Photonics and Optical Communication, Spring 2007, Dr. D. Knipp Photonics and Optical Communication (Course Number 300352) Spring 2007 Optical Source Dr. Dietmar Knipp Assistant Professor of Electrical Engineering http://www.faculty.iu-bremen.de/dknipp/ Optical Sources 1 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp Photonics and Optical Communication 5 Optical Sources 5.1 Introduction 5.2 Absorption and Emission of light 5.2.1 Spontaneous Emission 5.2.2 Stimulated Emission 5.3 Light emitting diodes versus laser diodes 5.4 Introduction to semiconductors 5.4.1 Structural Properties of Semiconductors 5.4.2 Energy Bands in Semiconductors 5.4.3 The pn-junction 5.4.4 Diodes under forward bias 5.5 Light emitting diodes (LEDs) 5.5.1 Direct and indirect Semiconductors 5.5.2 Device structures 5.5.3 Application of Light emitting Diodes 5.6 Lasers 5.6.1 Spontaneous Emission 5.6.2 Population inversion 5.6.3 Three and four energy level systems 5.6.4 Optical feedback and laser resonators Optical Sources 2 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.6.5 Threshold condition for laser oscillation 5.6.6 Requirements for lasing 5.7 Semiconductor Lasers 5.7.1 Stimulated emission and lasing in Semiconductors 5.7.2 Semiconductor Materials for lasing applications 5.7.3 Efficiency of LEDs and laser diodes 5.7.4 Laser Diode structures 5.7.4.1 Fabry Perot Homojunction laser diode 5.7.4.2 Double heterostructure laser diode 5.7.4.3 Quantum well lasers 5.7.4.4 Distributed Feedback (DFB) Lasers 5.7.4.5 Vertical Cavity Surface Emitting Lasers (VCSELs) References Optical Sources 3 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.1 Introduction The success of optical communication technology is stimulated by the development of optical fibers and optical fiber technology on one side and the invention of solid state lasers and laser diodes on the other side. Solid state lasers are compact, reliable and inexpensive. Optical communication systems with very high bandwidth-distance products can only be implemented by using lasers or laser diodes. Laser diode package and micrograph of inside of a laser package. Ref.: Infineon Optical Sources 4 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.1 Introduction In general the generation of light is caused by the transition of an electron form an energetically higher energy state to a lower energy state. The energy difference due to the transition of the electron leads to a radiative or a non- radiative process. We are of course interested in radiative processes as we like to “build” an optical source. The non-radiative processes typically lead to the creating of heat. The energy is simply dissipated by heat. In the case of a radiative process photons are emitted. The emission of light, can take place either spontaneously or it can be stimulated by the presence of another photon of the “right” energy. In order to understand the processes of light-generation, it is necessary to consider fundamental processes like structural and optical properties and energy levels in materials and the electronic device concepts. An understanding of the structural and optical properties is needed to actually understand the process of light generation and an understanding of the devices is needed to make use of such an effect. Optical Sources 5 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.2 Absorption and Emission of light The interaction of light and matter in the form of absorption and emission requires a transition from one discrete energy level to another energy level. The frequency and the wavelength of the emitted or absorbed photon is related to the difference in energy E, between the two energetic states, where h is the Planck constant h=6.626 x 10-34J, f is the frequency and λ is wavelength of the absorbed or emitted light. hc E = E − E = hf = Photon energy 2 1 λ Optical Sources 6 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.2 Absorption and Emission of light The figure illustrates transitions between two energy states. When a photon with the energy (E2-E1) is incident on the material an electron may be excited from the energy state E1 into an higher energy state E2 through the absorption of the photon. Alternatively, when the electron is initially on a higher energy level it can make a transition to a energetically lower state and the provided energy loss leads to the emission of a photon. Here the transition is assumed to be a radiative transition. Energy state diagram showing (a) absorption, (b) spontaneous emission, (c) stimulated emission. Ref.: J.M. Senior, Optical Fiber Communications Optical Sources 7 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.2 Absorption and Emission of light We have to distinguish between radiative and non-radiative processes. In the case of a non-radiative process the energy is dissipated as heat. The question