View with images and charts â€œSTUDY OF Optical Fiber Communication Systemsâ€? 1.1Objective: In Bangladesh application of Optical fiber as communication links has already started by the Bangladesh Railway from Dhaka to Chittagong however due to lack of Technical experts and demand it was not properly utilized, When the era of mobile telephone came to Bangladesh Grameenphone started to use the Optical fiber Link taking lease from. Bangladesh Railway after that E-mail Internet Video services companies started using Optical fiber. It is to be mentioned here that Grameenphone could use only the Grameenphone is using 2 cores out of 48 core fiber. Grameenphone has started its own optical fiber Communication systems laying cables along the Railway tracks of Bangladesh. Very recently Grameen phone has laid down Optical fiber from Dhaka to Kushtia along the Railway lines; we were installed directly with these projects of laying Optical fiber cable. Our thesis covers all the methods and procedures of laying the Optical fiber cable against the project. 1.2 Historical Perspective Communication engineers have airways dreamt of higher information Bandwidth. The use of visible optical carrier waves or light for communication has been common for many years. Prior to the nineteenth century all communication systems were of a very low data rate type and basically involved only optical or acoustical means, such as signal lamps or horns. One of the earliest known optical links was the use or a fire signal by Greeks in the eighth century B.C for sending alarms, call for help, or announcement of certain events. In the fourth century B.C the transmission distance was extended through the use of relay stations, and by approximately 150 B.C these optical signals were encoded in relation to the alphabet so that any message could then be sent. Improvements of these systems were not actively pursued because of technology. In 1880 Alexander Graham Bell reported the transmission of speech using a light beam. The photo-phone proposed by Bell four years after the invention of the telephone modulated sunlight with a diaphragm giving speech transmission over a distance of 200 meters. However, although some investigation of optical communication continued in the early part of the twentieth century but its use was limited to mobile, low capacity communication links. This was mainly due to two reasons-the lack of suitable light sources and the light transmission suffers from severe atmospheric turbulence. T.H. Maimon working in USA stimulated a renewed interest in optical communication in the early 1960's with the invention of the ruby laser. This device tor the first time provided a powerful coherent light source, together with the possibility of modulation at high frequency. However, this also suffered from atmospheric disturbances that restrict these systems to short distance application. The proposal for optical communication via dielectric wave-guides or optical fibers fabricated from glass to avoid degradation of the optical signal by the atmosphere made almost simultaneously in 1966 by C.K Kao and G.K. Hockman and wrest. Initially, the optical fibers exhibited very high attenuation (1000 dB/Km) and were therefore not comparable with the co-axial cables (5 to 10 dB/Km.) In figure the electromagnetic spectrum we can observe the relative frequencies and wavelength of these types of electromagnetic waves. It might be noticed that visible light extended from 0.4 to 0.7 micrometer that converts to bandwidth of 320 THz. Even if only one percent of these capabilities were available, it would still allow for 80 billions, 4 kHz voice channels.
10 10 10 10 10 10 10 10 10 10 10 10 10
Visible Fig: The electromagnetic Spectrum
In parallel with the development of the fiber wave guide, attenuation was also focused on the other optical components which would constitute the optical fiber communication. Since optical frequencies are embraced with very small wavelengths the development off all these optical components essentially required a new technology. Thus semiconductor optical sources such as injection lasers and LED's and detectors such as-photo diodes to a certain extent photo transistors were designed and fabricated to enable successful implementation of the optical fiber system. Initially, the semiconductor laser exhibited very short life times of at best a few hours, but significant developments in the device structure by Hartman, Dement, Hwaang and Kuhn in 1973 and by Goodwin, Pion, Bourne in 1977 enabled lifetimes greater than 1 000 hours and 7000 hours respectively. These devices were originally fabricated from alloys of Gallium Arsenide (AaIGaAs). Which emitted in the near infrared between 0.8 and 0.9 micrometer? Subsequently the mentioned wavelength was extended 1.1 to 1.6 micrometer region by the use of other semiconductor alloys for better performance and more lifetimes. In 1978 D.R.Smith, R.C. Hopper and Garrett published a comparison between an APD and a PiN photodiodes followed by low capacitance PINFET receivers for the 1.3 and 1.55 micrometers transmission windows would out-perfom an equivalent APD receiver. So the use of PIN receivers operating in the long wavelength transmission windows would be ideal for trunk route telephone links. It is also worthy this fiber type has quickly come to dominate system applications within telecommunication. Moreover, the lowest silica glass fiber losses of about 0.2 dB/Km are obtained in the other longer wavelength windows at 1.55 micrometers but unfortunately intramural dispersion is greater at thus wavelength thus the maximum bandwidth achievable with conventional single- mode fiber. To obtain both the low loss and low dispersion at the same operating wavelength, new advanced singlemode fiber structure have been realized. These are dispersion shifted dispersion flatted fibers. Since dispersion can be overcome using specially fabricated fibers, this generation of fiber optic systems has attracted much attention for high capacity, long span terrestrial and under sea communication links. The improvement in optical fiber system is extra-ordinary. Due to its high performance and strong reliability optical fiber communication systems are now widely employed both within telecommunication networks many other localized communication application areas. 1.3 Comparison Between Conventional Communication Systems And Optical Fiber Communication Systems: The block diagram of general communication system and optical fiber communication system is shown in fig. bellow:
Figure 1.2 (a) Conventional communication system. Information source
Optical fiber cable
Figure 1.2 (b) Optical fiber communication system. In a conventional communication system, the information source provides an electrical signal to a transmitter which converts the signal into a suitable form for propagation over the transmission medium. The transmission medium may consist of a路 pair of wires, a coaxial cable or a radio link through free space down which the signal is transmitted to the receiver, where it is transformed into the being passed to the destination. In this communication system, the information is attenuated in the transmission medium. For optical fiber communication system the information source provides an electrical signal to a transmitter comprising an electrical stage which drives an optical source to give modulation of light wave currier. The optical source which provided the electrical conversion may be either a semiconductor laser or LED. The transmission medium consists of an optical detector which drives a further electrical stage and hence provides demodulation for detection of the optical signal or the optical electrical conversion. In this communication system attenuation is negligibly small.
1.4 Present status of optical fiber communication in Bangladesh: The introduction of optical fiber communication system into the public network has stimulated investigation and application of the transmission techniques by public utility organizations, which provide their own communication facilities over moderately long distances. For examples these transmission techniques may be utilized on the railways and along pipe and electrical power lines. Bangladesh Railway is the first organization to introduce optical fiber communication in Bangladesh. It was constructed by GEC Telecommunication Limited in the year 1987 and Dhaka-Chittagong link was commissioned on 10 January 1989 funded by NORAD aided project. The total length of the whole railway optical link is about 1450 km. The maximum distance between repeaters is around 68 km and the minimum is 8 km. Number of transmitted channels is 30, 36 or 120 which depends on requirement and the speed of transmission is 8 Mbps. 1.5 Transmission Media: The transmission of an electrical signal requires a transmission medium, which normally takes the form of a transmission line. It determines the maximum number of bits (binary digits that can be transmitted per second or bps). Some common types of transmission media are as follows: 1. Two wire open line: A two wire open line is simplest transmission medium. Each wire is insulated from the other and both are open to free space. This type of line is adequate for connecting equipment that is up to 50 meters apart.
II. Twisted pair lines: In twisted pair line, a pair is twisted together. Twisted pair lines are suitable for bit rates in the order of 1 Mbps over short distances (less than 100 meters) and lower bit rates over long distances.
III. Coaxial cable: In twisted pair lines, more signal power is lost as a result of radiation effects. Coaxial cable minimizes this effect. Coaxial cable can be used with a number of different signal types. hut typically 10 Mbps over several 100 meters in perfectly feasible.
IV. Optical fiber: Optical fiber cable differs from above these transmission media in that it carries the transmitted information in the form of a fluctuating beam of light in glass fiber. Light waves have a must wider bandwidth then electrical wave enabling to achieve transmission rates of 100 Mbps.
1.6 Advantage of optical fiber communication: Several advantages come with taking the fiber optics route. Fiber optic cables have a much larger bandwidth capability than ordinary metal cables. Extra bandwidth means extra memory capability, which means that the amount of data they transfer is larger, and therefore more cost efficient. Fiber optic cables are less susceptible to interface as opposed to metal cables, leading to increased reliability and functionality. Their immunity to electromagnetic interference also includes nuclear electromagnetic pulses as well as electrical resistance. Fiber optic cables allow for data to be transferred digitally, as in computer data, rather than analogically. In addition to the advantages of having extra information bandwidth using like as the carrier signal, the optical fiber communication system have several other advantages over the conventional systems.
I. Enormous potential bandwidth: The optical carrier frequency in the range of 1013 to 1016 Hz (generally in the near infrared around 1014 Hz or 105 GHz) yields a far greater potential transmission bandwidth than metallic cables systems (i.e. Co-axial cable bandwidth up to around 500 MHz) or even millimeter wave radio systems (i.e. systems currently operating with modulation bandwidth of 700 MHz). So it has high bandwidth (10 MHz-Km over 1 THz-Krn) II. Low attenuation (0.2dB over 1 THz-Km) it in a fiber is markedly lower than that of coaxial cable or twisted pair and is constant over a very wide range- So transmission within wide range of distance is possible without repeaters etc.
