Photonic Devices and Systems

Robert G. Hunsperger

• Publisher: Routledge
• Year: 2017
• ISBN:
  9781351424844
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Optics & Light

This work describes all the major devices used in photonic systems. It provides a thorough overview of the field of photonics, detailing practical examples of photonic technology in a wide range of applications. Photonic systems and devices are discussed with a mathematical rigor that is precise enough for design purposes yet highly readable.

1

Introduction

Robert G. Hunsperger

University of Delaware Newark, Delaware

1. BACKGROUND

In the years following the invention of the transistor in 1947 [1] electrical engineers developed highly sophisticated electronic systems to perform such functions as telecommunications, digital computation, radio detection and ranging (radar), cable television (CATV), medical diagnosis and treatment, military ordnance delivery, and manufacturing process control. In all of these systems the information signal was carried by a current of electrons flowing in an electrically conducting material or by electromagnetic “radio” waves traveling through the air or in a waveguide. As electronic systems reached maturity, with performance being limited by fundamental asymptotes, engineers began to search for new ways to improve system performance further. They found that by using lightwaves to carry the information signal rather than an electric current or radio wave they could surpass the limitations of purely electronic systems in many applications.

Actually, the use of lightwaves is not a new concept. As early as 1880 Alexander Graham Bell invented the “photophone” [2], a telephonic device that transmitted its information signal over a beam of light. However, the photophone suffered from two serious limitations: neither a reliable light source nor a consistent light path was available. The light source that Bell used was sunlight and the optical path was the air. Of course, both of these vary from hour to hour and point to point, making reliable communication impossible.

It was not until 1960, with the invention of the laser [3], that a reliable light source for lightwave systems became available. The laser provided an intense source of monochromatic, highly collimated phase-coherent polarized light that could be used in a wide variety of system applications. At about the same time, the optical path problem was solved by the development of optical fiber waveguides [4]. Glass optical fibers with a waveguiding core of larger refractive index material surrounded by a cladding material with lesser index conduct lightwaves over a path with low (and constant) losses and phase shift.

The availability of laser sources and fiber waveguides made possible the development of sophisticated photonic systems in which the information signal is carried by an optical fiber waveguide in whole or in part by a beam of light. Photonic technology has developed steadily over the past several decades. Even the name photonics, which nicely parallels electronics and accurately describes a system in which the signal is carried by photons of light rather than by electrons has evolved over the same period of time. Early devices and systems employing lightwaves were called electro-optic. Over the years the terms optoelectronics, optical electronics, integrated optics, and quantum electronics have also been applied to this field. Each of these has been used to describe either the entire field or a part of the field, depending on the perspective of the particular author. This has led to some confusion of persons outside the field. Hopefully, this uncertainty in nomenclature will be resolved by consistent use of the term photonics, which seems to have evolved as the most popular name for the field as a whole.

Photonic devices and systems have been shown to have notable advantages over their electronic counterparts in many applications. For example, photonic telecommunications systems employing semiconductor laser sources and glass optical fiber waveguides have revolutionized long-distance communication. The basic elements of a photonic telecommunication system are shown in Fig. 1. A semiconductor laser diode is the light source. The information signal is impressed on the laser beam either by modulating the laser drive current directly or by using an external modulator. The modulated beam of light is carried by an optical fiber waveguide to a distant reception point where it is detected (usually by a semiconductor PIN or avalanche diode) and converted back to an electrical signal for further processing. Depending on the length of the fiber transmission path, one or more repeaters employing optical amplifiers may be needed. Systems of this type are capable of transmitting signals over hundreds of kilometers with digital data rates in the Gbit/s range [5]. The system bandwidth is several orders of magnitude greater than that available from a purely electronic telecommunications system. In addition to wider bandwidth, which translates into more transmission channels and greater signal-handling capacity, the photonic system offers substantially less signal attenuation and phase distortion than that resulting from transmission of electronic signals over a conducting cable. This means that far fewer repeaters are required, which greatly reduces system cost and complexity. Typical repeater spacing can be increased from a few kilometers to over 100 km [6]. The smaller number of repeaters required leads to reduced capital investment, lower maintenance cost, and improved system reliability.

