An Overview of DWDM

Seminar Report on

DENSE WAVELENGTH DIVISION MULTIPLEXING

( DWDM ) A Report Submitted in Partial fulfillment of the Requirements For the

Degree of

M.Sc- ELECTRONICS SCIENCE

Of

GAUHATI UNIVERSITY

By

ARINDAM DUTTA CHOUDHURY (Roll No- 93)

Dept of Electronics Science Gauhati University Guwahati- 781014

CCEERRTTIIFFIICCAATTEE �� “Dense Wavelength Division Multiplexing” � �� Arindam Dutta Choudhury �� ! � �� " #�$ % �� & �� Date- / 0 9 / 0 6

G u w ah ati -1 4 Prof (Mrs.) P. Dutta

Head of the Department

Dept. of Electronics Sc. Gauhati University

External Examiner Internal Examiner

Acknowledgement - At the onset I would like to take this opportunity to express my sincere gratitude to Dr. (Mrs.) P. Dutta, Head of the Department, Electronics Science Department, and Guwahati University for providing me with the facility and guidance for preparing my seminar topic. I would also like to thank all my teachers for providing valuable input to my seminar report. I would like to thank all my classmates for giving me moral support. I hope all the respected person would forgive me for any mistake that is likely to be committed by me during the seminar session. �� Roll no.-93 Fourth Semester Department of Electronics Science Gauhati University Year- 2006

IINNDDEEXX Section T op ic P a g e 1.0 Introduction 1 1.1 DWDM-What is it ? 2 1.2 Types of Multiplexing 3 1.3 Optical Multiplexing Technology 4 1.4 Optical Network 7 1.5 Transmitters 8 1.6 Modulator 10 1.7 Network Routes & Regeneration 11 1.8 Optical Amplifiers 11 1.9 Erbium Doped Amplifiers 12 1.10 ITU-T Grid 13 1.11 Receivers 13 1.12 Conclusion 14 1.13 Bibliography 15

IINNTTRROODDUUCCTTIIOONN The Changing Scenario- The last two decades have seen a phenomenal increase in the transmission of electronic information—a trend that is growing exponentially. These days we live in a global village and transfer information just by clicking any computer key. The reason behind such a dramatic change is due to the renascence in the world of Telecommunication. We can define Telecommunication as the process of exchange of information over a distance using some type of equipment. People have been using Copper wire for long years. But the advent of Optical Fibers changed the overall scenario. Since then Fiber Optic technology is growing at a furious pace. Optical fiber along with associated technology can carry information up to 50 Tbits/s, whereas today’s commercial links have transmitted far fewer than 100 Gbits/s ! Two major technological advances- Wavelength Division Multiplexing (WDM) and Erbium Doped Fiber (EDFA) have boosted the capacity of optical fiber . The emergence of Dense Wavelength Division Multiplexing (DWDM) technology is one the most important phenomenon in the development of Fiber Optic Transmission technology. The DWDM technology is based on two cutting edge technologies mentioned above. In our following discussion we briefly define the DWDM. We then examine the functions & components of it and conclude with a high level description of operation of DWDM system. It is the purpose of this text to present a brief but comprehensive aspect of DWDM technology.

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Author: Arindam Dutta Choudhury

1.1 DWDM- WHAT IS IT ? DWDM is just an extension of WDM technology. Basically it is a form of FDM- Frequency Division Multiplexing. The WDM is a technology which multiplexes multiple optical signals on a single Optical fiber by using different wavelengths(colors) of LASER/LED to carry different information or data.

Fig(1) : A schematic diagram to describe DWDM system. From fig(1) we can get a overview of working of DWDM system. The fiber optic cable guides the light/data from one end to other end. The signal/data is injected by LED or semiconductor LASER. Lasers produce light in a range called “ Window”. These windows occupy Near Infra Red at wavelength 850 nm to 1630 nm. These regions, called windows, lie between areas of high absorption. The earliest systems were developed to operate around 850 nm, the first window in silica-based optical fiber. A second window (S band), at 1310 nm, soon proved to be superior because of its lower attenuation, followed by a third window (C band) at 1550 nm with an even lower optical loss. Today, a fourth window (L band) near 1625 nm is under development and early deployment. Dense WDM common spacing may be 200, 100, 50, or 25 GHz with channel count reaching up to 128 or more channels at distances of several thousand kilometers with amplification and regeneration along such a route. As mentioned before DWDM is just an extension of WDM technology in terms of wavelengths, number of channels, & ability to amplify the optical signal. From now onwards we will use the terms WDM & DWDM interchangeably for convenience of our discussion.

