Free-Space Optics: Propagation and Communication

Free space optics is a telecommunications technique which is already being used for everyday exchange of information and has many advantages over other techniques (bandwidth, low cost, mobility of the equipment, security, etc.); within the next decade, it is likely to become an integral and essential part of data-processing architectures and telecommunications. A history of wireless optical telecommunications is given, together with a recapitulation of the application of the principles of electromagnetism to free-space optics. Coverage is also given to the transmitters and receivers of optical beams, whih are the basis of any optical communication system. These devices were responsible for the first truly significant advances in the performance of these systems. Special attention is given to the problems associated with the propagation of photons, both in the presence and absence of obstacles, since these are key issues in gaining an understanding of future telecommunication systems based on wireless optics. Finally, the authors considwer standards, as well as safety and confidentiality issues.

Table of Contents:
Introduction 11 Chapter 1. History of Optical Telecommunications 15 1.1. Some definitions 15 1.1.1. Telecommunication 15 1.1.2. Optical transmission 15 1.1.3. Radio or Hertzian waves 15 1.2. The prehistory of telecommunications 16 1.3. The optical air telegraph 17 1.4. The code 21 1.5. The optical telegraph 23 1.6. The heliograph or solar telegraph: a portable telecommunication system 25 1.7. Alexander Graham Bell’s photophone 26 Chapter 2. Basic Principles of Electromagnetism 29 2.1. Introduction 29 2.2. Maxwell’s equations in an unspecified medium 29 2.3. Propagation of electromagnetic waves in an isotropic and linear homogeneous medium 31 2.4. Energy associated with a wave 33 2.5. Propagation of a wave in a non-homogeneous medium 34 2.6. Coherent and incoherent waves 35 2.7. Relations between classical electromagnetism and geometrical optics 36 2.8. The electromagnetic spectrum 38 2.9. Units and scales 39 2.10. Examples of sources in the visible light and near visible light42 2.11. Conclusion 44 Chapter 3. Emission and Reception of Optical Beams 45 3.1. Foreword 45 3.2. Introduction 46 3.3. Radiometry: basic concepts 47 3.4. Optical spectral windows, materials and eye-safety 51 3.5. Transmitters 53 3.5.1. Broad spectrum incoherent light emitting diodes 54 3.5.1.1. Structures 54 3.5.1.2. Near and far field patterns 54 3.5.1.3. Spectral characteristics 57 3.5.1.4. Electrical and optical characteristics 58 3.5.2. Laser diodes: high radiant power output, coherent waves 58 3.5.2.1. Structures 58 3.5.2.2. “(Φ transmitted )/(Iinjected) characteristic”: static and dynamic 59 3.5.2.3. Spectra and near field patterns 58 3.5.2.4. Spectral and modal instabilities and light intensity noise 64 3.5.3. Use of amplifiers with “rare earth ion” doped fibers 65 3.6. Photodetectors 65 3.6.1. Optical spectral range and materials 66 3.6.2. Principle of operation and structures 67 3.6.2.1. Surface phenomena: optical reflection, charge mobility and current leakage 67 3.6.2.2. Absorption and conduction: semiconductor junctions 68 3.6.3. Responsivity, response time, junction capacity and dark current 70 3.6.4. Photomultipliers and semiconductor avalanche photodiodes 73 Chapter 4. Line of Sight Propagation 77 4.1. Influence of the propagation environment 77 4.1.1. Atmospheric absorption 78 4.1.2. Atmospheric scattering 79 4.1.3. Extinction and total spectral transmission 80 4.1.4. Earth’s atmosphere 80 4.1.4.1. Atmospheric composition 81 4.1.4.2. Aerosols 82 4.2. Visibility 83 4.2.1. Generalities 83 4.2.1.1. Definitions 83 4.2.1.2. Units and scales 85 4.2.1.3. Meteorology needs 85 4.2.1.4. Measurement methods 86 4.2.2. Visual estimate of the meteorological optical range 87 4.2.2.1. General 87 4.2.2.2. Estimate of the day time meteorological optical range 87 4.2.2.3. Estimate of the night time meteorological optical range 88 4.2.2.4. Estimate of the meteorological optical range in the absence of distant reference markers 90 4.2.3. Meteorological optical range measurement instruments 91 4.2.3.1. General 91 4.2.3.2. Instruments to measure the extinction coefficient 91 4.2.3.3. Measurement instruments of the scattering coefficient 93 4.2.3.4. Exposure and implantation of