Digital Filters for reducing the effects of Dispersion in optical communication links

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 MASTER In Electrical and Electronic Engineering Option: Telecommunications Title: Digital Filters for reducing the effects of Dispersion in optical communication links Presented by: - GRAINAT Youcef - MEBARKI Yaser Supervisor: Mr. Abdelkader ZITOUNI Registration Number:.../2016

2 Dedication First, we thank ALLAH for his blessings and reconcile and repaid during these years that have passed us to get to this stage of science, knowledge and culture. Without his mercy and sympathy we were not able to accomplish this work. We also dedicate this work to our lovely parents with deepest gratitude whose love and prayers have always of strength for us i

3 Acknowledgement For the ancestors who paved the path before us upon whose shoulders we stand. This is also dedicated to our family and to many friends who supported us on this journey, thank you. We would like first to express our deepest gratitude to our supervisor Mr. Abdelkader ZITOUNI for his unwavering support, collegiality and guidance throughout this project. Our thanks and appreciations go to our classmates in developing the project and to people who have willing helped us with their abilities. ii

4 Abstract The aim of this project is to use digital filters to reduce the effects of dispersion in modern WDM optical communication systems at two transmission speeds, 2.5 and 5 Gbps. For this purpose two types of filters [Gaussian Filter (GF) and Cosine Roll Off Filter (CROF)] have been tested by simulation using OptiSystem and Matlab in single mode fiber links at the operating wavelength of 1550 nm. From the obtained results a comparative study with respect to the conventional Dispersion Compensating Fibers (DCF) has been carried out. iii

5 Acronyms APD: Avalanche Photodiode BER: Bit-Error-Rate CD: Chromatic Dispersion CDMA: Code Division Multiple Access CDM: Code Division Multiplexing CFG : Chirped Fiber Grating CRO: Cosine Roll Off CROF: Cosine Roll Off Filter CWDM: Coarse Wavelength Division Multiplexing DC: Dispersion compensation DCF: Dispersion-Compensating Fiber DCM: Dispersion-Compensating Module DEMUX: DE-Multiplexer DWDM: Dense Wavelength Division Multiplexing EDFA: Erbium-Doped Fiber Amplifier FBG: Fiber Bragg Grating FDM: Frequency Division Multiplexing FSO: Free Space Optic FTT: Fiber To The FWHM: Full Width at Half Maximum IL: Insertion Loss IR: Infra-Red ISI: Inter Symbol Interference LED: Light Emission Diode iv

6 LASER: Light Amplification by Stimulated Emission of Radiation MUX: Multiplexer MZM: Mach-Zhender Modulator NRZ: Non-return to Zero OADM: Optical add/drop multiplexers OBPF: Optical Band Pass Filter OE: Opto-Electronic PMD: Polarization Mode Dispersion PON: Passive Optical network RZ: Return to Zero SDH: Synchronous Digital Hierarchy SMF: Single-Mode Fiber SONET: Synchronous Optical NETwork TDM: Time Division Multiplexing WDM: Wavelength Division Multiplexing OADM: Optical Add/Drop Multiplexer LAN: Local Area Network WAN: Wide Area Network v

7 Table of contents Dedication... i Acknowledgement... ii Abstract... iii Key words... Error! Bookmark not defined. Table of contents... vi List of tables... ix List of figures... x Introduction... 2 CHAPTER 1: Optical communication systems Introduction: Optical transmitter... 6 a) LED transmitters:... 6 b) LASER Diode transmitters: Mach-Zehnder Modulator: Optical fiber: Fiber cable types: Advantages of the optical fiber: Disadvantages of optical fibers: Optical receiver: vi

8 1.5. The optical amplifier: Optical fiber impairments: The attenuation: The dispersion: WDM system: CHAPTER 2: Dispersion compensation Introduction: Modal dispersion: Chromatic Dispersion: a- Material dispersion: b- Waveguide dispersion: Polarization Mode Dispersion: Techniques of dispersion compensation: Dispersion compensation fiber DCF technique: Fiber Bragg Grating technique: a- Advantages of FBG: b- The comparison between DCF& FBG techniques: Digital Filter: a- Gaussian Filter: b- Cosine Roll Off filter: CHAPTER 3: Simulation of dispersion compensation Introduction : vii

9 2. OptiSystem software: Application of OptiSystem: Bit Error Rate: Dispersion compensation scheme: Simulation: Transmitter structure: Channel structure: Receiver implementation: MATLAB component: Dispersion compensation: Using Gaussian filter: Using Cosine roll off Filter: Digital filter technique in 8-channel DWDM: CHAPTER 4: Conclusion 50 Appendix... I A- MATLAB component:... I B- Eye diagram:... I C- Transmission band of DWDM:... III References... IV viii

10 List of tables Table1.1 Comparison between LED and LASER diodes.7 Table2.1 comparison between DCF and FBG 25 Table3.1 System parameters...35 Table3.2 Results for Dispersion compensation using GF...39 Table3.3 Results for Dispersion compensation using CRO Filter..43 Table3.4 DWDM parameters..46 Table3.5 Important parameters for the three channels using GF.47 Table3.6 Important parameters for the three channels using CROF 49 Table4 Wavelength range of DWDM. III ix

11 List of figures Figure1.1 Optical system link 5 Figure1.2 Structure of Fiber Cable.7 Figure1.3 Signal propagation in an optical fiber....9 Figure1.4 Multimode fiber signaling Figure1.5 Single-mode fiber signaling.10 Figure1.6 Schematic diagram of a simple Doped Fiber Amplifier...13 Figure1.7 Rayleigh scattering in the fiber.15 Figure1.8 Spectral attenuation in silica fiber 16 Figure1.9 WDM operating principle.17 Figure2.1 Pulse broadening scheme.21 Figure2.2 polarization mode dispersion 22 Figure2.3 dispersion compensating fiber..22 Figure2.4 spectral response of FBG..24 Figure2.5 Impulse response for the Gaussian filter at 2.5 and 5 Gbps bit-rates for N= Figure2.6 Impulse response of CRO filter at 2.5 and 5 Gbps speeds...28 Figure3.1 Transmitter structure 33 Figure3.2 Channel implementation..34 Figure3.3 Receiver Structure 34 x