Figure 1 Generalized photonic telecommunications system.

Photonic telecommunication systems employing optical fiber waveguides also provide immunity from electromagnetic interference (EMI) and relative security from monitoring, unlike their electronic counterparts. Because of their inherent advantages, photonic systems have largely replaced electronic systems in long-distance telecommunication trunk lines, and now that replacement is being extended to the home or office of the subscriber.

Another example of a photonic system which provides improved performance over that of its electronic competitor is beam-of-light radar (sometimes called lidar), now being used by police to trap speeders. In this system a beam of infrared light from a semiconductor laser is focused on the vehicle, which reflects it back toward an infrared detector. The detector photodiode converts the lightwave signal into an electrical current waveform that is processed electronically to determine the speed of the vehicle. The advantage of this photonic system over previously used microwave radar systems lies in the focused nature of the beam of light. Microwave radar systems radiate enough stray electromagnetic energy for it to be detected up to a half mile away by radar detectors that can be bought in auto stores in most states. Over the years, as police radar systems have moved to higher and higher frequencies to thwart radar detector owners, the manufacturers have responded with new multiband detectors with the speed of a Department of Defense countermeasure expert. In fact, one can now purchase radar detectors that also respond to an infrared light beam. However, these detectors are not very effective because they do not give an alarm until the laser beam strikes the vehicle, at which time it is generally too late for the driver to respond.

Lightwave telecommunications and lidar are just two examples of photonic systems that are in wide use today. In the following chapters the fundamental principles upon which photonic devices and systems are based are explained and more examples of the advantages of photonic systems are provided.

II. ORGANIZATION OF THE BOOK

Since the laser is the light source that has made the field of photonics possible it seems natural to begin the book with a study of that device. Chapter 2 is intended to provide a basic understanding of how lasers operate and a description of their characteristics and capabilities. The concept of the laser as an oscillator operating at optical frequencies is presented. Basically, the laser, like an electronic oscillator, has two main components: a gain medium (amplifier) and a frequency-selective mechanism (filter) with positive feedback. The details of the form taken by these two basic elements, the gain medium and the optical resonator, are described for each of the most commonly used types of lasers: gas, solid-state, semiconductor diode, and liquid (dye). The conditions necessary for losses to be overcome and the lasing threshold attained are considered for both continuous-wave (CW) and pulsed laser operation. The commonly used methods for producing pulsed laser operations (gain switching, Q switching, cavity dumping, and mode locking) are all thoroughly explained. Techniques for stabilizing the laser output power and frequency and for controlling the transverse and axial modes in which it operates are described in Chapter 2.

Once the laser beam has been produced, the next step in most applications is somehow to impress an information signal onto it. This is done by modulation, which is the topic of Chapter 3. Modulation of the laser output in some cases can be accomplished internally by varying either the gain or index of refraction to produce amplitude or frequency modulation, respectively. This approach works very well in semiconductor lasers at frequencies up to about 2 GHz and has been widely used [7,8]. However, the demand for ever-wider bandwidth is forcing many modern photonic systems to be operated at higher frequencies at which internal modulation is limited by nonlinearity of response and frequency shift or “chirping.” Because of that, the emphasis in Chapter 3 is placed on external modulators, which are capable of operating at frequencies extending to tens of gigahertz. The optical properties of solid materials can be described by a complex index of refraction (n = n′ – in″), in which the real part characterizes the optical phase shifting that a wave experiences in passing through the material, and the imaginary part is indicative of the attenuation. The theory of these relationships is developed in Chapter 3 because most modulators operate on the principle of changing either n′ or n″ in response to an information signal. Since electro-optic modulators seem to offer better high-frequency performance than that of acousto-optic or magneto-optic devices, they are emphasized in this chapter. Electro-optic modulators are considered in a variety of types affecting either the phase and frequency or the amplitude of the optical beam. The introduction of multiple-quantum-well (MQW) structures to improve modulator performance is described. While electro-optic modulators are often employed as discrete devices to modulate a laser beam, it is also possible to monolithically integrate certain types of modulators with a laser diode on a single semiconductor substrate as part of a photonic integrated circuit (PIC). Techniques for accomplishing this integration and several examples of integrated laser/modulators are described in this chapter. (See Chapter 9 for a full discussion of photonic integrated circuits.)