III. Electrical immunity: No possibility of internal noise cross talk generation along with the immunity to ambient electrical noise, ringing echoes. No problems being used in explosive environments. Optical fibers form a dielectric wave- guide and are therefore free from electromagnetic interference (EMI), radio frequency interference (RFI) or switching transients giving electromagnetic pulses (EMP).
IV. Signal security (can't be easily tapped, any cross talk): The light from optical fibers does not radiate significantly and therefore they provide a high degree of signal security. So any attempt to acquire a message signal transmitted of optically may be detected. This feature is obviously attractive for military, banking and general data transmission (i.e. computer network) applications.
V. No hazards of short circuits in metal wires.
VI. Light in weight and small in size: Optical fibers have very small diameters which arc often no greater than the diameter of human hair. The smaller and much lighter characteristics allow for an expansion of signal transmission within mobiles, such as aircraft, satellites and even ships.
VII. Electrical Isolation: Optical fibers which are fabricated from. glass or sometimes a plastic polymer electrical insulator and the fibers create no arcing or spark hazard at abrasions of short circuits.
VIII. Ruggedness and flexibility: The can be bent of radii few cms or twisted without damage. IX. System reliability and ease of maintenance : The systems are more reliable for the transmitted signal strength with fewer repeats (long repeater spacing) and these enhance life times of 20 to 30 years. These properties also tend to reduced the maintenance time and cost.
X. Potential low cost: - The glass generally provides the optical fiber transmission medium is made from stand-not a scare resource. So in comparison with copper conductors optical fibers offers the potential for low cost line communication. XI. Immunity to adverse temperature and moisture condition. XII. No need for additional equipment to protect grounding and voltage problems. XIII. Problems in Lesser space applications such a space radiation shielding and line to line isolations. X 1 V. The optical fiber communication has high bit rate (100 Mbps to 10 Gbps). XV. No electrical connection is required between the sender and the receiver. Fundamentals of Optical Fiber 2.1 Basic Working Communication Principle Of Optical Fiber: Optical fiber consists of a core material whose refractive index is higher than that of the surrounding medium known as the "Cladding". The cladding supports the waveguide structure when sufficiently thick, substantially reducing the radiation loss into the surrounding air. Depending on the design of the fiber, from the optical source light is constrained to the core by either total internal reflection or refraction or refraction and light wave propagates from the transmitter and to the receiver end. n1
Figure 2.1 (a) Basic structure of optical fiber. The transmitter is a light source whose output acts as the carrier wave. Most of the optical communication links are used digital time division multiplexing. (TOM) techniques. But the easiest way to modulate a
carrier with a digital signal achieved by ASK, PSK, FSK. Optical ASK system is obtained by varying the source drive current directly which causes a proportional change in optical power. Optical fiber acts as transmission medium to carry the modulated light wave from the transmission end to the receiver end. At the receiving end, photo detector extracts the electrical signal from the modulated light wave. The photo detector current is directly proportional to the incident optical power. 2.2 Transmission Principle of Optical Signal In Fiber Optics: To guide light, an optical communication link must consist of a core of material whose refractive index is greater than the surrounding medium known as the cladding. Light is constrained to the core by total internal reflection. When the angle of reflection of any ray (which is incident into the fiber core) is 90째, then the angle of incident is called critical angle. If the angle of incidence is greater than the critical angle the light is reflected back into the originating dielectric medium, this reflection is called total internal reflection. The following figure shows the transmission of a light ray in an optical fiber via a series of total internal reflections at the interface of the silica core and the slightly lower refractive index silica cladding. Law index claddings High index Core n1 Low index cladding n1 Fig: 2.2 (a) Total internal refiection in an optical fiber The maximum angle to the axis at which light may enter into the fiber core in order to be propagated is called the acceptance angle. Any rays, which are incident into the fiber core at an angle greater than Oi. will be transmitted to the corc"-c1adding interface and will not be totally internally reflected. Figure shows two light rays entering an optical fiber. Refraction of both rays occurs on entry, however ray 1 fails to propagate in the fiber core because it hits the boundary at an angle less than the critical angle O c on the other hand, ray 2 enters the fiber at an angle Oi and then hits the boundary at Oc thus it will propagate successfully. If Oi is the maximum angle of incidence, then the numerical aperture, NA of the fiber is equal to the sine of Oi. We can find the NA by applying the SNELL'S LAW to ray 2. Thus n2 which results in lange n:
Figure 2.2 (b) The acceptance angle when launching light into an optical fiber. 2.3 Propagation Of Light In Tile Fiber: If light meets the inner surface of the cladding (the core - cladding interface) at greater than or equal to O c then total internal reflection occurs. (The angle of incidence at which total reflection first occurs is called the critical angle, Oc) Light waves incident at angles greater than Oc will also be totally reflected. So all the
energy in the ray of light is reflected back into the core and none escapes into the cladding. The ray then crosses to the other side of the core and, because the fiber is more or less straight, the ray will meet the cladding on the other side at an angle, which again causes total internal reflection. The ray is then reflected back across the core again and the same thing happens. In this way the light zigzags its way along the fiber. This means that the light will be transmitted to the end of the 1ibber. This is a sort of step index fibber. In the diagram we have shown the path of only one light beam. Low index cladding n2 Incident light wave Reflected Light wave Low index cladding n2 Figure 2.3 (a) Light wave propagation along a glass fiber core But practically it is not so. Light energy emanating from any practical point source, will have several paths with different angles of incidence at the boundary layer. It may also contain different colors with different frequencies. Then it is called step index multimedia propagation. Thus, the various light waves, traveling along the core, will have propagation paths of different lengths. Hence these will take different times to reach a given destination. Thus a distortion is produced & is called transit-time dispassion. The result of this dispersion the variation of successive pulses of light may overlap into each other, and thereby cause distortion of the information being carried. However, making the core diameter of the same order as the wavelength of the light wave to be propagated can minimize this defect. The resultant propagation is a single light wave. Which type of fibber is called a stepped index monomode fiber. Cladding Âľ2 Single Wave Light Source Core Âľ1 Figure 2.3 (b) Stepped index monomode propagation. Now we are going to discuss the propagation of multi wave light energy in graded index fibber shown in Figure 2.3 (c) with the individual waves being gradually refracted in the graded index core, instead of being reflected by the cladding. Thus waves traveling at different incident angles will travel different distances from the horizontal center axis .It is obvious that light waves with large angle of incident travel more paths than those with smaller angles. But we know that the decrease of refractive index allows a higher velocity of propagation. Thus all waves will reach a given point along the fibber at virtually the same time. As a result the transit time dispersion is greatly reduced. This type of light wave propagation is referred to as graded index multimode propagation. Cokl ua clocking Figure 2.3 (c) Graded index mulitimode propagation.
Basic configuration of optical fiber communication: An optical fiber link consists of optical source, optical fiber transmission medium, the photo detector and its associated receiver and connectors used to join individual fiber cables to each other and to the source and detector. A basic block diagram of a simple point-to- point optical fiber link is shown in figure: Electrical input Optical signal
Transmitter Drive circuit
Optical fiber Electrical Ouldut
Receiver Photo Delector
Figure 2.4 (a) Basic block diagram of optical fiber communication
I. Transmitter: An optical transmitter consists of a source which could be either a laser or LED, a means for efficiently coupling the output power into the transmission fiber, a modulation circuit and in the case of a laser, a level control circuit. In addition, for some applications spectral control is also necessary.
II. Receiver: The optical receiver has three functions, namely, (a) the conversion from optical to electrical signal, (b) amplification and (c) estimation of the message originally transmitted. However. all practical optical fiber optic communication systems use in' coherent (direct) detection. Thus, in optical fiber communication only optical power variation is detected. The first function of the receiver, namely, the conversion from optical to electrical signal is achieved by the use of photo-detectors together with their associated electronic circuit. The amplifiers must be such as to introduce minimum amount of noise and distortion. The receiver is highly sensitive low noise receiver. 2.5 Generalized Components of Optical Fiber Communication System: The principal components of a general optical fiber communication system for either digital of analog transmission are shown in the system block schematic. in the figure the transmit terminal equipment consists of an information encoder or signal shaping circuit preceding a modulation or electronic driver stage which operates the optical source. Light emitted from the source is launched into an optical fiber incorporated within a cable which constitutes the transmission medium is converted block into an electrical signal by an optical detector positioned at the input of the received terminal equipment. This electrical signal is then amplified prior to decoding or demodulation in order to obtain the information originally transmitted. The main components used in an optical communication system arc the followings-
Encoder/signat shaping circuit
Modulator / driver
Optical Source Transmit terminal
Transmission Medium Fiber
Optical Delector Receive terminal
Figure 2.5 (a) Generalized block diagram of optical fiber communication system. I. The optical source. II. A means to modulate the optical carrier from the source with the information with the signal to be transmitted. III. A medium for the transmission of the modulated signal. IV. The photo detector, which converts the received -optical power into corresponding electrical waveform. V. A modulator that recovers the original signal from the electrical waveform. Classification Of Optical Fibers 2.2.1. Fiber classification: A already stated earlier, an optical fiber is a piece' of very thin and almost absolutely pure glass. It is as thin as human hair. It is outside is made of a cladding of glass, which is also another type of glass, with slightly different chemical composition. Hence, it has different refractive index from that of the inner core. No single fiber design meets all application requirements, mainly due to many economic reasons. However, manufacturers have concentrated mainly on two broad classes of fibers, viz. I. Single mode fiber: II. Multimode fiber: Multimode step index fiber Multimode graded index fiber 3.1 Single Mode fiber: Because its core is so narrow Single Mode fiber can support only one mode. This is called the "Lowest Order Mode". Single mode fiber has some advantages over multimode. Buffer jacket Primary coating core
n1-1.46 n2= 1.46 Figure 3.1: Single mode fiber 2.2.2 Multimode fiber: Although it may seem from what we have said about total internal reflection that any ray of light can travel down the fiber, in fact, because of the wave nature of light,. only certain ray directions can actually travel down the fiber. These are called the "Fiber Mode". In a multimode fiber many different modes are supported by the fiber. This is shown in the diagram below.
