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.
