Absorption-Modulated Crossed-Optical Fiber-Sensor Platform for Measurements in Liquid Environments and Flow Streams

A new evanescent-wave fiber sensor is described that utilizes absorption-modulated luminescence (AML) in combination with a crossed-fiber sensor platform. The luminescence signals of two crossed-fiber reference regions, placed on opposite sides of the stretch of fiber supporting the absorbance sensor, monitor the optical intensity in the fiber core. Evanescent absorption of the sensor reduces a portion of the excitation light and modulates the luminescence of the second reference region. The attenuation is determined from the luminescence intensity of both reference regions similar to the Beer-Lambert Law. The AML-Crossed-Fiber technique was demonstrated using the absorbance of the Zn(II)-PAN(2) complex at 555 nm. A linear response was obtained over a zinc(II) concentration range of 0 to 20 mu M (approximately 0 to 1.3 ppm). A nonlinear response was observed at higher zinc(II) concentrations and was attributed to depletion of higher-order modes in the fiber. This was corroborated by the measured induced repopulation of these modes.