Introduction

Sandip Nandi1 and Dalia Nandi2

1Department of Electronics and Communication Engineering, Kalyani Government Engineering College, Kalyani, West Bengal, India

2Department of Electronics and Communication Engineering, Indian Institute of Information Technology Kalyani, West Bengal, India

Worldwide demand for enormous information transmission has created an immense need for expansion of capacity for higher generation communication networks. It is predicted that information traffic over the internet will double every year [1]. In this present scenario, Optical networks will be the favored solution to provide enhanced bandwidth for future communication system. Terabytes of information can be transferred between two nodes over the optical links. A switch is embedded in the optical communication system to route the information under the supervision of the control signals. The major limitation for high data communication over an optical link is the use of electronic switches, which will restrict the bandwidth of the link. With the passing of time, optical switches with their improved efficiency and low cost are replacing the traditional electronic switches to overcome the limitations. An optical switch is versatile in nature as it has a large number of usages in communication networks as well as in communication cores of modern‐day high configuration computers with data rates of 1000 Gbps. However, the application of optical switches in networks has some of its own challenges in respect of signal impairments and network parameters. This book deals with the present status, advantages, and future developments of optical switches and networks.

This chapter provides a brief introduction to optical communication networks and optical switches. We start the journey with the historical perspective of each topic. Then gradually we provide the essential background, and clarify the key terminologies. Finally we present the organization of the book along with a brief overview of each chapter.

A. Optical Communication Networks

A.1 Historical Perspective

Optical fiber communication has advanced dramatically over the past 50 years due to the enormous advancement of scientific research in the fields of photonics and electronics starting from mid‐1970s with the ground‐breaking invention of Laser and low‐loss optical fiber [2–4]. This advancement is widely reflected in the rate of data transmission increasing from megabits per second over several kilometers for the first ever point‐to‐point optical fiber connection to terabits per second wavelength division multiplexed optical communication networks. Also there is a continuous increase in productivity in terms of automation, flexibility, capacity, and cost reduction. Table 1 summarizes some significant developments in the progress of optical communication technology over the past few decades.

Today’s optical networking shows the broadcast communication networks which can provide increased capacity, routing, and restoration based on recent optical communication technologies. The appreciable increase in capacity using optical networks offers cost reduction for bit transmission compared to other long‐haul communication networks (e.g. wireless networks). This has a marvelous societal influence by revolutionizing the internet service. The optical network solution covers the entire transmission network range, from the main hub and metropolitan area network to the access network domain. Backbone networks and metropolitan area networks carry highly aggregated, high‐bit‐rate data traffic, ranging from the size of cities to entire continents. The access network carries different types of data streams in and out of private and commercial customers, and these data streams are multiplexed /de‐multiplexed to the central transmission network in nodes with fixed backhaul connections. The use of the optical network undoubtedly occupies a dominant position in the core and metropolitan area network segments, and recently it has also begun to expand to the access network segment.

Table 1 Historical developments in the field of optical communication technology.

YearSignificant developments
1880Telephone invented by Alexander Graham Bell.
1948Shannon’s Limit described by Claude Shannon [5].
1957Principle of Laser described first by Charles Townes and Arthur Schawlow [6].
1966Concept of glass fiber with cladding that can carry light without much radiation described by Charles Kao and Hockham. This is the first proposal of optical fiber communication [7].
1970First semiconductor lasers made separately by Zhores Alferov at Physical Institute in Leningrad and Mort Panish and Izuo Hayashi at Bell Labs.
1987First report of erbium‐doped optical fiber amplifier by David Payne at the University of Southampton [8].
1988First demonstration of soliton transmission over 4,000 km of single‐mode fiber by Linn Mollenauer of Bell Labs.
1991First model based description of erbium‐doped optical fiber amplifier by Emmanuel Desurvire and Randy Giles at Bell Labs [9].
1993First transmission of data at 10 Gb/s over 280 km of dispersion‐managed fiber Andrew Chraplyvy [10].
1996Introduction of viable wavelength‐division multiplexing (WDM) system.
2002Nonlinearity compensation in fiber optic transmission was introduced for phase‐modulated signals [11].
2003ITU‐T standardized gigabit‐capable passive optical networks.
2009Experimental demonstration of concept of Superchannel at 1.2 Tb/s [12].
2010ITU‐T standardized 10‐gigabit‐capable passive optical networks.
2011Implementation of spatial multiplexing for optical transport capacity scaling by Peter Winzer [13].
2012ITU‐T standardized flexible‐grid WDM [14].
2016ITU‐T specification for low‐loss low‐nonlinearity optical fibers [15].
2018Development of low‐loss MxN colorless‐directionless‐contentionless (CDC) wavelength‐selective switch (WSS) [16].
2019Demonstration of Super‐C‐band transmission with 6‐THz optical bandwidth.

