Research On Fiber Laser Ultrasound Hydrophones And Their Imaging Applications

Acoustic detection is a basic technology in the field of engineering,which play improtant roles in the military defense,resource exploration,medical imaging,marine fisheries and other fields.Fiber laser hydrophone is utilizes fiber to act as in-fiber acoustic sensing device,the acoustic wave is transformed into optical characteristic parameters to realize the detection of weak sound waves.At present,fiber optic hydrophones are relatively mature for low frequency acoustic(<20 kHz)and high frequency ultrasound(> 1 MHz).However,for the "intermediate frequency" band of20 k Hz-1MHz,due to the limitations of the fiber itself and the ability of the transducer,the detection capability is very limited,this band has always been a short slab.Optical fiber sensor utilizes short liner cavity laser to act as sensing device and use the wavelength or beat frequency demodulation mode to detect weak signal.Optical fiber sensor has a number of benefits,such as high sensitivity,miniature size,easily multiplexing and electromagnetic interference.In this thesis,a promising fiber laser techniques with novel structure is presented.The high sensitivity ultrasonic hydrophone for the band 20 kHz-1 MHz is realized by the beat frequency demodulation method,and the underwater imaging is realized based on its broadband response.The main work is as follows:(1)We encapsulated the orthogonal dual fiber laser into a cylinder with epoxy resin and the result shows that the sensitivity is promoted 100 times than bare fiber laser hydrophone.The reason for realizing sensitization is to increase the diameter of the fiber after encapsulation.Based on the scaling law between the acoustic wavelength and fiber diameter,the resonant frequency of the fiber laser is reduced to below 1MHz,thus realizing the sensitization mechanism.We used a 5mm diameter fiber laser ultrasonic hydrophone to detect the ultrasound below 1 MHz,the sound pressure sensitivity is 1.1kHz/Pa and the minimum detectable sound pressure is2mPa/Hz1/2 at a typical frequency of 200 kHz.Then we encapsulated three fiber laserultrasonic hydrophones with the diameter of 3.4,4.1 and 7.5mm,respectively.And studied their frequency response to 1MHz broadband ultrasound at the center frequency.This finding provides design guidance for fiber laser ultrasonic hydrophones,we can encapsulate fiber laser ultrasonic hydrophones with different diameters for specific frequencies,resulting in maximum ultrasonic sensitivity.Then we studied the direction and frequency noise of fiber laser ultrasonic hydrophone,and the experimental results were in good agreement with theoretical reasoning.(2)According to the principle of type B imaging in pulse echo imaging,we used 5mm diameter fiber laser ultrasonic hydrophone scanned while collecting the edge of the object.Through time-to-space mapping can obtain a define image.The axial spatial resolution and lateral resolution is 4.3cm and 4mm,respectively.This indicate that the study of 20 kHz-1 MHz frequency band can find applications in underwater search and targeting,as well as the arc discharge monitoring in transformers.