A high-fibre diet: how optical fibre is made

Manufacturing optical fibre requires lots of automation, plenty of process control, a seven-storey tower, and enough energy to power a small town, as E&T discovers.

Optical fibre has changed the world. Without it, today's broadband and long-distance network connections would be slower and more expensive - if they existed at all.

As you'd guess though, it is rather more than just a strand of glass. E&T found out just how much more when it visited the largest optical fibre manufacturing site in Europe, at Douvrin in north west France. Owned by Draka Communications, which also makes copper cables, Douvrin just makes fibres - other factories then take these and build them into the cables that feed our homes and offices, and indeed the cables that cross the world's ocean beds, linking the continents. Douvrin is also the place where Draka - which has consolidated the fibre-optic businesses of a stack of other companies, including ABB, Alcatel, Chromatic Technologies, Ericsson, ITT, Nokia, Phelps Dodge and Philips, and now has operations on four continents - industrialises its processes before transferring them to its other locations, says Philippe Vanhille, the company's vice president for optical fibre operations.

"This site does all the steps involved in making fibre, the others only do some of them," he says.

Those steps start out, appropriately enough for something that's going to pipe light like water, with a 60mm-wide glass tube. They end with a fibre 0.125mm across, at the heart of which is a core just nine micrometres (9µm) wide that actually carries the light beam.

How you get from one to the other is a highly automated process - the Douvrin site currently employs 375 people working shifts; in the late 1990s it required 1,000 people for about the same output, says Vanhille.

Laying down the core

The fibre core is the result of a chemical vapour deposition (CVD) process - the tube is placed on a lathe and passed through a heater which moves along the tube taking its section to around 2,000°C. At the same time, dopants such as silicon tetrachloride (SiCl4) and germanium tetrachloride (GeCl4) are blown through the middle, along with oxygen, and they vitrify onto the hot section of tube. Repeated applications of different dopant combinations build up in layers on the inside of the tube, creating layers of glass with the desired optical properties.

Draka uses three types of deposition process - modified, furnace and plasma. MCVD was inherited from Alcatel and uses an external burner, while FCVD uses an electrical induction heater and PCVD uses a gas plasma. Vanhille says PCVD is more productive because it heats the tube from the inside rather than the outside, so it wastes less dopant and damages the tube less, so stress points are less likely. "The risk is stress points, so getting rid of those is the manufacturer's priority," he adds. "Each large player has its own process - we don't make fibre in the same ways. Draka is the only one using the PCVD process for multi-mode and speciality fibres, for example."

The latter include radiation-hardened fibres for the nuclear and aerospace industries, plus fibres for non-telecoms use - optical fibre can also used as a mechanical sensor, for instance.

"M and F take 15 hours for 35 layers, PCVD can give us 5,000 [thinner] layers in eight to nine hours - more layers gives you a smoother index profile," Vanhille says. The index profile governs how well the glass transmits various frequencies of light and is especially important for bend-insensitive fibre, as the glass layers must guide the light so that it does not escape at the bends.

Collapsing the tube

The next step is to collapse the tube into a solid rod called a primary preform. This takes place in an induction furnace and requires perfect process control in order to get all the air out of the middle and produce a flawless result.

The primary preform is then overclad at high temperature with around 100mm of silica glass, and glass handles are welded onto the end to produce a finished preform.

Under polarised light and a Fresnel lens the preform's internal structure becomes visible, with the doped glass at the centre, and it is visually checked for flaws. Now this glass rod, perhaps 2m long and 120mm across, is ready to become hundreds of kilometres of fibre.

That takes place in yet another induction furnace, this one positioned over a 25m drop in a seven-storey tower. As the glass melts it drops like honey off a spoon, forming a long string. Human intervention is needed only at the very start, to take the droplet and start it moving downwards; from then on, precise control is essential to keep the glass melting at the correct rate.

At the bottom of the tower, pulleys and capstans keep the cooling fibre under tension - a process that pulls the melting glass into a 125µm fibre that moves at over 60km/h.

As the razor-sharp fibre falls past the fifth floor, sensors check its diameter, composition and speed. At the third floor it gets its first coat of smooth cushioning plastic, followed by a dose of UV light to cure the coating, and then on the first floor it receives its second plastic coat. This coat is harder, to protect the fibre from mechanical shock, and it is in one of the 12 colours that Draka offers.

Vanhille says that one of the things differentiating Draka from its competitors is that it colours its fibre right from here, whereas others make plain fibre and add a coloured casing later.

Adding colour before testing eliminates an element of mechanical risk and also improves visibility, he argues.

The downside is that Draka needs very good stock management, as it must match customer demand right back to the start of the production process.

The fibre is loaded onto 700km rolls as it is produced and sent for strain testing. To pass, it must survive being stretched to an additional 1 per cent of its length, after which it is put onto 25km or 50km spools for shipping. Of course, fibre does sometimes break, says Vanhille, in which case the company ends up with spools containing shorter lengths. He says these spools can still be sold to customers who want non-standard lengths, but it adds yet another element of stock-control complexity.

Hungry for energy and process control

There is a downside to all this - while fibre-optic communication links are far more energy-efficient than copper in use, all the heating and cooling involved in manufacturing fibre consumes huge amounts of energy.

Vanhille admits that the Douvrin plant uses some 90MW. "The main power consumer is the overcladding," he says. "We do consume a lot of energy, and we are putting work into reducing the bill - if there is a drawback [to optical fibre], that is it.

"It is still a very young process though, and it is changing all the time - we are always improving productivity and quality. It's all about process control, because if it drifts you have to identify that fast, so you need very qualified people. Then it's a matter of logistics."

He adds that the final part of the process - drawing the fibre - is perhaps the easiest to move to other factories. "Drawing and the final testing is also the most labour-intensive element, plus fibre is relatively cheap to ship. I see many people starting drawing factories in China, but no preform factories," he says.

"Sometimes we preform in Europe, draw in the US, and ship the cable in Europe - it is all about capacity management. Europe is good at specialist fibres.

"We need to have a certain proportion of preforms drawn close by [the preform factory] for a check, then we can move that part of the process out. But as soon as you change something you have to re-check of course - it is nearly a continuous loop."

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