DWDM principle

DWDM Principle

Contents Section 1 DWDM Overview 1.1 DWDM Technology Background 1.2 DWDM Principles Overview 1.3 DWDM Equipment Operating Modes 1.3.1 Two-fiber bi-directional transmission 1.3.2 Single fiber bi-directional transmission 1.3.3 Add and drop of optical signals 1.4 Application Modes of DWDM 1.5 Advantages of DWDM Section 2 DWDM Transmission Media 2.1 Optical Fiber Structures 2.2 Types of Optical Fiber 2.3 Basic Features of Optical Fiber 2.3.1 Physical Dimension (Mode field diameter) 2.3.2 Mode Field Concentricity Error 2.3.3 Bend Loss 2.3.4 Attenuation Constant 2.3.5 Dispersion Coefficient 2.3.6 Cutoff Wavelength 2.4 Types and Properties of Optical Fiber Cable 2.4.1 Types of Optical Fiber Cable 2.4.2 Properties of Optical Fiber Cable Section 3 DWDM Key Technologies 3.1 Lasers 3.1.1 Laser Modulation Modes 3.1.2 Wavelength Stability and Control of Laser 3.2 Erbium-doped Optical Fiber Amplifier (EDFA) 3.2.1 EDFA Operating Principle 3.2.2 Applications of EDFA 3.2.3 Gain Control of EDFA 3.2.4 Limitations of EDFA 3.3 DWDM Components 3.3.1 Optical Grating Type DWDM Component 3.3.2 Dielectric Film Type DWDM Component 3.3.3 Fused Conical Type DWDM Component 3.3.4 Integrated Optical Waveguide Type DWDM Component 3.3.5 Performances of DWDM Components i

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DWDM Principle

Section 4 DWDM Networking Design 4.1 Some Network Element Types of DWDM 4.1.1 Optical Terminal Equipment (OTM) 4.1.2 Optical Line Amplification Unit (OLA) 4.1.3 Optical Add/drop Multiplexing Unit (OADM) 4.1.4 Electrical Regeneration Unit (REG) 4.2 General Constitution of DWDM network 4.2.1 Point-to-point Networking 4.2.2 Chain Type Networking 4.2.3 Ring Type Networking 4.2.4 Network Management Information Channel Backup and Interconnection Capability 4.3 Factors To Be Considered in DWDM Networking 4.3.1 Dispersion Limited Distance 4.3.2 Power 4.3.3 Optical Signal-to-Noise Ratio 4.3.4 Other Factors 4.4 DWDM Network Protection 4.4.1 Protection Based on single Wavelength 4.4.2 Optical Multiplex Section (OMSP) Protection 4.4.3 Applications in Ring Networks 4.5 Analysis to The Examples 4.5.1 Networking Diagram (Physical Network Stations) 4.5.2 Networking Diagram (considering the dispersion limited distance of the lasers to divide the regenerator sections of the network) 4.5.3 Networking Diagram (considering the power of optical amplifiers to divide the optical regenerator sections) 4.5.4 Networking Diagram (considering OSNR)

ii

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DWDM Principle

Section 1 DWDM Overview

Section 1 DWDM Overview

Objectives: To master the concepts of DWDM. To know the background and technology characteristics of DWDM.

1.1 DWDM Technology Background With the dramatic increase of voice services and emergence of various new services, especially the quick change of IP technology, network capacity will inevitably be faced with critical challenge. Traditional methods for transmission network capacity expansion adopt space division multiplexing (SDM) or time division multiplexing (TDM). 1. Space Division Multiplexing (SDM) Space division multiplexing linearly expands the transmission capacity by adding fibers, and the transmission equipment is also linearly added. At present, fiber manufacture technology is quite mature. Ribbon optical fiber cables with tens of cores are rather prevalent and advanced connection technique for optical fiber simplifies cable construction. However, the increment of fibers brings much inconvenience to the construction and circuit maintenance in the future. Additionally, if the existing optical fiber cable lines have no sufficient fibers and require to lay new fiber cables for capacity expansion, engineering cost will increase in duplication. Moreover, this method doesn't sufficiently utilize the transmission bandwidth of the optical fiber and wastes the bandwidth resources. It's not always possible to lay new optical fibers to expand the capacity during the construction of communication networks. Actually, in the initial stage of the project, it is hard to predict the ever-growing service demands and to plan the number of fibers to lay. Hence, SDM method for capacity expansion is quite limited.

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DWDM Principle

Section 1 DWDM Overview

2. Time Division Multiplexing (TDM) TDM is a commonly used method for capacity expansion, e.g. multiplexing of the primary group to the fourth group of the traditional PDH, and STM-1, STM-4, STM-16 and STM-64 of current SDH. TDM technology can enhance the capacity of optical transmission information in duplication and greatly reduce the circuit cost in equipment and line. Moreover, it is easy to extract specific digital signals from the data stream via this multiplexing method. It is especially suitable for networks requiring the protection strategy of self-healing rings. However, TDM method has two disadvantages. Firstly, it affects services. An overall upgrade to higher rate levels requires to replace the network interfaces and equipment completely. Thus the equipment in operation must be interrupted during the upgrade process. Secondly, rate upgrade lacks of flexibility. Let's take SDH as an example, when a system with a line rate of 155Mbit/s is required to provide two 155Mbit/s channels, the only way is to upgrade the system to 622Mbit/s even though two 155Mbit/s are unused. For TDM equipment of higher rate, the cost is relatively high. Furthermore, 40Gbit/s TDM equipment has already reached the rate limitation of electronic devices. Even the nonlinear effects of 10Gbit/s rate in different fiber types will set various limitations to transmission. Currently, TDM is a commonly used capacity expansion method. It can implement capacity expansion via continuous system rate upgrade. When certain rate level is reached, other solutions must be found because of characteristic limitations of devices, lines, etc. All the basic transmission networks, whether using SDM or TDM to expand the capacity, adopt traditional PDH or SDH technology, i.e. utilizing optical signals on a single wavelength for transmission. This transmission method is a great waste of optical capacity because the bandwidth of optical fiber is almost infinite when compared to the single wavelength channel we currently use. We are worrying about the jam of networks, on the other hand huge network resources are being wasted. DWDM technology emerged under this background. It greatly increases the network capacity, makes full use of the bandwidth resources of optical fibers and cuts down the waste of network resources.