The era before 1990 is known for first‐generation optical networking, which includes the use of synchronous optical network (SONET/ SDH). This is used as a structure for standardizing line speed, coding technique, bitrate, network elements, and O&M functions. This first‐generation network runs on a single wavelength of each optical fiber and is opaque.

During the mid‐1990s, a new technology called wavelength division multiplexing (WDM) was proposed and implemented in optical networks, which greatly enhances the capacity of the fiber by providing a technique to transmit dozens of optical signals over a single fiber. It also implements a wavelength routing network through the use of electronic or all‐optical switching nodes. This multi‐wavelength optical routing scheme establishes the second generation of optical networks. After the introduction of WDM technology, the central backbone and the metropolitan optical network greatly depended on the high‐capacity wavelength division multiplexed link, while the traditional networks solely used the fixed capacity link between the nodes of networks with add/drop channels. Minimization of the number of repeaters correlating to minimizing the cost of the network is one of the common criteria for optimization. Hence network planning requires optimization of network design. The International Telecommunications Union (ITU) standardized a set of frequencies from which the wavelength should be chosen for the optimization.

However, due to a change in traffic or link/node failures, the network arrangement may change during the planning phase. Today the implementation of optical add/drop multiplexer and wavelength selective switching technique gives an extra level of opportunity to the execution of network design, permitting the restructuring of connections when there is any change in traffic. This approach has nominal interference with established traffic. In any case, a critical disadvantage actually stays unsettled and this is identified with the idle bandwidth issue because of the coarse and inflexible granularity of the framework.

The key element of the next generation of optical intelligent networks is the control plane, which is responsible for coordinating different network elements by introducing intelligent functions offering faster end‐to‐end link and traffic engineering with protection and restoration. Many regulatory bodies such as the Internet Engineering Task Force (IETF), the Optical Internetworking Forum (OIF), and ITU have been seriously chipping away at the applicable issues. Predicting standard of IP over WDM for the optical control plane can be well accomplished using generalized multiple protocol label switching (GMPLS). Although the central optical networks that are implemented over the past few years show good performance with respect to the present demands, there is a drawback regarding the very stringent specifications imposed on the equipment design for optical networks. This led to very complex design issues while improving the network performance. Hence future optical networks essentially require network dexterity and resource allocation flexibility that would empower a new generation of superior performance presented to the users with a lower cost. An upgraded version of GMPLS has been projected to launch in coming years which will allow more proficient use of network resources.

In addition to that, to improve the quality of service in the access domain of the network in terms of network availability, data loading speed etc., efforts are being made to shift the core infrastructure of connectivity to end users from the older copper or coaxial connection to a new optical fiber connection. The present day fiber‐to‐the‐home (FTTH) system reflects this trend to expand the range and number of users associated to each access point. Application of wavelength division multiplexing in optical access networks is still being researched, yet it is normal that soon it will be good to go.

Optical fiber networks give a high‐limit foundation to serving the developing traffic interest. In this regard, optical networking has an expanding crucial effect on our society and our personal satisfaction. Governments, research organizations, colleges, and the communications industry are putting vigorously efforts towards optical network‐related innovations with the objective of determining developments that fundamentally will work on the performance of the upcoming networks while simultaneously expanding the expense and energy productivity of the services to be implemented.

A.2 Essential Background

Optical networking uses an encoded light signal for transmitting information. Local area networks (LAN), wide area networks (WAN), and metropolitan area networks (MAN) find major uses for optical networks and they also have an application in transoceanic communication. Optical networks finds their application in a well‐organized and cost‐effective way, taking advantage of the distinctive properties of fiber. Here we provide, in brief, some essential backgrounds of optical networks.

A.2.1 Optical Networks

Commonly, optical networks are of three types: point to point link, star network, and ring network. Another variant, the passive optical network, find its application in one to multi point connectivity with the use of optical splitters. A brief description of common optical network topologies is given below.

