Helsinki University of TechnologyDepartment of Electrical and Communications Engineering

S-38.128 Telecommunications Technology, Special AssignmentLiisa Peltonen, 37881S22 November, 1998

Wavelength division multiplexing;an overview

1 Acronyms

3R Regeneration with retiming and reshaping

AOTF Acousto-optic tunable filter

APD Avalanche photodiode

AWG Arrayed waveguide grating

BER Bit error rate

CPM Cross-phase modulation

DCF Dispersion-compensating fiber

DSF Dispersion-shifted fiber

DFB Distributed feedback

EDFA Erbium-doped fiber amplifier

FWM Four-wave mixing

ISI Intersymbol interference

ITU International Telecommunication Union

LASER Light amplification by stimulated emission of radiation

LED Light-emitting diode

MAN Metropolitan area network

MZI Mach-Zehnder interferometer

MLM Multilongitudinal mode

NDF Nonzero dispersion fiber

NNI Network node interface

OADM Optical add-drop multiplexer

OOK On-off keying

OSNR Optical signal-to-noise ratio

OTDM Optical time division multiplexing

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OXC Optical cross-connect

PDFA Praseodymium-doped fiber amplifier

PDH Plesiochronous digital hierarchy

PDL Polarization-dependent loss

PMD Polarization-mode dispersion

POTS Plain old telephone service

QoS Quality of service

SBS Stimulated Brillouin scattering

SDH Synchronous digital hierarchy

SOA Semiconductor optical amplifier

SONET Synchronous optical network

SLM Single-longitudinal mode

SMF Single-mode fiber

SPM Self-phase modulation

SRS Stimulated Raman scattering

TFF Thin-film filter

TFMF Thin-film multicavity filter

TDM Time division multiplexing

WDM Wavelength division multiplexing

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2 List of Figures

Figure 1: The structure of an optical fiber..................................................................................6Figure 2: Services offered by a second-generation optical network.........................................13Figure 3: Optical layer networks...............................................................................................13Figure 4: Evolving broadband network layers..........................................................................15Figure 5: A unidirectional WDM link........................................................................................16Figure 6: Relationships between the ITU-T optical networking recommendations..................22Figure 7: A WDM broadcast and select network.......................................................................24Figure 8: A WDM wavelength routing network.........................................................................25Figure 9: A simple optical filter.................................................................................................26Figure 10: A wavelength multiplexer.........................................................................................26Figure 11: A wavelength add/drop multiplexer.........................................................................26Figure 12: A wavelength router.................................................................................................26Figure 13: A splitter...................................................................................................................27Figure 14: A combiner...............................................................................................................27Figure 15: A coupler..................................................................................................................27Figure 16: A Fabry-Perot filter.................................................................................................29Figure 17: A multilayer dielectric thin-film filter wavelength (de)multiplexer.........................30Figure 18: A Mach-Zehnder interferometer..............................................................................30Figure 19: An arrayed waveguide grating (AWG)....................................................................31Figure 20: A static routing pattern of a 4x4 arrayed waveguide grating..................................32Figure 21: Circulators: (a) three-port (b) four-port................................................................33Figure 22 : Optical fiber transmission with electrical regenerators.........................................34Figure 23: Block diagram of an Erbium-doped fiber amplifier (EDFA)...................................35Figure 24: A possible evolution scenario for optical network architecture..............................37

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3 Introduction

Optical fiber transmission has played a key role in increasing the capacity of telecommunication networks. The large, low-loss transmission capacity of fiber has made optical transmission to become the preferred means of high bit rate data transmission over long distances. However, as the demand for network capacity is rapidly increasing, even the current optical backbones are fast proving inadequate.

In many cases, it is relatively expensive to lay new fiber in order to increase network capacity. Wavelength division multiplexing (WDM) is a transmission technology which allows the capacity of existing optical links to be increased without installing new fiber, thereby enabling significant cost savings. In WDM, multiple signals at different carrier wavelengths are transmitted simultaneously over a single fiber. Today, point-to-point WDM links are already widely deployed in the US long distance networks, and the deployment has also started in Europe and Asia. The purpose of this paper is to give an overview of optical networks, wavelength division multiplexing and its key enabling technologies, and to briefly describe the fast-changing WDM deployment and standardization situation.

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4 Optical networks

Optical networks use optical fiber as a transmission medium. This chapter serves as an introduction to optical networks, covering the basics of optical fiber transmission, the evolution of optical networks, and the position of the optical layer in the communication network infrastructure.

4.1 Light propagation in an optical fiber

Basic knowledge of light transmission in an optical fiber is key to understanding both the significant advantages of using fiber as a propagation medium and the system limitations which need to be considered when designing optical communication systems. The basics of light propagation, pulse broadening effects caused by dispersion and nonlinear effects constraining the design of higher bit rate systems, are described in this section.

To begin with, light in a strict sense means the region of the electromagnetic spectrum that can be perceived by human vision. This visible spectrum contains approximately the wavelength 1

range of 0.4 m to 0.7 m. However, in the laser2 and optical communications fields, custom and practice have extended the usage of the term light to include a much broader portion of the electromagnetic spectrum, extending from the near-ultraviolet region of approximately 0.3 m through the visible region and into the mid-infrared region of approximately 30 m. [FED]

The medium used to guide the light signals in optical networks, the optical fiber, consists of a cylindrical core, which is surrounded by a cladding, as shown in Figure 1. Both the core and the cladding are primarily made of silica (SiO2). The refractive index3 of the core is made slightly higher than that of cladding by introducing certain impurities, or dopants, into the core and/or the cladding.

Cladding

Core

Figure 1: The structure of an optical fiber

A simplified understanding of the propagation of light in the fiber can be described with the help of ray theory. From the ray theory viewpoint, light propagates in the fiber due to a series of total internal reflections that occur in the core-cladding interface. However, ray theory is an

1 The wavelength () is related to the propagation velocity () and frequency () by = /.2 LASER = Light Amplification by Stimulated Emission of Radiation3 The refractive index of a material is the ratio of the speed of light in vacuum to the speed of light in that material.

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approximation that holds only when the signal wavelength is much smaller than the radius of fiber core.

A more general theory, applicable for all values of fiber radius, is the wave theory, which treats light as an electromagnetic wave, the propagation of which is governed by Maxwell’s equations. In wave theory, the propagation of light in any medium can be described by specifying the evolution of the associated electric and magnetic field vectors, denoted by E(r,t) and H(r,t), respectively, in space and time. [RS98] A more detailed look into the theory of electromagnetic waves is given e.g. by Cheng [Che89].

The electromagnetic waves travel partly in the core and partly in the cladding. The electric and magnetic field vectors in the core and the cladding must satisfy wave equations, which are second-order, linear partial differential equations:

2E + 2E = 02H + 2H = 0

Here is the angular frequency (rad/s) related to the light frequency f by = 2f, and are the magnetic and dielectric constants of the medium, respectively, and 2 is the Laplacian operator .

The solutions in the cladding and core, however, are not independent but are related by boundary conditions in the core-cladding interface. A fiber mode is every pair of solutions that satisfies these boundary conditions of the wave equations. Multi-mode fibers with core diameters of about 50 to 85 m can support more than one mode, and single-mode fibers, in which the radius of the core is of the order of the operating wavelength, can support only one mode. Single-mode fibers can carry more information than multi-mode fibers and are therefore the preferred guiding medium in high-bit rate optical communications over long distances. A more quantitative description of single- and multi-mode fibers is given e.g. in [KBW96].

The physical explanation of light propagation in a single-mode fiber follows from the difference of the refractive indices in the core and cladding. In any medium with a constant refractive index, a narrow light beam tends to spread due to a phenomenon called diffraction. The spreading can be counteracted by using an inhomogeneous medium in which the refractive index near the beam center (fiber core) is larger than at the beam periphery (fiber cladding), so that the beam center travels slightly slower than the beam periphery. This effectively provides continuous focusing of the light to counteract the spreading effect, and allows light to be guided in the medium and travel long distances with low loss. [RS98] A more quantitative description of light propagation in single-mode fibers using the wave theory approach is given e.g. in [KBW96].

4.2 Capacity limits of optical transmission

Optical fiber offers low-loss transmission capacity over an enormous frequency range of about 25 THz. [Bor97] Compared to the bandwidth available in other transmission media such as copper cable or free space, this is orders of magnitude more. Also the attenuation of silica is very low in wavelength regions dedicated to optical communications. These properties allow the transmission of signals over long distances at high bit rates before they have to be amplified or regenerated. Here lies the reason for the fact that optical communication systems are so widely deployed today.

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Like mentioned above, the intrinsic attenuation of silica is very low. With today’s technology, it is possible to fabricate optical fibers in which the attenuation of the signal traveling in the fiber is close to the theoretical limits due to scattering and absorption of light by silica molecules, less than 0.5 dB/km [Hew97]. The two low-loss regions are around the 1.3 m and 1.55 m wavelengths, the 1.55 m region having the lowest attenuation. Both low-loss regions, or optical windows, are used for communications. In some short-distance applications, such as computer interconnects, other wavelengths can be used as well [KBW96], [RS98]. Signal attenuation in optical fiber is therefore not considered a major limiting factor of optical transmission. Instead, two major effects which set limits on the feasible bit rates and transmission distances of today’s optical communication systems are dispersion and fiber nonlinearities, which are described in the following.

4.2.1 Dispersion

When a light pulse travels in an optical fiber, its different components (different modes and/or different frequencies) propagate at slightly different velocities. This distortion in general is called dispersion. As a result of dispersion, the pulse becomes broadened, and the signals in adjacent bit periods may overlap, a phenomenon called intersymbol interference (ISI).

