Research On Highly Sensitive And Miniaturized Fiber Optic Ultrasound Transducers And Their Imaging Applications

Ultrasound,as a kind of acoustic signal with a frequency greater than 20 k Hz,is widely used in medical imaging diagnosis,non-destructive testing and other fields.In recent years,with the development of science and the progress of society,the demand for medical imaging technology is rapidly increased.Several medical imaging technologies have been developed,including photoacoustic imaging and ultrasound imaging,which use ultrasound signals to diagnose the health of human tissues.As the core component of photoacoustic and ultrasound imaging system,the performance of ultrasound transducer directly determines the imaging quality of the imaging system.This dissertation focuses on the demand for high sensitivity,miniaturization and multiplexable optical fiber ultrasound transducer in high-performance photoacoustic/ultrasound imaging technology,and carries out innovative research from the optical fiber ultrasound sensing mechanism and structure,materials and demodulation technology.Three types of optical fiber ultrasound sensors are developed for passive ultrasound detection.An optical fiber ultrasound transducer with integrated transmitter and receiver is designed for active ultrasound detection.The main contents are as follows:(1)For the requirement of high sensitivity ultrasound detection,a linear microfiber ultrasound sensor packaged by PDMS is proposed.The dual sensitization mechanism based on the evanescent field of micro fiber and PDMS encapsulation is studied,and the microfiber ultrasound sensor with a diameter of several micros is designed and prepared.Based on the coherent detection technology,the microfiber ultrasound sensor achieves a sensitivity of 3.5m V/k Pa,which is 10 times higher than that of the single-mode fiber ultrasound sensor.(2)Aiming at the problems of limited response angle and difficulty in array detection of linear fiber optic ultrasound sensors,a fiber optic ultrasound sensor array based on phase modulation is proposed.The ultrasound sensing structure based on fiber end and polymer film is designed,whose diameter is only 125 μm.The sensing model of ultrasound-diaphragm vibration-optical phase modulation is established.The sensor exhibits a frequency response bandwidth of 14 MHz,a NEP of 0.46 k Pa and nearly omnidirectional directivity.A multi-channel parallel demodulation system based on coherent detection is built.The number of parallel multiplexed sensors is 30 in theory,and the parallel demodulation of 3 sensors is demonstrated experimentally.Based on the proposed sensor array,a photoacoustic imaging system is built,a lateral resolution of 165～217 μm and an axial resolution of 112～131 μm are achieved.Compared with the imaging system using a single sensor,the imaging time of the proposed imaging system is 3 times shorter.(3)In order to achieve low-cost multi-channel parallel ultrasound detection,a fiber optic wavelength tunable FPI ultrasound sensor array based on phtothermal effect is proposed.The influence factors of sensitivity and cavity length consistency of FPI sensor are analyzed theoretically,and the photothermal tuning mechanism of FPI sensor is proposed.A photothermal tuning ultrasound sensor array is designed and fabricated.The sensor exhibits a working wavelength tuning range of 15 nm with a heating laser power of 15 m W.A parallel demodulation system based on a fixed wavelength laser is built,and 53 sensors can be multiplexed in parallel theoretically.The parallel demodulation of 4 sensors is demonstrated experimentally,and the NEP is as low as 0.12 k Pa.Finally,the FPI optical fiber ultrasound sensor array is used for photoacoustic imaging,achieving a lateral resolution of 502 μm and an axial resolution of 133 μm.(4)According to the application requirement of active ultrasound detection,an integrated fiber optic ultrasound transducer is proposed.The mechanism of fiber optic ultrasound emission and ultrasound detection is analyzed.Based on the end face of double clad fiber with diameter of 125 μm,the optical fiber ultrasound transducer is designed with PDMS/carbon black composite material and plano-concave FPI structure.The transducer exhibits a high emission pressure of 450 k Pa and a broad emission bandwidth of 22 MHz,highly sensitive ultrasound detection is also demonstrated with a NEP as low as 72.8 Pa and a detection bandwidth of 16 MHz.Furthermore,the ultrasound imaging system based on the transducer achieves a lateral resolution of 25～58 μm and an axial resolution of 48～55 μm.