Modern Trends in Telecom & Information Superhighway

1. Modern Trends in Telecom & Information Superhighway Institute of Information & Communication Technologies (IICT), Mehran UET, Jamshoro

2. Objectives • Understand optical fiber propagation characteristics and transmission properties. • An understanding of the theory of optical sources including light-emitting diodes and laser diodes, and the methods for using these devices in optical fiber communication systems • Design such fiber optic links and relate the limitations in the performance to the limitations of the components and subsystems used; • Understand the modeling of photo detectors, including shot noise and avalanche noise. • Understand optical amplifiers and in particular their noise characteristics. • Understanding the principles and methods for constructing optical fiber communication systems, including techniques to increase the data rate and decrease transmission impairments.

3. Background of Optical Communications Age of Smoke Signals and semaphores!

4. Why Optical Communication? • Optical Fiber is the backbone of modern communication networks • Voice (SONET/Telephony) - The largest traffic • Video (TV) over Hybrid Fiber Coaxial (HFC) • Fiber Twisted Pair for Digital Subscriber Loops (DSL) • Multimedia (Voice, Data and Video) over DSL or HFC Information revolution wouldn’t have happened without the Optical Fiber

5. Information revolution

6. Why Optical Communication? • Physical limits for the bandwidth • Wire ~ 1 MHz = 106 Hz • Coaxial Cable ~ 10 GHz = 1010 Hz • Microwave (Wireless) ~ 100 GHz = 1011 Hz • Optical Fiber ~ 100 THz = 1014 Hz • Free space Optics ~ 1000 THz = 1015 Hz

7. Why Optical Communication?

8. Why Optical Communication? • Lowest attenuation  attenuationin the optical fiber (at 1.3 µm and 1.55 µm bands) is much smaller than electrical attenuation in any cable at useful modulation frequencies • Much greater distances are possible without repeaters • This attenuation is independent of bit rate • Highest Bandwidth (broadband)  high-speed • Single Mode Fiber (SMF) offers the lowest dispersion  highest bandwidth  rich content • Upgradability: Optical communication system can be upgraded to higher bandwidth, more wavelengths by replacing only the transmitters and receivers • Low Cost for fiber

9. Light in History • Light in Greek Times • In Greek times many believed that light came from visible objects toward the eye. However, Plato and many other Greeks believed that vision issued out from the eye. • Empedocles correctly believed that light traveled with finite speed. • Aristotle explained rainbows as a sort of reflection off of raindrops. • The mathematician Euclid worked with mirrors and reflection but did not know how to express it mathematically. • Ptolemy is the first recorded person to experiment with optics and collect data, but he believed in Plato's mistaken thought. • Light in Arabian Times • Ptolemy's work was further developed by the Egyptian scientist Ibn al Haythen, who was known to Europeans as Alhazen. • Alhazen who first drew ray diagrams • The Arabian mathematician Alhazen studied the refraction of light and disputed the ancient theory that visual rays emanated from the eye. He believed that the angles of incidence and refraction are related, but was unable to determine how they are related. • This relationship, now known as Snell's law, was established six hundred years later.

10. Light in Modern Times • Many advances in the study of light based on Alhazenwork were made in the 16th and 17th centuries by such renowned scientists as Galileo Galilei, Johannes Kepler, and Renes Descartes. • The Snell's law was discovered. • During the late 17th century, a debate grew over whether light behaved as a particle or a wave. Sir Isaac Newton and Laplace. • However, there were some who believed in a wave theory of light. The most notable among these was the Dutch scientist Christiaan Huygens who first wrote of light as a wave. • It was not until the early 19th century that the wave theory of light became widely accepted. This acceptance came in large part due to the work of the English doctor Thomas Young. (1801, “double-slit experiment”) • In the 1850s Fizeau and Foucault showed through measurements that light traveled slower through denser media. • In the same century, Augustin Fresnel and later James Clerk Maxwell, working on a wave theory of light explained phenomena such as polarization, interference, and diffraction. They also determined which part of the light will be reflected and which transmitted when light is reflected at a surface such as glass or water. • Maxwell's work seemed to have finally settled the issue of whether light was a wave or a particle, but the whole debate was reopened in the 20th century. • Scientists such as Albert Einstein, who described the Doppler effect for light, brought the particle theory back into the picture with quantum theory. • This time they postulated that light did not just behave as a particle or a wave, but had properties of both.

