  We describe and analyse the operation and stabilization of a Mach--Zehnder
interferometer, which separates the carrier and the first-order sidebands of a
phase-modulated laser field, and which is locked using the H\"ansch--Couillaud
method. In addition to the necessary attenuation, our interferometer
introduces, via total internal reflection, a significant polarization-dependent
phase delay. We employ a general treatment to describe an interferometer with
an object which affects the field along one path, and we examine how this phase
delay affects the error signal. We discuss the requirements necessary to ensure
the lock point remains unchanged when phase modulation is introduced, and we
demonstrate and characterize this locking experimentally. Finally, we suggest
an extension to this locking strategy using heterodyne detection.
