Study and Simulation of Dispersion Compensation Scheme Effects on the Performance of Optical WDM System

Transcription

1 People s Democratic Republic of Algeria Ministry of Higher Education and Scientific Research University M Hamed BOUGARA Boumerdes Institute of Electrical and Electronic Engineering Department of Electronics Final Year Project Report Presented in Partial Fulfilment of the Requirements for the Degree of Presented by: MASTER In Electrical and Electronic Engineering - Choufi Rafik - Gacem BelQassim Supervisor: Option: Telecommunications Title: Study and Simulation of Dispersion Compensation Scheme Effects on the Performance of Optical WDM System Mr. Abdelkader ZITOUNI Registration Number:.../2015

2 Acknowledgement Acknowledgement: We have taken efforts in this project. However, it would not have been possible without the kind support and help of many individuals. We would like to extend our sincere thanks to all of them. We are highly indebted to our supervisor, Mr.A.Zitouni for his guidance and constant supervision as well as for providing necessary information regarding the project and also for his support in completing the project. We would like to express our gratitude towards our parents for their kind co-operation and encouragement which help us in completion of this project. Our thanks and appreciations also go to our colleague in developing the project and people who have willingly helped us out with their abilities. ii

3 Abstract Abstract: The aim of this project is to study by simulation the different techniques used to reduce dispersion effects in modern WDM optical fiber communication system at two transmission speeds; 2.5 and 10 Gbps. Two approaches have been tested, dispersion compensation fiber (DCF) and fiber bragg grating (FBG) both at operating wavelength 1550 nm. The efficiencies of the two techniques have been compared and presented in our final result discussion. iii

4 Table of contents Table of Contents Dedication...i Acknowledgment... ii Abstract... iii Table of contents... iv List of tables... vii List of figures... viii List of Symbols...x Introduction...1 Chapter 1: Optical communication system 1. Structure of an Optical Communication System Optical Fiber Advantages of optical fibers Disadvantages of optical fibers compared to wires...4 A. Single-mode...4 B. Multi-mode...5 C. The advantages of single mode fiber...5 D. The disadvantages of single mode fiber...6 E. Multimode optical fiber s advantages and disadvantages Optical sources Product of Light...6 A. Spontaneous Emission...6 B. Stimulated Emission: Light Emission Diode LED Laser Diode Comparison of LEDs and Lasers Optical amplifier Erbium Doped Fiber Amplifiers (EDFAs) The Receiver Pin Photodetector APD photodetector Chapter 2: Signal Degradation in optical fiber 2. The Signal Degradation in Fiber Optic Optical Signal Attenuation iv

5 Table of contents Absorption A. Intrinsic Absorption B. Extrinsic Absorption: Scattering Loss Imperfection Loss A. Bending Loss B. Coupler and Splicing Loss Dispersion Dispersion in a Waveguide A. Material Dispersion B. Waveguide Dispersion Modal Dispersion Polarization Mode Dispersion Dispersion Compensation Dispersion Shifted Fiber Non-Zero Dispersion Shifted Fiber Dispersion Compensation Fiber Fiber Bragg Grating Chapter 3: DWDM System 3. Multiplexing and Demultiplexing (Channelization) Introduction Multiplexing Multiplexer (MUX) Demultiplexing Demultiplexer The Basic Types of Multiplexing Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM) Code Division Multiplexing (CDM) Dense Wavelength Division Multiplexing (DWDM) A. Description of DWDM B. Advantages DWDM C. Disadvantages of DWDM Modulation Summary v

6 Table of contents Chapter 4: study and simulation of dispersion compensation 4. Introduction: Dispersion Compensation Schemes Optisystem Applications Bit Error Rate (BER) Simulation Pre-, Post-, and Symmetrical Compensation by Using DCF: Description of System Structure A. The Modulated Transmitter B. The Transmission Line C. The Receiver Implementation Results and Discussion Dispersion Compensation by Using FBG-DCM Results and Discussion DCF-DCM Technique in 8-Channel DWDM Results and Discussion Conclusion Appendix A: Eye Diagram... I 7. Appendix B: Transmission Band of DWDM... II 8. References...III vi

7 List of figures List of figures: Figure 1.1 Optical communication system...3 Figure 1.2 Optical fiber structure...3 Figure 1.3 Critical angle...4 Figure 1.4 a) single mode b) multimode fiber...5 Figure 1.5 Light Emission Diode...7 Figure 1.6 Laser diode...8 Figure 1.7 Light of LED and LASER...9 Figure 1.8 Erbium Doped Fiber Amplifiers Figure 1.9: a) The schematic structure of an idealized pin photodiode b) The pin photodiode photodetection is reverse biased Figure 2.1 Attenuation spectrum for pure silica glass Figure 2.2 Rayleigh scattering Figure 2.3 Macro- and micro-bending in fibers Figure 2.4 Effects of material dispersion Figure 2.5 Effect of waveguide dispersion Figure 2.6 Total dispersion in standard fiber Figure 2.7 Modal dispersion in multimode fibers Figure 2.8 Polarization mode dispersion in single mode fibers Figure µm zero dispersion shifted fiber Figure 2.10 Different method of DCF a) Pre b) Post c) Symmetry compensation Figure 2.11 Fiber Bragg Grating Figure 3.1 Illustration of basic FDM with set of filters Figure 3.2 FDM as providing a set of independent channels Figure 3.3 The concept of time division multiplexing Figure 3.4 Concept of DWDM system Figure 4.1 The externally modulated transmitter implementation Figure 4.2 Transmission line Figure 4.3 The receiver implementation Figure 4.4 Dispersion pre-compensation Figure 4.5 Dispersion post-compensation Figure 4.6 Dispersion symmetry-compensation Figure 4.7 Eye Diagram of a) pre- b)post- c) symmetry- compensation scheme at 2.5Gbps for -16 dbm viii

8 List of figures Figure 4.8 Eye Diagram of a) pre- b)post- c) symmetry- compensation scheme at 2.5Gbps for 8 dbm Figure 4.9 Eye Diagram of a) pre- b)post- c) symmetry- compensation scheme at 10bit rate for 8 dbm laser power Figure 4.10 Q factor verses signal power at 2.5 Gbps bit rates for pre dispersion compensations Figure 4.11 Q factor verses signal power at 2.5 Gbps bit rates for post dispersion compensations Figure 4.12 Q factor verses signal power at 2.5 Gbps bit rates for symmetry dispersion compensations Figure 4.13 Q factor verses signal power at 10 Gbps bit rates for pre dispersion compensations Figure 4.14 Q factor verses signal power at 10 Gbps bit rates for post dispersion Figure 4.15 Q factor verses signal power at 10 Gbps bit rates for symmetrical dispersion Figure 4.16 Dispersion compensation post with FBG Figure 4.17 Eye Diagram of DCM compensation scheme at 10bit rate for -3 dbm laser power Figure 4.18 Eye Diagram of DCM compensation scheme at 10bit rate for 8dBm laser power Figure 4.19 Q factor verses signal power at 10 Gbps bit rates for DCM Figure 4.20 Q factor verses dispersion of FBG at 10 Gbps bit rates for -3 dbm Laser power Figure 4.21 Setup of 8-channel dwdm design Figure 4.22 Eye diagram of DCF-DCM compensation scheme at 10 Gbps for 8 dbm laser power for a) channel-1 b) channel -4 c) channel ix

9 List of tables List of tables Table 2.1 Comparison of DCF and FBG Table 3.1 W DM, CWDM and DWDM comparison Table 4.1 System parameters Table 4.2 Results of pre-, post - and symmetrical dispersion compensation Table 4.3 The system parameters Table 4.4 max Q-factor and min BER in channel 1, 4 and vii

