  We study a novel scheme named spontaneous parametric fiber loop (SPFL),
configured by deliberately intro- ducing dispersive elements into the nonlinear
Sagnac loop, and show it can function as a passive switch of photon pairs. The
two-photon state coming out of SPFL highly depends on the dispersion induced
phase difference of photon pairs counter-propagating in the loop. By properly
managing the dispersive elements, the signal and idler photons of a pair with a
certain detuning and bandwidth can be directed to the desired spatial modes of
SPFL. If the photon pairs are used to generate heralded single photons, the
SPFL can be viewed as a switch of single photons. Moreover, our investigation
about the dispersion based phase modulation is also beneficial for designing
all fiber sources of entangled photon pairs.