Chromatic dispersion

In a single-mode fiber, the dominant dispersion mechanism is chromatic dispersion, caused by different light frequencies traveling with different velocities. The wider the spectrum of the transmitted pulse, the greater the effect of chromatic dispersion. The early light transmitters such as light-emitting diodes (LEDs) or multilongitudinal mode4 (MLM) Fabry-Perot lasers emitted light over a fairly large spectrum of several nanometers (hundreds of GHz). Today, chromatic dispersion is significantly reduced with the use of narrow spectral-width single-longitudinal mode (SLM) distributed-feedback (DFB)5 lasers.

Chromatic dispersion is a characteristic of a fiber; different fibers have different chromatic dispersion profiles. It turns out that a silica-based, standard single-mode fiber (SMF) has essentially no chromatic dispersion in the 1.3 m optical window, but has significant dispersion in the 1.55 m window, which on the other hand has the lowest attenuation. However, fiber dispersion is a linear phenomenon and can therefore be compensated for by means of the transmission medium, and dispersion-shifted fiber (DSF), which has the zero-dispersion wavelength shifted to the 1.55 m window, has been developed for this purpose. DSF is suitable for single-channel systems operating at high bit rates (10 Gb/s and above) over long distances. However, DSF is not well suited to WDM systems, mainly due to the detrimental effects of four-wave mixing and other fiber nonlinearities.

The accumulated chromatic dispersion penalty increases with the link length. When the distances and bit rates increase, chromatic dispersion can be compensated for e.g. by using nonzero dispersion fiber (NDF), which has a small amount of dispersion in the 1.55 m

4 For a given laser, all the wavelengths that satisfy the condition that the length of the laser’s oscillation cavity must be an integral multiple of half the wavelength, are called the longitudinal modes of that laser. A multilongitudinal laser oscillates simultaneously in several longitudinal modes. [RS98]5 In a DFB laser, the light is fed back to the oscillation cavity in a distributed manner by a series of closely spaced reflectors. [RS98]

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window, thereby reducing the penalties due to nonlinearities but retaining most of the advantages of DSF. Also special dispersion-compensating fibers (DCFs) that provide negative dispersion in the 1.55 m range to enable a zero net dispersion are commercially available. However, a drawback of using DCFs is the additional loss they introduce to the system. [RS98]

Modal dispersion

In a multi-mode fiber, the energy of a pulse travels in different modes, each with a different velocity. The resulting dispersion mechanism is called modal dispersion. This was a problem especially in early telecommunication systems, which used multimode fibers along with light-emitting diodes (LEDs) or multilongitudinal-mode (MLM) Fabry-Perot lasers as transmitters.

Polarization-mode dispersion

Finally, polarization-mode dispersion (PMD) arises because the fiber core is not perfectly circular. This causes different polarizations6 of a signal to travel at different group velocities. PMD becomes an impediment in high-bit-rate systems operating at 10 Gb/s and above. [RS98]

4.2.2 Fiber nonlinearities

As long as the optical power of a signal traveling in an optical fiber is relatively small, the fiber can be considered a linear medium. However, when the signal levels get higher, fiber nonlinearities start imposing limitations on link length and/or bit rate of the system. The nonlinearities arise because the loss and refractive index of the fiber have a component dependent on optical power. In many cases, chromatic dispersion plays a key role in reducing the effects of nonlinearities: when a little chromatic dispersion is present in the fiber, the different interacting waves then travel with different group velocities. Nonzero dispersion fiber (NDF) is being installed to new WDM systems for this purpose.

The nonlinearities can be classified into two categories. The first is due to the scattering effects owing to the interaction of light waves with molecular vibrations in silica medium. Examples of this category are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS).

Effects in the second category arise because the refractive index of the fiber has an intensity-dependent component. Effects in this category include four-wave mixing (FWM), self-phase modulation (SPM) and cross-phase modulation (CPM).

Stimulated Brillouin scattering (SBS)

Stimulated Brillouin scattering (SBS) depletes the transmitted signal by producing gain in the direction opposite to signal propagation, that is, back toward the source. This often calls for the shielding of the transmitter with an isolator. SBS does not cause interaction between different

6 The state of polarization of light propagating in a single-mode fiber refers to the orientation of its electric field vector on a plane that is perpendicular to the direction of propagation.

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wavelengths as long as the wavelength spacing is greater than 20 MHz, but can cause significant distortion within a single channel.

Stimulated Raman scattering (SRS)

Stimulated Raman scattering (SRS) causes power to be transmitted from lower-wavelength channels to higher-wavelength channels, the gain coefficient being a function of wavelength spacing. Coupling occurs between two channels only if power is present both channels, that is, is both channels are sending 1 bits. The direction of coupling is both in the direction of propagation and the reverse direction. The penalties due to SRS are reduced when dispersion is present, because the signals in different channels then propagate at different velocities and the probability of pulses at different wavelengths overlapping at any point in the fiber is reduced. Considering that the channel spacing is fixed, the impairments due to SRS grow with the number of wavelength channels and the resulting total system bandwidth.

Four-wave mixing (FWM)

Four-wave mixing (FWM) induces signals at new frequencies that appear as crosstalk to the existing signals. The FWM effect is independent of the bit rate but is highly dependent on frequency channel spacing and is reduced when dispersion is present.

Self-phase modulation (SPM) and cross-phase modulation (CPM)

SPM and CPM arise when fluctuations in the optical power of a signal cause changes in signal phase. Thus, different parts of a pulse undergo different phase shifts, which causes pulse chirping7. This causes spectral broadening, which in turn increases dispersion penalties. The impairments due to SPM are significant mainly in high bit rate (over 10 Gb/s) systems. CPM becomes a problem if the wavelength channel spacing is tight (a few tens of GHz). [RS98]

4.3 Evolution of optical networks

The starting point of optical fiber communications technology can be dated back to the 1960s when the increasing voice traffic started to exhaust the wire pair circuits between the central offices of the telephone network, and a new, higher-capacity transmission medium was needed. Early experiments demonstrated the capability of waveguides to transport information encoded in light signals but it was not until the invention of the low-loss silica-based fiber in the 1970s that optical transmission really took off.

The early fibers were multi-mode fibers and were used along with LEDs or multilongitudinal mode (MLM) Fabry-Perot laser transmitters operating at 0.8 m and 1.3 m wavelength bands. The resulting system was thereby heavily degraded by modal dispersion and had to have electronic signal regenerators every few kilometers. The primary focus was on multiplexing digital voice circuits and the infrastructure was based on plesiochronous digital hierarchy (PDH).

7 The frequency of a chirped pulse varies with time.

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In the next generation of optical fiber communications systems deployed in the early 1980s, standard single-mode fiber (SMF) was used to eliminate modal dispersion. This enabled a substantial increase in the bit rates and distances between regenerators. MLM Fabry-Perot lasers in the 1.3 m band were used as transmitters. Modal dispersion was effectively eliminated and the distances between regenerators were primarily determined by fiber attenuation. Typically, the spacings between regenerators were about 40 km and the systems operated at bit rates of a few hundred Mb/s.

To attain longer spans between regenerators, the lower loss of the 1.55 m wavelength band motivated the deployment of systems operating in this optical window in the late 1980s. At this point, chromatic dispersion started to become a problem. To overcome these limitations, dispersion-shifted fiber (DSF) was developed. However, there already existed a large installed base of standard single-mode fiber for which it was not possible to apply this solution. Another solution to the problem was found by narrowing the spectrum of the transmitted pulse. Narrow spectral-width single-longitudinal mode (SLM) distributed-feedback lasers were deployed as transmitters, and it was possible to put 1-2 Gb/s transmission systems into use. In parallel, synchronous digital hierarchy (SDH) was standardized by ITU-T in 1988.

The development of optical amplifiers, specifically the Erbium-doped fiber amplifier (EDFA), in the late 1980s and the early 1990s marked the next major milestone in optical network evolution. It became possible to replace the electrical regenerators with EDFAs, which enabled significant cost savings. EDFAs also brought along another major benefits: since they operate in the optical domain they are transparent to bit rates and protocol formats. Thus, it became possible to upgrade the transmission system in bit rates by changing only the terminal equipment at each end of the link. Furthermore, EDFAs are able to amplify several wavelengths simultaneously, which enabled the transmission of several signals at different carrier wavelengths on a single fiber, but requiring only one amplifier.

A variety of optical networks emerged in the late 1980s and early 1990s. Examples of these networks include SONET (Synchronous optical network), which has largely replaced PDH in North America, forming the core of telecommunications infrastructure, and SDH (Synchronous digital hierarchy), SONET’s counterpart in Europe and Asia. The majority of today’s optical transport networks operate in the 1.55 m optical window, at bit rates of 2.5 Gb/s (SONET OC-48/SDH STM-16 signals) or lower.

The optical networks described above can be characterized as first-generation optical networks. Typical of these networks is that they use fiber solely as a transmission medium and do all processing of signals electronically. Therefore, the optical signals need to be converted back to their “native” electronic form for switching and routing. The electronic switching and routing functions offer a high degree of flexibility and network restorability, but are unable to match the high transmission capacity of optical fiber. In addition to being costly, the optical-to-electrical-to-optical conversions also introduce additional delay in the network. Therefore, as the first-generation optical networks were deployed, thoughts about using optics for more than just pure transmission started emerging. The idea of second-generation optical networks is that they attempt to perform more network functions in the optical domain than just the point-to-point transmission. These functions include routing and switching wavelengths, and eventually routing and switching optical packets. In all-optical networks, signals do not go through any optical-to-electrical-to-optical conversions, but all network functions are performed optically. WDM networks employing wavelength routing are actively being developed in research laboratories today and are soon expected to emerge as commercial products. Packet switching in the optical domain has not quite yet matured to the same level, limited by current state of optical switching technology and the lack of optical buffers. [Bor97], [RS98], [KBW96]

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4.4 The optical layer

To understand the role of second-generation optical networks in the layered network hierarchy it is useful to think of them as constituting an optical layer, a functionality which offers services to the higher network layers such as SONET/SDH, IP (Internet protocol) or ATM (Asynchronous transfer mode). In the layered network hierarchy, these electronic layers are the client layers of the optical layer, which acts as the server.