2.2.3 Stepped index fiber: The basic structure of stepped index fiber is shown in the fig. 3.2
Figure 3.2 Stepped index fiber It has two portions. Its structure is something like two concentric cylinders. inner cylinder is called the core. The outer cylinder may be made of air (Le. the core may be open to air). As the fiber core is open, the fiber as a whole will be mechanically weak. To overcome this, we should use a fiber of core diameter more than 200 pm. Step index fiber is so called because the refractive index of the fiber 'steps" up as we move from the cladding to the core of the fiber. Within the cladding the refractive index is constant, and within the core of the refractive index is also constant. 2.2.4 Graded index fiber: Graded Index Fiber has a different core structure from single mode and multimode fiber. Whereas in a step-index fiber the refractive index of the core is constant throughout the core, in a graded index fiber the value of the refractive index changes gradually from the center of the core onwards. In fact it has what we call a Quadratic Profile. This means that the refractive index of the core is proportional to the square of the distance from the center of the fiber. Buffer jacket Primary coating
Cladding Core n1-1.46 n2= 1.46
Figure 3.3 Graded index fiber Graded index fiber is actually a multimode fiber because it can support more than one fiber mode. But when we refer to "multimode" fiber we normally mean "step index multimode" .
So far we have described only glass-core fibers. But fibers of other compositions are now available. Plastic fibers (Plastic core) have been manufactured from a polymer perform drawing into a fiber. The losses associated with these fibers are usually in the hundreds of decibels. They operate at low temperature. We can use plastic fibers up to a maximum of 125 degree centigrade, while glass fibers can be used right up to a maximum temperature of 1000 degree centigrade. However, plastic fiber and cables have an inherent potential for many present and future applications. It is an ideal medium for sensors, process control and short distance communications. The characteristics of latest type of plastic fibers are the following: I. High light gathering capacity. II. Large core area. III. Low cost components- Fibers, cable, data links, LED. IV. Uses visible LED, which makes testing very easy - If see light LED is on. V. Easy to connectorize - cleave and crimp connectors perfect for assembly line or field installation, A fiber, having glass core and plastic cladding, is called "Plastic clad silica" or pCs fiber. The characteristics of such a fibers are the following: I. It has high NA. II. It has large core diameter. III. High attenuation. IV. Low bandwidth. The advantage of large core is the greater coupled power. The high value of NA permits the use of less expensive surface emitting LEDs. Other than high attenuation and low bandwidth, there are some major problems with plastic fibers: Plastic fibers have very poor mechanical strength They have low maximum temperature. 2.2.5. Other Latest Developed Types Of Optical Fibers: 2.2.6 High Purity Silicn Fiber (HPSUV): This type of fiber is suitable transmission of light in the range 180 to 800 It is good as well as cheap. It is sometimes coated with aluminum which gives very high mechanical strength and extra power handling capability, as aluminum dissipates heat more quickly. The aluminum coated fiber allows use up to 4:00째c and in a vacuum also. The main characteristics of High Purity Silica Fiber (HPSUV) Fiber type : Step index multimode. Core : High purity synthetic silica. Cladding coating : Doped Silica. Primary coating : Aluminum (HPSUVA) or Polymer. S. Optical Secondary coaling : Polymer. Numerical aperture : 0.24. Tensile strength : 7G pa (HPSUVA) Minimum Bend Radius : 40 times fiber radius. Operating temperature range : -196째C to 400째C. Humidity : 100%. Radiation resistance : Good. Guaranteed spot values of Attenuation : (1) 248 nm (KrF laser) < 1.2dB/m.
: (2) 308 nm (XeCl laser) < 0.26dB/m. 2.2.7 High Purity Silica (HPSIR): High purity silica type of fiber is similar to the HPSUV fiber with slightly different do pants to give it a longer wavelength capability in the near IR from 500 nm to 2600 nm. The same comments concerning strength and power handling apply. 2.2.8 Chalcogenide Fiber: The type of fiber is intended for transmission of light from 1 to 6 J.l.m. They have extremely low losses. We have two varieties of Chalcogenide fiber. Hence they are particularly suitable for medical applications. They are also useful for remote spectroscopy and a variety of industrial applications. The characteristics of such fibers are given below. 1. Fiber type 2. Core material 3. Primary coating 4. Clabbing material
: : : :
Step index multimobe. As2 S3 PTFE.
5. Secondary coating 6. Numerical Aperture 7. Minimum Bend radius 8. Operating temperature 9. Radiation resistance 10. Cover diameter
: : : : : :
PTFE or PVC. 0.3 to 0.5. <10 nm. -2000 C<T<1000C. Good.
As x S1-x
1. CHAL 100 - 100 µm. 2. CHAL 200 - 200 µm. 3. CHAL 300 300 µm.
11. Maximum internsity of transmitted Power
: 10 Watt ( CO Laser ). 2.2.9 Halide Fiber: These are the only known types of fibers that extend into the 15 J.l.m region. The most common application is for use with the C02 laser in medicine, to replace bulk-optics-delivery systems or in industry. One variety is silver Halide fibers. They are intended for transmission of light from 3 to 15J.l.m. They have low losses and arc currently the only known optical fibers for transmission of light from high power, long wavelength lasers like the C02 laser. They are very flexible and much marc convenient than normal mechanical delivery systems for these long wavelengths. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10
Numerical aperture Outer diameter with protective coating Core diameter Attenuation at 10.6 µm (CO2 aser) Attenuation at 5 to 6 µm. (CO2 Laser) Usable wavelength range Maximum Length (available) Yield strength Radius of elastic bending Operating maximum temperature
: : : : : : : : : :
<0.7. 0.36-1.5 µm. 0.1-1.2 µm. 0.5-1 .5 dB/m. < 2 dB/m. 3-15µm. < 15 m. 150- 170 MPa. > 0.4 cm. < 1000 C
. 2.2.10 Tapered Optical Fibers: Tapered fibers are useful for getting the maximum amount of power from a poor quality laser spot, into a fiber. The use of tapered optical fibers is an efficient low cost method of transforming n poor quality laser beam into a uniform output spot. The chief characteristics of tapered fibers are as follows: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10 . 11 . 12 .
Input and output ratios Input core diameter Output core diameter Taper length Total length Core material Primary coating Optional secondary coating Operating temperature Humidity : 100%.
: : : : : : : : : :
up to 5: 1 100 µm. to 4.0 mm. 50 µm to 1.5 mm. 1-3 meter. typical 3-10 meter. pure synthetic silica. Aluminium or acrylate. Epoxy acrylate. 196-400o C (Alocated). 100%.
i. cw up to 100 kw/cm2 ii. Pulse 145, 500 kw/cm2
2.2.11 Modes: Light propagates inside a fiber only a set of separates beam or rays, These different beams are called modes. The smaller the mode propagating angle, the lower. order mode. Mode propagating angles are a to ac, ac = II/2 - < ϕc.
