Design of Acoustical Bessel-Like Beam Formation by a Pupil Masked Soret Zone Plate Lens

Abstract

The image performance of acoustic and ultrasound sensors depends on several fundamental parameters such as depth of focus or lateral resolution. There are currently two different types of acoustic diffractive lenses: those that form a diffraction-limited spot with a shallow depth of focus (zone plates) and lenses that form an extended focus (quasi-Bessel beams). In this paper, we investigate a pupil-masked Soret zone plate, which allows the tunability of a normalized angular spectrum. It is shown that the depth of focus and the lateral resolution can be modified, without changing the lens structure, by choosing the size of the pupil mask. This effect is based on the transformation of spherically-converging waves into quasi-conical waves, due to the apodization of the central part of the zone plate. The theoretical analysis is verified with both numerical simulations and experimental measurements. A Soret zone plate immersed in water with D/2F = 2.5 and F = 4.5λ changes its depth of focus from 2.84λ to 5.9λ and the lateral resolution increases from 0.81λ to 0.64λ at a frequency of 250 kHz, by modifying the pupil mask dimensions of the Soret zone plate.

Analysis of Predistortion Techniques on Fresnel Zone Plates in Ultrasound Applications

“…The combination of the FZP lens and the pupil results in a very similar concept as the predistortion technique for the inner regions in the case of maximum predistortion ( ). The experimental increase in the focal depth (FLHM) reported in [ 33 ] is around , while the FWHM slightly decreases, as predicted in our simulations. We have carried out the corresponding simulation with our predistortion technique using the same lens parameters ( , , mm) and operating wavelength ( kHz) as those used in the experiment.…”

“…Both values are equal, which results in the disappearance of the third transparent Fresnel region, while the third opaque Fresnel region ends at the same radius where the fourth opaque Fresnel region begins without any discontinuity, becoming a larger opaque region. A very similar structure involving large inner opaque areas was previously proposed in [ 33 ]. In this work, the FZP was modified by placing a pupil device in front of the FZP lens.…”

“…The difference between our simulation result and that of the experiment can be due to several factors. In our opinion, the signal to noise ratio of the measurement presented in reference [ 33 ] is not very high, and this could have led to some measurement errors. The lateral resolution is slightly enhanced as the FWHM becomes smaller when predistortion is applied.…”

“…In order to experimentally validate the analytical model and the simulations shown in this paper, we rely on some previous work from our research group ([ 33 ]). This reference analyzes the use of a pupil in front of an FZP lens.…”

Optimization of sapphire capillary needles for interstitial and percutaneous laser medicine

“…5. One can observe that the large angle leads to forwardpropagated laser light; moreover, such pointed needle could provide some kind of a Bessel beam of a first order with its benefits of low-diffraction caustic; [55][56][57] in turn the small angle might lead to the extreme light confinement and, thus, to a specific localization of tissue exposure. It is interesting that the needle with l ¼ 2.0 mm combines diffused and forward components simultaneously.…”