Fibre Optics and Endoscopy

A modern endoscope lets a doctor see directly inside the body — down a throat, into a joint, along a length of intestine — without a single incision. The physics behind it is a simple optical trick, total internal reflection, applied with remarkable precision across thousands of hair-thin glass fibres bundled together.

Total internal reflection at the core-cladding interface

An optical fibre is a thin, flexible strand of glass or plastic designed to carry light along its length, even around bends, with very little loss. Its structure has two parts: a central core with a relatively high refractive index, surrounded by a cladding layer with a lower refractive index. This difference in refractive index between core and cladding is what allows the fibre to guide light at all.

Light travelling within the core that strikes the boundary with the cladding at an angle of incidence greater than the critical angle undergoes total internal reflection (TIR): instead of refracting out into the cladding and being lost, the light reflects entirely back into the core, at an equal angle, and continues travelling onward. Because the fibre is manufactured so that light entering it always strikes the core-cladding boundary at an angle exceeding the critical angle, this reflection happens repeatedly, every time the light ray reaches the boundary again further along the fibre — guiding the light efficiently along the fibre's entire length, including around gentle bends, with only a small amount of energy lost to absorption within the glass itself.

Coherent versus non-coherent bundles

A single optical fibre can only carry light — it cannot, by itself, carry a picture. To transmit an actual image, many individual fibres must be bundled together, and critically, how they are bundled determines what the bundle can do.

• Non-coherent bundle. The individual fibres are arranged randomly, with no consistent relationship between their positions at one end of the bundle and their positions at the other. A non-coherent bundle can still carry light perfectly well from one end to the other — useful simply for illumination — but it cannot preserve a spatial image, because the relative position of each fibre's input is not preserved at its output. Light entering at one point could exit almost anywhere across the far end.
• Coherent bundle. The individual fibres are arranged in exactly the same relative spatial order at both ends of the bundle. If an image is focused onto one end, each tiny fibre carries the brightness and colour information from just one small part of that image, and because the fibres' relative positions are preserved end to end, the same spatial pattern is faithfully reproduced at the other end. This preservation of spatial arrangement is precisely what allows a coherent bundle to transmit a genuinely recognisable picture, rather than a scrambled jumble of light.

The optical system of a flexible endoscope

A flexible endoscope is a medical instrument that combines both types of fibre bundle, each doing a distinct job, to allow direct visual examination inside the body without open surgery:

• A non-coherent bundle carries light from an external source, down through the instrument, to illuminate the area being examined — since this task only requires carrying light, not preserving an image, a non-coherent bundle is entirely adequate and is typically cheaper to manufacture.
• A separate coherent bundle carries the resulting image of that illuminated area back out of the body to an eyepiece or camera, where it can be viewed directly or displayed on a screen.
• A small lens system at the tip of the endoscope focuses the image of the area under examination onto the input end of the coherent bundle, ensuring a sharp image is what actually gets transmitted back.
• A flexible endoscope commonly also includes a working channel running alongside the fibre bundles, through which small surgical instruments (for example, for taking a tissue sample) can be passed to the site being examined.

Mark as complete