Parallel detection of amplitude-modulated, ultrasound-modulated optical signals

We investigated the effect of amplitude-modulated (AM) ultrasound (US) on acousto-optic (AO) signals. A phantom was exposed to both AM US and a green laser, and CCD measurements of speckle contrast were made with various exposure times. The results show that the AO signal oscillates at the AM frequency when the CCD exposure time is a fraction of the AM period and stops oscillating when the CCD exposure time is a multiple of the AM period. The AO signal decreases quickly as the AM frequency increases or peak-peak (pk-pk) amplitude decreases. With 4ms exposure time, 250Hz AM frequency and 1.27MPa pk-pk acoustic pressure, there is an ~30% increase in the AO signal compared with that of CW US. The increase in the signal is likely to be due to the particle oscillation and the induced shear wave as a result of the radiation force generated by the AM US.