10 List of Symbols List of symbols APD BER CWDM CDMA CDM DCF DCM DSF DWDM EAM EDFA EO ER FBG FDM GVD IF ISI ITU-T LED LASER MUX MZM NZ-DSF NRZ OADM OE Avalanche Photodiode Bit-Error-Rate Coarse Wavelength Division Multiplexing Code Division Multiple Access Code Division Multiplexing Dispersion-Compensating Fiber Dispersion-Compensating Module Dispersion Shifted Fiber Dense Wavelength Division Multiplexing Electro Absorption Modulator Erbium-Doped Fiber Amplifier Electro-Optical Extinction Ratio Fiber Bragg Grating Frequency Division Multiplexing Group Velocity Dispersion Intermediate Frequency Inter Symbol Interference International Telecommunication Union Light Emission Diode Light Amplification by Stimulated Emission of Radiation Multiplexer Mach-Zhender Modulator Non-Zero Dispersion-Shifted Fiber Non-return to Zero Optical add/drop multiplexers Opto-Electronic x

11 List of Symbols OFC PMD RZ SMF SSMF SONET TEChDC TDM WDM OADMs ROF LAN WAN IR SDH STM Optical Fiber Communication Polarization Mode Dispersion Return to Zero Single-Mode Fiber Standard Single-Mode Fiber Synchronous Optical Network Talbot Effect Chromatic Dispersion Compensation Time Division Multiplexing Wavelength Division Multiplexing Optical Add/Drop Multiplexer Radio Over Fiber Local Area Network Wide Area Network InfraRed Synchronous Digital Hierarchy Synchronous Transport Module xi

12 Introduction Introduction: Communication is an important part of our daily life. Everyday, we are using different types of communication services, such as voice, video, images, and data communication. As needs for those services increase, demands for large transmission capacity networks also increase. In order to fulfill the increasing demand for higher data rate and larger bandwidth, light-wave technology has been developed. The combination of photon and glass fiber provides a tremendous transmission capability improvement compared to transmission lines through electrons and copper wires. As a result fiber optic transmission technology will remain the key communication technology for the foreseeable future. Optical fiber has been used as physical medium by many networking technologies, such as FDDI, SONET, ATM and SDH due to its high bandwidth and low signal degradation. It is used by many telecommunications companies to transmit telephone signals, Internet communication, and cable television signals. However, infrastructure development within cities was relatively difficult and time-consuming, and fiber-optic systems were complex and expensive to install and operate. Due to these difficulties, fiber-optic communication systems have primarily been installed in long-distance applications, where they can be used to their full transmission capacity, offsetting the increased cost. Since 2000, the prices for fiber-optic communications have dropped considerably. The price for rolling out fiber to the home has currently become more costeffective than that of rolling out a copper based network. In this project, we have organized our work as follows: Chapter 1: Introduces optical communication systems and optical fiber technology, the components used in optical networks and their basic operations. The characteristic and types of optical fibers are also discussed. Chapter 2: Shows many types of dispersion in optical fibers and gives some techniques used for dispersion compensation. Chapter 3: In this chapter we are going to introduce multiplexing and DWMD system, and show its importance in transmitting multiple information streams over the fiber. 1

13 Introduction Chapter 4: provides two types of dispersion compensation, (Dispersion Compensation Fiber and Dispersion Module) the performance of the system is studied in a single channel in our simulation, at the end of our study we give a comparison between these two techniques. Conclusion: summarizes the outcome and states briefly the achievements of the project, and discusses possible future directions. 2

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15 Chapter 1 Optical Communication System 1. Structure of an Optical Communication System: A modern optical communication links depends mainly on four components: the optical fiber, the optical transmitter, the optical receiver and the optical amplifier. The optical transmitter is used to generate light signal and modulate information on the signal, the optical receiver receives the transmitted signal and convert it back to the carried information, the optical fiber is the transmission media of light, and the optical amplifier is used to extend the transmission distance. Figure 1.1 : Optical communication system Optical Fiber: Optical fiber consists of a very fine cylinder of glass core, through which light propagates. The core is surrounded by another layer of glass, named cladding, which is then wrapped by thin plastic jacket. The core has a slightly higher index of reflection than the cladding glass [1]. The ratio of refractive indices of core and cladding defines the critical angle : (1.1) where n 1 is the refractive index of the core and n 2 is the refractive index of the cladding. Figure 1.2: Optical fiber structure. 3

16 Chapter 1 Optical Communication System What makes fiber optics work is the total internal reflection, when a ray of light goes from the core to the core-cladding boundary at an angle larger than, the ray is completely reflected back to the core. Thus, light signal can be guided inside optical fibers, figure 1.3 explain it[30]. Figure 1.3 : Critical angle Advantages of optical fibers: Low weight and small size. Signals contain very little power. Low loss of signal (typically less than 0.3 db/km). Enormous potential bandwidth. Large data-carrying capacity (reaching speeds of up to 1.6 Tb/s in field deployed systems and up to 10 Tb/s in lab systems). Immunity to electromagnetic interference, including nuclear electromagnetic pulses. High electrical resistance, so safe to use near high-voltage equipment. No crosstalk between cables. Difficult to place a tap or listening device on the line, providing better physical network security Disadvantages of optical fibers compared to wires: High investment cost. Need for more expensive optical transmitters and receivers. Optical fibers are typically referred to as either single-mode or multimode. A. Single-mode: Single-mode fibers are optical fibers designed to support only a single mode per polarization direction for a given wavelength (figure 1.4-b), the core diameter is typically between 8 and 9 4

17 Chapter 1 Optical Communication System µm, while the cladding diameter is 125 µm. For single-mode fiber, a smaller core makes it possible restrict the light propagates in one mode or one path only down the fiber [2]. B. Multi-mode: Multimode fiber allows multiple paths, called modes, for the light to travel in them. It uses a large number of frequencies (or modes). The cable's core is larger than that of single-mode fiber (usually 50 or 62 µm). Multimode fiber is the type usually specified for LANs and WANs. It also depends on the wavelength used. At very long wavelength, even multimode fiber can propagate only a single mode. At short wavelengths, several modes of light may propagate in a single-mode fiber. So single-mode fiber have so called cutoff wavelength. Below the cutoff wavelength, a single-mode fiber becomes a multimode. Figure 1.4: a) Multimode fiber b) Single mode fiber. Multimode and single-mode fiber optic cables differ greatly in their design and purpose. While both cables use the same basic principles, each has its own advantages and disadvantages that make them ideally suited for a particular environment [3]. C. The advantages of single mode fiber: Exhibit the greatest transmission bandwidths. single-mode cable is capable of transmitting data at up to 40 Gb over hundreds of kilometers 5

18 Chapter 1 Optical Communication System Less modal dispersion per kilometer. Support transmission over long distance. D. The disadvantages of single mode fiber: Too much power is needed to be concentrated into a very small core diameter. Expensive and difficult to manufacture. LED s cannot couple enough light into a single mode fiber to facilitate long distance communication hence only lasers can be used. E. Multimode optical fiber s advantages and disadvantages: Exhibit the high transmission bandwidths. Useful for small distance because a modal dispersion is too high Optical sources : In the field of optical communications, an optical source is used to generate an optical frequency carrier, and the carrier is modulated according to the transmitted data and passing through the fiber to the receiver. This section discusses important light sources used in optical communications. Two basic light sources are used for fiber optics: lasers and light-emitting diodes (LED) Product of Light: In a general case, there is only one way that light can be produced, if an electron exists in a state of relatively high energy, then rapidly changes to a more stable state (lower energy), a photon is emitted. Emission of a photon can be spontaneous or stimulated by the passing of another photon of the proper energy [4]. A. Spontaneous Emission: Excited electron remains in low stability picoseconds. The released photon will have a stochastic phase and direction but its wavelength will be determined by the bandgap energy traversed. Spontaneous emission is the process of optical production observed in light emitting diodes. B. Stimulated Emission: Excited electron remains in low stability microseconds. 6

