Cisco ONS 15454 Engineering Planning Guide, Release 5.0
Appendix C, DWDM Primer
Downloads: This chapterpdf (PDF - 411.0KB) The complete bookPDF (PDF - 17.83MB) | Feedback

DWDM Primer

Table Of Contents

DWDM Primer

Comparison of SONET TDM and Wave Division Multiplexing (WDM)

Value of DWDM in the Metropolitan Area Network

DWDM System Functions

DWDM Components and Operation

Optical Fibers

Multimode and Single-Mode Fiber

Single-Mode Fiber Designs

Transmission Challenges

Attenuation

Fundamentals Technology Optical Fibers

Dispersion

Chromatic Dispersion

Polarization Mode Dispersion

Other Nonlinear Effects

Light Sources and Detectors

Light Emitters - LEDs and Lasers

Light Detectors

Optical Amplifiers

Erbium-Doped Fiber Amplifier (EDFA)

Constant Gain Mode

Gain Flatness

Transient Suppression

Low Noise

Saturation-Protection Internal VOA

DWDM Multiplexers and Demultiplexers

Techniques for Multiplexing and Demultiplexing

Optical Add/Drop Multiplexers

Interfaces to DWDM

Operation of a Transponder Based DWDM System

ITU Grid


DWDM Primer


This appendix provides an introduction into dense wave division multiplexing (DWDM) principles.

The following topics are covered in this chapter:

Comparison of SONET TDM and Wave Division Multiplexing (WDM)

Value of DWDM in the Metropolitan Area Network

DWDM System Functions

DWDM Components and Operation

Interfaces to DWDM

Comparison of SONET TDM and Wave Division Multiplexing (WDM)

SONET TDM takes synchronous and asynchronous signals and multiplexes them to a single higher bit rate for transmission at a single wavelength over fiber. Source signals may have to be converted from electrical to optical, or from optical to electrical and back to optical before being multiplexed. WDM takes multiple optical signals, maps them to individual wavelengths, and multiplexes the wavelengths over a single fiber. Another fundamental difference between the two technologies is that WDM can carry multiple protocols without a common signal format, while SONET cannot. Some of the key differences between TDM and WDM interfaces are graphically illustrated in Figure C-1.

Figure C-1 TDM and WDM Interfaces

Bandwidth, the chief driver in the long-haul market, is also a big driver in metropolitan area, access, and large enterprise networks. In these types of networks additional applications driving demand for bandwidth include storage area networks (SANs), which make possible the serverless office, consolidation of data centers, and real-time transaction processing backup.

Value of DWDM in the Metropolitan Area Network

DWDM has been very successful in the backbone. It was first deployed on long-haul routes in a time of fiber scarcity. Then the equipment savings made it the solution of choice for new long-haul routes, even when ample fiber was available. While DWDM can relieve fiber exhaust in the metropolitan area, its value in this market extends beyond this single advantage. Alternatives for capacity enhancement exist, such as pulling new cable and SONET overlays, but DWDM can do more. What delivers additional value in the metropolitan market is DWDM's fast and flexible provisioning of protocol- and bit rate-transparent, data-centric, protected services, along with the ability to offer new and higher-speed services at less cost.

The need to provision services of varying types in a rapid and efficient manner in response to the changing demands of customers is a distinguishing characteristic of the metropolitan networks. With SONET, which is the foundation of the vast majority of existing MANs, service provisioning is a lengthy and complex process. Network planning and analysis, ADM provisioning, Digital Cross-connect System (DCS) reconfiguration, path and circuit verification, and service creation can take several weeks. By contrast, with DWDM equipment in place provisioning new service can be as simple as turning on another lightwave in an existing fiber pair.

DWDM System Functions

At its core, DWDM involves a small number of physical-layer functions. These are depicted in Figure C-2, which shows a DWDM schematic for four channels. Each optical channel occupies its own wavelength.

Figure C-2 DWDM Functional Schematic

The system performs the following main functions:

Generating the signal - The source, a solid-state laser, must provide stable light within a specific, narrow bandwidth that carries the digital data, modulated as an analog signal.

Combining the signals - Modern DWDM systems employ multiplexers to combine the signals. There is some inherent loss associated with multiplexing and demultiplexing. This loss is dependent upon the number of channels but can be mitigated with optical amplifiers, which boost all the wavelengths at once without electrical conversion.

Transmitting the signals - The effects of cross-talk and optical signal degradation or loss must be reckoned with in fiber optic transmission. Controlling variables such as channel spacing, wavelength tolerance, and laser power levels can minimize these effects. Over a transmission link, the signal may need to be optically amplified.

