In the current design of the LSC ISC, a Mach-Zehnder is used to avoid the generation of sub-sidebands. We have to check if the MZ is really necessary. If so, the noise introduced by the MZ has to be evaluated. If the MZ noise is too much, we should consider alternative ways, such as filtering cavity to eliminate sub-sidebands or a clever modulation technique using polarization rotation.
MZ noise analysis by S. Kawamura and O. Miyakawa for the 40m interferometer MZ-40m.pdf
Peter Beyersdorf's Virtual MZ idea. VirtualMZ.pdf