The overall objective of the proposed research is to develop a low cost Polarization-Sensitive Optical Coherence tomography (PS-OCT) system for the early detection, diagnosis and monitoring of dental caries. New, more sophisticated diagnostic tools are needed so that if such carious lesions are detected early enough, they can be arrested/reversed by non-surgical means through fluoride therapy, anti-bacterial therapy, dietary changes, or by low intensity laser irradiation. Recent studies indicate that polarization sensitivity is necessary in order to quantify the severity of the caries lesions in occlusal and inter-proximal surfaces. Current PS-OCT designs are prohibitively expensive to manufacture and do not satisfy clinical use cost objectives. This Phase I proposal outlines a new and innovative PS-OCT design approach which shows substantial promise at meeting such cost objectives. It replaces the traditional Michelson design with an auto-correlator adapted with a novel polarization switching probe. The objectives of Phase I will be achieved through the following two specific aims. Aim 1 is the development and fabrication of a prototype PS-OCT system embodying the proposed design approach. Aim 2 is to demonstrate through in-vitro imaging with extracted human teeth that the prototype system has the requisite performance for monitoring the progression of tooth decay. A successful Phase I will lead to a Phase II effort which will focus on the design and development of a PS-OCT instrument and hand held probe suitable for in-vivo clinical evaluations. The Phase II objectives are: 1) The design provide high resolution imaging of incipient caries; 2) It can be transitioned to a manufacturing process meeting the cost objectives of market-wide acceptance in the dental field; and 3) The design is suitable to our already selected Phase III partner. Other commercial applications include medical imaging of oral cancer and pre-cancers, bi-refringent tissues and non-destructive evaluation of materials, laminations and composite structures.