Molecular species sensitive optical coherence tomography using coherent anti-stokes Raman scattering spectroscopy

We present our progress in developing a novel technique and instrument that images specific molecular species in biological tissues using Optical Coherence Tomography (OCT). Standard OCT instruments measure only the scattering from structural features, such as refractive index changes. We utilize Coherent Anti-Stokes Raman Scattering (CARS) Spectroscopy, a nonlinear optics technique that can selectively stimulate molecular groups, to gather compositional information from the sample. Being a coherent process, our instrument will produce interference between the nonlinear anti-Stokes signal produced in the sample and a reference molecular sample to both exclude background and nonresonant signals and range features in the tissue. Because of this, we will also gain the benefits of sensitivity that interferometry can provide. By utilizing the tunability of an optical parametric oscillator, we can address a range of molecular resonances from 1500 cm^-1 to 3500 cm^-1. This frequency range offers the possibility of measuring the distributions and densities of proteins, lipids, and nuclear material that we believe will be useful for determining the early presence of epithelial carcinomas. We demonstrate the principle of this imaging method by producing interference between two separately produced CARS signals from the same probe and Stokes beams.