Figure: Differcnt modes of optical fiber: 2.2.12 Types of mode: There are two types of mode in optical fiber systems. They are: a) Single mode fiber (SMF): Core diameter is very small (2µm to 10µm) Allows to propagates only one mode. b) Multi-mode fiber (MMF): 1. Core diameter is relatively large (50 µm to 100 µm) 2. Allows propagates of many modes. Number of modes of an optical fiber. N=V2/2 for step index fiber = V2/4 fo9r graded index fiber
Where, V = normalized cut off frequency = πd/λ (NA) Where, d = Core diameter. Optical Sources And Detectors 3.1 Optical sources: The optical source has fundamental function to convert electrical energy-iii the form of a current into optical energy (light) in an efficient manner, which allows the light output to be effectively launched or coupled into the optical fiber. For short links (<10 km) LED is suitable, but for mediun and long links LED is not suitable. To a large extent these to sources fulfill the major requirements for an optical fiber emitter, which are given as follows 1. A size and configuration compatible with launching light into an optical fiber. II. Most accurately track the electrical input signal to minimize distortion and noise. III. Preferably capable of simple signal modulation over wide bandwidth. IV. Must be capable of maintaining a stable optical output which is largely unaffected by changes in ambient conditions. V. It is essential that the source is comparatively cheap and highly reliable in order to complete with conventional transmission techniques. VI. Should emit light at wavelengths where the fiber has low losses and low dispersion and where the detectors are efficient. VII. Should have a very narrow spectral bandwidth in order to minimize dispersion in the fiber. There are three main light sources used in the field of fiber optics. Wideband continuous spectra sources (incandescent lamps) Monochromatic non-coherent sources (LED) Monochromatic coherent sources (LASERS) For optical communication purpose, highly monochromatic light sources are essential. Therefore, LED and Lasers are most commonly used. 3.2 Light emitting diode (LED): LED have the following advantages: I. Simpler fabrication, II. Low cost III. Reliability IV. Less temperature dependent, V. Simpler drive circuitry, V. Linearity. These advantages combined with the development of high radiance, reliability high bandwidth devices have ensured that the LED remains an extensively uses source for optical communication. Light Output P- type Laser
Figure: 2.7 (a) LED Planar Structure The simplest structure of LED is the planner structure, which is fabricated by either liquid- or vapor-phase epitaxial process over the whole surface of a GaAs substrate. This involves p type diffusion into the n type substrate in order to create the junction as shown in above figure. Forward current flow through the
junction gives Lambertian spontaneous emission and the device emits light from all surfaces. However, only a limited amount of light escapes the structure due to total internal reflection. LEDs have several drawbacks including: I. Generally lower optical coupled into a fiber, II. Usually lower modulation bandwidth, III. Harmonic distortion, 3.3 Laser: Light amplification in the laser occurs when photons colliding with an atom in the excited energy steps of a semiconductor material cause the stimulated emission of a second photon and then both these photon release two more. The stimulated process is shown in Bellows figure. Continuation of this process effectively creates avalanche multiplication, and when the electromagnetic wave associated with this photons are in phase, amplifier coherent emission is obtained. It is necessary to contain the photons with in the laser medium and maintain amplifying medium as show in figure. The optical cavity format is more analogous to an oscillator then an amplifier as it provides positive feedback of the photons by reflection at the mirrors at other end of the cavity. if one mirror is made partially transmitting useful radiation may escape from the cavity. Rapid decay Pumping
Figure 2.8 (a) Energy diagram for Lasing action Mirror
Figure 2.8 (b) Basic structure of a Laser 3.4 Optical detector: The detector is an essential component of an optical fiber communication system and is on the crucial elements that dictate the overall system performance. Its functions is to convert the received signal into an electrical signal, which is then amplified further processing, the following are the important requirements for photo detectors: I. High sensitivity at the operating wavelengths. High fidelity. 3.Large electrical response to the received optical signal. Short response time to obtain a suitable bandwidth. A minimum noise introduced by the detector.
Small size. High reliability. Low cost. Low bias voltages. Two conventional photo detectors are frequently used in optical fiber communication system. PIN (positive intrinsic negative) photodiode. APD (avalanche photodiode) 3.5 Pin Photodiode: To achieve a wider depletion region for operation at long wavelength, a PIN structure is created, where a lightly doped n type material acts as an intrinsic layer. Photons are absorbed. in the depletion region and electron-hole pairs are thus created, which constitute the photo-detector current, which shown iii bellow's figure. Metallic contact `1hf Antireflection Cutting
Figure 2.9.1 (a) Structure of PIN Photodiode 4.2.2 Optical amplifier: Optical amplifier, as the name implies, operate solely in the optical domain with no interconvcrsion of photons to electrons. Therefore, instead of using regenerative repeaters which, as currently implemented, require optoelectronic devices for source and detector, together with substantial electronic circuitry for pulse splicing, retiming and shaping, optical amplifiers can be placed at intervals along a fiber link to provide linear amplification of the transmitted optical signal. Optical amplifier delivers at its output a linearly amplified replica of the optical input signal. The optical amplifier is more then a component that can replace an optoelectric regenerator. In a conventional repeater the optical signal is first converted to an electrical signal that can be directly amplified by semiconductor electronic circuits. Using optical amplifiers the power budget limitations of optical passive networks may be overcome and such networks could be extended. Optical amplifiers, thus, an important role in 10 Gb/s light waves systems technology. Optical amplifier are also very important enhance flexibility, high degree of reliance, reliability of system configuration. Basically, two types of optical amplifiers have been developed namely Semiconductor amplifier and fiber amplifier. Semiconductor laser amplifiers: The semiconductor laser amplifier (SLA) is based on the conventional semiconductor laser structure where the output facet reflectivity's are between 30 and 40%. SLAs can be used in both nonlinear and linear modes of operation. Various types of SLA may be distinguished including the resonant or Fabry-Perot amplifier which is an oscillator biased below oscillation threshold, the traveling wave (TW) and the near traveling wave (NTW) amplifiers which are effectively single pass devices and the injection locked laser amplifier, which is a laser oscillator designed to oscillate at the incident signal frequency. This type of amplifier utilizes simulated emission from injected carriers semiconductor LEASER amplifiers can be grouped in three different types, such as: I. The injected bl6ck LEASER amplifier (ILL), which is basically a single node LEASER locked to a weak. input signal. II. The fabry-perot LEASER amplifier (FPLA) with biased blow the threshold.
III, The traveling wave amplifier (TWA), which has coated facets in order to suppress multiple reflections. (2) Fiber amplifiers: Fiber based optical amplificr employ have at present not progressed as far as those using semiconductor laser amplifiers. Hence, it is appears certain that fiber amplifiers will compliment the growing device technology associated with SLAs. These amplifiers are the following types: a) Rare earth doped amplifiers: Rare earth ion doped fiber has been used as a gain medium to amplify an external weak signal at a wavelength that is within the gain profile. Both the weak signal and pump are launched into the gain medium. Devices forming this function are termed as traveling wave amplifiers (or simply called Erbium Doped Fiber Amplifier or EDFA) b) Raman and Brillouin fiber amplifiers 3.6 Integrated optics: The multitude potential application areas for optical fiber communications coupled with the tremendous advances in the field have over recent years simulated a resurgence of interest in the area of integrated optics (IO). The concepts of IO involves the realization of optical and electro-optical Clements which may be integrated in large numbers on a single substrate. Hence, IO seeks to provide an alternative to the conversion of an optical signal back into the electrical regime prior to signal processing by allowing such processing to be performed on the optical signal. A major factor in the development of integrated optics is that it is essentially based on single mode optical waveguides and therefore tends to be incompatible with路 multimode fiber systems. Hence,IO did not make a significant contribution to first and second generation optical fiber systems. 3.6 Some integrated optical devices: The numerous developments in this field exclude any attempt to provide other than general examples in the major areas of investigation which arc pertinent to optical fiber communications. Hence, the application of IO in this area is to provide optical methods for multiplexing, modulation and routing. These various function may be performed with a combination of optical beam splitters, switches, modulators, filters, sources and detectors. 3.7 Optoelectronic integration: The integration of interconnected optical and electronic devices in an important area of investigation for applications within optical fiber systems. Monolithic optoelectronic integrated circuits (OEICs) incorporating both optical sources and detectors have been successfully realized for a number of years. The realization of OEICs has, however, lagged behind other developments in IO using dielectric materials such as lithium niobate. Compositional and structural differences between photonic devices and electronic circuits create problems in epitaxial crystal growth, planarization for lithography, electrical interconnections, thermal and chemical stability of materials, electrical matching between photonic and electrical devices together with heat dissipation. 3.8 Optical computation: With optical systems the situation is changed as they are capable of commutating many high bandwidth channels in parallel without interface. Thus parallel commutation can easily be provided within an optical computer system at relatively low cost. For some time work in optical computation has been directed towards particular requirements which arc necessary to provide a practical optical computing system. This include: High contrast Steady state bias External addressing Cascadability
Fan-out and fan-in Gain Arrays Speed and power 3.2.1 Fiber Connectors: Demountable fiber connectors are more difficult to achieve than optical fiber splices. This is because they must maintain similar tolerance requirements to splices in order to couple light between fibers efficiently, but they must accomplish it in a removable fashion. Additionally, the connector should ideally be a low cost component which can be fitted with relative ease. Hence optical fiber connectors may be considered in three major areas, which are: I. The fiber termination, which protects and locates the fiber ends; The fiber end alignment to provide optimum optical coupling; The outer shell, which maintains the connection and the fiber alignment. protects the fiber ends from the environment and provides adequate strength at the joint. Cylindrical ferrule connectors: The two fibers to be connected are permanently bonded in metal plugs known as ferules which have an accurately drilled central hole in their end faces where the stripped fiber is located. 3.2.2 Biconical ferrule connectors: A ferrule type connector which is widely used as part of jumper cable in a variety of applications in the United States is the biconical plug connector. The plugs are either transfer molded directly on to the fiber or cast around the fiber using a silica-loaded epoxy resin ensuring concentricity to within 5 Âľm. 3.2.3 Double eccentric connector: The double eccentric connector does not rely on a concentric fixed sleeve approach but is an example of an active assembly which is adjustable, allowing close alignment of the fiber axes. The mechanism consists of two eccentric cylinders within the outer plug. The optical fiber is mounted eccentrically within the inner cylinder. Therefore, when the two connector halves are mated it is always possible through rotation of the mechanism to make the fiber core axes coincide. This operation is performed on both plug using either an inspection microscope or a peak optical adjustment. 3.2.4 Expended beam connector: An alternative to connection via direct but joint between optical fibers is offered by the principle of the expended beam. Fiber connection utilizing this principle is a connector consisting of two lenses for collimating and refocusing the light from one fiber into other. 3.2.5 Optical connectors: Optical connectors are the means by which fiber optic cable is usually connected to peripheral equipment and to other fibers. These connectors are similar to their electrical counterparts in function and outward appearance but are actually high precision devices. The connector centers the small fiber so that its light gathering core lies directly over and in line with the light source (or other fiber) to tolerances of a few ten thousandths of an inch. Since the core size of common 50 micron fiber is only 0.002 inches, the need for such extreme tolerances is obvious. 3.2.6 Fiber couplers: An optical fiber is a device that distributes light from a main fiber into one or more branch fibers. Optical fiber couplers are often passive devices in which the power transfer takes place either: through the fiber core cross section by butt jointing the fibers or by using some form of imaging optics between the fibers (core interaction-type); or through the fiber surface and normal to axis by converting the guided core modes to both cladding and refracted modes which then enable the power sharing mechanism( surface interaction-type).
