CHAPTER 3 COMPONENT IN FIBER OPTIC COMMUNICATION Fiber
CHAPTER 3 COMPONENT IN FIBER OPTIC COMMUNICATION
Fiber Optic Sources Two basic light sources are used for fiber optics: � light-emitting diodes (LED) �laser diodes (LD)
Light-Emitting Diodes �An LED is form of junction diode that is operated with forward bias �Instead of generating heat at the PN junction, light is generated and passes through an opening or lens �LEDs can be visible spectrum or infrared
light-emitting diodes (LED) � Fiber optic sources must operate in the low-loss transmission windows of glass fiber. � LEDs are typically used at the 850 -nm and 1310 -nm transmission wavelengths � LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of their inherent bandwidth limitations and lower output power. � They are used in applications in which data rates are in the hundreds of megahertz. � Two basic structures for LEDs are used in fiber optic systems: surface emitting and edge emitting � The output spectrum of a typical LED is about 40 nm, which limits performance because of severe chromatic dispersion. � LEDs operate in a more linear fashion than do laser diodes. This makes them more suitable for analog modulation
Surface-emitting LEDs �Light emerges from top �Directed by device structure and packaging �Common for illumination LEDs +- +-
Edge-emitting LEDs �Light in junction plane �Emerges from side facet �Smaller emitting area +- +-
LED �Figure 8 -22 shows a graph of typical output power versus drive current for LEDs and laser diodes. �Notice that the LED has a more linear output power that makes it more suitable for analog modulation �Typical applications are local area networks, closedcircuit TV, and transmitting information in areas where EMI may be a problem.
Laser Diodes �Laser diodes generate coherent, intense light of a very narrow bandwidth �A laser diode has an emission linewidth of about 2 nm, compared to 50 nm for a common LED �Laser diodes are constructed much like LEDs but operate at higher current levels
Laser Diode Construction
Laser Diode �Laser diodes (LD) are used in applications in which longer distances and higher data rates are required. �Because an LD has a much higher output power than an LED, it is capable of transmitting information over longer distances. � Consequently, and given the fact that the LD has a much narrower spectral width, it can provide high bandwidth communication over long distances. �The LD has smaller N. A. also allows it to be more effectively coupled with single-mode fiber. �The difficulty with LDs is that they are inherently nonlinear, which makes analog transmission more difficult
Laser Diode �They are also very sensitive to fluctuations in temperature and drive current, which causes their output wavelength to drift. �In applications such as wavelength-division multiplexing, in which several wavelengths are being transmitted down the same fiber, the stability of the source becomes critical. �This usually requires complex circuitry and feedback mechanisms to detect and correct for drifts in wavelength. �The benefits, however, of high-speed transmission using LDs typically outweigh the drawbacks and added expense.
Laser Diode Laser diodes can be divided into two generic types �Gain-guided laser diodes work by controlling the width of the drive-current distribution; this limits the area in which lasing action can occur. �Index-guided laser diodes use refractive index steps to confine the lasing mode in both the transverse and vertical directions.
LED vs. laser spectral width Single-frequency laser (<0. 04 nm) Laser output is many times higher than LED output; they would not show on same scale Standard laser (1 -3 nm wide) LED (30 -50 nm wide) Wavelength
LED versus Laser Characteristic Output power Spectral width LED Lower Wider Laser Higher Narrower Numerical aperture Speed Cost Ease of operation Larger Smaller Slower Less Easier Faster More difficult
Light Detector �Optical detection occurs at the light wave receiver’s circuitry. �The photo detector is the device that receives the optical fiber signal and converts it back into an electrical signal. �The most common types of photo detectors are a) positive intrinsic negative photodiode ( PIN ) b) the avalanche photodiode (APD )
Light detector characteristic The important characteristics of light detectors are : 1. Responsitivity: Responsitivity is a measure of the conversion efficiency of a photodetector. 2. Dark current: Dark current is the leakage current that flows through a photodiode with no light input. 3. Transit time: Transit time is the time it takes a lightinduced carrier to travel across the depletion region. 4. Spectral response: Spectral response is the range of wavelength values that can be used for a given photodiode. 5. Light sensitivity: Light sensitivity is the minimum optical power a light detector can receive and still produce a usable electrical output signal.
PIN diode � The most common optical detector used with fiber-optic systems is the PIN diode � The PIN diode is operated in the reverse-bias mode � As a photodetector, the PIN diode takes advantage of its wide depletion region, in which electrons can create electron-hole pairs � The low junction capacitance of the PIN diode allows for very fast switching � PIN photodiodes are inexpensive, but they require a higher optical signal power to generate an electrical signal. � They are more common in short distance communication applications.
