Opto Electronics

1. Opto Electronics Syed Abdul Rehman Rizvi The Islamia University of Bahawalpur

2. Opt0 Electronics INSTRUCTOR: Syed Abdul Rehman Rizvi E-mail: engr.rehman@hotmail.com OFFICE: New Building, U.C.E.T PHONE: 03336371316 OFFICE HOURS: As per scedule I encourage you to make an appointments if time table conflicts with your schedule. REFERENCE TEXT: As indicated in the slides LECTURES: As per time table LABS: As per time table 2

3. GRADING: Assignments|Project : 3 Due at the beginning of class on due date. Quizzes (9-10) : 10 Quizzes will be given at random dates. Classroom participation : 2 Give full attenetion to ur teacher. viva voce : 5 Comprehensive. QUIZZES: Quizzes will be given at random dates throughout the term. Most of them will be pop quizzes. Late homework will be penalized with 20% of the grade for each day it is late. HOMEWORK: VIVA: There will be no make-up viva. ACADEMIC DISHONESTY: Violations of academic dishonesty will be sanctioned. It involve the use of any method or technique enabling a student to misrepresent the quality and integrity of his or her own academic work or the work of a fellow student. Students committing academic dishonesty will be reported to the appropriate college official and an appropriate disciplinary action will be initiated against him/her. 3

4. Objective To provide an understanding of the structure, operating principles and underlying physical concepts of optical communication systems (particularly fiber links), having emphasis on fundamental aspects, but taking care of engineering issues as well.

5. Text Book/Reference Books • Fiber-Optic Communication Systems by Govind P. Agrawal‡ Optical Fiber Communication, principles and practices (2nd edition) by John M. Senior • Understanding optical communication by Harry Dutton. ‡ Optical communication (Willy series in telecommunication and signal processing) by Robert M.Gagliardi, Sherman Karp. ‡ Optical Communication Systems by John Gawar

6. The starting point ! ‡ For thousands of years we have used light to communicate. ‡ Even in these high-tech days of satellite communications, ships still carry powerful lamps for signaling at sea. ‡ It was a well known ‘fact’ that, as light travels in straight lines, it is impossible to make it follow a curved path. ‡ Boston, USA, 1870. An Irish physicist by the name of John Tyndall gave a public demonstration of an experiment which not only disproved this belief but gave birth to a revolution in communications technology.

7. Expected ! ‡ His idea was very simple. He filled a container with water and shown a light into it in dark room. ‡ It was expected that the light would shine straight out of the hole and the water would would Curve downward as shown in Figure.

8. what actually happened ! ‡ The light stayed inside the water column and followed the curved path. He had found a way to guide light! ‡ The basic requirements still remain the same today — a light source and a clear material (usually plastic or glass) for the light to shine through. ‡ The light can be guided around any complex path as in Figure .

9. Applications of light guiding Road signs-A single light source can be used to power many optic fibers. Endoscopes. Hazardous areas. All at sea. Flexible lighting.(marking escape routes for fire fighters, mountain and mine rescue, underwater routes for divers, helicopter landing zones, oil refineries, planes, ships, tunnels. The list is almost endless)

10. Back ground - Need for Optical Fiber ‡ The development of worldwide telephone networks during 20th Century necessitated the use of coaxial cables instead of pairs wires for increased capacity. ‡ A 3-MHz system capable of transmitting 300 voice channels was put in to use in 1940. ‡ Then arises the frequency-dependent cable losses, which increase rapidly for frequencies beyond 10 MHz. ‡ This limitation led to the development of microwave communication systems in which an electromagnetic carrier wave with frequencies in the range of 1-10 GHz is used to transmit the signal by using suitable modulation techniques.

11. Cont. ‡ The first microwave system operating at the carrier frequency of 4 GHz was put into service in 1948. Since then, both coaxial and microwave systems have evolved considerably and are able to operate at bit rates ~100 Mb/s. ‡ A severe drawback of such high-speed coaxial systems was their small repeater spacing (~ 1 km) -- expensive to operate. ‡ Microwave communication systems -allow larger repeater spacing. ‡ Figure of merit for communication systems is the bit rate-distance product, BL, where B is the bit rate and L is the repeater spacing

12. BL Development

13. Cont ‡ An increase of several orders of magnitude in the BL product would be possible if optical waves were used as the carrier -- But neither a coherent optical source nor a suitable transmission medium was available during the 1950s. ‡ The invention of the laser and its demonstration in 1960 solved the first problem. ‡ Attention was then focused on finding ways for using laser light for optical communications. ‡ It was suggested in 1966 that optical fibers might be the best choice, as they are capable of guiding the light in a manner similar to the guiding of electrons in copper wires

14. Cont.. ‡ The main problem was the high losses of optical fibers. ‡ fibers available during the 1960s had losses in excess of 1000 dB/kmm. ‡ A breakthrough occurred in 1970 when fiber losses could be reduced to below 20 dB/km in the wavelength region near 1 µm. ‡ The reduction of loss made it possible to use optical fibers for communication. Which Around 1975. The enormous progress was further reduced to 0.2 was realized ! db/km in 1979.

