Diffraction in finite-amplitude focusing sound field and its application to acoustical imaging.

球面聚焦源发出的正弦波在向焦点传播的过程中会产生波形畸变，这主要是由于介质的弹性非线性，可以用非线性参数来描述。理论和实验证明，在焦后区域所见的二次谐波分量绝大部分是在焦后区域产生的，这是因为在焦前和焦后区域所产生的二次谐波分量受到经过焦点的声波相移的影响而相互抵消。将非线性参数与周围介质（水）非线性参数不同的样品插入焦点区域，相位参数<pi>/2弧度的二次谐波分量的幅值发生严重变化。这使得可以检测和可视化浸入焦点的样品的非线性参数与水的不同。另一方面，由于圆柱聚焦源的波传播不遭受类似的相移，在每个焦点区域产生的二次谐波分量不能在后焦点区域占主导地位。因此，只有球聚焦声可能适合于本研究项目。假设样品的线性参数（环境密度、声速和吸收系数）与周围介质的线性参数以及非线性参数不同，从理论上和实验上讨论了样品对声波的影响。结果表明，在焦后区域观测到的二次谐波分量不仅与非线性参数有关，而且与线性声学参数也有很大的关系。本文给出了在密度和声速都已知的情况下，非线性参数的估计方法。高斯聚焦源的实现有望使样品非线性参数的可视化成为可能。

A sinusoidal sound wave emitted by a spherically focusing source distorts in waveforms with propagation toward the focus mainly due to the elastic nonlinearity of the medium, which can be described with the nonlinearity parameter. It was theoretically and experimentally proved that the largest part of the second harmonic component seen at the post focal region was generated in the focal region, because the secound harmonic components generated in the preand post-focal region cancel each other affected by the phase shift of the sound wave passing through the focus. Inserting a sample whose nonlinearity parameter differs from that of surrounding medium (water) into the focal region, the magnitude of the second harmonic component with the phase parameter of <pi>/2 radians seriously changes. This leads to the possibility to detect and visualize how the nonlinearity parameter of samples immersed in the focus differs from water. On the other hand, since the wavf propagation of a cylindrically focusing source does not suffer a similar phase shift the second harmonic component generated at eh focal region cannot be dominant in the post focal region. Therefore only the spherically focused sound may be appropriate for the present research project. Assuming a general case where the linear parameters (the ambient density, the sound speed, and the absorption coefficient) of the sample differed from those of surrounding medium as well as the nonlinearity parameter, the influence of the sample on the sound wave was theoretically and experimentally discussed. The result shows that the second harmonic component observed in the post focal region strongly depends on the linear acoustic parameters as well as on the nonlinerity parameter. However the method to estimate the nonlinearity parameter when both the density and sound speed are known has been shown. The realization of a gaussian focusing source will hoprfully make possible to visualize the nonlinerity parameter of the sample.