Diode Lasers and Photonic Integrated Circuits

Larry A. Coldren, Scott W. Corzine, Milan L. Mashanovitch

• Publisher: Wiley
• Year: 2012
• ISBN:
  9781118148181
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Diode Lasers and Photonic Integrated Circuits, Second Edition provides a comprehensive treatment of optical communication technology, its principles and theory, treating students as well as experienced engineers to an in-depth exploration of this field. Diode lasers are still of significant importance in the areas of optical communication, storage, and sensing. Using the the same well received theoretical foundations of the first edition, the Second Edition now introduces timely updates in the technology and in focus of the book. After 15 years of development in the field, this book will offer brand new and updated material on GaN-based and quantum-dot lasers, photonic IC technology, detectors, modulators and SOAs, DVDs and storage, eye diagrams and BER concepts, and DFB lasers. Appendices will also be expanded to include quantum-dot issues and more on the relation between spontaneous emission and gain.

Chapter 1

Ingredients

1.1 Introduction

Diode lasers, like most other lasers, incorporate an optical gain medium in a resonant optical cavity. The design of both the gain medium and the resonant cavity are critical in modern lasers. A sample schematic of a laser cavity and its elements is shown in Fig. 1.1. In this case, an optional mode selection filter is also added to permit only one cavity mode to lase. The gain medium consists of a material that normally absorbs incident radiation over some wavelength range of interest. But, if it is pumped by inputting either electrical or optical energy, the electrons within the material can be excited to higher, nonequilibrium energy levels, so the incident radiation can be amplified rather than absorbed by stimulating the de-excitation of these electrons along with the generation of additional radiation. The resonant optical cavity supports a number of cavity standing waves, or modes. As illustrated in Figs. 1.1b and c, these occur where the cavity length is a multiple of a half wavelength. If the resulting gain is sufficient to overcome the losses of some resonant optical mode of the cavity, this mode is said to have reached threshold, and relatively coherent light will be emitted. The resonant cavity provides the necessary positive feedback for the radiation being amplified, so that a lasing oscillation can be established and sustained above threshold pumping levels. A typical diode laser light-pump current characteristic is shown in Fig. 1.1d. The threshold can be identified on an output light power vs. pump characteristic by a sharp knee, as illustrated in Fig. 1.1d.

Figure 1.1 (a) A schematic of a simple laser diode. (b) Necessary ingredients for a single-frequency laser cavity—two mirrors, a gain medium, and a mode selection filter, which is required only for single wavelength λ operation. (c) Spectral characteristics of laser elements that get superimposed for single mode operation: cavity modes are given by

, where the mode number m is an integer, and

is the effective index of refraction (d) Typical light-current diode laser characteristic.

For various applications, a single lasing mode inside a laser cavity is preferred. Different methods in cavity design can be used to favor the lasing of one mode relative to others. The response of the optical mirrors can be tailored to support a single mode. Often, additional optical filtering elements will be incorporated inside the resonant cavity, to insure single mode operation of the laser. Fig. 1.1c shows the spectral response of the various elements of this cavity. This resonant optical cavity is defined by two broadband mirrors, with flat spectral responses, which define a number of cavity modes. An additional mode filtering element, with a defined bandpass optical transfer function, is included. The optical gain medium has a certain spectral response, which, in combination with the spectral response of the filter, will define which cavity mode will be singled out. As in any other oscillator, the output power level saturates at a level equal to the input minus any internal losses.

Since their discovery, lasers have been demonstrated in solid, liquid, gas and plasma materials. Today, the most important classes of lasers, besides the widespread diode/(or semiconductor) lasers are, gas, dye, solid-state, and fiber lasers, the latter really being fiber-optic versions of solid-state lasers. The helium–neon gas laser, the widely tunable flowing-dye laser, the Nd-doped YAG (yttrium–aluminum–garnet) solid-state and the Er or Yb-doped silica fiber lasers are four popular examples. Figure 1.2 shows commercial examples of Nd-YAG and dye lasers, an Er-doped fiber amplifier (EDFA), as well as a packaged diode laser for comparison. The EDFA is used in fiber-optic systems to compensate losses, and with the addition of mirrors placed in the fiber, it can also become a laser. Diode lasers are distinguished from these other types primarily by their ability to be pumped directly by an electrical current. Generally, this results in a much more efficient operation. Overall power conversion efficiencies of ∼50% are not uncommon for a diode laser, whereas efficiencies on the order of 1% are common for gas and solid-state lasers, which traditionally have been pumped by plasma excitation or an incoherent optical flashlamp source, respectively. However, in recent years diode laser pumps have been used for both bulk solid-state lasers as well as fiber lasers, and wall plug efficiencies better than 25% have been achieved. Efficiencies of some gas lasers can be somewhat higher than that of the He-Ne laser, such as in the case of the CO2 gas laser, which has a typical efficiency of over 10%. Another type of gas laser, the so-called Excimer laser, uses transitions between highly excited atomic states to produce high-power ultraviolet emission, and these are used in the medical industry for a variety of surgical procedures as well as in the semiconductor industry for patterning very fine features. Dye lasers are almost always used in a research environment because of their relatively high maintenance requirements, and they are generally pumped by other high power bench-top lasers. Their appeal is that their output wavelength can be tuned by as much as 10% for a given dye and mirror set, and by changing these, wavelengths from the near IR through much of the visible can be provided from a single commercial product.