12 Figure3.4 MATLAB component for retransmitting of the received signal..34 Figure3.5 Dispersion compensation design using GF for a simple link...35 Figure3.6 Dispersion compensation design using GF twice 36 Figure3.7 Eye diagram at 2.5 Gbps for -10dBm laser power (a) after 100km (b) after 200km...36 Figure3.8 Eye diagram at 2.5 Gbps for 8dBm laser power (a) after 100km (b) after 200km..37 Figure3.9 Eye diagram at 5 Gbps for 8dBm laser power (a) after 100km (b) after 200km.38 Figure3.10 The maximum Q-factor vs power at 2.5 Gbps for 100km fiber length.38 Figure3.11.The maximum Q-factor vs power at 5 Gbps for 100km fiber length 39 Figure3.12 Dispersion compensation design using CROF twice 40 Figure3.13 Eye diagram at 2.5 Gbps for -10dBm laser power (a) after 100km (b) after 200km...40 Figure3.14 Eye diagram at 2.5 Gbps for 8 dbm laser power (a) after 100km (b) after 200km...41 Figure3.15 Eye diagram at 5 Gbps for 8 dbm laser power (a) after 100km (b) after 200km...42 Figure3.16 The maximum Q-factor vs power at 2.5 Gbps for 100km fiber length.42 Figure3.17 The maximum Q-factor vs power at 5 Gbps for 100km fiber length 43 Figure 3.18 Dispersion pre-compensation design 44 Figure3.19 Dispersion post-compensation design 44 Figure 1.20 Dispersion symmetry-compensation design.45 xi

13 Figure3.21 Eye diagram a)pre-b)post-c)symmetric compensation schemes at 2.5 Gbps for - 16dBm using DCF.45 Figure3.22 (8-channel) DWDM Design with GF 47 Figure3.23 Eye diagrams of GF compensation at 2.5 Gbps with 8dBm LASER power a) Channel-1 b)channel-4 c) Channel Figure3.24 (8-channel) DWDM System with CROF..49 Figure3.25 Eye diagrams of CROF compensation at 2.5 Gbps with 8dBm LASER power a) Channel-1 b) Channel-4 c) Channel Figure4 Eye Diagram interpretation II xii

14 Introduction Introduction 1

15 Introduction Introduction Optical fiber communication is a method of transmitting information from one place to another by sending pulses of light through optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. The potential bandwidth of optical communication systems is the driving force behind the worldwide development and deployment of light wave system. Like other communication systems optical communication system also faces problems like dispersion, attenuation and non-linear effects that lead to deterioration in its performance. Among them dispersion affects the system the most and it is tougher to overcome it as compared to other two problems. A pulse of light temporally dilates or compresses because of dispersion as a result of the fact that in all media except vacuum, the group velocity of light depends on its wavelength. This phenomenon occurs, of course, in every travelling wave that carries energy, and in the context of light, has been exploited, utilized, or mitigated ever since short-pulse lasers emerged. Modelocked lasers, for instance, depend critically on the dispersion-map of the path in which light travels in a laser cavity. Traditionally, optical techniques to control or compensate dispersion comprised devices that would provide wavelength-dependent delays, such as bulk-optic diffraction gratings or prisms. With the advent of practical, long-haul optical fiber links in the 1980s [1], the need to manage dispersion in an optical communication network arose. In addition to the temporal distortion effects that pulses from mode-locked lasers experienced due to dispersion, a pulse train in a communication link also had to contend with inter-symbol interference (ISI) arising from overlaps of adjacent pulses in a bit-stream. As a general rule of thumb, the amount of 2

16 Introduction dispersion tolerated by an optical transmission line varies inversely proportionally to the square of the data-rate. For the early 2.5-Gb/s systems, the dispersion tolerance was greater than 30000ps/nm. Hence a conventional single-mode fiber (SMF), with 17 ps/(km-nm) dispersion could be used for lengths as long as 2500 km without being limited by dispersion. However, as transmission lengths as well as bit rates grew, dispersion became the primary bottleneck that the optical network had to combat [2,3]. Thus, it is important to work out an effective dispersion compensation technique that leads to performance enhancement of the optical system. In this project we are going to present our work as follow: Chapter 1: Introduces some basics of optical communication systems, the optical and electrical components that are used in this project. Chapter 2: Shows the types of dispersion and some techniques that had been used to compensate it. Chapter 3: In this chapter we are going to simulate the different techniques that are shown in the 2 nd chapter and compare between them. Chapter 4: Conclusion that summarizes the outcome of this project and gives some suggestions about the future work. 3

17 OPTICAL COMMUNICATION SYSTEMS 1 CHAPTER 1 Optical communication systems 4

18 OPTICAL COMMUNICATION SYSTEMS 1 Introduction: First developed in the mid- 1970s, fiber-optics have played a major role in the advent of the Information Age by making a revolution in the telecommunications industry [1]. Fiber-optics is used to transmit telephone signals, Internet communication, and cable television signals. Using fiber-optic communication Internet speeds have reached 100 Peta-bits per second kilometer. Optical Fiber communication is a method for carrying information at a distance in the form of light. The light forms an electromagnetic carrier wave that is modulated to carry information. A standard Modern fiber-optic communication system consists of a transmitting device to convert the electrical signal into optical signal to send into the optical fiber, an optical fiber cable carrying the light, multiple kinds of amplifiers, and an optical receiver to recover the signal and convert it back into an electrical signal. The information transmitted is typically digital information generated by computers, telephone systems, and cable television companies. Figure 1.1 Optical system link Here we will examine the various components that make up a fiber optic communication system. 5

