Non-Invasively Monitoring Tunable 2-by-2 Mach-Zehnder Interferometers in Programmable Si Photonic Integrated Circuits

We present a non-invasive readout method for real-time monitoring the phase condition of a Si tunable 2 x 2 Mach-Zehnder-interferometer (MZI) beam splitter applied for a programmable photonic integrated circuit. This method, implemented by integrating an AC-coupled Wheatstone-bridge circuit on the MZI, can detect the phase-controlled splitting ratio of the MZI beam splitter, by examining the waveguide impedance variation due to surface absorption of the Si waveguide. The MZI is also integrated with microheaters for finely tuning the waveguide phase. Therefore, the phase condition of the MZI beam splitter can be observed and tuned by investigating the variable splitting ratio with respect to the driving current. A pi-phase shift was characterized. This Wheatstone-bridge circuit can be monolithically integrated into a large-scale MZI-based programmable photonic integrated circuit for phase error pre-calibration and phase management.