Reverse confocal polarized Raman spectroscopy (RCPRS) for tissue analysis

Confocal Raman spectroscopy (CRS) being one of the most widely used depth-sensitive techniques for measuring layer wise Raman characteristics of layered biological tissues faces two practical problems. First, the overall probing depth is limited in a given optical design of the CRS system and second, the objective lens used for focusing touches to surface of the target sample during probing of deeper sub-surface layers. To facilitate deeper probing in a given CRS system and to avoid lens-sample touching issue, an alternate scheme of depth-sensitive Raman measurement is presented. The scheme, named reverse confocal polarized Raman spectroscopy (RCPRS), uses an experimental arrangement of plane polarized illumination and orthogonal polarized detection in which depth-sensitive measurements are performed by moving the focal plane of the illumination beam away from the tissue surface unlike to CRS which obtains depth-separation by traversing across different depths of the target tissue. The performance of the RCPRS is evaluated using a non-biological phantom and a biological tissue. It is found that the introduction of polarization reduces the interference of the signals originating from the layers surrounding the target layer and thereby improving the depth-selectivity.