19 OPTICAL COMMUNICATION SYSTEMS Optical transmitter In order that data can be carried along an optical fiber cable, it is necessary to have a light source or optical transmitter. The optical transmitter generates an optical frequency carrier, and the carrier is modulated according to the transmitted data and passing through the fiber to the receiver. The latter is one of the key elements of any optical fiber communication systems and the choice of the correct one will depend upon the particular application that is envisaged. There are two main types of optical transmitters that are commonly used today. Both of them are based around semi-conductor technology: a) LED transmitters: LEDs are cheap and reliable. They emit only incoherent light with a relatively wide spectrum as a result of the fact that the light is generated by a method known as spontaneous emission [1]. A typical LED used for optical communications may have its light output in the range nm. In view of this the signal will be subject to chromatic dispersion, and this will limit the distances over which data can be transmitted. b) LASER Diode transmitters: Laser in acronym of Light Amplification by Stimulated Emission. These optical fiber transmitters are more expensive and tend to be used for telecommunications links where the cost sensitivity is nowhere near as great. The output from a laser diode is generally higher than that available from a LED, although the power of LEDs is increasing. Often the light output from a laser diode can be in the region of 100 mw.in addition to this the output is more directional than that of a LED and this enables much greater levels of coupling efficiency into the optical fiber cable[1]. 6

20 OPTICAL COMMUNICATION SYSTEMS 1 The following table summarizes the main differences between LEDs and LASERs: CHARACTERISTIC LED LASER DIODE Cost Low High Data rate Low High Distance Short Long Fibre type Multi-mode fibre Multi-mode and single-mode fibre Lifetime High Low Temperature sensitivity Minor Significant Table 2.1 Comparison between LED and LASER diodes 1.2. Mach-Zehnder Modulator: A Mach-Zehnder modulator is used for controlling the amplitude of an optical wave. The input waveguide is split up into two waveguide interferometer arms. If a voltage is applied across one of the arms, a phase shift is induced for the wave passing through that arm. When the two arms are recombined, the phase difference between the two waves is converted to an amplitude modulation [2]. This is a multi-physics model, showing how to combine the Electromagnetic Waves, Beam Envelopes interface with the Electrostatics interface to describe a realistic waveguide device Optical fiber: An optical fiber is a flexible filament of very clear glass or plastic and is capable of carrying information in the form of light, it is consisting of core, cladding and one or more protective 7

21 OPTICAL COMMUNICATION SYSTEMS 1 coatings such as(buffer, jacket )to keep it safe from environmental and mechanical damage as it shown in Figure1.2. Figure1.1 Structure of the Fiber Cable The phenomenon of total internal reflection, responsible for guiding of light in optical fibers, is known since the nineteenth century [2]. In fact, by Snell s law: n 0. sin θ i = n 1. sin θ r (1.1) Where n0 and n1 are the refractive indices of the air and the core, respectively - Applying the same law for the core-cladding media we get: sin φ c = n 2 n 1 (1.2) Where n2 is the refractive index of the cladding and φ c is the critical angle [1]. Figure 01.3: Signal propagation in an optical fiber 8

22 OPTICAL COMMUNICATION SYSTEMS Fiber cable types: Two main types of optical fiber used in optical communications include multi-mode fiber optics and single-mode optical fiber. a) Multimode fiber: Due to its larger diameter core ( 50 micrometers) and multiple mode; All multimode fibers are best suited for speeds between 10 Mbps to 10 Gbps in shorter length. allowing also less precise, cheaper transmitters and receivers to connect to it as well as cheaper connectors. Furthermore, because of its higher dopant content, multi-mode fibers are usually expensive and exhibit higher attenuation.it is typically used in campus network, shipboard, aircraft applications etc[3]. b) Single mode fiber: Figure 01.4: Multimode fiber signaling Single-mode fiber has a very small core diameter (<10 micrometers) and it is designed for the transmission of a single ray or mode of light as a carrier and is used for long-distance signal transmission. Single-mode fiber must be driven with a precision LASER transmitter, more expensive components and interconnection methods; but carries higher speed data with higher performance, much further for environment such as internet backbones, WANs etc [3]. 9

23 OPTICAL COMMUNICATION SYSTEMS Advantages of the optical fiber: Figure 1.5: Single-mode fiber signaling - The life of fiber is longer than copper wire. - Light transmission through optical fibers is unaffected by other electromagnetic radiation nearby since fiber optic is electrically non-conductive, so it does not act as an antenna to pick up the electromagnetic signals it can also be run in electrically noisy environments without concern as electrical noise will not affect fibre[19]. -Broad bandwidth: Fiber cables have a much greater bandwidth than metal cables. The amount of information that can be transmitted per unit time of fibre over other transmission media is its most significant advantage. With the high performance single mode cable used by telephone industries for long distance telecommunication, the bandwidth surpasses the needs of today's applications and gives room for growth tomorrow. - Low attenuation loss over long distances compared to coaxial cable or twisted pair: Attenuation loss can be as low as 0.2 db/km in optical fiber cables, allowing transmission over long distances without the need for repeaters.in comparison to copper; in a network, the longest recommended copper distance is 100m while with fibre, it is 2000m [4]. -Handing and installation costs of optical fiber is very nominal. Glass can be made more cheaply than copper because the raw materials for glass are plentiful, unlike copper. 10

24 OPTICAL COMMUNICATION SYSTEMS 1 - Material cost and theft prevention: Conventional cable systems use large amounts of copper. Global copper prices experienced a boom in the 2000s, and copper has been a target of metal theft. -There is no necessity for additional equipment for protection against grounding and voltage problems since fiber is a dielectric, it does not present a spark hazard. -Size: In comparison to copper, a fibre optic cable has nearly 4.5 times as much capacity as the wire cable has and a cross sectional area that is 30 times less. -Weight: Fibre optic cables are much thinner and lighter than metal wires. They also occupy less space with cables of the same information capacity. Lighter weight makes fibre easier to install. -An optical fibre has greater tensile strength than copper of the same diameter. It is flexible, bends easily and resists most corrosive elements attacking copper cable. - Security of information passed down the cable: Copper can be tapped with very little chance of detection. As they do not radiate electromagnetic energy, emissions cannot be intercepted. As physically tapping the fibre takes great skill to do undetected, fibre is the most secure medium available for carrying sensitive data Disadvantages of optical fibers: - Cables are expensive to install but last longer than copper cables. - Optical fiber are fragile and can be broken or have transmission loses when wrapped around curves of only a few centimetres radius. However, by encasing fibres in a plastic sheath, it is difficult to bend the cable into a small enough radius to break the fibre. -Very Expensive Receivers and transmitters are required. 11