11. Properties of Light Definition of light: That agent, force, or action in nature by the operation of which objects are rendered visible or luminous. Or light is an electromagnetic radiation that can produce a visual sensation. Properties of Light • Propagation Matter is not required for the propagation of light. • Reflection occurs at the surface, or boundary, of a regular medium. • Refraction, or bending, may occur where a change of speed is experienced. • Interference is found where two waves are superposed. • Diffraction, or bending around corners, takes place when waves pass the edges of obstructions. • Scattering occurs at the surface, or boundary, of irregular medium. • Absorption change of light into heat energy. Dual nature of light • Particle (or Corpuscular) Theory: Sir Isaac Newton believed that light consists of streams of tiny particles, which he called "corpuscles," emanating from a luminous source. • Wave theory: Christian Huygens - The wave theory treats light as a train of waves having wave fronts perpendicular to the paths of the light rays. Wave effects are insignificant in an incoherent, large scale optical system because the light waves are randomly distributed and there are plenty of photons.

12. The Electromagnetic Theory • The James Clerk Maxwell in 1865 predicted that heat, light, and electricity are propagated in free space at the speed of light as electromagnetic disturbances.

13. The Electromagnetic Spectrum • EM-Spectrum extend from 10 Hz to 1025 Hz. • All EM radiations travel in free space with constant velocity of 3 x 108 m/s. • Optical radiation lies between radio waves and x-rays on the spectrum.

15. Visible Spectrum/Visible Light • At x-ray and shorter wavelengths, electromagnetic radiation tends to be quite particle like in its behavior, whereas toward the long wavelength end of the spectrum the behavior is mostly wavelike. The visible portion occupies an intermediate position, exhibiting both wave and particle properties in varying degrees. • Their wave lengths range from approximately 7600 A to 4000 A. • The optical spectrum also extends into the near infrared and into the near ultraviolet. Although our eyes cannot see these radiations, they can be detected by means of photographic film.

16. Ultraviolet (UV) light Short wavelength UV light exhibits more quantum properties than its visible and infrared counterparts. UV light is arbitrarily broken down into three bands: • UV-A (or black light)is the least harmful and most commonly found type of UV light, because it has the least energy. It is used for its relative harmlessness and its ability to cause fluorescent materials to emit visible light - thus appearing to glow in the dark. Most phototherapy and tanning booths use UV-A lamps. • UV-B is typically the most destructive form of UV light, because it has enough energy to damage biological tissues, yet not quite enough to be completely absorbed by the atmosphere. UV-B is known to cause skin cancer. Since most of the extraterrestrial UV-B light is blocked by the atmosphere, a small change in the ozone layer could dramatically increase the danger of skin cancer. • Short wavelength UV-C is almost completely absorbed in air within a few hundred meters. When UV-C photons collide with oxygen atoms, the energy exchange causes the formation of ozone. UV-C is almost never observed in nature, since it is absorbed so quickly. Germicidal UV-C lamps are often used to purify air and water, because of their ability to kill bacteria. Common UV band designations

17. Infrared (IR) Light • Infrared light contains the least amount of energy per photon of any other band and therefore, an IR photon often lacks the energy required to pass the detection threshold of a quantum detector. IR is usually measured using a thermal detector such as a thermopile, which measures temperature change due to absorbed energy. • Since heat is a form of infrared light, far infrared detectors are sensitive to environmental changes - such as a person moving in the field of view. Night vision equipment takes advantage of this effect, amplifying infrared to distinguish people and machinery that are concealed in the darkness. • Infrared is unique in that it exhibits primarily wave properties. This can make it much more difficult to manipulate than UV and visible light. IR is more difficult to focus with lenses, refracts less, diffracts more, and is difficult to diffuse.

18. The most important ideas The most important ideas to note are: • Light travel slower through denser media. • Propagation Matter is not required for the propagation of light. • Reflection occurs at the surface, or boundary, of a regular medium. • Refraction, or bending, may occur where a change of speed is experienced. • Interference is found where two waves are superposed. • Diffraction, or bending around corners, takes place when waves pass the edges of obstructions. • Scattering occurs at the surface, or boundary, of irregular medium. • Absorption change of light into heat energy. • Electromagnetic waves span over many orders of magnitude in wavelength (or frequency). • The frequency of the electromagnetic radiation is inversely proportional to the wavelength. • The visible spectrum is a very small part of the electromagnetic spectrum. • Photon energy increases as the wavelength decreases. The shorter the wavelength, the more energetic are its photons.

19. Wavelength Standards • Frequency multiples of 50 GHz or 100 GHz • λ = c/fc = 299792458 m/s • λ [nm] = 299792.458/f [THz] • f = 195 THz → λ = 1537.397 nm • f =195.1 THz → λ = 1536.609 nm • f =195.2 THz → λ = 1535.822 nm • f =193.4 THz → λ = 1550.116 nm f 50 or 100 GHz ITU grid

20. Wavelength Ranges available for Communication Different types of sources/detectors/amplifiers are used in different bands

21. Example of a Problem How many 100 GHz-ITU Grid channels are covered by the conventional band (1530 – 1560 nm)? λ = 1530 nm → f = 195.943 THz λ = 1560 nm → f = 192.174 THz 192.2 192.3 192.4 . . . 195.7 195.8 195.9

22. Fiber Optic Communication System • Generic System • Transmitter and receiver module • Fiber-optic communication channel

23. OFC System • An optical fiber communication (OFC) system is similar in basic concept to any type of communication system, the function of which is to convey the signal from the information source over the transmission medium, to the destination. • For OFC, the information source (usually, an LED or laser) provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of the light wave carrier. • The transmission medium consists of an optical fiber cable and the receiver consists of an optical detector which drives a further electrical stage and hence provides demodulation of the optical carrier. • Photodiodes (p-n, p-i-n, or avalanche) and, in some instances, phototransistors and photoconductors are used as detectors.