Avalanche Photodiode � APD photodiodes are more sensitive to lower optical signal levels and can be used in longer distance transmissions. � They are more expensive than the PIN photodiodes and are sensitive to temperature variations � The avalanche photodiode (APD) is also operated in the reverse- bias mode � The creation of electron-hole pairs due to the absorption of a photon of incoming light may set off avalanche breakdown, creating up to 100 more pairs � This multiplying effect gives an APD very high sensitivity
Splices and Connectors � In fiber-optic systems, the losses from splices and connections can be more than in the cable itself � Losses result from: Axial or angular misalignment Air gaps between the fibers Rough surfaces at the ends of the fibers
Fiber-Optic Connectors �Coupling the fiber to sources and detectors creates losses as well, especially when it involves mismatches in numerical aperture or in the size of optical fibers �Good connections are more critical with single-mode fiber, due to its smaller diameter and numerical aperture �A splice is a permanent connection and a connector is removable
Type of connector Type Feature Ferrule Connector (FC) -Was designed for use in high vibration environment - provide non-optical disconnect performance -Designed with a threaded coupling for durable connection Straight Tip (ST) -Maintains the perfect alignment of the ends of the connected fibers required for efficient light transmission. - Mate with an interconnection adapter and is latched into place by twisting to engage a spring-loaded bayonet socket Application - datacom -Telecommunicati on - measurement equipment -multimode fiber optic LAN
Type of connector Type Feature Application Subscriber Connector (SC) - a standard- duplex fiber optic connector with a sqaure molded plastic body and push-pull locking features. -Data communication - CATV - telephony Subminiature (SMA) -robust fiber optic connector that is composed of a threaded coupling housing -can withstand high temperatures without experiencing hot spots - Compatible with TO-18 transmitter/emitter components - Medical - Industrial - Data / Telecom - FTTx - Mining - Oil exploration
Type of connector Type Feature Application Lucent / Local Connector -similar to a RJ 45 connector - Optimized for point to point interconnection and multi-channel routing application -Data / Telecom -Local Area - Network (LAN) - FTTH / FTTP -Cable TV
Optical Couplers �An optical device that combines or splits power from optical fibers �As with coaxial cable and microwaveguides, it is possible to build power splitters and directional couplers for fiber-optic systems �It is more complex and expensive to do this with fiber than with copper wire �Optical couplers are categorized as either star couples with multiple inputs and outputs or as tees, which have one input and two outputs
Type of coupler/adapter Type Feature Application ST -used to link different kinds of ST optical fiber components. - Mates with interconnection adapter and is latched into place by twisting to engage a spring-loaded bayonet socket -Premise installation Telecommunicatio n networks (LANs) Data processing networks SC - a snap-in (push-pull) connection design for quick patching of cables into rack or wall mounts. . -CATV Telecommunicatio n networks Local Area Networks (LANs)
Type of coupler/adapter Type Feature Application Fiber Distributed Data Interface (FDDI) - refer to a local area network standard such as Ethernet ar Token Ring. - Contain two ferrules in large, bulky plastic housing that uses a squeeze tab retention mechanism. -CATV Telecommunicatio n networks -Local Area Networks (LANs) FC -used to link the screw type FC optical fiber connections. - can be used alone or be mounted onto fiber optic patch panels. -CATV Telecommunicatio n networks - Local Area Networks (LANs)
Optical Switches and Relays �Optical switch is a switch that enables signals in optical fibers or integrated optical circuits (IOCs) to be selectively switched from one circuit to another. �The simplest type of optical switch moves fibers so that an input fiber can be positioned next to the appropriate output fiber �Another approach is direct the incoming light into a prism, which reflects it into the outgoing fiber. By moving the prism, the light can be switched between different output fibers �Lenses are necessary with this approach to avoid excessive loss of light
