Characterisation of laser-ultrasound signals from an optical absorption layer within a transparent fluid

Characteristics of laser-ultrasound signals are presented from photoacoustic interaction with a layered, optically absorbing medium surrounded by a transparent fluid. A thermoelastic model is presented describing the interaction, with signal predictions in the fluid arising from polymer transducer detection. By taking the optical absorption coefficient and finite layer thickness into account, the amplitude and shape of transient elastic waves are calculated for both forward and backward travelling directions. Additionally, wave interaction with the PVDF transducer has been characterised using a discrete-time algorithm for the transducer response. With just three constants to define transducer response characteristics, the response function may be used to predict voltage signals. Good agreement with experimental waveforms is demonstrated, so that the response function may form the basis of system modelling when miniature laser-ultrasound probes are used in various applications.