An Illustrated Dictionary of Optoelectronics and Photonics: Important Terms and Effects Safa Kasap University of Saskatchewan, Canada Harry Ruda University of Toronto, Canada Yann Boucher École Nationale d’Ingénieurs de Brest, France Ali Javan and his associates William Bennett Jr. and Donald Herriott at Bell Labs were first to successfully demonstrate a continuous wave (cw) helium-neon laser operation (1960). (Courtesy of Bell Labs, Lucent Technologies.) Concise Second Student Edition Version 1.3.1 (February 2002) Illustrated Dictionary of Important Terms and Effects in Optoelectronics and Photonics ( 1999 S.O. Kasap) 2 PREFACE The title An Illustrated Dictionary of Important Terms and Effects in Optoelectronics and Photonics reflects exactly the contents of this dictionary. It is not meant to be an encyclopedia in this field but rather a self- contained semiquantitative description of terms and effects that frequently turn up in optoelectronics and photonics courses at the undergraduate level. We have missed many terms but we have also described many; though, undoubtedly, our own biased selection. In our own view there is very little difference between optoelectronics and photonics, except that in the latter there may not be any electronics involved. Today, photonics is a more fashionable term than the old optoelectronics term which usually conjectures images of LEDs, solar cells, optoisolators etc, whereas photonics is closely associated with optical communications and optical signal processing. Please feel free to email us your comments for improving the dictionary for its next edition. Although we may not be able to reply individually, we do read all our email messages and take note of suggestions and comments. S.O. Kasap, Saskatoon, Canada H. Ruda, Toronto, Canada Y. Boucher, Brest, France “I know you believe you understand what you think I said, but I am not sure you realize that what you heard is not what I meant” Alan Greenspan Chairman of US Federal Reserve WARNING All material in this publication is copyrighted. It is unlawful to duplicate any part of this publication. © The authors reserve all rights. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the author . © S.O. Kasap, 1996, 1997, 1998 © S.O. Kasap, H. Ruda and Y. Boucher, 1999, 2000 Version 1.3 (October 2000) Illustrated Dictionary of Important Terms and Effects in Optoelectronics and Photonics ( 1999 S.O. Kasap) 3 Absorption coefficient α characterizes the loss of photons as light propagates along a certain direction in a medium. It is the fractional change in the intensity of light per unit distance along the propagation direction, that is, δ α =− I δ I x where I is the intensity of the radiation. The absorption coefficient depends on the photon energy or wavelength λ. Absorption coefficient α is a material property. Most of the photon absorption (63%) occurs over a distance 1/α and 1/α is called the penetration depth δ. Photon energy (eV) 5 4 3 2 1 0.9 0.8 0.7 1 108 Ge 1 107 In0.7Ga0.3As0.64P0.36 In Ga As Si 0.53 0.47 1 106 GaAs (m-1) InP 1 105 a-Si:H 1 104 1 103 0.2 0.4 0.6 0.81.0 1.2 1.4 1.6 1.8 Wavelength ( m) Absorption coefficient ( ) vs. wavelength ( ) for various semiconductors (Data selectively collected and combined from various sources.) Absorption is the loss in the power of an electromagnetic radiation that is traveling in a medium. The loss is due to the conversion of light energy to other forms of energy, e.g. lattice vibrations (heat) during the polarization of the molecules of the medium, local vibrations of impurity ions, excitation of electrons from the valence band to the conduction band etc. Acceptance angle, or the maximum acceptance angle is the largest possible light launch angle from the fiber axis. Light waves within the acceptance angle that enter the fiber become guided along the fiber core. If NA is the numerical aperture of a step index fiber, and light is launched from a α medium of refractive index no, then maximum acceptance angle max is given by α = NA sin max no Illustrated Dictionary of Important Terms and Effects in Optoelectronics and Photonics ( 1999 S.O. Kasap) 4 Total acceptance angle is twice the maximum acceptance angle and is the total angle around the fiber axis within which all light rays can be launched into the fiber. Acceptance cone is a cone with its height aligned with the fiber axis and its apex angle twice the acceptance angle so that light rays within this cone can enter the fiber and then propagate along the fiber. Acceptance cone α 2 max Optical fiber Acceptor atoms are dopants that have one less valency than the host atom. They therefore accept electrons from the valence band (VB) and thereby create holes in the VB which leads