How fast does light travel through a fibre optic cable?

When we talk about fiber optic cables, we’re referring to a broad spectrum of technologies, ranging from simple DIY projects using fishing lines and LEDs, to complex, high-bandwidth, transoceanic single-mode cables. For the purpose of this discussion, let’s focus on practical, short-range cables used for signal transmission in everyday devices like hi-fi audio equipment and computers.

The Basics of Light Travel in Fiber Optic Cables

Fiber optic cables can be made from a variety of materials, including plastic and glass. Plastic fibers are often used in shorter, multi-mode types of cables, but they tend to be quite lossy. More robust multi-mode cables typically use glass fibers.

The speed at which light travels through these cables is influenced by the refractive index of the material, a concept further explored in our guide on how cables are made. For instance, with a glass refractive index of 1.5, the critical angle for Total Internal Reflection (TIR) at the glass-air interface is 41.8 degrees from the surface normal, or 48.2 degrees from the fiber axis. Light at this angle would have the longest possible path of 1/cos48.2.

This results in a path that is 1.5 times the length of the fiber, which is exactly the refractive index. Additionally, the speed of light is slowed down by a factor of 1.5. Therefore, most of the light would take between a factor of n and a factor of n^2 times the air length time.

The Role of Cladding in Fiber Optic Cables

Most serious fiber optic cables, including multi-mode fibers made of glass, are “clad” fibers. These consist of a core glass with a refractive index of perhaps 1.6, coated with an outer layer of lower index, perhaps 1.5. This cladding separates the environment and contamination from the reflecting interface that confines the radiation.

In these clad fibers, the light travels in the core glass, with an index of 1.6. However, the effective critical angle is now arcsin (1.5/1.6)=69.6 degrees, so the maximum path length is now 1.6/1.5 (1.0667) times the fiber length, not 1.5 times.

The Impact of Numerical Aperture

The “numerical aperture” of the fiber, specified by the manufacturer, is a measure of the maximum acceptance angle of the cone of light which can propagate along the cable. This can provide valuable information about the fiber’s performance without knowing the fiber index.

The Importance of Fiber Cleanliness

Unclad fibers are highly sensitive to contamination. Even touching them with your fingers and getting sweat on the surface can cause serious loss of light out the side of the fiber, as discussed in our article on why coax cables fail. Therefore, it’s crucial to handle these fibers with care to maintain their performance.

In conclusion, the speed at which light travels through a fiber optic cable is influenced by a variety of factors, including the material of the fiber, the refractive index, the presence of cladding, and the cleanliness of the fiber. Understanding these factors can help us make the most of this remarkable technology.

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