How does the light stay in a glass wire?

Glass lets the light through: how to make a guide that preserves it? An optical fiber combines a core and a sheath. These two transparent regions do not have the same refractive index. Therefore, they do not modify the propagation of light in the same way.

The heart has a higher index than the sheath that surrounds it. Under guidance conditions, this difference confines the light instead of letting it radiate freely outwards. The device does not rest on a wire surrounded by a metal mirror. The boundary between the two glass compositions itself plays an essential role in guiding.

Containment does not eliminate all losses. Impurities can absorb or diffuse part of the signal; the very high purity of the materials helps to reduce them. Fiber doesn’t just send light through any glass. It combines optical architecture and carefully prepared material to transport information over long distances.