The light traveling down the fiber is a group of electromagnetic (EM) waves occupying a small band of frequencies within the electromagnetic spectrum, so it is a simplification to call it a ray of light. However, it is enormously helpful to do this, providing an easy concept – some framework to hang our ideas on. We do this all the time and it serves us well providing we are clear that it is only an analogy.

Magnetic fields are not really lines floating in space around a magnet, and electrons are not really little black ball bearings flying round a red nucleus.

Light therefore, is propagated as an electromagnetic wave along the fiber. The two components, the electric field and the magnetic field, form patterns across the fiber. These patterns are called modes of transmission. Modes means methods – hence methods of transmission. An optic fiber that carries more than one mode is called a multimode fiber (MM).

The number of modes is always a whole number.

In a given piece of fiber, there are only a set number of possible modes. This is because each mode is a pattern of electric and magnetic fields having a physical size. The dimensions of the core determine how many modes or patterns can exist in the core – the larger the core, the more modes.

The number of modes is always an integer; we cannot have incomplete field patterns. This is similar to transmission of motor vehicles along a road. As the road is made wider, it stays as a single-lane road until it is large enough to accommodate an extra line of vehicles whereupon it suddenly jumps to a two-lane road. We never come across a 1.15-lane road!

The number of modes is given (reasonably accurately) by the formula:

where NA is numerical aperture of the fiber and is the wavelength of the light source.

Let’s choose some likely figures as in Figure below and see the result.

Calculation as below:

The calculator gave 703.66 but we cannot have part of a mode so we have to round it down. Always round down as even 703.99 would not be large enough for 704 modes to exist.
Each of the 703 modes could be represented by a ray being propagated at its own characteristic angle. Every mode is therefore traveling at a different speed along the fiber and gives rise to the dispersion which we called intermodal dispersion.