FREE ESSAY ON FIBER OPTICS

FIBER OPTICS

Table of Contents
1.0 Intro to Fiber Optics
2.0 Fiber Constuction
1.0 Introduction to Fiber Optics
Today many communications companies are replacing their copper carrier wires with fiber
optic cables. A fiber optic cable is capable of transmitting laser light across thousands
of miles and can carry many more messages at the same time than the copper wire of
equivalent diameter. With the relentless pursuit of bandwidth, fiber optic cabling is
being deployed at an ever increasing rate. This cable, which uses glass to carry light
pulses, poses both advantages and challenges. The intent of this paper is to explain the
how's and why's of fiber optic cabling and to provide a set of solutions to the
challenges faced with it's use and give you an understanding of fiber optic cable
technology and its applications. Fiber optic cabling has much to offer, and in most
cases, its use will provide benefits which justify the implementation. 
Since the invention of the telegraph by Samuel Morse in 1838, there has been a constant
push to provide data at higher and higher rates. Today, the push continues. Just as
RS-232 attached terminals gave way to 10Mbps Ethernet and 4 and 16 Mbps Token Ring, these
are giving way to Fast Ethernet (100Mbps), FDDI (100Mbps), ATM (155Mbps), Fiber Channel
(1062Mbps) , Gigabit Ethernet (1000Mbps). With each of these increases in speed, the
physical layer of the infrastructure is placed under more stress and more limitations.
The cabling installed in many environments today cannot support the demands of Fast
Ethernet let alone ATM, Fiber Channel, or Gigabit Ethernet. Fiber Optic cabling provides
a viable alternative to copper. Unlike its metallic counterpart, fiber cabling does not
have the severe speed and distance limitations that plague network administrators wishing
to upgrade their networks. Because it is transmitting light, the limitations are on the
devices driving it more than on the cable itself. By installing fiber optic cabling, the
high cost of labor and the time associated with the cabling plant can be expected to
provide service for the projected future. 
Plastic Optical Fiber (POF) technology is making fiber even more affordable and easier to
install. Because the core is plastic instead of glass, terminating the cable is easier.
The trade-off for this lower cost and ease of installation is shorter distance
capabilities and bandwidth limitations. 
2.0 Fiber Construction 
Fiber optic cabling has the following components (starting in the center and working
out): core, cladding, coating, strength member, and jacket. The design and function of
each of these will be defined. The core is in the very center of the cable and is the
medium of propagation for the signal. The core is made of silica glass or plastic (in the
case of POF) with a high refractive index. The actual core is very small (compared to the
wire gauges we are used to). Typical core sizes range from 8 microns (millionth of a
meter) for single mode silica glass cores up to 1000 microns for multi mode POF. The
cladding is a material of lower index of refraction which surrounds the core. This
difference in index forms a mirror at the boundary of the core and cladding. Because of
the lower index, it reflects the light back into the center of the core, forming an
optical wave guide. This is the same effect as looking out over a calm lake and noting
the reflection, while looking straight down you see through the water. It is this
interaction of core and cladding that is at the heart of how optical fiber works. The
coating (also referred to as buffer or buffer coating) is a protective layer around the
outside of the cladding. It is typically made of a thermoplastic material for tight
buffer construction and a gel material for loose buffer construction. As the name
implies, in tight buffer construction, the buffer is extruded directly onto the fiber,
tightly surrounding it. Loose buffer construction uses a gel filled tube which is larger
than the fiber itself. Loose buffer construction offers a high degree of isolation from
external mechanical forces such as vibration. Tight buffer construction on the other hand
provides for a smaller bend radius, smaller overall diameter, and crush resistance. To
further protect the fiber from stretching during installation, and to protect it from
expansion and contraction due to temperature changes, strength members are added to the
cable construction. These members are made from various materials from steel (used in
some multi - strand cables) to Kevlar. In single and double fiber cables, the strength
members are wrapped around the coating. In some multi-strand cables, the strength member
is in the center of the bundle. The jacket is the last item in the construction, and
provides the final protection from the environment in which the cable is installed. Of
concern here is the intended placement of the cable. Different jackets provide different
solutions for indoor, outdoor, aerial, and buried installations.