19 Chapter 1 Optical Communication System The released photon has the same phase, direction and wavelength of the photon that stimulated it. The stimulated emission is used in Laser Diode Light Emission Diode LED: A Light-Emitting Diode (LED) is essentially a P-N junction solid-state semiconductor diode that emits light when a current is applied through the device. The essential portion of the Light Emitting Diode is the semiconductor chip. This chip is divided into two parts or regions separated by a boundary called a junction. The P-region is dominated by positive electric charges (holes) and the n-region is dominated by negative electric charges (electrons). The junction serves as a barrier to the flow of the electrons between the p and n-regions. The electrons move across the junction into the p-region. Band-gaps determine how much energy is needed for the electron to leap from the valence band to the conduction band. As an electron in the conduction band recombines with a hole in the valence band, the electron makes a transition to a lower-iying energy state and releases energy in an amount equal to the band-gap energy. This energy is released in photons [5].The LED structure is shown in figure 1.5. The larger the traversed band gap, the shorter the wavelength of emitted light. Refer to the equation (1.2) Where λ is the wavelength of emitted light, h is planck s constant, c is the speed of light in vacuum, and E photon is the energy of photon gained by traversing the bandgap. Figure 1.5: Surface Light Emission Diode. 7

20 Chapter 1 Optical Communication System There are two different coupling structures for LED: surface-emitting and edge-emitting. The first type couples light vertically away from the planar emitting surface and is call surfaceemitting or Burrus LED. The second type couples light out in parallel to the active layer and is called edge-emitting LEDs. Edge-Emitting LEDs have smaller linewidth than those of surfaceemitting diodes Laser Diode: LASER is acronym for Light Amplification by Stimulated Emission of Radiation. Semiconductor lasers are very similar to edge Light-Emitting Diodes in structure (figure 1.6). Laser diodes are a p-n junction semiconductor that converts the electrical energy applied across the junction into optical radiation. In both laser diodes and LEDs, the wavelength of the output radiation depends on the energy gap across the p-n junction. However, the output from a laser diode is highly coherent and can be collimated, while the output from an LED has many phases and is radiated in different directions [6]. Figure: 1.6 Laser diode Comparison of LEDs and Lasers: The advantages of LEDs over Laser include: Lower cost due to their simpler fabrication. More reliable because they are less sensitive to temperature change. They have better linearity since they require simplest circuitry. Low power versus laser. LEDs relatively wide emission. 8

21 Chapter 1 Optical Communication System The advantages of Laser over LEDs include: Compact size and high efficiency. Higher wavelength range. Small emissive area compatible wave fiber core dimension. Higher power output. Produce directional and coherent light which is the better option for optical communication Optical Amplifier: Figure 1.7: Emitted light of LED and LASER. An optical amplifier can boost the strength of optical signals so they can travel farther through optical fibers. They amplify light directly in optical form without converting the signal to electrical form [7] Erbium Doped Fiber Amplifiers (EDFAs): An Erbium-Doped Fiber Amplifier (EDFA) is a device that amplifies an optical fiber signal. A trace impurity in the form of a trivalent erbium ion is inserted into the optical fiber's silica core to alter its optical properties and permit signal amplification [8]. The device uses a short length of optical fiber doped with the rare-earth element erbium. When the signal-carrying laser beams pass through this fiber, external energy is applied, usually at IR wavelengths. This, so, called pumping excites the atoms in the erbium-doped section of optical fiber, increasing the intensity of the laser beams passing through. The beams emerging from the EDFA retain all of their original modulation characteristics, but are brighter than the input beams. Figure 1.8 illustrates the concept. 9

22 Chapter 1 Optical Communication System Figure 1.8: Erbium Doped Fiber Amplifiers. In fiber optic communication systems, problems arise from the fact that no fiber material is perfectly transparent. The visible-light or infrared (IR) beams carried by a fiber are attenuated as they travel through the material. This necessitates the use of repeaters in spans of optical fiber longer than about 100 km. A conventional repeater puts a modulated optical signal through three stages: Optical-to-electronic conversion. Electronic signal amplification. Electronic-to-optical conversion. Repeaters of this type limit the bandwidth of the signals that can be transmitted in long spans of fiber optic cable. This is because, even if a laser beam can transmit several gigabits per second (Gbps) of data, the electronic circuits of a conventional repeater cannot. Besides eliminating complex and inefficient conversion and electronic amplification stages, the EDFA allows the transmission of signals that employ wavelength-division multiplexing (WDM). This increases the realizable bandwidth relative to conventional repeaters still further The Receiver: Fiber optic receiver is used to transform the signal from light form to electrical form. Usually the fiber optic signal is transmitted via optical fiber and sent to the fiber optic receiver, the receiver can convert the form of the information from light to electrical, and then the electrical receiver transforms electrical signal back to its original form such as data, video or audio. The most common photodiodes used in optical system are Pin PhotoDetectors and Avalanche PhotoDetectors (APDs) [9]. 10

23 Chapter 1 Optical Communication System Pin Photodetector: The majority of optical detectors used in communication rely on the principle of ionization in a semiconductor material. In other words, for suitable material, a photon with enough energy will strike the surface of the semiconductor, rapidly migrates to an electrical contact where it swept out of the detector and most probably electrically amplified as shown in figure 1.9. For a frequency response in the GHz range, this implies that a very low amount of stray capacitance can exist within the photodetecor itself, which in turn implies that the detector is very small. To generate a photocurrent, we must ensure that the energy of the incident photon is equal to or greater than the band gap. The energy of the incident photon is given by the equation1.3 (1.3) where h is planck's constant. E g is energy of the band gap. c is velocity of light. is wavelength at wich the semiconductormaterial will function as a photodetector. e is charge of the electron [10]. Figure 1.9: a) The schematic structure of an idealized pin photodiode b) The pin photodiode photodetection is reverse biased. 11

24 Chapter 1 Optical Communication System APD Photodetector: The APD is a semiconductor device that, when reverse-biased, creates strong fields in the junction region. When a photon causes an electron-hole pair, the pair flows through the junction. Because of the strong fields in the junction, the electron gains enough energy to cause secondary electron-hole pairs, which in turn cause more. Thus a multiplication (or avalanche) process takes place (hence the name), and a substantial current is generated from few initial photons [11]. The gain M of an APD is expressed by equation 1.4: where I APD is the APD output current and I primary is the current due to photon-electrons conversion. (1.4) 12

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26 Chapter 2 Signal degradation in optical fiber 2. The Signal Degradation in Fiber Optic: As an optical signal pulse traveling inside a fiber, there are several factors that can degrade the data transmission. The longer the distance an optical pulse goes, the less chance the data can get to receiver end, the faster a pulse is being transmitted the worse the information can be recognized successfully. These are due to the attenuation and dispersion of propagation lightwave. The attenuation effect decreases the signal and the power and the dispersion effect distorts the shape of the pulse as lightwave propagation down a fiber Optical Signal Attenuation: Signal attenuation is a very important property in the design of a fiber optical communication system, because it largely determines the maximum transmission distance between a transmitter and receiver [14]. Attenuation is characterized by equation 2.1. Where P O is the optical power at the transmitter in Watts. L is distance in Km between the receiver and the transmitter. P L is the power of the optical pulse in Watts. There are three basic mechanisms causing signal attenuation in a fiber; they are Absorption, scattering and Imperfection losses of the optical energy Absorption: Absorption loss can be classified as two types: Intrinsic Absorption and Extrinsic Absorption. A. Intrinsic Absorption: (2.1) The intrinsic absorption is due to the material nature of absorbing specific wavelength region of light. The intrinsic absorption occurs in both the infrared and ultraviolet ranges as shown in figure 2.1. Fortunately, these intrinsic losses are mostly insignificant in the region where fiber systems are operated, but these losses limit the extension of fiber optic communication toward the ultraviolet as well as toward longer wavelength [15]. 13