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DWDM Principle

Section 1 DWDM Overview

1.2 DWDM Theory Overview DWDM technology utilizes the bandwidth and low attenuation characteristics of single mode optical fiber, adopts multiple wavelengths as carriers and allows them to transmit in the fiber simultaneously. When compared to common single channel systems, dense-WDM (DWDM) greatly increases the network capacity and makes full use of the bandwidth resources of optical fibers. Moreover, DWDM has many advantages such as simple capacity expansion and reliable performances. Especially, it can access various types of services and this gives it a bright prospective of application. In analog carrier communication systems, the frequency division multiplexing (FDM) method is often adopted to make full use of the bandwidth resources of cables and enhance the transmission capacity of the system, i.e. transmitting several channels of signals simultaneously in a single cable and, at the receiver end, utilizing band-pass filters to filter the signal on each channel according to the frequency differences among the carriers. Similarly, in optical fiber communication systems, optical frequency division multiplexing method can also be used to enhance the transmission capacity of the systems. In fact, this multiplexing method is very effective in optical communication systems. Unlike the frequency division multiplexing in analog carrier communication systems, optical fiber communication systems utilize optical wavelengths as signal carriers, divide the low attenuation window of optical fibers into several channels according to the frequency (or wavelength) difference of each wavelength channel and implement multiplexing transmission of multi-hannel optical signals in a single fiber. Since some optical components (such as narrow-bandwidth optical filters and coherent lasers) are currently not mature, it is difficult to implement ultra-dense optical frequency division multiplexing (coherent optical communication technology) of optical channels. However, alternate-channel optical frequency division multiplexing can be implemented based on current component technical level. Usually, multiplexing with a larger channel spacing (even in different windows of optical fibers) is called optical wavelength division multiplexing (WDM), and WDM in the same window with smaller channel spacing is called dense wavelength division multiplexing (DWDM). With the progress of technologies, nanometer level multiplexing can be implemented by using modern technologies. Even sub-nanometer level multiplexing can be implemented but merely with stricter component technical requirements. Hence, multiplexing of 8, 16, 32 or more wavelengths with smaller wavelength spacing is called DWDM.

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DWDM Principle

Section 1 DWDM Overview

The diagram of DWDM system structure and optical spectrum is shown in Figure 1-1. At the transmit end, optical transmitters output optical signals of different wavelengths whose accuracy and stability meet certain requirements. These signals are multiplexed via an optical wavelength multiplexer and sent to an erbium-doped optical fiber power amplifier (it is mainly used to compensate the power loss aroused by the multiplexer and enhance the launched power of the optical signals). After amplification, this multi-channel optical signal is sent to the optical fiber for transmitting. In the midway optical line amplifiers can be installed or not according to practical conditions. At the receiver end, the signals are amplified by the optical pre-amplifier (it is mainly used to enhance receiving sensitivity and prolong transmission distance) and sent to the optical wavelength de-multiplexer which separates them into the original multi-channel optical signals.

OTU

D M U

M O TU

U X

X OTU

Optical spectrum

Optical line amplifier

Optical booster amplifier

Single channel

Optical line amplifier

Optical pre-amplifier

Optical spectrum

Wavelength

Figure 1-1 The diagram of DWDM system structure and spectrum

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Wavelength

DWDM Principle

Section 1 DWDM Overview

1.3 DWDM Equipment Operating Modes 1.3.1 Two-fiber Bi-directional Transmission As shown in Figure 1-2, a single optical fiber implements only one directional transmission of optical signals. Hence the same wavelengths can be reused in two directions.

1

Detector 1

Optical source 1

WDM

Optical source N

N+1

Detector N

1 N

Detector 1

Optical source 1

WDM

Detector N

WDM 1 N

Optical source N

Figure 1-2 Two-fiber bi-directional transmission DWDM system

This kind of DWDM system can effectively exploit the huge bandwidth resources of optical fiber and expand the transmission capacity of a single optical fiber in several or tens of times. In long-haul networks, capacity can be expanded by adding wavelengths gradually according to the demands of practical traffic, which is very flexible. This is, under the condition that the actual fiber dispersion isn't known, also an approach to use multiple 2.5Gbit/s systems to implement ultra-large capacity transmission, avoiding adopting ultrahigh speed optical systems.

1.3.2 Single fiber Bi-directional Transmission As shown in Figure 1-3, a single fiber transmits optical signals of two directions simultaneously, and the signals in the two different directions should be assigned on different wavelengths.

1-7

N

N+1





2N

WDM





N

1

2N

DWDM Principle

Section 1 DWDM Overview

N

Optical source N

Detector 1

N+1

A single optical fiber

Detector N

WDM

Optical source N+1

N+1 2N

N+1





2N

N

WDM 1 N

Detector N+1

1



Optical source 1



1

Detector 2N

Optical source 2N

Figure 1-3 The DWDM system which adopts single fiber bi-directional transmission

Single fiber bi-directional transmission allows a single fiber to carry full duplex channels and, generally, saves one half of the fiber components of unidirectional transmission. Since signals transmitted in the two directions do not interact and create FWM (Four-Wave Mixing) products, its total FWM products are much less than two-fiber unidirectional transmission. However, the disadvantage of this system is that it requires a special measure to deal with the light reflection (including discrete reflection resulted by optical connectors and Rayleigh backward reflection of the fiber) to avoid multi-path interference. When the optical signal needs to be amplified to elongate prolong transmission distance, components such as bi-directional optical fiber amplifier and optical circulator must be adopted, but their noise factor is a little worse.

1.3.3 Add and Drop of Optical Signals

1 N OADM

Detector1

Optical source 1

1-8

OADM

Detector2

Optical source 2

2N

DWDM Principle

Section 1 DWDM Overview

Figure 1-4 Optical add and drop transmission

By utilizing optical add/drop multiplexer (OADM), optical signals of the wavelengths can be added and dropped in the intermediate stations, i.e. implementing add/drop of optical paths. This method can be used to implement ring type networking of DWDM systems. At present, OADM can only be made as fixed wavelength add/drop device (as shown in Figure 1-4) and thus the flexibility of this operating mode is limited.

