460 Chapter 15
around tight corners without any discernible increase in
the fiber’s attenuation. There are several names given to
these optical fibers such as bend insensitive or bend
resistant that can be somewhat misleading when it
comes to the selection of the fiber. The user may tend to
believe that the reduction of the bend radius will also
eliminate any mishandling, temperature extremes,
improper routing, or other external forces on the fiber.
However, the user should be aware that these factors
may not always be true. Selecting a reduced-bend
radius-fiber really achieves the improvements of bend-
ing the fibers for tighter bends in fiber panels, frames
and routing pathways like conduits, raceways and risers.
There is a common basic rule of thumb that the
maximum bend radius should be ten times the outside
diameter of the cable or approximately 1.5 inches,
whichever is greater. This reduced bend radius of the
fiber decreases the standard by about 50%, or to 15 mm,
without changing the fiber’s attenuation.
There have been fiber demonstrations showing a
reduced bend radius fiber patch cord and tying a tight
knot within the patch cord. Then the patch cord was
tested with the tight knot and revealed that no light
escaped and also no increase of attenuation was present.
These improvements for patch cords have been tremen-
dous, but when it comes to using reduced bend radius
for other applications such as in routing in higher densi-
ties or easy connector access they will become more
critical. Thus, always consult with the manufacturer’s
guidelines and specifications when selecting reduced
bend radius fibers.
Connector Loss. Connector loss is a function of the
physical alignment of one fiber core to another fiber
core. Scratches and dirt can also contaminate connector
surfaces and severely reduce system performance, but
most often the connector loss is due to misalignment or
end separation.
Several styles of fiber optic connectors are available
from major connector suppliers. Typically, each manu-
facturer has its own design and is generally not compat-
ible with those of other manufacturers. However, things
are constantly changing for the better so now all SMA-
and ST-type connectors are compatible.
Depending on connector type, different terminating
techniques are used:
- Epoxy and Polish—the fiber is epoxied in place in an
alignment sleeve, then polished at the ferrule face. - Optical and Mechanical—both lenses and rigid align-
ment tubes are commonly used. In addition, index
matching mediums may be employed.
The optical power loss of a connector-to-connector
interface typically runs between 0.1 dB and 2 dB,
depending on the style of the connector and the quality
of the preparation.Splice Loss. Two fibers may be joined in a permanent
fashion by fusion, welding, chemical bonding, or
mechanical joining. A splice loss that is introduced to
the system may vary from as little as 0.01 dB to 0.5 dB.Coupling Loss. Loss between the fiber and the signal
source or signal receiver is a function of both the device
and the type of fiber used. For example, LEDs emit
light in a broad spectral pattern when compared to laser
diodes. Therefore, LEDs will couple more light when a
larger core fiber is used, while lasers can be effective
with smaller core diameters such as in single-mode
systems.
Fiber core size is, therefore, a major factor in deter-
mining how much light can be collected by the fiber.
Coupled optical power increases as a function of the
square of the fiber core diameter.
The numerical aperture (NA) is the light gathering
ability of a fiber. Only light injected into the fiber at
angles greater than the critical angle will be propa-
gated. The material NA relates to the refractive indices
of the core and cladding(15-13)
where,
NA is a unitless dimension.We can also define the angles at which rays will be
propagated by the fiber. These angles form a cone,
called the acceptance cone, that gives the maximum
angle of light acceptance. The acceptance cone is
related to the NA(15-14)where,
T is the half-angle of acceptance, Fig. 15-15.The NA of a fiber is important because it gives an
indication of how the fiber accepts and propagates light.
A fiber with a large NA accepts light well; a fiber with a
low NA requires highly directional light.
In general, fibers with a high bandwidth have a
lower NA; thus, they allow fewer modes. Fewer modes
mean less dispersion and, hence, greater bandwidth.
NAs range from about 0.50 for plastic fibers to 0.21 forNA n 12 –= n 22T=sin1– NA
NA=sinT