Time-reversal Wavefront Engineering in Biological Media
In the optical domain, elastic scattering typically exceeds absorption by an order of magnitude or more. Thus, light scattering is predominately attributed to turbidity in biological tissues. The output light field of a laser beam incident on a very turbid medium looks like a grainy pattern (Figure 1). Although the output field looks completely random, it contains valuable information about the medium it has just passed through. This is because scattering can be thought of as the modulation of the incident light by a turbid medium.
Figure 1: A random speckle field
It is known that this process of 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 (Figure 2). Although it is not possible to record and play back the entire wavefront modulated by a non-trivial scattering object such as biological tissue, our group has shown that it is still possible to return the original input light field by OPC .
Figure 2: Schematic illustrating the principle of the optical phase conjugation
Our research efforts, thereafter, have been threefold:
1. Understanding the TSOPC phenomenon
In order to develop robust biophotonic tools using TSOPC, we have been and are continuing to devote efforts towards the understanding of this phenomenon and its limits in biological tissues [2-4].
Figure 3. We have experimentally reversed scattered light
a rabbit's ear by OPC.
Figure 4: We further characterized the OPC phenomena in tissue phantoms and
chicken breast tissues. We also observed that scattering effectively
increases the numerical aperture of the phase conjugate mirror, decreasing
the spot size of the reconstructed spot.
2. Developing and refining novel, robust phase conjugate mirrors (PCM)
Traditionally, there are several ways for generating an OPC field – four wave mixing (FWM), holography, and photorefraction, all of which utilize photorefractive crystals. While these crystals are usually easy to use, their main drawback is their low reflectivity, resulting in a low OPC efficiency. This is especially limiting in applications with thick biological tissues where the highly scattered wavefront cannot be efficiently collected by the limited numerical aperture of the PCM. To address these problems, our group has recently developed an optoelectronic PCM. We term this method “Digital optical phase conjugation” (DOPC). In this setup, a digital camera in a phase-shifting holography setup measures the scattered wavefront. A spatial light modulator (SLM) is aligned such that each pixel corresponds to its respective pixel on the digital camera and plays back the OPC wavefront . We continue to seek refinement (e.g. speed, efficiency, ease of setup) for this method and development of other novel methods for tissue turbidity suppression.
Figure 5: Experimental scheme of the DOPC system: SLM = spatial light modulator;
EO = electro-optic modulator
3. Utilizing TSOPC in biomedical applications
Most of our current projects aim to engineer modalities that can experimentally prove interesting TSOPC related phenomena with the end goal of applying them in a variety of biomedical applications such as photodynamic therapy, high resolution microscopy in turbid tissues, and deep tissue focusing, etc.
1. Yaqoob, Z., et al., Optical phase conjugation for turbidity suppression in biological samples. Nature Photonics, 2008. 2(2): p. 110-115.
2. McDowell, E.J., et al., Turbidity suppression from the ballistic to the diffusive regime in biological tissues using optical phase conjugation. Journal of Biomedical Optics, 2010. 15(2).
3. Cui, M., E.J. McDowell, and C.H. Yang, Observation of polarization-gate based reconstruction quality improvement during the process of turbidity suppression by optical phase conjugation. Applied Physics Letters, 2009. 95(12).
4. Cui, M., E.J. McDowell, and C.H. Yang, An in vivo study of turbidity suppression
by optical phase conjugation (TSOPC) on rabbit ear. Optics Express,
2010. 18(1): p. 25-30.
5. Cui, M. and C.H. Yang, Implementation of a digital optical phase conjugation system and its application to study the robustness of turbidity suppression by phase conjugation. Optics Express. 18(4): p. 3444-3455.