Imaging through Turbidity
Digital Phase Conjugation
In the context of optical imaging, the optical properties of biological tissue in the visible (400-700 nm) and near IR (700-1500 nm) regions are the most relevant. Most modern tissue optical imaging techniques use the optical window from 650 nm to 1300 nm where optical absorption is minimal so that light can penetrate relatively deep into tissue. In this range, absorption coefficient , the distance rate at propagating beam intensity is extinguished by absorption, varies between 0.1 and ~1 cm-1, so light can travel centimeters in depth with little attenuation . However in this optical window, scattering is much stronger in biological tissue, mostly due to scatterers whose sizes are comparable with the wavelength of light. The scattering coefficient, , describes the distance at which the illumination undergoes scattering events; it generally decreases at longer wavelength. For describing light diffusion, the reduced scattering coefficient is defined by , where is the anisotropy factor, the expected cosine of the scattering angle, generally equal to about 0.9 for biological media. The parameter represents the rate at which the light undergoes “isotropic” scattering events, or direction randomization. In the visible to Near IR region, with typically equal to about 100 cm-1, is approximately 10 cm-1 .
In the recent past, we have demonstrated that light scattering in biological tissue can be reversed using optical phase conjugation (OPC) . The phenomenon of OPC has been known for almost 40 years and has since been used in numerous laser-related applications. However, its use in suppressing turbidity in biomedical applications had remained largely unexplored. OPC overcomes light scattering in turbid or diffusing media without directly measuring the transmission matrix [2-5]. Instead, it relies upon a double-pass transmission of illumination through the sample. If a time-reversed reproduction of the full scattered wavefield (that is, ) is transmitted through the turbid medium, then the resulting output is , since , the scattering matrix is both symmetric and unitary (provided the absorption is negligible). That is, the scattering events are completely reversed, yielding a reproduction (up to complex conjugation) of the original illumination pattern.
In this project, we are developing electronically-controllable dynamic optical phase conjugation methods for turbidity suppression in biological samples. The goal is to accomplish near identical performance in image reconstruction as achieved in the photorefractive crystal based OPC setup [2, 4-5], but at appreciably fast rate.
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