Research On Key Technology Of Ultrahigh Sensitivity Microcavity Acoustic Sensing Based On Acousto-optic Dispersion Coupling

Acoustic sensors play a vital role in modern society,and their applications cover various fields such as non-destructive weak signal detection,medical and health diagnosis,underwater acoustic communication,etc.,which provide important support for major national scientific and technological progress and the main battlefield of national economy.Traditional piezoelectric acoustic sensor is limited in acoustic detection accuracy,and has drawbacks such as low detection sensitivity,susceptibility to electromagnetic interference,and low signal-to-noise ratio,which make it difficult to meet the needs of some special application scenarios.Therefore,it is particularly important to develop a new type of acoustic sensor with ultrahigh sensitivity,anti-electromagnetic interference and high signal-to-noise ratio.Whispering Gallery Mode（WGM）optical microcavity with extremely high quality（Q）factor and small mode volume significantly enhance light-matter interactions,making it an excellent platform for ultrahigh sensitivity optical sensing.Calcium fluoride（CaF₂）crystal microcavities exhibit the highest Q factor(theoretical Q>10¹⁴)in the near-infrared and visible spectral ranges due to their high purity and low material loss characteristics.Ultrahigh Q corresponds to the extremely narrow transmission spectrum,and higher solution resolution can be obtained either by detecting frequency shift or transmission spectrum broadening.Therefore,high Q CaF₂crystal microcavity as a sensitive unit becomes an ultrahigh sensitivity acoustic sensing solution.This thesis focuses on the verification of ultrahigh sensitivity microcavity acoustic sensing effect.The research is centered around three key technologies:acousto-optic dispersive coupling sensing mechanism,optimization of high-efficiency acoustic coupling structure,and fast switching of multi-mode locking.The performance and reliability of high sensitivity acoustic sensing are verified by outfield testing,and its application potential in the fields of non-destructive weak signal detection,speech recognition and separation,sound source localization and tracking,and underwater acoustic real-time communication is demonstrated.Finally,acoustic sensor with ultrahigh sensitivity is realized.The main research and results are as follows:1.Aiming at the problem that acoustic sensing mechanism of CaF₂crystal microcavity is still unclear,the acoustic-optic dispersive coupled sensing mechanism of acoustic wave-induced deformation effect of CaF₂crystal microcavity is revealedBased on the acoustic-photon coupling theory and the transmission equation,this paper uses the finite element method to study the acoustic field regulating the geometrical change of CaF₂crystal microcavity and the change of the refractive index caused by the elastic-optical effect,and establishes the CaF₂crystal microcavity coupling model under the action of acoustic field.Three different mechanisms of action are analyzed:a)the change of the radius due to the acoustic pressure;b)acoustic pressure induces the strain redistribution in the cavity,which changes the refractive index of the CaF₂crystal microcavity through the elastic-optical effect;c)acoustic pressure causes the change in the refractive index of the air by changing the air density in the acoustic-optic coupling evanescent field of the CaF₂crystal microcavity,which leads to the resonance frequency shift.The results show that the radius change is 1～2 orders of magnitude higher compared to the two mechanisms of refractive index change and air refractive index change.Therefore,CaF₂crystal microcavity acoustic mechanism is dispersion coupling sensing mechanism mainly based on the radius change due to the external acoustic pressure,which leads to the resonance spectrum shift.It provides a theoretical basis for ultrahigh sensitivity acoustic sensing.2.Aiming at the problem of insufficient response efficiency of acoustic-optic coupling in restricted space,the structural optimization of CaF₂crystal microcavity sensitive unit is carried out,which realized the enhancement of air acoustic sensing sensitivity by 2 orders of magnitudeBased on the finite element simulation method,the structural dimensions of the CaF₂crystal microcavity and the support sample are simulated,and the effects of the structural parameters of the sensitive unit on the radius change and the frequency response range are theoretically analyzed.The optimal structural parameters of 5.0 mm radius and 0.1 mm thickness.By establishing the microcavity acoustic sensing sensitivity formula,the sensitivity is proportional to the resonance frequency shift and the Q factor of the microcavity.The larger the radius,the thinner the thickness and the higher the Q factor of the microcavity,the higher the sensitivity.When the Q factor is 10⁸,the sensitivity can reach 11 V/Pa,and when the Q factor is 10⁹,the sensitivity can exceed 100 V/Pa,which is two orders of magnitude higher than the sensitivity of other microcavity acoustic sensors.3.Aiming at the problem of mutual constraints between ultrahigh sensitivity and wide dynamic response range,a multi-resonance mode switching locking method is proposed for the microcavity of columnar CaF₂crystals,which realizes the rapid switching between high sensitivity and wide dynamic rangeBased on the columnar CaF₂crystal microcavity supporting rich resonance modes with different Q factors（10⁶～10⁹）,fast switching of ultrahigh sensitivity and wide dynamic response range is achieved by the fast switching and locking of different modes.When Q factor is switched from 5.13×10⁸to 6.24×10⁶,the dynamic response range is improved by 38.3 d B.The test results of the acoustic sensing effect based on ultrahigh Q CaF₂crystal microcavity indicate that when the Q is 1.02×10⁸,the sensitivity is as high as11.54 V/Pa at 10 k Hz frequency,and the minimum detectable acoustic pressure is as low as 6.6μPa/Hz^1/2.Based on the above research,different experimental scenario tests are conducted and the results indicate that the microcavity acoustic sensing system can still respond to the human speech when there is a wall obstacle at a distance of 9 m away.Under the noisy environment in the rain,music and rain can be distinguished.Evan in environments where various instruments are operating simultaneously,signals can still be distinguished from noise,demonstrating that the ultrahigh sensitivity CaF₂crystal microcavity acoustic sensing system can be used for precise speech recognition and background noise environment sound source localization and tracking.4.In response to the problem of the loss of acoustic wave effective transmission energy and the deterioration of microcavity optical properties during the underwater testing process,the water-tightness and water-pressure-resistant encapsulation method of the microcavity of the high-Q CaF₂crystal has been optimized,and the Q factor of the encapsulated crystal is greater than 10⁸Based on the full encapsulation technology,the UV glue with low refractive index and low Young’s modulus is used as watertight encapsulation material,the device encapsulation of CaF₂crystal microcavity coupled with tapered fiber is realized.The size of the encapsulated device is 45 mm×28 mm×5 mm,which is small,easy to move,high Q factor,and the device can withstand the pressure of 1 MPa and the temperature of 150℃.The index test of underwater acoustic sensor is carried out based on the encapsulated device.The results show that the sensitivity reaches-120.4 d B at 500 Hz and the minimum detectable acoustic pressure is as low as 132μPa/Hz^1/2.Through the outdoor experimental scenario test,the results indicate that the encapsulated device can clearly display musical characteristics even at distances exceeding 5 m.The reconstructed speech signal has a higher signal-to-noise ratio than standard hydrophones,which fully proves that the CaF₂crystal microcavity encapsulated device with high Q factor is a new structure in the field of underwater acoustic detection,has ultrahigh sensitivity,excellent signal-to-noise ratio,high precision and underwater acoustic communication ability.