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.
FibreConneX 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 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. FibreConneX is the only one using the PCVD process for multi-mode and specialty 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 be 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 kilometers of fibre. That takes place in yet another induction furnace, this one positioned over a 25m drop in a sevenstorey 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 colors that FibreConneX offers.
PCVD deposition process
PCVD stands for Plasma activated Chemical Vapour Deposition. This process belongs to the Inside Deposition processes and is for the greater part comparable with the MCVD process. A gas and vapour mixture of SiCl4, GeCl4 and O2 is led into a quartz tube, during which SiO2 and GeO2 molecules are formed in a reaction zone, where they are deposited directly on the inside of the tube and form a transparent glass. So no soot is formed here, as opposed to the MCVD process; consequently, no sintering process takes place. As in the MCVD process, chlorine gas is formed, which must be discharged. The other differences with the MCVD process are that the chemical reactions and the forming of core glass take place in the PCVD process under the influence of a plasma and that the deposition of layers takes place in both directions.
A gas consists of molecules; molecules in turn are built up of atoms. An atom consists of a core, which is electropositive, with electrons revolving around it, which are electronegative. Atoms and molecules are electrically neutral, because the positive electric charge of the core or cores is equal to the total negative charge of the electrons. When such a neutral atom or molecule, through some cause or other, is given an electron (or several electrons) too many, the negative charge will consequently be greater than the positive charge. A “negative ion” has then been formed. An atom or molecule can also lose one or more electrons. In this case, the negative charge will become smaller and the positive charge will dominate; a “positive ion” has then been formed.
Under certain conditions, such as low pressure, high temperature, the absence of a strong electromagnetic field, a great electric voltage difference or a combination of these conditions, gas molecules can decompose into ions and electrons. The gas has then been ionized.
When a great many gas molecules have decomposed in this way, a great many ions and electrons will be intermingling. Electrons are much smaller than ions and also have a much smaller mass. They will therefore move much faster than ions. Such a gas, which has to a great extent been ionized and in which there is a considerable speed difference between the ions and the electrons, is called a plasma. The movements of electrons and ions can be increased by increasing the intensity of an existing electromagnetic field. Because of their lesser mass, the speed of the electrons is increased much more than that of the ions, increasing the difference in speeds considerably. Due to their much greater speed, the electrons will therefore, despite their lesser mass, represent a greater kinetic and collision energy than the ions. As a result of the many collisions, a great amount of heat is generated in the plasma. In this case, this is known as “non-isothermal plasma”. Here, non-isothermal means that the ions and electrons do not represent the same heat energy.
3. The PCVD process
In the PCVD process, a microwave resonator is fitted around the quartz tube. This resonator receives microwaves from a microwave generator, and consequently creates an electromagnetic field in the quartz tube. If this electromagnetic field is strong enough, this will cause the gas in the tube to be ionized. The speed of the electrons and ions will also be increased by this electromagnetic field. This will result in a non-isothermal plasma in the tube. The energy of this plasma, created in the tube itself, causes the necessary chemical reactions to take place (see below). The temperature of the plasma is approximately 2000 °C.
The reactions take place in various steps, ultimately resulting in:
(‘g’ stands for ‘gas’ and ‘s’ stands for ‘solid’)
GeCl4 (g) + O2 (g) -> GeO2 (s) + 2 Cl2 (g)
Contrary to the MCVD process, no SiO2 and GeO2 particles are created which form a soot layer, but separate molecules are created that are deposited on the inside of the tube to form glass there directly. Molecules created during the chemical reactions join to form particles; these particles can coagulate with other particles. This is undesirable; the glass must be built up through separate molecules “growing” together. That is why particles that threaten to coagulate are immediately “bombarded” apart by the fast electrons of the plasma to separate molecules, which can then “grow on to” the already formed glass.
The three phases (chemical reaction, deposition of layers and forming of glass) take place simultaneously in the PCVD process, both in the “forward” and “backward” stroke of the resonator. As the reactions take place under influence of the plasma, a high temperature in the quartz tube is not needed. Nevertheless, the tube is heated on the outside by means of a furnace.
Benefits for Single-mode and Multi-mode fibres
The PCVD process is a unique enabling technology for the industrialization of trenchassisted designs.
Trench design and PCVD are at the deep root of the success of the ITU-T G.657A2 FibreConneX BendBright-XS, the industry leading fibre for FTTX applications. For such Bend-insensitive application for single mode-fibres, trench-assisted design has been used to drastically decrease the macro- and micro-bending sensitivities of fibres while keeping almost same mode field diameter area and same cable cutoff wavelength as those of a standard single-mode fibre. Other deposition processes (xCVD or OVD) cannot simply cope with such demanding trench designs in a cost-effective way.
PCVD process shows excellent capability to manufacture high quality optical multi-mode fibres due to its precise control of the refractive index profile.
The resulting profile of graded index core consists of several thousand layers with high radial resolution, and accuracy and reproducibility can be easily achieved. The MaxCap and MaxCap-BB OM3 / OM4 and OM4+ multi-mode fibres be them of legacy or trench-assisted bend-insensitive designs are produced by the proprietary Plasma-activated Chemical Vapor Deposition process (PCVD), acknowledged worldwide as offering the best core profile accuracy in multi-mode fibre.
The PCVD process is also well suited for standard ITU-T G.652D fiber.