Performance Analysis of Dense Wavelength Division Multiplexing (DWDM)
System in the presence of Four Wave Mixing
Communications is the field of study concerned with the transmission of information through various means. It can also be defined as the inter-transmitting the content of data (speech, signals, pulses etc.) from one node to another.
1.1 Communication System:
Communication is a process of transferring information from one entity to another.
Communication processes are sign-mediated interactions between at least two agents which share a repertoire of signs and semiotic rules. Communication is commonly defined as “the imparting or interchange of thoughts, opinions, or information by speech, writing, or signs”. Communication systems convert information into a format appropriate for the communications. The basic principle of a communication system is illustrated in figure 1.1.
Figure1.1: Block diagram of basic communication system
Any communication system is composed of the following basic components:
(1) Transmitter:
A transmitter is used to manipulate the information from the message source and couples the message signal to the transmission medium (i.e. the channel). The message signal is used to modulate the carrier wave.
(2) Channel:
The channel includes the transmission medium and it may introduce random noise and distortion. Bridges the distance between the transmitter and receiver.
(3) Receiver:
The receiver extracts the message signal from the received signal and then converts it to a form suitable for the output transducer. The extraction process usually includes amplification, filtering and demodulation.
1.2 Historical perspective
People are prone to take for granted the fact that modern technology allows us to transmit data at nearly the speed of light to locations that are very far away. 200 years ago, it would be deemed preposterous to think that we could transmit web pages from China to Mexico in less than a second. It would seem equally preposterous to think that people with cell phones could be talking to each other, clear as day, from miles away. Today, these things are so common, that we accept them without even asking how these miracles are possible.
Communication systems of all types have “gone digital” and the primary advantage is maintenance of signal integrity during storage or transmission. The first significant application of digital transmission began in 1962 when the ATT Bell System installed the first T1 transmission system between telephone switching centers in Chicago. The system gave a 12-fold increase in transmission capacity on the wire pairs and it yielded high quality transmission with good noise performance when compared with analog transmission. The Morse telegraph was introduced in the 1860‘s which transmission rate: ?1bit/s and distance: Due to the application of relay stations: 1000km.
A renewed interest in optical communication was simulated in the early 1960s with the invention of laser. The period of 1965-1975 was devoted to develop the graded index fiber system, which utilized wavelengths of 850-900nm and achieved information rate in the range of 8-140 Mbps. First coaxial cable system was in 1940 with the capability to transmit 300 voice channels. The first microwave system was put into service in 1948 with a carrier frequency of 4GHz. Coaxial and microwave systems were operating at 100Mbit/s. High speed coaxial systems need repeater spacing of ?1km.
Recent trend is towards the use of 1500nm wavelength for long haul transmission system. Gradually the use of electromagnetic waves (EM Waves) became popular.
A new era in optical communication started after the invention of laser in 1960 by Maiman. The light waves from the laser, a coherent source of light waves having high intensity, high monochromaticity and high directionality with less divergence, are used as carrier waves capable of carrying large amount of information compared with radio waves and microwaves. Subsequently H M Patel, an Indian electrical engineer designed and fabricated a CO2 laser.
1.3 Introduction to Optical fiber communication system
Since the mid 90’s, optical fibers have been used for point to point communication at a very high speed. Optical communication is one of the newest and most advanced forms of communication by electromagnetic waves. In one sense, it differs from radio and microwave communication only in that the wavelengths employed are shorter (or equivalently, the frequencies employed are higher). The increasing demand of utilizing higher frequencies led to the development of optical communication. Optical communication is any form of telecommunication that uses light as the transmission medium.
Optical communications is preferred as other communications because optical cable uses crystal to transport light where the information is passed through. It is much faster than copper or other cable in existence. The chief advantage of optical fiber communication system for comes from properties of optical fiber which is medium used to convey information.
An optical communication system consists of a transmitter, which encodes a message into an optical signal, a channel, which carries the signal to its destination, and a receiver, which reproduces the message from the received optical signal. It’s a method of transmitting information from one place to another by sending pulses of light through an optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information.
Fig 1.2: Optical fiber communication system
The basic point-to-point optical communication system consists of three major element:
Optical source at the transmitter point, optical fiber cable as transmission medium and optical detector at the receiver terminal, as shown in fig 1.2 (a).
For optical fiber communications the system shown in Figure 1.2 (b) may be considered in slightly greater detail, as given in Figure. In an optical communications system, information from the source is encoded into electrical signals that can drive the transmitter. The transmitter consists of an LED or laser and is pulsed at the incoming frequency. The transmitter performs an EO conversion. The fiber acts as an optical waveguide. At the detector, the signals undergo an OE conversion, are decoded, and are sent to their destination. Fiber-optic system characteristics include attenuation, interference, and bandwidth characteristics. Fiber-optic systems are also secure from data tapping, and tampering can be detected far more easily than with metallic-based transmission medium or free-space propagation.