instruments 95 4.3. Atmospheric attenuation 96 4.3.1. Molecular absorption attenuation 96 4.3.2. Molecular scattering attenuation 98 4.3.3. Absorption attenuation by aerosols 99 4.4. Meteorological disturbances 100 4.4.1. Mie scattering attenuation 100 4.4.1.1. Theoretical aspect 100 4.4.1.2. Modeling 103 4.4.2. Cloud attenuation 110 4.4.3. Fog, haze and mist attenuation 112 4.4.4. Precipitation attenuation 114 4.4.4.1. Rain 114 4.4.4.2. Other hydrometeors (snow, hail, etc.) 119 4.4.5. Refraction and scintillations 123 4.5. Free-Space Optical links 127 Chapter 5. Propagation of an Optical Beam in Confined Space 129 5.1. Introduction 129 5.2. Various mechanisms of propagation 130 5.2.1. Line Of Sight links 131 5.2.2. Wide-LOS or cellular links 132 5.2.3. Reflected links 133 5.2.3.1. Specular reflection 134 5.2.3.2. Diffuse reflection 134 5.3. Propagation channel 139 5.3.1. Description of an infrared propagation channel 139 5.3.2. Channel characterisation 141 5.4. Modeling 143 5.4.1. Single reflection propagation model 143 5.4.2. Statistical model 145 5.4.2.1. Free-space loss 145 5.4.2.2. Impulse response 147 5.4.3. Exponential decay model 152 5.4.4. Ceiling bounce model 152 5.5. Additional power required to reach a given bit error rate 154 5.5.1. Additional power requirement and delay spread 154 5.5.2. One-Off Keying (OOK) modulation 154 5.5.3. Pulse Position Modulation (PPM) 157 5.5.4. Multiple-Subcarrier Modulation (MSM) 157 5.6. Optical noise 159 5.6.1. Tungsten filament light 159 5.6.2. Low frequency fluorescent light 159 5.6.3. High frequency fluorescent light 159 5.6.4. Infrared audio transmitters 160 5.6.5. Infrared TV remote control devices 160 5.6.6. Effects of daylight 161 5.6.7. Artificial light model 162 5.7. Comparison of infrared and radio media and conclusion 164 Chapter 6. Optical Communication 167 6.1. A reminder about digitization 167 6.2. Examples of laser applications outside optical communications 169 6.2.1. BTP sector 169 6.2.2. Industrial sector 169 6.2.3. Medical sector 170 6.2.4. General public sector 170 6.3. Inter-satellite or Earth-satellite optical communications 171 6.3.1. Earth-satellite optical communications 171 6.3.2. Inter-satellite optical communications 171 6.4. Free-space optical communications 173 6.4.1. Introduction – operating principles 173 6.4.2. Characteristics 175 6.4.2.1. Principal parameters 175 6.4.2.2. Secondary parameters 176 6.4.2.3. Examples of installed systems 176 6.4.3. Propagation times 178 6.4.4. Implementation recommendations 178 6.4.5. Legislation 179 6.4.5.1. The organisation of regulation activities in radio communications 179 6.4.5.2. Regulation of FSO equipment 181 6.4.6. Concept of quality of service and availability 183 6.4.6.1. Geometrical attenuation concept 183 6.4.6.2. Concept of the link margin 185 6.4.6.3. Availability and quality of service 186 6.4.7. FSO potential applications 193 6.4.7.1. Geographical approach 194 6.4.7.2. Applicative approach 194 Chapter 7. Safety and Confidentiality 197 7.1. Safety 197 7.1.1. Dangers 197 7.1.2. Concept of categories 199 7.1.3. Accessible Emission Limits (AEL) 200 7.1.4. Maximum Permissible Exposures (MPE) 200 7.1.5. The NOHD calculation 201 7.1.6. Conformity to standard IEC825/EN60825 202 7.1.6.1. Class 3B 202 7.1.6.2. Class 3R 203 7.1.6.3. Class 2 203 7.1.6.4. Class 1 203 7.2. Confidentiality 203 7.2.1. Transmitted data confidentiality 203 7.2.2. Confidentiality techniques 204 7.2.2.1. Cryptography 205 7.2.2.2. Public key and secret key cryptography 205 7.2.2.3. Quantum cryptography 206 7.2.2.4. Quantum telecommunications in free space 206 7.2.2.5. Non-encrypted links in confined space: contribution of artificial materials 207 Bibliography 209 Index 217

About the Author :
Olivier Bouchet works in the field of free-space optics at France Telecom Research and Development. Hervé Sizun is a specialist of the electromagnetic wave propagation at France Telecom. Christian Boisrobert is one of the designers of the first digital link using optical fibres in France at France Telecom. He is a professor at the University of Nantes. Frédérique de Fornal is the director of Research at CNRS and is Head of the Near Field Optics Group at the University of Bourgogne. Pierre-Noël Favennec works as a consultant engineer and is the author of numerous scientific books and papers. He is also the Chairman of URSI France.