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1.2 Types of Multiplexing - Multiplexing is sending multiple signals or streams of information through a circuit at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. Basic types of multiplexing include frequency division (FDM), time division (TDM), and wavelength division (WDM), with TDM and WDM being widely utilized by telephone and data service providers over optical circuits. Time Division Multiplexing Time-division multiplexing (TDM), as represented in Figure 3, is a method of combining multiple independent data streams into a single data stream by merging the signals according to a defined sequence. Each independent data stream is reassembled at the receiving end based on the sequence and timing. Synchronous Optical Network (SONET), Asynchronous Transfer Mode (ATM) and Internet Protocol (IP) utilize TDM techniques. In modern telecommunications networks, TDM signals are converted from electrical to optical signals by the SONET network element, for transport over optical fiber.

Figure 2: Time Division Multiplexing Wavelength Division Multiplexing WDM combines multiple optical TDM data streams onto one fiber through the use of multiple wavelengths of light. Each individual TDM data stream is sent over an individual laser transmitting a unique wavelength of light.

Figure 3: Wavelength Division Multiplexing

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1.3 Optical Multiplexing Technology - Optical multiplexing technologies, such as DWDM and WDM systems, have revolutionized the use of optical fiber networks. Different colors of light, called wavelengths, are combined into one optical signal and sent over a fiber-optic cable to a far-end optical multiplexing system. Optical Multiplexing Filters Figure 5 illustrates that a filter is a physical device that combines each wavelength with other wavelengths. Many technologies are used in multiplexing, including: • Thin-film filters • Bragg gratings • Arrayed waveguide gratings (AWGs) • Interleavers, periodic filters, and frequency slicers)

Figure 4: WDM Filters Thin-Film Filter The thin-film filter (TFF) is a device used in some optical networks to multiplex and demultiplex optical signals. The TFFs are devices that use many ultrathin layers of dielectric material coating deposited on a glass or polymer substrate. This substrate can be made to let only photons of a specific wavelength pass through, while all others are reflected. By integrating several of these components, you can then demultiplex several wavelengths. Figure 6 shows what happens with four wavelengths Release.

Figure 5: Thin-Film Filter Concept

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The first TFF section passes wavelength 1 and reflects 2, 3 and 4 to the second, which then passes 2 and reflects 3 and 4. This allows for demultiplexing or multiplexing of optical signals. Fiber Bragg Gratings A Bragg Grating is made of a small section of fiber that has been modified by exposure to ultraviolet radiation to create periodic changes in the refractive index of the fiber. The result, shown in Figure 7, is that light traveling through the Bragg Grating is refracted and then reflected back slightly, usually occurring at one particular wavelength.

Figure: 6 & 7: Fiber Bragg Grating R.I Time Delay Grating Length Wavelength Figure 8:(a)Grating Reflection Profile, (b)Time Delay Vs Wavelength Arrayed Waveguides In the transmit direction, the AWG mixes individual wavelengths, also called lambdas (λ) from different lines etched into the AWG substrate (the base material that supports the waveguides) into one etched line called the output waveguide, thereby acting as a multiplexer. In the opposite direction, the AWG can demultiplex the composite λs onto individual etched lines. Usually one AWG is for transmit and a second one is for receive. AWG provides multiplexing and demultiplexing of wavelength channels with spacing as low as 0.4 nm (50 GHz). Figure 8 illustrates the demultiplexing action or receive.

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Arrayed Waveguides Output Slab Waveguide

Composite Signal Individual Channel Signal Figure 9: Arrayed Waveguide (Demultiplexer) Demultiplexers With signals as precise and as dense as those used in DWDM, there needed to be a way to provide accurate signal separation, or filtration, on the optical receiver. Such a solution also needed to be easy to implement and essentially maintenance free. Early filtering technology was either too imprecise for DWDM, too sensitive to temperature variations and polarization, too vulnerable to crosstalk from neighboring channels, or too costly. This restricted the evolution of DWDM. To meet the requirements for higher performance, a more robust filtering technology was developed that makes DWDM possible on a cost effective basis- the Arrayed Waveguide Grating whose operation have been discussed already. The AWG can replace multiple Bragg Gratings, each Bragg Grating only supports one wavelength and occupies the same physical space as an 8-λ AWG. Multiple Bragg Gratings also cost more than a single AWG. For some applications, AWG offers a higher channel capacity at a lower cost per channel with a smaller footprint. This results in fewer components and provides for component integration (e.g., switching, variable optical attenuator). In The next chapter we first describe the components of DWDM network. Then we move to the detail analysis of each component’s role in the network.

