Hi Mohammad,
In this example, two sequences of electrical bits are combined together and are then converted to optical domain by the Mach-Zehnder modulator and transmitted into the line. On the receiver side, the interferometer converts the phase-keyed signal into an amplitude-keyed signal (I think) so that the data from both sequences can be demodulated by a pair of electrical amplitude demodulators and the data is obtained. If you need any further explanations, feel free to ask.