Output optical power can be achived by optical phase shift in one arm of Mach Zehnder interferometer. π/2 phase shift will lead to destructive interference and lower values of output intencity.
If we consider integrated structure of silicon Mach Zehnder modulator, the phase shift is induced by changing electron and hole concentration along with the effective refractive index.
See for exampleConference Paper Simulation of Mach–Zehnder modulator with ultra-responsive p...
The attached picture represents output power equation of MZM with a phase shifter in one arm. Indices 1 and 2 corresponds to MZM arms with and without the phase shifter. Index 3 denotes the phase shifter itself. L - length, betta - propagation constant, alfa - absorption coefficient.