25 OPTICAL COMMUNICATION SYSTEMS 1 -Transmission on optical fibre requires repeating at distance intervals. -Optical fibres require more protection around the cable compared to copper Optical receiver: By definition photodetectors convert light signals to electrical signals which can then be processed further. For optical fiber applications photodetectors work at standard wavelengths around and 1550nm. Suitable photodiodes may be either PIN diodes or avalanche photo diodes (APDs). In either case the operating wavelength determines the material used, for example Si being employed at nm and GE or alloys of In GA, As and P at 1330 nm[32]. PIN diodes and APDs are variations on a basic depletion layer photodiode in which reverse current is altered by absorption of light at the correct wavelength. APDs differ from PIN diodes in that APDs have gain so that with the correct circuitry better sensitivity can be achieved with APDs [4] The optical amplifier: Optical amplifier is a device that amplifies an optical signal directly, without first converting it to electrical form. Optical amplifiers are important in optical communication. There are several different physical mechanisms that can be used to amplify a light signal, which correspond to the major types of optical amplifiers. In doped fiber amplifiers and bulk lasers, stimulated emission in the amplifier's gain medium causes amplification of incoming light. An example of this type is the Erbium-Doped Fiber Amplifier (EDFA), 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. When the signal-carrying laser beams pass through this fiber, external energy is applied, usually at infrared (IR) wavelengths. This, so called pumping excites the atoms in the erbium-doped section of fiber optic, increasing the intensity of 12

26 OPTICAL COMMUNICATION SYSTEMS 1 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 [5]. Figure 1.6 Schematic diagram of a simple Doped Fiber Amplifier In semiconductor optical amplifiers (SOAs), electron-hole recombination occurs. In Raman amplifiers, Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. Parametric amplifiers use parametric amplification [4,5]. 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 Giga bits per second (Gbps) of data, the electronic circuits of a conventional repeater cannot [5]. 13

27 OPTICAL COMMUNICATION SYSTEMS 1 Besides eliminating complex and inefficient conversion and electronic amplification stages, the EDFA allows the transmission of signals that employ wavelength-division multiplexing (WDM) [4,5]. This increases the realizable bandwidth relative to conventional repeaters still further Optical fiber impairments: As we have said before the optical fiber may have some impairments such as attenuation and dispersion where: The attenuation: It is the loss of optical power as the light travels through the fiber; it is measured in db or in db/km as follow: α = P in P out L (2.1) Where Pout andpin is the output and the input power respectively in (db). And α is the attenuation parameter in (db/km); L is the length of the fiber in (km) [6]. Actually the attenuation is caused by many factors, among them: a) Rayleigh scattering: Figure1.7 Rayleigh scattering in the fiber 14

28 OPTICAL COMMUNICATION SYSTEMS 1 Basically, scattering losses are caused by the interaction of light with density fluctuations within a fiber. Density changes are produced when optical fibers are manufactured. b) Material absorption: It can be divided into two types of absorption, the intrinsic absorption and the extrinsic absorption. The intrinsic absorption is generally caused by the interaction of light with pure silica whereas the extrinsic absorption is caused the interaction of light with impurities in silica The dispersion: Figure1.8 Spectral attenuation in silica fiber It is defined as pulse spreading in an optical fiber. As a pulse of light propagates through a fiber, elements such as numerical aperture, core diameter, refractive index profile, wavelength, and laser line-width cause the pulse to broaden. Dispersion increases along the fiber length. The overall effect of dispersion on the performance of a fiber optic system is known as Inter symbol 15

29 OPTICAL COMMUNICATION SYSTEMS 1 Interference (ISI). Inter symbol interference occurs when the pulse spreading caused by dispersion causes the output pulses of a system to overlap [7]. 3. WDM system: 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. In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. 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). The term wavelength-division multiplexing is commonly applied to an optical carrier (which is typically described by its wavelength), whereas frequency-division multiplexing typically applies to a radio carrier (which is more often described by frequency). Since wavelength and frequency are tied together through a simple directly inverse relationship, in which the product of frequency and wavelength equals c (the propagation speed of light), the two terms actually describe the same concept.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 16

30 OPTICAL COMMUNICATION SYSTEMS 1 need to add additional fiber. Figure 1.9 WDM operating principle A WDM 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 [8], 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 [8]; this spacing is also the standard for ITU-T today. WDM System consists of the following Components: 17

31 OPTICAL COMMUNICATION SYSTEMS 1 -Optical transmitters/receivers. -DWDM mux/demux filters. -Optical add/drop multiplexers (OADMs). -Optical amplifiers. -Transponders (wavelength converters). 18

32 DISPERSION COMPENSATION 2 CHAPTER 2 Dispersion compensation 19

33 DISPERSION COMPENSATION 2 1. Introduction: Telecommunication systems change the intensity of light source in order to transmit information. Information is modulated and sent as a series of pulses representing binary encoded data. Data can be transmitted with few errors, as long as these pulses travel through the fiber without changing their shape. But usually, as they travel through the fiber, the pulses start to spread, losing their original shape and overlap each other becoming indistinguishable at the receiver input. Dispersion is the general term applied to this cause and this effect is known as inter-symbol interference. Dispersion was initially a problem when multimode step index fiber was introduced. Multimode graded-index fiber improved the situation, but when they are well graded some limitations are added to the information capacity of multimode fibers. Single-mode fiber eliminated the multipath dispersion and left only chromatic dispersion and polarization mode dispersion to be dealt with by engineers. Both of them cause distortion and broadening of pulse. Dispersion is generally divided into three categories: modal dispersion, chromatic dispersion and polarization mode dispersion Modal dispersion: Modal dispersion is defined as pulse spreading caused by the time delay between lowerorder modes and higher-order modes. Modal dispersion is problematic in multimode fiber, causing bandwidth limitation Chromatic Dispersion: Chromatic Dispersion (CD) is pulse spreading due to the fact that different wavelengths of light propagate at slightly different velocities through the fiber because the index of refraction 20