Following modulation, the signal carrying the beam must be conveyed to a desired destination. Sometimes this can be done through air, or through space, but most often it is accomplished by coupling the lightwaves into an optical fiber waveguide, as described in Chapter 4. In this chapter the basic theory of waveguiding is developed and the mode structure of waveguided light is explained. The various classifications of fibers—multimode, single mode, and polarization holding—are described and their performance characteristics are detailed. Even though attenuation in optical fiber waveguides is relatively low compared to that in electrical cable, it is still an important factor that much be considered; hence absorption and scattering losses are discussed in Chapter 4. The other important limiting factor in optical waveguide lightwave transmission is the pulse broadening and analog signal distortion that occurs because of dispersion, the effect of different optical modes, and the effect of different wavelengths of light traveling at different velocities within the fiber. Modal dispersion can be avoided by using single-mode fibers, but chromatic dispersion due to the spread in wavelengths over the laser emission linewidth and bulk material dispersion in the glass of the fiber must still be dealt with. Waveguide dispersion arising from differing propagation velocities for lightwaves in the fiber core and in the cladding is also present. Techniques for minimizing the net effect of dispersion at desired wavelengths are described in Chapter 4. The couplers that are used to couple optical fiber waveguides to each other or to devices such as lasers, modulators, and detectors are also important elements of the photonic system. A great many different types of couplers are available today for both singlemode and multimode systems. The most commonly used types are described in this chapter.

Since fiber waveguides must be extended over hundreds of kilometers in many applications and/or the lightwave signal must be divided between many separate channels at some point, even a relatively low attenuation of 0.2 dB/km may result in excessive overall attenuation, rendering signals undetectable. In such situations optical amplifiers can be added to the photonic system to restore the signal to acceptable levels. In Chapter 5 optical amplifiers that increase the power level of the lightwave beam without unduly distorting the modulation are described. These amplifiers make use of stimulated emission of photons, just as occurs in a laser, to amplify lightwaves passing through them. In this case feedback from resonant structures is either absent, suppressed, or is carefully controlled so as to augment the amplification. There are two basic types of optical amplifiers in use today in photonic systems: optical fiber amplifiers (OFAs) and semiconductor optical amplifiers (SOAs). In the OFA the gain medium is an optically active dopant atom such as erbium, which is introduced directly into a length of glass optical fiber. The SOA is basically a semiconductor diode laser operated below the lasing threshold. The fiber waveguide must be coupled to the SOA at its input and output faces. The principles of operation and performance characteristics of both OFAs and SOAs are presented in Chapter 5. Key factors such as gain, noise figure, pumping methods, power consumption, and modulation frequency response are discussed. In addition, the performance of the most popular type of OFA, the erbium-doped fiber amplifier (EFA), is compared to the InGaAsP diode SOA.

After being transmitted to its destination, the lightwave signal is ultimately converted back into an electronic current in most photonic systems. This conversion is accomplished by means of a photodetector, the topic of Chapter 6. The photodetectors in most photonic systems today are semiconductor devices, of either the p-n junction photodiode or photoconductive type. Hence this chapter begins with a discussion of semiconductor materials systems and the fundamental principles underlying the operation of semiconductor photodetectors. The basic properties that characterize their performance, such as responsivity, dark current, and response time, are enumerated and explained. Then all of the major types of photodetectors are discussed, including both photoconductive and photodiode types. Both discrete devices and arrays of multiple photodetectors are covered. A section on photodetector fabrication techniques is included in Chapter 6. The methods used to fabricate some of the specialized structures that are employed to improve photodiode performance are explained. Schottky barrier and guard-ring devices are included, along with diffus...