Research On Signal-to-Noise Ratio Enhancement Of Fiber-Optic Photoacoustic Microscopy

In photoacoustic tomography,the performance of an ultrasonic sensor is a key factor in the signal-to-noise ratio(SNR),which determines the image quality.Our group has deployed orthogonal-polarized dual-frequency fiber lasers as sensitive elements(with a noise-equivalent pressure,NEP of 43.6 Pa and a detection bandwidth of 15 MHz),and has realized fiber optic photoacoustic microscopy in vivo.However the SNR needs to be improved.This thesis studies and improves the performance of fiber-optic ultrasonic sensing system in both noise reduction and sensitization.As a result,the SNR of the imaging system has been significantly improved.In noise reduction,a "duplicate-and-average" method is proposed and demonstrated.The initial signal "reproduces" multiple identical signals through the buffer fiber,which are separated from each other in time.The ultrasonic waveform are averaged after the detection,which effectively reduces the noise from the detection system.The noise is reduced by 40%,and the background noise of the imaging system is reduced by 3.5 d B.The SNR has been increased by 1.6 times.In sensitivity increasement,research is carried out from the following two aspects: 1.Considering is the acoustic interaction between the silica fiber and incident ultrasound wave is weak,we proposed to apply polymer coating on the surface of the fiber to enhance the ultrasonic coupling.Applying a quarter-turn PMMA with a thickness of 25 ?m can increase the ultrasonic sensitivity by about 35%.2.To overcome the mismatch between the effective length of the ultrasonic interaction and the fiber laser sensor,we applieda plano-concave lens to convert the ultrasonic spherical wavefront into planar one,which increases the effective interaction length.The experimental result shows that this method can double the sensitivity of the ultrasonic sensor.Through the above sensitization and noise reduction methods,the signal-to-noise ratio of the optical fiber photoacoustic imaging system has been effectively improved,which paves the way to further applications including wearable and endoscopic imaging devices..