Optical and Microwave Technologies for Telecommunication Networks

By Otto Strobel

This is a self-contained book on the foundations and applications of optical and microwave technologies to telecommunication networks application, with an emphasis on access, local, road, cars, trains, vessels and airplanes, indoor and in-car data transmission as well as for long-distance fiber-systems and application in outer space and automation technology. The book provides a systematic discussion of physics/optics, electromagnetic wave theory, optical fibre technology, and the potential and limitations of optical and microwave transmission.

• Language: English
• Publisher: Wiley
• Release date: Mar 28, 2016
• ISBN: 9781119154594

Book preview

Preface

After human beings solved their most elementary problems of nutrition and availability of warming and protective clothes, they felt the need to communicate between each other. Even then, this communication improved the results of their labor. People first started by talking to each other at distances our ears are able to understand acoustically. The next step was visible communication limited by the resolution and focusing abilities of our eyes. Smoke signals, for example, were used during the day and fire beacons at night. The oldest written proof of optical communication is presented in AESCHYLOS'S (Aισχυλoς) play Agamemnon, written in the 5th century BC [1.25]. The news of the fall of Troy in 1200 BC, after years of siege by the Greeks, was reported to Agamemnon's wife Clytemnestra by fires which were lit on hills all the way from Asia Minor to Argos in Greece.

The first development of a useful optical telegraph happened to be during the time of the French Revolution. CLAUDE CHAPPE, a former Abbé, invented the semaphore. On top of a building a moveable beam was arranged, which carried a moveable arm at both ends; 192 different positions could be realized. In 1880, Alexander Graham Bell invented the photophone. The idea was that a light beam was modulated by acoustic vibrations of a thin mirror. The demodulation of the optical signal could be realized, for example, by utilizing the photoelectric effect in selenium.

All free space transmissions depend on good weather and undisturbed atmosphere. Some methods work only during the daytime, some only at night. An exception is free space transmission in outer space because, outside of the Earth's atmosphere, typical problems like natural disturbances by fog, rain or snow or artificially caused impurities do not inherently exist. However, even on Earth, it was desirable that communication is independent of environmental conditions. Therefore, some form of guidance of the light beam in a protective environment was necessary. There were ideas of guiding the light within a tube, whose inner walls reflect the light.

The development of the laser by Theodor Maiman, at the beginning of the 1960s, provided a light source which yields an entirely different behavior compared to the sources we had before. A short time after this very important achievement, diode lasers for usage as optical transmitters were developed. Parallel to that accomplishment in the early 1970s, researchers and engineers accomplished the first optical glass fiber with sufficiently low attenuation to transmit electromagnetic waves in the near infrared region. The photodiode as detector already worked, and thus, systems could be developed using optoelectric (O/E) and electrooptic (E/O) components for transmitters and receivers, as well as a fiber in the center of the arrangement. In 1966, Charles K. Kao and G.A. Hockam of Standard Telecommunications Laboratories in Harlow, England, published a paper in which they proposed the guidance of light within dielectric glass fibers. The immediate problem was the optical attenuation in fibers. Whereas, on a clear day, atmospheric attenuation is about 1 dB/km, the best glass then available showed an attenuation of about 1000 dB/km. To illustrate this, the optical power is reduced to 1‰ after a path of only 30 m. Kao and Hockham's main thesis was that if the attenuation could be reduced to 20 dB/km at a convenient wavelength, then practical fiber-optic communication should be possible.

In 1970, Corning Glass Works, USA, achieved this goal. By further refinement of fiber production, the attenuation coefficient could be reduced to below 0.2 dB/km in 1982. Fibers of commercial mass production today show an attenuation of approximately 0.2 dB/km. The optical power in such a fiber still amounts to about 1% after traveling a distance of 100 km.

In the 1970s and 1980s, reliable semiconductor light sources and detectors were developed. First field trials of fiber-optic links were very successful during the 1980s.

People often discuss the quality of systems in simple terms, such as good or bad. From the physical point of view, nothing is good or bad; it is as it is – the only question is what you need it for. For example, are we discussing a high-speed long distance system in the order of one-tenth of Gbit up to 100 Gbit/s (or more) with nearly no cost restriction, or are we talking about application in cars with 150 Mbit/s and about 10 m link lengths at low cost demands? These are completely different worlds and thus, for each demand, we have to find the proper solution.

In the last five decades, landline network communication has mainly been considered for application in telecom areas. The most well-known use is for high-speed, middle and long distance systems, as well as MAN and LAN networking; any last-mile application, including in-house communication to a single user's desk, needs to be connected to the rest of the world. Most recently, mobile communication, in particular cell phones (more recently smart phones), tablets, tablet PCs, laptops, PCs, etc., have been developed to replace cable-based phone calls, emails and Internet communication.

For about 20 years, Fiber-to-the-home (FTTH) has become the phrase on everybody's lips – the efforts to also bring optical communication into a single-family house. This did not happen until now for reasons of economy. However, because of the soaring use of the Internet, higher data rate needs increasingly occur in single-family houses, too. In order to permit a corresponding quantum leap, it remains absolutely essential to reduce costs for the participants. The keyword is opening up the last mile. Latest developments can help to achieve this aim.

In the last ten years, communication in transportation systems has become more and more in demand – for communication within a vehicle, from one vehicle to another and to land-line networks too. Development started in high-end cars with application in the infotainment area and has already reached airplanes and ships where sensor-relevant needs were also addressed. These techniques began with low data rates. Car communication technologies for the coming decade will also include high bit-rate systems up to the level of Gbit/s. Moreover, a new industry-standard, named communication in automation engineering, has been developed. By applying this technology, new perspectives could be opened up for data linking between tooling machines and central control units.

The idea of this book is to address a broad scope of readers, in order to give them an introduction to optical and microwave communication systems. For this reason, we not only present articles on state-of-the-art methods but also promising techniques for the future are discussed as well. On the one hand, it is important that the key differences between optical and non-optical systems are appreciated, yet on the other hand, similarities can be also seen. Moreover, a combination of these different physical techniques might lead to excellent results, which cannot be reached using them separately. Taking all these optical and microwave techniques, as well as GPS, together with high-speed high-data processing devices and appropriate software, may mean that the old human dream of easy worldwide communication (involving nearly unlimited data consumption), be it listening, seeing or reading, could be realized in the not too distant future.