Separating the received signals - At the receiving end, the multiplexed signals must be separated out. Although this task would appear to be simply the opposite of combining the signals, it is actually more technically difficult.

Receiving the signals - The demultiplexed signal is received by a photodetector.

In addition to these functions, a DWDM system must also be equipped with client-side interfaces to receive the input signal. This function may be performed by transponders. On the DWDM side are interfaces to the optical fiber that links DWDM systems.

DWDM Components and Operation

DWDM is a core technology in an optical transport network. The essential components of DWDM can be classified by their place in the system as follows:

On the transmit side, lasers with precise, stable wavelengths

On the link, optical fiber that exhibits low loss and transmission performance in the relevant wavelength spectra, in addition to flat-gain optical amplifiers to boost the signal on longer spans

On the receive side, photodetectors and optical demultiplexers using thin film filters or diffractive elements

Optical add/drop multiplexers and optical cross-connect components

These and other components, along with their underlying technologies, are discussed in the following sections.

Optical Fibers

The main job of optical fibers is to guide lightwaves with a minimum of attenuation (loss of signal). Optical fibers are composed of fine threads of glass in layers, called the core and cladding that can transmit light at about two-thirds the speed of light in a vacuum. Though admittedly an oversimplification, the transmission of light in optical fiber is commonly explained using the principle of total internal reflection. With this phenomenon, 100 percent of light that strikes a surface is reflected. By contrast, a mirror reflects about 90 percent of the light that strikes it.

Light is either reflected (it bounces back) or refracted (its angle is altered while passing through a different medium) depending upon the angle of incidence (the angle at which light strikes the interface between an optically denser and optically thinner material).

Total internal reflection happens when the following conditions are met:

Beams pass from a denser to a less dense material. The difference between the optical density of a given material and a vacuum is the material's refractive index.

The incident angle is less than the critical angle. The critical angle is the maximum angle of incidence at which light stops being refracted and is instead totally reflected.

The principle of total internal reflection within a fiber core is illustrated in Figure C-3. The core has a higher refractive index than the cladding, allowing the beam that strikes that surface at less than the critical angle to be reflected. The second beam does not meet the critical angle requirement and is refracted.

Figure C-3 Principle of Total Internal Reflection

An optical fiber consists of two different types of highly pure, solid glass (silica) - the core and the cladding-that are mixed with specific elements, called dopants, to adjust their refractive indices. The difference between the refractive indices of the two materials causes most of the transmitted light to bounce off the cladding and stay within the core. The critical angle requirement is met by controlling the angle at which the light is injected into the fiber. Two or more layers of protective coating around the cladding ensure that the glass can be handled without damage.

Multimode and Single-Mode Fiber

There are two general categories of optical fiber in use today, multimode and single-mode fiber. Multimode, the first type of fiber to be commercialized, has a larger core than single-mode fiber. It gets its name from the fact that numerous modes, or light rays, can be carried simultaneously through the waveguide. Figure C-4 shows an example of light transmitted in the first type of multimode fiber, called step-index. Step-index refers to the fact that there is a uniform index of refraction throughout the core; thus there is a step in the refractive index where the core and cladding interface. Notice that the two modes must travel different distances to arrive at their destinations. This disparity between the times that the light rays arrive is called modal dispersion. This phenomenon results in poor signal quality at the receiving end and ultimately limits the transmission distance. This is why multimode fiber is not used in wide-area applications.

Figure C-4 Reflected Light in Step-Index Multimode Fiber

To compensate for the dispersion drawback of step-index multimode fiber, graded-index fiber was invented. Graded-index refers to the fact that the refractive index of the core is graded - it gradually decreases from the center of the core outward. The higher refraction at the center of the core slows the speed of some light rays, allowing all the rays to reach their destination at about the same time and reducing modal dispersion.

The second general type of fiber, single-mode, has a much smaller core that allows only one mode of light at a time through the core (see Figure C-5). As a result, the fidelity of the signal is better retained over longer distances, and modal dispersion is greatly reduced. These factors attribute to a higher bandwidth capacity than multimode fibers are capable of. For its large information-carrying capacity and low intrinsic loss, single-mode fibers are preferred for longer distance and higher bandwidth applications, including DWDM.