4.4.1 Services

Theoretically, the optical layer may offer the same three types of services to higher network layers, as do the current electronically controlled networks. These services, shown in Figure 2, [RS98] can be offered as point-to-point or point-to-multipoint services.

The first service type is called a lightpath, which in a WDM network is an end-to-end connection between two nodes, set up by assigning a dedicated wavelength to the lightpath on each link in its path. Here, the whole capacity of the link is provided to the higher layer. Depending on the network capability, the lightpaths could be set up or taken down upon request of the higher layer, or the lightpaths offered could be permanent and set up when the network is deployed. The lightpath can be regarded as a circuit-switched service, like the plain old telephone service (POTS) provided by today’s telephone network.

The second service is the so-called virtual circuit, which is a circuit-switched connection between two nodes (as in ATM). Here, the capacity offered can be smaller than the full capacity available on a link or wavelength. Thus, some form of time division multiplexing must be incorporated in the network to combine multiple virtual circuits onto a wavelength in a WDM link (or onto the transmission bit rate in an OTDM (optical time division multiplexing) link).

The third service type a second-generation optical network can offer is a datagram service, which enables the transmission of short packets of information between the network nodes, without the overhead of setting up explicit connections (as in IP).

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Optical network

Lightpath

Virtual circuit/ datagram

Figure 2: Services offered by a second-generation optical network

At the moment, the limited technology in optical logic, buffering and gating makes the circuit-switched lightpath service the most practical choice of service. The lightpath is just the service enabled by current WDM systems. The other services are, however, nearing practicality. [RS98]

4.4.2 Sublayers

The optical layer can be further divided into sublayers. The draft version of ITU-T Recommendation G.872 (Architecture of Optical Transport Networks) [G872] defines a layered view of the optical layer itself. The definition is particularly well suited to describe WDM transport networks. Once the interfaces between the layers are defined, the vendors are able to provide standardized WDM technology, ranging from individual network elements through WDM links to whole WDM networks. The optical layer, its three sublayers (OCH, OMS and OTS) and the digital client layer are shown in Figure 3. [MB98]

Digital client layer (SONET/SDH, PDH, etc.)

Optical channel (OCH) layer

Optical multiplex section (OMS) layer

Optical transmission section (OTS) layer

Optical layer

Optical interfaces

Figure 3: Optical layer networks

The Optical channel (OCH) layer, as defined in the G.872, handles the end-to-end networking of lightpaths, or optical channels, and transparently conveys digital client information of

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varying formats (such as SDH, SONET, ATM or IP). The following capabilities are included in OCH:

optical channel connection rearrangement for flexible network routing optical channel overhead processes for ensuring the integrity of the optical channel

adapted information optical channel supervisory functions for enabling network level operations and

management functions, such as connection provisioning, quality of service (QoS) parameter exchange and network survivability

The Optical multiplex section (OMS) layer represents a point-to-point link along the route of a lightpath, or optical channel, and provides functionality for the networking of a multi-wavelength optical signal. The capabilities included in this layer are:

optical multiplex section overhead processes for ensuring the integrity of the multi-wavelength optical multiplex section adapted information

optical multiplex section supervisory functions for enabling section level operation and management functions, such as multiplex section survivability

The Optical transmission section (OTS) layer represents the functionality for transmission of optical signals of various types on optical transmission media. The capabilities of the OTS layer include also the supervision of optical amplifiers and repeaters when they are present in the network. The optical physical media serving the OTS does not contain any active components. [MB98], [G872]

4.4.3 Transparency

A major advantage of second-generation optical networks is that the services they offer can be designed to be transparent to bit rates and frame formats. This means that the aggregated bit rate over the transmission link does not have to be locked into a particular value, as for example the 2.5 Gb/s in a conventional SONET OC-48/SDH STM-16 system. In fact, the signal does not even have to adhere to SONET/SDH specifications, since no electrical processing is involved. Thereby, the second-generation optical networks will be able to carry diverse protocols and bit coding structures. [Fla98], [AlS98], [Ger96]

Another issue is how fast the optical transparency becomes feasible. Given the immaturity of devices that are able to do signal regeneration (especially retiming to reduce the effects of digital jitter and wander) and wavelength conversion optically today, at least some amount of optoelectronic conversion should be expected to exist in optical network architectures in the near future. [MB98]

The discussion of what layers will continue to exist in future transport networks continues. Alternative network architectures, shown in Figure 4 [Fla98], to the current IP-to-ATM-to-SONET/SDH-to-photonics approach are being searched. One of the notions has been that the optical layer (WDM) could completely replace the SONET/SDH layer and enable IP and ATM to plug directly into the optical layer. Also, because the main value of ATM lies in its ability to conserve a limited resource, that is, transmission capacity, it will be valuable only as long as that capacity is a scarce commodity. Furthermore, it seems that in a few years, IP traffic incorporating both data and voice, will continue to grow. Therefore, among the suggestions is

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also the notion of connecting directly from IP on the desktop to the optical layer. All and all, whatever the result of the evolution may be, none of it will happen overnight. Instead, various legacy networks will run parallel to the new optical networks for some time. It might even take 10 years before WDM along with the other technologies pull the unit cost of transmission so low that e.g. the statistical multiplexing provided by ATM becomes a wasted effort. [Tit97], [CI98], [Wir98]

Optical layer

SONET/SDH

ATM

IP

Broadband applications

Other

Figure 4: Evolving broadband network layers

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5 Wavelength Division Multiplexing (WDM)

5.1 Introduction

Wavelength division multiplexing (WDM) is a technology that enables the transmission of several optical signals simultaneously at different carrier wavelengths on a single fiber and the separation of the signals by wavelength at the receiving node.

By definition, the term wavelength division multiplexing can be used to denote any technique by which two or more optical signals having different wavelengths can be simultaneously transmitted in the same direction over one fiber, and then be separated by wavelength at the distant end. [FED]. Therefore, the term WDM is also used to refer to the technique where two wavelengths, one in the 1.3 m optical window and the other in the 1.55 m window have been multiplexed onto the same link, thereby doubling the link capacity. However, in this article, the term WDM refers to the technique where several, today typically from 16 to 32 signals having slightly different wavelengths in the same optical window (1.55 m) are used as carriers. Also the term dense wavelength division multiplexing (DWDM) is sometimes used to refer to the latter alternative.

A block diagram of a unidirectional WDM link is shown in Figure 5. The light sources, transmitters, one for each wavelength, are usually DFB (distributed feedback) lasers. The light sources are modulated with the electronic client signals (e.g. an STM-64 signal at 10 Gb/s), typically by on-off keying (OOK)8 them. The resulting optical information signals at different wavelengths are then combined to be transported over a single fiber in a multiplexer. The optical amplifiers, which are used along the fiber link, come in three different configurations and are able to amplify several WDM channels simultaneously. A power amplifier may be used in front of a transmitter to give a maximal increase of the output power, and an preamplifier may be used in front the receiver to increase sensitivity by providing high gain and the least amount of additional noise. A line amplifier, typically used in the middle of the link to compensate for link losses, is designed to provide a combination of the properties of power- and preamplifiers. After traversing the link, the signals enter a demultiplexer, which separates the individual signals at different wavelengths. Finally, the signals are received in photodetectors, which generate an electrical current proportional to the incident optical power.[RS98]

Transmitter Receiver

MultiplexerDe-

multiplexer

TransmitterTransmitter

Transmitter

Receiver

Receiver

... ...

Lineamplifier

Poweramplifier

Preamplifier

, . . . ,

Figure 5: A unidirectional WDM link

8 In on-off keying, a 1 bit is encoded by the presence of light, and a 0 bit is encoded by the absence of light.

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First-generation WDM systems supported from 4 to 16 signals at different wavelengths, each with a transmission capacity of 2.5 Gb/s. Today, the bit rate has typically remained the same, as the move to 10 Gb/s transmission is impeded by the complexity and thereby high price of required electronics and the penalties which arise from polarization-mode dispersion (PMD). However, the systems being deployed today typically offer from 32 to 40 wavelengths, and technological improvements will bring along systems that support even more wavelengths: as many as 100 wavelengths are expected in the near future. Experimental systems [Luc0398] that have a total transmission capacity of 1 Tb/s on a single fiber have already been built. In the future, WDM might be combined with optical signal processing techniques like optical time division multiplexing (OTDM) and optical packet switching to further increase the transmission capacity.

The set of wavelengths to be used in a WDM system depends on a combination of the underlying technology and the application; e.g. client signal bit rate, fiber type, optical filter technology, span distance between amplifiers and the overall target reach for the system set requirements for the wavelengths. The interchannel spacing is another issue to consider. On the other hand, it would be useful to have as large a spacing as possible, because this makes wavelength multiplexing and demultiplexing easier and alleviates the requirements set for the wavelength stability of components. Also, future upgrades to higher bit rates per channel may not be possible with very tight channel spacings. On the other hand, it would be desirable to have as many channels as possible, and as a result, tight channel spacings. For a given number of channels, when the interchannel spacing and the resulting total bandwidth is smaller, it is easier to have a flat optical amplifier gain profile over the set of wavelengths. Also the impairments due to stimulated Raman scattering (SRS) are smaller.