Forms of optical communication:
• Optical fiber communications
• Free-space optical communication
1.3.1 Optical fiber communication
Optical fiber communication is the most common type of channel for optical communications; however, other types of optical waveguides are used within communications gear, and have even formed the channel of very short distance (e.g. chip-to-chip, intra-chip) links in laboratory trials.
1.3.1.1 Components of optical fiber communication:
Ø Optical transmitters:
The transmitters in optical fiber links are generally light-emitting diodes (LED) or laser diodes. Infrared light, rather than visible light is used more commonly, because optical fibers transmit infrared wavelengths with less attenuation and dispersion. The signal encoding is typically simple intensity modulation.
It converts electrical signals into optical signals and launches the optical signals into an optical fiber. A fiber optic transmitter consists of an interface circuit, a source drive circuit, and an optical source. The interface circuit accepts the incoming electrical signal and processes it to make it compatible with the source drive circuit. The source drive circuit intensity modulates the optical source by varying the current through the source.
Ø Optical receivers:
Semiconductor photodiodes are used for the receivers in virtually all optical communication systems. There are two basic types of photodiodes in use:
•Photodiodes where, reverse biased junction is applied
• Avalanche photodiodes
1.3.2: Free-space optical communication
Free Space Optics (FSO) is an optical communication technology that uses light propagating in free space to transmit data between two points. The technology is useful where the physical connections by the means of fiber optic cables are impractical due to high costs or other.
1.4 Advantage of optical communication
Fiber optic communication has been growing at a phenomenal pace over the past twenty years, so rapidly, in fact, that its impact is increasingly felt in nearly all aspects of communications technology. The demand for transmission over the global telecommunications network will continue to at an exponential rate and only fiber optics will be able to meet up this challenge.
Communication using an optical carrier wave guided along a glass fiber has a number of extremely attractive features; hence it is useful to consider the merits and special features offered by optical fiber communication over more conventional electrical communications. The advantages are mainly as following:-
1. Wider bandwidth:
The optical carrier frequency is in the range 1013 to 1015 Hz while the radio wave frequency is about 106 Hz and the microwave frequency is about 1010 Hz. Thus number of bits per second is increased to a greater extent in the optical communication system.
2. Low transmission loss:
Due to the usage of the ultra low loss fibers and the erbium doped silica fibers as optical amplifiers, one can achieve almost lossless transmission. In the modern optical fiber telecommunication systems, the fibers having a transmission loss of 0.002 dB/km are used.
3. Dielectric waveguide:
Optical fibers are made from silica which is an electrical insulator. It is also suitable in explosive environments. Further the optical fibers are not affected by any interference originating from power cables, railway power lines and radio waves
4. Signal security:
The transmitted signal through the fibers does not radiate. Further the
Signal cannot be tapped from a fiber in an easy manner.
5. Small size and weight:
Fiber optic cables are developed with small radii, and they are flexible, compact and lightweight. The optical fiber cables are superior to the copper cables in terms of storage, handling, installation and transportation, maintaining comparable strength and durability.
6. System reliability and ease of maintenance
The reliability of the optical components is no longer a problem with predicted lifetimes of 20 t0 30 years now quite common. Both these factors also tend to reduce maintenance time and costs.
1.5 Review of Previous Work:
In recent years, the tremendous growths in Internet activities such as multimedia communications and networking have created an ever-increasing demand on network capacity. In the DWDM scheme, the existing fiber cables are used to carry increasing number of wavelengths instead of single wavelength carried by the fiber cables in the early 1990’s. Originally started at 4 wavelength channels per fiber in the mid 1990’s [19-21].In 1994 Chraplyvy described optical nonlinearities namely Stimulated Raman, Brillouin, Four Wave Mixing (FWM) and carrier induced phase noise in the context of light wave system limitations. Several methods have been proposed to mitigate the effect of FWM crosstalk, namely, arrangement of transmission fiber dispersion, unequal channel spacing (US) scheme, repeated unequal channel spacing (RUS) scheme, wavelength Shift Keying (WSK) [22-24].
In 1997, a number of new optical effects that result from degenerating four-wave mixing in transparent optical media was analyzed by A. Yariv et. al. [26]. In the same year David F. Geraghty et. al. presented a series of experiments evaluating several aspects of the performance of FWM in semiconductor optical amplifiers at bit rates of 2.5 and 10 Gb/s [27]. Error free de-multiplexing of 20 Gb/s data to 10 Gb/s was obtained, with 1.4 dB power penalty BER ¼ 10-9 by Hedekvist et al. who presented an all-optical time- division de-multiplexer with 22 dB conversion efficiency, using FWM at 1550 nm in a single-mode dispersion-shifted fiber [29]. Hwang et. al. in 1998, described the comparisons of power penalty due to FWM between equal channel spacing and the unequal channel spacing for the20-channelWDMsystem [30].Then in 1999, Shuxian Song showed that the influence of SPM and XPM on the FWM process becomes significant when the transmitted channel powers are large and the fiber dispersion or the channel spacing is small [28].