Figure C-5 Reflected Light in Single-Mode Fiber

Single-Mode Fiber Designs

Designs of single-mode fiber have evolved over several decades. The three principle types and their ITU-T specifications are:

Non-dispersion-shifted fiber (NDSF), G.652

Dispersion-shifted fiber (DSF), G.653

Non-zero dispersion-shifted fiber (NZ-DSF), G.655

As discussed earlier, there are four windows within the infrared spectrum that have been exploited for fiber transmission. The first window, near 850 nm, was used almost exclusively for short-range, multimode applications. Non-dispersion-shifted fibers, commonly called standard single-mode (SM) fibers, were designed for use in the second window, near 1310 nm. To optimize the fiber's performance in this window, the fiber was designed so that chromatic dispersion would be close to zero near the 1310-nm wavelength.

As optical fiber use became more common and the needs for greater bandwidth and distance increased, a third window, near 1550 nm, was exploited for single-mode transmission. The third window, or C band, offered two advantages: it had much lower attenuation, and its operating frequency was the same as that of the new erbium-doped fiber amplifiers (EDFAs). However, its dispersion characteristics were severely limiting. This was overcome to a certain extent by using narrower line width and higher power lasers. But because the third window had lower attenuation than the 1310-nm window, manufacturers came up with the dispersion-shifted fiber design, which moved the zero-dispersion point to the 1550-nm region. Although this solution now meant that the lowest optical attenuation and the zero-dispersion points coincided in the 1550-nm window, it turned out that there are destructive nonlinearities in optical fiber near the zero-dispersion point for which there is no effective compensation. Because of this limitation, these fibers are not suitable for DWDM applications.

The third type, non-zero dispersion-shifted fiber, is designed specifically to meet the needs of DWDM applications. The aim of this design is to make the dispersion low in the 1550-nm region, but not zero. This strategy effectively introduces a controlled amount of dispersion, which counters nonlinear effects such as four-wave mixing that can hinder the performance of DWDM systems.

Table C-1 provides dispersion ratings for five commonly used fiber types.

Table C-1 Fiber Dispersion Characteristics

Fiber Type
Manufacturer
Chromatic Dispersion [ps/(nm x km)]
PMD (ps/km 1/2)

SMF-28

Corning

17.0

<0.2 (0.1 typical)

LEAF

Corning

2.0 - 6.0 (1530 - 1565)

<0.1 (0.04 typical)

Metrocore

Corning

-10.0 - -10.0 (1530 - 1605)

<0.2 (0.1 typical)

Allwave

Lucent

Unspecified

<0.1

TrueWave RS

Lucent

2.6 - 6.0 (1530 - 1565)

<0.1


Transmission Challenges

Transmission of light in optical fiber presents several challenges that must be dealt with. These fall into the following three broad categories:

Attenuation - decay of signal strength, or loss of light power, as the signal propagates through the Fiber.

Chromatic dispersion - spreading of light pulses as they travel down the fiber.

Nonlinearities - cumulative effects from the interaction of light with the material through which it travels, resulting in changes in the lightwave and interactions between lightwaves.

Each of these effects has several causes, not all of which affect DWDM. The discussion in the following sections addresses those causes that are relevant to DWDM.

Attenuation

Attenuation in optical fiber is caused by intrinsic factors, primarily scattering and absorption, and by extrinsic factors, including stress from the manufacturing process, the environment, and physical bending. The most common form of scattering, Rayleigh scattering, is caused by small variations in the density of glass as it cools. These variations are smaller than the wavelengths used and therefore act as scattering objects (see Figure C-6). Scattering affects short wavelengths more than long wavelengths and limits the use of wavelengths below 800 nm.

Figure C-6 Rayleigh Scattering

Fundamentals Technology Optical Fibers

The intrinsic properties of the material itself, the impurities in the glass, and any atomic defects in the glass cause attenuation due to absorption. These impurities absorb the optical energy, causing the light to become dimmer (see Figure C-7). While Rayleigh scattering is important at shorter wavelengths, intrinsic absorption is an issue at longer wavelengths and increases dramatically above 1700 nm. However, absorption due to water peaks introduced in the fiber manufacturing process is being eliminated in some new fiber types.

Figure C-7 Absorption

The primary factors affecting attenuation in optical fibers are the length of the fiber and the wavelength of the light. Figure C-8 shows the loss in decibels per kilometer (dB/km) by wavelength from Rayleigh scattering, intrinsic absorption, and total attenuation from all causes.

Figure C-8 Total Attenuation Curve

Attenuation in fiber is compensated primarily through the use of optical amplifiers.

Dispersion

Dispersion is the spreading of light pulses as they travel down optical fiber. Dispersion results in distortion of the signal (see Figure C-9), which limits the bandwidth of the fiber.

Figure C-9 Principle of Dispersion

Two general types of dispersion affect DWDM systems. One of these effects, chromatic dispersion, is linear while the other, polarization mode dispersion (PMD), is nonlinear.