whether a transition is non-radiative or radiative depends on the involved species of carriers, the material itself, the level of impurities in the material, the temperature and the device structure. In the case of radiative emission we can than distinguish between spontaneous and stimulated emission. Optical Sources 8 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.2.1 Spontaneous Emission For most of the light sources the photons are emitted spontaneously (sun light, light bulb, halogen lamp). In a first step an electron is elevated to an energetically higher state which is usually unstable. In the second step the electron will spontaneously return to an energetically more stable state (which is typically the energetically lower state). This process is a statistical process which can happen very fast. As a consequence the spontaneously (or randomly) emitted photons are incoherent (very short coherence time) and the emitted spectrum has broad spectral width. Energy state diagram for spontaneous emission of a photon. Ref.: J.M. Senior, Optical Fiber Communications Optical Sources 9 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.2.2 Stimulated Emission The operating principle of a laser is based on stimulated emission. We speak about stimulated emission if the electron which enters an energetically higher state (excited state) remains in this state until it is “stimulated” by the presence of a photon to leave this higher energetically state and return to the more stable lower energetically state (ground state). One of the requirements for stimulated emission is that the electron can stay in its excited state a relatively long period of time (a few microseconds) before it changes its state spontaneously. In the case of spontaneous emission the electron stays in this excited state usually for a shorter period of time (picoseconds). In the case of stimulated emission the electron can be “stimulated” by the presence of a photon to emit its energy in the form of another photon. Energy state diagram for stimulated emission. Ref.: J.M. Senior, Optical Fiber Communications Optical Sources 10 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.2.2 Stimulated Emission In this case the energy of the incident photon has to be very close to the energy of the excited electron. Stimulated emission takes place when the emitted photon has the same energy (the same wavelength), phase and direction as that of the photon which stimulated it! Stimulated emission is the inverse process of absorption! 5.3 Light emitting diodes versus laser diodes In order to observe spontaneous or stimulated emission we have to excite the electrons first before they can return to a lower energetic states. Of course energy has to be provided to excite the electron. The energy can be provided by heat, absorption of photons (photoluminescence) or electrical current (electroluminescent). We are interested in the later case, where the energy is provided by an electrical current. Optical Sources 11 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.3 Light emitting diodes versus laser diodes In both types of devices the recombination of carrier is used to provide a photon flux. However, the emission of light in a light emitting diode is a spontaneous process, whereas it is a stimulated process in a laser diode. Therefore, the description of an LED (light emitting diode) is different from the description of a laser diode. The description of an LED is by far simpler than the description of a laser diode. In both cases a semiconductor diode is used, which operates under forward bias conditions. Furthermore, the same structure can be used to build an optical amplifier. Forward biased pn-diode operating as (a) LED, (b) semiconductor amplifier, (c) semiconductor injection laser. Ref.: Saleh & Teich, Fundamentals of Photonics Optical Sources 12 Photonics and Optical Communication, Spring 2007, Dr. D. Knipp 5.4 Introduction to semiconductors In order to get an understanding of semiconductor based optical light sources we have to review some of the basic semiconductor properties. We will concentrate in this lecture on the description of the basic operating principle of a pn-junction (pn diode) as the light emitting diode (LED) and the laser diode are based on such structure. 5.4.1 Structural Properties of Semiconductors First the structural properties of semiconductors will be discussed. The structural properties have a strong effect on the electronic and the optical properties of the material. In general we can distinguish semiconductors in terms of their structural properties. Semiconductors exist as crystalline or amorphous materials. Crystalline material exhibit a high degree of structural order, whereas amorphous materials are characterized by a random or partly random distribution of the atoms or molecules in the solid.