27 Chapter 2 Signal degradation in optical fiber. Figure 2.1: Attenuation spectrum for pure silica glass. B. Extrinsic Absorption: Extrinsic absorption is caused by atomic resonance of impurity particles in the fiber. The most important extrinsic absorption is due to water or hydroxyl ion (OH) bond [16]. Because the bond can absorb incident light at its resonant frequency and harmonics, there are absorption peak at wavelength of 2.8/ (n+1) µm Scattering loss: Scattering is a process whereby all or some of the optical power in a mode is transferred into another mode. There are four kinds of scattering loss in optical fiber: Rayleigh, Mie, Brillouin, and Raman scattering. Rayleigh is the most important scattering loss. During the manufacture process of glass fibers, some localized variation in density may happen due random motion of molecular. These material density variations may be modeled as small scattering objects embedded in an otherwise homogenous material [17]. Because these object are much smaller than the operating wavelength, when beam of light passing through these object, some of its energy is scattered and lost figure 2.2 illustrates the concept. 14

28 Chapter 2 Signal degradation in optical fiber The Rayleigh scattering loss can be approximated by the expression 2.2: (2.2) Where λ is in µm and the loss L is in db/km. As a result, the scattering loss is proportional to λ -4. Therefore, the use of short wavelength in fiber optic communication is severely restricted by Rayleigh scattering. Figure 2.2: Rayleigh scattering Imperfection Loss: Imperfection loss includes: Bending, Coupling, and splicing losses. A. Bending Loss: Bending loss occurs whenever an optical fiber undergoes a bend of finite radius of curvature. When a fiber is bent, partial energy radiates away through the evanescent field tail in the cladding. There are two types of bending Macro and Micro bending as shown in figure 2.3. If the fiber is sharply bent so that the light traveling down the fiber cannot make the turn and gets lost then it is Macro-Bending. When small bends in the fiber created by crushing, construction cause the loss then it is called Micro-Bending. Generally, bending loss is not significant and can be neglected, unless the bending curvature is too large [18]. Figure 2.3: Macro- and Micro-bending in fibers. 15

29 Chapter 2 Signal degradation in optical fiber B. Coupler and Splicing Loss : A light signal is also attenuated at a junction of two connected fiber either by a Coupler or Splicing. The loss is caused by some extrinsic or intrinsic reason. Extrinsic reasons include misalignment, tilt, end gap or bad end face quality. Intrinsic reasons are core ellipticity, mismatch in refractive index, or mismatch in mode field diameter. Typically, coupling loss is around 0.2 db and Splicing loss is around 0.05 db [19] Dispersion: Dispersion is the phenomenon whereby the index of refraction of a material varies with the frequency or wavelength of the radiation being transmitted through it. The term Chromatic Dispersion is often used to emphasize this wavelength dependence. The total dispersion in a waveguide or an optical fiber is a function of both the material composition (material dispersion) and the geometry of the waveguide (waveguide dispersion) [12] Dispersion in a Waveguide: When light is confined in an optical fiber or waveguide the index is a property of both the material and the geometry of the waveguide. The waveguide geometry changes the refractive index via optical confinement by the waveguide structure. The refractive index is therefore a function of both the material and waveguide contributions. The dispersion parameter of a waveguide such as an optical fiber is given by the total dispersion due to both the material and waveguide contributions. The total dispersion is the combination of the material dispersion and the waveguide dispersion and thus the dispersion parameter D of a waveguide is given by equation 2.3. ( ) (2.3) Where V G is Group Velocity, ω is the angular frequency, D M is material dispersion and D W is waveguide dispersion Material Dispersion: As well Known, when a light wave travels in vacuum, it moves at a velocity of light c = m/s. In any other medium, light waves travel at slower speed, given by v=, where n is the 16

30 Chapter 2 Signal degradation in optical fiber refractive index of the medium. For material used to make an optical fiber, the refractive index varies with the wavelength of light traveling inside a fiber. The term dispersion is used to describe the phenomenon of wavelength dependent velocity of propagation of an electromagnetic wave. When the velocity variation is caused by some property of the material, the effect is called material dispersion [20]. The material dispersion effect can be explained by considering the situation showed in figure 2.4. A finite linewidth optical source emits a pulse into a dispersive glass fiber. For simplicity, assume the input pulse is composed by three different single wavelength λ1, λ2, and λ3. The three pure color components travel at different velocities in the fiber. After propagating distance, they arrived at different time in the receiver end. As a result, the output pulse, the sum of three receiver single wavelength, becomes spreading. In a long enough fiber span the dispersion can be sufficiently large so that the adjacent pulses will overlap eventually, this result in inter symbol-interference (ISI) and producing a high bit error rate in communication. Figure 2.4: Effects of material dispersion. The material dispersion is then determined by taking the derivative of the group index N G of the material with respect to wavelength λ or equivalently the second derivative of the absolute index n with respect to wavelength as characterized by equation 2.4: ( ) (2.4) Waveguide Dispersion: Another basic type of dispersion is called waveguide dispersion, which is usually ignored in multimode fiber application. When a light signal is coupled into a single fiber, only about 80% of power is confined into the core, and the other 20% of power propagate in the cladding layer. Since 17

31 Chapter 2 Signal degradation in optical fiber the core and cladding have different refractive indices, the two modes of light travel at different speed. Since the light travels faster in the lower refractive index materials and slower in higher refractive index materials, the light propagating in cladding travels faster than the light confined in the core. Dispersion is then produced. The amount of waveguide dispersion depends on the design of fiber. Waveguide dispersion is function of the core radius, the refractive index different between core and cladding, and the shape of the refractive index profile [21]. Figure 2.5: Effect of waveguide dispersion. The contribution of the waveguide to the dispersion parameter depends on the confinement and propagation of the light in a waveguide and hence it is a function of the V parameter and the normalized propagation constant b. The waveguide dispersion can be calculated via knowledge of V and b: [ ] (2.5) In fact, the waveguide dispersion can be carefully designed to cancel out the material dispersion at particular wavelength in single mode fiber design. The resulting optical fibers are known as dispersion-shifted fibers (figure 2.6). Figure 2.6: Total dispersion in standard fiber. 18

32 Chapter 2 Signal degradation in optical fiber Modal Dispersion: Modal dispersion is a problem only when a multimode fiber is used. Multimode fiber allows different path or modes for the light to travel in them. The various modes intend to interact with each other in the big fibers. Since different modes propagate at different angles, each of them has a different axial group velocity along the fiber. In other words, they travel at different speed. This variation in the group velocities of the different modes results in a group delay spread or inter-modal distortion. The modal dispersion limits the speed and distance of an optical communication link. However, this dispersion mechanism can be eliminated when a single-mode fiber used. That is why some form of single mode fiber is always used in systems needing the highest speed and longest spanning [22]. Figure 2.7: Modal dispersion in multimode fibers Polarization Mode Dispersion: Polarization mode dispersion (PMD) describes a situation in which the electromagnetic wave components that make up an optical signal travel at different speeds within the fiber. This causes a multipath interference at the receiver. PMD is difficult to predict and may possibly vary with temperature and environment, the twisting of the cable as it was pulled and even between productions runs from the same manufacturer. The very high-speed systems are more prone to failing in the presence of significant level of PMD, figure 2.8 show effects of PMD [23]. Figure 2.8: Polarization mode dispersion in single mode fibers. 19

33 Chapter 2 Signal degradation in optical fiber 2.3. Dispersion compensation: As described in previous section, attenuation and dispersion effects can significantly limit the bit rate and the spanning distance of fiber optical communication. The war against attenuation can be won because the improvement of fiber manufacturing and the invention of EDFA. However, dispersion effects have to be taken into consideration as well. Since PMD is rarely observed, modal dispersion is taken cared by using single mode fiber, and waveguide dispersion can be controlled by fiber design, it is the material dispersion usually referred as the main factor limitation of optical network. In this section, several important fiber technologies used to provide dispersion compensation are described Dispersion Shifted Fiber: The standard single mode fiber deployed today is manufactured to optimize transmission at 1310 nm by effectively eliminating dispersion at that wavelength. The dispersion in the 1550 nm window far exceeds that for 1310 nm on standard fiber and hence is a limiting factor in single channel of DWDM systems operating in that window. Dispersion shifted fiber (DSF) differs from standard fiber in that the zero dispersion point in shifted from 1310 nm to 1550 nm by constructing a single mode fiber with a triangular-shaped refractive index variation (instead of a step-index or graded index variation). It is best suited for applications involving signal channel transmission at 1550 nm providing the benefits of zero dispersion as well as taking advantage of the lower attenuation occurring at the wavelength (figure 2.9). DWDM systems do not perform as well as single channel systems on DSF due to phenomenon known as four-wave mixing. Because a fiber s refractive index is nonlinear, two or more optical carriers can combine and produce several mixing products. This has a cascading effect and can result in unwanted products occurring at the operating carrier wavelength. This process is more intense when there is zero dispersion at the operating wavelength because the unwanted products will be moving at the same speed with the desired signal causing significant interference and thereby hampering system performance. There have been some successes with techniques that involve allocation and that avoid mixing product occurring at the signal wavelength; however, four-wave mixing remains a concern for DWDM using DSF [24]. 20

