Research On All-fiber Optic Ultrasonic Imaging

Ultrasound imaging has played an important role in biomedical applications,especially in clinical diagnostics.For invasive endoscopic imaging applications,conventional piezoelectric transducers typically have a relatively large device size and are difficult to penetrate into a narrow lumen structures.Therefore,the imaging capability has been limited.Aiming at this problem,this thesis proposes the development of all-fiber ultrasonic imaging technology based on the small size and flexibility of optical fiber.Based on the utilization of 125-micron optical fibers,a highintensity ultrasonic source and a high-sensitivity ultrasonic sensor were developed,respectively.The two were used together to form a miniaturized all-fiber ultrasonic probe,which can image biological tissue samples in a side-looking manner.The realization of the small-sized probe paves the way for the further development of fiber optic ultrasound endoscopy.This paper focuses on the fabrication of all-fiber ultrasonic probes and the realization of ultrasound imaging,which includes the following works.(1)On the optical ultrasonic excitation,the ultrasonic excitation film layer was deposited by dip coating on the fiber end face.At 2.46 μJ laser energy,the fiber ultrasonic source can generate an acoustic pressure of 0.95 MPa and a frequency bandwidth of 16.3 MHz.The film takes advantage of the strong optical absorption of carbon nanotubes as well as the high thermal expansion coefficient of the coated polymer,and has high ultrasonic generation efficiency and high damage threshold.(2)In the optical detection of ultrasonic waves,the high-sensitivity detection of ultrasonic waves is realized by using a dual-frequency fiber laser as sensing element.The incident ultrasonic waves induce fiber deformation and refractive index change,which subsequently causes a frequency shift of the output laser beat frequency.The experiment measured the sensor with a detection bandwidth of 19.5 MHz.(3)The above-mentioned optical fiber components with ultrasonic excitation and detection functions are used to form a 5 mm wide all-fiber ultrasonic probe.Two-and three-dimensional imaging of the swine tracheal tube has been realized by linearly scanning the probe and performing echo wave detection,signal processing and acoustic reconstruction.The tracheal contour and the inner layered structure can be distinguished in the imaging result.The axial and lateral resolutions are 150 μm and 62 μm,respectively.