Optical phase conjugation by dynamic holography for wavefront restoration in turbid media

Optical Phase Conjugation is a non-linear optical phenomenon that generates a phase conjugate replica of an incident beam. It has been widely used to suppress the effects of aberrations in optical systems such as resonators or imagetransmitting optical fibers. In this work, the possibility of using optical phase conjugation as a means of suppressing the effect of scattering in turbid media is analyzed, with the final aim to apply it to biological tissues. Firstly, light propagation through a slab representing a turbid sample was calculated by solving Maxwell's equations with the Finite-Difference Time-Domain method, in order to preserve all the information about the phase and coherence of the wavefront. The non-linear process that takes place within the phase conjugation mirror is described by coupledwave theory. A set of simulations was performed, and the results confirm the feasibility of using this effect to compensate the effect of scattering in turbid media. Subsequently, an experimental set-up was performed. In order to obtain a phase conjugation mirror, degenerate fourwave mixing was achieved by a real-time volume holography configuration. The pulsed laser source was a Nd3+:YAG laser at its second-harmonic (532nm). An ethanol solution of Rhodamine 6G was used as a non-linear medium. A lipidbased scattering sample was obtained by a solution of homogenized milk and distilled water, which provided us with an appropriate tissue phantom. The experimental results demonstrate scattering suppression, and constitute some preliminary measurements of an effect with a promising potential for a wide range of applications.This work has been partially carried out under the project TEC2006-06548/TCM of the Spanish Ministry of Science and Innovation