34 DISPERSION COMPENSATION 2 of glass fiber is a wavelength-dependent quantity; different wavelengths propagate at different velocities. follow: Figure2.1 Pulse broadening scheme CD consists of two parts: material dispersion and waveguide dispersion, we define them as a- Material dispersion: It is due to the wavelength dependency on the index of refraction of glass.in other words; the refractive index of the core varies as a function of wavelength [9]. b- Waveguide dispersion: It is due to the physical structure of the waveguide. In a simple step-index profile fiber, waveguide dispersion is not a major factor, but in fibers with more complex index profiles, waveguide dispersion can be more significant [9] Polarization Mode Dispersion: Polarization Mode Dispersion (PMD) occurs due to birefringence along the length of the fiber that causes different polarization modes to travel at different speeds which will lead to rotation of polarization orientation along the fiber [9-10]. 21

35 DISPERSION COMPENSATION 2 Figure 2.2 polarization mode dispersion 2. Techniques of dispersion compensation: In order to remove the broadening or the spreading of the light pulses, dispersion compensation is the most important feature required in optical communication systems, there are different techniques to compensate dispersion. The most commonly used techniques for DC are as follow: 2.1. Dispersion compensation fiber DCF technique: DCF is a loop of fiber having negative dispersion equal to the dispersion of the transmitting fiber. It can be inserted at either beginning (pre-compensation techniques) or end (postcompensation techniques) between two optical amplifiers. But it gives large footprint and insertion losses [10-12]. Figure2.3 dispersion compensating fiber Advantages of DCF: - They can be easily constructed and highly reliable. 22

36 DISPERSION COMPENSATION 2 - DCF provides continuous compensation over a wide range of optical wavelengths. - 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 technique: A Fiber Bragg Grating (FBG) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by adding a periodic variation to the refractive index of the fiber core, which generates a wavelength specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector. Fiber Bragg Gratings are made by laterally exposing the core of a single-mode fiber to a periodic pattern of intense ultraviolet light. The exposure produces a permanent increase in the refractive index of the fiber's core, creating a fixed index modulation according to the exposure pattern. This fixed index modulation is called a grating. At each periodic refraction change a small amount of light is reflected. All the reflected light signals combine coherently to one large reflection at a particular wavelength when the grating period is approximately half the input light's wavelength. This is referred to as the Bragg condition, and the wavelength at which this reflection occurs is called the Bragg wavelength. Light signals at wavelengths other than the Bragg wavelength, which are not phase matched, are essentially transparent [10]. This principle is shown in Figure

37 DISPERSION COMPENSATION 2 Figure2.4 spectral response of FBG Where: λ B = 2n eff Λ B And n eff is the refractive index of the Bragg. Optical FBG has recently found a practical application in compensation of dispersionbroadening in long-haul communication. In this, Chirped Fiber Grating (CFG) is preferred. CFG is a small all-fiber passive device with low insertion loss that is compatible with the transmission system and CFG s dispersion can be easily adjusted. CFG should be located in-line for optimum results [13, 14]. a- Advantages of FBG: - FBG has low insertion loss (IL). Typically, a 120-km FBG-DCM has an insertion loss in the range of 3 to 4 db, depending on its type [14]. - 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 display nonlinearity effects at rather low optical powers, the FBG-DCM will not introduce such effects even at the highest power levels present throughout 24

38 DISPERSION COMPENSATION 2 optical network. Dispersion requirements increase with higher bandwidth, the focus on dispersion compensation is high [11]. b- comparison between DCF& FBG techniques: - 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. It can be summarized in the following table: Characteristics DCF FBG Bandwidth Wide band, 20 nm Narrow band,0.1-5 nm Fiber length km cm Negative dispersion +15 to +25 ps/nm/km ps/nm/km Positive dispersion -80 to -120 ps/nm/km ps/nm/km Dispersion 16 ps/km/nm 17 ps/km/nm Bending loss db/km 0.14 db/km Reference ratio % % attenuation 0.8 db/km 0.2 db/km Non linear effects Some limitations no Insertion loss High Low Construction Complex Simple Overall cost of system High Low Table2.1 comparison between DCF and FBG 25

39 DISPERSION COMPENSATION Digital Filter: Digital filters using Digital Signal Processing (DSP) can be utilized for compensating the chromatic dispersion. They provide fixed as well as tunable dispersion compensation for wavelength division multiplexed (WDM) system. Popularly used filter is lossless all-pass optical filters for fiber dispersion compensation, which can approximate any desired phase response while maintaining a constant, unity amplitude response.other filters used for dispersion compensation are Gaussian filters, Super-Gaussian filters, Butterworth filters and microwave photonic filter [16]. In our project we will use the Gaussian filter and Cosine Roll Off filter. a- Gaussian Filter: The impulse response of a Gaussian filter is a Gaussian function given by h(t) = 1 t² e 2σ² Fourier transform H(f) = αe ln( 2)( f fc )2N σ 2π Where H(f) is the filter transfer function, α is the parameter Insertion loss, fc is the filter cutoff frequency, N is the parameter Order, and f is the frequency. For the time domain function, σ is the standard deviation.as we know, the Fourier transform of the Gaussian pulse is a Gaussian function. The Gaussian pulse is neither a Nyquist pulse nor an orthogonal pulse, but if we consider that a high portion of the pulse energy is included in one symbol time, this pulse can be approximately considered as both Nyquist and orthogonal pulses for sufficiently small σ [15, 16]. 26

