Studies On Photoacoustic Signal Detection System Based On Optical Fiber Interferometer

With the development of optical fiber communication,the research progress of optical fiber sensing technology is advancing by leaps and bounds,Optical fiber sensor based on optical fiber sensing principle is widely used in the measurement of various physical quantities,Compared with the previous electrical sensors,fiber optic sensors have obvious advantages,such as high sensitivity,corrosion r esistance,anti-interference and so on.When a laser illuminates an object,object absorb light,a sharp increase in the internal temperature of the object leads to morphological changes,producing ultrasonic(photoacoustic signal),this phenomenon known as the photoacoustic effect.Object material,structure is different,the photoacoustic signals produced by laser excitation are also different.The characteristics of the object can be studied by detecting the photoacoustic signal of the object.Since the photoacoustic effect has been found,the method of detecting the photoacoustic signal is also developing,the detection method can be divided into two kinds,one is an ultrasonic transducer,such as a piezoelectric ultrasonic transducer,as the ultrasonic transmission in the air has a greater loss,in general,ultrasonic transducers are used to detect the photoacoustic signal,and an ultrasonic coupling agent is required between the piezoelectric ultrasonic transducer and the object to be measured.This method of detection depends on the contact of the measured object,limiting the practicality of the method.With the development of optical technology,more and more optical methods have been proposed,especially the detection method based on optical fiber sensing technology has been widely concerned.In this paper,photoacoustic signal detection system based on optical fiber Mach-Zehnder interferometer,is an optical detection method,the key to this k ind of detection method is to solve the stability of the fiber interferometer.Due to the external environment caused by the optical fiber interferometer to produce random phase drift,resulting in the output interference signal fading,in order to solve the problem of random phase drift,we use direct-current phase tracking(PTDC)detection method.The output signal of the fiber interferometer contains the direct-current component and the alternating-current component.We use the low-pass filter to extract the direct-current component and the low frequency alternating-current component from the interference signal,and then applied by the integrator output on the phase modulator for phase compensation,so as to achieve a stable interference system and achie ve the purpose of photoacoustic signal detection.According to the principle of DC phase tracking detection technology,I set up a photo-acoustic signal detection system based on the optical Mach-Zehnder interferometer in the laboratory,and designed the p hase compensation circuit.I conducted a functional test for each module in the laboratory,and analyzed the results of the interference output before and after adding the feedback circuit system,experiments show that the feedback circuit system can achieve phase compensation wel.Then,I tested the system's signal detection capabilities,respectively,to detect the percussion signal,vibration signal of different frequency atomization film.Finally,the aluminum alloy plate and pencil core are used as the sample,using this system to detect the photoelectric signal produced by aluminum alloy plate and pencil core.Then,the ultrasonic transducer probe was used to detect the photoacoustic signal of the same sample,using to compare with the forme r.Experiments show that the system can realize the detection of photoacoustic signals,the detection bandwidth is about several hundred KHz to several MHz.The use of fiber optic sensors to detect the photoacoustic signal has a small size,easy to form a detection array,easy to achieve long-range detection and many other advantages,it is worth in-depth study.