Thin Fresnel zone plate lenses for underwater acoustics: Modeling and experiments

In situations where size, weight, and system complexity are important, Fresnel zone plate (FZP) lenses have potential advantages in acoustic systems. Numerical modeling and laboratory characterization of focusing by a Soret type FZP lens are presented in this study. Both rubber foam and resonant air cavity designs are considered for the opaque zones. This study extends previous work by examining a wider range of incident field angles. Measurements of a higher-order focus, more narrow than the primary focus but with reduced gain, are also presented that compare favorably with modeling predictions. Comparison is also made between standard diffraction theory predictions (Fourier acoustics) and finite-element, oblique wave scattering simulations for the axisymmetric FZP geometry. The latter accounts for edge effects and acoustic penetration through the lens material, and predicts a higher focal gain for oblique plane wave insonification. Finally, the directivity of an FZP lens and hydrophone combination is compared with that of an ideal pressure sensitive disk of equivalent diameter to assess gain vs. side-lobe tradeoffs. Calculations demonstrate 10 dB reduction of the first sidelobe level using the FZP with a 15% main lobe broadening.