Optics, Optoelectronics and Photonics

Summary

PHYSICAL PRINCIPLES OF PHOTODETECTION

Photodetection converts an optical signal into a signal of another form. Most photodetectors convert optical signals into electrical signals that can be further processed or stored. All photodetectors are square-law detectors that respond to the power or intensity, rather than the field amplitude, of an optical signal. The electrical signal generated by an optical signal is either a photocurrent or a photovoltage that is proportional to the power of the optical signal. Based on the difference in the conversion mechanisms, there are two classes of photodetectors: photon detectors and thermal detectors. Photon detectors are quantum detectors based on the photoelectric effect, which converts a photon into an emitted electron or an electron–hole pair; a photon detector responds to the number of photons absorbed by the detector. Thermal detectors are based on the photothermal effect, which converts optical energy into heat; a thermal detector responds to the optical energy, rather than the number of photons, absorbed by the detector. Because of this fundamental difference, the general characteristics of these two classes of photodetectors have a number of important differences.

The response of a photon detector is a function of the optical wavelength with a long-wavelength cutoff, whereas that of a thermal detector is wavelength independent. A photon detector can be much more responsive than a thermal detector in a particular spectral region, which typically falls somewhere within the range from the near ultraviolet to the near infrared. By comparison, a thermal detector normally covers a wide spectral range from the deep ultraviolet to the far infrared with a nearly constant response. Photon detectors can be made extremely sensitive. Some of them have a photon-counting capability that is not possible for a thermal detector. A photon detector can be designed to have a high response speed capable of following very fast optical signals. Most thermal detectors are relatively slow in response because the speed of a thermal detector is limited by thermalization through heat diffusion and by heat dissipation when the power of an optical signal varies. For these reasons, photon detectors are suitable for detecting optical signals in photonic systems, whereas thermal detectors are most often used for optical power measurement or infrared imaging. In this chapter, only the basic principles of photon detectors are discussed because our major concern is photodetection for photonics applications.

Summary

POPULATION RATE EQUATIONS

From the discussion in the preceding chapter, it is clear that population inversion is the basic condition for an optical gain. For any system in its normal state in thermal equilibrium, a low-energy level is always more populated than a high-energy level, hence there is no population inversion. Population inversion in a system can only be accomplished through a process called pumping by actively exciting the atoms in a low-energy level to a high-energy level. If left alone, the atoms in a system relax to thermal equilibrium. Therefore, population inversion is a nonequilibrium state that cannot be sustained without active pumping. To keep a constant optical gain, continuous pumping is required to maintain population inversion. This condition is clearly consistent with the law of conservation of energy: amplification of an optical wave leads to an increase in optical energy, which is possible only if the required energy is supplied by a source.

Pumping is the process that supplies energy to the gain medium for the amplification of an optical wave. There are many different pumping techniques, including optical excitation, electric current injection, electric discharge, chemical reaction, and excitation with particle beams. The use of a specific pumping technique depends on the properties of the gain medium being pumped. The lasers and optical amplifiers of particular interest in photonic systems are made of either dielectric solid-state media doped with active ions, such as Nd:YAG and Er:glass fiber, or direct-gap semiconductors, such as GaAs and InP. For a dielectric gain medium, the most commonly used pumping technique is optical pumping using either an incoherent light source, such as a flashlamp or a light-emitting diode, or a coherent light source from another laser. A semiconductor gain medium can also be optically pumped, but it is usually pumped by electric current injection. In this section, we consider the general conditions for pumping to achieve population inversion. Detailed pumping mechanisms and physical setups are not addressed here because they depend on the specific gain medium used in a particular application.

The net rate of increase of the population density in a given energy level is described by a rate equation. As we shall see below, pumping for population inversion in any practical gain medium always requires the participation of more than two energy levels.

Summary

The field of photonics has matured into an important discipline of modern engineering and technology. Its core principles have become essential knowledge for all undergraduate students in many engineering and scientific fields. This fact is fully recognized in the new curriculum of the Electrical Engineering Department at UCLA, which makes the principles of photonics a required course for all electrical engineering undergraduate students. Graduate students studying in areas related to photonics also need this foundation.

