History of Optical Fiber Attenuation

---

We
know that fiber optic technology has started a revolution in the human
communication process and optical fiber is the key component in fiber optics.
Light, of course, no doubt is the carrier of our communication signals. We have
been using light to transmit information for hundreds of years from now.
However, the new way of transmitting light through optical fibers opened up new
avenues of technological innovations that have started enhancing human life.

 

Similarly,
the invention of LASER (Light Amplification by Stimulated Emission of
Radiation) in the 1960s was one of the major leaps that made the transmission
of light through optical fibers possible. The laser can send a larger amount of
data than telephone, microwave, and copper telecommunication systems.

 

Scientists
have tried many different ways to transmit Laser light. They tried through
different types of glass fibers and watched which one gave out lower
transmission losses. The experiments span from space to glass tubes to solid
glass wires, and so on. Solid glass wires yielded promising results and thus
became a preferred medium for researchers.

 

Copper
wires were used for telecommunication and the initial results on solid glass
wires were not enough to replace the copper wires. Transmission loss in solid
glass wires, called Attenuation was high. Researchers have recorded 1,000 dB/km
for the optical fibers in their studies in the 1960s. Such loss levels were not
acceptable for transmission, and researchers continued their pursuit to purify
the glass in order to achieve lower loss.

 

In
1969, scientists concluded that the attenuation in glass wires can be brought
down by removing the impurities. They found that the impurities in glass wires
cause a signal loss in optical fibers. They devised techniques to purify the
glass by removing the impurities. It was not silica that was causing scattering
and absorption losses in a glass fiber, but the metals, metal oxides, and
hydroxyl ions cause high attenuation.

 

Scientists
in Corning’s laboratory in the United States made a multimode
optical fiber in 1970, that had a loss of 20 dB/km. That was an
amazing achievement in the history of optical fibers and attenuation. Charles
Kao, the father of fiber optics proposed ways to achieve attenuation as low as
4dB/km.

 

Corning
made a multimode fiber in 1972 that had an attenuation of 4 dB/km at 850nm.
This invention was a milestone achievement and encouraged researchers all over
the world to work for lower loss levels for silica glass fibers. Multimode
fibers were the first optical fibers to be used for telecommunication networks.
At 1300 nm, Multimode fibers exhibit an attenuation of around 1.5 dB/km. Those
were for the initial versions of multimode fibers having a core diameter of
62.5 micrometers.

 

Now,
we know that a multimode fiber having a 50-micrometer core diameter has an
attenuation of less than 2.5 dB/km at 850 nm and 0.5 dB/km at 1300nm.

 

Attenuation
is a wavelength-dependent characteristic. Attenuation at lower wavelengths is
higher and that at higher wavelengths is lower. Note that this statement is
true for a wavelength range from 800 nm to 1550 nm. From 1550nm onwards, the
attenuation will increase. Towards the end of the 1980s, researchers could
succeed in developing single mode fibers that could support the transmission of
higher wavelengths in order to achieve lower attenuation values.

 

Broadly
there are two basic types of optical fibers, multimode fibers, and single mode
fibers. Both types are used for telecom networks. Single mode optical fibers
are used for long haul networks and multimode optical fibers are used for short
haul networks.

 

Attenuation
in early single mode fibers, those classified as ITU-T G.652A fibers, at 1310
nm was 0.5 dB/km and 0.40 dB/km at 1550 nm. The second version of single mode
fiber, that is ITU-T G.652B showed improvements in attenuation at both wavelengths.
ITU-T G.652B fibers exhibited 0.4 dB/km at 1310 nm and 0.35 dB/km at 1550 nm.
Practically, these attenuation values are lesser than those specified by the
ITU-T.

 

G.652B
fibers offered by optical fiber manufacturers have attenuation values less than
0.35 dB/km at 1310nm and 0.20 dB/km at 1550nm. This is the case with ITU-T
G.652C and ITU-T G.652D fibers, though the ITU-T Recommendations specify 0.4
dB/km at both wavelengths for both types.

 

The
pursuit for lower losses for optical fibers has led to the invention of Low
loss optical fibers. Some manufacturers have achieved a level of 0.17 dB/km at
1550 nm and 0.32 dB/km at 1310 nm for G.652 category fibers.

 

Some
fiber manufacturers were able to achieve attenuation less than 0.16 dB/km for
cut-off shifted fibers, which are used for transoceanic ultra long haul
networks. The lowest attenuation reported so far is 0.1419 dB/km.