1
Introduction and Overview
By many accounts, the field of electro-optics is progressing at a faster pace than the field of computer electronics. For example, when we hear of breakthroughs in storage capacities and playback speeds, we know that optical media provide the best performance. With this in mind, anyone new to this field can easily become discouraged when trying to determine, for example, how the compact disc player stores and plays back this information. As with any other area of science, learning the basics will provide a good start to understanding a seemingly complex application. All optoelectronic devices such as diode lasers, LEDs, and photodiodes operate on the physical principle of the interaction of radiation and matter. This basic quantum process is responsible for the production of electromagnetic radiation.
So, why has the use of optoelectronic devices skyrocketed in recent years? One answer that quickly comes to mind is speed. As anyone who works with a personal computer knows, a machine built just a few years ago will have trouble running the latest software applications. The computer must have the ability to handle the increased information throughput that allows for the efficient transfer of electrical signals. Unfortunately, limits exist to how fast these signals can be transmitted in copper wire. As the frequency of the electronic signal increases, its ability to travel through a conductor decreases. This limitation is known as the skin effect. The consequence of the skin effect requires signals with oscillation frequencies in excess of about 100 Megahertz to be transmitted in coaxial cable. Electron flow through a conductor is not the only way to transmit information.
Optical signals, on the other hand, travel through transparent media that do not make good electrical conductors. These signals have no restriction such as the skin effect since photons propagate easily within these media. For example, near infrared optical energy oscillates at a frequency of about 300 Terahertz. This frequency is thousands of times greater than frequencies allowed to propagate electronically in a copper wire. To take advantage of this vast amount of bandwidth, the electronics must be carefully integrated to the optics. This is why we see optoelectronic components such as LEDs, diode lasers, and photodiodes used in electronic circuits in ever increasing numbers. Common everyday electronic devices depend upon these components for their operation. These devices include CD players, wireless remotes, range finders, and barcode scanners.
You will find this book to be different from most on this subject. This book uses a “back to basics” approach to introduce the reader to the science of electro-optics. One of the book’s objectives is to provide the reader with information on how to integrate optoelectronic components into useful electrical circuits. This information should not be presented in a cookbook fashion. The variations in the types of optoelectronic components available now are so diverse that it is impossible to cover every application. Instead, if the reader understands the physical concepts behind the operation of these components, this knowledge can then be applied to a particular situation. Both optical and electrical design issues will be discussed. Many circuit applications that use optoelectronic components will be presented and explained in detail.
It’s easy to become intimidated by data sheets, especially when they present data with polar plots using Greek symbols, and use terms such as D*. This book will guide you through the various parameters presented by most data sheets. Many of the optoelectronic components introduced in this book will use a manufacturer’s data sheet. Detailed explanations for each parameter will be given, and in some cases, actual circuit examples will be presented using the optoelectronic component. This will help the reader to understand data sheets, and to use them as effective design tools.
Data books and application manuals usually present schematic diagrams for the optoelectronic components that they specify. Unfortunately, this provides the circuit designer with only part of the information required to make a functioning circuit. In most cases, component placement, printed circuit board layout, and EMI shielding comprise the completed working circuit. This book will present these often overlooked details of circuit design when using optoelectronic components.
Since electro-optics involves the convergence of optics and electronics, it can best be understood when using both of these disciplines. An electro-optic system can be reduced to the basic sections as shown in Figure 1.1. These sections will be introduced separately in Part II of the book. The transmitter section contains an optoelectronic emitter such as an LED or diode laser. Optical elements such as lenses, apertures, and filters may also be used in the transmitter section with the main purpose being to affect light output characteristics. The emitted optical radiation then propagates through a medium to reach the receiver section. At the receiver, the detector and its associated electronics convert the input optical signal into an electrical signal for further processing.
In the first part of this book, we present the required theoretical background in optical physics. To start with, we consider the macroscopic view of light to explain phenomena such as reflection, refraction, diffraction, and interference using many examples. Maxwell’s equations provide us with a description for the propagation of light as electromagnetic radiation. Understanding the basic properties of electromagnetic radiation will help the circuit designer to optimize, for example, an optical receiver. The more he knows about what he is trying to sense, the easier his job will be. Next, the theory behind the production of light is studied by considering the interaction of radiation and matter. We must shift gears here to consider the microscopic view of light. In this realm, the photon becomes the basic quantum unit involved in this energy exchange. Electrons play a key role in photon production. This provides a natural place to begin the second part of the book.
Figure 1.1 Block diagram showing the basic parts of a general electro-optical system.