For readers not familiar with all these topics, there is coverage of many subjects of optical and microwave fundamentals. The book is intended to help undergraduate, graduate and PhD students with a basic knowledge of the subjects studying communication technologies. In addition, R&D engineers in companies should also find this book interesting and useful. This is true for novices as well as for experts checking certain facts or dealing with areas of expertise peripheral to their normal work.

I would like to express my appreciation to my former colleagues at Alcatel-Lucent Research Center (now Bell Labs Germany) for numerous helpful discussions. I also gratefully acknowledge my current colleagues at Esslingen University for much help, in particular Prof. Dr. Dr. h.c. R. Martin. Moreover, I have to mention my staff member Dipl.-Phys. H. Bletzer for active support in lab and manuscript preparation. For the latter, also many thanks to M.Sc. Marko Cehovski, who as my student also co-authored several publications – also Dipl.-Ing. Daniel Seibl, M.Sc. Jan Lubkoll, now with ASML Veldhoven/Niederlande. For this book, I was able to find a variety of R&D contributors from companies and universities all over the world: MSc. Werner Auer, FOP Faseroptische Produkte GmbH, Crailsheim, Germany (Chapter 4.1), Dr. Krzysztof Borzycki, National Institute of Telecommunications, Warsaw, Poland (Chapter 4.2 and 10.1), Dr. Ronald Freund, Dr. Markus Nölle, Fraunhofer Heinrich Hertz Institute, Berlin, Germany (Chapter 9.2), Dr. Ronald Freund, Dr. Nicolas Perlot, Fraunhofer Heinrich Hertz Institute, Berlin, Germany (Chapter 9.3), MSc. Marko Čehovski, Institut für Hochfrequenztechnik, Technische Universität Braunschweig, Germany (Chapter 9.5), Thorsten Ebach, eks Engel GmbH & Co. KG, Wenden, Germany (Chapter 9.6), Dr. Alicia López, Dr. M. Ángeles Losada, Dr. Javier Mateo, GTF, Aragón Institute of Engineering Research (i3A), University of Zaragoza, Spain (Chapter 10.2), Dr. Joaquín Beas, Dr. Gerardo Castañón, Dr. Ivan Aldaya, Dr. Alejandro Aragón-Zavala, Tecnológico de Monterrey, Mexico (Chapter 10.3), Prof. Dr. Zabih Ghassemlooy, Dr. Hoa Le Minh, Dr. Muhammad Ijaz, Northumbria University, Newcastle, UK, (Chapter 10.4), Dr. Riccardo Scopigno, MSc. Daniele Brevi, Multi-Layer Wireless Research Area, Istituto Superiore Mario Boella, Torino, Italy, (Chapter 10.5), Dr. Paolo Monti, Dr. Lena Wosinska, Dr. Richard Schatz, KTH Royal Institute of Technology, Stockholm, Sweden, Dr. Luca Valcarenghi, Dr. Piero Castoldi, Scuola Superiore Sant'Anna, Pisa, Italy, Aleksejs Udalcovs, Institute of Telecommunications, Riga Technical University, Riga, Latvia, (Chapter 10.6), Prof. Dr. Kira Kastell, Frankfurt University of Applied Sciences, Germany (Chapter 11.1), Prof. Dr. Vladimir Rastorguev, Dr. Andrey Ananenkov, Engineer Anton Konovaltsev, Prof. Dr. Vladimir Nuzhdin; Engineer Pavel Sokolov, Moscow Aviation Institute, National Research University, Russia (Chapter 11.2). Also Dr. rer. nat. Sebastian Döring from TU Braunschweig, Germany has to be acknowledged for actual contributions to recent research in organic Lasers (OLASERs). All R&D projects were carried out in cooperation with companies such as Alcatel-Lucent (Bell Labs Germany), HP, Agilent, Mercedes-Benz Technology, Siemens, Diehl Aerospace and Balluff Germany. The same holds for national and international universities, in particular concerning Bachelor, Master and PhD theses. Grateful acknowledgement to all university and company members involved.

Last but not least, a very big thank you to my family, especially to my wife Dorothee (who has to date suffered more than 40 years under an often absent professor), as well as my children Sven, Jana and Jasmin.

Otto A. Strobel

Esslingen/Germany, Summer 2015

1

Introduction

In this book, we present state-of-the-art and next-decade technologies for optical and related microwave transmission in telecom applications, high-speed long distance as well as last-mile and in-house communication. Furthermore, we have learned a lot about the needs of companies producing state-of-the-art systems. They use practical systems in order to compete in the market for such products. They want a complete solution to their demands, and they do not care if fiber optics are part of the solution or not. Consequently, nowadays, further physical techniques have to be developed: Wireless applications open up a new field of data transmission. High-speed wireless LED transmission offers short-range data transmission without EMI/EMC problems. Visible light communication using high power LEDs is an interesting technique. The first aim of using these light sources is to illuminate a room. However, at the same time, they can be modulated to transmit a data signal (Figure 1.6). Thus, optical and non-optical solutions, microwave-, Radio over Fiber- or even RADAR-systems have to be developed.

All these systems working in common will offer high-speed up- and downloads for offices, labs and private homes, and also for transportation systems such as cars, airplanes and ships.

