Abstract
Recently we presented acousto-optic (AO) probing of scattering media using addition or subtraction of speckle patterns due to tandem nanosecond pulses. Here we present a theoretical framework for ideal (polarized, noise-free) speckle patterns with unity contrast that links ultrasound-induced optical phase modulation, the fraction of light that is tagged by ultrasound, speckle contrast, mean square difference of speckle patterns and the contrast of the summation of speckle patterns acquired at different ultrasound phases. We derive the important relations from basic assumptions and definitions, and then validate them with simulations. For ultrasound-generated phase modulation angles below 0.7 rad (assuming uniform modulation), we are now able to relate speckle pattern statistics to the acousto-optic phase modulation. Hence our theory allows quantifying speckle observations in terms of ultrasonically tagged fractions of light for near-unity-contrast speckle patterns.
Highlights
The difference between the speckle patterns is plotted in figure 3(C) while the blurred speckle pattern that is integrated over one ultrasound cycle is shown in figure 3(D)
We have theoretically explored the relationship between phase modulations of light and the resulting speckle pattern modulations
We explored the effect of phase modulations on the difference between speckle patterns for a π rad ultrasound difference, and on the contrast of the speckle pattern integrated over a complete ultrasound cycle
Summary
Both photoacoustics and acousto-optic tomography (Resink et al 2012) combine the use of light and sound in turbid media. The detected amount of ultrasonically tagged light is an important quantity. It would be ideal if both measurements can be done in the same setup, with the same laser system. Acousto-optic measurements use a quasi-CW (continuous-wave) laser so that a slow optical detector, often a CCD (charge-coupled device) camera, can integrate over multiple ultrasound cycles. A camera is used for its high number of optical detector elements so that acquiring information for a great number of speckles is possible, increasing the SNR (signal-to-noise ratio) (Leveque et al 1999)
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