Lee Taehwa, Li Qiaochu, Guo L Jay
Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI48109, USA.
Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI48109, USA.
Sci Rep. 2016 Feb 12;6:21511. doi: 10.1038/srep21511.
Waves of any kinds, including sound waves and light waves, can interfere constructively or destructively when they are overlapped, allowing for myriad applications. However, unlike continuous waves of a single frequency, interference of photoacoustic pulses is often overlooked because of their broadband characteristics and short pulse durations. Here, we study cancellation of two symmetric photoacoustic pulses radiated in the opposite direction from the same photoacoustic sources near a free surface. The cancellation occurs when one of the two pulses is reflected with polarity reversal from the free surface and catches up with the other. The cancellation effect, responsible for reduced signal amplitudes, is systematically examined by implementing a thin transparent matching medium of the same acoustic impedance. By changing the thickness of the transparent layer, the overlap of the two symmetric pulses is controlled. For optimized matching layers, the cancellation effect can be significantly reduced, while the resulting output waveform remains unchanged. Similar to the planar absorber, different dimensional absorbers including cylinders and spheres also exhibit the cancellation between the outward and inward waves. This work could provide further understanding of photoacoustic generation and a simple strategy for increasing photoacoustic signal amplitudes.
任何类型的波，包括声波和光波，在重叠时都可能发生相长干涉或相消干涉，从而有无数的应用。然而，与单一频率的连续波不同，光声脉冲的干涉由于其宽带特性和短脉冲持续时间而常常被忽视。在这里，我们研究了在自由表面附近从同一光声源沿相反方向辐射的两个对称光声脉冲的相消情况。当两个脉冲中的一个从自由表面反射并极性反转后赶上另一个脉冲时，就会发生相消。通过使用具有相同声阻抗的薄透明匹配介质，系统地研究了导致信号幅度降低的相消效应。通过改变透明层的厚度，可以控制两个对称脉冲的重叠。对于优化的匹配层，相消效应可以显著降低，而产生的输出波形保持不变。与平面吸收体类似，包括圆柱体和球体在内的不同尺寸的吸收体也表现出向外波和向内波之间的相消。这项工作可以进一步加深对光声产生的理解，并提供一种增加光声信号幅度的简单策略。