In June, 2000 L. H. Spiekman et. al. have modulated eight DWDM channels in the wavelength region 1558-1570 nm, spaced at 200 GHz at 20 Gb/s and transmitted over 160 km using four in-line SOA’s [31]. Edmund Kueh et.al. presented an analytical evaluation of the impact of Four Wave Mixing on the Bit Error Rate performance of a WDM soliton transmission Link in 2002 [32]. In 2005, the effects of CPM and FWM in conjunction with various signaling formats in 16 channel systems are evaluated by S. W. Bang and D. G. Daut [33]. An investigation have done by degenerated and forwarded four-wave mixing effects in a self-defocusing photorefractive medium, in both one and two transverse dimensions in 2007 [34]. In the same year, Yasin M. Karfaa et. al. [35] presented a comprehensive theoretical study of four-wave mixing in optical fiber with exploring four fiber types. They evaluated the system performance through determining the average bit error rate relation with both of the frequency and wavelength of transmitted optical channels in the presence of four wave mixing.
1.6 Contribution of Thesis:
Optical dense wavelength division multiplexing (DWDM) systems using low dispersion fibers and erbium-doped fiber amplifier (EDFA) are very attractive to meet up the growing demand for broadband information distribution networks. At the spacing, the non-linear effects of the optical fiber can induce serious system impairments and modulation schemes are now being which are robust to both the linear and non-linear behavior of fiber. Four-Wave-Mixing (FWM) is another non linear effect that can limit the performance of WDM systems. According to the previous research works, it has been observed that maximum work has been done in the field of using four wave mixing for wavelength conversion. The investigation of four wave mixing effect on bit error rate and Q-factor with changing parameters has rarely been done.
Our effort was to analyze the impact of such influential system parameters on its performance in terms of BER; hence, comment on the possible optimization of such parameters. Other nonlinearities (i.e. Cross phase modulation, Stimulated Raman Scattering etc.) might also be destructive to system performance to an extent of major concern. But in the course of our work we have just limited our research only to FWM effects on WSK-DWDM system performance.
In this paper, an analytical approach is presented to evaluate the effect of FWM in single mode fiber on the bit error rate performance for WSK-DWDM binary system. The four wave mixing effect on BER at different channel spacing, fiber lengths is investigated. The graphs of BER show that higher channel spacing gives the better performance as compared to lower channel spacing. The bit error rate of a DWDM link is evaluated at a bit rate of 10 Gbit/s in the presence of FWM. The results show that at 10 Gbit/s the presence of unwanted light signals due to FWM degrades the performance of DWDM and the system suffers a penalty in terms of input power level per channel. The penalty at BER =10-9 is found to be -8dB and -3dB respectively for fiber length of 70km and 90km when the number of channels is 16. It has also been observed that on decreasing the number of input channels/users, the interference decreases and thus, the four wave mixing effect also decreases.
1.7 Thesis organization:
This thesis is divided into five chapters. The following is an outline of the contents of the thesis:
The 1st chapter presents a brief orientation to the optical communication, preview of previous works for dwdm with four wave mixing effect and contribution of thesis.
The 2nd chapter presents a brief introduction of different multiplexing techniques and different fiber nonlinear effects that cause the performance degradation of link performance which includes self phase modulation, cross phase modulation and four wave mixing. It also provides some background on why dense wavelength division multiplexing (DWDM) is an important innovation in optical networks and what benefits it can provide.
Chapter 3 focuses on the basics of a DWDM system Model analysis and different topologies and technologies used in DWDM. The technique of determining FWM induced noise power from their optical power spectral density is established. Then these noise terms are incorporated with the other noise terms to finally calculate the bit-error rate (BER) which is the most important parameter for measuring the performance of an optical link
Chapter 4 focuses on the link performance specially in terms of BER and absolute power transfer due to FWM. The effects of different BER parameters such as no of channel, channel spacing, no of fiber length are showed. All the parameters have been analyzed in the form of graphs.
Chapter 5 brings the conclusion of the total analysis and discuss about future development in this field.
2.1 Multiplexing
Multiplexing means many signals at a given time. The transmission of multiple optical signals (channels) over the same fiber is a simple way to increase the transmission capacity of the fiber against the fiber dispersion, fiber nonlinearity and speed of electronic components which limit the bit rate. So multiplexing techniques are followed.
Suppose for each channel the bit rate is 100 Gb/s and by accommodating 100 channels through multiplexing technique the total bit rate through a single fiber can be increased to
10 Tb/s (1 Tera = 1012): Thus the information carrying capacity of a fiber is increased by the multiplexing technique.
2.2 Optical Multiplexing Schemes
Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. In analog transmission, signals are commonly multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is divided into sub channels of different frequency widths, each carrying a signal at the same time in parallel. In digital transmission, signals are commonly multiplexed using time-division multiplexing (TDM), in which the multiple signals are carried over the same channel in alternating time slots.
