Silicon Photonic Crystal Membrane Ultrasonic Sensor

We present a suspended-element silicon photonic crystal membrane sensor with a diaphragm area of 12 mu m by 15 mu m suitable for free-space ultrasonic acoustic measurements. The sensor is based on the position-dependent directional coupling between neighbouring photonic crystal edges and exhibits compatibility with broadband optical sources, low insertion loss, and more freedom in the mechanical design. By tuning the wavelength such that both time-averaged outputs of the directional coupler were balanced, a differential detection scheme was implemented where common noise sources were suppressed while enhancing the sensor signal without additional photonic components. Using a coiled speaker, the dynamic acoustic response of the sensor from 10 kHz to 80 kHz was measured and we report a peak system response in the difference signal at 40 kHz with a sensitivity of 40 mu V/Pa. The compact size and improved mechanical frequency response of these integrated photonic sensors make them suitable for array applications, particularly for the study of fluid dynamics involving high Reynolds number phenomena.