Opto Electronics Syed Abdul Rehman Rizvi The Islamia University of Bahawalpur Opt0 Electronics INSTRUCTOR: Syed Abdul Rehman Rizvi E-mail: [email protected] 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 GRADING: QUIZZES: HOMEWORK: VIVA: 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 Comprehensive. : 5 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. 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 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. 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 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. 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. 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 . 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) 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. 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 BL Development 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 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 was further reduced to 0.2 Around 1975. The enormous progress was realized ! db/km in 1979. 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. Optical Comm Systems 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. 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 electromagnetic carrier wave that is modulated to carry information. 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. 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. 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. Advantages of Optical Fibre Thinner Less Expensive Higher Carrying Capacity Less Signal Degradation Light Signals Non-Flammable Light Weight 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.” 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. Areas of Application Telecommunications Computer network LA N,WAN Cable TV CCTV Optical Fiber Sensors Nuclear plant instrument Industrial process control system Fiber Optic Cable OPTICAL FIBER CONSTRUCTION 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 • The core, and the lower-refractive-index cladding, are typically made of high-quality silica glass, though they can both be made of plastic as well. Fiber Optic Layers • consists of three concentric sections plastic jacket glass or plastic cladding fiber core 29 Fiber Optic Cable 30 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 lightemitting diodes (LED) 31 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). 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 singlelongitudinal-mode lasers) Typical repeaters around 60-70 km spacing is 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 Internetinduced 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. DWDM System 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 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 If a ray of light enters from a material 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. of high 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. 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 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): φ Critical = arcSin ⎜ ⎛n 2 ⎞ ⎝n1 ⎠ ⎟ A worked example 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 Electromagnetic spectrum 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. NA = n 2 core −n 2 cladding Aceptance angle=sin-1 NA 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 coneH1of acceptance ? 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. NA - Step index fiber Numerical Aperture is a measure of the light gathering power of the fiber. The acceptance angle for an optical fiber is maximum angle to the axis at which light may enter the fiber in order to be propagated. It gives a relationship between the acceptance angle and the refractive indices of the three media involved, namely the core, cladding and air. The ray enters the fiber from a medium (air) of refractive index n0 , and the fiber core has a refractive index n1 , which is slightly greater than the cladding refractive index n2. using Snell’s law no sinθi = n1 sinθr Details are in class lecture Example. A silica optical fiber with a core diameter large enough to be considered by ray theory analysis has a core refractive index of 1.50 and a cladding refractive index of 1.47. Determine: (a) The critical angle at the core-cladding interface. (b) The NA for the fiber. (c) The acceptance angle in air for the fiber. Solution: (a)The critical angle φc at the core- cladding interface is given by Eq. sinφc = n2 / n1 φc = sin-1n2 / n1 = sin-1 1.47/1.50 = 78.50 (b); From Eq. The numerical aperture is NA = (n12 - n22) ½ = (1.502 - 1.472) ½ =(2.25 - 2.16) ½ =0.30 (c): Considering Eq the acceptance angle in the air θa is given by: θa=sin-1 NA = sin-1 0.30 =17.40 Intermodel dispersion (Multimode dispersion) The extent of pulse broadening can be estimated by considering the longest and shortest ray paths. The shortest path occurs for θ i = 0, and X is just equal to the fiber lenght 'L'.The longest path occurs for shown previously and has a lenght 'L/sin Φc . v = c / n1, Φc θi the time delay is given by ; ∆T =T Max −TMin n1 −L s x−L n2 Ln n −n = = 1 1 2n1 = = v v c cn2 n1 n1 L Ln 21 ∆ = cn 2 When ∆ <<1 L X= L/SinΦc SinΦc= n2/n1 X = L n1/n2 The tim e delay between the two rays taking the shortest and longest paths is a measure of broadening experienced by an impulse launched at the fiber input. We can relate ∆T to the information-carrying capacity of the fiber measured through the bit rate B., Requirement for minimal inter symbol interference: B ∆T < 1 where B = bit rate Names given to different rays The position and the angle at which the ray strikes the core will determine the exact path taken by the ray. There are three possibilities, called the skew, meridional and the axial ray as shown in Figure . If light enters a fiber from a practical light source, all three rays tend to occur as well as those outside of the cone of acceptance . The skew ray never passes through the center of the core. Instead it reflects off the core/cladding interface and bounces around the outside of the core. It moves forward in a shape reminiscent of a spiral staircase built from straight sections. The meridional ray enters the core and passes through its center. Thereafter, assuming the surfaces of the core are parallel, it will always be reflected to pass through the center. The axial ray is a particular ray that just happens to travel straight through the center of the core. Graded index fibers The refractive index - is not constant. Decreases gradually from its maximum value n1 at the core center to its minimum value n2 at the core-cladding interface. Most graded-index fibers are designed to have a nearly quadratic decrease and are analyzed by using α-profile, given by • where ‘a’ is the core radius. ‘ρ’ is the radial distance. • The parameter α determines the index profile. • A step-index - large α. A parabolic-index fiber corresponds to α= 2. Cont… Intermodal or multipath dispersion is reduced for graded-index fibers. Figure shows schematically paths for three different rays. The path is longer for more oblique rays. However, the ray velocity changes along the path because of variations in the refractive index. The ray propagating along the fiber axis takes the shortest path but travels most slowly as the index is largest along this path. Oblique rays have a large part of their path in a medium of lower refractive index. Suitable choice of the refractive-index profile leads to non-dispersive pulse propagation. The trajectory of a ray is obtained by where ρ is the radial distance of the ray from the axis. For ρ< a with α = 2, Eq. above reduces to an equation of harm onic oscillator and has the general solution; where p = (2∆/a2)1/2 and ρ0 and ρ’0 are the position and the direction of the input ray, respectively. All rays recover their initial positions and directions at distances z = 2mπ/p, where m is an integer. Such a complete restoration of the input implies that a parabolicindex fiber does not exhibit intermodal dispersion. The quantity ∆T/L, where ∆T is the maximum multipath delay in a fiber of length L, is found to vary considerably with α. Figure shows this variation for n1 = 1.5 and ∆ = 0.01. The minimum dispersion occurs for α= 2(1−∆) and depends on ∆ as The limiting bit rate-distance product is obtained by using the criterion ∆T < 1/B and is given by • The BL product of such fibers is improved by nearly three orders of magnitude over that of step-index fibers. • Indeed, the first generation of lightwave systems used graded-index fibers. Further improvement is possible only by using single-mode fibers.. • Graded-index fibers are rarely used for long-haul links. They have relatively large core, resulting in a high numerical aperture and high coupling efficiency - but exhibit high losses . • They can be used to transmit data at bit rates >1 Gb/s over short distances of 1 km or less (LAN). - Profile α The figures below expressing the range of refractive index profile of the fiber core as a variation of α. Allows representation of the step index fiber when α = ∞, a parabolic profile when α =2 and a triangular profile when α =1.