Fiber Specifications 
The most common size of multi mode fiber used in networking is 62.5/125 fiber. This fiber
has a core of 62.5 microns and a cladding of 125 microns. This is ideally suited for use
with 850nm and 1300nm wavelength drivers and receivers. For single mode networking
applications, 8.3/125 is the most common size. It's smaller core is the key to single
mode operation. 
Numerical aperture and acceptance angles are two different ways of expressing the same
thing. For the core / cladding boundary to work as a mirror, the light needs to strike at
it a small / shallow angle (referred to as the angle of incidence). This angle is
specified as the acceptance angle and is the maximum angle at which light can be accepted
by the core. Acceptance angle can also be specified as Numerical Aperture, which is the
sin of the acceptance angle (Numerical Aperture = sin (acceptance angle)).
Transmitter Specifications 
With a basic understanding of fiber construction, explanation of transmitters (the
devices that put the pulses of light into the fiber) is in order. From a general level,
there are three aspects of transmitters to discuss: 
1.type of transmitter 
2.wavelength of transmitter 
3.power of the transmitter 
Transmitters can be divided into 2 groups, lasers and LEDs. LEDs are by far the most
common as they provide low cost and very efficient solutions. Most multi mode
transmitters are of the LED variety. When high power is required for extended distances,
lasers are used. Lasers provide reliable light and the ability to produce a lot of light
energy. The drawbacks to lasers are their cost and electrical power consumption.
Equipment using high power lasers must provide cooling and access to a primary power
source such as 120V AC.
Transmitter types can also be broken down into single mode versus multi mode
transmitters. Multi mode transmitters are used with larger cable (typically 62.5/125
microns for most data networking applications) and emit multiple rays or modes of light
into the fiber. Each one of these rays enters at a different angle and as such has a
slightly different path through the cable. This results in the light reaching the far end
at slightly different times. This difference is arrival times are termed modal dispersion
and causes signal degradation. Single mode transmitters are used with very small cable
(typically 8/125 microns) and emit light in a single ray. Because there is only one mode,
all light gets to the far end at the same time, eliminating modal dispersion. 
The wavelength of the transmitter is the color of the light. The visible light spectrum
starts around 750nm and goes to 390nm. The 850nm transmitters common in multi mode
Ethernet can be seen because 850nm is the center of their bandwidth and they emit some
visible light in the 750nm range giving them their red color. The 1300nm and 1550nm
transmitters emit light only in the infrared spectrum. The difference in performance of
the various wavelengths is beyond the scope of this paper. What is important is an
awareness of the wavelengths and that the equipment on both ends of the fiber needs to be
matched. The final characteristic of transmitters is the output power. This is a measure
of the optical energy (intensity) launched into the fiber. It is measured in dBm. A
typical value for multi mode transmitters used in Ethernet is -15dBm. Single mode
transmitters have a wide range in power depending on the application.
Receiver Specifications 
With a knowledge of transmitters, what happens at the other end of the cable is
important. The light pulses are terminated and detected with a receiver. Receivers have
three basic considerations. These are: 
1.wavelength 
2.mode (single vs. multi) 
3.sensitivity 
Sensitivity is the counterpart to power for transmitters. It is a measurement of how much
light is required to accurately detect and decode the data in light stream. It is
expressed in dBm and is a negative number. The smaller the number (remember -40 is
smaller than -30) the better the receiver. Typical values range from -30dBm to -40dBm.
Receive sensitivity and transmitter power are used to calculate the optical power budget
available for the cable. This calculation is: Power Budget = Transmitter Power - Receiver
Sensitivity, Using the typical values given for multi mode Ethernet above, the power
budget would be: 15dBm = -15dBm - (-30dBm) The optical power budget must be greater then
all of the cable plant losses (such as attenuation, losses due to splices and connectors,
etc.) for the installation to work properly.
Connector Types
Figure A. - SC Connector Figure B. - ST Connector 
Many different connector styles have found their way into fiber optic networking. The SC
connector (Figure A) has recently been standardized by ANSI TIA/EIA-568A for use in
structured wiring installations. Many single mode applications are now only available in
the SC style. The ST connector (Figure B) has been the connector of choice for these
environments, and continues to be widely used. FDDI uses the MIC connector which is a
duplex connector. It is physically larger then the SC connector, and the SC connector is
gaining acceptance in the FDDI marketplace.