24. Networks • Local area networks (LAN) L ≤ 1 km • Metropolitan area networks (MAN) L ≤ 10 km • Wide area networks (WAN) L ≥ 100 km • LAN: provides communication access to users. Bit-rate requirement is relatively low. Main issue: scalable and reconfigurable architecture. • MAN: provides communication access within a city. Moderate Bit-rate requirements. Fixed architecture acceptable in many access • WAN: provides communication over long distances. High-bit rate is required. Use of multiplexing techniques. Compensation for optical losses and dispersion is major problem.

25. First Generation Fiber Optic Systems Purpose: • Eliminate repeaters in T-1 systems used in inter-office trunk lines Technology: • 0.8 µm GaAs semiconductor lasers • Multimode silica fibers • All the switching and processing is handled by electronics Limitations: • Fiber attenuation • Intermodal dispersion Deployed since 1974 • Examples • SONET (synchronous optical network), USA • SDH (synchronous digital hierarchy), international • FDDI (Fiber distributed data network)

26. Second Generation Fiber Optic Systems Opportunity: • Development of low-attenuation fiber (removal of H2O and other impurities) • Eliminate repeaters in long-distance lines Technology: • Use optics for switching and routing • 1.3 µm multi-mode semiconductor lasers • Single-mode, low-attenuation silica fibers • DS-3 signal: 28 multiplexed DS-1 signals carried at 44.736 Mbps Limitation: • Fiber attenuation (repeater spacing ≈ 6 km) Deployed since 1978

27. Third Generation Fiber Optic Systems Opportunity: • Deregulation of long-distance market Technology: • 1.55 µm single-mode semiconductor lasers • Single-mode, low-attenuation silica fibers • OC-48 signal: 810 multiplexed 64-kb/s voice channels carried at 2.488 Gbps Limitations: • Fiber attenuation (repeater spacing ≈ 40 km) • Fiber dispersion Deployed since 1982

28. Fourth Generation Fiber Optic Systems Opportunity: • Development of erbium-doped fiber amplifiers (EDFA) Technology (deployment began in 1994): • 1.55 µm single-mode, narrow-band semiconductor lasers • Single-mode, low-attenuation, dispersion-shifted silica fibers • Wavelength-division multiplexing of 2.5 Gb/s or 10 Gb/s signals Nonlinear effects limit the following system parameters: • Signal launch power • Propagation distance without regeneration/re-clocking • WDM channel separation • Maximum number of WDM channels per fiber Polarization-mode dispersion limits the following parameters: • Propagation distance without regeneration/re-clocking

29. Evolution of Optical Networks Regenerators/repeaters 0.85 LED or LD Receiver Multimode fibers 1.3 μm FP Laser Receiver Single mode fibers 1.55 μm DFB Laser Receiver Single mode fibers T1 T1 T2 DEMUX T2 MUX Optical amplifiers T3 T3

30. History of Attenuation

31. Three windows based on Wavelengths

32. Multiplexing Technologies • Time division multiplexing Low-speed data streams High-speed data streams Bit rate 10 to 40 Gbps

33. Wavelength Division Multiplexing λ1 λ2 Essentially the same as frequency division multiplexing λ3 MUX λ1 λ2 λ3 ... λN . . λN WDM: 32 wavelengths, 2.5 Gbps each → 80 Gbps commercially available now. Combination of WDM and TDM demonstrated to provide 2 Tbps over single mode fiber

34. What is Fiber Optics • Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. • They are arranged in bundles called optical cables. • If you look closely at a single optical fiber, you will see that it has the following parts: • Core (n1) - is a cylindrical rod of dielectric material. Light propagates mainly along the core of the fiber. • Cladding (n2)- Outer optical material surrounding the core that reflects the light back into the core. The index of refraction of the cladding material is less than that of the core material (n2 < n1). The cladding performs the following functions: • Reduces loss of light from the core into the surrounding air • Reduces scattering loss at the surface of the core • Protects the fiber from absorbing surface contaminants • Adds mechanical strength • Buffer coating -. For extra protection, the cladding is enclosed in an additional layer called the coating or buffer. It is a layer of material used to protect an optical fiber from physical damage and moisture. Refractive Index (n): The ratio of velocity of light in air ‘c’ to the ratio of velocity of light in any medium ‘v’.

