Introduction to Optical Communications :
Optical Fibers have moved from simple systems to bring light
to places, It is difficult to access systems that affect our lives such as
electronics and computers. Optical fiber have advantages are numerous as the
value of the loss and the light weight but the important feature is very high
bandwidth and reachable thousand of billions of bits per second. optical fiber
has been success in the field of communications and replacing the copper wire
in many uses such as the link between long-distance lines and across the sea .
The experience of using light in communications dates back
to 1880 when the inventor of telephone Alexander Graham experimented with the
transmission of sound by light for distance of 200 meters, and this device is
called Photophone .
T the major step in the development of optical fiber was in
the 1950’s when the laser was invented which has high energy and then used
glass as a medium, but the problem was in glass
is purity, and in 1970 the optical fiber was invented with a loss of 4.
so the energy of transmitted optical signal in this fiber loses by half after
1.2 Installation of fiber
optic and types :
Transmitting Light on
An optical fibre is a very thin strand
of silica glass in geometry quite like a human
hair. In reality it is a very narrow,
very long glass cylinder with special
characteristics. When light enters one
end of the fibre it travels (confined within the
fibre) until it leaves the fibre at the other end.
Two critical factors stand out :
1- Very little light is lost in its journey along the
Fibre can bend around corners and the light will stay within it and be
around the corners.
– optical fibre consists of two parts:
the core and the cladding. The core is
a narrow cylindrical strand of glass and the cladding is a
tubular jacket surrounding it.
as shown in the figure below the core has a (slightly)
higher refractive index than cladding .
Dispersion occurs when a pulse of
light is spread out during transmission
on the fibre. (In most communications
systems bits of information are sent as pulses of light. 1 =
light, 0 = dark) dispersion causes distortion .
Effect of Dispersion shown in figure . The
circles in the figure represent fibre loops. This is the conventional way to
indicate distance in system diagrams.
Optical fiber is divided into two basic types :
1- Single-Mode Fiber : small
core up to a diameter of 9?m, It
transmits infrared signals its wavelength ranging from 1550 – 1310 nm.
The core has a larger diameter of up to 6 ?m
and carries and transmits infrared signals with a wavelength ranging from 1310
– 850 nm.
– comparison between single and multi-mode :
then 10 ?m
than 50 ?m
than 3000 m
– 1310 nm.
– 850 nm
of the optical communications system :
Optical Source :
The function of the optical source is
to generate light energy in order to transmit traffic after modulating the
light signal with the electric signal. . Examples : light emitting diode (LED)
and diode lasers. They all behave as a diode . This means they need a forward
voltage that will result in a current flow in order to operate them. The LED
are cheaper than the lasers, they are used in low data rates and short distance
application . The major disadvantage of the LED is that they have a wide spectrum
width of their light output. Therefore, this main disadvantage causes high
dispersion as the light propagates over the fiber. . This is the reason why LED
cannot be used in long distance applications. However, laser has very narrower spectral width .So it uses
for long distance applications.
Modulated forms :
1.3 Optical Amplifiers And Erbium-doped fiber amplifier (EDFA) :
fiber amplifiers (DFAs) are optical amplifiers that use a doped optical fiber
as a gain medium to amplify an optical signal. They are related to fiber
lasers. The signal to be amplified and a pump laser are multiplexed into the
doped fiber, and the signal is amplified through interaction with the doping
ions. The most common example is the Erbium Doped Fiber Amplifier (EDFA).
Erbium-doped fiber amplifier (EDFA) :
amplifier is used to boost optical signals to higher power, often used both at
launch and within a signal network to maintain a high signal power. The
amplifier is based on erbium doped fiber, and can be incorporated directly into
an optical network, avoiding the need to convert optical signals to electrical
signals for amplification and re-launch .The core of a silica fiber is doped
with trivalent erbium ions and can be efficiently pumped with a laser at a
wavelength of 980 nm or 1,480 nm, and exhibits gain in the 1,550 nm region .
The signal passes along a short length of special fibre and is
amplified (by up to 1000 times, 30 dB) during its travel. The signal
and never leaves the fibre .
????? ????? ????
Fiber To The Home (FTTH) and its Topology:
network architecture refers to the design of a communication network and
provides a framework for the specification of the network from physical
components to service .
Point-to-multipoint topologies (P2MP) provide a single “feeder” fibre from
the central office (or POP) to a branching point and from there one individual,
dedicated fibre is deployed to the subscriber. A passive optical network
technology such as GPON uses passive optical splitters at the branching
point(s) and the Data is encoded so that users only receive data intended for
Point-to-point topologies (P2P) provide dedicated fibres between the Access Node (or POP) and
the subscriber. Each subscriber has a direct connection with a dedicated fibre.
The route from the central office (CO) to the subscriber will probably consist
of several sections of fibres joined with splices or connectors, but provides a
continuous optical path from the Access Node to the home. Most existing point-to-point FTTH deployments use Ethernet,
which can be mixed with other transmission schemes for business applications
(e.g. Fibre Channel). This topology can also include PON technologies by
placing the passive optical splitters in the Access Node .
are basically two different kinds of optical access networks, namely active
optical network (AON) and passive optical network (PON). (Aon) and (Pon) both
of them provide practical solutions for fiber to the home (FTTH) network
1.5 Active Optical Networks(Aon):
Active optical network,
also called point-to-point network, usually uses electrically powered switching
equipment such as a router or switch aggregator, to manage signal distribution
and direct signals to specific customers .
