A new acoustical imaging by means of frequency-differential hologram matrix

在传统的多频全息术中，发射频率为f的连续载波，测量接收信号的相位和幅度，得到与发射和接收换能器阵列相关的频率响应H(F)。然而，作为连续波声纳的一种新方案，我们引入了频差全息矩阵D(F)的概念。该方案可以补偿传播损耗，从而增强来自远区目标的微弱信号。为了测量FDHM，我们发送一个正弦信号调制的载波，然后使用正弦调制信号对接收信号进行相位检测和乘积检测。由发射机和接收机组成计算机控制的FDHM测量系统。然后，我们在256个不同的载波频率下对FDHM进行了测量，并对数据进行傅立叶变换，得到了目标的一维图像。实验结果表明，该方法消除了发射机和接收机之间的耦合效应，比频率响应法的结果提高了约15dB信噪比，补偿了远距离目标微弱信号的传播损耗。然后，针对基于线阵的FDHM二维成像系统进行了计算机仿真。结果表明，用32个换能器单元和32个载波频率组成的阵列测得的FDHM可以清楚地显示目标的图像。我们还建议使用二阶FDHM，因为它给出了具有均匀强度的目标的图像。

In the conventional multifrequency holography, a continuous carrier with frequency f is transmitted and the phase and amplitude of the received signal is measured to get the frequency response H(f) associated with transmitting and receiving transducer arrays. As a new scheme of CW sonar, however, we have introduced the concept of the FDHM ( frequency-differential hologram matrix) D(f). The scheme makes it possible to compensate the propagation loss, so that a weak signal from a far-region target can be enhanced.To measure the FDHM, we transmit a carrier frequency-modulated by a sinusoidal signal, and the received signal is first phase-detected and then product-detected using the sinusoidal modulation signal. Making up such transmitter and receiver, We constructed a computercontrolled system for measuring the FDHM. We then measured the FDHM at 256 different carrier frequency and the data was Fouriertransformed to get a one-dimensional image of a target. As a result, several merits of the method were experimentally proved, such as canceling of unwanted effect due to coupling between transmitter and receiver, improvement in signal to noise level by about 15 dB, compared with the result by the frequency response method, and the compensation of the propagation loss by which a weak signal from a target in the far region can be enhanced.Next, for designing a two-dimensional imaging system by means of the FDHM with respect to a linear array, a computer simulation has been carried out. It was shown that the FDHM measured with an array with 32 transducer elements and 32 carier frequencies gives a clear image of a target. We also propose to use the secon-order FDHM, because it gives an image of targets with uniform intensities.