  1. Point‐to‐point Link: At the very initial stage of optical networks, point‐to‐point transmission link between a pair of transmitter and receiver nodes was used. The transmitter node converts the information to be transmitted into an equivalent encoded optical signal by an electrical‐optical (EO) conversion process and transmits it over the optical fiber. The purpose of the receiver node is to get back the original information from the optical domain by optical‐electrical (OE) conversion process.
  2. Star Network: In this topology, multiple nodes can be connected simultaneously. Star couplers plays a major part in establishing a combined connection of multiple point‐to‐point links. The optical signal received by the star coupler is forwarded to all the output ports of the coupler. Here also EO conversion is performed at the transmitter and OE conversion is performed at the receiver. This finally build an optical single‐hop star network.
  3. Ring Network: This type of network interconnects pairs of adjacent nodes with point‐to‐point fiber links. Each node in the ring network also makes use of an OE and EO conversion process for incoming and outgoing signals, respectively. Ring networks find wide application in Fiber Distributed Data Interface (FDDI).

A.2.2 SONET/SDH

One of the key benchmarks for optical networks is Synchronous Optical Network (SONET), associated with the Synchronous Digital Hierarchy (SDH) standard. It was first notified in 1985 and found its complete form in 1988. It aims to specify some criteria for optical point‐to‐point link interfaces. These criteria permit (i) optical transmitters to use different carriers for transmission, (ii) directed optical interfacing, (iii) effortless access to branch signals, and (iv) the adding of new features in networks. To be specific, SONET explains the standard to be adopted for (i) the use of optical signals, (ii) the frame structure for synchronous time division multiplexing (TDM), and (iii) network strategy implementation and maintenance. SONET is essentially structured with digital TDM hierarchy with frame length of 125 μs. Ring network finds a wide application of SONET. The key elements to implement SONET in ring networks are add‐drop multiplexers (ADM) and digital cross connect system (DCS). ADMs essentially connect multiple SONET end devices and also combine and split network traffic at different speeds. DCS is superior to ADM in the sense that DCS can connect larger number of links than ADM as well as being able to add or drop a single SONET channel at any location.

A.2.3 Multiplexing

It is well understood that bandwidth is a major constraint in every communication system. Efficient use of bandwidth can be realized by sharing the bandwidth between multiple traffics with the use of the multiplexing technique. Optical networks also implement the multiplexing technique for the efficient use of huge optical fiber bandwidth. The major approaches for optical multiplexing are briefly discussed below:

  1. Optical TDM Network: Time Division Multiplexing (TDM) has been a well‐established technique for network design for more than 50 years. Production of a short optical pulse enables TDM to apply in optical networks at 100 Gbps. This high‐speed optical TDM (OTDM) needs significant attention to fiber transmission properties because dispersion can significantly limit the use of TDM to achieve a high bandwidth‐distance product. Also the electro‐optical bottleneck, which arises due to the modern faster processing technology at nodes in high speed networks, limits OTDM from full exploitation of bandwidth. OTDM networks are appropriate in short‐range network application. However, a long‐range network can deploy OTDM with the use of soliton transmission, where the nonlinear effect of fiber can eliminate the adverse dispersion effect. SONET/SDH is a significant example of OTDM networks.
  2. Optical SDM Network: An easy solution to the electro‐optical bottleneck is optical space division multiplexing (OSDM). Here a bunch of multiple parallel fibers replaces a single fiber for the network design. OSDM finds suitable application in short‐range transmission networks. However, OSDM becomes more costly and thus less practical for long‐range network applications.
  3. Optical WDM Network: The conventional frequency division multiplexing can also be applied in the optical domain and is called wavelength division multiplexing (WDM). Here the information from each node is sent on separate wavelength. Multiplexer and demultiplexer operate in the wavelength domain. Since WDM can operate at any random line rate maybe below the overall TDM rate, it can avoid the limitation of TDM. Furthermore, WDM can take complete advantage of fiber bandwidth thus can avoid the need for multiple fibers as needed for SDM, and therefore is cost‐effective. Hence, WDM emerges as the promising technology in optical networking. WDM fibers are installed between the nodes of optical network and can be implemented with or without electro‐optic conversion. Optical WDM networks are classified as (i) Opaque WDM Network, (ii) Transparent WDM Networks, and (iii) Translucent WDM Networks.

A.2.4 All‐Optical Networks

All‐optical networks (AON) work on the principle of optical transparency, i.e., the transmitter and receiver nodes can be directly connected optically by fully avoiding the intermediary nodes. Like SONET/SDH, AON is also based on circuit‐switched network. AON finds its application at the design level of network hierarchy. It implements all‐optical node configurations which do not use optical‐electrical conversion but instead work on the optical transparency principle. Switching and multiplexing in AON channels are accomplished by WDM. Like the ADM and DCS utilized in SONET/SDH, AONs also use a similar model known as optical add‐drop multiplexer (OADM) and optical cross‐connects (OXC). The AONs which make use of OADM and OXC are also stated as optical transport network (OTN). The use of optical bypassing in OADM and OXC makes AONs cost‐effective and thus distinguishes it as a superior network application in the optical domain. The network agility of AONs can be enhanced by the use of reconfigurable OADM (ROADM) and reconfigurable OXC (ROXC).

