Optical fibres

An optical fibre guides light along a transparent core surrounded by a layer called cladding. The core has a higher refractive index than the cladding. This difference allows light travelling inside the core to be reflected back into it at the core–cladding boundary.

Light can undergo total internal reflection at this boundary when its angle of incidence is greater than the critical angle. The angle of incidence is measured between the light ray and the normal: a line perpendicular to the boundary at the point where the ray meets it.

Repeated total internal reflections keep the light in the core as it travels along the fibre. The light can therefore follow the fibre around bends, although a bend that is too sharp can reduce the angle of incidence enough for light to escape.

Critical angle in an optical fibre

For a core with refractive index $$n_{\text{core}}$$ and cladding with refractive index $$n_{\text{cladding}}$$, the critical angle $$c$$ satisfies:

Because the core has the higher refractive index, light travelling in it can undergo total internal reflection when its angle of incidence at the boundary exceeds $$c$$.

Cladding and uses of optical fibres

The cladding helps confine light to the core and reduces unwanted transfer of light between adjacent fibres. Optical fibres are flexible and can transmit light over a distance with relatively low loss.

In medicine, fibres can guide light into the body to illuminate an area for imaging. They can also deliver laser light to an internal treatment site.

Flexible fibre-optic endoscopy

A flexible fibre-optic endoscope allows an internal region of the body to be illuminated and viewed. It uses one bundle of optical fibres to carry light into the body and another to carry an image back to the viewing end.

Illuminating the internal region

Light from an external source travels through a non-coherent bundle of fibres and illuminates the region being examined. The fibres do not need to keep the same relative positions at both ends of this bundle: their purpose is to deliver light, not to preserve an image.

Transmitting the image

An objective lens at the internal end forms an image of the illuminated region on the end of a second bundle. This is a coherent fibre bundle: its fibres retain the same relative positions at both ends.

Light from different points in the image enters fibres at corresponding positions. Because the arrangement is preserved along the bundle, those points remain in the correct spatial arrangement at the viewing end. An eyepiece or other viewing system allows the transmitted image to be observed.

Here, coherent describes the spatial arrangement of the fibres. It does not mean that the light is phase-coherent.

Viewing internal regions

A flexible endoscope can be passed through a natural opening or a small incision to view regions such as the stomach or large intestine. It provides direct visual information without requiring more invasive open surgery solely to inspect the region.