Enhanced detection of acousto-photonic imaging signals using a photorefractive crystal based system

Abstract

Acousto-photonic imaging is a dual-wave sensing technique where diffuse coherent light wave interacts with a superimposed acoustic field. A phase-modulated light wave emanates from the interaction region and carries with it information about the local opto-mechanical properties of the insonated media. The coherent nature of the light produces a speckle field. The modulation of the speckle field is spatially incoherent, yielding a small modulation depth when collecting multiple speckles or extremely low light levels when detecting at the single speckle level. We report preliminary results from a new detection scheme where the scattered laser light is mixed with a reference beam in a photorefractive crystal. The crystal serves as a dynamic holographic medium and a photorefractive grating is formed from which the reference beam diffracts. The diffracted reference beam has the same spatial structure as the scattered light, and the two interfere at the photodetector where the phase modulation on the scattered beam is converted to an intensity modulation. Measurements of the signals are presented for gel phantoms seeded with suspended polystyrene beads. The results exhibit qualitative agreement with a simple theoretical model. [Work supported by the Center for Subsurface Sensing and Imaging Systems via NSF ERC Award No. EEC-9986821.]