Types of multiplexing:
• Frequency Division Multiplexing (FDM)
• Time Division Multiplexing (TDM)
• Code Division Multiplexing (CDM)
• Wavelength Division Multiplexing (WDM)
2.2.1 Frequency Division Multiplexing (FDM)
In communication systems, Frequency Division Multiplexing (FDM) is a method in which each signal is allocated a frequency slot within the overall line/transmission bandwidth, In other words the total available frequency bandwidth on the transmission line is divided into frequency channels and each information signal occupies one of these channels the signal will have exclusive use of this frequency slot all the time (i.e. each subscriber occupies his/her own slot). Before transmission, the individual information signals are shifted up in frequency as shown on the following figure 2.1.
Figure 2.1: Basic Frequency Division Multiplexing
Individual signal can be extracted from the combined signal by appropriate electrical filtering or optical filtering at the receiver terminal. Hence FDM is usually done electrically at the transmit terminal prior to intensity modulation of single optical source.
2.2.2 Time Division Multiplexing (TDM)
Time Division Multiplexing (TDM) allows multiple conversations to take place by the sharing of medium or channel in time. It consists of interleaving multiple data streams into one higher bit rate stream. For example, 672 voice channels (DS0) at 64 Kbit/s are multiplexed into one T1 data stream at 1.544 Mbit/s. Eighty-four of these T1 channels are then multiplexed into a SONET (Synchronous Optical NET work) OC-3 signal at 155 Mbit/s.
Figure2.2: Basic Time Division Multiplexing (TDM) System
A channel is allocated the whole of the line bandwidth for a specific period of time. This means that each subscriber is allocated a time slot. When we discuss Pulse Code Modulation (PCM), we talk about sampling a signal in time. This is also done in Time Division Multiplexing (TDM). If we have a number of analog signals, each signal is sampled first. Then, the samples from each are combined and the composite signal is transmitted. Sampling is an essential component in TDM. Individual channels are sampled at higher rates [normally 8 kHz (i.e. 8 samples per cycle of 1 kHz)]. The samples are converted into digital signals and a series of zeros and ones is transmitted on the line.
2.2.3 Code Division Multiplexing (CDM)
Code division multiplexing (CDM) allows signals from a series of independent sources to be transmitted at the same time over the same frequency band. This is accomplished by using orthogonal codes to spread each signal over a large, common frequency band. At the receiver, the appropriate orthogonal code is then used again to recover the particular signal intended for a particular user.
The key principle of CDM is spread spectrum. Spread spectrum is a means of communication with the following features:
• Each information-bearing signal is transmitted with a bandwidth in excess of the minimum bandwidth necessary to send the information.
• The bandwidth is increased by using a spreading code that is independent of the information.
• The receiver has advance knowledge of the spreading code and uses this knowledge to recover the information from the received, spread-out signal.
Spread spectrum seems incredibly counterintuitive. We’ve spent most of this book studying ways to transmit information using a minimum of bandwidth. Why should we now study ways to intentionally increase the amount of bandwidth required to transmit a signal? By the end of this chapter you will see that spread spectrum is a good technique for providing secure, reliable, private communication in an environment with multiple transmitters and receivers. In fact, spread spectrum and CDM are currently being used in an ever-increasing number of commercial cellular telephone systems.
2.2.4 Wavelength Division Multiplexing (WDM)
Wavelength-division multiplexing (WDM) is a method of combining multiple signals on laser beams at various infrared (IR) wavelengths for transmission along fiber optic media. Each laser is modulated by an independent set of signals. Wavelength-sensitive filters, the IR analog of visible-light color filters, are used at the receiving end.
WDM is an optical technology that couples many wavelengths in the same fiber, thus effectively increasing the aggregate bandwidth per fiber to the sum of the bit rates of each wavelength. For example, 16 wavelengths at 10 Gbps per wavelength in the same fiber raise the aggregate bandwidth to 160 Gbps. An astonishing aggregate bandwidths at several terabits per second (Tbps) are possible!
WDM is similar to frequency-division multiplexing (FDM). But instead of taking place at radio frequencies (RF), WDM is done in the IR portion of the electromagnetic (EM) spectrum. Each IR channel carries several RF signals combined by means of FDM or time-division multiplexing (TDM). Each multiplexed IR channel is separated, or demultiplexed, into the original signals at the destination. Using FDM or TDM in each IR channel in combination with WDM of several IR channels, data in different formats and at different speeds can be transmitted simultaneously on a single fiber.
In early WDM systems, there were two IR channels per fiber. At the destination, the IR channels were demultiplexed by a dichroic (two-wavelength) filter with a cutoff wavelength approximately midway between the wavelengths of the two channels. It soon became clear that more than two multiplexed IR channels could be demultiplexed using cascaded dichroic filters, giving rise to Coarse wavelength-division multiplexing (CWDM) and Dense wavelength-division multiplexing (DWDM).
2.2.4.1 Coarse Wavelength Division Multiplexing (CWDM)
Coarse wavelength division multiplexing (CWDM) is a method of combining multiple signals on laser beams at various wavelengths for transmission along fiber optic cables, such that the number of channels is fewer than in dense wavelength division multiplexing (DWDM) but more than in standard wavelength division multiplexing (WDM). Prior to the relatively recent ITU standardization of the term, one common meaning for Coarse WDM meant two (or possibly more) signals multiplexed onto a single fiber, where one signal was in the 1550-nm band, and the other in the l3l0-nm band.