40 DISPERSION COMPENSATION 2 Its impulse response is given as follow: Figure 2.5 Impulse response for the Gaussian filter at 2.5 and 5 Gbps bit-rates for N=64 b- Cosine Roll Off filter: The transfer function of a CRO filter is given by α ; f < f 1 H(f) = 0.5. α 2. [1 + cos ( f f 1 r p. f FWHM. π)] ; f 1 < f < f 2 0 ; f > f 2 Where α is the parameter Insertion loss, f c is the filter cutoff frequency, and r p is the parameter Roll off factor [17]. The parameters f1 and f2 are: f 1 = (1 r p ). f c f 2 = (1 + r p ). f c 27

41 DISPERSION COMPENSATION 2 The parameter f FWHM is the Full Width at Half Maximum frequency (or the bandwidth at 50% of the maximum transmission) and it is given by: 2f c f FWHM = 1 + [ 2 π cos 1 ( 2 1) 1]. r p Figure 2.6 Impulse response of CRO filter at 2.5 and 5 Gbps speeds Remark: From figure2.5 and figure2.6 we see that if we increase the bit-rate we will have a large bandwidth even if we change the order of the GF or the Roll off factor of the CROF. 28

42 SIMULATION OF DISPERSION COMPENSATION 3 CHAPTER 3 Simulation of dispersion compensation 29

43 SIMULATION OF DISPERSION COMPENSATION 3 1. Introduction : The goal of this simulation is to study the dispersion compensation using two kinds of low pass digital filters which are Gaussian and cosine roll off filters and compare it with the previous techniques of dispersion compensation at two different speeds (Bit-Rates) 2.5Gbps and 5Gbps. 2. OptiSystem software: OptiSystem is an innovative 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, from analog video broadcasting systems to intercontinental backbones. OptiSystem is a stand-alone product that does not rely on other simulation frameworks. It is a system level simulator based on the realistic modeling of fiber-optic communication systems. It possesses a powerful new simulation environment and an hierarchical definition of components and systems. Its capabilities can be extended easily with the addition of user components, and can be seamlessly interfaced to a wide range of tools Application of OptiSystem: OptiSystem allows for the design automation of virtually any type of optical link in the physical layer, and the analysis of a broad spectrum of optical networks, from long-haul systems to MANs and LANs [17, 18]. OptiSystem s wide range of applications include: - Optical communication system design from component to system level at the physical layer. - CATV or TDM/WDM network design. - Passive optical networks (PON) based FTTx. 30

44 SIMULATION OF DISPERSION COMPENSATION 3 - Free space optic (FSO) systems. - 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. In addition to that, there are other software components in OptiSystem like MATLAB and C++ components which help the users to extent their simulation as much as possible. 3. Bit Error Rate: The Bit Error Rate (BER) is the number of bit errors per unit time. The bit error ratio (also BER) is the number of bit errors divided by the total number of transferred bits during a studied time interval. BER is a unitless performance measure, often expressed as a percentage. So, BER = number of bit errors total number of transfferd bits (3.1) The BER may be evaluated using stochastic computer simulations. If a simple transmission channel model and data source model is assumed, the BER may also be calculated analytically as follow: BER = 1 2 erfc ( Q 2 ) (3.2) Where: Q is the quality factor. And: erfc(x) = 2 π e t² dt (3.3) + x 31

45 SIMULATION OF DISPERSION COMPENSATION 3 From eq(3.2) and eq(3.3)we conclude that: the larger is the quality factor, the lower is the BER [23]. 4. Dispersion compensation scheme: 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 17ps/nm/km in the 1550 nm wavelength range for a standard single mode fiber (SMF). 2πc L < 16 D λ²b² (3.4) Where: L: Length of the fiber in km. C: Speed of light ( m/s). λ: Wavelength in meter. B: Bit rate in Gbps. D: material dispersion in ps/(km.nm). It means that when D=16 ps/(km.nm) at 2.5 Gbps, L 500km, whereas it drops to 30 km at 10 Gbps bit rate [23]. 5. Simulation: Firstly, we are going to simulate our work with simple optical fiber link using two digital filters (GF and CROF) and make the comparison between them and the other techniques. Then, we see our result in WDM system. 32

46 SIMULATION OF DISPERSION COMPENSATION Transmitter structure: The transmitter is a simple LASER transmitter modulated with NRZ generator by Mach- Zehnder modulator as it is shown in Figure3.1 Figure 3.1 Transmitter structure In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which ones are represented by one significant condition, usually a positive voltage, while zeros are represented by some other significant condition, usually a negative voltage, with no other neutral or rest condition. The pulses in NRZ have more energy than a return-to-zero (RZ) code, which also has an additional rest state beside the conditions for ones and zeros. NRZ is not inherently a self-clocking signal, so some additional synchronization technique must be used for avoiding bit slips; examples of such techniques are a run length limited constraint and a parallel synchronization signal [18] Channel structure: The channel consists of three main objects Optical Fiber, optical gain and optical band-pass filter such as Bessel OBPF as it is shown in figure3.2 33

47 SIMULATION OF DISPERSION COMPENSATION Receiver implementation: Figure 3.2 Channel implementation As we have mentioned in chapter 2, we can use PIN photo-detector as a receiver after that we connect it with an electrical low-pass filter as it is shown in figure MATLAB component: Figure3.3 Receiver Structure In order to retransmit the received signal we use the MATLAB component in OptiSystem as a digital comparator as it is shown in figure3.4 Figure 3.4 MATLAB component for retransmitting of the received signal 5.4. Dispersion compensation: The waveform of the transmitted pulses is usually changed before sending it into the channel. This process is done to limit the effective bandwidth of the transmission, and have a better 34

