Enhanced spontaneous backscattering in Brillouin dynamic gratings.

Spontaneous Brillouin backscattering, which accompanies the operation of Brillouin dynamic gratings (BDGs) setups, is investigated both theoretically and experimentally. It is shown that this noisy emission, which cannot be separated from the signal of interest, contains not only the probe spontaneous Brillouin backscattering but also a significant contribution from the spontaneous/stimulated acoustic field, originating from the high-frequency writing pump. In the absence of the low-frequency writing pump and for a strong enough high-frequency writing pump, the observed Stokes noise can exhibit an average backscattered power much higher than that from the probe alone.