The energy from the lasers in a CWDM system is spread out over a larger range of wavelengths than is the energy from the lasers in a DWDM system. The tolerance (extent of wavelength imprecision or variability) in a CWDM laser is up to ± 3 nm, whereas in a DWDM laser the tolerance is much tighter. Because of the use of lasers with lower precision, a CWDM system is less expensive and consumes less power than a DWDM system. However, the maximum realizable distance between nodes is smaller with CWDM.
2.2.4.2 Dense Wavelength Division Multiplexing (DWDM)
Dense wavelength division multiplexing (DWDM) is a technique that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. Dense wavelength-division multiplexing revolutionized data transmission technology by increasing the capacity signal of embedded fiber. This increase means that the incoming optical signals are assigned to specific wavelengths within a designated frequency band, and then multiplexed onto one fiber. This process allows for multiple video, audio, and data channels to be transmitted over one fiber while maintaining system performance and enhancing transport systems.
Figure 2.3: Block Diagram of a basic DWDM System
DenseWDM(DWDM) is a technology with a larger (denser) number of wavelengths coupled into a fiber (>40). However, as the number of wavelengths increases, several issues need attention, such as channel width and channel spacing, total optical power launched in fiber, nonlinear effects, cross-talk, span of fiber, amplification, and so on. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. Each channel carries a time division multiplexed (TDM) signal. In a system with each channel carrying 2.5 Gbps (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wave division multiplexing (WDM).
Since each channel is demultiplexed at the end of the transmission back into the original source, different data formats being transmitted at different data rates can be transmitted together. Specifically, Internet (IP) data, Synchronous Optical Network data (SONET), and asynchronous transfer mode (ATM) data can all be traveling at the same time within the optical fiber. DWDM promises to solve the “fiber exhaust” problem and is expected to be the central technology in the all-optical networks of the future.
Fig. 2.4: The basic concept of a DWDM system with many wavelength-channels in the same fiber
In order to meet the huge capacity demands imposed on the core transmission network by the explosive growth in data communications the number of optical channels in Dense-WDM optical networks is being increased. DWDM technology was made possible with the realization of several optical components. It is also expected that several optical functions will soon be integrated to offer complex functionality at a cost per function comparable to electronic implementation. The following provides a snapshot of what has enabled the DWDM technology to become reality.
| Optical fiber has been produced that exhibits low loss and better optical transmission performance over the wavelength spectrum of 1.3 µm and 1.55 µm. | |
| Optical amplifiers (EDFA) with flat gain over a range of wavelengths and coupled in line with the transmitting fiber boost the optical signal, thus eliminating the need for regenerators. | |
| Integrated solid-state optical filters are compact and can be integrated with other optical components on the same substrate. | |
| Integrated solid-state laser sources and photodetectors offer compact designs. | |
| Optical multiplexers and demultiplexers are based on passive optical diffraction. | |
| Wavelength selectable (tunable) filters can be used as optical add-drop multiplexers. | |
| Optical add-drop multiplexer (OADM) components have made DWDM possible in MAN ring-type and long haul networks. | |
| Optical cross-connect (OXC) components, implemented with a variety of technologies (e.g., lithium-niobate), have made optical switching possible.
2.3 Development of DWDM Technology Early WDM began in the late 1980s using the two widely spaced wavelengths in the 1310 nm and 1550 nm (or 850 nm and 1310 nm) regions, sometimes called widebandWDM. Fig 2.5 shows an example of this simple form of WDM. Notice that one of the fiber pair is used to transmit and one is used to receive. This is the most efficient arrangement and the one most found in DWDM systems. Figure 2.5 WDM with Two Channels
The early 1990s saw a second generation of WDM, sometimes called narrowband WDM, in which two to eight channels were used. These channels were now spaced at an interval of about 400 GHz in the 1550-nm window. By the mid-1990s, dense WDM (DWDM) systems were emerging with 16 to 40 channels and spacing from 100 to 200 GHz. By the late 1990s DWDM systems had evolved to the point where they were capable of 64 to 160 parallel channels, densely packed at 50 or even 25 GHz intervals. As Figure 2.6 shows, the progression of the technology can be seen as an increase in the number of wavelengths accompanied by a decrease in the spacing of the wavelengths. Along with increased densityof wavelengths, systems also advanced in their flexibility of configuration, through add-drop functions, and management capabilities. Figure 2.6 Evolution of DWDM
2.4 DWDM System Functions At its core, DWDM involves a small number of physical-layer functions. These are depicted in Figure 2.7, which shows a DWDM schematic for four channels. Each optical channel occupies its own wavelength. Figure 2.7 DWDM Functional Schematic
