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This Small Business Innovation Research (SBIR) Phase I project proposes an optical fiber sensor to determine concentrations of chloride ions in composite concrete structures such as buildings and bridges. Maintaining the integrity of this nation's infrastructure is a major challenge, and the incursion of chloride ions into concrete is a major cause of structure failure in the U.S. To solve this problem, Intelligent Optical Systems (IOS) will develop an innovative distributed intrinsic fiber optic sensor (DIFOS) that will consist of an optical fiber coated with a cladding that contains an indicator sensitive to chloride ions. The cladding changes its spectral properties in the presence of chloride, causing the fiber's optical properties to reflect chloride ion concentration. The sensor will detect chloride along its entire length, reliably indicating when structures need repair or replacement. Embedded or retrofit in a concrete structure, the sensor will detect the incursion of chloride ions and provide real-time monitoring of chloride diffusion within the structure. This sensor can be used to prevent structure degradation and potential catastrophic failures. The proposed system will be a valuable tool for safeguarding concrete structures. It will be used by civil engineering and construction companies. This sensor can be embedded into new structures during fabrication and into old structures that are being repaired. In addition, the concept of an optical fiber cladding that is sensitive to different analytes can be extended to smart structures such as aircraft, ships, and buildings to increase the safety of these structures and to decrease maintenance and inspection costs.