Chromatic Dispersion

Chromatic dispersion occurs because different wavelengths propagate at different speeds. The effect of chromatic dispersion increases as the square of the bit rate. In single-mode fiber, chromatic dispersion has two components, material dispersion and waveguide dispersion.

Material dispersion occurs when wavelengths travel at different speeds through the material. A light source, no matter how narrow, emits several wavelengths within a range. Thus, when this range of wavelengths travels through a medium, each individual wavelength arrives at a different time.

The second component of chromatic dispersion, waveguide dispersion, occurs because of the different refractive indices of the core and the cladding of fiber. The effective refractive index varies with wavelength as follows:

At short wavelengths, the light is well confined within the core. Thus the effective refractive index is close to the refractive index of the core material.

At medium wavelengths, the light spreads slightly into the cladding. This decreases the effective refractive index.

At long wavelengths, much of the light spreads into the cladding. This brings the effective refractive index very close to that of the cladding.

This result of the phenomenon of waveguide dispersion is a propagation delay in one or more of the wavelengths relative to others.

Total chromatic dispersion, along with its components, is plotted by wavelength in Figure C-10 for dispersion-shifted fiber. For non-dispersion-shifted fiber, the zero dispersion wavelength is 1310 nm.

Figure C-10 Chromatic Dispersion

Though chromatic dispersion is generally not an issue at speeds below OC-48, it does increase with higher bit rates due to the spectral width required. New types of zero-dispersion-shifted fibers greatly reduce these effects. The phenomenon can also be mitigated with dispersion compensators.

Polarization Mode Dispersion

Most single-mode fibers support two perpendicular polarization modes, a vertical one and a horizontal one. Because these polarization states are not maintained, there occurs an interaction between the pulses that results is a smearing of the signal. Polarization mode dispersion (PMD) is caused by quality of the fiber shape as a result of the manufacturing process or from external stressors. Because stress can vary over time, PMD, unlike chromatic dispersion, is subject to change over time. PMD is generally not a problem at speeds below OC-192.

Other Nonlinear Effects

In addition to PMD, there are other nonlinear effects. Because nonlinear effects tend to manifest themselves when optical power is very high, they become important in DWDM.

Linear effects such as attenuation and dispersion can be compensated, but nonlinear effects accumulate. They are the fundamental limiting mechanisms to the amount of data that can be transmitted in optical fiber. The most important types of nonlinear effects are stimulated Brillouin scattering, stimulated Raman scattering, self-phase modulation, and four-wave mixing. In DWDM, four-wave mixing is most critical of these types.

Four-wave mixing is caused by the nonlinear nature of the refractive index of the optical fiber. Nonlinear interactions among different DWDM channels create sidebands that can cause interchannel interference. In Figure C-11 three frequencies interact to produce a fourth frequency, resulting in cross-talk and signal-to-noise degradation.

Figure C-11 Four-Wave Mixing

The effect of four-wave mixing is to limit the channel capacity of a DWDM system. Four-wave mixing cannot be filtered out, either optically or electrically, and increases with the length of the fiber. Due to its propensity for four-wave-mixing, DSF is unsuitable for WDM applications. This prompted the invention of NZ-DSF, which takes advantage of the fact that a small amount of chromatic dispersion can be used to mitigate four-wave mixing.

Light Sources and Detectors

Light emitters and light detectors are active devices at opposite ends of an optical transmission system. Light sources, or light emitters, are transmit-side devices that convert electrical signals to light pulses. The process of this conversion, or modulation, can be accomplished by externally modulating a continuous wave of light or by using a device that can generate modulated light directly. Light detectors perform the opposite function of light emitters. They are receive-side opto-electronic devices that convert light pulses into electrical signals.

Light Emitters - LEDs and Lasers

The light source used in the design of a system is an important consideration because it can be one of the most costly elements. Its characteristics are often a strong limiting factor in the final performance of the optical link. Light emitting devices used in optical transmission must be compact, monochromatic, stable, and long-lasting.


Note Monochromatic is a relative term; in practice they are only light sources within a certain range. Stability of a light source is a measure of how constant its intensity and wavelength is.


Two general types of light emitting devices are used in optical transmission, light-emitting diodes (LEDs) and laser diodes, or semiconductor lasers. LEDs are relatively slow devices, suitable for use at speeds of less than 1 Gb/s, they exhibit a relatively wide spectrum width, and they transmit light in a relatively wide cone. These inexpensive devices are often used in multimode fiber communications. Semiconductor lasers, on the other hand, have performance characteristics better suited to single-mode fiber applications.