34 Chapter 2 Signal degradation in optical fiber Figure 2.9: 1.55 µm zero dispersion shifted fiber Non-Zero Dispersion Shifted Fiber: In order to support WDM systems, a fiber was developed that lowered the chromatic dispersion at 1550 nm but not to the extent that would encourage four-wave mixing as DSF does. This fiber is called Non-Zero Dispersion Shifted Fiber (NZDSF) because of the small non-zero amount of dispersion that occurs in the 1550 nm window. By now it may be apparent that there exists a sort of catch-22 situation with TDM and WDM systems. A large telecommunication provider desires to maximize the TDM rates placed on as many WDM optical carriers as possible to maximize the bandwidth of fiber optic facilities. Very high speed TDM requires zero dispersion; however, WDM or DWDM requires some small amount of dispersion to avoid mixing effects Dispersion Compensation Fiber: Another philosophy is to use standard single mode fibers in combination with a new type of fibers. Dispersion compensation fiber (DCF) is a new specialty fiber that has a very high negative value of dispersion suffered by 1550 nm signals that traverse standard single mode fiber. It is used as a sort of inline pre- or post-equalization in the form of a fiber spool of a particular length placed at one end of a link [25]. Dispersion compensation has a high negative dispersion -70 to -90 ps/nm.km and can be used to compensate the positive dispersion of transmitter fiber in C and L bands. Spans made of SMF and DCF are good source as their high local dispersion is known to reduce the phase matching. 21

35 Chapter 2 Signal degradation in optical fiber Fiber based compensation is done by three methods: Pre-compensation: In this method, the DCF of negative dispersion is placed before the SMF as shown in figure 2.10 (a). Post-compensation: In this method, the DCF of negative dispersion is placed after the SMF as shown in figure 2.10 (b). Symmetrical or mixed compensation: In this method, the DCF of negative dispersion is once placed before SMF and then placed after SMF as shown in figure 2.10(c). Figure 2.10: Different method of DCF a) Pre b) Post c) Symmetry compensation. 22

36 Chapter 2 Signal degradation in optical fiber Advantages of DCF: Advantages of DCF are that they can be easily constructed and highly re liable. DCF provides continuous compensation over a wide range of optical wavelengths. A DCF module should have low insertion loss, low polarization mode dispersion and low optical non-linearity. In addition to these characteristics DCF should have large chromatic dispersion coefficient to minimize the size of a DCF module Fiber Bragg Grating: Fiber grating is a section of fiber with a periodic variation in the refractive index of its core. Bragg diffraction effect occurs when an input wavelength equal to one-half of the repetition period Λ goes through the grating region. In the grating region, each change of index of refraction acts like a semi-reflection mirror, and only beams with the selected wavelength (Λ/2) are reflected and all the reflected beams add up in phase with each other. The reflected wavelength must satisfies the Bragg s law, which is (2.6) Where Λ is the grating period (distance between changes of refractive index), λ is the wavelength reflected, and n is the order of the Bragg diffraction [26]. Figure 2.11: Fiber Bragg Grating. Advantage of FBG as dispersion compensator: The most common advantage of FBG is low insertion loss (IL). Typically, a 120-km FBG- DCM has an insertion loss in the range of 3 to 4 db, depending on type. Furthermore, the FBG- DCM holds an advantage is that it has virtually constant IL versus span length, whereas the IL of the DCF-DCM grows linearly with span length. Residual dispersion is another key parameter for compensators. Due to the very flexible grating process developed by approximation, the chirp characteristics can readily be chosen according to fiber specifications. Although a DCF will 23

37 Chapter 2 Signal degradation in optical fiber display nonlinearity effects at rather low optical powers, the FBG-DCM will not introduce such effects even at the highest power levels present throughout optical network. Dispersion requirements increase with higher bandwidth, the focus on dispersion compensation is high. Comparison of DCF and FBG: The main differences between DCF and FBG are summarized in table 2.1 CHARACTERISTICS DCF FBG Bandwidth Wide band 20 nm Narrow band, nm Fiber length km cm Construction Complex Simple negative dispersion +15 to +25 ps/nm/km ps/nm/km positive dispersion 80 to 120 ps/nm/km ps/nm/km Dispersion 16pm/km/nm 17pm/km/nm Bending loss dB/km 0.14 db/km Reflectance ratio 99.99% 10-95% Attenuation 0.8 db/km 0.2 db/km Nonlinear effects Some limitations No Insertion loss High Low Overall Cost of system high Low Table 2.1: Comparison of DCF and FBG. DCF allows longer distances compared to FBG. DCF has higher bending loss, insertion loss and system cost. FBG has no non-linear effects while DCF has some limitations. Wider bandwidth bands are available in DCF. 24

38

39 Chapter 3 DWDM System 3. Introduction: Fiber optic communications systems are able to provide wide bandwidth cost effectively. In modern fiber optic communication systems, the dense wavelength division multiplexing DWDM technology has been developed to provide ultra-high bandwidth communications. In DWDM architecture, multiple laser sources operating at different wavelengths (λ) are used with each source s wavelength transmitting data at 2.5 Gbps for OC-48 interface or 10 Gbps for OC-192 interface. These independent wavelengths are placed very closely together at 0.8 nm=100ghz channel spacing in the 1550 nm range (ITU-T G.692 standard), with carrier wavelengths that are tuned to lie within a minimum in the optical fiber s attenuation curve Multiplexing and Demultiplexing (Channelization): Multiplexing: In data transmission, a function that permits two or more data sources to share a common transmission medium such that each data source has its own channel. In other word, a process where multiple analogue message signals or digital data streams are combined into one signal over a shared medium Multiplexer (MUX): Multiplexer is the equipment that enables several data streams to be sent over a single physical line. In DWDM system, multiplexer is a device for combining several channels to be carried by one fiber Demultiplexing: Refer to the separation of a combination back into separate information streams. The information carried over an optical fiber are separated into multiple channels Demultiplexer: Refer to a mechanism that implements the concept The Basic Types of Multiplexing: There are four basic approaches to multiplexing that each has a set of variations and implementations: Frequency Division Multiplexing (FDM), Wavelength Division Multiplexing (WDM), Time Division Multiplexing (TDM) and Code Division Multiplexing (CDM). 25

40 Chapter 3 DWDM System TDM and FDM are widely used. WDM is a form of FDM used for optical fiber. CDM is a mathematical approach used in cell phone mechanisms [27] Frequency Division Multiplexing (FDM): A set of radio stations or TV can transmit electromagnetic signals simultaneously. It is possible to send simultaneously multiple carrier waves over a single copper wire. A demultiplexer applies a set of filters that each extracts a small range of frequencies near one of the carrier frequencies. Figure 3.1 illustrates the organization. Figure 3.1: Illustration of basic FDM with set of filters. A key idea is that the filters used in FDM only examine frequencies. FDM mechanism will separate the frequency from others without modifying the signal. Advantage of FDM arises from the simultaneous use of a transmission medium by multiple pairs of entities. We imagine FDM as providing each pair with a private transmission path [29].Figure 3.2 illustrates the concept. Figure 3.2: FDM as providing a set of independent channels. Disadvantage of the frequencies of two channels are too close, interference can occur. Furthermore, demultiplexing hardware that receives a combined signal must be able to divide the signal into separate carriers. 26

