MadSci Network: Engineering

Re: How are Fiber Optics made?

Date: Thu Mar 19 18:54:45 1998
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Engineering
ID: 890183300.Eg


The manufacturing of glass optical fibers that are as strong as steel and 
yet bendable is a fascinating process that is somewhat similar to pulling  
melted taffy candy or Mozzarella cheese into strings. The details of The 
Manufacture and Physical Properties of Optical Fibers is discussed in 
detail with drawings on The Cooper Union School of Engineering web pages at 
the following URL:

Before reading this web site I'll give a quick overview of the process.

Depending on the application, optical fibers come in a number of  different 
forms. There are plastic fibers and glass fibers. Glass fibers are the 
technology used in most telecommunications applications and they are the 
type I'll discuss. 

Glass optical fibers also come in a number of configurations 
that depend on 1.) how long the fiber communications link is and 2) what  
data rate (number of light pulses per second) that the fiber must carry. 
The longest links, with the highest data rates, are the under sea cables 
that are thousands of miles/kilometers  long. They use STEP INDEX SINGLE 
MODE fiber.  Communication links less that 16 km (10 miles) long might use 
GRADED INDEX MULTIMODE fiber and very short, low data rate links, less than 
a mile/km long, might use STEP INDEX MULTIMODE fiber. 

All of the fiber types consist of a light guiding core of glass surrounded 
by a cladding layer of glass. For protection, the glass fiber is usually 
over coated with a thick layer of a plastic type material.  Most fibers 
have about the same outer diameter of glass ranging between 100 and 200 
micrometers (40 to 80 micro inches), about the diameter of human hair. 
However;  the light guiding glass cores are a only a few micrometers in 
diameter for the SINGLE MODE fibers used in undersea cables while the 
MULTIMODE fibers have light guiding cores about 50 micrometers (20 micro 
inches) in diameter. The light signals are trapped in the core glass 
because the core has a different chemical composition than that used in the 
cladding glass. The interface between the two types of glass causes an 
optical effect known as TOTAL INTERNAL REFLECTION. 

The manufacturing of optical fiber begins with the fabrication of a glass 
PREFORM that has the same ratio of core to clad glass diameters as the 
final fiber will have; however, the preform is thousands of times larger in 
diameter than the fiber. A typical  preform might have an outside diameter 
of 15 cm (6 inches)  and be 100 cm (40 inches) long.  The inner core glass 
will be between 50% and 1 % of the outside diameter depending on the type 
of fiber to be pulled. The large, heavy  glass PREFORM is slowly lowered 
into a circular doughnut shaped furnace which is also open at the bottom.

A fiber pulling machine is about 4 meters (12 feet) high and the preform is 
loaded into the top of the machine. As the perform glass is lowered into 
the top of a doughnut shaped furnace at a rate of a fraction of a 
centimeter(inch) per minute, the end of the preform begins to melt and it 
drops in a long string of glass from the bottom of the furnace much like a 
string of melted Mozzarella cheese or taffy candy behaves. The operator 
attaches the end of the long string of glass, which has cooled and 
solidified, onto a rotating take-up bobbin, a drum about 50 cm (20 inches) 
in diameter.  As the take-up bobbin rotates the glass string is pulled at a 
fast rate of about 100 cm (40 inches) per second. The faster the bobbin 
rotates the thinner the pulled fiber will be and the slower it rotates the 
thicker the fiber will be. The measuring device also controls the speed at 
which the preform is slowly lowered into the top of the oven

A highly accurate laser diameter measuring instrument is used to measure 
the diameter of the melted string of glass as it falls  from the furnace.  
The measuring device then controls the rotating speed of the bobbin to set 
the outer diameter of the fiber being pulled to the desired dimension. It 
takes several hours to pull the one meter long PREFORM into hundreds of 
kilometers of glass fiber!

What is amazing about the fiber pulling process is that if the outer 
diameter of the pulled fiber is accurately controlled, the diameter of the 
center light guiding core glass also maintains the correct diameter to a 
fraction of 1 %!

To simplify the manufacturing process the solidified fiber can also be 
coated with the protective plastic layer before it is wound on the take-up 
bobbin. Various other quality and strength tests can also be performed on 
the fast moving fiber before it is spooled onto the take-up bobbin. This is 
shown in the drawings in the referenced web pages.

Best regards, your Mad Scientist
Adrian Popa

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