Since the beginning of the 1960s, there has been a light source which yields a completely different behavior compared to the sources we had before. This light source is the LASER, Light Amplification by Stimulated Emission of Radiation. Basic work had been published already, in 1917 by Albert Einstein [1.1]. The first laser realized was the bulk-optic ruby laser, a solid state laser [1.2] developed by Theodor Maiman in 1960. A short time after this very important achievement, diode lasers for use as optical transmitters were developed [1.3]. At the beginning they were difficult to operate. They had to be cooled using nitrogen at –169°C. It took until 1970 to drive semiconductor diode lasers in a continuous wave (CW) mode at room temperature [1.4]. Parallel to that accomplishment, the use of dielectric optical waveguides as media for transmission systems was suggested. Charles Kao and George Hockam [1.5] can be regarded as inventors of fiber-optic transmission systems, as well as Manfred Börner [1.6]. Nowadays, their invention would not be regarded as anything remarkable. Take a light source as a transmitter, an optical fiber as a transmission medium, and a photodiode as a detector (Figure 1.1)! Yet, in the 1960s it was a revolution, because the attenuation of optical glass was in the order of 1000 dB/km corresponding to a factor of 10-100 over one kilometer. This was totally unrealistic for use in practical systems.

Image described by surrounding text.

Figure 1.1 Basic arrangement of a fiber-optic system

In the early 1970s, physicists and engineers in research laboratories developed the first optical glass fiber with sufficient low attenuation to transmit electromagnetic waves in the near infrared region (Corning Glass Works [1.7]). They achieved a value of about 20 dB/km, that is, after one kilometer there is still 1% of light at the detector from the light which was coupled into the fiber by the transmitter. Today's fibers have an attenuation of below 0.2 dB/km, which means the 1% value is still achieved after 100 km of the fiber.

The development of optical detectors started much earlier. Already in 1876, W. Adams and R. Day proved the separation of charge carriers in Selenium [1.8] by illumination of light. They discovered the inner photo effect, also named the photovoltaic effect. This effect is still the fundamental process exploited in modern photo detectors. First experiments with silica solar cells took place in the early 1950s, and new developments for practical use to transmit a data signal started in 1970 [1.9]. Thus, systems could be developed using optoelectric (O/E) and electrooptic (E/O) components for transmitters and receivers, as well as a fiber in the center of the arrangement. The main fields of application of such systems are found in the area of fiber-optic transmission and fiber-optic sensors (Figure 1.2).

Diagram shows optical transmission, electrical and optical measurement system of an electrical signal which includes electrical signals, physical parameters and sensor.

Figure 1.2 Application of fiber-optic systems

In the beginning, in both areas exclusively, the intensity of light was of interest. In analog as well as in digital systems, signal transmission is realized by modulation of the laser power. At the fiber end, an intensely sensitive receiver is used exclusively to detect the data signal. Regarding sensor systems, the measurement of the physical quantity of interest is also exclusively concerned with the power of the light. In this case the optical power is varied by exploiting a change of the fiber attenuation or other system components. Modern more sophisticated systems make use of the fact that an electromagnetic wave also carries information on frequency, phase and polarization.

The media to transport the propagation of light is named the waveguide. The most common waveguide is a glass fiber. The geometrical shape does not need to be strictly circular like a fiber, but can also be rectangular forming a planar waveguide. This was suggested by Stewart Miller in 1969 and thus the new field of integrated optics was born [1.10]. The aim of this technology is to integrate a maximum of components onto a single chip such as an integrated circuit – elements with a variety of different functions within the smallest space to avoid macro-optic devices like mirrors and lenses. Some integrated optic components are able to influence all parameters of an optical wave: amplitude (intensity), frequency, phase and polarization. However, the first optical transmission is much older. Native Americans for instance, were already communicating with smoke signals a long time ago [1.12,1.13]. Furthermore, it was a very sophisticated and modern system, because it was already a digital system, consisting of binary 1 and binary 0 (smoke/no smoke) (Figure 1.3).

Diagram shows two persons standing on the top of a hill transmitting binary codes, one and zero using a smoke signals to a receiver standing on the ground.

Figure 1.3 Digital optical transmission by use of smoke signals

Thus digital systems have already existed for a long time, providing the basics for information technologies, information processing and transmission. However, analog techniques are still of interest – physical quantities at the origin and the reception (e.g. human reception). But as soon as they are processed or transmitted nowadays, almost exclusively digital techniques are used. A further free space transmission has also been developed, the optical telegraph realized by Claude Chappe. He invented a semaphore set-up by means of movable bars able to produce several signs. But free space transmission on Earth suffers from atmospheric disturbance [1.11]. This also holds for free space laser transmission on Earth, which came to light in the 1960s after the invention of the laser. There are exceptions in outer space applications and for short distance air transmissions (Chapter 9.3).

The real breakthrough of optical data transmission systems came with the glass fiber, which had sufficient low attenuation for propagation of electromagnetic waves in the near infrared region. This low value of attenuation is one of the most attractive advantages of fiber-optic systems compared to conventional electrical ones.

Figure 1.4 depicts the attenuation behavior. In particular, we observe independence of the modulation frequency of fiber-optic systems in contrast to electrical ones which suffer from the skin effect. Yet it has to be confessed that there are different problems leading to a frequency limit, the dispersions: modal, chromatic and polarization mode dispersions must be mentioned (Chapter 3). Solutions of how to deal with these problems with sufficient success will be presented. In the end, the enormous achievable bandwidth must be highlighted. That leads to a high transmission capacity in terms of the product of achievable fiber bandwidth B and length L, also named transmission capacity, Ct. It is a figure of merit, as one of the most important goals is to maximize this product for every kind of data transmission with respect to the demands concerning its application.

Graph shows a coaxial line representing attenuation of a plastic fiber, a glass fiber and a silica glass as a function of modulation bandwidth.

Figure 1.4 Attenuation of coaxial cables and optical fibers

(1.1) numbered Display Equation

where:

B maximum achievable bandwidth and

L maximum achievable link length

In addition, low weight, small size, insensitivity against electromagnetic interference (EMI, EMC), electrical insulation and low crosstalk must be mentioned.

