A CMOS-compatible heterogeneous interferometer for chip-scale temperature sensing

We report a photonic temperature sensor with enhanced performance in both broad- and narrow-bandwidth optical measurements. The device consists of a heterogeneously in- tegrated Mach-Zehnder interferometer with arms composed of silicon and silicon nitride waveguides whose thermo-optic coefficients differ by an order of magnitude. The waveguides are fabricated in distinct layers of a monolithic device and guide light in a single transverse-electric mode. The resulting small bend radii enable compact sensing of temperatures local to integrated photonic components with a device footprint of 580 × 410 μm2. Furthermore, the dual layers of the sensor enable overlaying of the spiral arms of the in- terferometer over each other or other photonic circuit components. We measure a sensitivity of 324 pm/K, an over threefold enhancement compared to the measurement of an asymmetric Mach-Zehnder constructed of silicon waveguides on the same device. We additionally define a useful figure of merit for the side-of-fringe measurement regime which uses direct detection of a narrow linewidth laser and show that the reported device is also competitive on this metric.