41 Chapter 3 DWDM System Time Division Multiplexing (TDM): Time Division Multiplexing TDM is a digital technology that assigns time slots to each channel repeatedly. Multiplexing in time simply means transmitting an item from one source, then transmitting an item from another source one after the other, and so on, in such a way that they can be associated with the appropriate receiver. Figure 3.3: The concept of time division multiplexing. The allocation of the bandwidth is done by dividing the time axis into periods of fixed duration, and each user will transmit only during one of these periods determined. If it is done sufficiently and quickly, the receiving devices will not detect that, some of the circuit time was used to serve another logical communication path. TDM makes it possible to combine multiple channels of communication to flow down on one channel at higher flow rate. Figure 3.3 illustrates the concept [29] Code Division Multiplexing (CDM): Code Division Multiplexing is a technique in which each channel transmits its bits as a coded channel specific sequence of pulses. This coded transmission typically is accomplished by transmitting a unique time dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with different code, can be transmitted on the same fiber and asynchronously demultiplexed. CDM used in parts of the cellular telephone system and for some satellite communication, the specific version of CDM used in cell phones is known as Code Division Multi-Access (CDMA). CDM does not rely on physical properties such as frequency or time, CDM relies on an interesting mathematical idea, values from orthogonal vector spaces can be combined and separated without interference. CDMA is a channel access method utilized by various radio communication technologies. 27

42 Chapter 3 DWDM System Wavelength Division Multiplexing (WDM): Definition: WDM system is a technology that allows multiple information streams to be transmitted simultaneously over a single fiber using light wavelengths. This provides cost effective method to increase the capacity of existing networks without the need to add additional fiber. Figure 3.4: Concept of DWDM system. A. Description of WDM: A DWDM system uses a multiplexer at the transmitter to join the signal together and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously and can function as an optical add-drop multiplexer. The transmitter end of the communication channel has a finite limit to the maximum data that can be modulated onto a single wavelength. Multiplexing many such data streams on different wavelengths not only increases the net data rate but also circumvents the opto-electronic mismatch to a certain extent. Although the fiber can accommodate up to 50 Tbps of capacity, electronic systems that can modulate bit streams at such a high rate do not currently exist, this fundamentally causes the opto-electronic mismatch. Each modulated wavelength in the composite signal is called a channel and each channel is generally at a fixed spacing from its neighbors. In today s networks, each channel is usually 100 GHz or 50 GHz from its neighbors; this spacing is also the standard for ITU-T todays. Service provides started most DWDM services with 200 GHz spacing. That was a norm for a long while, until 100 GHz became feasible [28]. 28

43 Chapter 3 DWDM System WDM System Components: Optical transmitters/receivers. DWDM mux/demux filters. Optical add/drop multiplexers (OADMs). Optical amplifiers. Transponders (wavelength converters). WDM, CWDM and DWDM: WDM WDM technology uses multiple wavelengths to transmit information over a single fiber. WDM networks used just two wavelengths, 1310 nm and 1550 nm. Today's DWDM systems utilize 16, 32, 64, 128 or more wavelengths in the 1550 nm band. CWDM Coarse WDM (CWDM) has wider channel spacing (20 nm); low cost. DWDM DWDM has dense channel spacing (0.8 nm) which allows simultaneous transmission of more than 16 wavelengths (high capacity). Table 3.1: WDM, CWDM and DWDM. Each of these wavelengths provides an independent channel. The range of standardized channel grids includes 50, 100, 200 and 1000 GHz spacing. Wavelength spacing practically depends on: laser linewidth and optical filter bandwidth. DWDM Limitations: Theoretically large number of channels can be packed in a fiber. For physical realization of DWDM networks we need precise wavelength selective devices. Optical amplifiers are imperative to provide long transmission distances without repeaters and this is considered to be a problem. 29

44 Chapter 3 DWDM System Why DWDM? DWDM permits rapid network deployment and significant network cost reduction. Use of DWDM allows deployment of less fiber and hardware with more bandwidth being available relative to standard SONET networks. Reasons behind using DWDM can be summarized in the following points: Capacity upgrade of existing fiber networks (without adding fibers). Transparency: Each optical channel can carry any transmission format. Scalability: Buy and install equipment for additional demand as needed. Transmission Band of DWDM: As fiber optic networks have developed for longer distances and higher speeds, fibers have been used in new wavelength ranges, now called "bands," where fiber and transmission equipment can operate more efficiently. To take advantage of the lower loss at 1550 nm, fiber was developed for the C-band. As links became longer and fiber amplifiers began being used instead of optical-to-electronic-to-optical repeaters, the C-band became more important. With the advent of DWDM which allowed multiple signals to share a single fiber, use of this band was expanded. Development of new fiber amplifiers promise to expand DWDM upward to the L-band [24]. B. Advantages of DWDM: Low signal distortion and low power requirement. High bandwidth (about 50 Tbps). Easier network expansion. - No new fiber needed. - Just add a new wavelength. - Incremental cost for a new channel is low. - No need to replace many components such as optical amplifiers. DWDM systems capable of longer span lengths. C. Disadvantages of DWDM: Not cost effective for low channel numbers. - Fixed cost of mux/demux, transponder, other system components. 30

45 Chapter 3 DWDM System Introduces another element, the frequency domain, to network design and management. SONET/SDH network management systems not well equipped to handle DWDM topologies. DWDM performance monitoring and protection methodologies developing. DWDM is used on fiber optics to increase the capacity of a single fiber Modulation: Modulation is an important step of communication system. Modulation is the addition of information (or the signal) to an electronic or optical signal carrier. In other words, you can make a binary data stream superimpose on a carrier frequency. The motive behind modulation is to enable transport of data efficiently and without many errors. In an optical DWDM network, data is modulated onto the light that a laser diode emits. One way of modulation is making the output optical power of a laser diode proportional to the binary sequence of the data stream. There are two techniques for modulation by using optical lasers: direct modulation and external modulation. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals [13]. The type of modulation that is used in our work next is frequency modulation Summary: DWDM plays an important role in high capacity optical networks. Theoretically enormous capacity is possible. Practically wavelength selective (optical signal processing) components and nonlinear effects limit the performance. Passive signal processing elements like FBG, DCF are attractive Optical amplifications are imperative to realize DWDM networks. 31

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47 Chapter 4 Simulation of Dispersion compensation 4. Introduction: The purpose of this simulation is to study the techniques of the dispersion compensation in a single mode fiber SMF at a bit rate of 2.5 and 10 Gbps and to illustrate it using the OptiSystem software. There are many techniques which are used for dispersion compensation management. In this section, we are going to use first, DCF technique with standard single mode fiber. Then, we will use DCM method or Fiber Bragg Grating. 4.1 Dispersion Compensation Schemes: In this section, we will show how dispersion compensation schemes affect the system performance. The pulse broadening effect of chromatic dispersion causes the signals in the adjacent bit periods to overlap. This is called inter symbol interference (ISI). Broadening is a function of distance as well as dispersion parameter D. The material dispersion is given in ps/nm/km and changes from fiber to fiber. It is also a function of wavelength. D is usually about 17 ps/nm/km in the 1550 nm wavelength range for a standard single mode fiber (SMF). It is at a maximum of 3.3 ps/nm/km in the same window for a dispersion- shifted fiber (DSF). Nonzero dispersion fiber (NDF) has a chromatic dispersion between 1 and 6 ps/nm/km or -1 and -6 ps/nm/km [32]. (4.1) L: Length of the fiber in km. C: Speed of light (m/s). : Wavelength in meter. B: Bit rate in Gbps. D: material dispersion in ps/(km. nm). When D=16 ps/(km-nm) at 2.5 Gbps, L 500km, whereas it drops to 30 km at 10 Gbps bit rate. 32