Optical Cross Connect � Optical Cross-Connects (OXC) Wavelength Routing Switches (WRS) route a channel from any I/P port to any O/P port � Natively switch s while they are still multiplexed � Eliminate redundant optical-electronic-optical conversions DWDM Demux DWDM Fibers in DWDM Mux DWDM Fibers out All-optical DWDM Demux OXC DWDM Mux
MEMS Optical Switches �What is MEMS Micro-Electro-Mechanical System �What is MEMS optical switches Steerable micromirror array to direct optical light from input port to its destination port. System-in-a-chip
2 D MEMS Switches �Mirrors have only 2 positions (cross or bar) �Crossbar configuration �N 2 mirrors
3 D MEMS Switches �Mirrors can be tilted to any angles �N or 2 N mirrors accomplishing nonblock switching �Good scalability
Repeaters, Regenerators, and Optical Amplifiers �Boost signal after it fades with distance �Needed to span long distances more than 50 -100 km terrestrial often shorter distances in networks �Repeater: receiver-transmitter pair �Regenerator: Repeater plus signal clean-up �Optical amplifiers: amplify signal as light Current state of the art at 1530 -1620 nm �Optical regenerators: would be nice
Repeaters and regenerators Detector Electronic amplifier Transmitter Repeater Optional Detector Electronic amplifier Regenerator Thresholding & retiming Forward error correction Transmitter
Electro-optic repeaters �Receiver converts signal to electronic form �Electronics amplify signal, drive transmitter �Became obsolete Limited to one transmission format Designed for particular data rate One optical channel per repeater Erbium-doped fiber amplifiers are better �NOT obsolete for wavelength conversion
Why regenerators are still used �Optical amplifiers are analog devices Cannot remove noise or dispersion Contribute amplified spontaneous emission �Dispersion accumulates over long distances �Regeneration used at termination points �Most terrestrial systems <1000 km Terminate in switches or routers Signals redistributed �Regeneration is within the switch
Optical amplifiers �Directly amplify weak optical signal Stimulated emission from excited material Laser without a resonant cavity Optical signal makes single pass �Amplify all wavelengths in their range Compatible with WDM �Purely analog devices Require fine tuning to limit noise
Types of optical amplifiers �Erbium-doped fiber amplifiers: C band 1530 -1565 nm-most widely used L band 1570 -1620 nm �Thulium doped fibers, S band 1470 -1500 nm �Raman fiber amplifiers: broadband �Praseodymium-doped fiber amplifiers 1310 nm range �Semiconductor optical amplifiers �Cascaded
Erbium-fiber amplifier
Erbium fiber operation �Single pass produces gain �Optical isolators prevent feedback to laser �Noise from amplified spontaneous emission �Erbium gain is broad: 1520 -1630 nm Depends on erbium host Designs differ for different wavelength bands Long fibers for low-gain L band 1570 -1620 nm Short fibers for high-gain C band 1530 -1565 nm
Erbium-fiber gain
Cascaded �Amplifiers connected in series in electronics. �Two amplifiers are connected together by using coupling capacitor. �Used when topological conditions do not allow direct communication between module and gateway. �Their use can double or triple the communication distance between point within the limit of 3 repeater in cascaded.
Noise factor a) Thermal noise Noise due to thermal agitation of electron in a conductor. - It is present in all electronic devices and transmission media and is a function of temperature b) Shot noise - Caused by discrete nature of electrons a signal disturbance - The pulse start when the electron escapes from the cathode and end when the electron strikes the anode. -
Noise factor c) Dark Current Noise - The relatively small electric current that flow through photosensitive devices such as a photomultiplier tube, photodiode are charge coupled device even when no photon are entering the device. - Refer as reverse leakage current.
Signal to Noise Ratio SNR = S/N S – represents the information to be transmitted N – integration of all noise factors over the full system bandwidth SNR (d. B) = 10 log 10 (S/N)
Fiber Joints (Connections) �Point where two fibers are joined together � To allow light signal to propagate from one fiber into the other with as little loss as possible �Reasons for fiber joints: �Fibers and cables are not endless and therefore must eventually be joined. �Fiber may also be joined to distribution cables and splitters. �At both transmit and receive termination
Fiber Joints (Connections) Fiber optic cables terminated in 2 ways : �Connectors �Splices Splicing �Permanent connection of two optical fibers.