Figure C-12 shows the general principles of launching laser light into fiber. The laser diode chip emits light in one direction to be focused by the lens onto the fiber and in the other direction onto a photodiode. The photodiode, which is angled to reduce back reflections into the laser cavity, provides a way of monitoring the output of the lasers and providing feedback so that adjustments can be made.

Figure C-12 Typical Laser Design

Requirements for lasers include precise wavelength, narrow spectrum width, sufficient power, and control of chirp (the change in frequency of a signal over time). Semiconductor lasers satisfy nicely the first three requirements. Chirp, however, can be affected by the means used to modulate the signal.

In directly modulated lasers, the modulation of the light to represent the digital data is done internally. With external modulation, an external device does the modulation. When semiconductor lasers are directly modulated, chirp can become a limiting factor at high bit rates (above 10 Gb/s). External modulation, on the other hand, helps to limit chirp. The external modulation scheme is depicted in Figure C-13.

Figure C-13 External Modulation of a Laser

Two types of semiconductor lasers are widely used, monolithic Fabry-Perot lasers, and distributed feedback (DFB) lasers. The latter type is particularly well suited for DWDM applications, as it emits a nearly monochromatic light, is capable of high speeds, has a favorable signal-to-noise ratio, and has superior linearity. DFB lasers also have center frequencies in the region around 1310 nm, and from 1520 to 1565 nm. The latter wavelength range is compatible with EDFAs. There are many other types and subtypes of lasers. Narrow spectrum tunable lasers are available, but their tuning range is limited to approximately 100-200 GHz. Under development are wider spectrum tunable lasers, which will be important in dynamically switched optical networks.

Light Detectors

On the receive end, it is necessary to recover the signals transmitted at different wavelengths on the fiber. Because photodetectors are by nature wideband devices, the optical signals are demultiplexed before reaching the detector.

Two types of photodetectors are widely deployed, the positive-intrinsic-negative (PIN) photodiode and the avalanche photodiode (APD). PIN photodiodes work on principles similar to, but in the reverse of, LEDs. That is, light is absorbed rather than emitted, and photons are converted to electrons in a 1:1 relationship. APDs are similar devices to PIN photodiodes, but provide gain through an amplification process: One photon acting on the device releases many electrons. PIN photodiodes have many advantages, including low cost and reliability, but APDs have higher receive sensitivity and accuracy.

However, APDs are more expensive than PIN photodiodes, they can have very high current requirements, and they are temperature sensitive.

Optical Amplifiers

Due to attenuation, there are limits to how long a fiber segment can propagate a signal with integrity before it has to be regenerated. Before the arrival of optical amplifiers (OAs), there had to be a repeater for every signal transmitted. The OA has made it possible to amplify all the wavelengths at once and without optical-electrical-optical (OEO) conversion. Besides being used on optical links, optical amplifiers also can be used to boost signal power after multiplexing or before demultiplexing, both of which can introduce loss into the system.

Erbium-Doped Fiber Amplifier (EDFA)

By making it possible to carry the large loads that DWDM is capable of transmitting over long distances, the EDFA was a key enabling technology. At the same time, it has been a driving force in the development of other network elements and technologies.

Erbium is a rare-earth element that, when excited, emits light around 1.54 micrometers - the low-loss wavelength for optical fibers used in DWDM. Figure C-14 shows a simplified diagram of an EDFA. A weak signal enters the erbium-doped fiber, into which light at 980 nm or 1480 nm is injected using a pump laser. This injected light stimulates the erbium atoms to release their stored energy as additional 1550-nm light. As this process continues down the fiber, the signal grows stronger. The spontaneous emissions in the EDFA also add noise to the signal; this determines the noise figure of an EDFA.

Figure C-14 Erbium-Doped Amplifier Design

The key performance parameters of optical amplifiers are gain, gain flatness, noise level, and output power. EDFAs are typically capable of gains of 30 dB or more and output power of +17 dB or more. The target parameters when selecting an EDFA, however, are low noise and flat gain. Gain should be flat, because all signals must be amplified uniformly. While the signal gain provided with EDFA technology is inherently wavelength-dependent, it can be corrected with gain flattening filters. Such filters are often built into modern EDFAs.

Low noise is a requirement, because noise along with signal, is amplified. Because this effect is cumulative and cannot be filtered out, the signal-to-noise ratio is an ultimate limiting factor in the number of amplifiers that can be concatenated. In general, signals can travel for up to 120 km (74 mi) between amplifiers. At longer distances of 600 to 1000 km (372 mi to 620 mi) the signal must be regenerated. That is because the optical amplifier merely amplifies the signals and does not perform the 3R functions (reshape, retime, retransmit). EDFAs are available for the C-band and the L-band.

