Rajagopal, Srinath; (2020) Laser-generated, plane-wave, broadband ultrasound sources for metrology. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The accurate quantification of ultrasound fields generated by diagnostic and therapeutic transducers is critical for patient safety. This requires hydrophones calibrated to a traceable national measurement standard over the full range of frequencies used. At present, the upper calibration frequency range available to the user community is limited to a frequency of 60 MHz. However, there is often content at frequencies higher than this, e.g., through nonlinear propagation of high-amplitude pulses or tone-bursts for therapeutic applications, and the increasing use of higher frequencies in diagnostic imaging. To reduce the uncertainties and extend the calibrations to higher frequencies, a source of high-pressure, plane-wave and broadband ultrasound fields is required. This is not possible with current piezoelectric transducer technology, therefore laser-generated ultrasound is investigated as an alternative. This consists of an ultrasound wave generated by the pulsed laser excitation of a thin, planar, layer of light absorbing carbon-polymer nanocomposite materials. The work described in this thesis can be divided into three parts. The first part consisted of the fabrication of various nanocomposites in order to study the effect of different polymer types, composite thickness, laser fluence, and concentration of carbon nanotubes, on the ultrasound generated, as well as their stability. This included an investigation into the nonlinear propagation of MPa range laser-generated ultrasound, and the effect of the bandlimited hydrophone response, using a numerical wave solver (k-Wave). In the second part, the effects on the signal of acoustically reflective and matched backings (the substrates onto which the nanocomposite was coated) were studied. It was found experimentally that the backing material can significantly affect the pressure amplitude when the duration of the laser pulse is longer than the acoustic transit time across the thin nanocomposite layer. An analytical model was developed to describe how the signal generated depends on the backing material, absorbing layer thickness, and laser pulse duration. The model agreed well with measurements performed with a variable pulse duration fibre-laser. Finally, in the third part, a laser-generated, plane-wave, broadband ultrasound source device superficially resembling a standard piezoelectric piston source was designed, fabricated, and tested. The source produced quasi-unipolar pressure-pulse of 9 MPa peak-positive pressure with a bandwidth of 100 MHz, and the ultrasound beam is sufficiently planar to reduce uncertainties due to diffraction to negligible levels for hydrophones up to 0.6 mm in diameter.

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