A method and apparatus for monitoring and feedback control of a photonic switch such as 2×2 Mach-Zehnder Interferometer switch. Optical signals at an input and an output of the switch are monitored via optical taps. A sinusoidal time-varying phase shift is applied to one of the monitoring signals. An optical combiner then combines the monitoring signals. A photodetector monitors output of the optical combiner to provide a feedback signal. The amplitude of the feedback signal due to the time-varying phase shift increases with the amount of input signal present in the output signal. When the input signal is to be routed to the output (e.g. for a bar state), a controller manipulates the switch to maximize feedback signal amplitude. When the input signal is to be routed to a different output (e.g. for a cross state), the controller manipulates the switch to minimize feedback signal amplitude.