The most fundamental concepts in photonics are the nature of optical fields and the properties of optical materials because the entire field of photonics is based on the interplay between optical fields and optical materials. Any photonic device or system, no matter how simple or sophisticated it might be, consists of some or all of these functions: the generation, propagation, coupling, interference, amplification, modulation, and detection of optical waves or signals. The properties of optical fields and optical materials are addressed in the first two chapters of this book. The remaining nine chapters cover the principles of the major photonic functions.

This book is written for a one-quarter or one-semester undergraduate course for electrical engineering or physics students. Only some of these students might continue to study advanced courses in photonics, but at UCLA we believe that all electrical engineering students need to have a basic understanding of the core knowledge in photonics because it has become an established key area of modern technology. Many universities already have departments that are entirely devoted to the field of photonics. For the students in such photonics-specific departments or institutions, the subject matter in this book is simply the essential foundation that they must master before advancing to other photonics courses. Based on this consideration, this book emphasizes the principles, not the devices or the systems, nor the applications. Nevertheless, it serves as a foundation for follow-up courses on photonic devices, optical communication systems, biophotonics, and various subjects related to photonics technology. Because this book is meant for a one-quarter or one-semester course, it is kept to a length that can be completed in a quarter or a semester. Because it likely serves the only required undergraduate photonics course in the typical electrical engineering curriculum, it has to cover most of the essential principles.

With this self-contained and comprehensive text, students will gain a detailed understanding of the fundamental concepts and major principles of photonics. Assuming only a basic background in optics, readers are guided through key topics such as the nature of optical fields, the properties of optical materials, and the principles of major photonic functions regarding the generation, propagation, coupling, interference, amplification, modulation, and detection of optical waves or signals. Numerous examples and problems are provided throughout to enhance understanding, and a solutions manual containing detailed solutions and explanations is available online for instructors. This is the ideal resource for electrical engineering and physics undergraduates taking introductory, single-semester or single-quarter courses in photonics, providing them with the knowledge and skills needed to progress to more advanced courses on photonic devices, systems and applications.

Summary

An optical fiber is the core component of an optical fiber communication link. Popularly known as optical fiber cables, they are the most promising type of guided transmission medium for virtually all forms of digital and data communications applications. With optical fibers, electromagnetic light waves propagate through the media composed of a transparent material without using electrical current flow. Optical fibers are mostly made of glass or plastic material having properties such that the phenomena of total internal reflection takes place that enables light waves to propagate within it in a properly guided manner similar to that of electromagnetic waves through a metallic transmission medium. This chapter begins with an easy-to-understand ray model of the propagation of light through optical fibers. It is followed by a discussion on the concept of modes and the modal analysis of step-index as well as the graded-index type of fibers. Finally, the type of losses and dispersions are explained to assess the limitation of optical fibers.

Review of Optical Ray Theory

In essence, an optical fiber communications system is one that uses light (optical signal) as the carrier of analog or digital information signal. Propagating light waves, carrying information, through the earth's atmosphere is difficult and often impractical. The optical energy in a light wave follows narrow paths, called light rays or beams. For most practical applications, the light rays are used to describe a number of optical phenomena geometrically. In fact, ray theory is known as geometric optics. It is these rays (geometrical paths traversed by light) which actually carry the optical energy.

Velocity of Propagation

Electromagnetic energy, such as light waves, travels at a velocity of c = 3 × 108 m/sec approximately in free space (a vacuum). Moreover, the velocity of propagation is the same for all light frequencies in free space. However, it has been demonstrated that

Summary

The wireless optics holds great potential for fixed wireless communications as well as for other wireless applications. In spite of all other technology developments, wireless optics has promised an economical alternative to last mile connection solution in the wireless domain. The concept of wireless optics was originally developed over few decades ago by the military. An optical wireless technology, also known as free space optics (FSO) technology, provides broadband data communication links between line-of-sight (point-to-point) locations.