Fiber splices (contd. ) Need of splicing: �System design may require that fiber connections have specific optical properties (low loss) �Permit repair of damaged optical fibers �Cables are of limited lengths – 1 to 6 km. � To establish long-haul optical fiber links. �Splices might be required at building entrances, couplers, wiring closets, etc. Broadly classified into two types �Arc fusion splicing �Mechanical splicing
Splicing: Pre-requisite End preparation Stripping : � Stripping away all protection � Stripping their protective polymer coating � Thermal splicers are best Cleaning: � alcohol and wipes, or � ultrasonic cleaner Cleaving: � perfect fiber end face cut
Alignment Fiber splice alignment �Passive : relies on precision reference surfaces �Active : monitors splice loss or uses microscope
Arc Fusion Splicing �Localized heat melts or fuses the ends �Splice loss- direct function of angles and quality of fiber-end faces �Arc fusion- discharge of electric current across the gap between two electrodes
Arc Fusion Splicing � Fiber end placed between electrodes � Electric discharge melts or fuses the ends of each fiber � Initially, a small gap between fiber ends � Pre-fusion: short discharge of electric current, eliminates fiber defects from cleaving � Surface defects can cause core distortions or bubble formations � Fusion splice--ends pressed together, actively aligned, -longer and stronger electric discharge � Surface tension of molten glass tends to realign
Arc Fusion Splicing �Protecting the fiber: �Splice protection sleeve � Yields vary between 25 and 75% �Sophisticated fusion splices for low loss
Mechanical Splicing �Mechanical fixtures to align and connect optical fibers �Amount of splice loss stable overtime �Unaffected by changes in environmental or mechanical conditions �If high splicing loss results- splice reopened and fibers realigned
Mechanical Splicing (contd. ) Glass or Ceramic Alignment Capillary tube � Inner diameter of tube slightly larger than outer diameter of fiber � Transparent adhesive injected into the tube bonds the two fiber together � Adhesive also provides index matching � Relies on inner diameter of tube � Inner diameter should be appropriates
Mechanical splicing (contd. ) V-Grooved Splices � Open grooved substrate to perform fiber alignment � V-groove aligns the cladding surface of each fiber end � Transparent adhesive makes the splice permanent by securing the fiber ends to the grooved substrate � Transparent adhesives are epoxy resins that seal mechanical splices and provide index matching between the connected fibers. Fig. Open Vgrooved splice.
Mechanical splicing (contd. ) Spring V-Grooved Mechanical Splice � Two positioning rods � Two rods form a groove. This is used to align the fiber ends � Outer surface of each fiber end extends above the groove formed by the rod � A flat spring presses fiber ends into the groove � Transparent adhesive completes the process Fig. Spring V-grooved mechanical splice.
Mechanical splicing (contd. ) Rotary Splice � Fibers are mounted into a glass ferrule and secured with adhesives � The splice begins as one long glass ferrule that is broken in half during the assembly process. � Fiber inserted into each half of the tube, epoxied using ultraviolet cure epoxy. � The end face of the tubes are polished and placed together using the alignment sleeve. � Added mechanical stability � The rotary splice may use index matching gel within the alignment sleeve to produce low-loss slices.
Splicing Defect �Several defects can occur during splicing leading to useless splices �Great care needs to be taken
Dense Wavelength Division Multiplexing � It transmits multiple data signals using different wavelengths of light through a single fiber. � Incoming optical signals are assigned to specific frequencies within a designated frequency band. � The capacity of fiber is increased when these signals are multiplexed onto one fiber � Transmission capabilities is 4 -8 times of TDM Systems with the help of Erbium doped optical amplifier. � EDFA’s : increase the optical signal and don’t have to regenerate signal to boost it strength. � It lengthens the distances of transmission to more than 300 km before regeneration
Dense Wavelength Division Multiplexing A B C l 1 Wavelength Division Multiplexer Fibre Wavelength Division Demultiplexer l 2 l 3 l 1 l 2 l 1 + l 2 + l 3 X Y Z Ÿ Multiple channels of information carried over the same fiber, each using an individual wavelength Ÿ Dense WDM is WDM utilizing closely spaced channels Ÿ Channel spacing reduced to 1. 6 nm and less Ÿ Cost effective way of increasing capacity without replacing fiber Ÿ Commercial systems available with capacities of 32 channels and upwards; > 80 Gb/s per fiber
Simple DWDM System l 1 T 1 Wavelength Division Multiplexer Fibre Wavelength Division Demultiplexer l 2 TN l. N l 1 l 2 l 1 + l 2. . . l. N R 1 R 2 RN � Multiple channels of information carried over the same fiber, each using an individual wavelength � Unlike CWDM channels are much closer together � Transmitter T 1 communicates with Receiver R 1 as if connected by a dedicated fiber as does T 2 and R 2 and so on
Is DWDM Flexible? � DWDM is a protocol and bit rate independent hence, data signals such as ATM, SONET and IP can be transmitted through same stream regardless their speed difference. � The signals are never terminated within the optical layer allows the independence of bit rate and protocols, allowing DWDM technology to be integrated with existing equipment in network. � Hence, there’s a flexibility to expand capacity within any portion of their networks.
