1.1Introduction to Optical Communications :Optical Fibers have moved from simple systems to bring lightto places, It is difficult to access systems that affect our lives such aselectronics and computers.
Optical fiber have advantages are numerous as thevalue of the loss and the light weight but the important feature is very highbandwidth and reachable thousand of billions of bits per second. optical fiberhas been success in the field of communications and replacing the copper wirein many uses such as the link between long-distance lines and across the sea .The experience of using light in communications dates backto 1880 when the inventor of telephone Alexander Graham experimented with thetransmission of sound by light for distance of 200 meters, and this device iscalled Photophone .T the major step in the development of optical fiber was inthe 1950’s when the laser was invented which has high energy and then usedglass 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 after800 m. 1.2 Installation of fiberoptic and types :· Transmitting Light ona FibreAn optical fibre is a very thin strandof silica glass in geometry quite like a humanhair. In reality it is a very narrow,very long glass cylinder with specialcharacteristics.
When light enters oneend of the fibre it travels (confined within thefibre) until it leaves the fibre at the other end.Two critical factors stand out :1- Very little light is lost in its journey along thefibre.2- Fibre can bend around corners and the light will stay within it and beguidedaround the corners.- optical fibre consists of two parts: the core and the cladding. The core isa narrow cylindrical strand of glass and the cladding is atubular jacket surrounding it.as shown in the figure below the core has a (slightly)higher refractive index than cladding .
– Dispersion Dispersion occurs when a pulse oflight is spread out during transmissionon the fibre. (In most communicationssystems bits of information are sent as pulses of light. 1 =light, 0 = dark) dispersion causes distortion . Effect of Dispersion shown in figure . Thecircles in the figure represent fibre loops.
This is the conventional way toindicate distance in system diagrams. Optical fiber is divided into two basic types :1- Single-Mode Fiber : smallcore up to a diameter of 9?m, Ittransmits infrared signals its wavelength ranging from 1550 – 1310 nm. 2- Multi-ModeFiber: The core has a larger diameter of up to 6 ?mand carries and transmits infrared signals with a wavelength ranging from 1310- 850 nm.- comparison between single and multi-mode : Single- mode Multi- mode The core Less then 10 ?m grater than 50 ?m distance Less than 3000 m Maximum 2000m source LD LED network WAN LAN wavelength 1550 – 1310 nm. 1310 – 850 nm price High Low · Componentsof the optical communications system : · Optical Source :The function of the optical source isto generate light energy in order to transmit traffic after modulating thelight signal with the electric signal. .
Examples : light emitting diode (LED)and diode lasers. They all behave as a diode . This means they need a forwardvoltage that will result in a current flow in order to operate them. The LEDare cheaper than the lasers, they are used in low data rates and short distanceapplication . The major disadvantage of the LED is that they have a wide spectrumwidth of their light output. Therefore, this main disadvantage causes highdispersion as the light propagates over the fiber. .
This is the reason why LEDcannot be used in long distance applications. However, laser has very narrower spectral width .So it usesfor long distance applications. · Modulated forms : 1.
3 Optical Amplifiers And Erbium-doped fiber amplifier (EDFA) : Dopedfiber amplifiers (DFAs) are optical amplifiers that use a doped optical fiberas a gain medium to amplify an optical signal. They are related to fiberlasers. The signal to be amplified and a pump laser are multiplexed into thedoped fiber, and the signal is amplified through interaction with the dopingions. The most common example is the Erbium Doped Fiber Amplifier (EDFA). Erbium-doped fiber amplifier (EDFA) :Anamplifier is used to boost optical signals to higher power, often used both atlaunch and within a signal network to maintain a high signal power. Theamplifier is based on erbium doped fiber, and can be incorporated directly intoan optical network, avoiding the need to convert optical signals to electricalsignals for amplification and re-launch .The core of a silica fiber is dopedwith trivalent erbium ions and can be efficiently pumped with a laser at awavelength 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 isamplified (by up to 1000 times, 30 dB) during its travel. The signalnever becomeselectricaland never leaves the fibre .????? ????? ????????????? 1.4Fiber To The Home (FTTH) and its Topology:Thenetwork architecture refers to the design of a communication network andprovides a framework for the specification of the network from physicalcomponents to service .1-Point-to-multipoint topologies (P2MP) provide a single “feeder” fibre fromthe central office (or POP) to a branching point and from there one individual,dedicated fibre is deployed to the subscriber. A passive optical networktechnology such as GPON uses passive optical splitters at the branchingpoint(s) and the Data is encoded so that users only receive data intended forthem. (P2MP) 2-Point-to-point topologies (P2P) provide dedicated fibres between the Access Node (or POP) andthe subscriber. Each subscriber has a direct connection with a dedicated fibre.
The route from the central office (CO) to the subscriber will probably consistof several sections of fibres joined with splices or connectors, but provides acontinuous 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 byplacing the passive optical splitters in the Access Node . (P2P) thereare basically two different kinds of optical access networks, namely activeoptical network (AON) and passive optical network (PON).