The second part of the text builds upon the theory and practical examples previously studied to show how to integrate optoelectronic components into useful circuits. The electrical schematic is just one of the many steps involved in this design process. Most importantly, we must consider both the optical and electrical design issues. Electrical design issues given in this book will be specific to optoelectronic devices. The basic circuit theory contained in most electrical design manuals concentrates on devices such as thermistors, thermocouples, and other resistance-varying components. Optoelectronic components, such as photodiodes, vary their current production with input optical energy. Also, a photodiode’s internal capacitance varies with reverse-bias voltage. Thus, circuit techniques such as biasing will be different in both cases. Other important considerations include component selection and placement, RFI/EMI shielding, transient protection, environmental factors, lenses, and frequency response.
After we investigate the fundamental operation of optoelectronic components, the next step is to consider important areas such as modulation techniques, circuit board layout issues, and light detection. In the last chapter, we will combine what we have learned from all of these areas to discuss some common electro-optical system designs. Below, we give a brief discussion of the material covered in each chapter of this book.
Part I
Historical Development (Chapter 2)
This chapter provides a brief history pertaining to the development of modern electro-optics. The classical view according to Isaac Newton will be discussed. This classical view required modification when Max Planck made some observations that could not be explained by classical physics. This was the first step necessary for the acceptance of modern quantum theory.
Light and the Electromagnetic Spectrum (Chapter 3)
The nature of light will be introduced here. Maxwell’s equations will be used to explain light’s wave-like nature as it propagates through space. The electromagnetic spectrum will also be discussed. In this spectrum of electromagnetic energy, radio waves have the longest wavelengths. Cosmic rays possess the shortest wavelengths.
Reflection and Refraction (Chapter 4)
The physical laws governing reflection and refraction will be discussed using simple geometrical diagrams with plane waves. As we know, mirrors and lenses display these important optical phenomena. We will show many examples using Snell’s law. After this discussion, fiber optic waveguide will be studied. The physical principles involved with lightwave transmission will be introduced.
Interference (Chapter 5)
Interference phenomena display light’s wave-like nature. A practical device, known as the Fabry-Perot interferometer, will be detailed. Since diode lasers also use this basic structure in their operation, many examples will be considered describing how this interferometer works. Other applications considered in this chapter include thin films, interference filters, and non-reflecting glass. These optical components can be found in many electro-optical systems.
Diffraction (Chapter 6)
This chapter continues our study of wave optics. Since light is a wave phenomenon, it will bend around an obstacle such as the edge of a slit. A similar effect can be seen with water waves as they bend around an obstacle in their path. Diffraction effects must be controlled when designing optical systems. These effects can be both beneficial and counterproductive. We will see how diffraction limits the performance of any optical system. Many important applications of diffraction will be discussed in detail. Diffraction gratings take advantage of light’s wave-like nature to separate polychromatic light into its spectral components. A CD player uses a diffraction grating to help keep the read head on a particular track without skipping. In Chapter 14, we will consider the application of the diffraction grating in the CD tracking control system. In Chapter 9, a diffraction grating will be used in the construction of a low cost spectroscope.
Polarization of Light (Chapter 7)
According to Maxwell’s equations, light is composed of electric and magnetic fields arranged orthogonally to each other. Oscillations occur perpendicular to the direction of propagation, thus making light a transverse wave phenomenon. Optical components such as polarizers and quarter-wave plates prove light’s transverse wave nature. Laser light can be modulated in a communications system by controlling its polarization state. In the operation of an electro-optic modulator, the polarization of laser light is controlled after it leaves the laser. We will use this modulation example to serve as an introduction to the modulation process in diode lasers.
Light and Thermal Radiation (Chapter 8)
We find, after studying the last few chapters, that the wave theory of light is not sufficient to describe all observations. It also has a particle-like nature. All objects at a temperature above absolute zero emit radiation. When matter and energy are in equilibrium, the spectral distribution of this emitted radiation follows a blackbody curve. The prominent feature of this curve is a peak intensity at a specific wavelength determined by the temperature of the object. This blackbody curve, first discovered by Max Planck, contributed greatly to our understanding of electromagnetic radiation. The shape of this curve can only be explained when considering the particle nature of light. Light is composed of discrete energy packets known as photons.
