Axial versus Side Illumination of a Fluorescent Cladding Optical Fiber

Maximum number of excitation light sources Dependent on the fiber diameter: in general, only one due to the small fiber diameter. Using 5 mm diameter LEDs and a 10 cm long fiber, it is possible to accomodate a total of 20 light sources increasing the amount of signal accordingly. Amount of excitation light available Available from the evanescent wave tail which is a very small portion of the total light. All the light provenient from the source which is at least 1,000 times more than the light in the evanescent field: Type of signal Mostly from evanescent photons excited by the evanescent tail of the excitation light. From both homogeneous and evanescent photons that couples into the fiber core as leaky and bound modes, respectively. Estimate amount of signal (original excitation light intensity assumed to be one). Because evanescent wave absorption and coupling are simultaneously required, amount of the fraction of light coupled light is between 10 and 10 depending on how fluorescent sources are distributed in the cladding. Greater than 10: signal mostly due to leaky modes that propagate inside the fiber. By tapering the fiber, these leaky modes can be coupled into bound modes further increasing signal sensitivity. Signal to Noise ratio Light from excitation source can be used to generate fluorescent signal but also acts as a source of noise. Original light that was not absorbed is at least four orders of magnitude more intense than fluorescence masking its intensity. Sorting between signal and noise requires high performance filters that are expensive and decrease signal even further. Due to side illumination inherent “filtering” effect, most of the excitation light refracts away from the core. As a result excitation light serves mostly as a source of signal, rather than noise.