I also want to add that in an analog system that uses an MZM-based fiber-optic link to convey a signal without converting it to a new microwave frequency, the output fundamental signal strength is maximum when the MZM is biased at any voltage that is halfway between a maximum and minimum transmission point on the transfer function curve-a so-called “quadrature” bias point . Quadrature bias points occur when the optical phase difference at the output of the two paths in the Mach-Zehnder interferometer is an odd-integer multiple of 90 degrees. Maintaining the MZM bias at quadrature is very important in microwave systems with bandwidths wider than one octave, because only at quadrature are second-order distortion products minimized.
Check the following link for further query.
http://www.lightwaveonline.com/articles/print/volume-18/issue-5/features/bias-controllers-for-external-modulators-in-fiber-optic-systems-53469212.html