This is partly answered in my response, I just grouped it in with answer 2. I’m having a hard time finding a good reference on the web that explains clearly how a mach zehnder modulator works. But basically it comes down to splitting an optical signals into 2 branches and inducing a phase shift in one of them via an electrical signal. Higher voltage means a higher phase shift is induced. When the two signals are combined again they can experience constructive or destructive (or anything in between) interference.
Regards