[Robcat (OP)]

Could someone explain what is the relationship between the diameter of a fibre optic cable and its ability to carry information.
How fine is best,      or why not one glass rod of the same diameter as a bunch of fibre?
It probable only needs one expert to kindly answer this question.     Please!

[evan_au]

I am assuming you are talking about glass fibers - plastic fiber (core diameter about 0.5mm) has such high attenuation that you can't use it for lengths beyond a few meters, and it can't carry high-speed signals because it is "multi-mode" (see below).

The most common two types of glass fiber are "single-mode fiber" (core diameter about 10μm) and "multi-mode fiber" (core diameter about 50μm).
The very narrow diameter of single-mode fiber means that light can only take one path through the fiber, so light that starts together reaches the end together. This allows for transmission of high-speed signals which are received undistorted*.

The much wider diameter of multi-mode fiber means that light can take multiple paths through the fiber, and each path takes a slightly different delay to reach the end. This means that high-speed signals will be scrambled when they reach the end of a long fiber. Multimode fiber allows easier alignment of the fibers, and so it was frequently installed for short fiber runs, up to say 500m.

However, everyone now expects higher speeds from all their installed fiber, and so recent Ethernet standards have allowed higher speeds over multimode fiber by having electronics track all the multiple paths that are being received, and delaying and combining them all so that you can get a clean, high-speed signal out of multimode fiber. Achieving this requires immensely powerful computation, doing billions of calculations per second (and dissipates a bit of heat), but it's still cheaper than installing new single-mode fiber.

*For really long-distance communication (hundreds to thousands of km), slightly different wavelengths of light from the laser travel at slightly different speeds in the fiber; this is called "dispersion", and results in some distortion even in single-mode fiber. This can be reduced by transmitting around a wavelength where the fiber has minimum dispersion, or by using a short length of fiber which has the opposite dispersion to the main cable, producing undistorted signals at the end.

[Robcat (OP)]

Thank you