3.2.7 Three and four port couplers: Several methods are employed to fabricate three and four port optical couplers. The lateral offset method relics on the overlapping of the fiber end faces. Light from the input fiber is coupled to the output fibers according to the degree of overlap. Hence the input power can be distributed in a well defined proportion by appropriate control of the amount of lateral offset between the fibers 3.2.8 Star couplers: Star couplers distribute an optical signal from a single input fiber to multiple output fibers. The two principal manufacturing techniques for producing multimode fiber star couplers are the mixer-rod and the fused biconilal taper (FBT) methods. 3.2.9 Wavelength division multiplexing couplers: Wavelength division multiplexing (WDM) devices are specialized coupler type which enables light from two or more optical sources of differing nominal peak optical wavelength to be launched in parallel into a single optical fiber. 3.2.10 Splicing: Optical fibers have to be joined together to make longer lengths of fiber or existing fiber lengths which have been broken have to be repaired. Also the ends of the fiber have to be fitted with convenient connectors (terminations) to allow them to be easily plugged into equipment such as power meters, data transmitters, etc. Unlike electrical cables where all that is needed is to solder lengths of cable together, the process of joining two fibers (splicing) or terminating the end of a fiber is more complex and requires special equipment. Splicing is the process of joining the two bare ends of two fibers together. The ends of the fiber must be precisely lined up with each other; otherwise the light will not be able to pass from one fiber across the gap to the other fiber. There are four main alignment errors and any splicing technique is designed to deal with ends of these errors. Possible alignment errors during splicing There are four alignment errors in splicing optical fibers. These are: I. Lateral, 11. Axial, 111. Angular IV. Poor end Finish. These are illustrated in the diagrams below.
Figure 5.1 (a) Axil Misalignment
Figure 5.1 (b) Angular Misalignment
Figure 5.1 (e) Lateral Misalignment
Figure 5.1 (d) Poor End Finnish
Above these there are some factors that control the loss to a cable introduced by the connectors 3.2.11 Core Size Mismatch:
This type of mismatch occurs only when the outgoing fiber core size is smaller than that of the incoming fiber. Because the portion of the incoming light from the incoming core, will be reaching the outgoing core and rest is lost, thereby increasing the total loss of the fiber. Profile mismatch; This means that the shape of the incoming and outgoing fiber ends is not the same as per example, one end may be circular while the other end is elliptic. This is basically one type of area mismatch loss and losses are very small in most standard fibers. 3.2.12 Foreign Substance: If dirt or foreign material gets entry in to a splice or connector, during or after assembling, this may either increase its losses or even may block it completely. Proper care is to be taken during installation to avoid introduction of dirt. Splicing can be done successfully in even very dirty environment by using specially designed devices and proper care. There are two main types of splicing: Fusion Splicing and; Mechanical Splicing 5.4 Fusion Splicing: In fusion splicing the ends of the fibers are aligned either manually using micromanipulators and a microscope system for viewing the splice, or automatically
Figure 5.2 fusion Splicing Either using cameras or by measuring the light transmitted through the splice and adjusting the positions of the fibers to optimize the transmission the ends of the fibers are then melted together using a gas flame or more commonly an electric arc. 3.2.13 Mechanical Splicing: In mechanical splicing the two fiber ends are held together in a splice. This consists of some device usually made of glass which by its internal design automatically brings the two fibers into alignment. The openings at each end of the device are usually fluted to allow the fibers to be guided into the capillary where the alignment takes place. The splice is fist filled with optical cement whose refractive index is the same as that of the core of the fiber. After the fibers have been entered into the splice they are adjusted to give the optimum transmission of light. At this point they are clamped in position and the whole assembly is exposed to ultra-violet light which cures the cement. Mechanical splices are best used for multimode fiber. Some splices now exist which are suitable single mode fiber, but have a loss of 0.1 dB. This is five times the loss of the best fusion splice. Optical fiber systems 4.1 Optical transmitter circuit: The unique properties and characteristics of the injection laser and the light emitting diode (LED) which make them attractive sources for optical fiber communications. It is useful to consider these differences, as well as the limitations of the two source types, prior to discussion of transmitter circuits for various applications. a) Source limitations: Power Linearity Thermal Response
Spectral width Nonzero extinction ratio b) LED drive circuits: Digital transmission Analog transmission c) Laser drive circuits: It reduces the switch-on delay and minimizes any relaxation oscillations. It allows easy compensation for changes in ambient temperature and device ageing. 3. It reduces the junction heating caused by the digital drive current since the on and off currents are not widely different for most lasers. 4.2 Optical receiver circuits: The noise performance for optical fiber receivers incorporating both major detector types (the p-i-n and avalanche photodiode). The general receiver consisting of the elements often referred to as a linear channel because all operations on the, received optical signal may be considered to be mathematically linear. Preamplifier Automatic gain control (AGC) Equalization 6.3 Digital system planning considerations: The majority of digital optical fiber communication systems for the telecommunication network or local data application utilize binary intensity modulation of the optical source. So we choose to illustrate the planning consideration for digital transmission based on the modulation technique. Base band as PCM transmission using source intensity modulation is usually designated as PCM-IM. 4.3 The regenerative repeater: In the case of the long-haul, high capacity digital systems, the most important . Overall system performance parameter are the spacing of the regenerative repeaters. So it' is useful to consider the performance of the digital repeater, especially as it is usually designed with the same optical components as the terminal equipment. 4.4. The optical transmitter: The average optical power launched into the fiber from the transmitter depends upon the type of source used and the required system bit rate. The factor compares the optical power available from an injection aser and an LED for a transmission over a multimode fiber with a core diameter of 50 Âľm and a numerical aperture of 0.2. Typically, the LED launches around 1 m W, whereas usually the LED is limited to about ) 100 ÂľW. 4.5 The optical receiver: The basic optical receiver converts the modulated light coming from the optical fiber back into a replica of the original signal applied to the transmitter. The detector of this modulated light is usually a photodiode of either the PIN or the Avalanche type. This detector is mounted in a connector similar to the one used for the LED or LD. Since the amount of light that exits a fiber is quite small, optical receivers usually employ high gain internal amplifiers. Because of this, optical receivers can be easily overloaded. For this reason, it is important only to the size fiber specified for use with a given system. The only time any sort of receiver "mismatching" might be considered is when there is so much excessive loss in the fiber that the extra 5 to15 dB of light coupled into a multimode fiber by a single-mode light source is the only chance to achieve proper operation. However, this is an extreme case and is not normally recommended.
Demodulation schemes: Basic receiver configurations for optical heterodyne and homodyne detection are shown in figure. Optical detector
IF amp filter
Base hand anp filter
Signal Optical detector
Base band amp. filter
Figure 6.1 Basic coherent receiver configurations: (a) optical heterodyne; (b) optical homodyne receiver. AFC
The receiver configurations are as follows: Heterodyne synchronous detection Heterodyne non-synchronous detection Homodyne detection Phase diversity reception 4.6 Multiplexing Technique: In a simple optical fiber communication, a single light source transmits to a single detector over a single optical fiber. The disadvantage of such a conventional system is that the fiber or the cable cost may be greater than that of the other components and, therefore, techniques are needed to make better use of single fiber by increasing its information carrying capacity.
