Abstract
We experimentally demonstrate and theoretically analyze high Q-factor (~107) capillary-based optical ring resonators for non-contact detection of air-coupled ultrasound. Noise equivalent pressures in air as low as 215 mPa/√Hz and 41 mPa/√Hz at 50 kHz and 800 kHz in air, respectively, are achieved. Furthermore, non-contact detection of air-coupled photoacoustic pulses optically generated from a 200 nm thick Chromium film is demonstrated. The interaction of an acoustic pulse and the mechanical mode of the ring resonator is also studied. Significant improvement in detection bandwidth is demonstrated by encapsulating the ring resonator in a damping medium. Our work will enable compact and sensitive ultrasound detection in many applications, such as air-coupled non-destructive ultrasound testing, photoacoustic imaging, and remote sensing. It will also provide a model system for fundamental study of the mechanical modes in the ring resonator.
Highlights
Ultrasound detection is one of the most widely used methods to non-invasively examine internal and external structures of samples
Unlike conventional piezoelectric or MEMs-based non-contact ultrasound detectors[10, 12, 13, 27, 28], optical detectors do not suffer from geometry-dependent electrical noise, and are immune towards electromagnetic interference[14, 23]
The optical beam carries the information of the ultrasound pressure to the detector, and aforementioned absorption/propagation loss can be neglected
Summary
Ultrasound detection is one of the most widely used methods to non-invasively examine internal and external structures of samples. Ultrasound detection, usually requires an acoustic impedance matching layer (such as water, gel, or solid) between the sample and the ultrasound detector Such requirement is due mainly to the high acoustic coupling loss at the sample/air (or detector/air) boundary, and the large acoustic absorption of air at ultrasonic frequencies, both of which effects significantly reduce the intensity of air-coupled pressure waves received by the ultrasound detector[7,8,9]. In the beam deflection method, ultrasound-induced refractive index shift deflects the propagating optical beam, and the amount of beam deflection is recorded using a quadrant photodiode In both cases, the optical beam carries the information of the ultrasound pressure to the detector, and aforementioned absorption/propagation loss can be neglected. Since the resonators directly detect ultrasound waves, the surface roughness of the sample causes less problem than interferometric detectors discussed previously
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