Hi Zhang Gaolu,
Do not get confused with the two bias voltage. Each MZM will have two bias voltages These are required for operating the MZM in Peak, Quadrature and null point of operation. Depending on the point of operation you have to set the voltages. the single drive means only one RF voltages. i.e. Among the two arms of Mach-Zehnder interferometer structure, RF voltage is applies at only one arm. Please go through the literature of MZM. I can suggest you go through the book “Broadband optical modulators” edited by Antao Chen and Edmund Murphy.
Half-wave voltage means the voltage required to make a phase shift of pi/2.
for the 3rd Question I would suggest you go through the book I mentioned above. Then only you will understand.
Regards,
Dhiman