Contactless ultrasound detection using an optical ring resonator

We develop an air-couple ultrasound detector based on an optical fluidic ring resonator (OFRR) suspended on a Ushaped holder. The OFRR is a glass capillary with an outer diameter of approximately 130 μm and a wall thickness in the order of 1~10 μm. The circular cross section of the OFRR supports the high-Q whispering gallery mode (WGM) that circulates along the circumference. Incoming ultrasound pressure results in a small refractive index change in the glass wall and geometrical change in the OFRR shape, both of which in turn lead to a spectral shift in the WGM that can be sensitively detected owing to WGM with high optical Q-factors (>10 7 ). Due to the suspension nature of the OFRR, the ultrasound detection can be carried out in air, which is advantageous in comparison with other ultrasound detections that require acoustic coupling media such water, gel or solid. The sensitivity can be tuned and optimized by changing the diameter and wall thickness. Besides the optical detection, we also demonstrate optomechanical ultrasound mixing, in which optomechanical vibration is first excited within the OFRR that subsequently modulates the ultrasound wave. Our work will lead to the development of a new type of air-coupled ultrasound detector that can be used for photo-acoustic imaging, non-invasive ultrasound detection of external objects, and ultrasound detection/characterization of internal objects (such as particles and liquids) flowing inside the capillary.