Study Of Novel Photoacoustic Transducer Based On MEMS Technology

High amplitude,high frequency and wide band ultrasound signal can significantly improve the effectiveness and resolution of ultrasound,which has a broad demand in modern ultrasound applications,such as medical imaging and treatment,non-destructive testing etc.In view of application requirements,the photoacoustic transducer shows great advantages over traditional electroacoustic counterpart.Different from the working principle of electroacoustic transducer depending on mechanical vibration,the photoacoustic transducer relies on the transient thermal expansion and contraction caused by absorbing modulated light,called photoacoustic effect,to radiate ultrasound.With the development of laser technology and material science,the new high-performance photoacoustic transducer based on composite structure is attracting more and more interest in the world.The current researches mainly focus on high efficiency photoacoustic materials,miniaturized optical fiber integrated devices and typical biomedical applications.However,less attention has been paid to the fabrication methods and device structures.Specifically,for the focused photoacoustic transducer,conventional preparation process is a bit complex,difficult to control and demonstrates poor universality.Most importantly,its performance is fixed once being fabricated,largely limiting its applicability.Aiming to treat these issues,this dissertation explores new device structure design and fabrication process as well as related application for the photoacoustic transducer based on the MEMS processing technology.The main work and innovations are described as follows:MEMS processing technology is briefly introduced and the research status about the photoacoustic transducer is summarized.The physical mechanism of photoacoustic conversion is studied systematically.Based on the classical analysis theory of the photoacoustic transducer and the one-dimensional simplified model,the general approximate expression of the time-domain acoustic pressure harmonic response is derived.And on this basis,the effects of backing material,optical absorption coefficient and laser pulse width on photoacoustic signal are analyzed through numerical solution and software simulation.A new fabrication strategy for focused photoacoustic transducer with fixed focal length using polydimethylsiloxane(PDMS)backing material is proposed.It's simple yet effective and universal for fabricating photoacoustic lens with different performances.By combining the mature plane process with the elastomer injection molding process,the thickness and uniformity of photoacoustic conversion layer can be controlled very well,addressing the key technical challenges of the current photoacoustic lens preparation.The whole process is described in detail,the function and characteristics of the key process are analyzed.Taking candle soot as light absorbing medium,two different fixed-focus photoacoustic lens samples(9.0mm apertrue,6.3mm focal length and 12.0mm aperture,8.0mm focal length)with the same PDMS backing are made.The test results show same frequency responses(5.3MHz central frequency and 134%-6dB bandwidth),good sound field focusing ability and high photoacoustic conversion efficiency?8.0×10^-3 Pa/(W/m²),verifying the working effectiveness,control reliability,operation convenience and application universality of the preparation strategy successfully.A dynamic focus tunable photoacoustic transducer with suspended membrane structure is designed and fabricated.Through integrating the MEMS pneumatic actuation structure with suspended flexible photoacoustic conversion film,the photoacoustic transducer with dynamically and continuously modulated output sound field is realized for the first time,filling the technique gap in photoacoustic area.Moreover,this structure also demonstrates attractive capability in generating acoustic signal with distinctly improved negative sound pressure.The optimization design of the structure parameters of the suspended membrane is carried out.The feasibility and superiority of the structure design have been proved by a comprehensive sound field scanning test,in which the range of acoustic focal length of 10.0mm aperture device(3.75MHz central frequency and 88%-6dB bandwidth)is 6.0?8.5mm.At the acoustic focus,a quasi unipolar acoustic signal with a ratio of negative pressure to positive pressure of 1.5 is obtained,which is the maximum ratio that can be realized by photoacoustic transducer at present,solving the technical difficulty in providing large negative sound pressure associated with traditional photoacoustic transducers.An all-optical pulsed laser energy detector based on photoacoustic transducer is designed and fabricated.As a new application scenario of photoacoustic transducer,a novel design of all-optical pulsed laser energy detector is presented.For proof of concept demonstration,the successful preparation and experimental characterization of the prototype device have been provided.Based on the polymer MEMS process,an integrated packaging design is also carried out to realize a more compact pulsed laser energy probe.Furthermore,another portable pulsed laser energy probe,which directly uses the thermal expansion of the composite photoacoustic conversion layer to construct the F-P cavity sensor unit,is proposed.Its corresponding sample preparation and preliminary feasibility verification are completed as well.