Fiber Optics

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.
Low-OH fibers have a lower attenuation in the NIR and are useful between 400-2500 nm [NIR-fiber]. A high OH-content reduces the formation of imperfections of irradiation in the UV. High-OH fibers are optimized for the range 190-1100 nm [UV-fiber]. Solarization occurs below 240 nm, which leads to increasing attenuation of the fiber in this wavelength range. Therefore, for applications below 240 nm, solarization-stable [SB] fibers are required. They have a very low additional attenuation in the range between 190 and 250nm. After a 2h “burn-in” phase where the attenuation increases by approximately 20%, and measurements in the deep UV can be performed under stable conditions.
 However, the usable fiber length has to be limited due to increased damping in the lower UV region and the NIR above 2000 nm.
tec5 offers high-quality UV-conductive high-OH fused-silica glass fibers [also as solarization-resistant [SB] version] and IR-conductive low-OH fused silica glass fibers for industrial use. Each individual fiber is coated with glass finish [max. 120°C] or polyimide [high temperature up to 300°C]. Special industrial grade metal-silicone tubes with strain-relief offer maximal protection from extraneous factors as mechanical stress and chemical substances. Nevertheless, fibers should be handled with care, especially during installation. Mechanical stress can cause tear or breakage of the fibers. The minimum bend radius is 60mm for fiber bundles and 300 x the diameter of the fiber for mono-fibers [e.g. 18cm for a 600-micron fiber]. This bend radius should not be exceeded in order to ensure an optimal longevity.