48 Chapter 4 Simulation of Dispersion compensation 4.2 Optisystem: Optisystem is an optical communication system simulation package that designs, tests, and optimizes virtually any type of optical link in the physical layer of a broad spectrum of optical networks, it is a system level simulator based on the realistic modeling of fiber-optic communication systems [27]. 4.3 Applications: Optical communication system design from component to system level at the physical layer. TDM/WDM network design. Radio over fiber (ROF) systems. SONET/SDH ring design. Transmitter, channel, amplifier, and receiver design. Dispersion map design. Estimation of BER and system penalties with different receiver models. Amplified system BER and link budget calculations. 4.4 Bit Error Rate (BER): In digital transmission, the number of bit errors is the number of received bits of a data stream over a communication channel that have been altered due to noise, interference, distortion or bit synchronization errors. The performance criteria for digital receivers if governed by the bit-errorrate (BER), defined as the probability of incorrect identification of a bit by the decision circuit of the receiver [34]. The BER is expressed by equation (4.2) The relationship between quality factor and BER is given by equation 4-3. (4.3) Where is the error function: (4.4) The larger the Q-factor, the lower the BER will be. 33

49 Chapter 4 Simulation of Dispersion compensation 4.5 Simulation: We start first by studying dispersion reduction techniques in a single channel, then, it will be studied in 8-channel DWDM Pre-, post-, and symmetrical -compensation by using DCF: Pre-, post-, and symmetrical compensation configurations are shown in figure 4.4, figure 4.5 and figure 4.6. In our simulations, we have used optical amplifiers after each fiber to compensate for the span loss. The dispersion parameter of SMF is 120 km long and D=16 ps/nm-km. Therefore, total accumulated dispersion is = 1920 ps/nm. This dispersion can be compensated by using a 24 km long DCF with -80 ps/km-nm dispersion. Total transmission distance is 120 2=240 km for each case. In the post- compensation case, DCF is placed after SMF. In the symmetrical compensation case, fiber placement follows the sequence of DCF, SMF, SMF and DCF. Parameter Value Bit-Rate 2.5 and 10 Gbps Length of SMF 120 Km Length of DCF 24 Km Dispersion coefficient of SMF 16 ps/nm/km Dispersion coefficient of DCF -80 ps/nm/km Gain of Inline EDFA 20 db Gain of Inline EDFA 12.8 db Attenuation factor of SMF 0.2 db/km Attenuation factor of DCF 0.6 db/km Table 4.1: system parameters Description of System Structure: A. The Modulated Transmitter: A transmitter is a core equipment of the fiber optic transmitter consisting of optical source Laser, electrical pulse generator and the optical modulator. The modulated optical transmitter structure designed using OptiSystem software is shown in figure 4.1 Figure 4.1: The modulated transmitter. 34

50 Chapter 4 Simulation of Dispersion compensation B. The Transmission Line: The transmission line is composed of single mode fiber (SMF), dispersion compensation fiber (DCF) or fiber bragg grating (FBG) and EDFAs amplifier. As shown in figure 4.2. Figure 4.2 : Transmission line. C. The Receiver Implementation: An optical receiver is composed of BER analyzer, PIN Photodetector and bessel low pass filter. As shown in figure 4.3. Figure 4.3: The receiver implementation. 35

51 Chapter 4 Simulation of Dispersion compensation The following figures represent the design of dispersion compensation scheme. Figure 4.4 : Dispersion pre-compensation design. Figure 4.5 : Dispersion post-compensation design. Figure 4.6 : Dispersion symmetry-compensation design. 36

52 Chapter 4 Simulation of Dispersion compensation Results and Discussion: a) b) c) Figure 4.7: Eye diagram of a) pre- b)post- c) symmetry- compensation scheme at 2.5 Gbps for -16 dbm laser power. Discussion: Case 1: From table 4.2 and eye diagrams figure 4.7, all of the eye diagrams are very bad at -16 dbm signal power for 2.5 Gbps, quality factor is low and BER is large. So, there is bad sensitivity to timing error (large) and huge amount of distortion. We can say that these techniques are not good for dispersion compensation at -16 dbm. 37

53 Chapter 4 Simulation of Dispersion compensation a) b) c) Discussion: Figure 4.8.: Eye diagram of a) pre- b)post- c) symmetry- compensation scheme at 2.5 Gbps for 8 dbm laser power. Case 2: From table 4.2 and eye diagrams figure 4.8, we can see that the eye diagrams are very good at 8 dbm signal power for 2.5 Gbps, since signals have better tolerance for noise (eye is open and sharp), in addition to this, from the table, we observe that post compensation scheme is better than the symmetrical and pre-compensation schemes because it has better eye diagram shape and larger quality factor. In this case the pre-compensation scheme is the worst. Obviously, the dispersion compensation is much better at 8 dbm than what is at -16 dbm, which means that the greater the signal power the better the dispersion compensation will be. 38

54 Chapter 4 Simulation of Dispersion compensation b) a) c) Figure 4.9: Eye diagram of a) pre- b)post- c) symmetry- compensation scheme at 10 Gbps for 8 dbm laser power. Discussion: Case 3: From table 4.2 and eye diagrams figure 4.9, good results, in this simulation, of eye diagrams are obtained at 8 dbm signal power for 10 Gbps. We have also a good Q-factor (large), a good BER (low). 39

55 Chapter 4 Simulation of Dispersion compensation Figure 4.10: max Q-factor versus signal power at 2.5 Gbps bit rates for pre dispersion compensation. Figure 4.11: max Q-factor versus signal power at 2.5 Gbps bit rates for post dispersion compensation. Figure 4.12: max Q-factor versus signal power at 2.5 Gbps bit rates for symmetry dispersion compensation. 40

56 Chapter 4 Simulation of Dispersion compensation Figure 4.13: max Q-factor versus signal power at 10 Gbps bit rates for pre dispersion compensation. Figure 4.14: max Q-factor versus signal power at 10 Gbps bit rates for post dispersion. Figure 4.15: max Q-factor versus signal power at 10 Gbps bit rates for symmetrical dispersion compensation. 41

57 Chapter 4 Simulation of Dispersion compensation Case 1 Case 2 Case 3 Pre Post Symm Pre Post Symm Pre Post Symm Max Q-factor Min BER e e e -264 Eye height -4.38e e e e e e e e e -4 Threshold 3.7e e e e e e e e e -5 Decision inst Table 4.2: Results of pre-, post- and symmetrical dispersion compensation. After this simulation, it is found that: The dispersion compensation at 10 Gbps is worse than the dispersion compensation at 2.5 Gbps bit rate when the signal power is 8 dbm. At higher bit rate the symmetrical compensation scheme is better than the post and the pre-compensation scheme. The graphs of figures 4.10 to figure 4.15 highlight these results. The graph of figures from 4.10 to 4.12 are for Q-factor versus signal power at 2.5 Gbps bit rate for pre-, post -, and symmetrical dispersion compensations. Pre-, post-, and symmetrical dispersion compensation is almost similar from -16 dbm to 3 dbm, after that (3 dbm to 8 dbm) the post-compensation becomes better; however the quality factor is going to increase for the three kinds of compensation. Figures from 4.13 to 4.15 represent graphs of Q-factor versus signal power at 10 Gbps bit rate for pre-, post- and symmetrical dispersion compensations. Pre-, post- and symmetrical dispersion compensation is almost similar from -16 dbm to 0 dbm, then, the quality factor goes to be greater for symmetrical dispersion compensation from 0 dbm to 8 dbm, and reaches its maximum value at 8 dbm. It is clear that the quality factor is larger at smaller bit rates for pre-, post- and symmetrical dispersion compensation; besides, the post-compensation is better at smaller bit rates, but the symmetrical compensation will be better when larger bit rate is used. 42