Constant Gain Mode

Constant amplification per wavelength is important for bandwidth-on-demand wavelength services. As wavelengths are added/dropped from an optical fiber, small variations in gain between channels in a span can cause large variations in the power difference between channels at the receivers. Constant gain mode is achieved using an automatic control circuit that adjusts pump power when changes in input power are detected.

Gain Flatness

Figure C-15 illustrates the importance of an EDFAs gain-flattening filter. With the first fiber (a), channels having equal power going into a cascaded network of amplifiers have vastly different powers and optical signal-to-noise ratio (SNR) at the output - without a gain flattening filter. In contrast, with the second fiber (b), the EDFAs reduce this effect by introducing a gain-flattening filter within each amplifier.

Figure C-15 Gain Flattening Filter

Transient Suppression

Transients in the performance of EDFAs are inevitable whenever the number of signals or the relative power of signals change. The amount of time required by an amplifier to recover from a change indicates the suitability of the amplifier for add/drop applications. Some EDFAs can reconfigure rapidly to ensure constant gain and gain flatness. The lower transient suppression implied on the lower transient delay makes it suitable for dynamic channel addition and subtraction (add/drop).

Low Noise

Noise increases whenever a gain occurs in an optical system. The predominant source of noise in EDFAs is Amplified Spontaneous Emission (ASE). An EDFA with a low-noise figure of < 6.0 dB ensures better OSNR performance for cascaded amplified networks.

Saturation-Protection Internal VOA

Saturation-protection internal VOA is an internal variable optical attenuator that is placed before the EDFA to attenuate the channel and composite power going into the amplifier gain block. The purpose of the VOA is to protect the EDFA from being driven into saturation. The VOA can be adjusted from 1 dB to 10 dB. Since the EDFA saturation input power is -6 dBm, the internal VOA allows a higher-power input to the amplifier with higher power (up to +4 dBm more). The VOA can be adjusted through software to control the gain block input to -6 dBm or less. For conditions where the gain block is in the normal operating region (i.e. non-saturated), some EDFAs can operate as a variable-gain amplifier.

DWDM Multiplexers and Demultiplexers

Because DWDM systems send signals from several sources over a single fiber, they must include some means to combine the incoming signals. This is done with a multiplexer, which takes optical wavelengths from multiple fibers and converges them into one beam. At the receiving end the system must be able to separate out the components of the light so that they can be discreetly detected. Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them to individual fibers. Demultiplexing must be done before the light is detected, because photodetectors are inherently broadband devices that cannot selectively detect a single wavelength.

In a unidirectional system (see Figure C-16), there is a multiplexer at the sending end and a demultiplexer at the receiving end. Two systems (back-to-back terminals) would be required at each end for bi-directional communication, and two separate fibers would be needed.

Figure C-16 Multiplexing and Demultiplexing in a Unidirectional System

In a bi-directional system, there is a multiplexer/demultiplexer at each end (see Figure C-17) and communication is over a single fiber, with different wavelengths used for each direction.

Figure C-17 Multiplexing and Demultiplexing in a Bidirectional System

Multiplexers and demultiplexers can be either passive or active in design. Passive designs are based on prisms, diffraction gratings, or filters, while active designs combine passive devices with tunable filters. The primary challenges in these devices are to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength.

Techniques for Multiplexing and Demultiplexing

A simple form of multiplexing or demultiplexing of light can be done using a prism. Figure C-18 demonstrates the demultiplexing case. A parallel beam of polychromatic light impinges on a prism surface; each component wavelength is refracted differently. This is the "rainbow" effect. In the output light, each wavelength is separated from the next by an angle. A lens then focuses each wavelength to the point where it needs to enter a fiber. The same components can be used in reverse to multiplex different wavelengths onto one fiber.

Figure C-18 Prism Diffraction Multiplexing

Another technology is based on the principles of diffraction and of optical interference. When a polychromatic light source impinges on a diffraction grating (see Figure C-19), each wavelength is diffracted at a different angle and therefore to a different point in space. Using a lens, these wavelengths can be focused onto individual fibers.

Figure C-19 Waveguide Grating Diffraction

Arrayed waveguide gratings (AWGs) are also based on diffraction principles. An AWG device, sometimes called an optical waveguide router or waveguide grating router, consists of an array of curved-channel waveguides with a fixed difference in the path length between adjacent channels (see Figure C-20). The waveguides are connected to cavities at the input and output. When the light enters the input cavity, it is diffracted and enters the waveguide array. There the optical length difference of each waveguide introduces phase delays in the output cavity, where an array of fibers is coupled. The process results in different wavelengths having maximal interference at different locations, which correspond to the output ports.

