TM
CONCERNS REGARDING THE
USE OF VERY SHORT LENGTHS
OF FIBER OPTIC CABLE
MARKET: General Fiber Optics DOC. NUMBER: T-20
Overview:
It is occasionally desirable to connect very
short lengths of optical cable between fiber optic
transmission modules for specific applications
such as high voltage isolation systems, when tra-
versing secure or hazardous environments or for
simple bench-testing of equipment. This TECH-
nique describes steps which should be taken
to reduce the chance of transmission problems.
attenuated however since the transmission of the
cladding glass is poor. Remember, it’s normal
function is only to reflect light back into the core
from it’s surface so it does not have to be very
transparent. For very short lengths of fiber how-
ever significant light can still travel through this
second path.
What does this mean to us? The allowable at-
tenuation range (or loss budget) for most fiber
optic transmission systems is normally specified
from 0 dB to some maximum. These figures natu-
rally assume only core light, not core + cladding
light. If the fiber is very short, a couple of feet for
example, more light can pass than expected and
the fiber optic receiver “front end circuitry” may
actually overload. This will cause distortion in an
analog system and intermittent or even complete
loss of data in a digital system. Furthermore,
since the composition of the cladding glass is not
controlled to the degree that the critical core glass
is, the amount of cladding light can even vary
within the same fiber.
Details:
As shown in the diagram, light normally trav-
els through the core of a fiber optic cable by ei-
ther being reflected from the core/cladding inter-
face of a step-index fiber or by being “bent” by
the varying index of refraction of the glass in a
graded-index fiber. Attenuation figurers (dB/Km)
for fiber cable assume that this is the only path,
as one would naturally expect. This is not com-
pletely true however since there is another path
to consider.
Whenever light passes through any transpar-
ent media with different refractive indices, it
To solve this potential problem the industry has
bends. In a step-index fiber the core glass has a developed what is known as a “0 dB reference
higher index of refraction than the cladding glass fiber”. This fiber simply consists of 10 meters
and the light is reflected back into the core. In (about 33 feet) of standard fiber optic cable wound
the same fiber any light enter-
ing the cladding glass will en-
counter the outside world or air.
Air has a refractive index that
is equal to 1 (which is less than
that of the cladding glass). As
a result light will be reflected
back into the cladding at the air
interface as well and proceed
to travel down the fiber as if
everything were normal. Light
in this second path is quickly
55 Cabot Court, Hauppauge, NY 11788
Phone: (631)273-0404 Fax: (631)273-1638 www.commspecial.com
Questions? Comments? Email: TECHniques@commspecial.com
Fiberlink is a trademark of Communications Specialties, Inc.
© 2000, Communications Specialties, Inc.