15. Cont.. At about the same time, GaAs semiconductor lasers, operating at room temperature, were demonstrated . ‡ The simultaneous availability of compact optical sources and a low-loss optical fibers led to a worldwide effort for developing fiber-optic communication systems. ‡ Figure shows the increase in the capacity of lightwave systems realized after 1980 through several generations of development. ‡ The commercial deployment of lightwave systems followed the research and development phase closely. The progress has indeed been rapid as evident from an increase in the bit rate by a factor of 100,000 over a period of less than 25 years. ‡ ‡ Transmission distances have also increased from 10 to 10,000 km over the same time period. As a result, the bit rate-distance product of modern lightwave systems can exceed by a factor of 107 compared with the first-generation lightwave systems.

16. Optical Comm Systems

17. Optical Communication Systems ‡ Optical communication systems differ in principle from microwave systems only in the frequency range of the carrier wave used to carry the information i.e. 200 THz & 1 GHz respectively. ‡ An increase in the information capacity is expected i.e. 1o,ooo times. ‡ Optical communication system consists of a transmitter, a commmmunication channel and a receiver. ‡ Optical communication systems can be classified as guided and unguided. ‡ In the guided lightwave systems the optical beam emitted by the transmitter remains confined, using optical fibers. ‡ In the unguided optical communication systems the optical beam emitted by the transmitter spreads in space, similar to spreading of microwaves. ‡ Unguided optical systems are less suitable for broadcasting applications than microwave systems because optical beams spreads mainly in the forward direction because of their short wavelength.

18. Fiber-optic communication • This is a method of transmitting information from one place to another by sending light through an optical fiber. • The light forms an electromagneticcarrier wave that is modulated to carry information.

19. Fiber-optic communication The process of communicating using fiber-optics involves the following basic steps: • Creating the optical signal using a transmitter, • relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, • and receiving the optical signal and converting it into an electrical signal.

20. Evolution of Fiber • 1880 – Alexander Graham Bell • 1930 – Patents on tubing • 1950 – Patent for two-layer glass wave-guide • 1960 – Laser first used as light source • 1965 – High loss of light discovered • 1970s – Refining of manufacturing process • 1980s – OF technology becomes backbone of long distance telephone networks in NA.

21. OPTICAL FIBER • An optical fiber (or fibre) is a glass or plastic fiber that carries light along its length. • Light is kept in the "core" of the optical fiber by total internal reflection.

22. Advantages of Optical Fibre • Thinner • Less Expensive • Higher Carrying Capacity • Less Signal Degradation • Light Signals • Non-Flammable • Light Weight

23. Advantages of fiber optics • Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber • Immunity to Noise - Immune to electromagnetic interference (EMI). • Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline. • High Security - Impossible to “tap into.”

24. Advantages of fiber optics • Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation) • Reliability - More resilient than copper in extreme environmental conditions. • Size - Lighter and more compact than copper. • Flexibility - Unlike impure, brittle glass, fiber is physically very flexible.

25. Areas of Application • Telecommunications • Computer network • LA N,WAN • Cable TV • CCTV • Optical Fiber Sensors • Nuclear plant instrument • Industrial process control system

26. Fiber Optic Cable

27. Core – thin glass center of the fiber where light travels. Cladding – outer optical material surrounding the core Buffer Coating– plastic coating that protects the fiber. OPTICAL FIBER CONSTRUCTION

28. OPTICAL FIBER • The core, and the lower-refractive-indexcladding, are typically made of high-quality silica glass, though they can both be made of plastic as well.

29. plastic jacket glass or plastic cladding fiber core Fiber Optic Layers • consists of three concentric sections

30. Fiber Optic Cable

31. App. Of Fiber Optic Cable • Relatively new transmission medium used by telephone companies in place of long-distance trunk lines • Also used by private companies in implementing local data networks • It require a light source with injection laser diode (ILD) or light-emitting diodes (LED)

32. Five Generations of Light wave Systems ‡ First generation „ Operating near 800 nm and used GaAs semiconducor laser, commercially available in 1980 „ Operated at bit rate of 45 Mbps and repeater spacing of about 10 km (larger compared that of coaxial cable) „ Dec the instl and maintenance cost ‡ Second generation „ Operating near 1300 nm where fiber loss is 1 db/km (typically 0.5 db/km) and fiber exhibit minimum dispersion. „ Uses InGaAsP semiconductor lasers and detectors. (newly developed) „ Available in early 80s „ By 1987 commercially available systems were operating at bit rates of up to 1.7 Gbps and repeater spacing of about 50 km(SMF).