The system performs the following main functions: Ø 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. Ø 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. Ø Transmitting the signals— The effects of crosstalk and optical signal degradation or loss must be reckoned with in fiber optic transmission. These effects can be minimized by controlling variables such as channel spacings, wavelength tolerance, and laser power levels. Over a transmission link, the signal may need to be optically amplified. Ø 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. Ø 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 is performed by transponders. On the DWDM side are interfaces to the optical fiber that links DWDM systems. 2.5 Advantages of DWDM Dense Wavelength Division Multiplexing (DWDM) is an important approach for utilizing the large available bandwidth in a single mode optical fiber.. From both technical and economic perspectives, the ability to provide potentially unlimited transmission capacity is the most obvious advantage of DWDM technology. The current investment in fiber plant can not only be preserved, but optimized by a factor of at least 32. As demands change, more capacity can be added, either by simple equipment upgrades or by increasing the number of lambdas on the fiber, without expensive upgrades. Capacity can be obtained for the cost of the equipment, and existing fiber plant investment is retained. Bandwidth aside, DWDM’s most compelling technical advantages can be summarized as follows: Ø Transparency: Because DWDM is a physical layer architecture, it can transparently support both TDM and data formats such as ATM, Gigabit Ethernet, ESCON, and Fiber Channel with open interfaces over a common physical layer. Ø Scalability: DWDM can leverage the abundance of dark fiber in many metropolitan area and enterprise networks to quickly meet demand for capacity on point-to-point links and on spans of existing SONET/SDH rings. Ø Dynamic provisioning: Fast, simple, and dynamic provisioning of network connections give providers the ability to provide high-bandwidth services in days rather than months. Ø Bit rate transparency: Optical channels can carry any transmission format in DWDM system. Thus the different wavelengths from different systems can be transmitted simultaneously and independently over the same fiber without need for a common ATM, Gigabit Ethernet etc over a common layer. Thus DWDM system can transport any type of optical signal. Ø Quick deployment: The DWDM technology is, generally, deployed using existing fibers. The time required for laying new fiber is much more as compared to equipment deployment time. Hence, the deployment of dwdm systems can be done quickly. Ø Economical: The DWDM system is cheaper as compared to overall cost of laying new fiber for increasing transmission capacity. In DWDM system, one optical amplifier is used for amplification of all the channels, hence per channel cost is drastically reduced as compared to providing regenerator for individual channels in SDH network. Ø Wavelength routing: In DWDM system, by using wavelength sensitive optical routing devices, it is possible to route any wavelength to any station. Thus it is possible to use wavelength as other dimension, in addition to time and space in designing transmission network. Ø Wavelength switching: In DWDM system, wavelength switching can be accomplished by using OADM, optical cross connect and wavelength converters. Thus, it is possible to reconfigure the optical layer using wavelength switched architecture. |
2.6 Limitations of DWDM System
Even after the vast use of dense wavelength division multiplexing (DWDM), there are some limitations of the system. The current generation of light wave systems benefit from increased transmission distance by using optical amplification and increased capacity by using dense wavelength division multiplexing (DWDM) technology. The reach of present systems is limited by the noise contributed by the used amplifiers, combined with linear and nonlinear effects from transmission. The performance of DWDM networks are strongly influenced by both linear and nonlinear phenomena that determines the signal propagation inside the fiber. However linear propagation effects are compensated and there is a class of nonlinear effects that pose additional limitations in DWDM systems.
2.6.1 Linear Effects
2.6.1.1 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, Rayleighscattering, 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 2.8). Scattering affects short wavelengths more than long wavelengths and limits the use of wavelengths below 800 nm.
Figure 2.8 Rayleigh Scattering
Attenuation due to absorption is caused by the intrinsic properties of the material itself, the impurities in the glass, and any atomic defects in the glass. These impurities absorb the optical energy, causing the light to become dimmer (see Figure 2.9). 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 are being eliminated in some new fiber types.
Figure 2.9 Absorption
The primary factors affecting attenuation in optical fibers are the length of the fiber and the wavelength of the light. Figure 2-10 shows the loss in decibels per kilometer (dB/km) by wavelength from Rayleigh scattering, intrinsic absorption, and total attenuation from all causes.
Figure 2.10 Total Attenuation Curve
2.6.1.2 Dispersion
Dispersion is the spreading of light pulses as they travel down optical fiber. Dispersion results in distortion of the signal (see Figure 2.11), which limits the bandwidth of the fiber.
Figure 2.11 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.
2.6.1.2.1 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 2.12 for dispersion-shifted fiber. For non-dispersion-shifted fiber, the zero dispersion wavelength is 1310 nm.
Figure 2.12 Chromatic Dispersion
2.6.1.2.2 Polarization Mode Dispersion
Polarization mode dispersion (PMD) is caused by ovality 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.
2.6.2 Nonlinear Effects
Optical fiber communication networks employing DWDM suffer performance degradation because of fiber non-linear. 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 [13]. They are the fundamental limiting mechanisms to the amount of data that can be transmitted in optical fiber. The nonlinear effects in fibers can be broadly classified into two categories: Stimulated scattering effects and Kerr effects. The scattering effects including Stimulated Brillouin scattering (SBS) and Stimulated Raman Scattering (SRS) [14] are due to the interaction of light waves with molecular or sound waves in fiber. The Kerr nonlinearities are self-phase modulation (SPM), Cross-phase modulation (XPM) and Four-wave mixing (FWM) which become important for long distance transmission. In DWDM, four-wave mixing is the most critical of these types.