A.2.5 Optical Transport Network

The International Telecommunication Union (ITU) designed the Optical Transport Network (OTN), a next‐generation standard protocol, as a replacement to SONET/SDH. It is also called a “digital wrapper”, which can give a productive and internationally acknowledged approach to multiplex various services in optical domain. Different traffic types, such as Ethernet, storage, and digital video, as well as SONET/SDH, can be transmitted over a single Optical Transport Unit frame thanks to OTN’s enhanced multiplexing capacity. Traditional WDM transponder‐based networks offer substantial advantages over OTN‐based backbones and metro cores, including enhanced efficiency, dependability, and wavelength‐based private services. The IP‐over‐OTN infrastructure also provides improved management and monitoring, fewer hops, better service protection, and lower equipment acquisition costs. OTN plays a significant role in making the network an open and programmable platform, allowing transport to become as important as computation and storage in intelligent data center networking, in addition to scaling the network to 100G and beyond.

B. Optical Switching in Networks

B.1 Historical Perspective

The nineteenth century was the revolutionary phase in communication technology because the first ever electrical communication was implemented with the evolution of the telegraphy system. At the same time the concept of switching was also introduced in communication. Although initially the switching was manual, gradually it was improved to the semi‐automatic mode known as store and forward message switching [17]. The same principle of switching was adopted to further developed communication systems like telephony, facsimile, etc. till the computer network was evolved specifically for data communication. When computer networks were implemented, two new techniques of switching, circuit switching and packet switching, were developed gradually for better and hassle‐free communication in and around 1960 [18–20]. Although earlier message switching was found to be efficient compared to circuit switching in terms of bandwidth utilization, it was not the better solution for modern data communication networks. In this scenario packet switching was found to be a better way because of its simplified data storage capability and the requirement for less of a retransmission process. Therefore it is understood that switching is an inherent process for communication technology to gain its application. The limitations of electrical communication systems have led people to move to optical communication systems. The optical communication systems have proven to be much more advantageous over the electrical systems as they overcome problems or limitations like bandwidth, speed, security, reduced system noise, and several other factors which are undesirable for a faithful and sustainable communication system [21, 22]. At the same time, the idea of optical switching gained attention and within three decades from 1970 to 2000, several attempts were made to design optical switches [23–25]. The invention of WDM technology to cope with the growing demand for internet creates a challenge in designing optical switching over a multi‐wavelength channel. One of the problems lies in the optical processing. The optical processors that have been developed till now can only process low‐speed signals or signals with low bit rates. The processors need electrical signals which are converted from optical signals are processed and then converted back to optical signals. This is done using optical‐electrical‐optical switches or OEO switches [26, 27]. Now this conversion and back‐conversion takes up a lot of power and time, which are not desirable for high‐speed systems. As a result, optical switching has been promoted as a remedy to these issues and a key to switching alleviation [28–30]. New optical switching technologies have been created as a consequence of extensive research and development, and existing ones have been improved as a result of advancements in material science and manufacturing processes. Many of these technologies became feasible options for implementation in networks. Obviously, the move to the optical domain would provide the network many significant benefits. Optical switching is the backbone of optical network and is vital for the network architecture of tomorrow. As such, it is the answer to many network challenges and an essential component of the long‐term solution.

B.2 Essential Background

An optical switch is a device which can switch the optical signals between different circuits operating in optical domain. The capacity of the optical networks can precisely and completely be utilized with the help of optical switch technology. This section gives a brief overview of the basic optical switching technology.

B.2.1 Optical Switching in Networks

Electronics handle all of the additional functions in SDH/SONET networks, including multiplexing, cross‐connection, add/drop, and control. Additionally, some equipment suppliers have created cross‐connects based on electronic switching matrices with optical interfaces (OEO systems) and marketed them as optical cross‐connects and/or optical switching systems. Switching systems based on optical switching fabrics, on the other hand, are known as OOO systems. While OOO systems are commonly referred to as transparent or all‐optical, their OEO counterparts are frequently referred to as opaque. Transparency refers to a network’s ability to convey any sort of data regardless of protocol and encoding schemes, data speeds, or modulation scheme. Transparency is inherent in optical networks, because data is transferred and switched end‐to‐end in the optical domain. Electrical networks, in contrast to transparent networks, are opaque since their performance is based on signal type and specifications. This is due to their capacity to read and analyze the signals they transmit, which is advantageous in many applications. WDM technology and optical switching on the one hand, and electronic control on another, are merged in a partial or complete transparent network. This integrates the greatest aspects of optics and electronics.

