WT7001/FREE SPACE OPTICAL COMMUNICATION

WT7001/FREE SPACE OPTICAL COMMUNICATION
UNIT I FUNDAMENTALS OF FSO TECHNOLOGY Introduction – Maxwell’s Equations – Electromagnetic wave propagation in free space-alternate bandwidth technologies – Fiber Vs FSO- Fiber Access – Overview of FSO Optical Transmitters – Receivers – Subsystems – Pointing, Acquisition and Tracking – Line of sight analysis. TEXTBOOKS 1. Heinz, Phd. Willebrand, “Free Space Optics,” Sams, 1st Ed., 2001. 2. Morris Katzman, “Laser Satellite Communication,” Prentice Hall Inc., New York, 1991 Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

WT7001/FREE SPACE OPTICAL COMMUNICATION UNIT I
INTRODUCTION: MAXWELLS EQUATIONS Differential Form: where D=ϵE is the electric displacement vector, B=μH is the magnetic flux density, Jc is the conduction current density, E is the electric field and H is the magnetic field intensity. μ and ϵ is the permeability and permittivity of the medium, respectively. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: MAXWELLS EQUATIONS Integral Form: where D=ϵE is the electric displacement vector, B=μH is the magnetic flux density, Jc is the conduction current density, E is the electric field and H is the magnetic field intensity. μ and ϵ is the permeability and permittivity of the medium, respectively. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: ELECTROMAGNETIC WAVE EQUATION We investigate the propagation of light in free space without considering the origin of light using Maxwell’s equations. Maxwell’s equation for a source-free medium (i.e.; the charge density (ρv) and conduction current densities (Jc) in the medium are zero) are, where D=ϵE is the electric displacement vector, B=μH is the magnetic flux density, E is the electric field and H is the magnetic field intensity. μ and ϵ is the permeability and permittivity of the medium, respectively. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: ELECTROMAGNETIC WAVE EQUATION In order to decouple E and H in Maxwell’s third and fourth equations, we use curl function . From third Maxwell’s equation, Substituting B=μH and interchanging the space and time derivatives, we get, Assuming a non magnetic, homogenous medium such as glass rod (μ=μ0 and is not a function of space) Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: ELECTROMAGNETIC WAVE EQUATION Now substituting Maxwell’s fourth equation in above equation, we get, The vector identity is given by, So the above equation can be simplified to, Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: ELECTROMAGNETIC WAVE EQUATION For a homogenous medium, ϵ is not a function of space. From Gauss’s law, Substituting in above equation, we get, The above equation is called as the Wave Equation for the Electric Field. If we do similar analysis for the magnetic field we get the same wave equation for the magnetic field, Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: ELECTROMAGNETIC WAVE EQUATION To analyze the propagation of light in free space, we have to solve the wave equation with appropriate boundary conditions. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: Basic block diagram of Communication: Optical fiber communication: Information Source Transmitter Channel Receiver Destination Noise source Modulation Demodulation Information Source LED/LASER Optical Fiber Photodetector Destination Noise source Modulation Demodulation Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: Free space optical communication: Information Source LASER Free Space Receiver Destination Noise source Modulation Demodulation Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: Before optical communication, the most of the communication was in radio and microwave domain which has frequency range orders of magnitude lower than the optical frequency. Electromagnetic spectrum: Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: A good communication needs to have following criteria: Large Bandwidth (BW) Good Signal to Noise ratio (SNR) i.e; low loss To send more and more information on a channel, bandwidth needs to increase. Since the bandwidth (BW) of a system is more or less proportional to the frequency of operation (f0), use of higher frequency facilitates larger BW. where Q is the quality factor (for an electrical circuit, Q is independent of frequency: typically 100 to few 100s). Therefore, Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: The BW at optical frequencies is expected to be 3 to 4 orders of magnitude higher than that at the microwave frequencies (1 GHz to 100 GHz). Typical voice channel will require a bandwidth of about 4 kHz. After sampling and quantization we may require typically 64 kHz. Bandwidth ∝ No of users (for simultaneous communication). Microwave frequency (1 GHz) No of simultaneous users: 1.56 x 104 (1 x 109/64 kHz) Optical frequency (100 THz) No of simultaneous users: 1.56 x 109 (100 x 1012/64 kHz) Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: Major questions on optical communication are: Do we have medium to carry light from one place to another? Air medium- But it is not lossless. Information can be carried to certain distant (Typically few km) Glass (used in prisms, lens for physics experiments can transport light to certain distances)- But it has high attenuation (1000 dB/km). DB scale: (Logarithmic scale) 10 dB represent reduction in power by a factor of 10 20 dB represent reduction in power by a factor of 100 Researchers found that 1000 dB/km attenuation of glass is due to impurities present in it. After first level purification of glass, attenuation loss reduced to 20 dB/km (comparable with waveguides in microwave regime). Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: Major questions on optical communication are: Do we have sources which can carry information? By varying the properties of light such as intensity, frequency, phase and polarization. Tube lights or light bulbs are not suitable sources to carry information because the data has to be varied at a very fast rate. Rate at which an optical source can be switched on and off depends on its spectral width (White light: 400 nm to 750 nm, LASER: few nm). Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: TRANSMISSION MEDIA Twisted pair Co-axial cable (kbps) 3 kHz to 300 GHz Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: TRANSMISSION MEDIA Microwave Link Satellite Communication (few 100 MHz) (1/r2) Television (7 years) Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: TRANSMISSION MEDIA Satellite communication Fiber optic communication Point to multi-point Point to point BW (approx GHz) BW (approx THz) Maintenance free Needs maintenance Short life (approx 7 to 8 Years) Long life No upgradability Upgradable Mobile, air, sea On ground only Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: ADVANTAGES OF OPTICAL FIBER COMMUNICATION Ultra high bandwidth (THz) Low loss (0.2 dB/km) Low Electromagnetic interference (EMI) Security of transmission Low manufacturing cost (Mostly technology cost, Raw material (Si) is abundant in nature) Low weight, low volume Point to point communication (Disadvantage) The optical transmission medium is the best in a sense that it has ultra wide bandwidth and very low attenuation. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

