Study On In-line Hollow Core Photonic Bandgap Fiber Fiber-optic Fabry-Perot Interferometer And Its Coupling Model With Spherical HIFU Acoustic Field

In recent years,the research on high intensity focused ultrasound(HIFU)is more and more in-depth.Traditional traveling wave focusing method has been perfected in theory,but the acoustic pressure generated by traveling wave focusing collector is far lower than people expected.The latest spherical HIFU transducer breaks through the traditional HIFU focusing mode of traveling wave focused ultrasonic transducer,the focal field size is further compressed and the focus acoustic pressure is further improved,it can achieve more accurate therapeutic effect in the field of biomedical engineering.At the same time,spherical HIFU acoustic field provides an extreme experimental condition,and its major innovative research in various fields is no less than other extreme conditions,such as high temperature,low temperature,high pressure,vacuum,strong magnetic field,strong radiation and so on.Therefore,it is particularly important to accurately measure and demodulate the acoustic field characteristics of the spherical HIFU acoustic field,especially the acoustic field distribution,the focus position,the size of the acoustic focal region and the maximum focal acoustic pressure.In the previous works of our team,Wang et al.(Opt.Lett.37,2046-2048(2012))reported an in-line silica capillary tube all-silica fiber-optic Fabry-Perot interferometric(ILSCT-ASFPI)sensor for detecting HIFU field with the advantages of temperature insensitivity,high signal-to-noise ratio and so on.After that,Wang et al.(Appl.Phys.Lett.103,044102(2013))reported a tip-sensitive all-silica fiber-optic Fabry–Perot(TAFOFP)ultrasonic hydrophone for detecting HIFU field.However,there are no reports about the measurement and demodulation of spherical HIFU acoustic field at present.The reasons are that the research about the coupling principle of spherical HIFU acoustic field is not explicit and no effective means to measure the ultra-high sound pressure of spherical HIFU acoustic field.Accordingly,on the basis of our previous works,in this thesis,an in-line hollow core photonic bandgap fiber fiber-optic Fabry-Perot interferometer(ILHCPBF-FOFPI)and its coupling model with spherical HIFU acoustic field are studied,the multi-operating point nonlinear demodulation method based on ILHCPBF-FOFPI is improved,a ILHCPBF-FOFPI acoustic pressure demodulation experimental system is built and experimentally verified.In addition,the possible errors of the ILHCPBF-FOFPI are analyzed,and the corresponding test system is built to verify the experiment.The major research works and achievements completed in this thesis include:1.Based on principles of optics,considering the fact that the two ends of the actual Fabry-Perot cavity are not parallel,the parameter of wedge angle is introduced to analyze the wedge interference principle of ILHCPBF-FOFPI,especially the influence of the initial wedge angle and the measured wedge angle on the interference output signal,which lays a foundation for the later the acoustic pressure measurement system research.2.Based on principles of acoustics,a coupling mechanism of ILHCPBF-FOFPI with spherical HIFU acoustic field is analyzed detailly,especially the reflection and transmission of the ultrasonic wave in 5 heterogeneous media interfaces is analyzed in detail.And a coupling mode of ILHCPBF-FOFPI with spherical HIFU acoustic field is established and simplified so that it can be used in engineering.3.Through research and repeated tests,the parameters suitable for the fusion of hollow-core photonic bandgap fiber and SMF are obtained,and the ILHCPBF-FOFPI is fabricated,and the corresponding system is built for testing.Then,the multi-operating point nonlinear demodulation method based on ILHCPBF-FOFPI is improved,and a corresponding experimental system is built to verify it,which proves that this method is very effective.Finally,the ILHCPBF-FOFPI acoustic pressure measurement experimental system for the spherical HIFU acoustic field is built.The acoustic field distribution,the size of the acoustic focal region and the maximum focal acoustic pressure are experimentally verified.The experimental results verify the correctness of the coupling model and acoustic pressure demodulation method of ILHCPBF-FOFPI with spherical HIFU acoustic filed,and the ILHCPBF-FOFPI acoustic pressure demodulation experimental system can correctly describe the spherical HIFU acoustic field characteristics.4.The loss error of ILHCPBF-FOFPI is analyzed,and the measurement method of the initial wedge angle of ILHCPBF-FOFPI is proposed.The influence of changing the wedge angle on the measurement result on the basis of the initial wedge angle is analyzed,which lays the foundation for achieving more accurate ILHCPBF-FOFPI sensing.The research works in this thesis provide a new method for acoustic field characteristics and acoustic pressure measurement of spherical HIFU acoustic field,and it provides a theoretical basis.