Abstract: The optical sensor of the present invention detects both the existence and concentration of substances by changing from light leakage mode to wave guide mode when the sensor is exposed to the substances to be detected. The changes in the mode results in a large change in optical output. This change is measured and the substance is identified and/or measured with high sensitivity. The change in light leakage mode to wave guide mode of the sensor is possible by having a clad material around the core material, with the clad material decreasing in the index of refraction when exposed to the substance to be detected. When the index of refraction of the clad becomes less than that of the core material, the sensor changes from the light leakage mode and operates in the wave guide mode. Changes in light intensity output from the sensor is measured over time, and correlated to the substance to be detected.