Fiber design

As we saw in chapter 1, fibre optics has revolutionized the way the world is connected. It is the best method of quickly and efficiently transmitting data over long distances. But what exactly is an optical fibre made of? What properties make it unique?

Fibre Structure

An optical fibre is essentially a very narrow tunnel which guides light from a starting point to its destination. It does this by trapping the optical signal within its boundaries and pushing it from a transmitting end to a receiving end. Even if the fibre is bent, very little light is lost along the way.

The fibre itself consists of three layers: the core, the cladding, and the jacket (see Figure 1).

Figure 1

Light Characteristics

The colours we see (i.e. the visible spectrum) account for only a small portion of the different forms of light that exist, each represented by different wavelengths. The light of interest in optical communications, infrared light, is typically invisible to us because its wavelength is longer than the visible limit. The way the light travels across the fibre is determined by many factors:

Reflection
This refers to the ray of light being mirrored as it travels from the core onto the cladding and back through the core. The “bouncing” of light through an optical fibre relies on this mirroring principle (see Figure 2).

Figure 2

Refraction

This refers to the bending of a ray of light as it travels from one transmission medium through another with a different refraction index (see Figure 3).

Figure 3

Propagation Delay

Rays of light that are sent straight through the fibre (Figure 4) will travel faster than rays that are sent at an angle (Figure 5) as these must ‘bounce’ their way to the other side, thus taking a longer path and arriving later.

Bandwidth

This is defined as the width of the frequency range that can be transmitted by an optical fibre. In other words, it is the maximum amount of information that can be transmitted over a fibre.

Velocity

The speed at which light travels through the fibre is also determined by the refractive indexes of the glass used to make the fibre.

The light that goes into the fibre on the transmitting end is not exactly the same light that comes out on the receiving end. Indeed, the signal is transformed along its journey depending on the characteristics of the fibre.

Fibre Characteristics

The physical properties of the fibre can alter the optical signals that travel through it in many ways:

Attenuation

This refers to the loss of signal strength over the length of the fibre due to impurities in the glass. Also, variations in the uniformity of the glass cause the light to scatter, which contributes to attenuation (see Figure 6).

Figure 6

Maximum Power

This refers to the amount of power that can be sent on a fibre.

Polarization

Unpolarized light (such as sunlight) vibrates in multiple planes. The process of polarization transforms this light into polarized light which only vibrates in a single plane.

Dispersion

This occurs when an optical signal is spread out as it is transmitted along the fibre. A short pulse elongates and eventually merges with the pulse behind it, rendering recovery of a reliable bit stream impossible.

Noise

One benefit of fibre is that it is immune to noise emanating from outside the system. However, it can be affected by many types of noise from inside the system itself.

Understanding these concepts is merely the tip of the iceberg in grasping the full potential of fibre optics. Not all fibres are created equal, and as we will see in the next chapter, different types of fibre have different pros and cons and are most relevant for different applications.