Optical fibres are made of different types of glass, designed to bend and guide light along their hair-thin length. Although they are solid glass, light interacts with them similarly to how water flows down a pipe.
They begin life as inflexible glass rods which are then heated and softened in a furnace so that they can be stretched thinner in a process called “drawing down” to make them the diameters we need.
To achieve more complex functions, it is necessary to make fibres with intricate structures. We do this by taking the rods of glass and stacking them into the structure we want before drawing them down as a bundle.
To make fibres that can carry entire images, thousands of individual light guides are needed. Each light guide acts like a pixel, guiding a single spot of light from one end to the other. We begin by stacking up a few of these light guides in a square and drawing them down, then we stack the result and draw that down. This process is repeated until we have rods with hundreds of elements within them. These square rods are then put into a glass tube and the gaps filled with other glass to make sure nothing moves around as this structure is drawn down one final time to make the fibre.
This is what you see in the picture; 9 rods, each of which has many hundreds of individual guiding elements or “cores”, contained within a glass tube and supported by packing glass to await being drawn down to make a fibre.
This image was produced by illuminating the bundle from behind using a supercontinuum light source, and taking the shot with a phone camera. The preform here has an outer diameter of 22 mm. Once it has been drawn down, it makes an imaging fibre just 550 micron in diameter.