1.4 Application Modes of DWDM Generally, DWDM has two application modes: ⌧ Open DWDM ⌧ Integrated DWDM

The feature of open DWDM system is that it has no special requirements for multiplex terminal optical interfaces as long as they meet the optical interface standards defined in ITU-T G.957. The DWDM system adopts wavelength conversion technology to convert the optical signal of multiplex terminal into specific wavelength. Optical signals from different terminal equipment are converted into different wavelengths meeting the ITU-T recommendation, then multiplexed. Integrated DWDM system, without adopting wavelength conversion technology, requires that the optical signal wavelengths of the multiplex terminal meets DWDM system specifications. Different multiplex terminal transmits different wavelengths meeting the ITU-T recommendation. Thus, when connected to the multiplexer, these wavelengths occupy different channels and multiplexing is implemented. Different application modes can be adopted according to the demands of engineering. In practical applications, open DWDM and integrate DWDM can be mixed.

1.5 Advantages of DWDM The capacity of optical fiber is huge. However, traditional optical fiber communication systems, with one optical signal in a single fiber, only exploited a little part of the abundant bandwidth of optical fiber. To effectively use the huge bandwidth resources of optical fiber and increase its transmission capacity, a new

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DWDM Principle

Section 1 DWDM Overview

generation optical fiber communication technology based on dense-WDM (DWDM) has emerged. DWDM technology has the following features: 1. Ultra-large capacity The transmittable bandwidth of currently commonly used conventional fiber is very wide, but the utilization ratio is still low. By using DWDM technology, the transmission capacity of a single optical fiber is increased by several, tens of or even hundreds of times when compared to the transmission capacity of single wavelength systems. Recently, NEC Company, Japan, implemented a 132×20Gbit/s experimental DWDM system with a transmission distance of 120km. This system, with a total bandwidth of 35nm (1529nm~1564nm) and a channel spacing of 33GHz, can transmit 40 million telephone calls. 2. Data rate "transparency" DWDM systems conduct multiplexing and de-multiplexing in terms of optical wavelength differences and are independent to signal rates and modulation modes, i.e. transparent to the data. Hence, they can transmit signals with completely different transmission characteristics and implement combination and separation of various electrical signals, including digital signals and analog signals, and PDH signals and SDH signals. 3. Utmost protection of the existing investment during system upgrade During the expansion and development of the network, it is an ideal approach to implement capacity expansion without the need to rebuild the optical fiber cables and with the only requirement of replacing the optical transmitters and receivers. This is also a convenient way to introduce broad-band services (such as CATV, HDTV and B-ISDN). Furthermore, any new services or new capacity can be introduced simply by adding an additional wavelength. 4. High flexibility, economy and reliability of networking When compared to the traditional networks using electrical TDM networks, new communication networks based on DWDM technology are greatly simplified in architecture and have clear network layers. Dispatching of various services can be implemented simply by adjusting the corresponding wavelengths of the optical signals. Because of the simple network architecture, clear layers and convenient service grooming, the flexibility, economy and reliability of networking are obvious. 1-10

DWDM Principle

Section 1 DWDM Overview

5. Compatibility with all optical switching It is foreseeable that, in the realizable all optical networks in the future, processing such as add/drop and connection of all telecommunication services is implemented by changing and adjusting the optical signal wavelengths. So DWDM technology is one of the key technologies to implement all optical networks. Moreover, DWDM systems can be compatible with future all optical networks. It is possible to implement transparent and highly survivable all optical networks based on the existing DWDM system.

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DWDM Principle

Section 2 DWDM Transmission Media y

Section 2 DWDM Transmission Media

Objectives: To master basic structures and types of optical fibers. To know basic characteristics of optical fibers.

2.1 Optical Fiber Structures The kernel of optical fiber used in communication systems consists of a cylindrical glass core and a glass cladding. The outermost layer is a plastic wear-resisting coating. The whole fiber is cylindrical. The typical structure of optical fiber is shown in Figure 2-1.

Coating

Cladding

n2

Figure 2-1 The typical structure of optical fiber

2-12

n1

Core

Section 2 DWDM Transmission Media y

n2 n1

2b 2a

DWDM Principle

2b 2a

n2 n1

2b 2a

n2 n(r)

Figure 2-2 Three typical types of optical fibers

Thickness of the core and refractive indexes of the core material and cladding material are critical to the properties of the fiber. Figure 2-2 shows three typical optical fibers. As can be seen from this figure, there are two typical refractive index distributions in the fiber core-cladding cross-section. One is that the refractive index radial distributions of the core and the cladding are uniform, and the change of refractive index at the core-cladding boundary is a step function. This fiber is called step-index fiber. The other one is that the refractive index of the core is not a constant. It gradually decreases as the radial coordinate of the core increases until it equals to the index of the cladding. Hence this fiber is called graded-index fiber. The common feature of this two fiber cross-section is that the refractive index of the core n1 is larger than that of the cladding n2. This is also a necessary condition for the optical signal to transmit in the fiber. For a step-index fiber, total internal reflection can occur at the core-cladding boundary and the light wave can propagate along the core. For a graded-index fiber, the continuous refraction occurs to the light wave in the core, forming a light ray similar to the sine-wave through the fiber axis and guiding the light wave to propagate along the core. The tracks of the two light rays are shown in Figure 2-2. With the difference of the diameter size of the core of step-index and graded-index fibers, the number of modes transmitted in the fiber is different. Hence, step-index fiber or gradedindex fiber can be classified into single mode fiber and multimode fiber according

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Section 2 DWDM Transmission Media y

to the number of transmission modes. This is also a classification method of optical fiber. The core diameter of a single mode fiber is very small and, generally, less than 10µm, and the core diameter of a multimode fiber is relatively large and often equal to 50µm. However, there is little difference between the profiles of these two types of fiber. The diameters of fibers with a plastic jacket are less than 1mm.