Glass fiber systems are used in the near infrared range, right above the wavelengths we can see with our eyes. As optoelectronic components for light sources, we apply GaAlAs (Gallium-Aluminium-Arsenide) LEDs and laser diodes for wavelengths in the 850 nm region and InGaAsP (Indium-Gallium-Arsenide-Phosphide) devices for the long wavelengths of about 1200 nm to over 1600 nm. Photodiode materials of interest are well known, Si for 850 nm, and Ge and InGaAsP for the long wavelength range (Figure 6.40). However, an optical communication system is more than a light source, a fiber, and a photodiode. There is a laser driver circuit necessary to provide a proper high-bit-rate electric signal; this driver, combined with a laser or an LED, builds the optical transmitter. Also the photodiode (pin or APD – Avalanche Photodiode), together with the front-end amplifier, forms the optical detector, also called the optical receiver. This front-end amplifier consists of a very highly sophisticated electric circuit. It has to detect a high bandwidth operating with very few photons due to a large fiber length and it is struggling with a variety of noise generators. In addition, there are further electric circuits to be taken into account, such as circuits for coding, scrambling, error correction, clock extraction, temperature power-level, and gain controls. If the desired link length cannot be realized, a repeater consisting of a front-end amplifier and a pulse regenerator will be inserted. This pulse regenerator is necessary to restore the data signal before it is fed to a further laser driver followed by another laser (Figure 9.2). Alternatively, an optical amplifier can be used, in particular the Erbium-doped fiber amplifier is a great success (Figure 8.2).

Moreover, instead of unidirectional systems, we need bi-directional ones (Figure 9.9); that is, it is not sufficient that for a telephone link a person at one side of the link is able to speak, but at the other side another person can only listen; the system does not operate the other way round. To overcome this inconvenient situation, optical couplers on both sides of the link are inserted (Figures 4.53 and 4.54). The two counter propagating optical waves superimpose undisturbed, and they separate at the optical couplers on the other side of the link and reach the according receivers. To improve the transmission capacity drastically, wavelength selective couplers are applied, called multiplexers and demultiplexers. Several laser diodes operating at different wavelengths are used as transmitters; their light waves are combined by the multiplexer and on the other link end they are separated by the demultiplexer. This set-up is named the wavelength division multiplex system (WDM). If we apply this arrangement again in the two counter propagating directions, we achieve a bi-directional WDM system. The transmission capacity then rises by the number N of the channels transmitted over one single fiber.

For about 20 years now, last mile communication has been discussed. The idea of fiber to the home (FTTH) has also been discussed, but until now this did not happen because it was too expensive. Latest improvements could help to achieve this special communication. All-plastic PMMA-fibers (poly methyl methacrylate) have been developed, named Polymer Optical Fibers (POF), which feature in contrast to the previous PMMA-fibers at a considerably lower attenuation [1.17] (Chapter 10.2). As an economic alternative to the application of semiconductor lasers, another important step is the development of cheap high-speed LEDs [1.18] or low-cost VCSELs (Vertical Cavity Surface Emitting Lasers) [3.1), which can be modulated fast enough.

Combinations of fiber-optic with mm-waves systems or coaxial cable systems have been developed for the local area network as a possible alternative. As another alternative to cable systems, the declared dead free space transmission could be also revived with distances in the 100 m range. For this purpose, the light emerging from a fiber is fed to a lens and formed into a parallel beam. At the reception site it is again coupled into a fiber or directly to a detector.

This conjunction of fiber-optic transmission and free space may successfully be used in sky scraper areas, where air distances lie in the 100 m range and cable systems would need to be in kilometres. Wireless data transmission is also an interesting option for distances in the 10 m range. Connections between, for example, a PC, printer, scanner or adjacent participants in intranets (LANs, Local Area Networks, see below) should not be bridged by interfering electrical cables. Further developments in the microwave range have been developed, such as the recent well-known Bluetooth systems.

Moreover, besides typical point-to-point connections, network systems are necessary. In nearby zones, for example inside a business house, the commonly used term is LANs (Local Area Networks); in the local net or metro region it is MANs (Metropolitan Area Networks), [9.85–9.87]. The network topologies are bus, star or ring structures (Chapter 9.5.3).

Furthermore, free space transmission is gaining a particular renaissance in outer space, because outside of the Earth's atmosphere typical problems such as natural disturbances by fog, rain, snow or artificially caused impurities do not inherently exist. Therefore, laser free space connections between satellites have been already tried and tested successfully (Chapter 9.3).

In the last 10 years, communication in automotive systems became of great interest (Chapter 11.1). Currently, optical data buses in vehicles are almost exclusively used for infotainment (information and entertainment) applications. The Media Oriented Systems Transport (MOST) is the optical data bus technology used nowadays in cars with a data rate up to 150 MBit/s (Figure 11.5 [1.14]). The development of infotainment applications in cars began with a radio and simple loudspeakers. Today's infotainment systems in cars include but are not limited to ingenious sound systems, DVD-changers, amplifiers, navigation and video functions. Voice input and Bluetooth interfaces complement these packages. Important and basic logical links of these single components are already well known from a simple car radio. Everybody probably knows the rise of volume in the case of road traffic announcements. However, the integration of more and more multimedia and telematic devices in vehicles led to a large increase in data traffic demands. In particular, for luxurious classes, a huge need for network capacity and higher complexity by integration of various applications have to be taken into account. MOST 150 operates with LEDs, a POF and silicon photodiodes. However, to enable the next step towards autonomous driving, new bus systems with higher data rates will be required.

Another serious challenge arises in protecting new generation aircrafts, particularly against lightning strikes [1.15]. This is because new airplanes will be built using carbon-fibers to reduce the weight of the fuselage. Therefore, these airplanes will lose their inherent Faraday cage protection against lightning, cosmic radiation and further electrostatic effects (Figure 1.5).

Graph shows a coaxial line representing attenuation of a plastic fiber, a glass fiber and a silica glass as a function of modulation bandwidth.

Figure 1.5 Lightning strike in an airplane [1.22]. Source: Reproduced with permission of Denny Both, Piranha.dl 3d Animation, Berlin

In order to avoid failures in signal transmission in the physical layer, the electrical copper wires should be well protected. But this solution is too expensive and increases the weight of the cables [1.15]. A reasonable solution is to use glass or plastic fibers as transmission media in these new airplanes. Since the FlexRay bus protocol [1.16] is more adequate for avionic applications, it should be adapted for this kind of transmission. Thus, this solution is cost-efficient and offers more safety in the aviation domain. A promising solution for higher sophisticated systems could be the use of optical data transmission based on new laser types, such as VCSELs and the application of Polymer Cladded Silica (PCS) fibers. This enables EMC compatibility and paves the way for the future.