35. Types of Optical Fiber • Single mode • Single mode step index • Multi mode • Multimode step index • Multimode graded index • Dispersion Shifted/Non-dispersion shifted • Silica/fluoride/Other materials Mode: A set of electromagnetic wave

36. Types of Fibers nc step-indexmultimode n1 nc nc step-indexsinglemode nc nc GRIN nc

37. Single Mode Step Index • A small-core optical fiber through which only one mode will propagate. • The typical diameter is about 3.5 x 10-4 inches or 9 microns. • Step-index Fiber: Fiber that has a uniform index of refraction throughout the core that is a step below the index of refraction in the cladding. 

38. Multi-mode Fiber • Multimode (MM) Fiber: An optical fiber that has a core large enough to propagate more than one mode of light. The typical diameter is about 2.5 x 10-3 inches or 62.5 microns. • Multimode Step-Index Fiber: Fiber that has a uniform index of refraction throughout the core that is a step below the index of refraction in the cladding and allows more than one mode of light.  • Multimode Graded-Index Fiber: A multimode graded-index fiber has a core of radius (a). Unlike step-index fibers, the value of the refractive index of the core (n1) varies according to the radial distance (r). The value of n1 decreases as the distance (r) from the center of the fiber increases.

39. Multimode Graded Index Fiber Step-index Multi mode fiber Graded-index Multi mode fiber Step-index Single mode fiber

40. Typical dimensions

41. Transmission of Light through Optical Fibers • The transmission of light along optical fibers depends not only on the nature of light, but also on the structure of the optical fiber. • Two theories are used to describe how light is transmitted along the optical fiber. • Ray theory, uses the concepts of light reflection and refraction and treat light as a simple ray. The advantage of the ray approach is that you get a clearer picture of the propagation of light along a fiber. The ray theory is used to approximate the light acceptance and guiding properties of optical fibers. • Mode theory, treats light as electromagnetic waves. The mode theory describes the behavior of light within an optical fiber. The mode theory is useful in describing the optical fiber properties of absorption, attenuation, and dispersion.

42. Ray theory • Two types of rays can propagate along an optical fiber, meridional rays and skew rays. • Meridional rayspass through the axis of the optical fiber. • Meridional rays are used to illustrate the basic transmission properties of optical fibers. • Skew raysare rays that travel through an optical fiber without passing through its axis.

43. Refractive index • The refractive index is the ratio of the speed of light in a vacuum, c, to the speed of light in the medium, v. • Since the speed of light in a medium is always less than it is in a vacuum, the refractive index is always greater than one. In air, the value is very close to 1. • The refractive index varies with the wavelength of light. • In a homogeneous medium, that is, one in which the refractive index is constant, light travels in a straight line. Only when the light meets a variation or a discontinuity in the refractive index will the light rays be bent from their initial direction. • The light travelling into an optically denser medium (with higher refractive index) would be bent toward the normal, while light entering an optically rarer medium would be bent away from the normal.

44. Light in optical fibers • If one were to use a fiber consisting of only a single strand of glass or plastic, light could be lost at any point where the fiber touched a surface for support. • To eliminate this possibility, the central light carrying portion of the fiber, called the core, is surrounded by a cylindrical region, called the cladding. • Since the refractive index difference between the core and the cladding is less than in the case of a core and air, the critical angle is much bigger for the clad fiber. The index of the cladding n2, is still less than the index of the core n1, because total internal reflection will occur only when n1 > n2.

45. Total Internal Reflection • The light in a fiber-optic cable travels through the core (hallway) by constantly bouncing from the cladding (mirror-lined walls), a principle called Total Internal Reflection. • When light is incident upon a medium of lesser index of refraction, the ray is bent away from the normal, so the exit angle is greater than the incident angle. Such reflection is commonly called "internal reflection". The exit angle will then approach 90° for some critical incident angle θc , and for incident angles greater than the critical angle there will be total internal reflection. What is Frustrated TIR? And where it is useful?

46. Ray theory Snell’s Law • The angle of incidence θ1 and refraction θ2 are related to each other by Snell’s law, which states: Critical Angle • The critical angle can be calculated from Snell's law by setting the refraction angle equal to 90°.

47. Total internal reflection

48. Transmission of light through perfect optical fiber The transmission of a light ray in a perfect optical fiber

49. Phase change due to TIR • In addition, when light is totally internally reflected, a phase change δ occurs in the reflected wave. This phase change depends on the angle θ1 ˂ θc according to the relationships here δN and δp are the phase shits of the wave components normal and parallel to the place of incidence, respectively, and n = n1/n2.

50. Relative refractive index • It defines the difference in the core and cladding refractive indices.