A Novel Opto-Mechanical Accelerometer in Low-Loss Silicon Nitride Photonics Platform With High Dynamic Range Achieved via Heterodyne Detection

Integrated photonics has emerged as a popular platform for performing a wide range of measurements with high sensitivity. Here an optical accelerometer in a silicon nitride photonics platform was designed, and the fabricated device’s performance was measured. Light is first split into a reference and sensing arm. The sensing arm is a waveguide loop on top of a cantilever beam. When the cantilever is subjected to acceleration, a strain field is generated within the sensing arm that leads to a proportional optical phase change. Light passing through a reference arm is frequency-shifted using an acousto-optic modulator (AOM). Light from the sensing arm, which undergoes phase modulation due to cantilever vibrations, is combined with the frequency-shifted light from the reference arm. After passing the combined beams through a photodiode, demodulation of the voltage waveform into the in-phase and quadrature signals enables the phase from the sensing arm to be measured over many optical fringes. This approach extends the intrinsic dynamic range (IDR) of the optical interferometric readout beyond the limits of conventional interferometric sensors with an improved dynamic range of more than 40 dB compared with a conventional method with no active feedback. A calculated minimum detectable signal (MDS) of ${190}~\text {ng}/\sqrt {\text {Hz}}$ was achieved while a further improvement was discussed. The prototype devices are fabricated using a low-loss silicon nitride photonics platform, all on an SOI wafer with a Si device layer. The process enables two post-optical-waveguide fabrication processes; front-side and backside deep silicon etch (DSE). The buried oxide (BOX) of the SOI serves as a controlled etch stop for both top and backside silicon etch processes. We use this process to form thin and flexible cantilevers comprising only $50~\mu \text {m}$ thick silicon.