Optical evanescent field absorption sensors

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Stewart G., Jin W., Culshaw B., Prospects for fibre-optic evanescent-field gas sensors using absorption in near-infrared, Sensors and Actuators B 1997 38-39 42-47. 

Schwotzer G. et. al., Optical sensing of hydrocarbons in air or in water using UV absorption in the evanescent field of fibers, Sensors Actuators B 1997 38 150-153. 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

Absorbance evanescent-based sensors are based on the absorption or dispersion of light outside the core. They rely on light attenuation in the evanescent field following the Beer-Lambert law (ATR sensors), but owing to the low intensity of the field, they offer poor sensitivity. This can be improved because the effective optical path length can be increased, especially when using optical fibers, capillary [62] or planar waveguides [114]. 

Keywords Absorption ARROW waveguide Biosensor Chemical sensor Confinement Dielectrics Evanescent field Fiber optics Fluorescence Hollow core Liquid-core Loss Optical mode Photonic crystal Refractive index Resolution Scattering Sensitivity Total internal reflection Waveguide... 

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