2.2 Types of Optical Fiber Since the single-mode optical fiber has advantages of low internal attenuation, large bandwidth, easy upgrade and capacity expansion and low cost, it is internationally agreed that DWDM systems will only utilize single mode fiber as transmission media. At present, ITU-T has defined four types of single mode optical fiber with different design in Recommendations G.652, G.653, G.654 and G.655. G.652 fiber is currently a single mode fiber for extensive use, called 1310nm property optimal single mode fiber and also called dispersion unshifted fiber. According to the refractive index cross section of the core, it can also be divided into two categories: matched cladding fiber and depressed cladding fiber. They have similar properties. The former is simple in manufacturing but has relatively larger macrobend loss and microbend loss while the later has larger connection loss. G.653 fiber is called dispersion shifted fiber or 1550nm property optimal fiber. By designing the refractive index cross section, the zero dispersion point of this kind of fiber is shifted to the 1550nm window to match the minimum attenuation window. This makes it possible to implement ultrahigh speed and ultra long distance optical transmission. G.654 fiber is cut-off wavelength shifted single mode fiber. This kind of fiber is mainly designed to reduce the attenuation at 1550nm. Its zero dispersion point is still near 1310nm. The dispersion at 1550nm is relatively high, up to 18ps/(nm.km). So single longitudinal mode laser must be used to eliminate the affect of the dispersion. G.654 fiber is mainly used for submarine optical fiber communication with very long regenerator section distance. G.655 fiber, a nonzero dispersion shifted single mode optical fiber, is similar to G.653 fiber and preserves certain dispersion near 1550nm to avoid four-wave mixing phenomenon in DWDM transmission. It is suitable for DWDM system applications. Except for the above-mentioned four types of standardized fiber, a large effective area fiber suitable for higher capacity and longer distance has emerged. Its zero

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Section 2 DWDM Transmission Media y

dispersion point is near 1510µm and its effective area is up to 72 square µm. Therefore, it can effectively overcome the nonlinear affects and is especially suitable for DWDM system applications based on 10Gbit/s.

Thinking: Which type of optical fiber is widely laid at present?

2.3

Basic Features of Optical Fiber

2.3.1 Physical Dimension (Mode Field Diameter) The fiber core diameter of a single mode fiber is 8~9µm in the same magnitude as the operating wavelength 1.3~1.6µm. Because of the optical diffraction effect, it is not easy to measure the exact value of the fiber cord diameter. In addition, since the field intensity distribution of the fundamental mode LP01 isn't confined within the fiber core, the concept of single mode fiber core diameter is physically meaningless and should be replaced with the concept of mode field diameter. Mode field diameter measures the concentrate level of the fundamental mode field spatial intensity distribution within the fiber. The nominal mode filed diameter of G.652 fiber at 1310nm wavelength area should be 8.6~9.5µm with a deviation of less than 10%, and the nominal mode filed diameter of G.655 fiber at 1550nm wavelength area should be 8~11µm with a deviation of less than 10%. The cladding diameter of both types of above-mentioned single mode optical fibers is 125µm.

2.3.2 Mode Field Concentricity Error Mode field concentricity error refers to the distance between the mode field center and the cladding of the interconnected fibers. Fiber connector loss is in proportion to the square of the mode field concentricity error. So reducing mode field concentricity error is one of the key factors to reduce the fiber connection loss and should be strictly controlled in process. The mode field concentricity error of the two types of single mode optical fibers G.652 and G.655 shouldn't be greater than 1. Generally, it should be less than 0.5. 2-15

DWDM Principle

Section 2 DWDM Transmission Media y

2.3.3 Bend Loss Bend of the optical fiber will cause radiation loss. Actually, bend arises to an optical fiber in two cases. One is that the curvature radius of the bend is much larger than the diameter of the fiber (e.g. this kind of bend may occur when the fiber cable is laid). The other case is microbend. There are many causes for microbend. Microbend, limited to process conditions, may be caused during the production process of the fiber and the cable. Microbends of different curvature radiuses are randomly distributed along the fiber. The bent fiber with larger curvature radius can transmit fewer modes than the straight fiber, and a part of modes are radiated out from the fiber to cause loss. The randomly distributed fiber microbend will result in mode coupling in the fiber and cause energy radiation loss. Bend loss of the fiber is inevitable because it can't be guaranteed that no bend in any form will occur to the fiber and the cable during production or utilization process. Bend loss is related to the mode field diameter. The bend loss of G.652 fiber shouldn't be larger than 1dB at 1550nm wavelength area, and the bend loss of G.655 fiber shouldn't be larger than 0.5dB at 1550nm area.

2.3.4 Attenuation Constant Attenuation in optical fiber is mainly determined by three types of loss: absorption loss, scattering loss and bend loss. Bend loss, as described above, has no great effect on the attenuation constant in fiber. So, it is absorption loss and scattering loss that mainly determine the attenuation constant in fiber. Absorption loss is caused by the fiber material where excessive metal impurity and OH- ion absorb the light to result in loss. Scattering loss is often caused in the case that a part of optical power is scattered outside the fiber when uneven refractive index distribution local area emerges within the fiber and causes light scattering because of the micro-change in fiber material density and uneven density of compositions such as SiO2, GeO2 and P2O5. Or, scattering loss can be aroused if some defect occurs or some bubbles and gas scabs are remained at the core-cladding boundary. The physical dimension of these structural defects is much larger than the lightwave, causing wavelength independent scattering loss and upward shifting the whole curve of fiber loss spectrum. However, this kind of scattering loss is much less than the former one. Combining the above losses, the attenuation constant of single mode fiber at 1310nm and 1550nm wavelength areas is 0.3~0.4dB/km (1310nm) and 0.17~0.25dB/km (1550nm), respectively. As defined in ITU-T Recommendation

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Section 2 DWDM Transmission Media y

G.652, the attenuation constant at 1310nm and 1550nm should be less than 0.5dB/km and 0.4dB/km, respectively.

