 An
Overview of Fiber Optic Technology
The use of fiber
optics in telecommunications and wide area networking has
been common for many years, but more recently fiber optics
have become increasingly prevalent in industrial data communications
systems as well. High data rate capabilities, noise rejection
and electrical isolation are just a few of the important characteristics
that make fiber optic technology ideal for use in industrial
and commercial systems.
Most often used
for point-to-point connections, fiber optic links are being
used to extend the distance limitations of RS-232, RS-422/485
and Ethernet systems while ensuring high data rates and minimizing
electrical interference. Conventional electrical data signals
are converted into a modulated light beam, introduced into
the fiber and transported via a very small diameter glass
or plastic fiber to a receiver that converts the light back
into electrical signals. Fiber's ability to carry the light
signal, with very low losses, is based on some fundamental
physics associated with the refraction and reflection of light.
Fiber Optic Principles
Whenever a ray of light passes from one transparent
medium to another, the light is affected by the interface
between the two materials. This occurs because of the difference
in speeds that the light can travel through different materials.
Each material can be described in terms of its refractive
index, which is the ratio of the speed of light in the material
to its speed in free space. The relationship between these
two refractive indices determines the critical angle of the
interface between the two materials.
There are three actions that can happen when
a ray of light hits an interface. Each action depends on the
angle of incidence of the ray of light with the interface.
If the angle of incidence is less than the critical angle,
the light ray will refract, bending toward the material with
the higher refractive index. If the angle of incidence is
exactly equal to the critical angle the ray of light will
travel along the surface of the interface. If the angle of
incidence is greater than the critical angle, the ray of light
will reflect.
The refractive index of vacuum is considered
to be 1. Often, we consider the refractive index of air also
to be 1 (although it is actually slightly higher). The refractive
index of water is typically about 1.33. Glass has a refractive
index in the range of 1.5, a value that can be manipulated
by controlling the composition of the glass itself.
Fiber Optic Characteristics
Optical fibers allow data signals to propagate
through them by ensuring that the light signal enters the
fiber at an angle greater than the critical angle of the interface
between two types of glass. As shown in Figure 1, optical
fiber is actually made up of three parts. The center core
is composed of very pure glass, with a refractive index of
1.5. Core dimensions are usually in the range of 50 to 125
um. The surrounding glass, called cladding, is a slightly
less pure glass with a refractive index of 1.45. The diameter
of the core and cladding together is in the range of 125 to
440 um. Surrounding the cladding is a protective layer of
flexible silicone called the sheath.
When light is introduced into the end of an optical fiber,
any ray of light that hits the end of the fiber at an angle
greater than the critical angle will propagate through the
fiber. Each time it hits the interface between the core and
the cladding it is reflected back into the fiber. The angle
of acceptance for the fiber is determined by the critical
angle of the interface. If this angle is rotated, a cone is
generated. Any light falling on the end of the fiber within
this cone of acceptance will travel through the fiber. Once
the light is inside the fiber it 'bounces' through the core,
reflecting inward each time it hits the interface.
Figure 1 illustrates how light rays travel through the fiber,
reflecting off the interface. If the physical dimensions of
the core are relatively large individual rays of light will
enter at slightly different angles and will reflect at different
angles. Since they travel different paths through the fiber,
the distance they travel also varies. As a result they arrive
at the receiver at different times. A pulse signal sent through
the fiber will emerge wider than it was sent, deteriorating
the quality of the signal. This is called modal dispersion.
Another effect that causes deterioration of the signal is
chromatic dispersion. Chromatic dispersion is caused by light
rays of different wavelengths traveling at different speeds
through the fiber. When a series of pulses is sent through
the fiber, modal and chromatic dispersion can eventually cause
the pulse to merge into one long pulse and the data signal
is lost.
Another characteristic of optical fiber is attenuation.
Although the glass used in the core of optical fiber is extremely
pure, it is not perfect. As a result light can be absorbed
within the cable. Other signal losses include bending and
scattering losses as well as losses due to connections. Connection
loses can be caused by misalignment of the ends of the fiber
or end surfaces not properly polished.
