Optical FibresDylan Adlard

Optical fibres are used in high-speed communications, such as cable TV and high-speed broadband services. They are also used in medical endoscopes. It is a cable that has a fine high-quality glass core and it surrounded by glass cladding with a lower refractive index that the core. Light sent in one end thus undergoes repeated total internal reflection (only if the angle of light shone is greater than the critical angle), and emerges at the other end. This light is the information sent down bundles of optical fibres. Fibre optic cables carry more signals than traditional copper cable telephone lines. The fibre is transparent so that the light cannot be absorbed in its journey. As well as this, the glass must have a higher refractive index than the cladding.

The cladding has lower refractive index to reduce light loss from the core.

Modal dispersion (multipath): Light travelling along the axis of the core travels a shorter distance per metre of fibre than light that repeatedly undergoes total internal reflection. If a pulse were too long, it would merge with the next pulse. To prevent this, the core must be very narrow.

Pulse dispersion: occurs if white light is used instead of monochromatic light. Speed of light in the glass depends on the wavelength of the light in the glass. The speed difference causes pulses of white light in the fibre to become longer, as violet would fall behind red. Therefore, the best light to use is monochromatic.

Medical endoscope: It contains two bundles of fibres. The endoscope is inserted into the body, which is illuminated through one of the fibres. A lens positioned on the other end of the fibre bundle is used to form an image on that end. The light forming this image travels to the other end where the image is observed. The fibre bundle needs to be coherent fibres at relative ends must be in relative positions.