Figure C-20 Arrayed Waveguide Grating

By positioning filters, consisting of thin films, in the optical path, wavelengths can be sorted out (demultiplexed). The property of each filter is such that it transmits one wavelength while reflecting others. By cascading these devices, many wavelengths can be demultiplexed (see Figure C-21).

Figure C-21 Multi-Layer Interference Filters

Of these designs, the AWG and thin film interference filters are gaining prominence. Filters offer good stability and isolation between channels at moderate cost, but with a high insertion loss. AWGs are polarization-dependent (which can be compensated), and they exhibit a flat spectral response and low insertion loss. A potential drawback is that they are temperature sensitive such that they may not be practical in all environments. Their big advantage is that they can be designed to perform multiplexing and demultiplexing operations simultaneously. AWGs are also better for large channel counts, where the use of cascaded thin film filters is impractical.

Optical Add/Drop Multiplexers

Between multiplexing and demultiplexing points in a DWDM system, as shown in Figure C-17, there is an area in which multiple wavelengths exist. It is often desirable to be able to remove or insert one or more wavelengths at some point along this span. An optical add/drop multiplexer (OADM) performs this function. Rather than combining or separating all wavelengths, the OADM can remove some while passing others on. OADMs are a key part of moving toward the goal of all-optical networks.

OADMs are similar in many respects to SONET ADM, except that only optical wavelengths are added and dropped, and no conversion of the signal from optical to electrical takes place. Figure C-22 is a schematic representation of the add-drop process. This example includes both pre- and post-amplification; these components that may or may not be present in an OADM, depending upon its design.

Figure C-22 Selectively Adding and Removing Wavelengths

Interfaces to DWDM

Most DWDM systems support standard SONET/SDH optical interfaces to which any SONET compliant client device can attach. On the client side there can be SONET/SDH terminals or ADMs, ATM switches, or routers. Transponders are used to convert incoming optical signals into the precise ITU-standard wavelengths to be multiplexed.

Within the DWDM system a transponder converts the client optical signal back to an electrical signal and performs the 3R functions (see Figure C-23). This electrical signal is then used to drive the WDM laser. Each transponder within the system converts its client's signal to a slightly different wavelength. The wavelengths from all of the transponders in the system are then optically multiplexed.

In the receive direction of the DWDM system, the reverse process takes place. Individual wavelengths are filtered from the multiplexed fiber and fed to individual transponders, which convert the signal to electrical and drive a standard interface to the client.

Figure C-23 Transponder Functions

Using the ONS 15454 with its OC48ELR ITU optics cards reduces or eliminates the need for transponders. This architecture provides a cost-effective solution for Metro DWDM network applications.

Operation of a Transponder Based DWDM System

Some DWDM systems transponders are optical-electrical-optical (OEO) devices that transforms (maps) an incoming wavelength into a DWDM wavelength. Using the ONS 15454 OC48ELR ITU optical cards reduces or eliminates (based on your channel plan) the need for transponders. Figure C-24 shows a DWDM system with transponders.

Figure C-24 DWDM System with Transponders

The following steps describe the system shown in Figure C-24:

1. The transponder accepts input in the form of standard single-mode or multimode laser. The input can come from different physical media and different protocols and traffic types.

2. The wavelength of each input signal is mapped to a DWDM wavelength.

3. DWDM wavelengths from the transponder are multiplexed into a single optical signal and launched into the fiber. The system might also include the ability to accept direct optical signals to the multiplexer; such signals could come, for example, from a satellite node.

4. A post-amplifier boosts the strength of the optical signal as it leaves the system (optional).

5. Optical amplifiers are used along the fiber span as needed (optional).

6. A pre-amplifier boosts the signal before it enters the end system (optional).

7. The incoming signal is demultiplexed into individual DWDM lambdas (or wavelengths).

8. The individual DWDM lambdas are mapped to the required output type (for example, OC-48 single-mode fiber) and sent out through the transponder.

ITU Grid

For WDM system interoperability, the operating center frequency (wavelength) of channels must be the same at the transmitting and at the receiving end. The ITU-T currently recommends 81 channels in the C-band starting from 1528.77 nm, and incrementing in multiples of 50 GHz, to 1560.61 nm. Table C-2 lists the ITU frequencies and wavelengths.