B.2.2 Optical Switching in Practice

Optical switching, like electrical switching, is divided into two types: optical circuit switching (OCS) and optical packet switching (OPS). Switching takes place at the granularity of an optical circuit in OCS. Because traffic load dominates the network and circuit switching is not optimal for traffic volume, the research community has placed a strong emphasis on OPS, which performs optical switching at the packet level. Although OPS is often envisaged as an electronically controlled system, it confronts problems in this optical form, such as the requirement for fast adaptable optical switching technologies, optical packet segmentation, and synchronization. Optical Burst Switching (OBS) was developed over the years as a balance between OCS and OPS, and has subsequently attracted a lot of research attention. Because OBS is packet‐based, it may be more bandwidth‐efficient than OCS. The necessity to read/write packet headers in the form of light as well as the extremely tiny switching granularity are both challenges for OPS. OBS minimizes the load on control by combining packets into bigger bursts. The requirement for optical header reading/writing is eliminated by sending data and control separately and handling control electronically. OBS, on the other hand, has its own technological and design issues.

B.2.3 Optical Switch Technology

Different classifications exist for optical space switching technologies. The optical electrical optical (OEO) switch and the all‐optical switch (OOO) switch are the two most common types. Optical switching technologies can also be categorized depending on the fundamental physical effect that causes the switching process to occur. Numerous technology categories may be observed in this situation. Each of these classes contains a variety of technology types based on how the physical effect is exploited, the device design, the material used, and other factors. Some of these physical effects include: electro‐optic, acousto‐optic, thermo‐optic, magneto‐optic, liquid crystal‐based, SOA‐based, and many more. It should be emphasized that in many of the above category, regardless of the physical effect liable for the switching process, which is used as the foundation for this category, external control of the switching device is electrical.

On the other hand, optical signals are now used to transmit nearly all information due to their fast bit rate data transfer and huge bandwidth. Photonics has played a crucial part in this development since it allows for improved light‐matter interaction to regulate optical signals for specific applications. Because there is no need to transform optical‐electrical/electrical‐optical signals at the interfaces, all‐optical switches enable extremely efficient data transfer. This reduces power consumption and enhances operation speed. Photonic crystal‐based switches play an important role in switching operations to satisfy the rising need for high data rate, bandwidth, low loss, and low power consumption.

C. Organization of This Book

In light of the realities discussed above, this book comprehensively and cohesively presents recent developments in optical switching and its application. Most of the recent challenges involved in the commercialization of optical switching are examined and discussed in this book. The book is organized in three parts. Part A provides the basic introduction of optical switching. Then we have presented the device technology for optical switching in Part B. Part B contains 13 chapters based on different device physics adopted for designing optical switches. A number of switching mechanisms associated with optical communication systems are summarized in a brief in Chapter 1. Chapter 2 gives a review of current electro‐optic switches in terms of operating principle, fabrication material, and device structure along with the performance issues and subsequent challenges. The principles of thermo‐optical switches including thermo‐optic effect, trade‐off between switching time and power consumption, cross‐talk, as well as other merits are discussed in Chapter 3. Chapters 4 and 5 also describe the principle, design, and application area of magneto‐optic and acousto‐optic switches respectively. Further, some special switch fabrication, with underlying theory, principle of operation, and application area are described in this book. To be specific, Chapter 6, 7, and 8 focus on MEMS‐based optical switches, SOA‐based optical switches and liquid crystal optical switches, respectively. Chapters 9 and 13 specifically deal with photonic switches. Two special chapters (Chapter 10 and 12) are added in this book to illustrate the quantum and non‐linear effect in designing optical switches. An additional Chapter 11 is added which focuses on the optical electrical optical (OEO) effect and its advancement. To summarize, Part B of this book will cover the wide range of possibilities of optical switch design and fabrication, and will certainly provide comprehensive guidance to students and researchers.

Part C of this book emphasizes the application of optical switches in optical networks. Five chapters are included in this section to discuss all the possibilities and challenges of the application of optical switches in networks. Starting with the switch fabric control in Chapter 14, this section gradually examines the reliability of switches in networks in Chapter 15 along with the detailed discussion of challenges and mitigation processes for protection and restoration of optical switches in networks covered in Chapter 16. Chapter 17 provides an illustrative idea for the application of optical switches in high‐performance computing, which is currently a hot topic in computer networks. Finally, a special Chapter 18 is provided in this book with the discussion of software for optical network modelling, which will immensely help students and new researchers in this field to acclimatize with the available software.

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