Principles of fibre-optics

The fundamental requirement of all radiation-transfer techniques in remote spectroscopy is that the radiation be transferred from the spectrometer to the sample, probe the reactions or transformations of interest and then return the modified beam of radiation to the spectrometer for the measurement of intensity at each wavelength in the spectral region of interest, all without any contributions from the transfer medium and with little loss in energy. While fibre-optics meets these requirements under favourable circumstances, there are potential artefacts and limitations, which may be understood from the principles of operation. 

It is noted that only for values of 6 greater than 6 will there be no refracted ray such that light is totally internally reflected at the interface and propagates within the medium of 

The intrinsic absorption properties of the fibre material at the wavelength, X, of the radiation. This is usually expressed in optical power loss, a, in dB/m, per unit length of fibre, L, which is related to the spectrophotometric absorbance of the material, A (Equation (3.21)), by the relation 

The absorption of the evanescent wave. The evanescent wave is the component of the totally internally reflected wave that penetrates into the medium of lower refractive index (i.e. the cladding) by a distance that depends on the angle of incidence. This is illustrated in 

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