Unambiguous identification of waveguide loss and coupling coefficients using add-drop ring resonators

Waveguide propagation loss and coupling coefficients are key parameters that must be measured routinely following fabrication of integrated photonic chips. While multiple methods exist to measure these parameters, existing methods are either sensitive to off-chip coupling uncertainty and occupy a large footprint (cutback method) or cannot unambiguously distinguish waveguide losses from coupling coefficients (all-pass ring resonator method). In this work, we show that by performing two spectral measurements of an add-drop ring resonator, the waveguide loss and ring-waveguide coupling coefficients can be recovered and unambiguously identified. We perform uncertainty analysis and show that this method recovers waveguide loss and coupling coefficients with lower uncertainty than alternative methods in many situations. Finally, we perform several experimental demonstrations of the technique, highlighting its increased robustness to defects compared to the cutback method and its capability to measure waveguide losses and coupling coefficients on various waveguide platforms. As the add-drop ring resonator used has a relatively small footprint (roughly 200 µm x 100 µm) and unambiguous results can be obtained through measurement of individual resonances, this method represents a convenient tool for integrated photonic process development and fabrication monitoring.