High resolution optical low coherence reflectometry in biological tissues
Optical low coherence reflectometry (OLCR) is a technique that permits to measure the absolute position and to determine optical properties. It is based on an interferometric method that uses a probe light characterized by a broad spectrum. Small volumes located at the inside of the sample can be studied individually and with a high sensitivity. Transmission of part of the light in the sample is the only requirement. Technological improvements that happened in the last decade allowed an increase of the performances of OLCR. This technique has a large potential in numerous fields of application, for example for the characterization of optical waveguides. In this work is presented the application of OLCR to biological tissues. The final goal of this study is the use OLCR as a medical imaging technique or as clinical diagnosis tool. Simultaneously, the light - biological tissue interaction is investigated for a better understanding. The principle and the limits of this technique are first exposed. The performances characteristic of a reflectometer are mainly described in term of dynamic range and of spatial resolution. The propagation of partially temporal coherent light in complex media and the resulting OLCR signal are described. The instrumental specificity involved in OLCR are then globally discussed. A more precise description is given for the reflectometers with which the results presented here were obtained. These reflectometers were built in the laboratories where the measurements are performed. Highly diffusive biological tissues were investigated with OLCR. This field of application of OLCR was totally unexplored when we started with these measurements. Two different reflectometers were used. One is characterized by a high dynamic range, limited by the quantum behavior inherent to any light detection. The other is characterized by a high spatial resolution, that allows the discrimination of structural details of very low dimension. This spatial resolution is the highest published for OLCR up to now. Results obtained with these performances allows one evaluate the potential for the application of OLCR in the biomedical field.