to p > n and hence to a p-type semiconductor. Acousto-optic modulator makes use of the photoelastic effect to modulate a light beam. Suppose that we generate traveling acoustic or ultrasonic waves on the surface of a piezoelectric crystal (such as LiNbO3) by attaching an interdigital electrodes onto its surface and applying a modulating voltage at radio frequencies (RF). The piezoelectric effect is the phenomenon of generation of strain in a crystal by the application of an external electric field. The modulating voltage V(t) at electrodes will therefore generate a surface acoustic wave (SAW) via the piezoelectric effect. These acoustic waves propagate by rarefactions and compressions of the crystal surface region which lead to a periodic variation in the density and hence a periodic variation in the refractive index in synchronization with the acoustic wave amplitude. Put differently, the periodic variation in the strain S leads to a periodic variation in n owing to the photoelastic effect. We can simplistically view the crystal surface region as alternations in the refractive index. An incident light beam will be diffracted by this periodic variation in the refractive index. If the acoustic wavelength is Λ, then the condition that gives the angle θ for a diffracted beam to exist is given by the Bragg diffraction condition, 2Λsinθ = λ/n where n is refractive index of the medium. Suppose that ω is the angular frequency of the incident optical wave. The optical wave reflections occur from a moving diffraction pattern which moves with a velocity vacoustic. As a result of the Doppler effect, the diffracted beam has either a slightly higher or slightly lower frequency depending on the direction of the traveling acoustic wave. If Ω is the frequency of the acoustic wave then the diffracted beam has a Doppler shifted frequency given by ω′ = ω ± Ω When the acoustic wave is traveling towards the incoming optical beam, then the diffracted optical beam frequency is up-shifted, i.e. ω′ = ω + Ω. If the acoustic wave is traveling away from the incident optical beam then the diffracted frequency is down-shifted, ω′ = ω − Ω. It is apparent that we can modulate the frequency (wavelength) of the diffracted light beam by modulating the frequency of the of the acoustic waves. (The diffraction angle is then also changed.) Illustrated Dictionary of Important Terms and Effects in Optoelectronics and Photonics ( 1999 S.O. Kasap) 5 Acoustic absorber Induced diffraction grating Diffracted light Incident light Traveling acoustic waves Acoustic create a harmonic variation wave Through light in the refractive index and fronts thereby create a diffraction grating that diffracts the Piezoelectric incident beam through an crystal angle 2 . Interdigitally electroded Modulating RF voltage transducer Incident optical beam Diffracted optical beam Acoustic A A' wave x x B B' n min O n max P Q Acoustic n min sin sin wave fronts O' nmax vacoustic n n n nmin nmax nmin max Simplified Actual Consider two coherent optical waves A and B being "reflected" (strictly, scattered) from two adjacent acoustic wavefronts to become A' and B'. These reflected waves can only constitute the diffracted beam if they are in phase. The angle is exaggerated (typically this is a few degrees). Activation energy is the potential energy barrier that prevents a system from changing from state to another. For example, if two atoms A and B get together to form a product AB, the activation energy is the potential energy barrier against the formation of this product. It is the minimum energy which the reactant atom or molecule must have to be able to reach the activated state and hence form the product. The probability that a system has an energy equal to the activation energy is proportional to − the Boltzmann factor: exp( EA/kT), where EA is the activation energy, k is the Boltzmann constant and T is the temperature (Kelvins). Active device is a device that exhibits gain (current or voltage or both) and has a directional function. Transistors are active devices whereas resistors, capacitors and inductors are passive devices. Active region is the region in a medium where direct electron hole pair (EHP) recombination takes place. For LEDs it is the region where most EHP recombination takes place. In the laser diode it is Illustrated Dictionary of Important Terms and Effects in Optoelectronics and Photonics ( 1999 S.O. Kasap) 6 the region where stimulated emissions exceeds spontaneous emission and absorption. It is the region where coherent emission dominates. Airy disk is the central white spot in the Airy ring pattern Airy rings represent a diffraction pattern from a circular aperture illuminated by a coherent beam of light.