 'revolver' fiber. We
compare the vibrational coherent anti-Stokes Raman signal of N$_2$, at 2331
cm$^{-1}$, generated in ambient air (no fiber present), with the one generated
in a 2.96 cm of a revolver fiber. We show a 170 times enhancement for the
signal produced in the fiber, due to an increased interaction path. Remarkably,
the N$_2$ signal obtained in the revolver fiber shows near-zero non-resonant
background, due to near-zero overlap between the laser field and the fiber
cladding. Through our study, we find that the revolver fiber properties make it
an ideal candidate for the coherent Raman spectroscopy signal enhancement.