INTRODUCTION: The demand for high bandwidth in metropolitan networks on short timelines is increasing. Further, requirements of flexibility and cost effectiveness of service provisioning (some connections could be temporary, whereas others are long term) have caused an imbalance. This problem is often referred to as the “last mile bottleneck” or “connectivity bottleneck”. Free-space optics is a solution to encounter connectivity bottleneck. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

ALTERNATIVE BANDWIDTH TECHNOLOGIES: The first most obvious choice for addressing the bandwidth shortage is fiber. But the digging, cost to lay that fiber, and time to market are the most prohibitive factors. In addition, after you lay fiber, it becomes a sunk cost; if the customer leaves, it becomes almost impossible to recover that cost. Even though fiber is technologically superior to free-space optics, it is significantly more costly. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

ALTERNATIVE BANDWIDTH TECHNOLOGIES: A second choice is radio frequency (RF). This technology is mature and has been widely deployed. RF based networks require immense investments to acquire the spectrum licenses, yet they cannot scale to optical capacities. (The ceiling today is 622 Mbps.) However, RF-based networks can go longer distances. When compared to free-space optics, RF does not make economic sense. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

ALTERNATIVE BANDWIDTH TECHNOLOGIES: A third alternative is all the copper-based technologies, such as cable modems, T1s, and DSLs. It is not a viable alternative for solving the connectivity bottleneck. The problem is bandwidth scalability. Copper’s distance per segment and throughput is inherently limited; therefore, its potential to solve the connectivity bottleneck is also limited. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

