Fiber-tip Microbubble Based Acoustic Sensor With Wide Bandwidth And High Sensitivity

Acoustic/ultrasound detection plays important roles in areas of military defense,industry,and biomedicine.Compared with electric acoustic sensors,fiber-optic acoustic sensors?FOASs?have the advantages of high sensitivity,light weight,immunity to electromagnetic interference?EMI?,and capability of remote detection and multiplexing.A typical implementation of FOAS is to attach thin deflectable diaphragms to the fiber tips to construct a Fabry-Perot?F-P?cavity.Acoustic can be detected by demodulating the change of optical signal caused by the deformation of diaphragms.A F-P cavity based FOAS can realize a high acoustic sensitivity by means of optimizing the diaphragm material or reducing the thickness of diaphragm.However,it has always faced the problems of complexity of preparation,reliability,and applicability due to heterogeneous compatibility of fiber and diaphragm.In order to solve the above problems,we proposed a scheme of FOAS based on a fiber-tip microbubble structure,which realized acoustic detection with high sensitivity and wide frequency bandwidth.And it was utilized as a functional element for photoacoustic imaging.Here's how it would work:heating light is delivered to a metal-coated optical fiber facet in the water and is absorbed by the metal film,then a microbubble which serves as a F-P cavity is photothermally generated at the facet.The microbubble interferometrically detects the pressure-induced bubble deformation for acoustic detection.Because the air/liquid interface instead of thin elastic diaphragm is used for in situ acoustic measurement,the scheme not only avoids the requirement of heterogenous compatibility and effectively solves the problems of complex diaphragm fabrication processes,but also has the advantages of high sensitivity and strong controllability.This article mainly focuses on the fabrication,acoustic response characteristics and photoacoustic imaging applications of the microbubble based FOAS,and the main research work is as follows:1.Fabrication,control and stabilization processes of microbubble.We studied the dynamic process of photothermal generation of microbubble on the fiber tip,clarified the dependence of the microbubble size and the heating light power,and grasped the basic rule of optically controlling the microbubble size.A reflection spectrum with high contrast was obtained by optimizing the thickness of the endface metal film.The microbubble structure was stabilized by fast servo-control of the heating light power and the diameter of the microbubble fluctuates within 0.5 nm when stabilized.2.Study of acoustic response characteristics.The acoustic responses of microbubbles with different radii to acoustic waves with different frequencies were measured.When the microbubble radius is 39?m,the acoustic pressure sensitivity is 0.2 nm/Pa,the operating frequency bandwidth is 60 kHz,and the minimum detectable acoustic pressure is 1 mPa/Hz^1/2in a frequency range of the operating bandwidth.We found that the resonant frequency of an oscillating bubble in water exhibits an inverse dependence on the microbubble radius.Based on this,we adjusted the heating power so that the microbubble radius was continuously adjusted from 100?m to 5?m,and the operating frequency was tuned from 2 kHz to 450kHz.3.Photoacoustic imaging applications.A microbubble with radius of⁶.5?m and operating frequency bandwidth of³50 kHz was utilized as an ultrasonic sensor to measure the photoacoustic signals point by point during the mechanical scanning process.The acquired phtoacoustic signals were used for image reconstruction based on the filter back-projection?BP?algorithm and a resolution of 2.2 mm was achieved by using the microbubble.