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1.4 Optical Network - Figure 10 shows an optical network using DWDM techniques that consists of five main components:

1. Transmitter (transmit transponder) : • Changes electrical bits to optical pulses • Is frequency specific • Uses a narrowband laser to generate the optical pulse

2. Multiplexer / Demultiplexer : • Combines/separates discrete wavelengths • Works as a Add/Drop component

3. Amplifier : • Pre-amplifier boosts signal pulses at the receive side • Post-amplifier boosts signal pulses at the transmit side (post amplifier) • and on the receive side (preamplifier) • In line amplifiers (ILA) are placed at different distances from the source • to provide recovery of the signal before it is degraded by loss.

4. Optical fiber (EDFA as media) : • Transmission media to carry optical pulses • Many different kinds of fiber are used • Often deployed in sheaths of 144–256 fibers

5. Couplers/ Splitters • Combines the pump and signal wavelengths into the fiber

6. Isolators • Isolator prevents the amplifier fiber from back reflected light

7. Optical Filters • Flattens the fiber gain and removes the pump wavelength from the

transmission fiber 8. Receiver (receive transponder) :

• Changes optical pulses back to electrical bits • Uses wideband laser to provide the optical pulse.

Figure 10: DWDM Network

Fig 10 gives a detail picture of DWDM system. In the next chapter we will discuss about the transmitter Section in detail . 7


1.5 Transmitters - From so far we have discussed about the DWDM Systems it is clear that we need Individual transmitters that generates specific wavelengths and these wavelengths are combined for transmission over a Optical fiber by a multiplexer . This is how today’s DWDM system works . Transmitters use lasers as the signal source. It Is desirable To have a single laser module tunable over the range of wavelengths Instead of having an array of lasers . Before going deep into this matter Let’s analyze the requirements of transmitter system .

Transmitter requirements in DWDM Networks : Quality Of generated Light –

• The Linewidth has to be as narrow as a possible. Indeed, to attain 40 channel Multiplexing we need a Channel spacing of hundred GHz. Thus, The linewidth Cannot be more than one GHz To avoid Channel crosstalk .

• The sidemode Suppression ratio (SMR) Has to be as high as Possible. All these Side Modes and have to be suppressed to avoid any Channel crosstalk .

• A laser has to operate in a single Longitudinal mode. • Chirp Has to be eliminated . This is unacceptable in DWDM System . • Direct modulation is unsuitable In DWDM System .

Stability – • Variation in output power result in variation in linewidth— a detrimental

phenomenon. In addition, Power instability Causes several hidden problems associated with such nonlinear effects in single mode optical fiber as Four- wave Mixing(FWM) And Stimulated scattering .

• Variation in peak wavelength are unacceptable because we need to keep the channel spacing staple, Otherwise we will mix Channel and lose information . More precisely the instability of a Peak wavelength causes variation in the channel crosstalk level .

• The Relative Intensity nice (RIN) Has to be minimized because this is another form of output- power instability. Since one of the major reasons for RIN is Beck- reflection, a laser has to be suitably protected .

Reliability • Laser source must be reliable .

Power consumption • We want to minimize power consumption to reduce the heat radiated by a Laser

diode . But This requirement becomes critical for DWDM Transmitters, Where many laser diodes operates under very congested conditions .

Tunability Having 128 - channel s Systems commercially available - It becomes necessary to have a Laser which has high tuning Capacity . this is the most crucial issue in today’s DWDM Network . The Transmitters use lasers as the signal source

• The key features of tunable lasers are The tuning speeds and the ability to emit several wavelengths simultaneously. We have to bear in mind that light sources are necessary for DWDM Networks not only as Transmitters but also as elements of Add/ Drop Components .

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There are various types of Lasers that have been developed optical network transmission. We have- 1. Fabry-Parot laser diode, 2.Distributed- Feedback (DFB) Laser diode, 3. Vertical

cavity-surface-emitting Lasers (VCSEL), 4. Sectional distributed – Bragg Reflection tunable Lasers, 5. Integrated cavity Lasers as an option Which can be applied in DWDM system .

