The Effect of the Cladding Refractive Index on an Optical Fiber Evanescent-Wave Sensor

Evanescent field interactions can be used to provide a variety of sensing modalities in optical fibers with a modified cladding. However, the evanescent field interaction with the surrounding environment is strongly dependent on the refractive index of the modified cladding region. This can lead to difficulties due to dispersion in the refractive index, particularly in fluorescence based sensors where the excitation and emission wavelengths are separated. Here, a broadband supercontinuum light source has been used to characterize the refractive index dependence of the sensor response over a wide wavelength range. The critical effect of the cladding refractive index on the performance of an optical fiber evanescent wave sensor is demonstrated for both amplitude and wavelength modulated situations. In principle, this approach can be used to predict the performance of the sensor over the full wavelength range of the broadband source. The results also suggest that residues from the original cladding of the fiber cause an intrinsic loss, which reduces the sensitivity at low levels of extrinsic absorption. The integrity of the interface between the core and the modified cladding is therefore an important parameter to be addressed in practical sensing applications.