Table C-2 ITU Grid 

Frequency (GHz)
Wavelength (nm)
Frequency (GHz)
Wavelength (nm)
Frequency (GHz)
Wavelength (nm)
Frequency (GHz)
Wavelength (nm)

196.100

1528.77

195.050

1537.00

194.000

1545.32

192.950

1553.73

196.075

1528.97

195.025

1537.20

193.975

1545.52

192.925

1553.93

196.050

1529.16

195.000

1537.40

193.950

1545.72

192.900

1554.13

196.025

1529.36

194.975

1537.59

193.925

1545.92

192.875

1554.34

196.000

1529.55

194.950

1537.79

193.900

1546.12

192.850

1554.54

195.975

1529.75

194.925

1537.99

193.875

1546.32

192.825

1554.74

195.950

1529.94

194.900

1538.19

193.850

1546.52

192.800

1554.94

195.925

1530.14

194.875

1538.38

193.825

1546.72

192.775

1555.14

195.900

1530.33

194.850

1538.58

193.800

1546.92

192.750

1555.34

195.875

1530.53

194.825

1538.78

193.775

1547.12

192.725

1555.55

195.850

1530.72

194.800

1538.98

193.750

1547.32

192.700

1555.75

195.825

1530.92

194.775

1539.17

193.725

1547.52

192.675

1555.95

195.800

1531.12

194.750

1539.37

193.700

1547.72

192.650

1556.15

195.775

1531.31

194.725

1539.57

193.675

1547.92

192.625

1556.35

195.750

1531.51

194.700

1539.77

193.650

1548.11

192.600

1556.55

195.725

1531.70

194.675

1539.96

193.625

1548.31

192.575

1556.76

195.700

1531.90

194.650

1540.16

193.600

1548.51

192.550

1556.96

195.675

1532.09

194.625

1540.36

193.575

1548.71

192.525

1557.16

195.650

1532.29

194.600

1540.56

193.550

1548.91

192.500

1557.36

195.625

1532.49

194.575

1540.76

193.525

1549.11

192.475

1557.57

195.600

1532.68

194.550

1540.95

193.500

1549.32

192.450

1557.77

195.575

1532.88

194.525

1541.15

193.475

1549.52

192.425

1557.97

195.550

1533.07

194.500

1541.35

193.450

1549.72

192.400

1558.17

195.525

1533.27

194.475

1541.55

193.425

1549.92

192.375

1558.38

195.500

1533.47

194.450

1541.75

193.400

1550.12

192.350

1558.58

195.475

1533.66

194.425

1541.94

193.375

1550.32

192.325

1558.78

195.450

1533.86

194.400

1542.14

193.350

1550.52

192.300

1558.98

195.425

1534.05

194.375

1542.34

193.325

1550.72

192.275

1559.19

195.400

1534.25

194.350

1542.54

193.300

1550.92

192.250

1559.39

195.375

1534.45

194.325

1542.74

193.275

1551.12

192.225

1559.59

195.350

1534.64

194.300

1542.94

193.250

1551.32

192.200

1559.79

195.325

1534.84

194.275

1543.13

193.225

1551.52

192.175

1560.00

195.300

1535.04

194.250

1543.33

193.200

1551.72

192.150

1560.20

195.275

1535.23

194.225

1543.53

193.175

1551.92

192.125

1560.40

195.250

1535.43

194.200

1543.73

193.150

1552.12

192.100

1560.61

192.225

1535.63

194.175

1543.93

193.125

1552.32

192.075

1560.81

192.200

1535.82

194.150

1544.13

193.100

1552.52

192.050

1561.01

192.175

1536.02

194.125

1544.33

193.075

1552.73

192.025

1561.22

192.150

1536.22

194.100

1544.53

193.050

1552.93

191.000

1561.42

192.125

1536.41

194.075

1544.72

193.025

1553.13

191.975

1561.62

192.100

1536.61

194.050

1544.92

193.000

1553.33

191.950

1561.83

192.075

1536.81

194.025

1545.12

192.975

1553.53

191.925

1562.03

           

191.900

1562.23


While this grid defines a standard, users are free to use the wavelengths in arbitrary ways and to choose from any part of the spectrum. In addition, manufacturers can deviate from the grid by extending the upper and lower bounds or by spacing the wavelengths more closely, typically at 50 GHz, to double the number of channels. The closer the spacing, the more channel cross-talk results. In addition, the impact of some fiber nonlinearities, such as FWM, increases. Spacing at 50 GHz also limits the maximum data rate per wavelength to 10 Gb/s. The implications of the flexibility in implementation are twofold:

There is no guarantee of compatibility between two end systems from different vendors.

There exists a design trade-off in the spacing of wavelengths between number of channels and maximum bit rate.