In this book we mainly give an introduction to optical transmission. Emphasis is on fiber transmission systems, working with basic components. The reader should be familiar with the fundamental optical techniques for communication systems. Moreover, for more comprehensive considerations there are further components to be dealt with, for example the optical amplifier to enhance the link length. In order to achieve this, Erbium and Raman amplifiers (Chapter 8) [8.1,8.2] have been developed to overcome the problem of attenuation in fibers. Due to the above-mentioned dispersions, there are signal distortions in optical fibers. The systems suffer from pulse broadening (Figure 3.21) leading to bandwidth reduction with impact on the transmission capacity, the product of bandwidth and fiber length. Solutions to overcome or at least to reduce these problems are discussed in Chapter 3.2.

Furthermore, wireless applications open new fields for data transmission [1.19]. High speed wireless LED transmissions offer short and middle range data transmission without EMI/EMC problems (Figure 1.6). Higher bandwidths than non-optical wireless applications will offer high-speed up- and downloads in offices, labs and private homes.

Diagram shows a cubic structure with a white light LED panel, reflections and line of sight in the downward direction.

Figure 1.6 Room lighting with inherent data modulation and transmission

Also, car-to-car communication could be an interesting scenario (Figure 11.1 [1.20]). In particular, safety relevant applications are of great interest (Iizuka 2008, [1.20]). The catch-word is Pre-crash safety by VL-ISC: Visible Light Image Sensor Communication. Human reaction time is a problem in security, as in difficult circumstances it could last much longer than a sensor does. The vehicle in front might suddenly start braking. Thus, depending on the brake pressure, immediately its stoplights will give information to the following vehicle. In critical cases the following vehicle will automatically start emergency braking to avoid a collision or at least to avoid serious damage.

A further new development is shown in Figure 11.2, a red-light-to-car communication. In this case, the red light tells a waiting driver that he will get a green signal in 50 seconds. Another example would be that a car is approaching a red light and the driver gets the information 10 seconds before he can even see it. While the driver is waiting at the red light, the system receives the red interval from the traffic light in order to take the decision to stop the engine. The outcome would be a gas-mileage and CO2 reduction of more than 5% (November 2008). In addition, the driver does not have to fix his eyes on the traffic light permanently. These systems can predict idling intervals with accuracy to solve unnecessary engine stops and starts. Including information about the green, amber and red time zones, the traffic volume can be better regulated.

Moreover, a new industry standard, named communication in automation engineering, has been developed. Applying this technology has opened up new perspectives for data linking between tooling machines and a central control unit: Industry 4.0 is a collective term for technologies and concepts of value chain organization (Chapter 9.6).

Finally, non-optical techniques have to be taken into account (Chapters 10.5 and 11.2). For example, WLAN and even RADAR systems can be used in automotive application, to guarantee more safety in limited optical visibility situations like heavy rain, fog or snow.

As mentioned above, it has to be underlined that nowadays the user wants a complete solution for his demands and he does not stop to ask if that is fiber optics or whatever. Moreover, he also wants combinations of other physical techniques with or without fibers, so Optical wireless communications, Optical and Non-Optical Solutions, and Microwaves in Radio over Fiber (RoF) have to work together.

2

Optical and Microwave Fundamentals

The propagation of electromagnetic waves in transmission media is very important for optical transmission techniques as well as for fiber-optic sensor applications. The spectrum of electromagnetic waves varies from long-wave radio waves to short-wave cosmic radiation. The area which is interesting in fiber optics and sensor techniques spans from visible light to the near infrared region (Figure 2.1).

Diagram shows wavelengths, frequencies and electromagnetic radiations along with the spectrum of colors.

Figure 2.1 Infrared (IR) and visible region (VIS) including ultraviolet (UV) is called the field of optical radiation

The related physical area is called Optics. In a closer sense, almost exclusively, electromagnetic waves in the visible area are named light but often the IR- and UV-regions are included in the term too. The propagation can take place in free space, air and outer space or in guided media. The electromagnetic wave propagation device is called the optical waveguide. The most well-known type of optical waveguide is a glass fiber. Instead of glass, it is also possible to use a transparent plastic material, a polymer optical fiber (POF). Moreover, it is not imperative that the waveguide shows a round cross-sectional shape like the fiber does. For example, it can be inserted in a plane substrate. Thus, a two-dimensional waveguide structure will be achieved, which finds its application in the field of integrated optics.

2.1 Free Space Propagation of Electromagnetic Waves

Electromagnetic waves appear as a periodic spatiotemporal excitation of field quantities of a physical field transporting physical energy. The electric field vector ( ) oscillates perpendicularly to the magnetic field vector ( ) and moreover, both fields are perpendicular to the wave propagation direction (Figure 2.2); such a wave is called a transversal wave.

Image described by caption.

Figure 2.2 Electric ( ) and magnetic field vector ( ) of an optical wave at a certain time with propagation in the x-direction

The mathematical description of electromagnetic wave propagation is based on Maxwell's theory of the electromagnetic field [2.1]. For Maxwell's equations include:

(2.1) numbered Display Equation

(2.2) numbered Display Equation

(2.3) numbered Display Equation

(2.4) numbered Display Equation

where:

Magnetic induction

Dielectric displacement

Current density and Electric charge density

There are material equations which take into account the media properties that the waves are propagating:

(2.5) numbered Display Equation

(2.6) numbered Display Equation

(2.7) numbered Display Equation

where:

κ Specific conductivity

ϵ Dielectric constant (Permittivity)

ϵ0 Free space dielectric constant (Permittivity)