λ λ λ
λ λ MUX
Figure 6.2 Multiplexing and demultiplexing technique. This is being achieved by wavelength division multiplexing technique (WDM). In this WDM technique optical signal from different light sources are simultaneously transmitted through the same optical fiber
while the message integrity of each signal is preserved from subsequent conversion to corresponding electrical signals. As a matter of fact, the WDM techniques make use of LED's or lasers, as light sources but each source must emit at a different wavelength. The light from each source is combined for transmission down the same fiber using a device called optical multiplexer. The photo detectors at the far end of the fiber are broadband devices with respect to wavelengths being received, that is, they respond to many different wavelengths but cannot distinguish one from another. Another device is, therefore, needed to separate the light into its component wavelengths. Such a device is called demultiplexer. Various types of multiplexing that are used in optical fiber communication are as follows: I. Time Division Multiplexing (TDM) II Frequency Division Multiplexing (FDM) III. Wavelength Division Multiplexing (WDM) IV. Wavelength Add-Drop Multiplexing (WADM) 4.7 Time Division Multiplexing: In optical time division multiplexing, a high bit rate data steam is constructed directly by timer multiplexing several low bit rate optical signal is demultiplexed to several lower bit rate optical signal before detection and conversion to the electrical domain. Fig. indicates the principle of optical time division multiplexing. In an electrically time multiplexed system, multiplexing is done in electrical domain, before the optical-toelectrical (O/E) conversion. For n base band channels, each of bit rate B, the multiplexed bit rate is nB. The main sources of error are termed as electronic bottle-necks occur in the MUX and in the O/E converter and the DMUX, where the electronics must operate at the full multiplexed bit rate
M U X
| D M U X
Control singnal Figure 6.3 Principle of optical TDM. In the optically multiplexed system the electronic bottlenecks are removed by removing the E/O and OLE converters into the base and channels. Multiplexing carried out after the E/O conversion. All electronic devices associated with signal processing operate only at the base band bit rate; a control signal is needed to drive the demultiplexer. 4.8 Frequency Division Multiplexing: In this system, signals from different channels are modulated with different frequencies and these modulated signals are again modulated signals are modulated. With a higher carrier frequencies are transmitted from one placed to another. Let ten modulated signals f1, f2, ........................ f10, sueh as
................................................................... f1 f2 Where these are again modulated with a high frequency F such as
Figure 6.4 FDM techniques. 4.9 Wavelength Division Multiplexing (Wdm): WDM is considered to be a means for achieving large-scale concurrency on a single fiber. With WIDM, although the rate at which one user transmits information is limited by electronics, multiple users can transmit simultaneously on different wavelengths. Direct detection today provides approximately 10-20 wavelengths per fiber. The WDM utilizes the full information capacity of the optical fiber. In a normal point to point optical fiber communication link one fiber is needed with one optical source and one detector. The light sources work on a very narrow spectrum; say at 0.85 to 1 .3 etc. The optical fiber has very small attenuation in wavelength rangeO.8Sp to I .6 and several light sources working on a properly spaced peak emission wavelength could be simultaneously used with a single tiber provide the arrangement is such that no cross takes with individual sources. Such a process of sending the information is known as wavelength division multiplexing. I/p1 I/p2
Optical Source 1 (λ1)
Optical Detector 1 (λ1)
I/p2 Optical Source 2 (λ1)
Optical Detector 2 (λ1)
I/pN Optical Source 3 (λ1)
single optical fiber
Optical Detector N(λ1)
Figure 6.5 (a) Block diagram of a simple WDM system with N optical sources (unidirectional) The WDM is similar to FOM used in microwave radio and satellite systems. Output processed could be achieved either of the two processes shown in fig. 6.5 (a) or fig. 6.5 (b). WDM system, shown in fig. 6.5 (a) has N optical sources being used to transmit signals with a simple fiber. Optical Soufcel (λ1)
WDM device Optical Derec for 2 (λ1)
Optical Derec for 1 (λ1)
WDM device Optical Soufcel 2 (λ1)
Figure 6.3 (b) Principle of a bi- directional WDM systems. 4.10 Wavelength Add-Drop Multiplexing (Wadm): The multi wavelength optical networking (MONET) program's goal is to demonstrate the feasibility of a transparent multi wavelength optical transport layer is composed of seven different types of network c1cmellts (NE's) that provide all necessary network functionalities. A wavelength add-drop multiplexer! (WADM) is one of the keys. NE's which is used for selectivity dropping and inserting optical signals into the wavelength division multiplexed (WDM) network. The WADM that" was used in this work consists of a preamplifier, a multiplexer Demultiplexer (MUX! DMUX) pair, 2x2 optical switches for signal adding/dropping, dervo-controlled attenuators for power equalization and a booster amplifier. Fig: shows the block diagram of a WADM. Amplified spontaneous emission (ASE) noise due to the erbium-doped fiber amplifiers (EDFA's) is filtered inside each WADM by the MUXIDMUX pair. However, the part of amplified spontaneous emission (ASE) noise inside the pass band of the MUXIDMUX pair accumulates and eventually degrades the optical signal-to-noise ratio (SNR) at the drop location.
Figure 6.6 (a) the block diagram of a WADM. 4.11 Modulation Format: 4.12 Amplitude Shift Keying (ASK): Several techniques may be employed to amplitude modulate an optical signal. Digital intensity modulation used in direct detection systems is a form of amplitude shift keying in which the received signal is simply detected using the photo detector as a squire law device. It is apparent that the simplest approach to ASK is by direct modulation of the laser drive current.
Data Signal Carrier Phase Coherent PSK
Figure 6.7 (a) Amplitude shift deying (ASK) 4.13 Frequency shift keying (FSK): Frequency shift keying or FSK modulation uses one tone to represent a 0 and another tone to represent a 1 as shown in figure. The frequency deviation property of a directly modulated semiconductor laser can be usefully employed with wideband frequency shift keying (FSK) coherent optical systems. Hence optical FSK has the advantage that it does not necessarily require an optical modulator, thus allowing higher launch powers.
Data Signal Carrier Carrier2 FSK
Figure 5 (a) Frequency shift keying (FSK) 4.14 Phase shift keying (PSK): In the simplest form of phase modulation called' differential phase-shift keying or DPSK, the phase of a constant frequency sine-wave carrier of perhaps 1700 Hz is shifted by 1 800 to represent a change in the data from a 1 to a 0 or a change in the data from a 0 to a 1. Although rarely employed, optical phase modulation can be achieved by direct current modulation of a semiconductor laser into which external coherent laser light is injected. When the injected laser frequency is exactly tuned to the modulating signal frequency, the output signal phase relative to the modulating signal phase change of ir/2 is obtained when the injected laser frequency is exactly tuned to the modulating frequency; the output signal phase relative to the modulating phase is zero. A relative phase change of 900 is obtained when the injected laser frequency is detuned away from the modulated light frequency.
Data Signal Carrier Phase Coherent PSK
Figure 5.6.3 (a) Phase shift keying (PSK) Polarization Shift keying (PoISK): An additional modulator form which has been investigated within coherent optical fiber communication involves use of the polarization characteristics of the transmitted optical signal. The digital transmission implementation of such polarization modulation is known as polarization shift keying. 4.15Direct Detection Optical Receiver (ASK direct technique): In this technique at first ASK modulated signal fall upon the photo detector. Characteristic of ASK signal is such that when bit 1, light intensity is high and when bit o light intensity. Also approaches zero. Photo detector produces an electrical signal corresponding to the modulated optical signal. So a preamplifier is used in next stage to improve its amplitude. In order to further increase the strength of the signal a post amplifier is used. Then a low pass filter is used to remove to noise and harmonies. To equalize the pulse shape an equalizer is used in the next stage. To sample the pulse a sampler is used. Comparator is used to compare the sampled data to threshold value and gives the desired data at the output. Direct detection receiver performance consideration
4.2.1 Noise: Noise is a term generally used to refer to any spurious or undesired disturbances that mask the received signal in a communication system. In optical fiber communication systems we are generally concerned with noise due to spontaneous fluctuations rather than erratic disturbances which may be a feature of copper based systems. Due to spontaneous fluctuations, there are main types of noise in optical fiber communication systems; Thermal noise: This is the spontaneous fluctuation due to thermal interaction between, say, the free electrons and the vibrating ions in a conducting medium, and it is specially prevalent in resistors at room temperature. b) Dark current noise: When there is no optical power incident on the photo detector a small reverse leakage current still flow from the device terminals. This dark current contributes to the total system noise and gives random fluctuations about the average particle flow of the photocurrent. Thus the dark current noise i2d is given by: ยก2d = 2eB/d C) Quantum noise: The quantun nature of light and the equation for the energy of this quantum or photon was stated as E = hf. The quantum behaviour of electromagnetic radiation must be taken into account at optical frequencies since hf > KT and quantun fluctuations dominate over thermal fluctuations. d) Digital signaling quantum noise: For digital optical fiber systems it is possible to calculate a fundamental lower limit to the energy that a pulse of light must contain in order to be detected with a given probability of error. The probability of no pairs being generated when a light pulse is present may be obtained and is given by: P (0/1) = exp (-Zm) Thus in the receiver described P(O/I) represents the system error probability p(e) and therefore: P(e) = exp(-zm) e) Analog transmission quantum noise: In analog optical fiber systems quantum noise manifests itself as shot noise which also has Poisson statistics. The shot noise current is on the photocurrent Ip is given by: i2s = 2eB/p 4.2.2 Receiver noise: This is dependent on both the method of demodulation and type of device used for detection. The conditions for coherent detection are not met in IM/DD optical fiber systems. Thus heterodyne and homodyne detection, which are very sensitive techniques and provide excellent rejection of adjacent channels, are not used as the optical signal arriving at the receiver tends to be coherent. In practice the vast majority of installed optical fiber communication systems use incoherent or direct detection in which the variation of the optical power level is monitored and no information is carried in the phase or frequency content the signal. Optical signal
Amplifier Electronic signal Detector load dais Noise
Figure: 7.1 Block schematic of the front end of an optical receiver showing the various sources of noise. a) p-n and p-i-n photodiode receiver:
The two main source of noise in photodiodes without internal gain are dark current noise and quantum noise, both of which may be regarded as shot noise on the photocurrent. b) Receiver capacitance and bandwidth: Consider the equivalent circuit shown in fig , the total capacitance for the front end of an optical receiver CT is given by: CT= Cd +Ca Where Cd is the detector capacitance and ca is the amplifier input capacitance.
