Improving Depth of Focus for Single Element Annular Transducers
Date Issued
2012
Date
2012
Author(s)
Huang, Kai-Han
Abstract
The hypothesis of this research is that frequency encoded aperture can be used to improve depth of focus (DOF). Frequency encoded aperture refers to the transducer design where different positions on the aperture have different frequency response. To test the hypothesis, a non-uniform thickness single element annular transducer (SEAT) is proposed. The sub-elements have different frequency response and thus can be controlled by different excitation signals. Because it is spatially separable, dynamic focusing can be achieved to some extent and thus DOF can possibly be improved. In this study, we design two types of coding signals which is up-frequency coding signal (frequency increased with time) and down-frequency coding signal (frequency decreased with time). When we choose the up-frequency coding signal, the curvature of delay profile increase and the focal depth becomes shallower. Also, when we choose the down-frequency coding signal, the delay profile decrease and the focal depth becomes deeper. With this method, SEAT can achieve dynamic focusing.
According to the simulation results, when the bandwidth of SEAT is around 80%, the DOF of SEAT can be improved by 83% with the up-frequency coding signal and 133% with the down-frequency coding signal. But, the focus point still remains un-change. The lateral resolution maintains at 100μm while using up and down-frequency coding signal. When the bandwidth of SEAT is reduced to 10%, the DOF can be improved around 280% and the focal point can be shifted by 0.6mm. The tradeoff is that the lateral beam width of SEAT is increased. Based on the experiment results, the DOF of an 80% bandwidth SEAT can be improved around 55% with the up-frequency coding signal and 64% with the down-frequency coding signal. The lateral resolution of SEAT still maintains at 100μm. In this study, we demonstrate the frequency encoded aperture improves the DOF of SEAT. However, dynamic focusing on SEAT can be only achieved when the bandwidth of SEAT is sufficiently low.
Subjects
SEAT
frequency encoded aperture
up-frequency coding signal
down-frequency coding signal
DOF
dynamic focusing
Type
thesis
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