  Correlated signal and idler photon pairs with small detuning in the telecom
band can be generated through spontaneous four-wave mixing in dispersion shift
fibers. However, photons originated from other nonlinear processes in optical
fibers, such as Raman scattering and self-phase modulation, may contaminate the
photon pairs. It has been proved that photons produced by Raman scattering are
the background noise of photon pairs. Here we show that photons induced by
self-phase modulation of pump pulses are another origin of background noise.
After studying the dependence of self-phase modulation induced photons in
signal and idler bands, we demonstrate that the quantum correlation of photon
pairs can be degraded by the self-phase modulation effect. The investigations
are useful for characterizing and optimizing an all fiber source of photon
pairs.