Amplifier Detector Amplifier Figure: 7.2 The equivalent circuit for the front end of an optical fiber receiver. 4.2.3 Receiver structures: A full equivalent circuit for the digital optical fiber receiver, in which the optical detector is represented as a current source idet shown in figure. The noise sources (it , iTS and iamp) and the immediately following amplifier and equalizer, are also shown. Equalization compensates for distortion of the signal due to the combined transmitter, medium and receiver characteristics. The equalizer is often a frequency shaping filter. Deflector and bias
Equalizer V out
Figure 7.3 A full equivalent circuit for a digital optical fiber receiver including the various noise sources. a) Low impedance front end: Three basic amplifier configurations are frequently used in optical fiber communication receivers. The simplest, and perhaps the most common, is the voltage amplifier with an effective input resistance Ra. However, RL may be modified to incorporate the parallel resistance of the detector bias resistor Rb and the amplifier input resistance Ra. The modified total load resistance RTL is given by: RTL = RbRa /(Rb+Ra) b) High impedance front end: The second configuration consists of a high input impedance amplifier together with a large detector bias resistor in order to reduce the effect of thermal noise. The detector output is effectively integrated over a large time constant and must be restored by differentiation. This may be performed by the correct equalization. c) The trans impedance front end: This configuration largely overcomes the drawbacks of the high impedance front end by utilizing a low noise, high input impedance amplifier with negative feedback. The device therefore operates as a current mode amplifier where the high input impedance is reduced by negative feedback. 4.2.4 FET Preamplifiers:
The lowest noise amplifier device which is widely available is the silicon field effect transistor (FET). Unlike the bipolar transistor, the FET operates by controlling the current flow with an electric field produced by an applied voltage on the gate of the device rather than with a base current. Thus the gate draws virtually no current, except for leakage, giving the device extremely high input impedance. a) Gallium arsenide MESFETs: Silicon FETs have a limited useful bandwidth; much effort has been devoted to the development of high performance microwave FETs since the mid-1970s. These FETs are fabricated from gallium arsenide and, being Schottky barrier devices, are called GaAs metal Schottky field effect transistors (MESFETs). They overcome the major disadvantage of silicon FETs. Thus in optical fiber communication receiver design they present an alternative to bipolar transistors for wideband operation. b) PIN-FET hybrid receivers: The p-i-nl FET, or PIN-FET, hybrid receiver utilizes a high performance p-i-n photodiode followed by a low noisc preamplifier often based on a GaAs MESFET, the whole of which is fabricated using thick film integrated circuit technology. 4.2.5 High performance receivers: The noise performance is a major design consideration providing a limitation to the sensitivity which may be obtained with a particular receiver structure and component mix. However. two other important receiver performance criteria were also outlined in the aforementioned sections, receiver bandwidth and dynamic range. Low noise performance combined with potential high speed operation has been a major pursuit in the hybrid integration of p-i-n photo diodes with GaAs MESFETs. The optimization of PIN-FET receiver designs for sensitivity and high speed operation has been investigated. Also a wideband (10GHz) low noise device using discrete commercial components has recently been reported. In addition, new high speed, low noise transistor types are under investigation for optical receiver preamplifiers Losses And Limitations Of Optical Fiber 5.1 Loss Of Optical Power: Attenuation is the decrease in light power during the light propagation along an optical fibber. It is the loss of power and means 'Loss of Optical Power' in the fiber itself. This loss may arise from different sources e.g. 1. Material absorption, II. Scattering, III. Mode coupling, IV. Leaky modes, V. Bending of the fiber, VI. Radiation induced attenuation, VII Defective construction losses, VIII. Inverse square law losses, IX. Transmission Losses and' X. Core and cladding losses Losses are expressed in decibels per kilometer (dB/ Km). The normal range of attenuation is from 0.154 dB/Km at operating wavelength 1550 nm for single mode fiber to over 10 dB/ m for plastic fiber. 5.2 Material Or Impurity Loss: It is due to the absorption of light by the fiber material. It includes absorption due to light interacting with the molecular structure of the material as well as loss due to material impurities. The loss due to atomic structure of the material itself is relatively small. Losses due to impurities can be reduced by better manufacturing. In the fabrication of various types of fibers, we use Ge02, P205, B203 etc, as dopants in silica, in order to modify the refractive index. While B produces strong absorption peaks at 3.2 Âľm, P205
produces the same at 3.8 Âľm. However in both these cases, absorption-tails extend below 1 .3 Âľm. That is why Boron and Phosphorous based dopants are not used in the low loss, single mode fiber. 5. 3 Light Scattering: Light is scattered by the molecules of the material due to structural imperfection and impurities. The scattered light does not propagate down the fiber. The glass, which is used in the fabrication of fibers, has many microscopic in homogeneities and material density fluctuations of the silica material contents. As a result, a portion of light passing through the glass fiber gets scattered. This phenomenon is called "Rayleigh scattering" the losses due to this scattering effect vary inversely with the fourth power of the wavelength. Scattered light
Figure : Optical obstacle in fiber. 5.4 Absorption Loss: This form of loss is caused by very nature of the core material and varies inversely to the transparency of the material. Besides, in some materials absorption losses are not uniform across the light spectrum, and are regarded to be wavelength-sensitive. 5.5 Leaky Modes: The losses due to leaky modes arise due to a irregularities in wave-guide geometry, so these can be regarded as wave-guide Âˇscattering. These leaky modes have radiative components that result in cladding power losses. So these can be minimized by surrounding the thin cladding layer on the fiber core by a third layer of pure silica, which has an index of refraction higher than that of the cladding, but lower than that of the core. This third additional silica coating will not only give additional strength to the fiber, but side by side also remove the partial refraction rays from the leaky modes, as well as remove the passed rays from cut-off modes by total refraction. 5.6 Bending Losses: Whenever a fiber deviates from a straight- line path, radiative losses occur. These losses are prominent for improperly installed single mode optical cable. These losses can be classified in to two types; these are Micro-bending and Macro-bending. 5.7 Micro bending: It is caused by curvature of the fiber axis. 5.8 Macro bending: It is caused by bending the entire optical fiber. 5.9 Radiation Induced Losses: When the glass molecular matrix interacts with electrons, neutrons, gamma rays and x-rays, the structure of the glass molecules is altered and fiber darkens. This introduces additional losses, which increase with amount, type, and dose and exposure time of radiation.
