Temperature Effects on a Fiber-Optic Evanescent Wave Absorption Sensor

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Abstract
A coiled fiber-optic chemical sensor has proven to be effective for the remote detection of volatile organic compounds, such as trichloroethylene (TCE), 1,1-dichloroethylene (DCE), and gasoline, in aqueous solutions. The analyte diffuses into the hydrophobic cladding and evanescent wave absorption spectra are measured in the near-infrared (1600–1850 nm) without the presence of the water absorption bands. In order for fiberoptic chemical sensors to operate effectively in remote environments, the influence of temperature on the sensor response must be known. The C-H bonds of the polysiloxane cladding material also have absorption bands in the near-infrared (NIR). Changes in temperature will change the density (i.e., concentration of C-H bonds) and refractive index of the cladding. Due to these effects, a temperature change of only 3°C from the reference has been shown to significantly alter the background absorbance. The temperature-dependent background absorption is found to be linear with the slope, and the values are proportional to the absorption coefficient of the cladding material. The intercept of the absorbance vs. temperature plot is found to follow the first derivative of the fiber sensor transmission spectrum. Evanescent wave absorption spectra of TCE solutions have been corrected for temperature.