Figure 1.2 Examples of solid-state (upper left), dye (upper right), and fiber laser (bottom left) systems compared to a packaged diode laser chip (bottom right). To function, the diode laser also requires some drive electronics, and this increases its net size somewhat.

Because of their longer cavities gas, dye, solid-state and fiber lasers also tend to have more coherent outputs than simple semiconductor lasers. However, more sophisticated single-frequency diode lasers can have comparable linewidths in the low megahertz range.

Another major attribute of diode lasers, their high reliability or useful lifetime, has led to their widespread use in important applications such as fiber-optic communications systems. Whereas the useful life of gas or flash-lamp-pumped solid-state lasers is typically measured in thousands of hours, that of carefully qualified diode lasers is measured in hundreds of years. Recent use of diode lasers to pump solid-state and fiber lasers may, however, provide the best advantages of both technologies, providing high reliability, improved efficiency, and low linewidth.

Net size is another striking difference between semiconductor and other lasers. Whereas gas, solid-state and fiber lasers are typically tens of centimeters in length, diode laser chips are generally about the size of a grain of salt, although the mounting and packaging hardware increases the useful component size to the order of a cubic centimeter or so. The diode lasers are mass-produced using wafer scale semiconductor processes, which makes them really inexpensive compared to all other types of lasers. The semiconductor origins of diode lasers allows for semiconductor integration techniques to be applied, and for multiple building blocks to be defined along the common waveguide, yielding functionally complex devices and opening a new field of photonic integrated circuits. Diode lasers with integrated optical amplifiers, modulators and similar other functions have been realized. In addition, monolithic widely tunable diode lasers and transmitters have been conceived and developed, in a footprint much smaller than that of external-cavity widely tunable lasers. Arrays of diode lasers and transmitters have been commercialized as well, for both optical pumping, and telecom purposes.

Diode lasers are used in many consumer products today. Examples are illustrated in Fig. 1.3. The most widely used diode lasers on the planet by far are those used in CD/DVD players, DVD ROM drives and optical mice. These diode lasers produce light beams in the red part of the visible spectrum at a wavelength of 0.65 μm. Recent improvements of the diode lasers emitting in the blue visible part of the spectrum have allowed for higher density DVD discs to be developed, resulting in the Blu-ray Disc technology, operating at 0.405 μm. Visible red diode lasers have replaced helium-neon lasers in supermarket checkout scanners and other bar code scanners. Laser printers are commonly used to produce high-resolution printouts, enabled by the high resolution determined by the wavelength of the diode laser used (780 nm or lower). Laser pointers, patient positioning devices in medicine utilize diode lasers emitting in the visible spectrum, both red and green.

Figure 1.3 Examples of the most common products that utilize diode lasers. (left) red laser in a DVD player shown in laptop computer; (center-top) blue laser in a Blu-ray Disc player; (center-bottom) red laser in a laser printer; (right-top) red laser in a bar-code scanner; (right-bottom) red (and sometimes green or blue) laser in a pointer.

In fiber-optic communication systems, diode lasers are primarily used as light sources in the optical links. For short reach links, a directly modulated diode laser is used as a transmitter. For longer reach links, diode lasers are used in conjunction with external modulators, which can be external to the diode laser chip, or integrated on the same chip. Complex diode laser-based photonic integrated circuits are currently deployed in a number of optical networks. In addition, Erbium doped fiber amplifiers, a key technology that is utilized for signal amplification in the existing fiber-optic networks, has in part been enabled by the development of high power, high reliability diode pump lasers.

There are many other areas where diode lasers are utilized. In medical applications, diode lasers are used in optical coherence tomography, an optical signal acquisition and processing method allow...

Table of contents

• Cover
• Series Page
• Title Page
• Copyright
• Preface
• Acknowledgments
• List of Fundamental Constants
• Chapter 1: Ingredients
• Chapter 2: A Phenomenological Approach to Diode Lasers
• Chapter 3: Mirrors and Resonators for Diode Lasers
• Chapter 4: Gain and Current Relations
• Chapter 5: Dynamic Effects
• Chapter 6: Perturbation, Coupled-Mode Theory, Modal Excitation, and Applications
• Chapter 7: Dielectric Waveguides
• Chapter 8: Photonic Integrated Circuits
• Appendix One: Review of Elementary Solid-State Physics
• Appendix Two: Relationships between Fermi Energy and Carrier Density and Leakage
• Appendix Three: Introduction to Optical Waveguiding in Simple Double-Heterostructures
• Appendix Four: Density of Optical Modes, Blackbody Radiation, and Spontaneous Emission Factor
• Appendix Five: Modal Gain, Modal Loss, and Confinement Factors
• Appendix Six: Einstein's Approach to Gain and Spontaneous Emission
• Appendix Seven: Periodic Structures and the Transmission Matrix
• Appendix Eight: Electronic States in Semiconductors
• Appendix Nine: Fermi's Golden Rule
• Appendix Ten: Transition Matrix Element
• Appendix Eleven: Strained Bandgaps
• Appendix Twelve: Threshold Energy for Auger Processes
• Appendix Thirteen: Langevin Noise
• Appendix Fourteen: Derivation Details for Perturbation Formulas
• Appendix Fifteen: Multimode Interference
• Appendix Sixteen: The Electro-Optic Effect
• Appendix Seventeen: Solution of Finite Difference Problems
• Index
• Wiley Series
• End User License Agreement