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

当弱色散层被插入到聚焦区域中时，由于二次谐波频率的声速略高，以及由于衍射引起的相位延迟，对于通过该层的二次谐波分量发生相位提前。相位失配和相消干涉的影响减弱，导致焦后区二次谐波分量的振幅增大。将不同含油率的油水混合物层作为弱分散液体进行了实验，并与非分散情况进行了比较，验证了理论预测的正确性。当聚焦超声应用于有限振幅法来估计分散液体中的非线性参数B/A时，它可能被高估。介绍了在同一聚焦系统中测量基频和二次谐波声速差的方法和精确测量B/A的方法。作为色散样品的一个例子，测量了凝胶状态下油水混合物的非线性声学参数B/A。通过将不同量的明胶粉末与少量甲醛一起混合到温热的混合物溶胶中来获得凝胶。B/A值不因凝固而改变。为了实现小体积样品的B/A测量方法，还试制了20 MHz聚焦高斯光源。将厚度为0.19 mm的LiNbO_3压电板粘结到由合成石英玻璃制成的固体声透镜的平面上，并通过光刻技术制造具有星星形状的电极。通过对基波近场的观测和非线性二次谐波的传输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.