Research On Emission Characteristic Control And In Situ Acoustic Detection Technology Of Photoacoustic Transducer

Photoacoustic transducers based on the principle of photoacoustic effect can generate high-intensity,high-frequency,and wide-bandwidth ultrasound.Compared with traditional electroacoustic devices,they have the characteristics of anti-electromagnetic interference,making them highly regarded in the field of ultrasonic technology.Currently,the research on photoacoustic transducers mainly focuses on three aspects: efficient photoacoustic conversion materials,device structure and fabrication process,and application research based on photoacoustic transducers.Although remarkable achievements have been made,there is still a big gap between the ultrasound technology based on photoacoustic transducers and the widely used electroacoustic devices,due to the immature development.This is mainly reflected in two aspects: the emission characteristics control of the device and the ultrasound reception function.Specifically,(1)The realization of the focused ultrasound is limited to the strategy of using the geometric focusing characteristics of the spherical or conical photoacoustic source,the corresponding fabrication process is complex,and it is difficult to obtain stable emission performance;(2)Lack of effective MPa-level dynamic spatial ultrasound field control technology;(3)Currently,most photoacoustic transducers can only be used as ultrasound emission sources and lack in-situ ultrasound detection function.In view of the abovementioned problems,this thesis selects candle soot nanoparticles and polydimethylsiloxane as photoacoustic conversion materials.Starting from the principle of photoacoustic conversion,through device structure design,fabrication process exploration and theoretical analysis,this thesis studies the emission characteristics control and in-situ ultrasound reception function of photoacoustic transducers.The main contents and innovations include:The photoacoustic transducer’s research status and spatial ultrasound field modulation technology are investigated and summarized.From the principle of photoacoustic conversion,the time-domain solution of the photoacoustic signal is deduced under the one-dimensional approximated model.The relationship between the photoacoustic signal waveform and the structural parameters of the photoacoustic conversion layer is obtained.The related theory of ultrasound transmission and non-diffraction field is studied,providing a theoretical reference for explaining the acoustic phenomena found in the subsequent device characterization.A focused photoacoustic transducer with a planar structure is designed and fabricated.A well-developed planar process is used to pattern the photoacoustic conversion region into a ring shape by combining the masking technique.Under single pulse excitation,it can generate a focused ultrasound field with a lateral diameter of several hundred microns and a focal depth of several centimeters,solving the fabrication issues of the traditional focused photoacoustic transducers.The influence of ring size on its focusing and frequency characteristics is explored through experiments.And the intrinsic relation between focusing characteristics and ring size is successfully revealed by adopting the theory of non-diffracting X-waves.A photoacoustic transducer with binary dynamic acoustic pressure amplitude control capability is proposed and realized.Referring to the deduced universal temporal expression of photoacoustic signal,an original transducer structure with integrated suspended film and pneumatic control cavity is designed.Driven by external air pressure,the acoustic backing of the photoacoustic conversion film can be switched between air and glass.Under the same excitation light condition,the amplitude difference of output ultrasound pressure under different backing can reach an order of magnitude.Further array design of the device unit structure is carried out to preliminarily verify the function of this scheme for MPa-level spatial dynamic ultrasound pressure amplitude control,solving the technical bottleneck of spatial ultrasound field control of high sound pressure in the field of photoacoustic transducer.A forward-viewing all-optical ultrasound probe with integrated self-receiving function on one optical fiber is designed and realized.An innovative process based on capillary effect is developed,and high-efficiency optoacoustic conversion layer and high-contrast Fabry-Perot(FP)optical interference cavity structure for transmitting and receiving ultrasound are synchronously prepared at the end face of the capillary.With the double-clad optical fiber that can simultaneously transmit excitation light and detection light,an all-optical ultrasound probe with ultrasound transmission and reception functions is constructed.The acoustic emission and reception characteristics of the probe are systematically studied,and its self-receiving capability is successfully verified by pulse-echo experiments,which will be helpful to construct more compact photoacoustic functional devices and simplify their fabrication.