33. Cont.. ‡ Third generation „ Fiber has minimum loss at 1550 nm (realized in 1979 but dispersion was considerably large) „ Displayed more dispersion arround 1550nm „ Dispersion shifted fibers could overcome the dispersion problem , designed to have minimum dispersion around 1550 nm. „ In 1990 commercially available systems were operating at 2.5 Gbps and capable of operating at 10 Gbps. (DSF with single- longitudinal-mode lasers) „ Typical repeaters spacing is around 60-70 km

34. Cont.. ‡ Fourth generation „ A drawback of third generation 1.55µmis that the signal is regenerated periodically by using electronic repeater. „ The fourth generation makes use of optical amplifiers(1989) for increasing the repeater spacing and WDM for increasing the bit rate. „ It employs erbium-doped fiber amplifiers(1990), 60 - 100 km apart. „ Several WDM systems were deployed across the Atlantic and Pacific oceans during 1998-2001 in response to the Internet- induced increase in the data traffic; they have increased the total capacity by orders of magnitudes. ‡ Fifth generation „ Concerned with finding the fiber dispersion problems „ Optical amplifiers have solved the loss problem but made the dispersion problem worse „ Solution is based on the concept of optical solitons - optical pulses that preserve their shape during propagation by counteracting the effect of dispersion through the fiber nonlinearity.

35. DWDM System

36. Refraction ‡ Imagine shining a flashlight. The light waves spread out along its beam. ‡ As we move further from the light source, the wavefront gets straighter and straighter. ‡ At a long distance from the light source, the wavefront would be virtually straight. ‡ In a short interval of time each end of the wavefront would move forward a set distance. ‡ If we look at a single ray of light moving through a clear material the distance advanced by the wavefront would be quite regular.There is a widely held view that light always travels at the same speed. ‡ This ‘fact’ is simply not true. The speed of light depends upon the material through which it is moving. In free space light travels at its maximum possible speed, close to 300 million meters or nearly eight times round the world in a second

37. Refractive index !! ™ When it passes through a clear material, it slows down by an amount dependent upon a property of the material called its Refractive index. ™ For most materials that we use in optic fibers, the refractive index is in the region of 1.5. Refractive Index = Speed of light in free space/Speed of light in material Lower refractive index = higher speed

38. If a ray of light enters from a material of high refractive index to another material with a lower index, in which it would move faster. W e can see that the distances between the successive wave crests, or the wavelength, will increase as soon as the light moves into the second material. ‡ The direction that the light approaches the boundary between the two materials is very significant.

39. Snell’s law Willebrord Snell, a Dutch astronomer, discovered that there was a relationship between the refractive indices of the materials and the sine of the angles. He made this discovery in the year 1621. Snell’s law states the relationship as: n1sin φ1 = n2sin φ2 Where: n1 and n2 are the refractive indices of the two materials, and sin φ1 and sin φ2 are the angles of incidence and refraction respectively. Snell's law will apply to the refraction of light in any situation, regardless of what the two media are.

40. Example Calculate the angle shown as φ2 ,The first material has a refractive index of 1.51 and the angle of incidence is 38° and the second material has a refractive index of 1.46. Starting with Snell’s law: n1sinφ1 = n2sinφ2

42. Critical angle - Light Guiding As the angle of incidence in the first material is increased, there will come a time when, eventually, the angle of refraction reaches 90° and the light is refracted along the boundary between the two materials. The angle of incidence which results in this effect is called the critical angle.We can calculate the value of the critical angle by assuming the angle of refraction to be 90° and transposing Snell’s law: n1sin φ1 = n2sin90° As the value of sin90° is 1, we can now transpose to find sin φ1, and hence φ1, (which is now the critical angle): ⎛n 2 ⎞ φ = arcSin ⎜ ⎟ Critical ⎝n 1 ⎠

43. A worked example

44. Total internal reflection The critical angle is well-named as its value is indeed critical to the operation of optic fibers. ‡ At angles of incidence less than the critical angle, the ray is refracted. ‡ However, if the light approaches the boundary at an angle greater than the critical angle, the light is actually reflected from the boundary region back into the first material. The boundary region simply acts as a mirror. This effect is called total internal reflection (TIR). ‡ The effect holds the solution to the puzzle of trapping the light in the fiber. If the fiber has parallel sides, and is surrounded by a material with a lower refractive index, the light will be reflected along it at a constant angle -shown as ø in the Figure . ‡ ‡ Any ray launched at an angle greater than the critical angle will be propagated along the optic fiber

46. Electromagnetic spectrum

48. Numerical aperture ‡ The numerical aperture of a fiber is a figure which represents its light gathering capability. ‡ The acceptance angle also determines how much light is able to enter the fiber and so we must expect an easy relationship between the nummerical aperture and the cone of acceptance as they are both essentially measurements of the same thing. ‡ The formula for the numerical aperture is based on the refractive indices of the core and the cladding. 2 2 NA = n −n core cladding Aceptance angle=sin-1 NA

49. Example Let’s try the short cut and see how it works out using values of ncore = 1.5, and n cladding = 1.48 What will happen if incident angle is more than cone of acceptance ? H1

50. Geometrical- Optics description In its simplest form an optical fiber consists of a cylindrical core of silica glass surrounded by a cladding whose refractive index is lower than that of the core. Because of an abrupt index change at the core-cladding interface, such fibers are called step-index fibers. In a different type of fiber, known as graded-index fiber, the refractive index decreases gradually inside the core.