2.2.3 Operational Principles

Figure 15.  Fibre Modes

Figure 15 illustrates the three different kinds of optical fibre.

•  Multimode Step-Index

•  Multimode Graded-Index

•  Single-Mode (Step-Index)

The difference between them is in the way light travels along the fibre. The top section of the figure shows the operation of “multimode” fibre. There are two different parts to the fibre. In the figure, there is a core of 50 microns (μm) in diameter and a cladding of 125 μm in diameter. (Fibre size is normally quoted as the core diameter followed by the cladding diameter. Thus the fibre in the figure is identified as 50/125.) The cladding surrounds the core. The cladding glass has a different (lower) refractive index than that of the core, and the boundary forms a mirror.

This is the effect you see when looking upward from underwater. Except for the part immediately above, the junction of the water and the air appears silver like a mirror.

Light is transmitted (with very low loss) down the fibre by reflection from the mirror boundary between the core and the cladding. This phenomenon is called “total internal reflection”. Perhaps the most important characteristic is that the fibre will bend around corners to a radius of only a few centimetres without any loss of the light.

Multimode Step-Index Fibre

Figure 16.  Multimode Step-Index Fibre

The expectation of many people is that if you shine a light down a fibre, then the light will enter the fibre at an infinitely large number of angles and propagate by internal reflection over an infinite number of possible paths. This is not true. What happens is that there is only a finite number of possible paths for the light to take. These paths are called “modes” and identify the general characteristic of the light transmission system being used. This is discussed further in 2.3.4, “Propagation Modes” on page 44. Fibre that has a core diameter large enough for the light used to find multiple paths is called “multimode” fibre. For a fibre with a core diameter of 62.5 microns using light of wavelength 1300 nm, the number of modes is around 400 depending on the difference in refractive index between the core and the cladding.

The problem with multimode operation is that some of the paths taken by particular modes are longer than other paths. This means that light will arrive at different times according to the path taken. Therefore the pulse tends to disperse (spread out) as it travels through the fibre. This effect is one cause of “intersymbol interference”. This restricts the distance that a pulse can be usefully sent over multimode fibre.

Multimode Graded Index Fibre

Figure 17.  Multimode Graded index Fibre

One way around the problem of (modal) dispersion in multimode fibre is to do something to the glass such that the refractive index of the core changes gradually from the centre to the edge. Light travelling down the center of the fibre experiences a higher refractive index than light that travels further out towards the cladding. Thus light on the physically shorter paths (modes) travels more slowly than light on physically longer paths. The light follows a curved trajectory within the fibre as illustrated in the figure. The aim of this is to keep the speed of propagation of light on each path the same with respect to the axis of the fibre. Thus a pulse of light composed of many modes stays together as it travels through the fibre. This allows transmission for longer distances than does regular multimode transmission. This type of fibre is called “Graded Index” fibre. Within a GI fibre light typically travels in around 400 modes (at a wavelength of 1300 nm) or 800 modes (in the 800 nm band).

Note that only the refractive index of the core is graded. There is still a cladding of lower refractive index than the outer part of the core.

Single-Mode Fibre

Figure 18.  Single-Mode Fibre. Note that this figure is not to scale. The core diameter is typically between 8 and 9 microns while the diameter of the cladding is 125 microns.

If the fibre core is very narrow compared to the wavelength of the light in use then the light cannot travel in different modes and thus the fibre is called “single-mode” or “monomode”. There is no longer any reflection from the core-cladding boundary but rather the electromagnetic wave is tightly held to travel down the axis of the fibre. It seems obvious that the longer the wavelength of light in use, the larger the diameter of fibre we can use and still have light travel in a single-mode. The core diameter used in a typical single-mode fibre is nine microns.

It is not quite as simple as this in practice. A significant proportion (up to 20%) of the light in a single-mode fibre actually travels in the cladding. For this reason the “apparent diameter” of the core (the region in which most of the light travels) is somewhat wider than the core itself. The region in which light travels in a single-mode fibre is often called the “mode field” and the mode field diameter is quoted instead of the core diameter. The mode field varies in diameter depending on the relative refractive indices of core and cladding,

Core diameter is a compromise. We can’t make the core too narrow because of losses at bends in the fibre. As the core diameter decreases compared to the wavelength (the core gets narrower or the wavelength gets longer), the minimum radius that we can bend the fibre without loss increases. If a bend is too sharp, the light just comes out of the core into the outer parts of the cladding and is lost.

You can make fibre single-mode by:

•  Making the core thin enough

•  Making the refractive index difference between core and cladding small enough

•  Using a longer wavelength

Single-mode fibre usually has significantly lower attenuation than multimode (about half). This has nothing to do with fibre geometry or manufacture. Single-mode fibres have a significantly smaller difference in refractive index between core and cladding. This means that less dopant is needed to modify the refractive index as dopant is a major source of attenuation.

It’s not strictly correct to talk about “single-mode fibre” and “multimode fibre” without qualifying it - although we do this all the time. A fibre is single-moded or multi-moded at a particular wavelength. If we use very long wave light (say 10.6 nm from a CO₂ laser) then even most MM fibre would be single-moded for that wavelength. If we use 600 nm light on standard single-mode fibre then we do have a greater number of modes than just one (although typically only about 3 to 5). There is a single-mode fibre characteristic called the “cutoff wavelength”. This is typically around 1100 nm for single-mode fibre with a core diameter of 9 microns. The cutoff wavelength is the shortest wavelength at which the fibre remains single-moded. At wavelengths shorter than the cutoff the fibre is multimode.

When light is introduced to the end of a fibre there is a critical angle of acceptance. Light entering at a greater angle passes into the cladding and is lost. At a smaller angle the light travels down the fibre. If this is considered in three dimensions, a cone is formed around the end of the fibre within which all rays are contained. The sine of this angle is called the “numerical aperture” and is one of the important characteristics of a given fibre.

Single-mode fibre has a core diameter of 4 to 10 μm (8 μm is typical). Multimode fibre can have many core diameters but in the last few years the core diameter of 62.5 μm in the US and 50 μm outside the US has become predominant. However, the use of 62.5 μm fibre outside the US is gaining popularity - mainly due to the availability of equipment (designed for the US) that uses this type of fibre.