Instead of a single, standard set of WDM carrier wavelengths, the ITU-T participants have defined a frequency grid for WDM channels, in which the adjacent channels are separated by 100 GHz. As defined in the ITU-T draft Recommendation G.692 (ex G.mcs), “Optical interfaces for multichannel systems with optical amplifiers”, the reference frequency is at 193.1 THz, in the middle of the 1.55 m fiber and EDFA passband. As the usable wavelength band of EDFAs is from 1530 nm to 1564 nm, it is possible to place a maximum of 43 channels on a fiber with the 100 GHz frequency grid. This frequency grid is based on what has been feasible with existing technology; when the technology improves, the frequency spacing can be reduced and a larger number of wavelength channels will become possible. Indeed, several vendors have already proposed modifications to permit smaller channel spacings, for example 50 GHz and 25 GHz, to the existing WDM channel frequency recommendations. At least the 50 GHz spacing is possible with today’s technology in some applications, so it is likely that smaller and even irregular channel spacings will be included in future recommendations. [MB98], [RS98], [CD98]

5.2 Advantages

The economical benefits of deploying WDM technology are numerous. At its simplest, a WDM system can be considered as a parallel set of optical channels, each using a slightly different wavelength, but sharing the same physical transmission medium. By using WDM, it is thereby possible to increase the capacity of existing networks without expensive re-cabling and thereby significantly reduce the cost of network capacity upgrades.

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An alternative way to increase the capacity would be to increase the transmission bit rate of today’s electronically time division multiplexed (TDM)9 systems. The highest transmission bit rate in commercially available TDM systems today is 10 Gb/s, and 40 Gb/s TDM technology is being developed in research laboratories. However, some experts believe that while the talk about the possibility of 40 Gb/s TDM transmission (SONET OC-768) continues, time division multiplexing architectures like SONET are about to reach their capacity limits what comes to cost-efficiency. TDM and WDM are, however, two complementary approaches and the question of which combination to use is a complicated one with many parameters affecting the right choice. It is clear however, that the next generation of networks is likely to involve increased use of WDM along with TDM.

Also, methods are being developed where time division multiplexing and demultiplexing could be performed optically, by an approach called optical time division multiplexing (OTDM). Experiments have demonstrated the multiplexing/demultiplexing of several 10 Gb/s streams into/from a 250 Gb/s stream. However, the feasibility of commercial OTDM systems is still several years away and OTDM networks therefore represent a longer-term approach. [Sar94], [Tit97], [RS98]

As opposed to purely time division multiplexed systems, WDM offers several advantages. With WDM, it is possible to increase the transmission capacity by keeping the bit rate the same, but adding more (wavelength) channels which all operate at the original bit rate. Keeping the bit rate low has its benefits. For example, polarization-mode dispersion (PMD) becomes a problem in systems operating at 10 Gb/s and above, but does not cause impediments at 2.5 Gb/s and lower bit rates. Also, receiver sensitivities are typically lower at higher bit rates, which implies that signals need amplification sooner. Adding new amplifiers or regenerators makes span engineering more complex and causes significant costs. [Rya98]

In WDM systems, it is possible to modularly increase the transmission capacity by adding additional wavelength channels when capacity increases are needed, whereas TDM systems require large upfront installations. For example, the capacity of a four-channel WDM system can be stepwise increased from 2,5Gb/s to 10 Gb/s in steps of 2.5 Gb/s, but a 10 Gb/s TDM system must by installed all at once.

A major advantage of WDM in many cases is that WDM systems can be designed to be transparent to different bit rates and protocol formats. Another useful property of WDM networks is that they are able to do wavelength routing. Here, the path of a signal through the network is determined by the signal’s wavelength and origin, as well as the states of network switches and wavelength converters. Therefore, with wavelength routing a transparent lightpath can be provided between two network nodes. [RS98]

The invention of a suitable optical amplifier, namely the Erbium-doped fiber amplifier (EDFA), described in more detail in Section 5.6.4, has been a key factor in making the use of WDM economical. The EDFAs enable direct amplification of optical signals without the use of electronic regenerators. The gain of an EDFAs is relatively flat over a rather wide wavelength span around the 1.55m window, which means that a single EDFA can amplify a nearly arbitrary number of wavelengths simultaneously. This enables significant cost savings especially in long distance networks, because it becomes possible to replace N fiber pairs and electronic regenerators along each fiber with just one fiber pair (running N wavelengths) and a single EDFA for both fibers. [Rya98]

9 In TDM, two or more information channels are combined onto a common transmission medium by interleaving the pulses representing bits from different channels into different time slots.

18

The economical advantages of WDM in local and metropolitan area networks are not quite as obvious as in long distance networks. However, at least Ericsson10 and Telia11 have already tested a combination of Ericsson’s WDM system and Gigabit Ethernet technology in a local area network spanning a distance of 22 km. According to some experts, when realized as commercial product, this type of solution might prove to be 50-90 percent cheaper than the traditional technology used to handle local area traffic. [HC98], [ET9808]

5.3 Deployment

The first WDM deployments occurred among the long distance operators in the United States of America. The deployment has also started in Europe and Asia and WDM is also being applied to undersea networks. Today, the WDM deployment situation is changing very fast, suppliers announcing new products and awarded contracts on an almost daily basis.

It is easiest to deploy WDM for use with new line systems, because it will then be possible to define the optical interface requirements. The architecture of existing optical network (SONET/SDH), facility locations and fiber characteristics all constrain the deployment of WDM in legacy systems. The existing legacy systems can gain access to WDM transport by the use of transponders at WDM-client interface. A transponder is a device, which converts the various wavelengths existing in the legacy system to a common set of WDM carrier wavelengths. In the WDM systems deployed today, the client technology loaded onto WDM has mainly been SONET or SDH. [Low98]

5.3.1 North America

In large scale, the deployment of WDM technology in the US started during 1996. The main target was to relieve network congestion, which had happened unexpectedly rapidly. Throughout 1996 and 1997 the emergence of WDM technology has been extremely rapid, with the sales of WDM systems growing from perhaps $50 million in 1995 to $1 billion in 1997. According to some forecasts, the growth is expected to continue, reaching $4 billion in 2001. The point-to-point WDM links are widely deployed in the US long distance networks today. All major long-distance service providers, e.g. Sprint12 and Worldcom13, are utilizing WDM as a standard part of their networks, and all major optical transmission systems suppliers are offering WDM products. The client technology loaded onto the WDM layer has been almost exclusively SONET (OC-48/2.5 Gb/s and OC-192/10Gb/s), with few PDH systems being carried. [Rya98], [Low98], [BP98]

5.3.2 Europe

In Europe, WDM deployment started in 1997, when the capacity of existing fiber exhausted. Compared to the United States, the deployment started later, mainly due to the more modest

10 http://www.ericsson.com/11 http://www.telia.com/12 http://www.sprint.com/13 http://www.wcom.com/

19

traffic growth cycle and shorter intercity distances and thereby lower regenerator costs. However, at least both of the United Kingdom’s largest operators, British Telecom14 (BT) and Cable and Wireless Communications15 (CWC), have announced WDM system deployments. Also, in Finland, Sonera16 (previously Telecom Finland) has ordered an ERION (Ericsson Optical Network) WDM system from Ericsson. Telia of Sweden have plans to introduce 16-wavelength WDM in its national optical network in 1998. Also Telefónica de Espana17 and Telenor18 are thought to have deployed WDM systems. Most major suppliers, at least Lucent19, Ericsson, Ciena20, Pirelli Cables & Systems21, Nortel22, and Alcatel23 are already offering WDM solutions to European operators. [Low98]

5.3.3 Asia

In Asia, China has been in the forefront of WDM deployment. NEC24 has received an order for a WDM system to increase the capacity of a 2500-km long inter-provincial trunk network linking Beijing, Shenyang and Harbin. Ordered by China Telecom, the system will carry STM-16 signals over 8/16 wavelengths. Also Lucent and Alcatel have signed WDM projects in China. Lucent has also signed a contract to supply and install a WDM system in the Korean Dacom's25 backbone network. This is the first deployment of WDM technology in Korea. In some of the developing countries of Asia, WDM implementation might happen a bit slower due to low the penetration of optical networks. [ASL98], [NEC0798], [Alc0798], [Luc0897], [Luc1197]

5.3.4 Undersea networks

WDM is being deployed also in the international undersea networks, which form an important part of the global telecommunication network. Although WDM is being used to double and triple the capacity of some existing transoceanic links, like the trans-Atlantic TAT-12/13 (capacity doubled from 5 Gb/s to 10 Gb/s per fiber), the advantages of WDM can be most fully realized in the design and implementation of new submarine networks. Undersea networks spanning distances of over 8000 km with up to 16 wavelengths and an STM-16 signal (2.5 Gb/s) on each wavelength are now possible. Some of the most significant networks utilizing WDM technology are described further in this section.

In addition to capacity increase, WDM also brings along significant enhancements to the networking capabilities and flexibility of undersea networks. Typical of undersea networks is that the number of fiber pairs in the network is limited by the physical constraints of the undersea cable and the design of repeater housing, including the number of optical amplifiers 14 http://www.bt.com/15 http://www.cwc.com/16 http://www.sonera.fi/17 http://www.telefonica.es/18 http://www.telenor.no/19 http://www.lucent.com/20 http://www.ciena.com/21 http://www.pirelli.com/cables/index.htm22 http://www.nortel.com/23 http://www.alcatel.com/24 http://www.nec-global.com/25 http://www.dacom.co.kr/home.html

20

in the repeaters. For economical and technical reasons, only repeaters with up to four pairs of optical amplifiers are manufactured today. When an optical wavelength layer with a wavelength routing capability is added into an undersea network, the network functionality is much less constrained by the number of physical fiber paths in the cables. Wavelengths can be added and dropped independently, and it is possible to build networks which are more flexible and have more landing points, with less physical fiber.

