The Structure of Fibre Optic Explained


Fibre optic cable is the cream of the crop when it comes down to network cabling quality. It’s faster, and more secure than twisted paired copper, but it’s also more expensive. But for some applications, particularly those with very heavy data loads, where bandwidth is an issue and speed a premium consideration Fibre optic is also useful where outside interference is a problem, it’s heads and shoulders above anything else on the market. It can also carry the data far greater distances before the signal requires any sort of boost.

3 Main Types of Fibre Optic Cable

There are three main types of Fibre Optic cabling available today. They are:

Single mode

Single mode fibre-optic cable consists of one single strand of glass fibre material that has a diameter between 8.3 and 10 microns. To put this into some sort of perspective, one micron is 1/250th of the diameter of a single human hair.


Multimode cable consists of multiple strands of glass fibres. The diameter of the complete group of fibres is somewhere between 50 to 100 microns. Each individual fibre has the ability to carry a different independent signal from all of the other fibres within the same cable bundle.

Plastic Optic Fibre (POF)

Plastic optic fibre is a more recent arrival, and as the name suggests, is a plastic based material. In terms of specification, the performance is similar to single mode cable, but costs significantly less.

How Fibre Optic Cables Work

A single strand of fibre optic cable operates rather like a light guide, channelling the light that is introduced at one end of the cable, throughout its length to the other end. The light source itself can be generated by either Laser, or LED (Light Emitting Diode).

The source of the light, as it is introduced into the cable, pulses on and off. It travels along the cable length in this pulsed format until it reaches a light sensitive receiver at the other end of the cable, whereby it is then converted back into binary code consisting of digital ones and zeros that corresponds to the original signal.

Any sort of light, including laser, is subject to some loss of strength as it traverses the fibre optic cable. This loss comes about through the dispersion and scattering of the light inside the cable. The quicker that the laser fluctuates, the more risk there is of dispersion. In certain instances light strengtheners, which are known as repeaters, may have to be introduced.

The Cross Section of a Strand of Fibre Optic Cable

If you were to see a cross section of single strand of fibre optic cable, it would appear as 5 concentric circles rather like a bull's-eye. The outer circle is either a PVC (Non-Plenum), or a Fluoride Co-Polymer (Plenum) jacket; the next circle in is known as the Aramid Strength Member; the next is a Thermoplastic overcoat or Buffer; the next is simply a fibre coating, and the final, inner circle is the silica glass itself.

An optical fibre link has the ability to carry up to 10,000,000 digital bits of information each and every second along its length. This is enough to carry tens of thousands of different telephone conversations. The light signal is transmitted along 2 concentric layers of high quality silica glass. The outer cable prevents any light spillage.

Because of the physics of the cable (i.e. the cladding having a lower refractive index than the silica glass), the light remains concentrated at the core. As technology improves, in terms of Lasers and LEDs, it is envisaged that fibre-optic cable networks may one day be able to carry trillions of bits of data per second along their lengths.

Fibre-optic cable capacity

Simple mode fibre-optic cable transmits signals faster, and up to 50 times further than multimode cable. It does however cost more. The singleton feature of single mode cable means that there is virtually no distortion from overlapping light pulses, guaranteeing top performance.

Will multimode fibre, light waves become dispersed into various paths (known as modes) as they travel along the cable’s length. When the length of the cable is more than 3000 feet, the signal can become distorted and may lose its integrity.

Grading fibre-optic cable

In comparison to Cat-5 cable, fibre-optic cable is easier to grade because of its immunity to electrical interference. The only measurements to be checked are:

·Attenuation - measured in dB/km - signal strength loss

·Graded refractive index measurement - measured in nanometres and recording how much light the fibre carries

·Propagation delay - speed of transmission

·TDR (Time Domain Reflectometry) - pulses that allow faults to be isolated along the cable length

Fibre Optic Cables Test Devices

In terms of testing fibre-optic cable, there are many devices on the market today. They work by shining a light down one end of the cable and measuring it at the other end for signal strength as against the strength at the light source. The latest types of cable testers also enable the cable to be checked for compliance against specific standards.

Need advice on the type of cable that’s best suited to your business? Why not give us a call on +44 (0)8450 740 530today and discuss your options. We will be more than happy to help you arrive at the perfect solution, no matter what the size of your company.

Photo: Roshan Nikam