Wireless optics (sometimes called fibreless optics) is an optical, wireless, line-of-sight, point-to-point, broadband technology. It basically uses infrared transmission instead of RF in the unlicensed higher frequency spectrum above 300 GHz. FSO signals are transmitted using low-power infrared beams (invisible) through free space, thereby limiting the coverage range. Another constraint on its usage is that the radiated power under any circumstances must not cross the specified limits in order to avoid any visual impairment to the human eye.

Wireless optics transceivers are generally installed in the middle or upper floors, or on an open roof of the building so that a clear line-of-sight transmission path between two stations is available. However, in some applications, these transceivers can be mounted behind a window in an existing office/home building.

In short, we can say that wireless optics is a matured technology based on line-of-sight propagation which uses optical light for transmission of user information such as voice, data, image, or video in open space. It allows optical connectivity without the use of optical fiber cables. Wireless optics system requires a light source such as LED or a LASER at the transmitter end which is capable of emitting a highly-focused light beam. The LASER beams are preferred in wireless optics because of its advantages as offered in optical fiber communications. Obviously, the only difference is the transmission medium – free space (unguided) as compared to optical fiber (guided). We know that the light travels all the way through air only at a much faster speed than through the optical fiber cable. Wireless optics, also referred to as open-air photonics or optical wireless or free-space photonics (FSP), deals with propagation of modulated optical beams in visible infrared (IR) range over air to provide broadband wireless communications.

Summary

Recent advances in fiber optic technology and optoelectronic devices led to emergence of fiber optic sensors. With the availability of highly sensitive detectors and material loss almost approaching to negligible, it is possible to sense even slight variations in intensity level, phase shift and wavelength from external distresses on the optical fiber itself. This is the fundamental concept of fiber optic sensors. The primary function of a fiber optic sensor is to measure or monitor a physical quantity such as temperature, pressure, corrosion, humidity, and similar environmental factors. The basis of measurement is the net effect on its intensity modulation, operating wavelength, phase angle of the incident optical ray, or polarization of the light propagating through the optical fiber.

Advantages of Fiber Optic Sensors

Summary

When all the spectral components are separated from an optical signal, it is termed dispersion. It usually occurs when optical signals travel along optical fiber from transmitter to receiver in an optic–fiber communication link. Dispersion causes distortion in the transmitted optical signal (analog or digital transmission) along the optical fiber. As the optical pulses travel along the optical fiber channel, when digital modulation is used in transmitting optical signals, the dispersion phenomenon causes the broadening of optical pulses. There are different types of dispersion effects such as modal dispersion (in multimode fiber, transmitted optical pulse tends to spread due to time delay between lower- and higher-order propagation modes, causing bandwidth limitation), chromatic dispersion (combination of material dispersion as well as waveguide dispersion that results in spreading of transmitted optical pulse as they travel through the optical fiber), and polarization mode dispersion. Material dispersion happens because of variations in the fiber core refractive index with respect to operating wavelength, and waveguide dispersion happens due to nature of the physical structure of the optical fiber. Due to variations in the fiber core refractive index, different wavelengths of the light beam would travel at somewhat different velocities of light. As a result, an optical pulse gets broadened, causing dispersion. With the introduction of optical amplifiers (as discussed in the previous chapter) as in-line amplifiers in an optic–fiber link, the signal attenuation due to fiber is no more the major concern for achieving desired performance for optical fiber communication systems. However, they aggravate the dispersion problems. In order to achieve the lowest attenuation, there is a need of implementing efficient dispersion management techniques. There are different varieties of optical fibers available including dispersion compensating fibers. With an objective of controlling the spread of transmitted optical pulse in optical fiber communications systems, the dispersion management (also known as dispersion compensation) techniques must be applied.

This chapter focuses on dispersion management in optical fiber communications. The discussion begins with the need of dispersion management because dispersion-induced pulse broadening imposes the severe limitations on the performance of the system. This is followed by detailed discussions on different techniques of dispersion management as pre-compensation as well as post-compensation. The discussion is carried forward by describing various types of dispersion-compensating fibers including fiber Bragg gratings. Finally, different methods of fabrication of chirped fiber gratings are covered.