48 SIMULATION OF DISPERSION COMPENSATION 3 control over the inter symbol interference caused by the channel. So the pulse-shaping filter determines the spectrum of the transmission. Pulse-shaping filters need to satisfy certain criteria so that the filter itself does not introduce ISI. Common criteria for evaluating filters are Nyquist ISI criterion and orthogonal pulse criterion. The Nyquist pulses are appropriate for sampling receivers whereas the orthogonal pulses are usually used as pulse-shaping filters in the transmitter and as matched filters in the receiver. In our simulation we ll use the following data: PARAMETER Bit-Rate Fiber length Gain of the optical amplifier Band-width of the BPOF Attenuation of the Fiber VALUE 2.5 and 5 Gbps 100 km 20 db 40 GHz 0.2 db/km Using Gaussian filter: We simulate the following system: Table 3.1 System parameters Figure 3.5 Dispersion compensation design using GF for a simple link 35

49 SIMULATION OF DISPERSION COMPENSATION 3 Figure 3.6 Dispersion compensation design using GF twice 1) Results and discussions: Case1: we adjust the bit rate at 2.5Gbps with 10dBm laser power Figure 3.7 Eye diagram at 2.5 Gbps for -10dBm laser power (a) after 100km (b) after 200km Discussions: For this case we see from table3.2 and figure3.7. That we have low BER with high Q- factor, also we have good eye diagrams, so we can say that this technique is very good for compensating dispersion at low power LASER (-10dBm). 36

50 SIMULATION OF DISPERSION COMPENSATION 3 Case2: we take the same bit rate as the 1 st case with 8dBm laser power Figure 3.8 Eye diagram at 2.5 Gbps for 8dBm laser power (a) after 100km (b) after 200km Discussions: These results in this case are better than the first case because of the increased power of the LASER (8dBm) for the same speed (2.5Gbps). As we see in figure3.8 the eye diagrams are opened much well than that in the 1 st case also we see Table3.2 we have zero BER and high Q- factor. 37

51 SIMULATION OF DISPERSION COMPENSATION 3 Case 3: we test the design in figure3.6 at 5Gbps bit rate with 8dBm laser power Figure 3.9 Eye diagram at 5 Gbps for 8dBm laser power (a) after 100km (b) after 200km Discussions: In this case we have set the bit rate at 5 Gbps for 8 dbm LASER power, from Table3.2 and figure3.9 the eye diagrams are well opened, we have also good Q-factor (large) and low BER. We have also: Figure 3.10 The maximum Q-factor vs power at 2.5 Gbps for 100km fiber length 38

52 SIMULATION OF DISPERSION COMPENSATION 3 Discussions: Figure3.11.The maximum Q-factor vs power at 5 Gbps for 100km fiber length From the results of this simulation we see that the dispersion compensation at 5Gbps is worse than that at 2.5Gbps. The graphs shown in figure3.10 and figure3.11 illustrates the maximum Q-factor versus the signal power in (dbm), both of them shows that the greater is the power of the signal, the larger is the maximum Q-factor. Case1 Case2 Case3 Parameters (a) (b) (a) (b) (a) (b) Minimum BER 5.243e e e e-123 Maximum Q-factor Threshold 2.16e e Eye height 8.865e e Decision inst Table 3.2 Results for Dispersion compensation using GF 39

53 SIMULATION OF DISPERSION COMPENSATION Using Cosine roll off Filter: In this case we are going to use CRO filter instead of GF as follow: Figure 3.12 Dispersion compensation design using CROF twice 1) Results and discussions: Case1: we test the design in figure3.12 at 2.5Gbps with -10dBm laser power Figure 3.13 Eye diagram at 2.5 Gbps for -10dBm laser power (a) after 100km (b) after 200km 40

54 SIMULATION OF DISPERSION COMPENSATION 3 Discussions: From the eye diagrams in figure3.13 we have good results of eye openings in low LASER s power (-10dBm) at speed of 2.5 Gbps, and from table3.3 we have also high Q-factor with very low BER. In this case, we have got approximately the same results in that of GF dispersion compensation. Case2: we keep the same speed as the 1 st case with 8dBm laser power Figure 3.14 Eye diagram at 2.5 Gbps for 8 dbm laser power (a) after 100km (b) after 200km Discussions: As you see in figure 3.14 the eye diagrams are well opened if we increase the power of LASER to 8dBm, and from table3.3 we have low BER with large Q-factor. 41

55 SIMULATION OF DISPERSION COMPENSATION 3 Case3: we adjust the bit rate at 5Gbps with 8dBm laser power Figure 3.15 Eye diagram at 5 Gbps for 8 dbm laser power (a) after 100km (b) after 200km Discussions: If we increase a little bit the bit-rate to 5 Gbps and let the power of LASER at 8 dbm, we ll have a well opening of the eye diagrams as it is shown in figure3.15 and from table3.3 as usual we ve low BER and high Q-factor. Figure3.16 The maximum Q-factor vs power at 2.5 Gbps for 100km fiber length 42

56 SIMULATION OF DISPERSION COMPENSATION 3 Discussions: Figure3.17 The maximum Q-factor vs power at 5 Gbps for 100km fiber length From the graphs in figure3.16 and figure3.17 we observe a good behavior of the Q-factor from -10 to 2 dbm of LASER power at the two speeds, but if we increase the signal power the maximum Q-factor goes down, in other words, the greater is the signal power, the worse quality factor is. Case1 Case2 Case3 Parameters (a) (b) (a) (b) (a) (b) Minimum BER 3.75e e e e e e-87 Maximum Q-factor Threshold 3.89e e Eye height 8.55e e Decision inst Table 3.3 Results for Dispersion compensation using CRO Filter 43

57 SIMULATION OF DISPERSION COMPENSATION Using DCF: In this case we are going to see the technique in which we use Dispersion Compensating Fiber (DCF) as it is mentioned in the following figures: Figure 3.18 Dispersion pre-compensation design Figure3.19 Dispersion post-compensation design 44

58 SIMULATION OF DISPERSION COMPENSATION 3 Figure 2.20 Dispersion symmetry-compensation design Figure3.21 Eye diagram a)pre-b)post-c)symmetric compensation schemes at 2.5 Gbps for -16dBm using DCF [31] 45