After introducing light’s particle-like nature in Chapter 8, the time has come to investigate the energy exchange process. The atomic structure, as presented by Neils Bohr, will be used for this purpose. Energy can be absorbed or emitted by an atom in discrete amounts only. These discrete energy packets, known as photons, are fundamental to our study of electro-optics. The construction of a simple spectroscope using a diffraction grating will be presented for the purpose of viewing these photons, collectively. When optical energy is emitted from a source, the discrete energy will show up as emission lines in the spectroscope. We use the spectroscope to determine the wavelength, and thus the energy, of these photons.
Part II
Semiconductor Light Sources (Chapter 10)
In this chapter, we will begin our study of the basic electro-optic system shown in Figure 1.1. The transmitter contains a light source that may produce photons by the interaction of radiation and matter. There are two processes to be considered here: spontaneous and stimulated emission. Light sources convert electrical current (electrons) into photons. Light emitting diodes (LEDs) and diode lasers use semiconductor materials in this photon production process. We use the particle nature of light in describing this process. The operation of the LED can be characterized by the process of spontaneous emission. The diode laser’s unique geometry allows lasing to occur by the process of stimulated emission, another way that radiation interacts with matter. We will use the concepts developed in the previous chapters to more fully understand the process involved with the amplification of light. This process can be more fully understood when we consider the wave and particle natures of light. The particle nature helps to explain the process of stimulated emission within the laser structure, while the wave nature helps to explain how a tuned cavity (Fabry-Perot interferometer) is used to amplify light. These semiconductor light sources have the ability to be switched on and off very quickly. This property allows them to modulate the light they emit. The optical and electrical properties of LEDs and diode lasers will be considered here.
Optical Transmitters (Chapter 11)
An optical transmitter uses a semiconductor light source with electronic circuitry to produce a desired optical output. The electronic circuit controls the electron-to-photon conversion process. The subsequent light output must usually be modulated in some fashion for effective information transfer. We will investigate ways to modulate these light sources using electronic components. The electro-optic modulator introduced in Chapter 7 will serve as a guide to understanding modulation using electronic circuits. Many circuit examples will be discussed in detail, including diode laser modulation. These optoelectronic devices require relatively large amounts of current to produce the light levels required for reliable operation. Switching large amounts of current at fast rates produces emission known as electromagnetic interference or EMI. When using these devices, careful attention must be paid to circuit location, grounding, and shielding. Established practices on how to minimize this unwanted noise will be discussed.
Photodetectors (Chapter 12)
The block diagram of the receiver in Figure 1.1 contains a photodetector that converts incident photons into electrons. The process involved here is very much the reverse of the spontaneous emission process discussed in Chapter 10. Optoelectronic components such as photodiodes, phototransistors, optocouplers, and photoconductors will be considered. The basic structure and operation of the photodiode will be studied by considering parameters such as quantum efficiency, spectral response, dark currents, capacitance, noise sources, and response time. The circuit designer must know the various trade-offs available when integrating these devices into useful circuits. For example, to achieve the fastest response time, the photodiode must be reverse-biased by a specified voltage. This practice produces a dark current within the device that shows up as excess noise, reducing the signal-to-noise ratio. Thus, a trade-off between sensitivity and speed of response exists in this situation. Many common trade-off situations will be studied so that they can be applied to the circuit design examples in the next chapter.
Optical Receivers (Chapter 13)
Showing the reader how to integrate a photodetector into a useful receiver circuit is one of the main objectives of this chapter. The electronic circuitry in the receiver provides the current-to-voltage conversion required before signal processing begins. This signal voltage must then be amplified to a useful level. We will start with the basic photodiode circuit discussed in Chapter 12 by looking at its voltage-current transfer curve. This transfer curve tells us how the optical input affects the electrical output. Simple operational amplifier and hybrid circuits will be presented in a way that allows the reader to adapt to his own application. Other topics covered include optimization techniques, bandwidth requirements, circuit board layout precautions, and EMI protection against internal and external noise. Many typical optical considerations are also discussed.
Electro-Optic Systems (Chapter 14)
This chapter considers the completed system as outlined by Figure 1.1. Four interesting examples are presented in this chapter. The operation of the CD player involves many of the electrical and optical concepts discussed in this book. The optical path in a typical CD player uses photodiodes for many important functions. We will see that the diode laser makes an excellent optical source for this application. For the next example, we will consider the design of an optical data link between, for example, a personal computer and a printer. A recent development in infrared data transmission has resulted in a standard protocol known as Infrared Digital Association (IrDA). A typical IrDA transceiver design will be considered by studying both the electrical and optical design issues. These design issues are common to most electro-optical systems. Chapter 11 provides the background on the IrDA signaling format. The other systems presented in this chapter include an integrated light sensor and a fiber optic communications system.