2.3.5 Dispersion Coefficient Dispersion in optical fiber refers to a physical phenomenon of signal distortion caused when various modes carrying signal energy or different frequencies of the signal have different group velocity and disperse from each other during propagation. Generally, three kinds of dispersion exist in optical fiber. 1) Modal dispersion: This is caused when the fiber carries multiple modes of the same frequency signal energy and different mode has different time delay during transmission. 2) Material dispersion: Because the refractive index of the fiber core material is a function of the frequency, signal components of different frequency propagate at different velocities along the fiber. This causes dispersion. 3) Waveguide dispersion: In the fiber, for a signal carrying different frequencies in the same mode, dispersion is caused because of different group velocities during propagation. These three types of dispersion are called chromatic dispersion. ITU-T G.652 defines a zero dispersion wavelength range of 1300nm~1324nm and a maximum dispersion slope of 0.093ps/(nm2.km). In the wavelength range of 1525~1575nm, the dispersion coefficient is approximately 20ps/(nm.km). ITU-T G.653 defines a zero dispersion wavelength 1550nm and a dispersion slope of 0.085ps/(nm2.km) in the wavelength range of 1525~1575nm where the maximum dispersion coefficient is 3.5ps/(nm.km). The absolute value of the dispersion coefficient of G.655 fiber should be within 0.1~6.0 ps/(nm2.km) in the range of 1530~1565nm.

Technical details: The following figure shows the dispersion characteristics of several types of fiber.

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Section 2 DWDM Transmission Media y

G.655 fiber with positive dispersion coefficient

G.653 fiber

Dispersion coefficient (ps/nm km)

17

G.652 fiber

G.655 fiber with negative dispersion coefficient

1310

1550

2-18

Wavelength

(nm)

DWDM Principle

Section 2 DWDM Transmission Media y

2.3.6 Cutoff Wavelength To avoid modal noise and dispersion penalty, the cutoff wavelength of the shortest optical fiber cable in the system should be less than the shortest operating wavelength of the system. The cutoff wavelength condition can guarantee single mode transmission in the shortest cable and suppress the occurrence of higher order modes or reduce the power penalty of the generated higher order mode noise to an negligible degree. At present, ITU-T has defined three types of cutoff wavelengths. 1) Cutoff wavelength of primary coating fiber in jumper cable shorter than 2m. 2) Cutoff wavelength of 22m cable optical fiber. 3) Cutoff wavelength of 2~20m jumper cable. For G.652 fiber, the cutoff wavelength is 2~20m jumper cable, and

1260nm in 22m cable,

1260nm in

1250nm in jumper cable shorter than 2m. For G.655

fiber, the cutoff wavelength is

1480nm in 22m cable,

coating fiber of jumper cable shorter than 2m, and

1470nm in primary

1480nm in 2~20m jumper

cable.

2.4

Types and Properties of Optical Fiber Cable

2.4.1 Types of Optical Fiber Cable In terms of the structure, optical fiber cable can be classified into four types: loose jacket twist type, skeleton type, central beam nominal type and ribbon optical fiber cable. According to the laying methods, optical fiber cable can be classified into plow-in optical cable, optical fiber cable for installation in duct, aerial optical cable, submarine optical cable and office optical cable, etc. According to application situation, traffic demands and capacity expansion demands, the core number of optical fiber cable is classified into 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34 and 36, and can be increased in even number.

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Section 2 DWDM Transmission Media y

2.4.2 Properties of Optical Fiber Cable 1. Mechanical property: An optical fiber cable should possess certain mechanical property that makes it withstand items including tension, bruise, impulsion, repeated bending, twisting, flexure, hook hang, kink, reeling, etc. 2. Protective property: Optical fiber cable should possess property of moisture proof and water proof. Additionally, it should meet some requirements including protection of termite, rat and insect gnawing, corrosion, lightning, etc.

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Section 3 DWDM Key Technologies y

Section 3 DWDM Key Technologies

Objectives: To understand the requirements and solutions of DWDM optical resources. To understand DWDM optical amplification technology. To understand DWDM multiplexing and de-multiplexing technology.

3.1 Lasers Laser, whose function is to generate laser, is an important component of DWDM system. At present, lasers used in DWDM system are semiconductor laser LD (Laser diode). The operating wavelengths of DWDM systems are relatively dense. Generally, the wavelength spacing is from several nanometer to sub-nanometer. Hence, the laser diode is required to operate in a standard wavelength and possess good stability. On the other hand, the non-electrical regeneration distance of DWDM systems is increased from 50~60km of single SDH system transmission to 500~600km. lasers of the DWDM system are required to adopt lasers more advanced in technology and excellent in performance in order to elongate the dispersion limited distance of the transmission system and overcome fiber nonlinear effects {such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), self-phase modulation (SPM), cross-phase modulation (XPM), modulation instability and four-wave mixing (FWM)}. In summary, lasers of DWDM system have two major features: 1. Relatively large dispersion tolerance; 2. Standard and stable wavelength.

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Section 3 DWDM Key Technologies y

3.1.1 Laser Modulation Modes At present, optical fiber communication systems for extensive use employing intensity modulation — direct detection system. There are two types of intensity modulation for lasers, i.e. direct modulation and indirect modulation. 1. Direct modulation Direct modulation: It is also called internal modulation, i.e. directly modulating the laser and changing the launched lightwave intensity by controlling the injection current. LED or LD sources used in traditional PDH and SDH systems of 2.5Gbit/s or below employ this modulation method. One character of direct modulation is that the launched power is in proportion to the modulation current. It has advantages of simple structure, low loss and low cost. Since it changes the length of the laser resonant cavity, the variation of modulation current will cause a linear variation of the emitting laser wavelength corresponding to the current. This variation, called modulation chirp, is actually a kind of wavelength (frequency) jitter inevitable for direct modulation sources. The chirp broadens the bandwidth of the emitting spectrum of the laser, deteriorates its spectrum characteristics and limits the transmission rate and distance of the system. Generally, for conventional G.652, the transmission distance is transmission rate

100km and the

2.5Gbit/s.

For DWDM system without optical line amplifier, direct modulated lasers can be considered to save the cost. 2. Indirect modulation Indirect modulation: This modulation method is also called external modulation, i.e. modulating the laser indirectly and adding an external modulator in its output path to modulate the lightwave. In fact, this modulator works as a switch, as shown in Figure 3-1.