Types of Fibers
Optical fibers are manufactured in three main
types: multi-mode step-index, multi-mode graded-index, and
singlemode. Multi-mode step-index fiber has the largest diameter
core (typically 50 to 100 um). The larger distance between
interfaces allows the light rays to travel the most distance
when bouncing through the cable. Multi-mode fibers typically
carry signals with wavelengths of 850 nm or 1300 nm. The diagram
below shows how a narrow pulse introduced to the fiber becomes
wider at the receiving end.
Multi-mode step-index fiber (a) is comparatively
easy to splice and terminate due to the large diameter fiber.
It is also relatively inexpensive to manufacture compared
to other types. However, it tends to be too slow for most
purposes and it not common in modern systems.
Multi-mode graded-index fiber (b) is constructed
in such a way that the refractive index between the core and
cladding changes gradually. This causes the light rays to
bend gradually, as well. The resulting pattern of reflections
tends to be more uniform and dispersion is reduced. This provides
improved performance for a moderate increase in cost. Gradedindex
fibers provide wider bandwidth than step-index fibers.
Single-mode fibers (c) give the highest performance
of the three types. Manufactured using a very small diameter
fiber (typically 8 um), when light is introduced into the
fiber reflections are kept to a minimum by the dimensions
of the core. Light travels virtually straight through the
core and pulses introduced at one end are reproduced at the
other end with very little dispersion. Typically, single-mode
fibers carry signals with wavelengths of 1320 nm or 1550 nm.
Single-mode fiber is relatively expensive, however, and is
more difficult to splice and terminate since the core must
be aligned very accurately.
Single-mode fibers offer much lower attenuation
than multi-mode fibers. At typical single-mode fiber will
attenuate a 1310 nm signal less than 0.5 dB per kilometer.
A typical multi-mode graded-index fiber will attenuate the
same signal about 3 dB per kilometer. Single-mode fiber is
most often used in applications with high bandwidth requirements
over long distances. Some Ethernet fiber optic equipment can
increase distances from two kilometers using multi-mode fiber
to about 70 kilometers over single-mode fiber.
Fiber Optic Cable Construction
Even though optical fiber seems quite flexible,
it is made of glass, which cannot withstand sharp bending
or longitudinal stress. Therefore when fiber is placed inside
complete cables special construction techniques are employed
to allow the fiber to move freely within a tube. Usually fiber
optic cables contain several fibers, a strong central strength
member and one or more metal sheaths for mechanical protection.
Some cables also include copper pairs for auxiliary applications.
Signal Sources and Detectors
To use fiber optic cables for communications,
electrical signals must be converted to light, transmitted,
received, and converted back from light to electrical signals.
This requires optical sources and detectors that can operate
at the data rates of the communications system.
Signal Sources
There are two main categories of optical signal sources:
- Light Emitting Diodes
- Infrared Laser Diodes
Light emitting diodes (LED) are the less expense, but lower
performance device. These are used in lower-cost applications
where lower data rates and/or shorter distances are required.
Infrared laser diodes operate at much higher speeds, dissipate
higher power levels and require temperature compensation
or control to maintain specified erformance levels. They
are also more costly.
Signal Detectors
Signal detectors also fall into two main categories:
- PIN Photodiodes
- Avalanche Photodiodes
Similarly to sources, the two types provide much different
cost/performance ratios. PIN photodiodes are more commonly
used, especially in less stringent applications. Avalanche
photodiodes on the other hand, are very sensitive and can
be used where longer distances and higher data rates are
involved.
Splicing, Joining and Terminating Optical Fibers
In practical situations fiber optic cables exhibit signal
power losses based on both the fiber and connections from
the fiber to sensors or other fiber segments. Typically fiber
losses run at about 10 dB per kilometer.