59 SIMULATION OF DISPERSION COMPENSATION 3 Discussions: If we compare the eye diagrams in figure3.21 with those that are in figure3.7 and figure3.13 [31] we see that DCF does not give good results at 2.5 Gbps for low power. However, we observe that digital filters (GF or CROF) have the capability to compensate dispersion. So, they can be used in that case Digital filter technique in 8-channel DWDM: Previously we have seen that digital filtering gives good results in a single channel, Now, we are going to see this technique in DWDM Systems. We ll test three channels (1 st, 4 th and 8 th channel) of the DWDM System. We ll set the same parameters as in the single channel. PARAMETER Bit-Rate Fiber length Gain of the optical amplifier 2.5 Gbps 100 km 20 db VALUE Attenuation of the Fiber LASER diode frequencies 0.2 db/km From to THz (100 GHz spacing) Table3.4 DWDM parameters a) Using Gaussian Filter: The design of the system is given in figure

60 SIMULATION OF DISPERSION COMPENSATION 3 Figure 3.22 (8-channel) DWDM Design with GF a) Results and discussions: Parameters Channel -1 Channel -4 Channel -8 Maximum Q-factor Minimum BER Table3.5 Important parameters for the three channels using GF 47

61 SIMULATION OF DISPERSION COMPENSATION 3 Figure3.23 Eye diagrams of GF compensation at 2.5 Gbps with 8dBm LASER power a) Channel-1 b)channel-4 c) Channel-8 Discussions: From table3.5 and eye diagrams of figure3.23 we see that there is high Q-factor and zero BER, which means that GF gives good results at 2.5 Gbps for 8dBm laser power. 48

62 SIMULATION OF DISPERSION COMPENSATION 3 b) Using Cosine Roll Off Filter: We keep the same parameters as in the first method but we change GF by CROF as follow: Figure 3.24 (8-channel) DWDM System with CROF a) Results and discussions: Parameters Channel -1 Channel -4 Channel -8 Maximum Q-factor Minimum BER 2.29e e e-60 Table3.6 Important parameters for the three channels using CROF 49

63 SIMULATION OF DISPERSION COMPENSATION 3 Figure3.25 Eye diagrams of CROF compensation at 2.5 Gbps with 8dBm LASER power a) Channel-1 b) Channel-4 c) Channel-8 Discussions: From table3.6 and eye diagrams presented in figure3.25 we ve got as usual good results at speed of 2.5Gbps and a Laser power of 8dBm, but if we compare these results with those obtained when using GF, the latter gives the best results (higher Q-factor, zero BER). 50

64 CONCLUSION 4 CHAPTER 4 Conclusion 51

65 CONCLUSION 4 There are number of techniques to compensate the chromatic dispersion of an optical signal travelling along the optical fiber. The dispersion compensation using digital filters is the most effective way of compensating it. In this report, we presented a comparative analysis of different digital filters and DCF for chromatic dispersion compensation as it is considered to be the most dominant mechanism of dispersion up to transmission rates of 40Gbps in single mode operation. We have made our study at two different (2.5Gbps and 5Gbps) transmission rates. We have built then simulate first a simple optical communication link then a more realistic WDM link, using the OptiSystem Software and Matlab as a co-simulator tool. The obtained results for dispersion compensation in terms of quality factors, bit error rates and eye diagrams show that first, the Gaussian Filter (GF) gives the best compensation compared to Cosine Roll Off Filter (CROF). Second and at low power level rates where nonlinear effects are neglected, the digital filtering technique is more efficient than the conventional DCF technique. Finally, our results are in a good agreement with those published in literature. The GFs have proved to be a cost-effective choice compared to CROF, both of them are easy to implement with less complexity than DCFs. The eye diagrams, Q-factors and BERs show that all the techniques that have been simulated can reduce significantly the Chromatic Dispersion (CD) resulting in an important increase of the overall performance of the system in particular the transmission bandwidth. We suggest as future work for the next coming students, to complete this study by testing other types of filters such as Super-Gaussian filters, Butterworth filters and microwave photonic filter at higher transmission rates. 52

66 Appendix A- MATLAB component: As we have said before the software OptiSystem has an opened space of simulation, among them MATLAB simulator, we have used MATLAB component as a digital comparator and we have written the following program: OutputPort1 = InputPort1;% we have only one input port and one output port if(inputport1.typesignal == 'Electrical')% both input and output are electrical R = length(inputport1.sampled.signal);% R is the length of input sampled signal Th=max(InputPort1.Sampled.Signal)/2;% Th is the threshold referred to the maximum if( R > 0 ) for i=1:r if abs(inputport1.sampled.signal(i))<=th OutputPort1.Sampled.Signal(i)=0; else OutputPort1.Sampled.Signal(i)=1; end end end end B- Eye diagram: As stated previously, the data eye diagram is a representation of a high speed digital signal that allows key parameters of the electrical quality of a signal to be quickly visualized and determined. The requirements for high speed data signals mentioned in the previous section are some of the key metrics that can be measured using eye diagrams. Eye Diagrams are used to characterize a high speed signal source or transmitter (receiver testing usually requires bit error rate testing) [21]. I

67 Figure 4 Eye Diagram interpretation All of the measurement results are the statistical average of the samples of the waveform at the point shown. The measurements are defined as follows: - One Level: The one level in an eye pattern is the mean value of a logic one. The actual computed value of the one level comes from the histogram mean value of all the data samples captured inside the middle 20% (40 to 60% points) of the eye period. - Zero Level: The zero level in an eye pattern is the mean value of a logic zero. The zero level is computed from the same 40 to 60% region of the baseline area during the eye period as the one level. - Eye Amplitude: Eye amplitude is the difference between the one and zero levels. The data receiver logic circuits will determines whether a received data bit is a 0 or 1, based on the eye amplitude. - Eye Height: Eye height is a measure of the vertical opening of an eye diagram. An ideal eye opening measurement would be equal to the eye amplitude measurement. For a real eye diagram measurement, noise on the eye will cause the eye to close. As a result, the eye height measurement determines the eye closure due to noise. The signal to noise ratio of the high speed data signal is also directly indicated by the amount of eye closure. II