1.6 Passive Optical Networks(pon)
PON doesn’t contain electrically
powered switching equipment, instead it uses fiber optic splitters to guide
traffic signals contained in specific wavelengths. The optical splitters can
separate and collect optical signals when they run through the network .
The PON equipment comprises of an optical line terminal (OLT)
in the point of presence (POP) or central office. One fibre runs to the passive
optical splitter and a fan-out connects a maximum of 64 end-users with each
having an optical network unit (ONU) at the point where the fibre terminates .
The ONU is available in several versions, including an MDU(multi-dwelling
unit ) version suitable for multiple subscribers for in-building applications
and incorporates existing in-building cabling (CAT5/Ethernet or xDSL) .
Advantages of PON includes reduced fibre usage (between POP
and splitters), absence of active equipment between the OLT and ONU.
It is important to note that the last part of the network,
between the last splitter and the end-user, is the same for a point-to-point or
a PON solution: every home passed will be connected with one (or more) fibres
up to the point where the last splitter is to be installed, this is also known
as a fibre concentration point (FCP) or fibre flexibility point (FFP).
– Multiplexing :
A process where multiple analog message signals or digital
data streams are combined into one signal over a shared medium .
– Optically :
Time division multiplexing .
division multiplexing .
– Wavelength Division
Multiplexing (WDM) :
multiplexing, WDM, has long been the technology of choice for transporting
large amounts of data between sites. It increases bandwidth by
allowing different data streams to be sent simultaneously over a single
optical fiber network. In this way WDM maximizes the usefulness of fiber
and helps optimize network investments.
Multiple channels of
information carried over the same fiber , each using and individual wavelength
.WDM is way of transmitting more data by separating channels by color.
Ø . Instead of
using multiple fibers for each and every service, a single fiber can
be shared for several services.
1.7 PON active
1- optical line terminal (OLT)
2- optical network unit (ONU)
The OLT boards can handle up to 16,384 subscribers
(based on 64 users per GPON connection) .
OLT boards can also provide up to 768 point-to-point
connections (Active Ethernet) for applications .
There are a number of different types of ONU available to suit
the location :
1.8 Passive Optical Networks(Pon) and active
Optical Networks(Aon) :
– PON uses passive
components that only need less maintenance and do not need power, which
contributes to that PON building is cheaper than that of AON .
– (AON) networks can cover a range to
about 100 km, a (PON )is typically limited to fiber cable runs of up to 20 km.
– PON uses passive optical splitters at the branching
point(s) and , AON uses electrically powered switching equipment such as a
Optical Networks(pon) type:
3- GPON 4- EPON
Broadband PON (BPON), which is one
type of PON, can support high-speed voice, data and video services to
subscribers residential homes and small businesses. 1490nm wavelength is
utilized in the downstream traffic direction in order to transmit data and
voice services . The same fiber that is used to transmit the downstream traffic
is also used to transmit the upstream traffic. 1310nm is the wavelength used to
transmit data and voice services by the ONT for the upstream. In order to send
video service in the downstream direction from the OLT, 1550nm wavelength is
required. Video distribution is one main application for this one-way
downstream traffic .
– one single OLT is able to achieve
22 BPON lines that can deliver data, voice, and video to 702 clients if the
optical splitter is a 1:32 ratio, as can be shown below .
BPON and two other types of PON :
2- Optical System for Passive Optical Network (BPON) :
1- WDM Transmitter
: Has maximum 8 optical output .
– We see the output spectrum maximum spectrum at wavelength
1.55µ and power of the light -12dbm
as shown below :
circulator bidirectional : three ports if light enters
port 1 it is emitted from port 2, but if some of the emitted light is reflected
back to the circulator, it does not come out of port 1 but instead exits from
port 3 and used to separate optical signals that travel in opposite directions
in an optical fiber.
bidirectional optical fiber : used to separate optical signals that
travel in opposite directions in single
optical fiber .
Length : 20 km –
Attenuation : 0.2 db/km – dispersion : 16.75 ps/nm/km
We see the output spectrum the maximum spectrum at wavelength
1.55µ and minimum at 1.3 µ as shown below :
splitter bidireccional 1*N :
4- optical network
unit (ONU) :
Convert an optical signal to an
Filters the signal with a Bessel
frequency transfer function
Used in upstream due to TDMA
5- 3R Regenerator :
This component regenerates an
electrical signal. It generates the original bit sequence, and a modulated
electrical signal to be used for BER analysis. It is a subsystem based on the
Data Recovery component and a NRZ Pulse Generator.
In this Figure The first output port
is the bit sequence, the second one is a modulated NRZ signal and the last
output is a copy of the input signal. These three signals can be connected
directly to the BER Analyzer, avoiding additional connections between
transmitter and the receiver stage.
So By using the 3R Regenerator, there
is no need for connections between the transmitter and the BER Analyzer. This
is especially important for WDM systems, where you have with multiple
transmitters, receivers and BER Analyzers .
7- BER Analyzer :
the ultimate measurement of the performance for
optical communication system is bit error rate (BER). BER can be defined as the
the correct received bits to the whole number of bits
transmitted. So BER Analyzer allows the
user to calculate and display the bit error rate (BER) of an electrical signal
8- optical time domain visualize for upstream
– We see the output
spectrum for upstream ,the maximum
spectrum at wavelength 1.3µ and minimum at 1.55 µ
as shown below :