The dispersion of a light beam is based upon the principle of total reflection. Total reflection occurs at the interface between two media of different refractive indexes at the transition of a light beam from an optically denser material into an optically thinner material. An optical step index fiber consists of a core with a refractive index of n1 and a cladding with a refractive index of n2, whereas the relationship n1 > n2 is required for light transfer.
The numerical aperture [NA], is a basic optical characteristic of a specific fiber configuration and can be calculated as a function of the sine of the maximum accepted angle of incidence αmax of light rays into an optical fiber. The refractive index of the fiber core and cladding define the critical angle. The NA of standard fibers for spectroscopy is 0.22 [±0.02], which equals a maximum acceptance angle αmax of approx. 12.5 degrees [flare angle = 25 deg.].
The OH-content of a fiber determines its optical absorption behavior due to OH-vibrations.