(Aon) and (Pon) bothof them provide practical solutions for fiber to the home (FTTH) networkconnection (P2P) 1.5 Active Optical Networks(Aon):Active optical network,also called point-to-point network, usually uses electrically powered switchingequipment such as a router or switch aggregator, to manage signal distributionand direct signals to specific customers . 1.6 Passive Optical Networks(pon):PON doesn’t contain electricallypowered switching equipment, instead it uses fiber optic splitters to guidetraffic signals contained in specific wavelengths. The optical splitters canseparate 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 passiveoptical splitter and a fan-out connects a maximum of 64 end-users with eachhaving an optical network unit (ONU) at the point where the fibre terminates .
The ONU is available in several versions, including an MDU(multi-dwellingunit ) version suitable for multiple subscribers for in-building applicationsand incorporates existing in-building cabling (CAT5/Ethernet or xDSL) .Advantages of PON includes reduced fibre usage (between POPand 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 ora PON solution: every home passed will be connected with one (or more) fibresup to the point where the last splitter is to be installed, this is also knownas a fibre concentration point (FCP) or fibre flexibility point (FFP).- Multiplexing :A process where multiple analog message signals or digitaldata streams are combined into one signal over a shared medium . – Optically : · Time division multiplexing .· Wavelength division multiplexing . – Wavelength DivisionMultiplexing (WDM) :Wavelength divisionmultiplexing, WDM, has long been the technology of choice for transportinglarge amounts of data between sites. It increases bandwidth byallowing different data streams to be sent simultaneously over a singleoptical fiber network.
In this way WDM maximizes the usefulness of fiberand helps optimize network investments.Multiple channels ofinformation carried over the same fiber , each using and individual wavelength.WDM is way of transmitting more data by separating channels by color. Ø . Instead ofusing multiple fibers for each and every service, a single fiber canbe shared for several services. 1.7 PON activeequipment :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-pointconnections (Active Ethernet) for applications . There are a number of different types of ONU available to suitthe location :• indoor applications• outdoor applications• business applications• MDU applications 1.8 Passive Optical Networks(Pon) and activeOptical Networks(Aon) :- PON uses passivecomponents that only need less maintenance and do not need power, whichcontributes to that PON building is cheaper than that of AON . – (AON) networks can cover a range toabout 100 km, a (PON )is typically limited to fiber cable runs of up to 20 km.
– PON uses passive optical splitters at the branchingpoint(s) and , AON uses electrically powered switching equipment such as arouter . 1.9 PassiveOptical Networks(pon) type:1-APON 2-BPON 3- GPON 4- EPON 1.9.1 BroadbandPON (BPON):Broadband PON (BPON), which is onetype of PON, can support high-speed voice, data and video services tosubscribers residential homes and small businesses.
1490nm wavelength isutilized in the downstream traffic direction in order to transmit data andvoice services . The same fiber that is used to transmit the downstream trafficis also used to transmit the upstream traffic. 1310nm is the wavelength used totransmit data and voice services by the ONT for the upstream. In order to sendvideo service in the downstream direction from the OLT, 1550nm wavelength isrequired. Video distribution is one main application for this one-waydownstream traffic . – one single OLT is able to achieve22 BPON lines that can deliver data, voice, and video to 702 clients if theoptical 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 : 2- circulator bidirectional : three ports if light entersport 1 it is emitted from port 2, but if some of the emitted light is reflectedback to the circulator, it does not come out of port 1 but instead exits fromport 3 and used to separate optical signals that travel in opposite directionsin an optical fiber. 3- bidirectional optical fiber : used to separate optical signals thattravel 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 networkunit (ONU) : Photodetector :Convert an optical signal to anelectrical signalBessel Filter:Filters the signal with a Besselfrequency transfer functionDynamic Y-Select:Used in upstream due to TDMAmechanism 5- 3R Regenerator :This component regenerates anelectrical signal. It generates the original bit sequence, and a modulatedelectrical signal to be used for BER analysis. It is a subsystem based on theData Recovery component and a NRZ Pulse Generator. In this Figure The first output portis the bit sequence, the second one is a modulated NRZ signal and the lastoutput is a copy of the input signal.
These three signals can be connecteddirectly to the BER Analyzer, avoiding additional connections betweentransmitter and the receiver stage.So By using the 3R Regenerator, thereis no need for connections between the transmitter and the BER Analyzer. Thisis especially important for WDM systems, where you have with multipletransmitters, receivers and BER Analyzers .
7- BER Analyzer :the ultimate measurement of the performance foroptical communication system is bit error rate (BER). BER can be defined as theratio ofthe correct received bits to the whole number of bitstransmitted. So BER Analyzer allows theuser to calculate and display the bit error rate (BER) of an electrical signalautomatically. 8- optical time domain visualize for upstream 9- We see the outputspectrum for upstream ,the maximumspectrum at wavelength 1.3µ and minimum at 1.55 µ as shown below :10-11-