In our discussion we will analyze the distributed feedback laser- DFB as it is widely used today.

Figure 11: Tunable Laser Figure 11 shows a schematic diagram of a DFB laser. To reduce the spectral width, we need to make a diode that merely radiate only one longitudinal mode. This has been done with distributed feedback laser( DFB) diodes. Such lasers have tuning speeds in the orders of tens of nanoseconds which is needed in DWDM communication. The principle of their operation is as follows- The injection current changes the carrier densities in the active region of Laser and, in so doing, alters the Refractive Index of the active medium. Variation in RI is equivalent to altering the optical path of a laser cavity. Thus, a change in the driving current leads to variation in the radiating wavelength because it changes the resonant condition of the laser diode. This is the basic mechanism leading to the tenability of the Semiconductor laser diode.

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1.6 Modulator - The modulator changes the laser signal by either pulsing it off and on or by changing the phase of the signal so that it carries information. DWDM systems typically use phase modulation. Each variation represents a 1 or a 0.

Figure 12: Laser Signal Sources Amplifiers and Regeneration Amplifiers are defined as type 1R, 2R, or 3R. • 1R—Reamplify • 2R—Reamplify and reshape • 3R—Reamplify, reshape, and retime Figure 13 illustrates the effect on a degraded optical signal once it has been 1R, 2R, or 3R regenerated.

Figure 13: Regeneration

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1.7 Network Routers and Regeneration- Figure 14 shows that optical networks can have 1R, 2R, and 3R devices. The 1R device only amplifies the signal received. A 2R device provides amplification and reshaping of the waveform to provide some data recovery. The 3R device provides amplification and reshaping and requires a time source so that it can provide retiming for the transponder. Asynchronous input transponders do not depend on timing and cannot be retimed. Such transponders commonly support non-SONET rates and have a SONET output that is internally clocked by the transponder. By observation you see that 3R devices include 1R and 2R as well as 3R.

Figure 14: Network Regeneration DISADVANTAGES- Repeaters are Acceptable signal boosters for point to point link , such as TAT-8, If where bit rate and signal format are determined by use of a single transmitter. However, repeaters don’t work for Fiber-optic networks, where many transmitters send signals to many Receivers at different bit-rate and in different formats. Thus the need for Optical Amplifier arises.

1.8 OPTICAL AMPLIFIERS - Optical amplifiers simply strengthens the optical signals. Optical amplifiers works without having to convert optical signal into electrical from and back . this features leads to two great advantages . First, optical amplifiers support any bit rate and signal format because again they simply amplify the received signal. Thus optical amplifiers are Transparent to any Bit-rate and signal format. Secondly, they support not just a single wavelength, as repeaters do, but the entire range of wavelengths. For example EDFA amplifies all wavelengths from 1530nm to 1660nm. The shift from TDM to DWDM links and from point to point links to a complicated network couldn’t have been done without Optical amplifiers. 11


Two major classes of optical amplifiers are in use today: Semiconductor Optical amplifiers, and Fiber Optical amplifiers. We will discuss only the Fiber Optical amplifiers in this text. Fiber amplifiers, specifically EDFA, are the workhorses in today’s DWDM networks. Optical amplifiers are categorized in terms of the function they perform. These are- Boosters, in-line amplifiers, pre-amplifiers. It is shown in Fig-15 (a), (b), (c) below.

1.9 Erbium-Doped Fiber Amplifiers - Erbium-doped fiber amplifiers (EDFAs) provide the gain mechanism for DWDM amplification, depicted in Figure 15. DWDM systems use erbium amplifiers because they work well and are very efficient as amplifiers in the 1500 nm range. Only a few parts per billion of erbium are needed. Light is pumped in at around 1400 nm (pump laser diode) to excite the erbium ions, and then the incoming 1500-nm light signal from the source system is amplified.

Figure 16: EDFA Figure 16 shows an erbium-doped fiber amplifier (EDFA) and is the last active component in the DWDM system. An EDFA is the heart of an erbium-doped amplifier but it’s not the only

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component. To make an EDFA work and to improve the gain and noise characteristics of an active fiber, other components are needed. The other components are- Pump diode laser, Coupler, Isolator, optical filter. The general configuration of EDFA system is given in the fig-16. The operation of components were discussed in page 7. Fiber Bands Three optical frequency bands are used today for fiber-optic DWDM networks. The bands are: • C-band (conventional) has a range from 1530 nm to 1570 nm. • L-band (long wavelength) has a range from 1570 to 1625 nm. • S-band (short wavelength) has a range from 1450 to 1500 nm.