Fiber Advantages 
Fiber provides several advantages to Ethernet and Fast Ethernet networks. The most common
advantage and therefore use of fiber is to overcome the distance limitations of coaxial
and twisted pair copper topologies. Ethernet being run on coax (10Base2) has a maximum
distance limitation of 185m, and Ethernet being run on twisted pair (10BaseT and
100BaseTX) has a limitation of 100m. Fiber can greatly extend these distances with
multi-mode fiber providing 2000m and single-mode fiber supporting 5km in half duplex
environments, and much more (depending on transmitter strength and receiver sensitivity)
in full duplex installations. Ethernet running at 10Mbps has a limitation of 4 repeaters,
providing some leniency in the solutions available for distance, however, Fast Ethernet
only allows for 2 repeaters and only 5m of cable between them. 
As Fast Ethernet becomes more ubiquitous, the need for fiber optic cabling will grow as
well. When distance is an issue, fiber provides what may be the only solution. Even when
using coaxial cable or twisted pair (shielded or unshielded), some electrical noise may
be emitted by the cable. This is especially true as connectors and ground connections age
or weaken. In some environments (medical for example), the potential risk associated with
this is just not acceptable, and costs of alternative cable routings too high. Because
fiber optic cabling uses light pulses to send the signal, there is NO radiated noise.
This makes it perfectly safe to install this cabling in any sensitive environment.
Optical fiber adds additional security protection as well. There are no emissions to pick
up and decode, and it is not feasible to tap into it for the purposes of eavesdropping.
This makes fiber optic cabling ideal for secure network installations. 
Another problem that is common when using copper cabling is other electrical noise
getting into the desired electrical networking signal. This can be a problem in noisy
manufacturing environments or other heavy industrial applications. The use of optical
fiber provides a signal that will be completely unaffected by this noise. In some
instances, fiber provides the advantage that it can withstand more tension during the
cable pulling. It is also smaller in size then twisted pair cables and therefore takes up
less room. Compared to Category 5 UTP, most duplex fiber optical cable can also endure a
tighter bend radius while maintaining specified performance.
Fiber Challenges 
Fiber optical cabling is not a cure-all however, there are some challenges to be
resolved. The first (and probably the best known), is the cost of termination. Because of
the need for perfect connections, splices and connections must be carefully cut and then
polished to preserve the optical characteristics. The connectors must also maintain a
very high level of precision to guarantee alignment of the fibers. 
The second problem that is encountered when installing fiber cabling is that legacy
equipment does not support fiber connections. Very few desktop computers have a fiber
network interface, and some critical network equipment does not offer a fiber interface.
In Ethernet, the size of the collision domain can effect the use of fiber. In a half
duplex (shared media) environment, no 2 devices can be separated by more then 512 bit
times. While the transmission of a signal is faster through fiber than copper, only about
11% faster and not enough to make a significant difference. This limitation means that
there are times when the signal quality and fiber are sufficient to carry the signal but
the distance and network design rule out it's use.
Fiber Solutions 
Fortunately, the problems are not without solutions. As fiber deployment increases, the
economy of scale for the manufacturers is driving costs down. Also, much work is being
done to further reduce these costs, Plastic Optical Fiber is an example of one such
development. 
The need to connect to legacy equipment and infrastructure also has a solution. By using
copper to fiber media converters, fiber can be connected to almost any legacy
environment. Equipment equipped with an AUI port can also make use of fiber transceivers
as well. Media converters are devices (usually small enough in size to fit in the palm of
your hand) which take in signals from one media type and send it out on another media
type. For those instances when collision domain restrictions preclude the use of fiber, a
2 port bridging device (such as Transition Networks Bridging Media Converter) with
10/100-Base-T(X) on one port and fiber on the other can be used. Bridges by definition
break collision domains, and when connected to a server, workstation, or another bridge
can operate in Full Duplex mode. In this mode, there are no limitations imposed by
collision domains, and the distance attainable is solely a function of the fiber cable;
and transmitters and receivers.
Summary 
Fiber optic cabling is rapidly becoming the most viable choice for data networking
infrastructure. With the cost of cable, connectors, installation, and equipment becoming
competitive with traditional copper solutions, fiber should be given serious
consideration. Transition Networks' complete line of fiber connectivity products are
specifically designed to ease this migration to fiber. Once installed, fiber optic
cabling will future proof your cabling infrastructure, providing support for even the
fastest most demanding protocols.
Work Cited
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