Thin Fresnel zone plate lenses for focusing underwater sound
Thin Fresnel zone plate lenses for focusing underwater sound
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DOI：
10.1063/1.4926607
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发表时间：
2015-07-06
影响因子：
4
通讯作者：
Layman, Christopher N.
中科院分区：
文献类型：
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作者：
Calvo, David C.;Thangawng, Abel L.;Layman, Christopher N.
索雷特型菲涅尔波带片(FZP)透镜通过不透明屏幕中穿过开放环形区域的波的建设性干涉来产生焦点。对于频率低于MHz的水下声音，使用高阻抗、致密材料阻挡声音的大型FZP将具有实际劣势。我们从实验和数值上研究了一种替代方法，即在薄层(0.1lambda)透明橡胶衬底上粘贴软硅橡胶泡沫塑料制成的薄层(0.4lambda)声学不透明带。文中还提出了一种具有较高增益的超薄(0.0068 nm)FZP，并对其进行了模拟，它使用低体积分数的气泡状谐振空气环腔来构造不透明区域。在200 kHz下的实验室测量表明，橡胶泡沫可以准确地模拟为声阻抗约为水的1/10的有损流体。测量的聚焦增益高达20分贝，与正常入射和倾斜入射时的理论预测一致。对于低像差透镜，测量的焦半径为0.68lambda(峰零比)，与瑞利衍射极限预测值0.61lambda/na(NA=0.88)一致。
A Fresnel zone plate (FZP) lens of the Soret type creates a focus by constructive interference of waves diffracted through open annular zones in an opaque screen. For underwater sound below MHz frequencies, a large FZP that blocks sound using high-impedance, dense materials would have practical disadvantages. We experimentally and numerically investigate an alternative approach of creating a FZP with thin (0.4 lambda) acoustically opaque zones made of soft silicone rubber foam attached to a thin (0.1 lambda) transparent rubber substrate. An ultra-thin (0.0068 lambda) FZP that achieves higher gain is also proposed and simulated which uses low-volume fraction, bubble-like resonant air ring cavities to construct opaque zones. Laboratory measurements at 200 kHz indicate that the rubber foam can be accurately modeled as a lossy fluid with an acoustic impedance approximately 1/10 that of water. Measured focal gains up to 20 dB agree with theoretical predictions for normal and oblique incidence. The measured focal radius of 0.68 lambda (peak-to-null) agrees with the Rayleigh diffraction limit prediction of 0.61 lambda/NA (NA = 0.88) for a low-aberration lens.