ALTERNATIVE BANDWIDTH TECHNOLOGIES: The fourth, most viable alternative is free-space optics. FSO represents the most optimal solution in terms of technology (optical), bandwidth scalability, speed of deployment (hours versus months) and cost effectiveness (at least one fifth). Fiber less laser driven technology No licensing and frequency coordination required Operating wavelength range nm nm Provides a line of sight link FSO links are full duplex and easy to install Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: Because FSO and fiber optics enable similar bandwidth transmission abilities, it is important to compare them. Light can be transmitted either through free space or a confined medium. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS A fiber-optic cable carries a light signal from point A to point B, but the light signal must be generated first. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS A light source converts an electrical signal carrying voice/data/video content into an optical signal. The process by which the electrical signal is mapped onto the optical signal is called modulation. The light source can perform the modulation (self-modulation) or do so with the aid of an external shutter, called a modulator. For a digital signal—that is, a stream of 0s and 1s—modulation can be achieved by simply turning the light source on and off in response to an electrical 1 or 0. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources may vary depending on a variety of factors, such as the type of fiber used, the data rate, and cost. Light sources that possess a number of key characteristics: Brightness: All other factors being equal, the brighter (high flux or radiant power) the light coming out of the source, the farther it can travel through the fiber before requiring amplification/regeneration, and the more cost efficient the transmission becomes. Given the high cost of light amplifiers and regeneration equipment, bright light sources are imperative in transmission systems. It must emit many photons within a narrow band of wavelengths. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: Highly focused: The core of the fiber that carries the light is extremely small: less than the diameter of your hair. If the light beam from the source diverges too quickly, most of the light will not enter the fiber core and will be wasted. Thus, the area over which the light source emits must be small compared to the area of the fiber core.. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: High modulation speeds: In directly modulated applications, where the light source turns on/off in response to the incoming electrical signal, the light source must be able to do so at speeds of millions/billions of times per second. In externally modulated applications, in which a high-speed shutter is placed in front of the light source, this property is not critical. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: Wavelength matching with fiber: In certain wavelengths, light suffers the least loss in a fiber medium; these are called transmission windows of the fiber. To maximize the distance that light can travel through fiber, the light source must emit at wavelengths within the transmission window of fiber. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: Wavelength matching with fiber: History on attenuation of Glass Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: Wavelength matching with fiber: History on attenuation of Glass Initially in early 1970s, the optical fiber had a low loss window around 800 nm. Also the semiconductor optical sources were made of GaAs which emitted light at 800 nm. Due to compatibility of the medium properties and the sources, the optical communication started in 800 nm band so called the ‘First window'. As the glass purification technology improved, the true silica loss profile emerged in 1980s. The loss profile shows two low loss windows, one around 1300 nm (0.4 dB/Km) and other around 1550 nm (0.2 dB/Km). In 1980s the optical communication shifted to 1300 nm band , so called the ‘Second Window’. This window is attractive as it can support the highest data rate due to lowest dispersion. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: Wavelength matching with fiber: History on attenuation of Glass In 1990s the communication was shifted to 1550 nm window, so called ‘Third Window’ due to invention of the Erbium Doped Fiber Amplifier (EDFA). The EDFA can amplify light only in a narrow band around 1550 nm. This band has higher dispersion, meaning lower bandwidth. However, this problem has been solved by use of so called ‘dispersion shifted fibers’. Both 1300 nm and 1550 nm band have approximately 100 nm bandwidth each. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: Reliable: In today’s communications systems, a single strand of fiber carries millions of telephone calls and other mission-critical data. A failure of the light source can terminate all these calls and halt the transmission of mission-critical data. Reliability of the light source is critical. In undersea networks, where a repair trip can be expensive and time consuming, any deployed device must pass the test of fault-free operations for at least 25 years. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FIBER OPTICS Light sources that possess a number of key characteristics: Small: Light sources need to be small. Efficient: The light source must be able to convert the electrical signal into light efficiently without generating too much heat. Two types of light sources fulfill all of these requirements: light-emitting diodes (LEDs) and laser diodes. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FREE SPACE OPTICS Free-space optics, as the name implies, means the transmission of optical signals through free space or air. Such propagation of optical capacity through air requires the use of light. Light sources can be either LEDs or lasers (light amplification by stimulated emission of radiation). FSO is a simple concept that is similar to optical transmission using fiber-optic cables. The only difference is the medium. Interestingly enough, light travels faster through air (approximately 300,000 km/s) than it does through glass (approximately 200,000 km/s), so free-space optical communications could be classified as optical communications at the speed of light. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FREE SPACE OPTICS- ENVIRONMENTAL CHALLENGES TO TRANSMISSION THROUGH THE AIR Whereas fiber-optic cable is a predictable medium, free space, as an open medium, is less predictable (atmospheric attenuation is one example). Because of this unpredictability, it is more difficult to control the transmission of optics through free space. This unpredictability affects the system availability and maximum design capacities. FSO is also a line-of-sight technology, which means that the points that interconnect have to be able to see each other without anything in between. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FREE SPACE OPTICS- ENVIRONMENTAL CHALLENGES TO TRANSMISSION THROUGH THE AIR The main issues creating potential compromise of a link include the following: Fog: The major challenger to free-space optical communications is fog. To further qualify, it is dense fog that affects FSO connectivity. Fog is water vapor in the form of water droplets that are only a few hundred microns in diameter. These droplets are able to modify light characteristics or completely hinder the passage of light through them through a combination of absorption, scattering, and reflection. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FREE SPACE OPTICS- ENVIRONMENTAL CHALLENGES TO TRANSMISSION THROUGH THE AIR The main issues creating potential compromise of a link include the following: Absorption: Absorption occurs when suspended water molecules in the terrestrial atmosphere extinguish photons, causing a decrease in the power density of the beam (attenuation) and directly affecting the availability of a system. Absorption occurs more readily at some wavelengths than others. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FREE SPACE OPTICS- ENVIRONMENTAL CHALLENGES TO TRANSMISSION THROUGH THE AIR The main issues creating potential compromise of a link include the following: Scattering: Unlike absorption, scattering results in no energy loss, only directional redistribution of energy (multipath effects) that can cause a significant reduction in beam intensity, particularly for longer link distances. Three main types of scattering exist: Rayleigh, Mie, and Raman scattering. Mie scattering, a scattering mechanism that becomes important when the particle size and wavelength are similar, is the main attenuation process to impact FSO system performance. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FREE SPACE OPTICS- ENVIRONMENTAL CHALLENGES TO TRANSMISSION THROUGH THE AIR The main issues creating potential compromise of a link include the following: Physical obstructions: Birds can temporarily block the beam, but this tends to cause only short interruptions, and transmissions are easily resumed. Multibeam systems address this issue. Building sway: The movement of buildings can upset receiver and transmitter alignment. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: FREE SPACE OPTICS- ENVIRONMENTAL CHALLENGES TO TRANSMISSION THROUGH THE AIR The main issues creating potential compromise of a link include the following: Scintillation: Heated air rising from the ground creates temperature variations among different air pockets. This can cause fluctuations in signal amplitude, which lead to “image dancing” at the receiver end. The most familiar effects of scintillation in the atmosphere are the twinkling of stars. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: OTHER POINTS OF COMPARISON Whereas it takes months—if not years—to enable fiber-optic communications, free-space optical communications can be implemented in a matter of weeks or even days at a fraction of the cost. As mentioned earlier, fiber deployments are a sunk infrastructure, which is lost when the customer leaves the building or decides to cancel the service. In contrast, FSO is a redeploy able platform, thereby proposing a zero sunk cost model. Furthermore, because of FSO’s flexibility and ease of deployment in multiple architectures, it offers an economic advantage over fiber optics. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