58 Chapter 4 Simulation of Dispersion compensation Dispersion Compensation by Using FBG-DCM: We will now show how the amount of compensating dispersion affects system performance. We will use an Ideal Dispersion Compensation FBG as the dispersion compensation module (figure 4.16). In this case, we selected a post-compensation scheme because it is simple compared to the symmetrical compensation scheme. All schemes perform similar in low power regions. The total accumulated dispersion of the SMF is = 1920 ps/nm. We swept the total dispersion of FBG from -30 to ps/nm. The bit rate is set to 10 Gbps. In this simulation, we want to investigate the dispersion limited performance of the system. To avoid triggering fiber nonlinearity, we keep the received power at -3 dbm. Effects of residual dispersion to nonlinear effects will be considered in other examples. Figure 4.20 shows Q factor versus accumulated dispersion. Parameter Bit-Rate Length of SMF Dispersion coefficient of SMF Dispersion coefficient of FBG Gain of EDFA Noise figure of EDFA Attenuation factor of SMF Value 10 Gbps 120 Km 16 ps/nm/km ps/nm/km 22.8 db 4 db 0.2 db/km Table 4.3: The system parameters. 43

59 Chapter 4 Simulation of Dispersion compensation Figure 4.16 Dispersion post-compensation design with FBG Results and Discussion: The following figure shows the eye diagram for input power -3 dbm. Figure 4.17: Eye diagram of FBG-DCM compensation scheme at 10 Gbps bit rate for -3 dbm laser power. 44

60 Chapter 4 Simulation of Dispersion compensation Figure 4.18: Eye diagram of FBG-DCM compensation scheme at 10 Gbps bit rate for 8 dbm laser power. Figure 4.19: Max Q-factor versus signal power at 10 Gbps bit rates for FBG-DCM. 45

61 Chapter 4 Simulation of Dispersion compensation Figure 4.20: Max Q-factor versus dispersion of FBG at 10 Gbps bit rates for -3 dbm Laser power. Discussion: It can be seen in figure 4.17 that the eye diagram is open at -3 dbm, but it is not sharp, so, there is considerable distortion in the signal. At 8 dbm (figure 4.18) we obtain encouraging results, since the eye is open and sharp which means that there is less additive noise in the signal. Also the intersymbol interference (ISI) is lower. It is evident that the FBG works better when the input power is set to 8 dbm. According to the graph shown in figure 4.19, we observe that the quality factor increases, as the signal power increases until it reaches its maximum value (33) at 6 dbm, after that, this quality factor goes to decrease, so the high signal power degrades the system performance. From the results that we obtained in section and the results we obtained it is found that: At low power the FBG has better dispersion compensation as it is compared to DCF. At high power DCF works better than FBG since it has larger quality factor. 46

62 Chapter 4 Simulation of Dispersion compensation From the graph shown in figure 4.20 we observe that the quality factor is maximum at accumulated dispersion of ps/nm, this is due the fact that the FBG-DCM has positive dispersion of ps/nm and the total dispersion of the optical fiber is ps/nm, so = -80 ps/nm and this the lowest accumulated dispersion at which the quality factor is, certainly, maximum DCF-DCM Technique in 8-Channel DWDM: We have seen in the previous that DCF technique works well in single channel. In the next simulation, we will use this technique for 8-channels and study the performance of the system in the first (channel 1), middle (channel 4) and last channel (channel 8). We will use the same parameters for symmetrical dispersion compensation scheme that we have used in the single channel. The design of the system is shown in figure Parameter Bit-Rate Length of SMF Length of DCF Dispersion coefficient of SMF Dispersion coefficient of DCF Gain of Inline EDFA Gain of Inline EDFA Attenuation factor of SMF Attenuation factor of DCF Value 10 Gbps 120 Km 24 Km 16 ps/nm/km -80 ps/nm/km 20 db 12.8 db 0.2 db/km 0.6 db/km Table 4.4: dwdm system parameters. Lasers diode frequencies are from to THz with spacing frequency of 100 GHz. 47

63 Chapter 4 Simulation of Dispersion compensation Figure 4.21: setup of 8-channel dwdm design. Result and Discussion: The following figures represent eye diagrams. The power laser is 8 dbm for each channel and the bit rate is 10 Gbps. We take three channels (channel-1, channel-4 and channel-8) as samples and study the dispersion compensation. The values of the Q-factor and BER are shown in table 4.5. Channel-1 Channel-4 Channel-8 Max Q-factor Min BER Max Q-factor Min BER Max Q-factor Min BER e e e -195 Table 4.5: max Q-factor and min BER in channel 1, 4 and 8. 48

64 Chapter 4 Simulation of Dispersion compensation a) b) c) Figure 4.22: Eye diagram of DCF-DCM compensation scheme at 10 Gbps for 8 dbm laser power for a) channel-1 b) channel-4 c) channel-8. 49

65 Chapter 4 Simulation of Dispersion compensation Discussion: After simulation we obtained good results. As shown in figure 4.22, the eye diagrams are all open, meaning that our signals have tolerance for noise. However, the maximum quality factor is slightly different from one channel to another as shown in table 4.5, where it is better in the center channel (channel-4), while it is not that good at the first channel and the last one (channel 8). The performance of the system is better in the center channel. When we compare these results with the results found in section 4.5.3, we observe that the maximum quality factor for single channel is 34.9 and the maximum quality factor using DWDM system (multiple channels) is for center channel. As expected, though DWDM system, which is a complex system, has achieved very good results, it has, slightly, more noise than single path which, insignificantly, hampers the performance of the system. 50

66 Conclusion Conclusion: The first objective of this work is to study the impairments due to dispersion on the performance of a WDM optical fiber communication system. The second objective is to compare the efficiency of two common techniques to reduce these impairments. We have investigated two techniques based on introducing along the optical link a compensation to the total dispersion using a component having a negative dispersion parameter. Commonly two different types of such compensating components are practically used; the DCF (Dispersion Compensating Fiber) and the FBG-DCM (Fiber Bragg Grating). To reach this goal, we have built then simulate an optical communication link using the Optisystem software. The simulation results (eye diagrams, Q factor and BER) show that both techniques can reduce significantly the chromatic dispersion resulting in an important increase of the overall performance of the system. At low power level applications the FBG-DCM technique has been found more efficient than the DCF technique. In addition the FBG-DCM has proved to be a cost-effective choice compared to the DCF technique, since this last introduces a more important degradation of the overall system performance of the form of attenuation which imposes the use of optical extra amplifying units. At high power level applications however, the DCF technique exhibits superior efficiency with regard to dispersion reduction and is therefore, considered to be the best choice for this type of applications. The obtained results show also that at low data bit-rates, the post-dcf version of the DCF technique gives the best compensation compared to the pre and symmetric versions. However, as the data bit-rate increases the symmetric-dcf version becomes the best. Our results are in good agreement with those published in literature. Finally, we suggest as further work for the next coming students, to investigate other techniques of dispersion compensation such as Electronic Dispersion Compensation (EDC) and digital filters then, carryout a general comparative study. 51

67 Appendices Appendix A: Eye Diagram In telecommunication, an eye pattern, also known as an eye diagram, is an oscilloscope display in which a digital data signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails. It is an experimental tool for the evaluation of the combined effects of channel noise and intersymbol interference on the performance of a baseband pulse-transmission system. It is the synchronized superposition of all possible realizations of the signal of interest viewed within a particular signaling interval. Several system performance measures can be derived by analyzing the display. If the signals are too long, too short, poorly synchronized with the system clock, too high, too low, too noisy, or too slow to change, or have too much undershoot or overshoot, this can be observed from the eye diagram. An open eye pattern corresponds to minimal signal distortion. Distortion of the signal waveform due to intersymbol interference and noise appears as closure of the eye pattern. Interpreting Measurements: Eye-diagram feature What it measures Eye opening (height, peak to peak) Additive noise in the signal Eye overshoot/undershoot Eye width Eye closure Peak distortion due to interruptions in the signal path Timing synchronization & jitter effects Intersymbol interference, additive noise Table 1: Eye diagram interpreting. I

68 Appendices Figure 1 : Eye diagram interpreting. Appendix B: Transmission Band of DWDM Band Name Wavelengths (nm) Description O-band Original band, PON upstream E-band Water peak band S-band PON downstream C-band Lowest attenuation, original DWDM band, compatible with fiber amplifiers, CATV L-band Low attenuation, expanded DWDM band U-band Ultra-long wavelength Table 2: DWDM band wavelength range. II