SEA-ME-WE-3, a major part of one of the most complex fiber-optic undersea communications systems in the world, is a submarine cable network extending from Germany via the Mediterranean to Singapore and Australia and thereby linking South East Asia (SEA), Middle East (ME) and Western Europe (WE). The SEA-ME-WE-3 is scheduled to be ready by March 1999. It will be the first network to use WDM technology to do undersea routing of wavelengths. The network, which is owned by about 100 telecommunications administrations, has a trunk and branch cable topology, which consists of a main trunk cable in deep water, off the continental shelf, and branch cable connections to landing points on the shore via underwater wavelength add/drop multiplexing branching units. The branching units contain wavelength selective elements, which are able to add and drop individual wavelengths to and from branch cables. The network will have two pairs of fiber and a capacity of up to eight wavelengths on each fiber. Each wavelength will be carrying an STM-16 (2.5 Gb/s) signal, and the maximum capacity will therefore be 40 Gb/s.

Although the trans-Atlantic TAT 12/13, completed in 1996, has been since upgraded to two wavelengths on each fiber and negotiations are being made to extend the upgrade to 3 wavelengths equaling 15 Gb/s, the need for transmission capacity across the Northern Atlantic will outpace this supply. The Atlantic Crossing –1 (AC-1) is built to answer this capacity need. The AC-1, scheduled to be in service in January 1999, is a self-healing SDH ring network with four separate undersea cable segments and landing points in the United States, United Kingdom, Netherlands and Germany. The AC-1 has four fiber pairs and will initially carry 2.5 Gb/s STM-16 signals on four wavelengths on each fiber, totaling a bi-directional transport capacity of 40 Gb/s.

China-US will be the first direct cable link between the United States and China. Scheduled to enter service by the end of 1999, the China-US is a trans-Pacific SDH ring network carrying traffic on four fiber pairs. The network is designed to carry eight wavelengths at 2.5 Gb/s (STM-16) on each fiber, which totals a bi-directional transport capacity of 80 Gb/s.

Atlantis-2, Columbus-3 and Americas-2, scheduled to be in service by July 1999, are separate WDM systems but will together form a ring around the South Atlantic Ocean. Unlike e.g. in SEA-ME-WE-3, the multiplexing and demultiplexing of wavelengths is done on shore, which allows the underwater branching units to be passive devices. Therefore, it is possible to increase the transmission capacity just by changing the shore-based terminal equipment after the system has become operational. [TM98]

5.4 Standardization

The deployment of WDM systems has occurred before the standards have been developed. However, international standardization is essential for enabling a multivendor interoperable optical network infrastructure. In the previous ITU-T (International Telecommunication Union – Telecommunication Standardization Sector) study period (1993-1996) a set of optical

21

networking recommendations was prepared. These recommendations focus primarily on optically amplified SDH systems and SDH point-to-point WDM line systems. The standardization will, however, need to cover a larger scope and go beyond SDH-specific applications to include the aspects related to other client systems of the optical layer (PDH, ATM, IP etc.). Also, applications more complex than just the point-to-point systems need to be covered. As such, international standardization efforts for optical networking, including WDM technology, are currently in progress in the ITU-T. During the 1997-2000 Study Period, the goal of ITU-T is to define a complete set of optical networking recommendations. The scope of this effort includes:

architecture of optical transport networks (draft G.872, ex G.otn): a logical view of implementation-independent functionality that needs to be supported by the network [G872]

network requirements (draft G.873 (ex G.onr)): optical network requirements and reference configurations [G873]

equipment functions (G.oeg, G.oef): general and functional characteristics of optical equipment

management aspects (G.onm): how the network should be managed information model (G.oni): a management view of network elements structures mappings (G.ons): the definition of the information associated with the network

node interface (NNI) physical layer (G.onp): optical characteristics of equipment interfaces and transmission components (G.onc): the transmission-related aspects of components and subsystems framework (G.onf): the coordination of all ITU-T activities on optical networking

Once the optical transport network requirements and architecture are defined, it is possible to move on to other recommendations. The relationships between the ITU-T optical networking recommendations are illustrated in Figure 6.

Network requirementsand architecture

(G.873 (ex G.onr), G.872 (ex G.otn))

Equipmentfunctions

(G.oeg, G.oef)

Informationmodel(G.oni)

Managementaspects(G.onm)

Structuresmappings(G.ons)

Framework(G.onf)

Physicallayer

(G.onp)

Components(G.onc)

Figure 6: Relationships between the ITU-T optical networking recommendations

The recommendations are developed using a phased approach, which takes into account the maturity of the technology and the application requirements of the market. In the first phase, the emphasis is on point-to-point WDM line systems. In the second phase, the scope is extended to include optical add/drop multiplexing and optical cross-connection systems. The third phase further extends to optical layer survivability and so on. All the other recommendations, except G.872 (ex G.otn), G.873 (ex G.onr) and G.onf are handled according to the phased approach.

The standardization of optical networking is a demanding challenge with many open questions, like should the networks be allowed to be optically transparent or opaque (to what degree are

22

electro-optical conversions allowed), how the operation, administration and maintenance are handled, what is the role of optical protection switching, and is transverse 26 or longitudinal27

compatibility required. Also the fact that rapid progress has been made in the development of optical networking technology and the pace only seems to be accelerating, causes difficulties for standardization: the standards are always at risk of lagging behind the current state of the art, thereby not being widely deployed. [MB98], [CHH94]

5.5 Network architectures

WDM has applications beyond the simple increase of the capacity of point-to-point links. In WDM networks, wavelengths become an integral part of the network infrastructure. Network topologies, which are made possible by the multiwavelength technology, can be classified into two broad categories: simple broadcast and select networks and the more sophisticated wavelength routing networks. These two architectural types can be combined with each other as well as other types to generate a broad range of network architectures.

5.5.1 WDM broadcast and select networks

In a WDM broadcast and select network, shown in Figure 7, multiple network nodes are connected to a passive device, which broadcasts the signals sent by the nodes to all the nodes in the network. The passive device is an optical star coupler, which combines the signals from all the nodes and delivers a fraction of the power from each signal to each outport. In each node, there is a tunable optical filter for selecting the desired wavelength for reception.

Laser

Passivestar

coupler

Node N

Node 2Node 1

Receiver Receiver

Laser

Receiver

Laser

. . .

, . . . ,

, . . . , , . . . ,

26 Transverse compatibility = equipment from one vendor can be connected to equipment from any other vendor.27 Longitudinal compatibility = equipment from different vendors is not guaranteed to interwork. Instead, for any given application there is a choice of vendors from which to choose the equipment. Only one vendor’s equipment can be used per application.

23

Figure 7: A WDM broadcast and select network

This network architecture is simple and suitable for use in local- and metropolitan-area networks. Broadcast and select networks have, however, two drawbacks. First, they waste optical power since the power of each transmitted signal is evenly divided between all the nodes in the network. Second, each node requires a distinct transmission wavelength, and the number of nodes is therefore limited to the number of available wavelengths. For this reason, broadcast and select networks are not scalable. [HN96], [RS98]

5.5.2 WDM wavelength routing networks

A more practical network architecture today is the wavelength routing network, in which the network nodes, which contain wavelength-selective elements, are capable of routing different wavelengths at their input ports to different output ports. This way, the wavelength determines the path, which a signal takes through the network, and a signal at a particular wavelength can be routed directly to its destination instead of being broadcast to all the network nodes. This eliminates unnecessary divisions of signal power. In a static wavelength routing network, the wavelength selective elements are static components and the path that a particular signal takes is uniquely determined by the wavelength of the signal and the port through which it enters the network. In a reconfigurable network, the nodes contain switches and/or dynamic wavelength converters and the routing patterns at the nodes can be changed. Static routers can be built using wavelength multiplexers and demultiplexers, while dynamic routers will in addition require optical switching technology, which is still immature today. [Bra90], [RS98]

Wavelength routing networks became a major research area in the early 1990s when people realized the benefits of having an optical layer. They are now being introduced as commercial products in local-exchange and interexchange networks. A wavelength routing network enables the setting up of several simultaneous lightpaths, which are transparent because they remain optical across the network, and which can use the same wavelength in different, non-overlapping parts of the network. Therefore, it is possible to reuse the capacity of the network spatially. An example in Figure 8 shows three simultaneous lightpaths. Lighpaths between nodes 1 and 5 and between nodes 3 and 4 do not share common links and can therefore be set up using the same wavelength, 1. The lightpath between nodes 2 and 4, however, shares a common link with the lightpath between nodes 1 and 5, and must be therefore be set up using a different wavelength, 2. In this example, all the lightpaths use the same wavelength on every link in their path. This is the constraint that a network designer has to deal with if the network does not have wavelength conversion capabilities. If, for example node 4 had a wavelength conversion capability, it would be possible to set up an additional lightpath between the nodes 3 and 5 with only the two existing wavelengths, using 2 from node 3 to node 4, and 1 from node 4 to node 5. [RS98], [HN96], [Ger96]

24

Node 1

Node 4

Node 5

Node 3Node 2

Figure 8: A WDM wavelength routing network

5.6 Key components

As a point-to-point technology, WDM has changed from a laboratory technique to a commercially viable technique in a relatively short time. Whether or not wavelength division multiplexing technology can be successfully used to build true all-optical networks, depends heavily on the available optical device technology. The following sections describe some of the key technologies that can be used to construct higher level components with a networking functionality in the optical domain, like optical filters, wavelength (de)multiplexers, add/drop multiplexers and wavelength routers. A simple optical filter, shown in Figure 9, selects one wavelength for transmission and rejects all others. Optical filters are key components used in constructing wavelength multiplexers and demultiplexers. They are also used to provide gain equalization and noise filtering in optical amplifiers.

