Measurement of the spectral directivity of optoacoustic and ultrasonic transducers with a laser ultrasonic source

Abstract

Comprehensive characterization of wideband ultrasonic transducers and specifically optoacoustic detectors is achieved through the analysis of their frequency response as a function of the incident angle. The tests are performed under well-defined, repeatable operating conditions. Backillumination of a blackened, acoustically matched planar surface with a short laser pulse creates an acoustic impulse which is used as a wideband ultrasonic source. Upon illumination with a short laser pulse, the bandwidth of our source shows a -6 dB point of 12 MHz and a low-frequency roll-off around 300 kHz. Using proprietary software, we examine thoroughly the planarity of the emitted wave front within a specified amplitude cutoff and phase incoherence. Analysis of the angular dependence of the frequency response yields invaluable directivity information about the detector under study: a necessary component toward accurate optoacoustic image reconstruction and quantitative tomography. The laser ultrasonic source we developed is the main feature of our directivity measurement setup. Due to its simplicity, it can easily be adapted to various calibration devices. This paper focuses on the development and characterization of the flatness and the bandwidth of our wideband ultrasonic source.

Figure 1

Optoacoustic delta-pulse source. In the first part (a), the laser light travels in air through expanding optics. Then, in the second half of the source (b), light travels through Rexolite until it converts into sound on a highly absorbing layer.

Figure 2

Optoacoustic delta-pulse source impulse measured with GL 0200, amplified 7.1 times.

Figure 3

Frequency content of the optoacoustic delta-pulse source impulse. The −6 dB point occurs around 12 MHz.

Figure 4

Planarity of the acoustic wave front 0.5 mm from the source. The contour indicates the area inside of which the amplitude of the signal is within a 6 dB window. The phase incoherence is limited to 15% at 10 MHz.

Figure 5

Planarity of the acoustic wave front 50 mm from the source. The contour indicates the area inside of which the amplitude of the signal is within a 6 dB window. The phase incoherence is limited to 15% at 10 MHz.

Figure 6

Schematics of the directivity setup. The whole assembly is placed on top of a water tank in a way to fully immerse the detector. The mounting block is designed to allow positioning of a variety of detectors exactly onto the axis of rotation. The source is then held in place, also under water, centered onto the sensitive area of the transducer. The detector can then be rotated and the angle of incidence, with respect to the face of the source, is read on the protractor.

Figure 7

Directivity data for our GL 0200 detector. The source’s intrinsic response was deconvolved from the measured impulse, and the curves were normalized individually to emphasize the frequency roll-off.