The invention relates to an optical waveguide device that includes a waveguide phase modulator (WPM). A waveguide monitoring structure is coupled optically in parallel with the WPM so as to form a Mach-Zehnder interferometer therewith for producing monitor light indicative of a phase shift imparted by the WPM. The waveguide monitoring structure includes a first optical tap for tapping off a fraction of light entering the first WPM for providing first tapped-off light, a second optical tap for tapping off a fraction of light exiting the first WPM for providing second tapped off light, and an ancillary phase modulator for modulating the optical phase of the first or second tapped-off light so as to modulate the intensity of the monitor light in dependence upon the first phase shift. A feedback circuit controls the phase shift imparted by the WPM based on a modulation index of the monitor light.