Laser Ultrasonic Sensor Based On Microfiber Knot Resonator

China has a vast territory and rich reserves of oil and gas resources.With the rapid growth of national economy and energy consumption,the demand for oil and natural gas is increasing.Exploring the laws of oil and gas resources storage and studying the geological structure of energy storage play an important role in guiding the scientific and effective development and acquisition of oil and gas resources.Seismic physical model imaging technology is a tool to simulate the seismic wave exploration situation.It replaces seismic waves with ultrasonic waves,restores the process of seismic wave exploration under ideal conditions,makes up for the problems of repetitiveness and lack of stability in field exploration,facilitates us to summarize and conclude the law of seismic wave propagation,and it establishes a bridge between field exploration and theoretical calculation.The traditional electrical excitation and reception of ultrasonic waves has certain limitations and defects.It is too large,vulnerable to electromagnetic interference and narrow frequency band.Optical fiber sensing technology and laser ultrasonic technology have shown their advantages in multiple neighborhoods.The study of ultrasound sensors that can be used for laser ultrasound imaging of seismic physics models is more conducive to obtaining anti-interference,multi-dimensional,high-resolution,and efficient imaging results.In this paper,an ultrasonic transducer based on a microfiber knot resonator is developed to address the problems of traditional piezoelectric transducers and some fiber optic ultrasonic sensors,such as insufficient detection ability of high-frequency ultrasonic signals and narrow response bandwidth.Based on microfiber technology,it has a smaller sensing size and has excellent detection capabilities for high-frequency and broadband ultrasonic signals.We also use a new packaging technology to protect the sensor,make it more suitable for underwater detection environment,and improve the stability.In this paper,we analyze the principle of microfiber knot resonator sensing theoretically and characterize the performance of our sensor experimentally.Experimental results show its good response to high-frequency ultrasonic signals and its stability for underwater detection.The imaging results prove the feasibility of the sensor used in seismic physical model imaging.In addition,in order to obtain stable laser ultrasound signals,a new photoacoustic material for laser ultrasound seismic physics model imaging,namely graphene heat dissipation film,is proposed in this paper.We characterized the physical and chemical properties of this material,and also explored its photoacoustic properties and the effects of various factors on the photoacoustic signal through a combination of theoretical analysis and experiments.The experimental results show that it has good photoacoustic characteristics.In the following tests,it is used in the seismic physical model,which shows that it can well excite the ultrasonic wave carrying the model information.This paper concludes with a preliminary exploration of laser ultrasound seismic physics model imaging by applying a microfiber knot resonant ultrasound sensor to laser ultrasound technology.We used the encapsulated microfiber knot resonant ultrasound sensor as the receiver.A graphene heat dissipation film was attached to the surface of the designed seismic physical model,and the photoacoustic signal was excited by nanosecond pulsed laser irradiation.The sensor receives the ultrasonic echo signal that carrying the internal information of the model,and forms a clear image of the characteristics of the model through data processing,which verifies that the sensor has a good response to laser ultrasound.