Is DWDM Expandable? � “ DWDM technology gives us the ability to expand out fiber network rapidly to meet growing demands of our customer”, said Mike Flynn, group President for ALLTEL’s communications operations. � DWDM coupled with ATM simplifies the network, reduce network costs and provide new services. � They can add current and new TDM systems to their existing technology to create a system with virtually endless capacity expansion
DWDM System Characteristics � Well-engineered DWDM systems offer component reliability, system availability, and system margin. Although filters were often susceptible to humidity, this is no longer the case. � An optical amplifier has two key elements: the optical fiber that is doped with the element erbium and the amplifier. � Automatic adjustment of the optical amplifiers when channels are added or removed achieves optimal system performance. � In the 1530 - to 1565 -nm range, silica-based optical amplifiers with filters and fluoride-based optical amplifiers perform equally well. � The system wavelength and bit rate can be upgraded but planning for this is critical.
DWDM Components � Transmitter : Laser with precise stable wavelength. � Link: Optical fiber that exhibits low loss and transmission performance in relevant wavelength spectra. � Receiver: Photo detectors and Optical demultiplexers using thin film filters or diffractive elements. � Optical add/drop multiplexers and optical cross connect components.
DWDM component –Mux/demux DWDM terminal demultiplexer �The terminal demultiplexer breaks the multiwavelength signal back into individual signals and outputs them on separate fibres for client-layer systems (such as SONET/SDH) to detect.
DWDM component -OADM Optical Add/drop multiplexer (OADM) �Between multiplexing and demultiplexing points in a DWDM system, there is an area in which multiple wavelengths exist.
DWDM component -OSC Optical Supervisory Channel (OSC) � The OSC carries information about the multi-wavelength optical signal as well as remote conditions at the optical terminal or EDFA site. � The “out–of–band” Optical Supervisory Channel (OSC) allows the supervision of all the NEs along the WDM path; moreover it gives some order–wires (data channel and voice channel) to the users. � Out-of-band, means the OSC is using a different band than the DWDM system is normally running in, which normally would be the U-band. � ITU standards suggest that thee OSC should utilize an OC-3 signal structure, though some vendors have opted to use 100 Megabit Ethernet or another signal format.
DWDM System with optical amplifier Receivers DWDM Multiplexer Optical fibre Power Amp Transmitters Line Amp Receive Preamp DWDM De. Multiplexer 200 km ŸEach wavelength behaves as if it has it own "virtual fibre" ŸOptical amplifiers needed to overcome losses in mux/demux and long fiber spans
DWDM System with Add-Drop Add/Drop Mux/Demux DWDM Multiplexer Power Amp Transmitters Optical fibre Receivers Line Amp Receive Preamp DWDM De. Multiplexer 200 km • OADM can drop a number of incoming wavelengths and insert new optical signals on these wavelengths. The remaining wavelengths of the WDM link are allowed to pass through. • The wavelengths that it adds/drops can be either statically or dynamically configured.
DWDM Coupler Ÿ 8 wavelengths used (4 in each direction). 200 Ghz frequency spacing ŸIncorporates a Dispersion Compensation Module (DCM) ŸExpansion ports available to allow denser multiplexing
DWDM versus TDM ŸDWDM can give increases in capacity which TDM cannot match ŸHigher speed TDM systems are very expensive
DWDM Standards ŸITU Recommendation is G. 692 "Optical interfaces for multichannel systems with optical amplifiers" ŸG. 692 includes a number of DWDM channel plans ŸChannel separation set at: Ø 50, 100 and 200 GHz Ø equivalent to approximate wavelength spacings of 0. 4, 0. 8 and 1. 6 nm ŸChannels lie in the range 1530. 3 nm to 1567. 1 nm (so-called C-Band) ŸNewer "L-Band" exists from about 1570 nm to 1620 nm ŸSupervisory channel also specified at 1510 nm to handle alarms and monitoring
Optical Spectral Bands 2 nd Window O Band 5 th Window E Band S Band 1200 1300 1400 1500 Wavelength in nm C Band L Band 1600 1700
Applications of DWDM � DWDM is ready made for long-distance telecommunications operators that use either point-topoint or ring topologies. � Building or expanding networks � Network wholesalers can lease capacity, rather than entire fibers. � The transparency of DWDM systems to various bit rates and protocols. � Utilize the existing thin fiber � DWDM improves signal transmission
Advantages � Robust and simple design � Works entirely in the Optical domain � Multiplies the capacity of the network many fold � Cheap Components � Handles the present BW demand cost effectively � Maximum utilization of untapped resources � Best suited for long-haul networks
Disadvantages �Dispersion Chromatic dispersion Polarization mode dispersion �Attenuation Intrinsic: Scattering, Absorption, etc. Extrinsic: Manufacturing Stress, Environment, etc. �Four wave mixing Non-linear nature of refractive index of optical fiber Limits channel capacity of the DWDM System PREPARED BY: MAIZATUL ZALELA BINTI MOHAMED SAIL
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