A new approach to the detection of opto-acoustic signals with the use of single-scattered optical fields in low scattering media is presented with the ultimate goal of performing opto-acoustic confocal microscopy in human skin. The method is based on the generation of heterodyne optical signals using opto-acoustic generated signals as a source of particle displacement. A focused, pulsed laser is used to generate an opto-acoustic signal and the particle displacement detection is performed with a coherent confocal microscope. The goal is to improve the depth of imaging obtained by confocal microscopy while maintaining its resolution. The use of optical coherent detection of teh Doppler shifts generated by teh ultrasound filed positioned at the optical focus gives us information about the amplitude and phase of the the scattered optical field. The results of interferometric measurements at different depths into scattering phantoms are presented.
Acousto-photonic imaging (API) is a dual-wave sensing technique in which a diffusive photon wave in a turbid medium interacts with an imposed acoustic field that drives scatterers to coherent periodic motion. A phase-modulated photon field emanates from the interaction region and carries with it information about the local opto-mechanical properties of the insonated media. A technological barrier to API has been sensitivity - the flux of phase-modulated photons is very small and the incoherence of the resulting speckle pattern reduces the modulation of the scattered light leading to low sensitivity. We report preliminary results from a new detection scheme in which a photorefractive crystal is used to mix the diffusively scattered laser light with a reference beam. The crystal serves as a dynamic holographic medium where the signal beam interferes with the reference beam, creating a photorefractive grating from which beams diffract. In addition, the phase modulation is converted to an amplitude modulation so that the API signal can be detected. Measurements of the API signal are presented for gel phantoms with polystyrene beads used as scatterers, showing a qualitative agreement with a simple theoretical model developed.