Evaluation of ultrasonic transducer response and structural integrity using coded photoacoustic imaging

A novel technique for ultrasonic transducer photoacoustics imaging is presented. The technique is based on the analysis of the photoacoustic response of the light-absorbing layer placed on the transducer surface or, if the transducer layer or piezoelement absorbs light, directly on transducer surface. The focal spot of the pulsed laser is focused on the transducer surface, and the voltage generated on the output is used as a sensitivity metric at that location. Scanning in the xy plane produces a 2D sensitivity map. Use of pulsed laser diodes enables spread spectrum excitation which improves SNR and immunity to electromagnetic interference. A broadband photoacoustic response can be obtained with bandwidth limited by the antialiasing filter to 0.3-30 MHz. Experimentally evaluated lateral resolution for 50 ns pulse excitation was 148 µm or 203 µm depending on beam orientation. For 5-30 MHz chirp excitation after inverse filter PuC plus 0.2-25 MHz window resolution was 73 µm or 83 µm depending on beam orientation. The axial resolution in case of chirp excitation and pulse compression using inverse filter is down to 20 ns, which corresponds to 46 µm in PZT-5 ceramics or about 20 µm in PVDF or PVC. Signal decomposition into pulse positions allows for measuring the acoustic thickness of the transducer layers or resonant frequency of piezoelement. Capabilities are demonstrated on PZT transducers with a λ/4 matching layer and on PVDF strips array transducers.