Phase and amplitude modulation with acoustic holograms

Michael D. Brown

doi:10.1063/1.5110673

Abstract

Acoustic holograms are a low cost method for generating arbitrary diffraction limited pressure distributions in 3 dimensions. However, at present, the creation of complex fields using this approach is limited by the inability of these holograms to independently modulate both the phase and amplitude of an incident wave. In this work, it is shown that this limitation can be circumvented by using two phase holograms, designed using an iterative Fourier transform algorithm, to form the phase conjugate of a back-propagated target pattern over a predefined surface. An experimental test-case, designed to generate the letters “UCL” with the uniform amplitude and phase, is prepared to demonstrate the feasibility of this technique. Field measurements from this sample show that the modulation of both the phase and amplitude of the acoustic field can be achieved with this approach.

Highlights

A range of algorithms have been described which can be used to calculate phase holograms that generate fields with the desired amplitude distribution.10 these typically allow the phase over the target pattern to vary freely
It is shown that this limitation can be circumvented by using two phase holograms, designed using an iterative Fourier transform algorithm, to form the phase conjugate of a back-propagated target pattern over a predefined surface
An in-line geometry attaching the first hologram directly to the planar transducer and modulating the amplitude over a parallel plane containing the second hologram which adjusts the phase on transmission is simple to register and implement (Fig. 1(a) in the supplementary material and Fig. 1)

Summary

Introduction

A range of algorithms have been described which can be used to calculate phase holograms that generate fields with the desired amplitude distribution.10 these typically allow the phase over the target pattern to vary freely. At present, the creation of complex fields using this approach is limited by the inability of these holograms to independently modulate both the phase and amplitude of an incident wave.

Objectives

Results

Conclusion