ϵr Relative dielectric number

μ Permeability

μ0 Induction constant

μr Relative permeability

Several helpful simplifications are obtained in optics and consequently for wave propagation in optical waveguides. Concerning the used wavelength areas, the attenuation is very small, particularly in glass fibers (Chapter 3.1). Thus it can be neglected for actual considerations and thus the waveguide will be treated as free of absorption. Furthermore, glass is a non-conductive material, and there are no charge carriers either. Moreover, the magnetic induction in the waveguide is approximately the same in a vacuum, so it follows that:

numbered Display Equation

Therefore a simplification of Maxwell's Eqs (2.1) to (2.4) is gained by the application of the laws of vector analysis following from Eqs (2.1) to (2.4) [2.2]:

(2.8) numbered Display Equation

(2.9) numbered Display Equation

Applying the Laplace operator Δ to each Cartesian component of the and vector, respectively:

numbered Display Equation

the propagation velocity v (phase velocity) of an electromagnetic wave in the media is given by:

(2.10)

numbered Display Equation

where:

c Light velocity in vacuum

n Refraction index

ω Angular frequency

f Frequency

k0 Value of the wave vector in vacuum (k0 = 2π/λ)

λ Wavelength of the electromagnetic wave

Now Eqs (2.8) and (2.9) can be recalculated and regarding the electric and the magnetic field strength the following differential equations, the wave equations, are given by:

(2.11) numbered Display Equation

(2.12) numbered Display Equation

Next, solutions to these differential equations have to be found for the vectors and . The most general solution is (e.g. the electric field strength):

numbered Display Equation

where:

is the phase of the searched wave, t is the time and vector = (x/y/z) is the position vector.

For the time dependence applies:

(2.13)

numbered Display Equation

Such a solution is called a harmonic wave. The spatial design of the wave can have different forms, for example a cylindrical wave or a spherical wave. In practice, plane waves are important because adequate approximations are often allowed. A scalar wave will be obtained if the vector of the electric field exclusively oscillates in the direction of one local coordinate (linearly polarized wave, see below). The following equation applies to a planar harmonic scalar wave propagating in the x-direction (Figure 2.3, the -field is not illustrated):

(2.14) numbered Display Equation

Electric field versus coordinate x graph shows a sinusoidal curve of amplitude A and wavelength lambda.

Figure 2.3 Intensity of the scalar electric field E of an optical wave versus local coordinate x

where:

A amplitude of the wave

where:

λ Wavelength

A Amplitude

2.2 Interference

Waves having fixed phase relations between themselves are called coherent waves. A spatial and time coincidence of two or several waves with adequate polarization leads to a characteristic superposition which is called interference [2.3,2.4]. In the following, the superposition of two waves (E1 and E2) at a fixed position are considered. For each of both waves it holds that:

(2.15) numbered Display Equation

where:

Ai Amplitude of the wave i

ωi Angular frequency of the wave i

ϕi Phase of the wave i

At a certain point of the superposition area, the total electric field strength E results from the sum of the single fields. A detector is used for observation, for example, with the eye, a photographic film or a photodiode. The visual sense is attained by the recognition of a specific intensity. Concerning a photographic film, an optical density appears and regarding the photodiode, we achieve a corresponding photocurrent. All detectors mentioned measure the absolute value of the square of the electric field strengths, often called intensity. It is not the field strength itself that will be observed. The reason is that the frequency of the wave is in the order of 200 to 300 THz (terahertz), which is about 3 to 4 orders of magnitude too high for any detector to be resolved. What we see is something similar, like the effective value (root mean square) of a 230 VAC current coming out of a socket. This intensity I is precisely described in physics as the energy flux density, which is the absolute value S of the pointing vector:

(2.16a) numbered Display Equation

The unit is W/m².

For I follows by time-related averaging of the pointing vector:

(2.16b) numbered Display Equation

and thus:

(2.16c)

numbered Display Equation

where:

E* conjugate complex of E

Thus follows:

(2.17)

numbered Display Equation

The first two terms in Eq. (2.17) represent information regarding the amplitude. In contrast, the third term, the interference term, gives us information about the amplitude as well as frequency and phase. Thus, the last term can be used for extraordinary applications in data transmission (Chapters 3.2.5 and 9.4). In particular, Eq. (2.17) tells us that in the absence of a second wave (A2 = 0), there is no information about frequency and phase. Exclusively information about amplitude could be gained by applying such absolute values of the square detectors mentioned above. A lot of applications superimpose two (or more) partial waves coming from the same light source, that is, their frequencies are equal. Therefore Eq. (2.17) can be simplified to:

(2.18) numbered Display Equation

Figure 2.4 illustrates the intensity as a function of the phase difference Δϕ = ϕ1 − ϕ2of two waves with adequate amplitude (A1 = A2 = A). For Δϕ = 0, 2π, 4π, ... a maximum of intensity can be seen. This is called constructive interference. For Δϕ = π, 3π, 5π, ... both waves are annihilated. This is a case of destructive interference.

Intensity versus phase difference graph shows a sinusoidal curve of larger amplitude 4A sup(2).

Figure 2.4 Intensity of two superimposing waves versus phase difference Δϕ

where:

A Amplitude of each wave

2.3 Coherence

Concerning the derivation of Eq. (2.17), there is the assumption that the superposing waves are completely coherent. However, this does not generally hold. The interference capability [2.5] of waves which come from light sources of finite elongation and finite spectral width can be described by the idea of partial coherence. The quantitative degree for partial coherence, the coherence function γ, indicates the conditions between complete random phase correlation and fixed phase coupling of the two waves. There is 0 ⩽ γ ⩽ 1; thus, incoherent and coherent superposition is described as borderline cases of the coherence function, where γ = 0 is the incoherent superposition, 0 < γ < 1 is the partial coherent superposition and γ = 1 is the coherent superposition

Hence, considering the coherence function, the following is derived from Eq. (2.17):

(2.19)

numbered Display Equation

where:

Δf Spectral width of the light source

For γ = 1 and A1 = A2 = A follows for the maximum intensity Imax  ∼ 4A², and for the minimal intensity, Imin  = 0. For γ = 0 and A1 = A2 = A, the result is Imax  = Imin  ∼ 2A² (Figure 2.4). Thus, γ can be interpreted as the contrast K from the interference phenomenon:

(2.20) numbered Display Equation

If a high interference contrast should be achieved, γ → 1 must be obtained. The narrower the spectral width of the used light source, the larger the value of the coherence function γ. This means the spectral width of the light source should be as narrow as possible; it would be ideal if γ = 1. Thus, it follows that the light source is allowed to oscillate in a single frequency. That is called rigorous monochromasia.

where:

f0 Center frequency

Δf Spectral width of the light source (FWHM: full width at half maximum)

In this case, the distribution of the spectral power density p(f) can be described by a delta function (Figure 2.5, broken line):

(2.21) numbered Display Equation

Spectral power density versus frequency graph shows a curve with P sub(max) at the peak and delta f at half P sub(max).

