Research

Turbidity Suppression by Optical Phase Conjugation (TSOPC) in Biological Media

Elastic optical scattering in biological tissues typically dominates over absorption by an order of magnitude or more. Being the dominant light-matter interaction process, scattering prevents tissue from being transparent, and scattered light is generally regarded as poor in imaging information. This is due to severe deterioration of the incident light field, caused by disordered amplitude and phase modulation of its wavefront as it propagates through the tissue.

It is known that elastic optical scattering is a deterministic and time reversible process. In other words, if we can record the phase and amplitude of the propagating scattered light field completely and reproduce a back-propagating optical phase conjugate (OPC) field, this field should be able to retrace its trajectory through the scattering medium and return the original input light field (Fig. 1).

Figure 1: Schematic illustrating the principle of the optical phase conjugation.

Optical phase conjugation refers to a phenomenon by which a light field can be made to back propagate. A light field ‘reflects’ from a phase conjugate mirror (PCM) in such a way that the spatial amplitude variations are preserved but the signs of the phase variations are reversed. There are several ways for generating OPC field – four wave mixing (FWM), holography, and photorefraction. Holography and photorefraction are additionally interesting because they allow the original light field to be recorded and an OPC copy to be played back at a later time (Fig. 2). FWM is advantageous for real-time OPC.

Figure 2: Recording and playback of the light wavefront passing through tissue.

Our current research in this direction is focused on understanding this novel phenomenon and its limits, and more importantly, developing a robust biophotonic tool based on TSOPC. We aim at using this method for a variety of biomedical applications, such as tissue density heterogeneity determination, photodynamic therapy, etc.

Figure 3. This set of experimental data illustrates the feasibility of TSOPC.