As Alistu mentioned Mach-zehnder modulator is used at the transmitter for modulating the signal. ie, electric signal is applied to both the plates of the MZI for controlling the amplitude of an optical wave. The input waveguide is split up into two waveguide arms. If a voltage is applied across one of the arms, a phase shift is induced for the wave passing through that arm. When the two arms are recombined, the phase difference between the two waves is converted to an amplitude modulation.
Whereas in case of Mach-Zehnder interferometer the light is spitted into two parts and then MZI is used to measure the relative phase difference between the two beams, here delay element can be used or a different path length is used. Basically I have seen the use of mach-Zehnder interferometer in the receiver side.
Regards