Abstract
In this work, the use of binary amplitude holography is investigated as a mechanism to focus broadband acoustic pulses generated by high peak-power pulsed lasers. Two algorithms are described for the calculation of the binary holograms; one using ray-tracing, and one using an optimization based on direct binary search. It is shown using numerical simulations that when a binary amplitude hologram is excited by a train of laser pulses at its design frequency, the acoustic field can be focused at a pre-determined distribution of points, including single and multiple focal points, and line and square foci. The numerical results are validated by acoustic field measurements from binary amplitude holograms, excited by a high peak-power laser.
Highlights
The photoacoustic effect occurs when a time varying optical source, which can be coherent or incoherent, is incident on an optically absorbing material
It is shown using numerical simulations that when a binary amplitude hologram is excited by a train of laser pulses at its design frequency, the acoustic field can be focused at a pre-determined distribution of points, including single and multiple focal points, and line and square foci
The acoustic fields generated by binary amplitude holograms when excited by a set of laser pulses spaced at their design frequency were simulated using the k-Wave toolbox
Summary
The photoacoustic effect occurs when a time varying optical source, which can be coherent or incoherent, is incident on an optically absorbing material. The incident photons are absorbed and converted to heat by non-radiative de-excitation This causes a small temperature rise resulting in local pressure increase which generates an acoustic pulse.. If the optical intensity is modulated sinusoidally, the resulting acoustic spectrum has a single peak at the modulation frequency.17,18 Both spatial and temporal modulation have been combined in several works. Spectral control of the acoustic field can be achieved by the use of multiple pulses applied with a high PRF (MHz) This could enable optically generated ultrasound to become a practical modality for 2-D ultrasound arrays. The properties of the acoustic fields that can be generated by the use of pulsed lasers and binary amplitude holograms are explored in detail. The third section validates the numerical model and demonstrates that this approach can be applied experimentally with the use of patterned optical absorbers and a Q-switch laser
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