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Section 3 DWDM Key Technologies y

Constant light source

Optical modulator

Optical signal output

Electric modulation signal input

Figure 3-1 The structure of external modulated laser

The constant laser is a highly stable source continuously emitting lightwave with fixed wavelength and power. It isn't affected by the electric modulation signal during emitting, so no modulating frequency chirp occurs and the line breadth of its optical spectrum keeps at minimum. According to the electric modulation signal, the optical modulator processes the highly stable light from the constant laser light in a way of either passing through or blocking. During the modulation process, the spectrum characteristics of the lightwave won't be affected. This guarantees the quality of the spectrum. Lasers adopting indirect modulation are relatively complex with high loss and cost, but its modulating frequency chirp is very low. It can be used in systems whose transmission rate is

2.5Gbit/s and transmission distance longer than 300km.

Hence, in DWDM systems with optical line amplifiers, the lasers at the transmit end are generally indirectly modulated. Commonly used external modulators are photoelectric modulator, acoustooptic modulator and waveguide modulator. The basic operating principle of photoelectric modulator is crystal linear photoelectric effect. Photoelectric effect refers to the phenomenon that electric field causes the variation of the refractive index of a crystal. A crystal that is able to generate the photoelectric effect is called photoelectric crystal. Acoustooptic modulator is made by utilizing the acoustooptic effect of the dielectric. Acoustooptic effect refers to the phenomenon that the dielectric changes under the pressure of an acoustic wave when it propagates through the dielectric. This change causes the variation of the refractive index of the dielectric and affects the transmission characteristics of the lightwave.

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Section 3 DWDM Key Technologies y

Waveguide modulator is manufactured from titanium (Ti) diffused LiNbO2 substrate material on which waveguide is made via photoetching method. It has many advantages such as small in dimension, light in weight and facile for optical integration. According to the integration and separation conditions of the laser and the external modulator, external modulated lasers can be classified into two categories: integrated external modulated laser and separated external modulated laser. As a maturing technology, integrated external modulation becomes the development trend of DWDM lasers. The commonly used modulator is electroabsorption modulator which, small and compact and integrated with the laser, meets most application requirements in performances. Electroabsorption modulator, a kind of loss modulator, operates at the boundary wavelength of the material absorption region. When the modulator isn't biased, the wavelength from the laser is out of the absorption range of the modulator material. Thus the launched power of this wavelength is maximum and the modulator is turned on. When the modulator is biased, the boundary wavelength of the material absorption region shifts and the wavelength from the laser is within this region. Thus the launched power is minimum and the modulator is turned off, as shown in Figure 3-2. Biased Unbiased Absorption region

Absorption region 1

0

0

is the absorption side wavelength of unbiased modulator is the absorption side wavelength of biased modulator 0 is the operating wavelength of the constant light source 1 2

Figure 3-2 Variation of the absorption wavelength of an electroabsorption modulator

Electroabsorption modulator can be manufactured by utilizing the same technical process as semiconductor laser. Therefore, it is easy to integrate the laser and the modulator, suitable for batch production. So its development speed is high. For example, InGaAsP optoelectronic integrated circuit monolithically integrates a 3-24

2

DWDM Principle

Section 3 DWDM Key Technologies y

laser and an electroabsorption modulator on a single chip that is put on a thermoelectric cooler (TEC). This typical optoelectronic integrated circuit is called electroabsorption modulated laser (EML). It can support transmission of 2.5Gbit/s signal over 600km, far exceeding the transmission distance of directly modulated lasers. Its reliability is similar to that of standard DFB lasers with an average life span of 20 years. Separated external modulated laser generally uses constant output laser (CW) + LiNbO3 Mach-Zehnder external modulator. This modulator separates the light input into two equal signals that, respectively, enter the two branches. These two branches employ electrooptic material whose refractive index changes with the magnitude of the external electrical signal applied to it. Change of the refractive index of the optical branches will result in variation of the signal phases. Hence, when the signals from the two branches recombine at the output end, the combined optical signal is an interfering signal with varying intensity. Via this method, the information of the electrical signal is transferred onto the optical signal. Thus optical intensity modulation is implemented. The frequency chirp of separated external modulated laser can be zero. Moreover, its cost is relatively low when compared to electroabsorption modulated external laser.

3.1.2 Wavelength Stability and Control of Laser In DWDM system, wavelength stability of lasers is a critical problem. According to ITU-T G.692, deviation of the central wavelengths shouldn't be greater than one fifth (±1/10) of optical channel spacing, i.e. the deviation of the central wavelengths shouldn't be greater than ±20GHz in a system with a channel spacing of 0.8nm. Because the optical channel spacing is very small (can be as low as 0.8nm), DWDM system has strict requirements to the wavelength stability of the lasers. For example, a 0.5nm variation of wavelength can shift an optical channel to another one. In practical systems, the variation should be controlled within 0.2nm. The specific requirement is determined according to the wavelength spacing, i.e. the smaller the spacing, the higher the requirement. So the lasers should adopt strict wavelength stabilization technology. Fine tuning of the wavelength of integrated electroabsorption modulated laser is mainly implemented by adjusting the temperature. The temperature sensitivity of the wavelength is 0.008nm/

. The normal operating temperature is 25

adjusting the chip temperature from 15

to 35

. By

, the EML can be set up to a

specific wavelength with an adjustable range of 1.6nm. The chip temperature is adjusted by changing the drive current of the cooler and using a thermal resistance as feedback. Thus the chip temperature is stabilized and stays at a constant value. 3-25

DWDM Principle

Section 3 DWDM Key Technologies y

According to the correspondent characteristics of wavelength and chip temperature, distributed feedback laser (DFB) controls its wavelength by controlling the temperature of the laser chip to achieve wavelength stability. For DFB laser, the wavelength-temperature coefficient is about 0.02nm/ central wavelength meets the requirement within the range of 15

