Re: what are the advantages and disadvantages of fibre optic cables

Greetings:

References:
1. Mad Science Archives Question: How big is the field of fibre optics?
Could light compete with electricity?

http://www.madsci.org/posts/archives/1999- 09/938144585.Eg.r.html

2. Mad Science Archives Question: How are fiber optics made?

http://www.madsci.org/posts/archives/1998- 03/890381007.Eg.r.html

3. J. C. Palais, Fiber Optic Communications, 4th edition.
Prentice Hall, NJ, 1998.

Fiber optics are replacing wire transmission lines, such as twin lead,
coaxial cable and microwave wave guides, for most telecommunications
applications that are longer than a few hundred meters (yards). In the
1970s, the first commercial fiber optic links were used for long distance
telecommunications between cities and in undersea cables. As the technology
matured in the 1980s and equipment costs were reduced, fiber optic links
were extended to major buildings and industrial and educational campuses.
Since the mid 1990s the thrust has been to install fiber to the block
and now in the 21st century fiber optics are being extended to the home
(FTH) or fiber to the premises (FTP). Much of this history is discussed in
Reference 1.

There are a number of factors that have caused the replacement of metal
transmission lines with optical fibers. The major factor for switching to
optical fiber is that glass fibers have significantly less attenuation
(signal loss) per kilometer (per mile) than metal transmission lines. Also
as the bandwidth of the information carried by the metal transmission lines
increases into the microwave frequency region (gigahertz), the attenuation
of transmission lines becomes ever greater. This is not true for fiber
optics, the attenuation is not dependent, to first order, on the bandwidth
of the information.

One problems with optical fibers is that the electrical information signals
must be converted (modulated) to light at the transmitter and then down
converted (demodulated) from light back to electrical signals at the
receiver. Wire transmission lines do not necessarily have these extra pair
of conversions.

While optical fibers can have very low error rates (bit error rates) better
than one part in a billion for digital signals, they are limited in dynamic
range (power from the noise floor to the maximum signal strength)for analog
signals when compared to metal transmission lines. For example analog radio
and television signals using amplitude modulation (AM) and frequency modulation
(FM) techniques. Thus most, but not all, optical fiber communication systems
use digital signals.

Electromagnetic interference (EMI) from man made sources (machinery, near by
transmitters etc.) and natural sources (solar storms) can become a significant
problem of interference for metal transmission lines while the light in
optical fibers is immune to EMI. Also, the magnetic fields from signals in
metal transmission lines can be monitored by near by equipment while it is
necessary to actually tap light out of a fiber to monitor the signal. Thus
the information carried in optical fibers can be more secure than metal
transmission lines.

As discussed in Reference 2., optical fibers have diameters measured in
micrometers while metal transmission lines typically have diameters measured
in centimeters. Thus optical fibers are much smaller and much lighter in
weight per kilometer (per mile) than metal transmission lines. However,
optical fiber connectors and splices are much more difficult to fabricate
compared to metal based transmission lines. This need for great accuracy in
mating optical fibers requires costly precision equipment and skilled
technicians when compared to connecting wire transmission lines.

Finally metal transmission lines can be bent into tight almost square corners
within equipment while sharp bends place great stress on the glass fibers
causing them to break in time. This time to breaking is also dependent on the
how much moisture is in the glass fibers environment. Thus for long life, it is
best the keep the radius of fiber optic bends to greater than 15 centimeters
(6 inches). This makes it difficult to interface optical fibers with
equipment boxes and electronic circuits. Reference 3. is a low cost
general reference in the field.

Best regards, Your Mad Scientist
Adrian Popa