Measurement Of Acoustic Pressure At Focus Of The Nonlinear Hifu Field Based On Laser Deflection

BackgroundThe sound pressure at focus is an important parameter of High Intensity Focused Ultrasound(HIFU)field,has an important guiding significance to the development and application of acoustic pressure of the focus detection of HIFU equipments.In recent years,with the rapid development of HIFU technology,focused ultrasound focal point pressure can reach 108 Pa order,as the pressure increases,nonlinear enhancement the effect of sound field,the sound field is more complex,greatly increased the difficulty of measuring the acoustic pressure of the focus,there is no measurement of the nonlinear sound field method can accurately complete the ideal.The current method for high intensity focused ultrasound acoustic pressure of the focus of quantitative detection of a hydrophone method(including piezoelectric hydrophone,hydrophone,hydrophone),acousto-optic diffraction method,laser measurement method.The piezoelectric hydrophone detection method is suitable for measuring lowpower under the pressure,temperature and cavitation due to nonlinear effect in high power under the focal point caused by the sensor sensitivity decreases,even damage the sensor due to the addition of,and hydrophone in sound field,the sound field distribution.The interference method for measuring high pressure as the main method of fiber optic hydrophone,although the method of measuring sound pressure can reach 107 ~ 108 Pa,but need to be placed in optical fiber sensor measurement this,not only interfere with the distribution of the focused sound field,but also in the water due to the sensor,is easy to produce cavitation in the cavitation and the measurement results and the actual sound The pressure is not the same,and easy to damage the optic fiber sensor.The acousto-optic diffraction method refers to when the acoustic wave propagation in water medium,will form a phase or amplitude grating in the far-field diffraction pattern will appear stable,the intensity distribution of diffraction is determined by the Bessel function of the corresponding order,then deduces the relationship with.Although this method can measure the pressure of the high pressure,but it is mainly through the intensity distribution of the diffraction fringe to determine the size of the pressure,and the intensity distribution of interference,and the intensity distribution and assumptions of the model,and the relationship between the sound pressure is quite complex,so it is rarely the method in actual measurement using laser vibrometer method.Is the use of water waves caused by vibrationpropagation in water,velocity and pressure of water medium on the measurement of the pressure.Although the use of laser measurement can reach very high spatial resolution,but this method.The complex pressure increases may damage the reflective membrane,so it is not suitable for the detection of high pressure.To solve the above problems,the need to seek a does not affect the distribution of the sound field measurement does not cause damage to the instrument,and the method of convenient measuring nonlinear HIFU field acoustic pressure of the focus.ObjectiveIn this paper,a rapid and nondestructive testing method for the sound pressure of a nonlinear HIFU field is discussed.Research methodBased on the theory of Raman-Nath acousto optic interaction,when a narrow parallel beam perpendicular to the incident through a certain frequency ultrasonic field,the sound field of the medium whose refractive index changes periodically with time,the beam will undergo periodic deflection,the deflection angle(or distance)from a distant spot spot image on the screen is received the obtained.Based on the acousto-optic interaction theory,firstly,laser light passing through the offset and the acoustic pressure of the focus of the focus of nonlinear HIFU field theoretically,and then in different driving power,through the acquisition of laser spot image forming device can change the focus of the field,andcalculated under different power peak pressure corresponding to the length of focus spot.In order to verify the feasibility of the theoretical model,the optical fiber hydrophone is used to measure the sound pressure at the same condition.The results of the two methods are compared under the same conditions Results1.The focal length of the laser formed through the focal point increases with the increase of the driving power of the transducer.The peak pressure can be obtained rapidly by measurement the length of the deflection laser spot.2.Compare the acoustic pressure of the focus from the two methods of measuring the value found in the driving power of 110 W in the relative error is less than 5%;the drive power is less than 250 W,the relative error is less than 15%;the driving power is greater than 250 W,the relative error is greater than 15%,but less than 20%.ConclusionIn this paper,a laser detection method is proposed,which can be used to calculate the sound pressure of a nonlinear HIFU sound field,and the peak sound pressure can be calculated quickly and the measurement efficiency can be obtained quickly after laser detection.