A wavelength multiplexer, illustrated in Figure 10, combines input signals at different wavelengths onto a common output, and a demultiplexer does the opposite. Wavelength (de)multiplexers are used in WDM terminals and in larger wavelength add/drop multiplexers and wavelength routers. A wavelength add/drop multiplexer, shown in Figure 11, is able to extract some of the wavelengths in the system and to inject them back (carrying a different information signal). The function of static, 2-input, 2-output wavelength router is shown in Figure 12. In this example, the router exchanges the wavelengths 1 and 4 between the input ports and routes them to output ports.

Opticalfilter

Figure 9: A simple optical filter

25

Wavelengthmultiplexer

Figure 10: A wavelength multiplexer

Wavelengthadd/drop

multiplexer

Figure 11: A wavelength add/drop multiplexer

Wavelengthrouter

Figure 12: A wavelength router

In addition to optical filters, couplers and circulators are needed to construct wavelength (de)multiplexers, add/drop elements and routers. The couplers, optical filters and circulators are described in the following, along with the optical amplifier technology which has had a significant impact on WDM deployment, the Erbium-doped fiber amplifier (EDFA). Other components used in WDM networks, but not described here, include lasers, photodetectors and isolators. Lasers are used as transmitters to convert an electrical signal stream into pulses of light for transmission over optical fiber. Photodetectors receive the light pulses at the far end of the link and generate an electrical current proportional to the optical power of the incident light pulses. Isolators are used in front of lasers and optical amplifiers to prevent performance-degrading reflections from entering them.

Most components described here are passive devices, which means that they do not require an electrical source of energy for their operation. Therefore, they are not dependent on electronics or electro-optical conversions and the limits on transmission speed they pose. Instead, they are able to operate at the same high speed as the transmission medium, the optical fiber and are transparent to signal formats. Thus, as various networking technologies battle for market share in the coming years, the transparent optical networking technology can grow along. The prices of WDM components are still relatively high, but as more companies enter the competition and optical components can be mass-produced more economically, prices are expected to decrease between 10% to 50% annually. [LWX1098], [RS98]

5.6.1 Couplers

26

Passive couplers are some the simplest elements used in optical communication systems. Coupler is a general term used to denote all devices that are used to combine and split optical signals. A 12 splitter, in Figure 13, is a coupler which divides the power of an optical signal on one fiber onto two fibers. A 21 combiner, in Figure 14 does the opposite: it combines the power of two optical signals from two fibers onto a common fiber. In general, a 22 coupler in Figure 15 is a 21 combiner followed by a 12 splitter. The coupler broadcasts the signals from the two input fibers onto the two output fibers. When the coupling length l, in Figure 15, is adjusted so that half the power from each input appears at each output, the coupler is called a 3-dB coupler.

Input

Output 1

Output 2

Figure 13: A splitter Figure 14: A combiner

Input 2

Input 1 Output 1

Output 2l

Figure 15: A coupler

27

Couplers can be constructed by simply fusing two fibers together or by using waveguides in integrated optics. Couplers are the building blocks of various optical components. An nn star coupler used in broadcast and select networks, described in Section 5.5.1, is a generalization of the 22 coupler. It divides the power from each input equally among all the outputs, and can be constructed by suitably interconnecting a number of 3-dB 22 couplers. Couplers are also the key components used to construct Mach-Zehnder interferometers, which can be used as optical filters and (de)multiplexers. [Bor97], [RS98]

5.6.2 Optical filters

Optical filters are key components in WDM networks. They allow the manipulation of wavelengths just like time slots are manipulated in time division multiplexed networks. Several optical filtering technologies are available, all utilizing the property of interference between optical waves. In addition, some filters utilize the diffraction property of light, which is the property by which light from a source tends to spread to all directions. The technologies include:

Gratings Bragg gratings Fiber gratings Fiber Bragg gratings Fabry Perot filters Multilayer dielectric thin-film filters Mach-Zehnder interferometers Arrayed waveguide gratings Acousto-optic tunable filters

Gratings

The term grating is used to denote almost any device whose operation involves interference among multiple optical signals which originate from the same source, but have different relative phase shifts (an exception to this is the etalon, in which the multiple optical signals are generated by letting the light traverse a single cavity repeatedly). In gratings, the relative phase shift between two optical waves from the same source is achieved, when the waves are made to traverse two paths of different lengths. In optics, gratings have been used for decades to separate light into its constituent wavelengths. [RS98]

Bragg gratings

In general, any periodic perturbation (usually a periodic variation of the refractive index) in the grating medium serves as a Bragg grating. Bragg gratings are widely used in optical communication systems, e.g. to construct multiplexers and add/drop elements. The principle of the Bragg grating also underlies the operation of acousto-optic tunable filter, where, the Bragg grating is caused by the propagation of an acoustic wave in the propagation medium. [RS98]

Fiber gratings

Fiber gratings are written in fibers using the photosensitivity property of the fiber. This is done by exposing the fiber to an interference pattern of ultraviolet (UV) light. In the spots where the UV light waves add in phase, the refractive index of the fiber is permanently raised, and the interference pattern is thereby transformed into a refractive index pattern in the fiber. Fiber Bragg gratings are suitable for e.g. filtering, add/drop functions and compensating for accumulated dispersion. [Oue98]

Fabry-Perot filters

A Fabry-Perot filter, also called Fabry-Perot interferometer, is an etalon, or a cavity formed by two highly reflective mirrors, which are placed parallel to each other, as shown in Figure 16. Light from an input fiber enters the left surface of the cavity at right angle. After traversing the cavity, part of the light passes through the right facet and leaves the cavity, while part of the light is reflected backwards, towards the left facet. After a number of reflections, those wavelengths for which the cavity length is an integral multiple of half the wavelength (so that the round trip through the cavity is an integral multiple of the wavelength) add in phase. These wavelengths, called the resonant wavelengths, propagate through the cavity, while the remaining wavelengths destructively interfere.

Fabry-Perot cavity

Reflections

Input signal

Output signal

Figure 16: A Fabry-Perot filter

Fabry-Perot filters are commercially available, compact components, which have been used in several optical network testbeds. Their main advantage over some other filters is that they can be tuned to select different wavelengths. This is done by mechanically tuning the cavity length. The tuning time is, however, on the order of a few milliseconds, which generally makes the Fabry-Perot filter unsuitable for packet-switched applications. [RS98], [Bor97]

Multilayer dielectric thin-film filters

A multilayer dielectric thin-film filter (TFF) is a Fabry-Perot filter, where the mirrors surrounding the resonant cavity are constructed using multiple reflective dielectric thin-film layers. The device is a bandpass filter, letting the resonant wavelength pass through and reflecting all others. A multilayer thin-film multicavity filter (TFMF) consists of two or more resonant cavities, which are separated by reflective dielectric thin-film layers. Adding more cavities has the effect of flattening and sharpening the passband, both desirable features for a filter.

When cascaded, these filters, each passing a different wavelength and reflecting all others, can be used as (de)multiplexers, as shown in Figure 17. When used as a demultiplexer, the first filter in the cascade passes one wavelength and reflects all the others to the second filter. The second filter passes another wavelength and reflects all the others to the third filter, and so on.

Lens

Lens

Lens

Lens

Lens

Lens

Lens

Lens

Lens

Fiber Fiber

Fiber

Glass substrate

, ... ,

Narrowbandfilter

Figure 17: A multilayer dielectric thin-film filter wavelength (de)multiplexer

The TFMFs have several features that make them an attractive option to be used as (de)multiplexing devices. They are becoming widely used in today’s commercial WDM systems, mainly because they

enable a very flat passband and sharp transitions from passband to stop band are remarkably stable in regard to temperature variations have low loss are insensitive to the polarization of the signal [Hok98], [RS98]

Mach-Zehnder interferometers

Mach-Zehnder interferometers (MZI) are used as filters or (de)multiplexers. They resolve different wavelengths by making use of two interfering paths of different lengths. Basically, a Mach-Zehnder interferometer consists of two 3-dB directional couplers with a path length difference between the arms, as shown in Figure 18.

Input 2

Input 1 Output 1

Output 2

Path length differenceL

Figure 18: A Mach-Zehnder interferometer

Devices based on this principle have been constructed for some decades. There are better technologies, such as dielectric multicavity thin-film filters, available for realizing narrowband filters, but MZIs are useful as wide band filters, for example in separating wavelengths in the 1.3m band and in the 1.55m band. Narrow band MZI filters can be constructed by cascading several stages, but this leads to larger losses. Furthermore, the passband of narrow band MZI filters is not flat. MZIs are also useful as 21 multiplexers and 12 demultiplexers; for constructing larger (de)multiplexers there are better technologies available. [RS98]

Arrayed waveguide gratings

An arrayed waveguide grating (AWG) is a generalization of the Mach-Zehnder interferometer. It consists of two multiport star couplers connected by a waveguide grating array, as shown in Figure 19. The grating array consists of curved waveguides with a constant difference in length between any two adjacent waveguides. When the input multiwavelength signal enters the first coupler, it diffracts and enters the grating array. In the array, because the distances they travel are different, the signals in each channel are phase-shifted by a different amount, which causes interference at the second coupler. This process results in different signals having interference maxima at different locations; the locations of the second coupler’s output ports.

1

1

2

2

3

3

.. .

.

.

.

N

N

Inputwaveguides

OutputwaveguidesArrayed waveguides

Starcoupler

Starcoupler

L = constant

Figure 19: An arrayed waveguide grating (AWG)

When only one output or input port is used, the AWG can act as an N1 multiplexer or a 1N demultiplexer. Compared to a suitably interconnected chain of MZIs, which also realizes the (de)multiplexing function, it is preferable to use an AWG due to its flatter passband and lower loss. An AWG is also easier to implement on an integrated-optic substrate, which is usually silicon. The waveguides are usually made of silica, Ge-doped silica or SiO2-Ta2O5.