FIBER VERSUS FSO: OTHER POINTS OF COMPARISON Another important aspect to take note is the environmental benefits of free-space optics. Fiber requires digging of trenches, which may cause pollution, cutting of trees, and destruction of historical landmarks. FSO does not; therefore, it is friendly to the environment. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

THE ROLE OF FSO IN THE NETWORK Communications is the act of exchanging information among two or more individuals; a network is the physical infrastructure that enables this exchange to take place across space and time. The distance over which two people can communicate is constrained by the limits of the human senses—how far your voice can carry or how far your eye can see—audio or visual information needs to be converted into formats suitable for transmission over distances that defy these limits. The physical infrastructure that enables the transmission of voice, video, and data comprise the communications network. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

THE ROLE OF FSO IN THE NETWORK As mentioned previously, you can carry communications content between two points in space in multiple ways: RF Wireless transmission over the airwaves, electrical signal transmission over copper and coaxial cable, light transmission over fiber-optic cable, and now light over air using free-space optics. Optical networking involves the process of carrying communications content—voice, video, and data—over light signals, whether it be on fiber or air. Take the example of a telephone call. When you speak into the handset, your voice is converted by the telephone into an electrical signal. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

THE ROLE OF FSO IN THE NETWORK Optical networking involves taking this electrical signal as input and doing three things: converting it into a stream of light pulses, carrying the light signal over a fiber-optic cable or air to its destination, and reconverting the light pulse back into its electrical format. Dr. V. Sathiesh Kumar Department of Electronics, MIT, India

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