Experimental results for two-wave mixing experiments in a single Bi12T
iO20 fibre-type crystal at a wavelength of 632.8 nm under transverse a
.c. electric field are reported. An exponential gain factor GAMMA = 4.
7 cm 1, which is much higher than the absorption coefficient (0.6 cm-1
), is obtained. It is observed that the signal beam enhancement is in
fact signal beam depletion for spatial frequencies higher than F(B), w
hich is determined by fibre geometry. An unusually strong dependence o
f response time on the recording beams intensities ratio reflects the
nonlinearities of the multi-two-wave mixing process.