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6.1.3 The double-Crucible Method

Figure 228.  Double-Crucible Method

In the double-crucible method, a pair of platinum crucibles set one inside the other hold molten silica containing the desired dopants etc. Not shown in the diagram, the whole assembly is housed inside a furnace to keep the temperature at the optimum level. This is often done in an inert (nitrogen) atmosphere. The inner crucible holds the core glass and the outer crucible, the cladding. The crucibles are continually replenished by the insertion of feed rods of silica (as shown). Fibre of the desired cross-section is drawn from an outlet in the base of the assembly.

The double-crucible method of fibre manufacture was important in the past but is not in commercial use today. It was very attractive because it is a continuous process and therefore offers potentially very low cost operation. However, it has not been able to deliver the required precision in control of fibre dimensions and characteristics.

6.1.4 Vapour deposition Techniques

The major methods of fibre manufacture create a solid rod (called a preform) from which the fibre is drawn in a batch process. Preforms are created by depositing silica (including dopants) from reactions between gasses at high temperatures. There are four general processes:

1.  Outside Vapour deposition (OVD)
2.  Vapour Axial deposition (VAD)
3.  Modified Chemical Vapour deposition (MCVD)
4.  Plasma-activated Chemical Vapour deposition (PCVD)

Of the above, VAD and OVD are the most widely used processes for making single-mode fibre although MCVD is also sometimes used. MCVD is used extensively for specialty fibres such as in fibre made for pigtailing and in doped fibres for amplifiers etc. MCVD is also extensively used in making GI multimode fibre. In manufacturing single mode fibre, the VAD process is almost exclusively used in Japan. OVD and MCVD are used in Europe and America. Outside Vapour deposition (OVD)

Figure 229.  Outside Vapour deposition

The basic process involved in OVD is called “flame hydrolysis”. SiCl4 reacts with oxygen to produce silica (SiO2) and HCl. This reaction takes place within an oxy-hydrogen flame.

  At normal temperatures SiCl4 is a relatively volatile liquid.
  Oxygen is passed through the silicon chloride to form a mixture of SiCl4 vapour and oxygen.
  The SiCl4-O2 mixture is then fed to an oxy-hydrogen burner such that it is introduced into the flame.
  Minute particles of molten silica (called a “soot” are formed in the flame.
  A rod of either carbon or a metal is rotated in a lathe and the silica depositing flame is moved back and forth along it.
  The silica is deposited evenly along the rod.
  Dopants such as germanium are introduced into the flame to create the required variations in the refractive index.
  When sufficient material has built up on the rod the process is stopped and the rod removed. The glass body so formed is a porous conglomerate of silica particles stuck together.
  The preform is then heated (sintered) to coalesce the material into a solid glass tube. The tube is then heated further and collapsed to form a rod.
  In principle you could draw fibre directly from the collapsed silica preform.
  In practice it is a waste of time to deposit all of the cladding in this way. When the preform has built up a sufficient thickness for the core and a small part of the cladding the process is usually stopped. This preform is then sintered and inserted into a silica tube. The assembly is then heated and collapsed together creating a preform with the appropriate profile.

Of course the above process must take place in either a vacuum or an inert atmosphere. Since the process produces HCl and water a mist of hydrochloric acid is also produced which must be neutralised and disposed of.

This process has three problems:

1.  It is difficult to remove all the water (OH groups) from the formed glass. Thus the resulting fibre tends to have a large absorption peak around the 1385 nm wavelength region. Considerable success was achieved at removing most of the OH but it was never as good as some competing processes.
2.  The fibre produced tends to have a depression in the refractive index along its axis. This is also true for some other processes (such as MCVd) but the effect is bigger in this one.
3.  It is a batch process and preforms produced this way are limited in size. This means that the ultimate cost of the process is likely to be high.

OVd was the first of the vapour deposition processes and is important for that reason but is not in commercial use today. Vapour-Phase Axial deposition (VAD)

VAD is a very important process and accounts for a large proportion of world fibre production. It was originally intended to be a continuous process which would have a lot lower cost than the batch processes. Today it is still a batch process. However it produces large preforms which can be drawn to lengths of up to 250 km of fibre.

The basic mechanism used in VAD is flame hydrolysis (like OVD). It was discovered that if you mix GeCl4 as a dopant into the SiCl4-O2 feed the proportion of germania (GeO2) deposited with the silica varies with the temperature of the flame! So if you control the flame temperature you can control the proportion of dopant deposited. Further if a wide flame is used with a temperature gradient across it you can get a graded proportion of germania deposited. This is difficult to do in practice but since it is a very successful commercial process we know that these difficulties have been overcome!

Figure 230.  Vapour Axial deposition (VAD)

The process is shown in Figure 230.

  At the bottom of the figure flame hydrolysis torches produce silica soot (including dopants) which are deposited across the base of a relatively wide preform.
  Other dopants can be included instead of or in addition to germanium. POCl and BBr are often used.
  As the soot is deposited the preform is pulled upwards so that the torches themselves do not move.
  Further up the preform other torches (or sometimes an electric furnace) are used to sinter the preform and form a solid glass preform rod.
  The whole process is of course enclosed and an atmosphere of pure oxygen with SOCl2 vapour is used to capture the water produced by the torches and prevent it from entering the glass.
  The resultant rod is used as the feed for the fibre drawing process.

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