When all the N input and output ports are used, the AWG can also be used as a static NN router, where the route of a signal is determined by its wavelength and input port. Different signals at same wavelength can be simultaneously input to different input ports and still not interfere with each other at the output ports. An arbitrary routing pattern is not, however, possible but a number of static routing patterns can be achieved by a suitable choice of wavelengths. The most useful type of routing pattern is illustrated in Figure 20, which shows the routing pattern of a static 44 wavelength router. [AlS98], [Bor97], [Fid97], [RS98]

, , ,

, , ,

, , ,

, , ,

, , ,

, , ,

, , ,

, , ,

Figure 20: A static routing pattern of a 4x4 arrayed waveguide grating

Acousto-optic tunable filters

The acousto-optic tunable filter (AOTF) is one the several optical devices whose operation is based on the interaction of sound and light. In an AOTF, a Bragg grating, or a periodic variation of density, results from the propagation of an acoustic wave in the medium. Thereby, by varying the wavelength of the acoustic wave, an AOTF can act as a tunable filter.

A special feature of the AOTF is that by launching multiple acoustic waves simultaneously, the energy of signals at multiple wavelengths can be exchanged between the two input and output ports of the AOTF, and by varying the wavelengths of the acoustic waves, the routing pattern can be changed. Therefore, the AOTF holds out a promise of dynamic routing. At the moment, however, the AOTF has not quite proved to especially useful either as a tunable filter or a dynamic router, mainly because of the high level of crosstalk present in the device. [RS98]

5.6.3 Circulators

Circulators are passive, nonreciprocal devices, as opposed to couplers and most other passive optical devices that are reciprocal28. The principle of operation of a circulator involves utilizing the different (horizontal and vertical) polarization modes of the light propagating in fiber. Typically, circulators have three or four ports, as shown in Figure 21, (a) and (b). In a three-port circulator, the signal input to port 1 is sent to port 2, the signal input to port is sent to port 3, and the signal input to port 3 is sent to port 1. Circulators are useful in constructing add/drop multiplexers. [RS98]

28 A reciprocal device works the same way if its inputs and outputs are reversed.

1

2

3

1

2

3

4

(a) (b)

Figure 21: Circulators: (a) three-port (b) four-port

5.6.4 Optical amplifiers

Repeatered vs. amplified systems

In an optical communication system, optical signals from the transmitter are attenuated as they propagate through fiber. After some length, this causes the signal to become too weak to be detected. Therefore, before this happens, the signal has to be restored. The conventional way to accomplish this has been to electrically regenerate the signal, that is, to receive and re-transmit it. This is the process accomplished by regenerative repeaters, or regenerators, which convert optical signals to electrical signals, clean them up, and convert them back to optical signals for onward transmission.

A conventional regeneration technique for digital data is regeneration with retiming and reshaping (3R). In 3R, the bit clock is extracted from the signal, and the signal is reclocked. This effectively eliminates transparency to bit rates and frame formats since acquiring the clock usually requires knowledge of both of these. Furthermore, as separate regenerating equipment is required for each wavelength channel, this is an inherently costly solution A diagram of conventional 3R regeneration is shown in Figure 22. [KBW96] Here, MOD stands for electrical signal modulator, and E/O and O/E are electrical-to-optical and optical-to-electrical signal converters, respectively. [RS98]

3R E/OO/EFiber

Transmitter ReceiverE/OMOD O/E MOD

Electrical

Reshapingcircuit

Regeneratingcircuit

Retimingcircuit

Optical

Inputsignal

O/E:Photodetector

E/O:Laserdiode

Outputsignal

Optical

3R: Reshaping, Retiming, Regenerating

RepeaterTransmitter terminal Receiver terminal

Electrical signal

Optical signal

Figure 22 : Optical fiber transmission with electrical regenerators

Many of the most relevant advances in optical communications can be traced to the invention of optical amplifiers, which were relatively unknown before 1980. The original motivation for the widespread research was then to replace the costly electrical regenerators on long-haul transoceanic systems, in which the regenerators were placed every 50 km along the fiber. Unlike in electrical repeaters, in optical amplifiers, the signals remain in optical form during amplification. The key physical phenomenon behind signal amplification in optical amplifiers is the stimulated emission of radiation by atoms in the presence of an external electromagnetic field. Optical amplifiers consist of an active medium that has its carriers inverted into an excited energy level, thus enabling an externally input optical field to initiate stimulated emission and achieve coherent gain.

Optical amplifiers offer several advantages compared to regenerators. Because in optical amplifiers the signals remain in optical form throughout the process, the optical amplifiers are ideally transparent boxes, which provide gain and are insensitive to bit rate, modulation format, power and the wavelengths passing through it. Also, optical amplifiers are able to amplify several wavelengths simultaneously in the 1530nm – 1564 nm band, and typically only one amplifier is therefore needed to amplify a number of WDM channels. [KBW96]

Even if the operation principle of optical amplifiers is the same as that of lasers (except that amplifiers do not need a cavity whereas lasers need one for oscillation), from the first attempt in the 1960s it took about 30 years before efficient, low-noise fiber optical fiber amplifiers were developed in 1987. The keys to the construction of efficient amplifiers were the following: [Sud97]

1. the use of the efficient transitions of rare-earth29 ions such as Erbium (Er3+), which provide high quantum efficiency with small or no nonradiative transition probability

29 Rare-earth elements: a series of 15 transition metals, beginning with lanthanum (atomic number 57) and ending with lutetium (atomic number 71) along with scandium (atomic number 21) and yttrium (atomic number 39). The electronic configuration of the elements in the series is very similar and is based on the gradual filling of the 4f subshell along the series. [Sud97]

2. the use of laser light with very narrow spectral width as the pumping source, which leads to very efficient pumping for the narrow absorption band of rare-earth ions

3. finding the energy levels and wavelengths suitable for efficient pumping, for example 1.48 and 0.98 m

4. developing a fabrication method for low-loss rare-earth doped fibers

Erbium-doped fiber amplifiers

One of the key technologies in enhancing WDM deployment has been the Erbium-doped fiber amplifier (EDFA). In fact, most newly installed optical fiber transmission systems today use EDFAs instead of regenerators. EDFAs, like other optical amplifiers, are rather simple devices, consisting mainly of a length of doped optical fiber, a pump laser (the only active component) and a component that combines the pump signal with the transmitted data signal (see Figure23). EDFAs work by using the pump laser to activate, or excite, the rare earth ions in the doped fiber coil acting as the gain medium. In an EDFA, the gain medium is doped with ionized atoms, Er3+, of the rare-earth element Erbium. A weak signal, which needs amplification, encounters the excited ions, stimulates them to release their excess energy and thereby boost the signal’s amplitude as it propagates through the amplifier. [Hew97], [RS98]

Figure 23: Block diagram of an Erbium-doped fiber amplifier (EDFA)

Several factors have made the EDFA amplifier to become the preferred amplifier to use in today’s optical communication systems. These include: [RS98]

a) the availability of compact and reliable high-power semiconductor pump lasersb) the fact that an EDFA is an all-fiber device which makes it polarization independent and

easy to couple light in and out of itc) the simplicity of the deviced) the fact that an EDFA introduces no crosstalk when amplifying WDM signals

Optical amplifiers are not, however, ideal devices and a number of impairments must be taken into account when designing an optically amplified WDM link: [KBW96], [RS98]

The usable EDFA band is from 1530 nm to 1564 nm, but the gain spectrum is not flat over the entire region: some wavelengths receive more gain than others. The problem becomes more serious when a number of amplifiers are cascaded.

In addition to providing gain, the EDFAs introduce noise to the system. The additional noise causes degradation in receiver sensitivity: a higher signal power is required at the receiver in order to maintain a desired bit error rate (BER).

The gain depends on the total input power: for high input powers the amplifier has a tendency to saturate, which causes the gain to drop.

The EDFAs allow fiber dispersion and nonlinear effects to accumulate unimpeded.

Other types of optical amplifiers

The principal of operation of Praseodymium-doped fiber amplifiers (PDFAs) is very similar to EDFAs. PDFAs offer some promise in amplifying signals in the 1.3 m band with a pump wavelength of 1017 nm, where pump lasers are not yet well developed. PDFAs may, however, become commercially available in the next few years.

Semiconductor optical amplifiers (SOAs), which preceded EDFAs, are not as good as EDFAs for use as amplifiers, but are finding other applications in switches and wavelength converters. [RS98]

5.7 Future

Because the evolution of optical networks is strongly dependent on the available technology, it is difficult to foresee exactly how the optical networks will evolve in the future and what will be WDM’s role. Some scenarios are, however, given in the following.

5.7.1 Network architectures

According to one scenario, the progress of optical network architectures will be similar to the manner in which the current SONET/SDH networks evolved. This is illustrated in Figure 24. First, the point-to-point WDM transmission systems are deployed, which has already happened. Next, a limited amount of flexibility is introduced into WDM systems. This is done by deploying static optical add/drop multiplexers (OADMs) and the use of WDM protection switching. This step has also been taken already, e.g. in undersea WDM links where wavelength selective branching units are used. WDM technology suppliers have also started to offer add/drop equipment for terrestrial links. Next, the WDM link architecture will evolve to ring topology networks, which contain dynamic OADMs. Then, the use of dynamic OADMs will enable ring networks with full connectivity, similar to today’s SONET/SDH rings. The next step in the evolution might be the interconnection of several optical rings with an overlaying mesh topology. Here optical cross-connects (OXCs) and OADMs are used to interconnect the rings. [Lag98], [Low98]

Transmitter

... ...

1996 1998 2000 2002

Tech

nolo

gy e

volu

tion

WDM transmission

Transmitter

... ...