-35

and its . This

temperature feedback control method completely depends on the chip temperature of the DFB laser. At present, MWQ-DFB laser technical process can guarantee that the wavelength deviation meets the requirements of DWDM system in the life span (20 years) of the laser. Except for the temperature, laser drive current can also affect the wavelength. The sensitivity is 0.008nm/mA, smaller than the affect of the temperature in one order. In some cases, its effect is negligible. Additionally, package temperature may also affect the device wavelength (e.g. temperature conduction brought by wires from the package to laser platform and inward radiation from the package shell will also affect the device wavelength). In a well-designed package, its effect can be controlled to minimum. The above methods can effectively solve the problem of short-term wavelength stability. However, they are incapable of dealing with long-term wavelength variation caused by factors such as laser aging. It is ideal to directly utilize a wavelength sensitive component for wavelength feedback control of the laser. The theory is shown in Figure 3-3. Standard wavelength control of this type of scheme and reference frequency disturbance wavelength control are promising and being developed. Optical output LD

LD control circuit

Wavelength sensitive component

For wavelength control

Signal processing

Figure 3-3 Theory for wavelength control

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For wavelength monitoring

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Section 3 DWDM Key Technologies y

Thinking: Why does the DWDM system set strict requirements to the wavelength stability?

3.2 Erbium-doped Optical Fiber Amplifier (EDFA) As a key component of new generation optical communication systems, erbium doped fiber amplifier (EDFA) has many advantages such as high gain, large output power, wide operating optical bandwidth, polarization independence, low noise factor and amplifying characteristic independent to system bit rate and data format. It is an indispensable key component of high capacity DWDM systems.

3.2.1 EDFA Operating Theory To amplify optical power, some passive optical components, pump source and erbium-doped fiber are combined together according to specific optical structure. Then EDFA optical amplifier is formed. Figure 3-4 shows a typical optical structure of dual-pumping source erbium-doped optical fiber amplifier.

Optical splitter ISO

Signal input

WDM

WDM Optical coupler

TAP EDF

Optical isolator

Pumping laser

PD

Pumping laser ISO

Signal output

TAP

PD Optical detector

Figure 3-4 Typical internal light path of EDFA

As shown in Figure 3-4, signal light and pump light from the pumping laser are combined via a DWDM multiplexer, then they are sent to the erbium-doped fiber

3-27

EDF

DWDM Principle

Section 3 DWDM Key Technologies y

(EDF). The two pumping lasers form a two-stage pump. Excited by the pumping light, the EDF yields the amplification function. Therefore, the function of amplifying the optical signal is implemented. 1.Erbium-doped optical fiber (EDF) Erbium-doped optical fiber (EDF), doped with Er3+ of a given density, is the kernel of the optical fiber amplifier. To illustrate its amplification principle, we need to begin with the energy level diagram of Er3+. The outer-shell electrons of Er3+ have three-level structure (E1, E2 and E3 in Figure 3-5), where E1 is ground state, E2 is metastable state and E3 is high level, as shown in Figure 3-5.

E3 excited state Decay Pump light 1550nm signal light

E2 metastable state

1550nm signal light E1 ground state

Figure 3-5 EDFA energy level diagram

When high energy pumping lasers are used to excite the EDF, lots of bound electrons of the erbium ions are excited from the ground state to the high level E3. However, the high level is not stable and erbium ions are soon dropped to the metastable state E2 via a radiationless decay process (i.e. no photon is released). E2 level is an metastable energy band on which particles' survival span is relatively long. Particles excited by the pumping light gather on this level via nonradiative transition. Thus, population inversion distribution is implemented. When an optical signal of wavelength 1550nm passes through this erbium-doped fiber, particles in the metastable state are transited to the ground state via stimulated radiation and generate photons identical to the photons of the incident signal light. This greatly increases the quantity of the photons in the signal light, i.e. implementing the function of continuous amplifying the signal light transmitted in the EDF. 2. Optical coupler (WDM)

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Section 3 DWDM Key Technologies y

Optical coupler, as its name implies, has function of coupling. It couples the signal light and the pumping light and sends them into the erbium-doped fiber. It, also called optical multiplexer, usually employs optical fiber fusible cone multiplexer. 3. Optical isolator (ISO) Optical isolator (ISO), a kind of component utilizing Faraday magnetooptical effect, allows only unidirectional light transmission. Along the light path, the functions of the two isolators are as follows: The input isolator can block the backward ASE in the EDF, keep it from interfering the transmitters of the system and from generating larger noise when it is reflected at the input end and reenter the EDF. The output isolator prevents the amplified optical signal, when reflected at the output end, from reentering the EDF, consuming particles and affecting the amplification characteristics of the EDF. 4. Pumping laser (PUMP) Pumping laser, the energy source of EDFA, provides energy for amplifying the optical signal. Generally, it is a semiconductor laser with output wavelength of 980nm or 1480nm. When passing through the EDF, the pumping light pumps the erbium ions from low level to high level. Thus population inversion is formed. When the signal light passes through, the energy will be transferred to it. Hence, optical amplification is implemented. 5. Optical splitter (TAP) The optical splitter used in the EDFA is a one by two component. Its function is to tap off a small part of the optical signal for monitoring the optical power of the main channel. 6. Optical detector (PD) The PD is an optical power detector. Its function is to convert the received optical power into photocurrent via photoelectric conversion. Hence, it monitors the input and output optical power of the EDFA module.

3.2.2 Applications of EDFA According to its location in the DWDM optical transmission network, EDFA can be classified into booster amplifier (BA), line amplifier (LA) and preamplifier (PA). 1. Booster amplifier (BA) Booster amplifier is installed behind the transmitters of terminal equipment or regeneration equipment, as shown in Figure 3-6. The major function of the booster 3-29

DWDM Principle

Section 3 DWDM Key Technologies y

amplifier is to boost the launched power and elongate transmission distance by enhancing the optical power injected into the fiber (generally above 10dBm). So in some documents, it is also named as power booster amplifier. Here, its noise characteristic requirement is not high. The major requirement is linear power amplification characteristic. Generally, booster amplifier works in the saturation range of gain or input power in order to enhance the conversion efficiency from pumping source power to optical signal power. Repeating section D W D M

D W D M

BA

e q u ip

equip ment

m e n t

Optical fiber connector

Figure 3-6 Location of the amplifier in the regenerator section

2. Line amplifier (LA) Line amplifier is located in the middle of the whole regenerator section, as shown in Figure 3-7. This is an application form to insert the EDFA into the optical fiber transmission link and amplify the signal directly. A regenerator section can be configured with multiple line amplifiers according to the demands. Line amplifier is mainly applied in long-haul communication or CATV distribution networks. Here, the EDFA is required to have high small-signal gain and low noise factor.