Figure 2.5 Spectral power density p(f) versus frequency f

with p(f) = dP/df

where:

P Optical power of the light source

Real light sources, also high coherent lasers, at best are quasi monochromatic. The light source has to be considered as time coherent. The corresponding monochromasia requirement then results from the center frequency f0 and the spectral width Δf (Figure 2.5, continuous line). The radiation of such a light source over this major time interval can be treated as coherent, as long as the induced phase difference due to the cut-off-frequency (Figure 2.5) is not larger than π compared to the center frequency:

(2.22) numbered Display Equation

where:

tcoh Coherence time

The coherence length Lcoh is given by:

(2.23) numbered Display Equation

The contrast of an interference phenomenon disappears if the optical path difference of two partial waves is equal to its coherence length (minor changes of this definition are known in literature). The optical path difference Δg relates to the phase difference Δϕ by the following:

(2.24) numbered Display Equation

The optical path length g results from the product of the geometrical length L and the refraction index n of the media in which the waves propagate:

(2.25) numbered Display Equation

Equation (2.23) shows that the larger the coherence of a light source the narrower is its spectral width. In light sources which exhibit a continuous spectrum, the coherence function γ(Δϕ) decays according to a 1/e-function. If there is no continuous spectrum, such as in a bulb or an LED, but a mode structure such as in a multimode laser or a super luminescent diode, again a mode structure is obtained for the coherence function γ(Δϕ) [2.6] (coherence function and spectrum are linked by the Fourier transformation [1.13,2.7].), that is, with increasing phase difference Δϕ, the amount of the coherence function varies from high to low values and completely disappears at large phase differences. Figure 2.6 shows the measured mode spectrum of a super luminescent diode, and Figure 2.7 the corresponding coherence function.

Spectral power density versus lambda graph shows oscillation with higher amplitudes between 841 and 851 nanometers.

Figure 2.6 Spectral (longitudinal) modes of a super luminescence diode

Coherence function of difference in optical path versus optical path difference graph shows narrow spikes with the highest peak at zero.

Figure 2.7 Coherence function γ of the luminescence diode according to Figure 2.6

where:

p(f) spectral power density.

Δg optical path difference:

(2.26) numbered Display Equation

Examples for spectral width and coherence length are shown in Table 2.1. Regarding large spectral widths, the units of wavelengths are chosen, because otherwise large numerical values would be generated:

Table 2.1 Spectral width and coherence length

2.4 Polarization

Hitherto, there was the assumption that the vector of the electromagnetic field perpetually oscillates in the direction of a single local coordinate (Figure 2.8). However, this limitation is not generally accepted.

Image described by caption.

Figure 2.8 Electromagnetic wave propagating in the z-direction, while the -vector oscillates constantly in the x-direction

The peak of the -vector can be oriented in a different direction at a later point in time. Hence, light as an electromagnetic wave has to be treated like a vector [2.8]:

(2.27) numbered Display Equation

with

where:

Axi, Ayi Amplitudes of the wave i in x- and y-direction, respectively

ωi Angular frequency of the wave i

Δψi Phase difference between Exi and Eyi

The waves propagate in the z-direction. Thus, the total scalar treatment in Eq. (2.16) has to be described as a vector:

(2.28)

numbered Display Equation

where:

Hermetic conjugate vector of

It follows that:

(2.29) numbered Display Equation

Within the interference term appears the scalar product of two vectors . This product becomes zero if both vectors are perpendicular. To realize a constructive interference, a polarization adaption of the superposing waves must take place. Therefore, the polarization plays an important role in all fiber-optic communication and sensor challenges which work with coherent techniques.

The polarization describes the special behavior of the -vector. By projecting the arrowhead of the -vector onto a plane perpendicular to the propagation direction over a sufficient long-time period, an ellipse will usually be gained (Figure 2.9). Form and position of the ellipse in this plane are given by the general ellipse Eq. [2.9]:

(2.30)

numbered Display EquationGraph shows elliptical polarization where a represents major axis, b represents minor axis and p represents the angle between x-axis and the major axis in first quadrant.

Figure 2.9 Ellipse of polarization

where:

Ax, Ay Amplitudes in x- and y-direction, respectively

a, b Major and minor ellipse half-axis, respectively

The inclination (inclination angle ρ) of the ellipse half-axes and the axial ratio b/a are determined by the amplitudes of both waves Ax and Ay, as well as by their optical path difference Δψ:

(2.31) numbered Display Equation

with a, b as major and minor ellipse half-axis, respectively:

(2.32) numbered Display Equation

Clockwise polarization states are attained, that is the arrowhead of the electromagnetic field vector shows a clockwise rotation on the ellipse:

(2.33) numbered Display Equation

Counter clockwise polarization states are gained. Linearly polarized light will be obtained as a special case of the polarization ellipse (Figure 2.10):

numbered Display EquationImage described by caption.

Figure 2.10 Electromagnetic wave propagating in the z-direction., The -vector oscillates constantly in a plane given by the amplitudes in the x-direction and y-direction respectively

With Eq. (2.29), it follows that:

(2.34) numbered Display Equation

Thus, a straight line with positive and negative slopes, respectively according to b/a = 0 or b/a → ∞.

As another special case, circularly polarized light will be gained (Figure 2.11):

numbered Display EquationImage described by caption.