2.6.2.1 Self-Phase Modulation (SPM)
SPM is single-channel nonlinearity. Phase modulation of an optical signal by itself is known as self-phase modulation (SPM). SPM is primarily due to the self-modulation of the pulses. Self phase modulation is a phenomenon, which occurs when the refractive index of the fiber core becomes power dependent and whereby a wavelength can spread out into adjacent wavelength by itself. Generally, SPM occurs in single-wavelength systems. At high bit rates, however,SPM tends to cancel dispersion. SPM increases with high signal power levels. In fiber plant design, a strong input signal helps overcome linear attenuation and dispersion losses. However, consideration must be given to receiver saturation and to nonlinear effects such as SPM, which occurs with high signal levels.
Fig 2.13: As a pulse (with a spectral distribution) (A) travels in a non-linear medium,it affects its refractive index (B), the slope of which (C), according to the Kerr effect, causes spectral broadening and shape distortion.
SPM results in phase shift and a nonlinear pulse spread. As the pulses spread, they tend to overlap and are no longer distinguishable by the receiver. The acceptable norm in system design to counter the SPM effect is to take into account a power penalty that can be assumed equal to the negative effect posed by XPM. A 0.5-dB power margin is typically reserved to account for the effects of SPM at high bit rates and power levels.
2.6.2.2 Cross-Phase Modulation (XPM)
Cross-phase modulation (XPM) is a multi-channel nonlinear optical effect where one wavelength of light can affect the phase of another wavelength of light through the optical kerr effect. CPM results from the different carrier frequencies of independent channels, including the associated phase shifts on one another [38]. The induced phase shift is due to the walkover effect, whereby two pulses at different bit rates or with different group velocities walk across each other. As a result, the slower pulse sees the walkover and induces a phase shift. The total phase shift depends on the net power of all the channels and on the bit output of the channels. Maximum phase shift is produced when bits belonging to high-powered adjacent channels walk across each other.
Figure 2.14 Cross-phase modulation (XPM) in DWDM System
Cross-phase modulation can be used as a technique for adding information to a light stream by modifying the phase of a coherent optical beam with another beam through interactions in an appropriate non-linear medium. This technique is applied to fiber optic communications.
XPM limits the maximum allowable power into the fiber and maximum allowable transmission length of a DWDM link. Increasing the fiber effective area will improve XPM and all other fiber nonlinearities.
2.6.2.3 Four-Wave Mixing
Four-wave mixing (FWM) occurs when two or more frequencies of light propagate through an optical fiber together. Providing a condition known as phase matching is satisfied, light is generated at new frequencies using optical power from the original signals. Generation of light through four-wave mixing has serious implications for the rapidly expanding telecommunications field of dense wavelength division multiplexing (DWDM).
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. The generation of new frequencies from two or three input signals is shown schematically in the diagram 2.15 below. The equation that defines the frequency of a FWM product is also shown.
Four wave mixing is due to changes in the refractive index with optical power called optical Kerr effect. In FWM effect, two co-propagating wave produces two new optical sideband wave at different frequencies. When new frequencies fall in the transmission window of original frequency it causes severe cross talk between channels propagating through an optical fiber.
Origin of FWM:
Figure 2.15: (a) Additional frequencies generated through FWM in the partially degenerate and (b) non-degenerate case
FWM originates from the weak dependence of the fiber refractive index on the intensity of the optical wave propagating along the fiber through the third order non linear susceptibility.
If three signal waves with frequencies fi , fj, fk are incident at the fiber input , new waves are generated whose frequencies are
fijk= fi+ fj – fk ( i, j, k = 1, 2, 3 ) ………………[2.1]
Here we exclude fijk with i=k or j=k where interruptions from other channels to signal do not happen. As a result, we will examine FWM lights with the frequency of f321, f312, f213, f332, f331, f223, f221, f113, and f112 which are shown in Fig. 2.5.3.3. Note that the number of the FWM lights is enhanced drastically with an increase in the number of channels.
This is called “four wave mixing” since three waves interfere to provide a fourth wave. Again it is sometimes called “our photon mixing”.