OADM OADM

WDM transmission with add/drop

OADMOADM

OADM

OADM

WDM rings withnode addressing

WDM rings withfull connectivity

Interconnected ringsand mesh topologies

OADMOADM

OADM

OADMOADM

OADM

OADM

OADM

OADM

OXC

OADMOXC

OXC

OXC

OXC

OADM

Figure 24: A possible evolution scenario for optical network architecture

5.7.2 WDM in metropolitan area and access networks

While the WDM technology is maturing in transport networks, the drive to bring WDM downstream, closer to the end user, has become the next goal. Future access networks need to offer cost-effective, high transmission capacity support for the increasing number of new broadband end-customer services. In the next few years, WDM is expected to migrate from public backbones to metropolitan area networks (MANs) and access networks. These applications represent an emerging WDM market that differs from the long-distance applications in that the spans between terminals are shorter and that there are more add/drop points. Therefore, there is usually no need for signal amplification in the middle of the links, only in front of the terminals. Instead, the add/drop functionality is essential. Also, due to the nature of metropolitan networks, the WDM system interfaces need to support a variety of transport signals in these applications. WDM vendors have already anticipated the downstream trend by developing “metro” systems [Cie98], which have been engineered and priced differently than the long-distance systems. [AlS98], [Fid97], [HC98]

6 Summary

Optical networks have been essential in providing the large transmission capacity of today’s telecommunication networks. The traffic amounts have, however, grown rapidly, to the stage where the capacity needs surpass the available capacity. As laying new fiber is a relatively expensive alternative for increasing capacity, other methods have been developed. While the traditional optical networks operate with one wavelength, wavelength division multiplexed (WDM) networks are able to transport several signals on separate wavelengths over a single, existing fiber, thus offering an economical way to upgrade network capacity. In WDM, the optical bandwidth of a link is split into fixed, non-overlapping spectral bands, each band constituting a wavelength channel that can, independently of other bands, be used for a specific bit rate and transmission technique.

In the past few years, WDM technology has been widely deployed in long distance point-to-point links. Currently, WDM systems are evolving from point-to-point links to true networks where individual wavelength channels may be added and dropped. Also, the wavelength division multiplexing technology is expected to migrate from long distance applications to metropolitan and access networks.

To avoid the costly and capacity-constraining optoelectronic conversions in future’s networks, most experts believe that all-optical networking, where all network functions are performed in the optical domain, has to be the next goal. However, the dynamic optical technologies required to support the all-optical networks need to mature significantly before this stage can be reached.

As the evolution of optical networking in general depends to a great extent on the available technology, it is not totally clear what the role of WDM will be in the future. Most likely, WDM will be used together with time division multiplexing (TDM) or optical time division multiplexing (OTDM) techniques to further increase transmission capacities in various types of networks. Undoubtedly, much optical networking research and development activity will take place in the coming years. So far, significant strides have certainly been made with WDM in the progress towards a telecommunications market where transmission capacity is a commodity item, distances are irrelevant and the provided information and services represent the highest value-added element.

7 References

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AlS98 Al-Salameh, Daniel Y. et al.Optical Networking.Bell Labs Technical Journal, January-March 1998. 23pp.http://www.agile.com/ideas2/perspectives/bltj/jan-mar1998/pdf/paper04.pdf(version current on September 16, 1998)

ASL98 D-WDM standards underway.Asian Sources Library, 1998.http://www.asiansources.com/LIBRARY/MAGAZINE/TS/9809/WDM01.HTM(version current on November 13, 1998)

Bra90 Brackett, Charles A.Dense Wavelength Division Multiplexing Networks: Principles and Applications.IEEE Journal on Selected Areas in Communications, August 1990. Vol. 8. No. 6, pp.948-964. ISSN 0733-8716.

Bor97 Borella, M. S. & Jue, J. P. & Banerjee, D. & Ramamurthy, B. & Mukherjee, B.Optical Components for WDM lightwave Networks.Proceedings of the IEEE, vol. 85, no. 8, pp. 1274-1307, August 1997.http://ortega.cs.ucdavis.edu/~byrav/Professional/Proc.ps(version current on September 14, 1998)

BP98 Butler, Robert K. & Polson, David R.Wave-Division Multiplexing in the Sprint long Distance Network.IEEE Communications Magazine, February 1998. Vol. 36, No. 2, pp. 52-55. ISSN 0163-6804.

CD98 Cortez, Steve & Dickerson, Michael.Enabling the all-optical network.America’s Network, February 1, 1998. http://www.americasnetwork.com/issues/98issues/980201/980201_optical.html(version current on November 17, 1998)

Che89 Cheng, David K.Field and Wave Electromagnetics.USA: Addison-Wesley Publishing Company, Inc., 1989. 703pp. ISBN 0-201-52820-7.

CHH94 Cochrane, Peter & Heckingbottom, Roger & Heatley, David.The Hidden Benefits of Optical Transparency.IEEE Communications Magazine, September 1994. Vol. 32, No. 9, pp. 90-97.ISSN 0163-6804.

CI98 High-fibre instant recipe.Communications International – Live, 1998.http://www.totaltele.com/cilive/issue/nov/art10.htm(version current on November 22, 1998)

Cie98 Ciena MultiWave Metro Dense Wavelength Division Multiplexing System.Ciena product information, 1998.http://www.ciena.com/products/mwmetro.html(version current on November 22, 1998)

ET9808 Ericsson och Telia har gjort hemliga tester.Elektroniktidningen 98-08.http://www.et.se/elektronik/arkiv/1998/9808/17.html(version current on September 8, 1998)

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Fid97 Fidorra, F. et al.WDM in the Access Network and Key Components.http://www.hhi.de/JB/Jb96/net_jb96/3WDM_O1.htm(version current on September 11, 1998)

Fla98 Flanigan, Barry.Fibre-rich Futures: The SDH Story.Telecommunications, International Edition. February 1998. Vol. 32, No.2. pp. 37-40. ISSN 0-278-4831.

G872 Determined version of ITU-T Recommendation G.872 (ex G.otn): Architecture of optical transport networksContribution to T1 Standards Project – T1X1.5.ftp://ftp.t1.org/pub/t1x1/x15.98/8x150730.pdf(version current on November 13, 1998)

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Ger96 Gerstel, Ori.On the Future of Wavelength Routing Networks.IEEE Network, November/December 1996. Vol. 10, No. 6, pp. 14-20. ISSN 0890-8044.

HC98 Hatton, Patricia V. & Cheston, Frank.WDM Deployment in the Local Exchange Network.IEEE Communications Magazine, February 1998. Vol. 36, No. 2, pp. 56-61. ISSN 0163-6804.

Hew97 Hewak Dan.Travelling Light. ORC Research Review #1460.University of Southampton, Optoelectronics Research Center.http://holly.orc.soton.ac.uk/orchelp/pubs/1460dh.html(version current on September 9, 1998)

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1 ACRONYMS.........................................................................................................................2

2 LIST OF FIGURES..............................................................................................................4

3 INTRODUCTION.................................................................................................................5

4 OPTICAL NETWORKS......................................................................................................6

4.1 LIGHT PROPAGATION IN AN OPTICAL FIBER.....................................................................64.2 CAPACITY LIMITS OF OPTICAL TRANSMISSION.................................................................7

4.2.1 Dispersion.................................................................................................................8Chromatic dispersion...........................................................................................................................................8Modal dispersion.................................................................................................................................................9Polarization-mode dispersion..............................................................................................................................9

4.2.2 Fiber nonlinearities..................................................................................................9Stimulated Brillouin scattering (SBS)...............................................................................................................10Stimulated Raman scattering (SRS)..................................................................................................................10Four-wave mixing (FWM)................................................................................................................................10Self-phase modulation (SPM) and cross-phase modulation (CPM).................................................................10

4.3 EVOLUTION OF OPTICAL NETWORKS..............................................................................104.4 THE OPTICAL LAYER.......................................................................................................12

4.4.1 Services...................................................................................................................124.4.2 Sublayers................................................................................................................134.4.3 Transparency..........................................................................................................14

5 WAVELENGTH DIVISION MULTIPLEXING (WDM)..............................................16

5.1 INTRODUCTION...............................................................................................................165.2 ADVANTAGES.................................................................................................................175.3 DEPLOYMENT.................................................................................................................19

5.3.1 North America........................................................................................................195.3.2 Europe....................................................................................................................195.3.3 Asia.........................................................................................................................205.3.4 Undersea networks.................................................................................................20

5.4 STANDARDIZATION.........................................................................................................215.5 NETWORK ARCHITECTURES............................................................................................23

5.5.1 WDM broadcast and select networks.....................................................................235.5.2 WDM wavelength routing networks.......................................................................24

5.6 KEY COMPONENTS..........................................................................................................255.6.1 Couplers.................................................................................................................275.6.2 Optical filters..........................................................................................................27

Gratings.............................................................................................................................................................28Bragg gratings...................................................................................................................................................28Fiber gratings.....................................................................................................................................................28Fabry-Perot filters.............................................................................................................................................28Multilayer dielectric thin-film filters.................................................................................................................29Mach-Zehnder interferometers..........................................................................................................................30Arrayed waveguide gratings..............................................................................................................................31Acousto-optic tunable filters.............................................................................................................................32

5.6.3 Circulators..............................................................................................................325.6.4 Optical amplifiers...................................................................................................33

Repeatered vs. amplified systems.....................................................................................................................33Erbium-doped fiber amplifiers..........................................................................................................................35Other types of optical amplifiers.......................................................................................................................36

5.7 FUTURE...........................................................................................................................365.7.1 Network architectures............................................................................................36

5.7.2 WDM in metropolitan area and access networks...................................................37

6 SUMMARY.........................................................................................................................38

7 REFERENCES....................................................................................................................39