Measurement of co- and cross-polarized Raman spectra in silica fiber for small detunings.

We report measurement of co- and cross-polarized Raman gain spectra at the zero-dispersion wavelength of standard dispersion-shifted fiber for detunings down to 0.17 THz (5.7cm-1) on both Stokes and anti-Stokes sides by using a photon-counting technique. This technique separates the Raman scattering from the four-photon scattering. In addition, the use of a pulsed pump eliminates Brillouin scattering and the use of a Sagnac loop rejects the pump photons that spectrally spread into the detection band due to self-phase-modulation.