When the two-photon absorption of a high intensity pump beam takes place in a semiconductor optical amplifier there is an
associated fast phase change of a weak probe signal. A scheme to realize fast all-optical XOR logic function using twophoton
absorption induced phase change has been analyzed. Rate equations for semiconductor optical amplifiers, for input
data signals with high intensity, configured in the form of a Mach-Zehnder interferometer has been solved. The input
intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change.
The model shows that both XOR operation and pseudorandom binary sequence generation at 250 Gb/s with good signal to
noise ratio is feasible.