5.10 Inverse Square Law Losses: In all light systems, there is the possibility of losses, caused by spreading of the beam. If you take a flashlight and point it at a wall, and measure the illuminance per unit area at the wall at a distance of, say, one meter, and then back off to twice the distance then measure again, you will find that the illuminance had dropped to one- fourth. In other words, the illuminance per unit area is inversely proportional to the square of the distance (I Ix). 5.11 Transmission Losses: These losses are caused by light, which is caught in the cladding material of clad optical fibers. This light is either lost to the outside, or is trapped in the cladding layer and is thus not available for propagation in the core of fiber. 5.12 Temperature Dependence of fiber Losses: Temperature extremes have an adverse effect on fiber losses. Tight tube designs and clad made of plastic are normally usable down to â€“ 10 0C. If temperature is below -100C., differential. Thermal expansion between polymer coatings and glass causes stresses which cause micro-bending losses. Loose tube and glass-clad are used J between - 10Â°C to - 50Â°C. The central element of the fiber determines the amount of cable contraction with temperature and called 'Stiffering". 5.13 Core and cladding Losses: In a fiber the core and cladding have different refractive indices, as they have different compositions. So the core and cladding have different attenuation coefficients (c & Ud). 5.14 Limitation Of Optical Fiber Communication: The use of fibers for optical communication does have some drawbacks in practice. Hence to provide a balanced picture, these advantages or limitations must be considered especially at bit rate higher than 5Gbps. These limitations are as follows: 5.15 Stimulated Raman scattering: It is an interaction between light and vibrations of silica molecules causes attention of short wavelength channels in wavelength multiplexed systems. 5.16 Stimulated Brillion Scattering: It is an interaction between light waves and sound waves cause frequency convention and reversal of propagation direction of light: 5.17 Fiber four-waves mixing: Third order interaction distortion where by two or more optical waves at different wavelength mixed to produce new waves at other wavelength. 5.18 Self phase modulation: It is process of change in signal phase due to change in intensity of the signal due to group velocity dispersion. 5.19 Cross phase modulation: It is an interaction via non-linear refractive index between intensity of one light wave and optical phase of other light waves. 5.20 Dispersion:
When a light contains different frequency components are traveled through the fiber, the each frequency component will has their own group velocity. So at the end of the fiber light will be dispersed and this effect is known as dispersion. Other wise light pulse broading inside a fiber is known as dispersion. Dispersion of the transmitted optical signal causes for both digital and analog transmission along optical fibers. There are three types of dispersion such as materials dispersion, modal dispersion and wave guide dispersion. 5.21 Intra modal dispersion: Intra modal or chromatic dispersion may occur in all types of optical fiber results from the finite spectral line width of the optical source. a) Material dispersion: Pulse broadening due to material dispersion results from the different group velocities of the various spectral components launched into the fiber from the optical source. b) Wave guide dispersion: The wave guiding of the fiber may also create intra modal dispersion. This results from the variation in group velocity with wave length for a particular mode 5.22 Inter modal dispersion: Pulse broadening due to inter modal dispersion results from the propagation delay differences between modes within a multimode fiber. a) Multimode step index fiber: Using the ray theory model, the fastest and slowest modes propagating in the step index fiber may be represented by the axial ray and the extreme meridional ray (which is incident at the core-cladding interface at the critical angle) respectively. b) Multimode graded index fiber: Inter modal dispersion in multimode fibers is minimized with the use of graded index fibers. Hence, multimode graded index fibers show substantial bandwidth improvement over multimode step index fibers. c) Modal noise: The inter modal dispersion properties of multimode optical fibers create another phenomenon which affects the transmitted signals on the optical channel. Application 6.1 Trunk network: The trunk or toll network is used for carrying telephone traffic between major communications. Hence these are generally a requirement for the use of transmission system, which have a high capacity in order to minimize cost per ekc. The transmission distance for trunk system can vary enormously from under 20 km to over 300 km and occasionally to as much as 1000 km. Therefore, transmission systems which exhibit low attenuation路 and hence give a maximum distance of unlimited operation are the most economically variable. In this context Optical Fiber system with their increased bandwidth and repeater spacing often a distinct advantage. 6.2 Local and Rural Networks: The local and rural network as subscribers to the local switching center or office. Virtually all local and rural telephone networks utilized a star configuration based on copper conductors for full duplex speech transmission. There is substantial interest in the possibility if replacing the existing narrow band local and rural network twisted pairs with optical fibers. These can also be utilized in the star configuration to provide wide band services to the subscriber together with the narrow band width speech channel. The cost of optical fiber cable may be reduced towards the cost of copper twisted pairs with large scale production required for the local and rural networks.
6.3 Submerged System: Under sea cable systems are an integral part of the international communication networks. They find application on shorter routes especially in Europe on longer routes, such as across the Atlantic, They provide route diversity in consumption with satellite links. The number submerged cable route and their capacities are steadily increasing and hence there is a desire to minimize costs per channel. In this context digital optical fiber communication systems appear to offer substantial advantages over current analogous FDM and digital pcM coaxial cable systems. Research and development of single mode fiber submerged cable system is progressing in a no. of countries including the UK, France, USA and Japan. 6.4 In Military Applications: 6.5 Mobiles: One of the most promising areas of military application for optical fiber communication is with in military mobiles such as aircraft, ships and tanks. The small size and weight of optical fiber provide and effective solution to space problems in these mobiles which are increasingly equipped with sophisticated electronic. Also the wide band nature of optical fiber transmission will allow the multiplexing of a no of signals in to a common bus. Furthermore the immunity of optical transmission to electromagnetic interference in the often noisy environment of military mobile is tremendous advantage. 6.6. Communication links: The other major area for the application of optical fiber communications in the military sphere includes both short and long distance communication links. Short distance optical fiber system may be utilized to connect closely spaced items of electronic equipment in such areas as operation rooms and computer installations. A large no of these systems have already been installed in military installations in the UK. It's sure that due to possibility overriding, considerations such as size weight, deploy ability, survivability and security. Optical fiber communication will be widely used in military applications in the future. 6.7 Civil, consumer and industrial applications: 6.8 Civil: In these applications, although high capacity transmission is not usually required, optical fibers may provide a relatively low cost solution, also giving enhanced protection in harsh environment, especially in relation to EMI and EMP. Experimental optical fiber communication systems have been investigated within a number of organizations in Europe, North America and Japan. For instance, British Rail has successfully demonstrated a 2Mbit/s system suspended between the electrical power line gantries over a 6km route in Cheshire. 6.9 Consumer: A major consumer application for optical fiber is within automotive electronics. Work is progressing within the automobile industry towards this end together with the use of microcomputers for engine and transmission control as well as control of convenience features such as power windows and scat controls. Optical fiber communication links in this area provide advantage of reduced size and weight together with the elimination of EMI. 6.10 Industrial: Industrial uses for optical fiber communications cover a variety of generally on-remises applications within a single operation site.
digital transmission at rates from d.c to 20Mbit/s, synchronous or asynchronous, having compatibility with a common logic family(i.e. TTL or CMOS), being independent of data format and with the bit error rates less than 10-9; analog transmission from d.c to lOMHz, exhibiting good linearity and low noise; transmission distances from several meters up to a maximum of kilometers, although generally 1Km. will prove sufficient, a range of environments from begin to harsh, and often exhibiting severe electromagnetic interference from industrial mach Cable lay-out method: At the time of making trench lay-out it should be taken in mind the position of bridge and bend road. The splicing point should be given considering this point. The two sides of the bridge there should be placed handhold and splicing point .The distance between two handhold and splicing point should be declarer in meter in the drawing. The line will be marking by line and straight. The position of underground cable and railway office their lay-out plans. Cable Installation: The optical Fiber cable has been made by blowing machine at the time of blowing A man should be kept the front handhold. 25m cable should be kept in the handhold and 30m cable should be kept where the splicing point. For beginning the next blowing, the blowing machine should be placed next the handhold function will be method. Reading of the cable should be recorded at the closure terminal, at splicing loss more then o.3 will be taken. At the time of splicing the joint point will have to heating by giving heat will smove. The fiber will have to arrange fairly in cloggier tray. If the fiber number is 12 then they have separate by 6 in each arrangement. If this work function for 48 fiber will have to finished. The trench will have to kept between two closure and heaved to kept fairly by closure rubber band. The point of closure have to close by heating. They indicate the direction of cable by arranging circle order in the handhold ammoniating striker have to given and at last the open path of the handhold will have to close by the handhold key. List of the Instruments: Blowing machine Optical fiber cable Splicing machine Round cutter Alcohol Tissue Closure Heat sing Heat sings tape Smove Cutting player Andy cutter
Final Checking: When the lying of the cable is complete next important point is to check weather there is any loop. This is done by passing laser beam from one tower to another. If there are us any leakage or loss or any other technical problem is easily detected by this method. 8.1 Discussions:
The demand for fiber optics remains at an all-time high, especially in regard to local area networks. Many telephone companies are replacing their traditional lines with the date-intensive fiber optic cables. Almost all communications companies in existence today are utilizing or implement fiber optic cable systems. We know light wave transmission via optical fiber is beneficial, due to its huge number of advantages, Such asEnormous potential bandwidth. Small size & weight, Electrical installation. Signal security. Low transmission. Ruggedness & Flexibility. System reliability and ease of maintenance. Potential low cost, etc. These are not all: the most significant are the reduced power consumption exhibited by optical fiber systems in comparison with their metallic counter parts and their ability to provide for an expansion in the system capability often without fundamental and costly changes to the system configuration. For instance, a system may be upgraded by simply changing from a LED to an injection laser source, by replacing a pin photodiode with an APD detector, or alternating by operating at a longer wavelength without replacing the fiber cable. 9.1 Conclusion: The main disadvantage of fiber optic cables is their cost. Expensive to install and more fragile than their metal counterparts, fiber optic cables are difficult to split as well. This makes them more difficult to work with and install onsite, Some optical fibers are subjected to "fiber fuse", an occurrence caused when too much light reaches an imperfection in the line that destroys connectivity. "Fiber fuse" can be minimized with defection circuitry at a transmitter. Some fiber optic cables also can't carry electrical power to operate terminal devices, buit this feature is becoming passes with the wider availability of mobile phones, wireless PDAs, and other remote devices. Although we get some disadvantages Optical communication, Such as: The flexibility of the base fibers. Some problems involved with joining low T-couplers. Some doudt in relation to the long-term reliability of optical fiber in the presence of moisture. The small size of the fibers & cables which creates some difficulties with splicing & forming connector. An independent electrical power feed is required for any repeaters, etc. But the technology has developed. So both continuing developments and experience with optical fiber system are generally reducing these problems. References: Optical Fiber Communication By-John M. Senior Optical Communication System By-John Gower Optical Fibers and Fiber Optic Communication System By-Subir Kumar Sarkar Optical Fiber Telecommunications
By-Ivan P. Kaminow & Thomas L, Koch Fiber Optic Communications