Let four channels consist frequencies fa, fb, fc and fd. The detailed explanation of FWM lights produced is as follows:
TABLE 2.1: The FWM frequency combinations
| Row No. | I=a, j=b, k=c | i=b, j=c, k=d | i=c, j=d, k=a | i=a, j=b, k=d |
| 1 | fabc=fa+f-fc | fbcd=fb+fc-fd | fcda=fc+fd-fa | fabd=fa+fb-fd |
| 2 | fbca=fb+fc-fa | fcdb=fc+fd-fb | fdac=fd+fa-fc | fbda=fb+fd-fa |
| 3 | facb=fa+fc-fb | fbdc=fb+fd-fc | fcad=fc+fa-fd | fadb=fa+fd-fb |
| 4 | faab=fa+fa-fb | fbbc=fb+fb-fc | fcca=fc+fc-fa | faab=fa+fa-fb |
| 5 | faac=fa+fa-fc | fbbd=fb+fb-fd | fccd=fc+fc-fd | faad=fa+fa-fd |
| 6 | fbbc=fb+fb-fc | fccb=fc+fc-fb | fdda=fd+fd-fa | fbbd=fb+fb-fd |
| 7 | fbba=fb+fb-fa | fccd=fc+fc-fd | fddc=fd+fd-fc | fbba=fb+fb-fa |
| 8 | fcca=fc+fc-fa | fddb=fd+fd-fb | faac=fa+fa-fc | fdda=fd+fd-fa |
| 9 | fccb=fc+fc-fb | fddc=fd+fd-fc | faad=fa+fa-fd | fddb=fd+fd-fb |
From the above table we can easily see that for each element in the last 6 rows there is an extra repetition. There are 24 elements in the last 6 rows. If we eliminate each extra repetition we will have 24-12=12 elements. From the upper 3 rows we get 3×4=12 individual combinations. So we get a total of 12+12=24 elements. So if we calculate using the previous equation , we will get erroneous results. To eliminate this problem of calculating the total lights produced by FWM we developed an equation which gives the desired no of FWM lights without any error.
The equation is as follows:
Number of FWM lights = [2.2]
Where, n is the number of channels.
Impact of FWM:
FWM process can b a serious limiting factor for long haul multichannel communication systems.The effect of four-wave mixing is to limit the channel capacity of a DWDM system. The effects of FWM are pronounced with decreased channel spacing of wavelengths and at high signal power levels. High chromatic dispersion also increases FWM effects. FWM also causes interchannel cross-talk effects for equally spaced WDM channels. 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.
Fig 2.16: No. of FWM Lights with increasing No. of Channels
Furthermore, the nature of the FWM crosstalk (power depleted in bit Is and noise generated in both bit Is and Os) results in an extremely high bit error rate (BER) in digital systems. Thus four wave mixing is the most sensitive to system parameters.
The problem of designing a plan of unequal spacing for FWM suppression can be reduced to an integer linear programming (ILP) problem by dividing the optical bandwidth into equal slots of bandwidth ?f large enoughto avoid appreciable overlap between spectra in adjacent slots.
2.6.2.4 Stimulated Raman Scattering (SRS)
When light propagates through a medium, the photons interact with silica molecules during propagation. The photons also interact with themselves and cause scattering effects, such as stimulated Raman scattering (SRS), in the forward and reverse directions of propagation along the fiber. This results in a sporadic distribution of energy in a random direction.
The SRS causes a power transfer between WDM channels, but it can be mitigated by the gain equalization schemes [9]. SRS refers to lower wavelengths pumping up the amplitude of higher wavelengths, which results in the higher wavelengths suppressing signals from the lower wavelengths. One way to mitigate the effects of SRS is to lower the input power. In SRS, a low-wavelength wave called ‘Stoke’s wave’ is generated due to the scattering of energy. This wave amplifies the higher wavelengths. The gain obtained by using such a wave forms the basis of Raman amplification. The Raman gain can extend most of the operating band (C- and L-band) for WDM networks. SRS is pronounced at high bit rates and high power levels. The margin design requirement to account for SRS/SBS is 0.5 dB.
2.6.2.5 Stimulated Brillouin Scattering (SBS)
For intense beams (e.g. laser light) travelling in a medium such as an optical fiber, the variations in the electric field of the beam itself may produce acoustic vibrations in the medium via electrostriction. The beam may undergo Brillouin scattering from these vibrations, usually in opposite direction to the incoming beam, a phenomenon known as stimulated Brillouin scattering (SBS).
Stimulated Brillouin scattering (SBS) is due to the acoustic properties of photon interaction with the medium. When light propagates through a medium, the photons interact with silica molecules during propagation. The photons also interact with themselves and cause scattering effects such as SBS in the reverse direction of propagation along the fiber. In SBS, a low-wavelength wave called ‘Stoke’s wave’ is generated due to the scattering of energy. This wave amplifies the higher wavelengths. The gain obtained by using such a wave forms the basis of Brillouin amplification. The Brillouin gain peaks in a narrow peak near the C-band. SBS is pronounced at high bit rates and high power levels. The margin design requirement to account for SRS/SBS is 0.5 dB.
The impact of SBS in a long-haul DWDM system is not critical due to lower channel power, but it is an important consideration in cable television transmission systems.
2.7 Some Compensation Techniques
For the upcoming ultra high bit rate optical network, nonlinearities like XPM and FWM can be the most threatening factor that would limit the performance of an optical link. Therefore, finding some suitable techniques for compensation can really be promising for the establishment of future optical network with high bit rate demand. A technique to compensate is to use dispersion compensating fiber (DCF).
It is recently reported that short period dispersion managed fiber (SPDMF) could suppress FWM efficiently with minimal dispersion accumulation [5, 9]. Unlike the conventional dispersion managed fibers, SPDMF could be accommodated easily within a single cable since it is composed of short positive and negative dispersion fibers.