Repeating section

D W D M

LA

equip ment

D W D M equip ment

Optical fiber connector

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Section 3 DWDM Key Technologies y

Figure 3-7 Location of the line amplifier in the regenerator section

3. Pre-amplifier (PA) Pre-amplifier is located at the end of the regenerator section but in front of the optical receiving equipment, as shown in Figure 3-8. The main function of this amplifier is to amplify the small signal attenuated along the link and enhance the receiving sensitivity of the optical receiver. Here the main problem is noise. The main noise in EDFA is amplified spontaneous emission (ASE). This noise makes the optoelectronic detector output three noise components, i.e. extra shot noise due to the increase of optical power, signal-ASE beat noise and ASE-ASE beat noise. By using a narrow-band optical filter (1nm bandwidth), most ASE-ASE beat noise can be filtered and extra shot noise can be reduced. But the signal-ASE beat noise can't be filtered. Despite of this, the noise characteristic of EDFA is greatly improved by adopting the optical filter. The pre-amplifier greatly improves the sensitivity of receivers employing direct detection. For example, the sensitivity of an EDFA receiver of 2.5Gbit/s can be up to -43.3dBm. An improvement of about 10dB is achieved when compared to the receivers employing direct detection without EDFA.

R e p e a t in g s e c tio n

D W D M

PA

e q u ip m ent

O p tic a l f ib e r c o n n e c t o r

Figure 3-8 Location of the pre-amplifier in the regenerator section

Β

Tricks:

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Section 3 DWDM Key Technologies y

BA, PA and LA differ from each other in that their locations in the DWDM network are different. However, the most important difference lies in their input optical power and gain: BA: relatively high input optical power and low gain; PA: relatively low optical power and low gain, similar to BA; LA: relatively low input optical power, similar to PA, but its gain larger than BA.

3.2.3 Gain Control of EDFA 1. EDFA gain flatness control In DWDM systems, the more the optical channels multiplexed, the more the optical amplifiers needed in cascading. This requires that a single amplifier occupies a wider and wider bandwidth. However, EDFA based on ordinary pure silicon optical fiber has a very narrow flat gain range between 1549 and 1561nm, a range of approximately 12nm. And the gain fluctuation between 1530 and 1542nm is very large, up to about 8dB. When the channel arrangement of the DWDM system exceeds the flat gain range, channels near 1540nm will suffer severe signal-to-noise degradation and normal signal output can't be guaranteed. To solve the above-mentioned problem and adapt to the development of DWDM systems, a gain flattened EDFA based on aluminum-doped silicon optical fiber is developed. It greatly improves the operating wavelength bandwidth of the EDFA and suppresses gain fluctuation. The up-to-date mature technology can achieve 1dB gain flattened range which almost expands to the whole erbium pass-band (1525nm~1560nm). Basically, it has solved the problem of gain unflatness of ordinary EDFA. Figure 3-9 compares the gain curves of non-aluminum-doped EDFA and aluminum-doped EDFA.

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DWDM Principle

Section 3 DWDM Key Technologies y

1525nm-1565nm non-aluminum-doped EDFA Gain

1525nm-1565nm aluminum-doped EDFA Gain

Figure 3-9 Improvement of EDFA gain curve flatness

Technically, the range of 1525nm~1540nm in EDFA gain curve is called blue band area and the range of 1540nm ~1565nm is called red band area. Generally, red band area is preferential when the transmission capacity is less than 40Gbit/s.

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DWDM Principle

Section 3 DWDM Key Technologies y

Technical details: Performance comparison of EDFA gain unflatness and flatness is given in Figure 3-10.

Cascading amplification of amplifier gain unflatness

Cascading amplification of amplifier gain flatness

Figure 3-10 Diagram of EDFA gain flatness

2. EDFA gain-locking EDFA gain-locking is an important problem because the WDM system is a multiwavelength working system. When certain wavelengths are dropped, their energy will be transferred to those undropped signals due to gain competition. Thus the power of other wavelengths increases. At the receive end, abrupt increment of the electrical level is possible to cause error. In limiting case, if seven wavelengths of eight wavelengths are dropped, all the energy will concentrate to the one wavelength left and its power may be up to about 17dBm. This will result in strong nonlinear effects or receiving power overload of the receiver, and this will also cause lots of errors. There are many gain-locking technologies for EDFA. One typical method is to control the gain of pumping laser. The internal monitoring electric circuit of the EDFA controls the output of the pumping source by monitoring the input-output power ratio. When some signals of the input wavelengths are dropped, the input power will decrease and the output-input power ratio will increase. Via the

3-34

DWDM Principle

Section 3 DWDM Key Technologies y

feedback circuit, the output power of the pumping source will be reduced in order to keep the gain (output/input) of the EDFA. Hence, the total output power of the EDFA is reduced and the output signal power is kept stable. The process is shown in Figure 3-11.

INPUT

OUTPUT

TAP

PUMP

PIN

TAP PIN

Non-linear control

Figure 3-11 Gain-locking technology of controlling the pumping laser

Another method is saturation wavelength. At the transmit end, except for the eight operating wavelengths, system sends another wavelength as saturation wavelength. In normal cases, the output power of this wavelength is very small. When some line signals are dropped, the output power of the saturation wavelength will automatically increase in order to compensate the energy of the lost wavelengths and maintain the output power and gain of the EDFA. When the multi-wavelength line signals are restored, the output power of the saturation wavelength will correspondingly decrease. This method directly controls the output of the saturation wavelength laser, so its speed is faster than controlling the pumping source.

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DWDM Principle

Section 3 DWDM Key Technologies y

Technical details:

falling wavelength

>1dB

adding wavelength >1dB

Figure 3-12 NO Gain-locking when EDFA falling wavelength and adding wavelength

Falling wavelength

adding wavelength