Figure 2.11 Electromagnetic wave propagating in the z-direction. The -vector rotates while the propagation takes place on a circle with radius A around the z-axes

With Eq. (2.29), it follows that:

(2.35) numbered Display Equation

Thus, it is a circle according to b/a = 1.

In addition, non-polarized light has to be mentioned, such as during wave propagation in the z-direction the -vector statistically oscillates in the x- and y-direction, respectively. All further polarization states can be reduced to combinations of the states described above. It is also possible that light is partially polarized. This will be characterized by the degree of polarization, which specifies the relation between polarized light intensity and the sum of polarized and non-polarized intensity.

Positioning a linear polarizer in an optical path exclusively with an oscillation direction of the transverse wave fitting to the polarizer inclination direction will be transmitted [2.10] (Figure 2.12a).

Diagram on left shows a polarizer, an analyzer with vectors E sub(x) and E sub(y) together with a wave propagation in the z-direction. Transmitted intensity versus beta graph on right shows a curve decreasing from 0 to 90 degrees and then increases.

Figure 2.12 (a) Linearly polarized light wave with polarizer and analyzer (b) Transmitted intensity It = I after the analyzer according to Malus' law

β angle between transmission direction of polarizer and analyzer.

To determine the direction of the oscillation plane, a second polarizer will be used, which in this case is called the analyzer. If the transmission direction will be rotated by an angle β compared to the angle of the polarizer exclusively, the projection Et of the -vector will be transmitted by the analyzer:

(2.36) numbered Display Equation

where:

E0 Value of the -vector in front of the analyzer

The transmitted intensity It after the analyzer is following from Eq. (2.16). The result is given by the Malus' Law shown in Figure 2.12b:

(2.37) numbered Display Equation

where:

I0 Intensity in front of the analyzer

β angle between transmission direction of polarizer and analyzer.

By introducing a transparent-optical inactive substance [2.1] between two crossed polars, there is no light transmission through the system in Eq. (2.37). An important effect, which is strongly connected to polarization, is the Faraday effect [2.10].

where:

L Length

I Current

H Magnetic field

α Angle between input and output polarization direction

Concerning certain substances, the polarization direction of the linearly polarized wave is rotated by an angle α by the aid of the Faraday effect applying a magnetic field parallel to the propagation direction of the light wave; a Faraday rotator is gained (Figure 2.13), which applies to:

(2.38) numbered Display Equation

Diagram shows Faraday's effect on the rotator in the direction of polarization of a light wave in parallel to magnetic field, while rotating at an angle alpha.

Figure 2.13 Faraday rotator

where:

H Value of the magnetic field

L Length along the magnetic field takes effect

VVerdet constant

The Verdet constant is the material constant of the Faraday effect; it shows how much the oscillation direction will be rotated by the applied magnetic field. The magnetic field can be generated by an electric coil as well as by a permanent magnet. For the magnetic field strength, which is excited by means of an electric coil:

(2.39) numbered Display Equation

where:

N Number of coil windings

I Value of the current through the coil

In addition, introducing a Faraday rotator into the optical path according to the arrangement referred to Figure 2.12, the polarization direction of a linearly polarized light wave rotates by angle α (Figure 2.13). The transmitted intensity by the analyzer varies according to Malus' Law (Eq. (2.37)).

The Faraday effect as the non-reciprocal effect can occasionally be used for a few applications in optical communication and sensor techniques. Both polarizers will be adjusted to an angle of 45 degree to each other, according to the input and output polarization directions of the Faraday rotator, respectively (Figure 2.13). For this arrangement, the total intensity will be transmitted if the Faraday rotator rotates about the polarization plane by 45 degree in the correct transmission direction of the analyzer.

Assuming a reflection of the light wave at a following optical component only, light having the correct polarization direction passes the analyzer in the reverse direction, that is, the reflected light wave and the analyzer must have the same polarization orientation. Due to its non-reciprocity, the Faraday rotator does not turn back the oscillation plane, but continues the rotation by an angle of 45 degrees. The oscillation plane of the reflected light is then oriented by an angle of 90 degree with respect to the transmission direction of the input polarizer. Thus, the light wave cannot pass the polarizer; an optical isolator is gained. Instead of an electric coil, a permanent magnet can also be applied. Optical isolators with a suppression of about 60 dB are available.

2.5 Refraction and Reflection

Refraction and reflection occur when a light wave transition from a media having a refraction index n1 to a media with a different refraction index n2 ≠ n1 takes place.

Figure 2.14 illustrates the incidence of a plane wave on an interface between such media.

Diagram shows generation of wavefronts due to the incidence of a wave on a media with incident and refracted angles alpha sub(1) and alpha sub(2).

Figure 2.14 Refraction and reflection of a light wave at an interface

where:

n1, n2 Refraction indices in front and behind of an interface, respectively

α1 Angle of incidence and reflection, respectively

α2 Angle of refraction

With respect to the interface, the wave front in media 1 is inclined by an angle α1. The wave vector simplified by the light ray is oriented perpendicularly to the wave front and forms the angle α1 with the vertical of the interface (Figure 2.14). If the wave approaches the interface in this case, the left-hand side of the wave front penetrates earlier into media 2 in comparison to the right-hand side (view from wave front). From Eq. (2.10), the wave velocity in media 1 is given by v1 = c/n1. In Figure 2.14, it is assumed that the refraction index of media 2 is higher than that of media 1. Therefore, it can be written as n2 > n1 and v2 = c/n2 < v1. The part of the wave front which has already penetrated into media 2 moves slower than the one left behind in media 1. The wave front inclines and changes its propagation direction. The distance of the following phase fronts has become smaller. The wave vector and the vertical to the interface then include the angle α2.

Regarding the transition from an optical thinner to a denser media (n1 < n2), the light ray is refracted toward the vertical plane of the interface. Vice versa, in the case that n1 > n2, it is refracted off. The correlation between angles and refraction indices is described by the law of refraction, Snell's law [2.10]:

(2.40) numbered Display Equation

However, in general,