1.10 ITU-T Grid - One of the most important aspect is to ensure the compatibility of equipments from different Equipment-manufacturers with the service from different service providers. The International Telecommunications Union-Telecommunications Standardization Sector (ITU-T) established a set of standards for telecommunications that drives all optical DWDM systems today. Systems are based on an absolute reference to 193.10 THz that corresponds to a wavelength of 1552.52 nm with individual wavelengths spaced in steps of 50 GHz or a wavelength step of 0.41 nm from the reference. All land-based DWDM systems follow this standard. This Frequency-Grid starts from 196100GHz (1528.77 nm) to 192100GHz (1560.61 nm). Note that channels can be spaced not only equally but also unequally because Four-Wave Mixing (FWM) is severely detrimental to equally spaced channels .

1.11 RECEIVERS - The basic requirement for WDM receivers is the ability to operate within entire window of wavelength range used in today’s WDM networks. The P-I-N photodiodes and APD photodiodes are said to have very high spectral characteristics. The electronics associated with them are also available with appropriate Bandwidth. But this statement is not fully true. The problem starts with a network where we have to obtain multiple access routes to WDM networks. In such a case, we need a receiver capable of choosing an individual channel from the many transmitted over the network. There are two approaches to selecting a desired channel (wavelength) from a networks main steam : Tunable Transmitter- Fixed Receiver (TTFR) and Fixed transmitter-tunable Receiver (FTTR) In the second approach, the receiver is tuned to pick up d4esired wavelength while each transmitter emits a fixed wavelength. (Fig-17). This approach is used primarily in DWDM.

Fig- 17: DWDM network

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Tx1

Tx2

Rx1

Rx2

DWDM Network


1.12 CONCLUSION - Network Management ~ A critical yet often under appreciated part of any telecommunications network is the management system— whose reliability is especially vital in the complex and high capacity world of DWDM. Indeed, dependable and easily accessible network management services increasingly will become a distinguishing characteristic of high performance, high-capacity systems. Today’s leading DWDM systems include integrated, network management programs that are designed to work in conjunction with other operations support systems (OSSs) and are compliant with the standards the International Telecommunication Union (ITU) has established. By meeting ITU standards and utilizing a Q3 interface, the system ensures that end users retain high Operations, Administration, Maintenance, and Provisioning (OAM&P) service.

Applications for DWDM ~ • DWDM is ready made for long-distance telecommunications operators that use either point–to–point or ring topologies. The sudden availability of 16 new transmission channels where there used to be one dramatically improves an operator’s ability to expand capacity and simultaneously set aside backup bandwidth without installing new fiber. • This large amount of capacity is critical to the development of self-healing rings, which characterize today’s most sophisticated telecom networks. By deploying DWDM terminals, an operator can construct a 100% protected, 40 Gb/s ring, with 16 separate communication signals using only two fibers. • Operators that are building or expanding their networks will also find DWDM to be an economical way to incrementally increase capacity, rapidly provision new equipment for needed expansion, and future–proof their infrastructure against unforeseen bandwidth demands. • Network wholesalers can take advantage of DWDM to lease capacity, rather than entire fibers, either to existing operators or to new market entrants. DWDM will be especially attractive to companies that have low fiber count cables that were installed primarily for internal operations but that could now be used to generate telecommunications revenue.

The Future of DWDM ~ In the space of two years, DWDM has become recognized as an industry standard that will find acceptance in any carrier environment. Deployment of DWDM will allow new services to come on-line more quickly, help contain costs so that prospective customers can more easily afford new services, and readily overcome technological barriers associated with more traditional solutions. Its acceptance will drive the expansion of the optical layer throughout the telecommunications network and allow service operators to exploit the enormous bandwidth capacity that is inherent in optical fiber but that has gone largely untapped—until now.

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1.13 BIBLIOGRAPHY -

• Fiber-Optic Communications Technology -- Maynbaev and Scheiner • Fiber Optic Communications

-- Harold Kolimbiris

• DWDM - ATG’s Communications & Networking Technology Guide Series

• DWDM Tutorial- FUJITSU Corporation’s study material

• Fiber Optic Association – Home Page.

• Linkitionary.com

• Wikipedia

• www.Google.com

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arindam

Sticky Note

the author can be reached at, [email protected]

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An Overview of DWDM