Optical fibres

Optical fibers are based on the principle of total internal reflection that was observed in a water jet by Swiss physicist Jean Daniel Colladon in the early 1840s, and later by French physicist Jacques Babinet who observed the same phenomenon in a glass rod.  The first practical application was the use of total internal reflection as a medium for light propagation in fountain lighting. In the 20^th century, research focused on developing fiber bundles for image transmission, to be used in internal medical examination. The first fiber optic semi-flexible gastroscope was patented by  Basil Hirschowitz, C. Wilbur Peters and Lawrence E. Curtiss in 1956, and Curtiss himself produced the first glass-clad fibers. In 1965 Charles K. Kao' and George A. Hockham, working at STL, the research center of the British company STC (Standard Telephones and Cables) were the first to suggest that the attenuation in the fibers available at the time was actually caused by glass impurities that could be removed, rather than by physical effects such as scattering. They promoted the idea that fibers would be a practical and efficient communication medium when the attenuation could be reduced below 20 dB/km. This was achieved in 1970 by researchers Robert D. Maurer, Donald Keck, Peter Schultz and Frank Zimar, working for American glassmaker Corning Glass Works (now Corning Inc.). They produced a fiber with 17 dB/km attenuation by doping silica with titanium.

Total internal reflection

An optical fiber is a medium for telecommunication that includes a core surrounded by a cladding, both made from silica glass SiO₂ doped with various materials in order to obtain a refractive index of the core n1 greater than the refractive index of the cladding n2. The difference between these two refractive indexes, thanks to the phenomenon of total internal reflection, allows light to propagate along the fiber.

Attenuation (or signal loss)

The optical fiber is totally immune to electromagnetic interference, but light attenuations can occur due to impurities in the glass (loss due to absorption) and to physical effects inside the glass fiber itself (scattering). Such attenuations are measured in dB/km and can vary according to the wavelength of the light beam.

Attenuation can also be caused, in addition to light absorption and scattering, by excessive bending of the fiber during use: when a critical bend radius of the fiber is reached, the light ray will partially disperse in the cladding as its angle of incidence becomes higher than the acceptance angle of the fiber.

Numerical aperture

Another important fiber parameter is the Numerical Aperture (NA): it represents the maximum acceptance angle of the fiber for an incident ray.  If the angle of the intersection is too large, the light ray will not be totally reflected back into the core but partially lost in the cladding.

Applications for optical fibers

Optical fibers are used in telecommunications for data transmission over long distances and at increasing speeds. Technological developments in the last 20 years brought an increasing use of optical fibers in lightning as well as image and energy transmission both in the medical field (surgery, endoscopy, physiotherapy) and in the industrial sector (systems for vision, cutting, welding/soldering, drilling etc.).