Analysis of nonlinear focused sound field in dispersive media and its application to measurement.

当在焦区插入弱色散层时，由于二次谐波的声速稍高，以及由于衍射引起的相位延迟，通过该层的二次谐波分量发生了相位提前。然后减弱相位失配和相消干涉的影响，使焦后区域的二次谐波分量幅值增大。将含有不同油份的油水混合物作为弱分散液体的实验结果与非分散情况进行了比较，验证了理论预测的有效性。将聚焦超声应用于有限幅值法估计色散液体中的非线性参数B/A时，可能会高估B/A。给出了用同一聚焦系统测量基频和二次谐波声速差和精确测量B/A的方法。以分散样品为例，测量了油水混合物在凝胶状态下的声学非线性参数B/A。该凝胶是将不同量的明胶粉末与少量甲醛混合到加热的混合溶胶中得到的。B/A值不会因凝固而改变。为了实现小体积样品的B/A测量方法，还进行了20 MHz聚焦高斯源的试制。将0.19 mm厚的LiNbO_3压电板粘接在由合成二氧化硅玻璃制成的固体声学透镜的平面上，并用光刻技术制作了星形电极。通过对基频近场和非线性二次谐波传输的观测，证实了聚焦高斯光束的形成。

When a weakly dispersive layer is inserted into the focal region, a phase advance occurs for the second harmonic component passing through the layer due to slightly higher sound-speed for second harmonic frequency, together with the phase delay due to diffraction. Then the effects of the phase-mismatching and destructive-interference are weakened to result in amplitude increase of the second harmonic component in the postfocal region. The experiment for a layer of water-oil mixture with various oil fractions employed as a weakly dispersive liquid is compared with non-dispersive case and demonstrates the validity of theoretical prediction. When a focused ultrasound is applied to the finite-amplitude method to estimate the nonlinearity parameter B/A in dispersive liquid, it can, therefore, be overestimated. The methods used to measure the sound-speed difference between the fundamental and the second harmonic frequencies using the same focusing system and to measure the B/A accurately are presented. As an example of dispersive samples, the acoustic nonlinearity parameter B/A in water-oil mixtures at the gel state has been measured. The gel is obtained by mixing various amounts of gelatin powder together with small amount of formaldehyde to the warmed mixture sol. The B/A value is not changed by the solidification. To realize the B/A measurement method for small volume samples, the trial production of a 20-MHz focusing Gaussian source has also been conducted. A piezoelectric plate of LiNbO_3 of 0.19-mm thickness is bonded to a plane surface of solid acoustic lens made from synthetic silica glass, with an electrode fabricated through photo-etching technique to have a shape of star. The formation of focused Gaussian beam is confirmed through observation of near field of the fundamental and propagation of the nonlinearly generated second harmonic.