Summary

Light wave at higher frequency range of electromagnetic spectrum (3 × 1011–3 × 1016 Hz) is used for transmission of information through fibers as transmitting medium in optical fiber communications. It can offer a large bandwidth (more than 50 THz) for data transmission. The emergence of fiber optics as a dominant technology for long-distance broadband services is discussed in this chapter. The basic configuration of optical fiber communication system comprises of an information source, a voltage-to-current converter, an optical source, a channel coupler, an optical fiber channel, an optical repeater, an optoelectronic detector, an electronic receiver, and the output device. The need, advantages, disadvantages, and wide range of applications of optical fiber communication are covered so as to generate interest to know more about the subject in subsequent chapters.

Historical Development

Optical fiber communication has been developed over the last two centuries. The first optical communication system, known as the ‘optical telegraph’, was invented in the 1790s by French engineer Claude Chappe. In 1880, Alexander Graham Bell patented the photophone—an optical telephone device for transmission of speech using a beam of light. During the 1920s, an experiment was conducted by J. L. Baird of England and C. W. Hansell of the USA, for transmission of images for TV/Facsimile systems using arrays of uncoated fiber cables. In the 1950s, A. V. Heel and H. Hopkins protected a bare glass fiber by covering it with a transparent cladding having lower index of refraction. As a result, crosstalk between fibers was greatly reduced in addition to providing protection from contamination. This led to the development of the flexible fiberscope, which is widely used in the medical field.

Note: In 1956, N. S. Kapany of England coined the term ‘fiber optics’. The initial applications of optical fiber were not in communications at all, because the early fibers were too lossy. Bundles of fibers were used for medical imaging to view inaccessible parts of the human body.

By 1960, attenuation of the order of 1 dB/m was achieved with glass-clad fibers. This was acceptable for medical imaging applications but not for voice/data transmissions. The invention of the lasers in the 1960s marked the beginning of a new era in modern optics, called Photonics.

Summary

The purpose of a receiver in an electronic communication system is to extract the information sent by the corresponding transmitter with as minimum a carrier power level as possible. The primary function of an optical receiver in an optical fiber communication link is to convert the received optical signal into an equivalent electrical signal and recover the data. One of the main components of an optical receiver is a photodetector that converts incident optical signals into electric signals using photoelectric effects. High Sensitivity, dynamic range, fast response (i.e., acquisition time), high reliability, low noise, compatible size with that of fiber, and low cost are some of the important requirements of a photodetector. These requirements are best met by semiconductor photodetectors that convert an optical signal transmitted via optical fiber cables to equivalent electrical signals for further processing to achieve the desired output. The type of photodetectors suitable for three optical spectrum ranges of 800–900 nm, 900–1100 nm, and 1100–1600 nm vary in the material used for their fabrication as well as assembly techniques. A p–i–n photodiode is an ideal semiconductor photodetector device, because it can provide high quantum efficiency, fast response and capability to operate at higher modulation frequencies. The minimum received optical power that can be detected by a photodetector is limited by noise. A fully integrated single beam optical receiver comprises of a semiconductor photodiode, preamplifier in the electric domain, digital logic circuits, and an off-chip electronic driver circuit. This chapter discusses all the important aspects of photodetectors and optical receivers. The discussion begins with basic concepts behind the photo detection process, followed by description of different types of photodetectors usually used by optical receivers. Next, the components used in an optical receiver unit are explained. Finally, different types of noise sources in optical receivers that limit the signal-to-noise ratio, the receiver sensitivity parameter and its degradation are covered in sufficient detail.

Requirements for a Photodetector

In an optical fiber communication system, a semiconductor photodetector is the basic component in an optical receiver, as shown in Fig. 4.1.

Optics, Optoelectronics and Photonics

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Partial List of Symbols

Appendix A - Symbols and Notations

11 - Photodetection

Summary

8 - Optical Amplification

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Preface

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Index

5 - Optical Interference

Principles of Photonics

5 - Optical Amplifiers

Frontmatter

Tables

2 - Basics of Optical Fibers

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Appendix C - Wireless Optics

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Appendix A - Fiber Optic Sensors

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6 - Dispersion Management Techniques

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1 - Introduction

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Dedication

Appendix D - Model Test Papers

4 - Optical Receivers

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Appendix E - Abbreviations and Acronyms

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