Whenever a fiber must be terminated the goal is to produce
a perfectly transparent end to the fiber. The end should be
square, clear and physically mated to the receiving optical
device. In some cases cables are permanently joined by welding
or gluing the ends of the fiber together. Others mechanically
align the fibers and use a transparent gel to couple the signal
at the interface.
Connectors
Early fiber optic connections involved cutting the fiber,
epoxying a special connector, and polishing the end of the
fiber. This operation required special tools and testing equipment
to ensure a good connection. While this technique is still
used, devices used to cut, align and join fibers have been
improved and simplified. Connection losses vary, depending
on the type of connection but typically range from 0.2 dB
to 1 dB.
There are several standard connector types used
to join or terminate fiber optic cables. These include:
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Used in inter/intra building, security, Navy and
industrial applications (also used with some B&B
Electronics products)
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Used in data communications and telecommunications
applications (also used with some B&B Electronics
products)
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Used in data communications and telecommunications
applications
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Used in some fiber optic networks
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Used where high density interconnections are required
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Planning A Fiber Optic Link
The most important consideration in planning
the fiber optic link is the power budget specification of
the devices being connected. This value tells you the amount
of loss in dB that can be present in the link between the
two devices before the units fail to perform properly. This
value will include line attenuation as well as connector loss.
Power Budget Example
For B&B Electronics’ 9PFLST Port-Powered
RS-232 Fiber Optic Modem the typical connector-to-connector
power budget is 12.1 dB. Because 62.5/125 µm cable typically
has a line attenuation of 3 dB per km at 820 nm, the 12.1
dB power budget translates into 2.5 miles (4 km). This assumes
no extra connectors or splices in the link. Each extra connection
would typically add 0.5 dB of loss, reducing the possible
distance by 166 m (547 ft.) Your actual loss should be measured
before assuming distances. When the 9PFLST is used without
external power, the power available to the Fiber Optic transmitter
may be less than the typical value. The link should be tested
with the 9PFLST in place with a variable attenuator to check
the optical power budget of the whole system.
Advantages of Fiber Optic Cables
Noise Immunity
Noise immunity is one of the most useful features of fiber
optics in industrial applications. In environments where electromagnetic
interference is prominent and unavoidable, fiber optics are
unaffected. While cables are normally contained in protective
sheaths and often run inside conduit, there is no need to
physically isolate fiber optic cables from electrical cables.
This makes cable routing simpler.
Electrical Isolation
The problem of ground loop noise and common mode potential
differences is eliminated by the use of fiber optic cables.
Field signals, generated by devices floating at high potentials,
can be coupled to other equipment at much lower potentials
without the risk of damage. This is particularly desirable
in industrial applications.
Low Error Rates
When properly designed to provide adequate signal levels at
the receiving end of the link, a fiber optic system provides
very low bit error rates.
Safe for Use in Hazardous Areas
Fiber optic links can be used to couple signals into areas
with potentially explosive atmospheres without a risk to delivering
or storing sufficient energy to ignite an explosion. This
makes fiber optic technology particularly useful when designing
intrinsically safe systems.
Wide Bandwidth
Fiber optic cables can carry very wide bandwidth signals,
well into the GHz range. Many individual, lower bandwidth
signals can be multiplexed onto the same cable. In commercial
systems fiber optic cable often carries a mixture of signal
types, including voice, video and data all on the same fiber.
Low Signal Attenuation
Optical fibers do exhibit some attenuation due to
absorption and scattering. However, this attenuation is relatively
independent of frequency, a factor that is significant in
copper cables.
Light Weight, Small Diameter
Because many signals can be multiplexed onto one fiber, cables
tend to be smaller and lighter. This makes installation easier.
No Crosstalk
Since fibers do not pick up electromagnetic interference,
signals on adjacent cables are not coupled together.
Inherent Signal Security
For applications where signal security is a concern,
optical fiber is an excellent solution. Fiber optic cables
do not generate electromagnetic fields that could be picked
up by external sensors. It is also more difficult to 'steal'
signals by spicing into optical fibers than it might be with
conventional copper wiring.
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