“STUDY OF Optical Fiber Communication Systems”
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.
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 single-mode 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.
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 formfor propagation over the transmission medium. The transmission medium may consist of a· 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 105GHz) 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 Cladding
n2 Core
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 90°, 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 Oc 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: CLADDING
CORE
RAY CLADDING
RAY
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 Oc 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 m2
Single Wave
Light Source
Core m1
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.
2.4 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
Optical fiber
Electrical
Ouldut
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-
Information input Information output
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 Refractive index cladding
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 Refractive index
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):
Thistype 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 4:00°c 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 : -196°C to 400°C.
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 | : | Step index multimobe. |
| 2. Core material | : | As2 S3 |
| 3. Primary coating | : | PTFE. |
| 4. Clabbing material | : | As x S1-x |
| 5. Secondary coating | : | PTFE or PVC. |
| 6. Numerical Aperture | : | 0.3 to 0.5. |
| 7. Minimum Bend radius | : | <10 nm. |
| 8. Operating temperature | : | -2000 C<T<1000C. |
| 9. Radiation resistance | : | Good. |
| 10. Cover diameter | : | 1. CHAL 100 – 100 mm. |
| 2. CHAL 200 – 200 mm. | ||
| 11. Maximum internsity of transmitted Power | 3. CHAL 300 300 mm. |
: 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. | Numerical aperture | : | <0.7. |
| 2. | Outer diameter with protective coating | : | 0.36-1.5 mm. |
| 3. | Core diameter | : | 0.1-1.2 mm. |
| 4. | Attenuation at 10.6 mm (CO2 aser) | : | 0.5-1 .5 dB/m. |
| 5. | Attenuation at 5 to 6 mm. (CO2 Laser) | : | < 2 dB/m. |
| 6. | Usable wavelength range | : | 3-15mm. |
| 7. | Maximum Length (available) | : | < 15 m. |
| 8. | Yield strength | : | 150- 170 MPa. |
| 9. | Radius of elastic bending | : | > 0.4 cm. |
| 10. | Operating maximum temperature | : | < 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. | Input and output ratios | : | up to 5: 1 |
| 2. | Input core diameter | : | 100 mm. to 4.0 mm. |
| 3. | Output core diameter | : | 50 mm to 1.5 mm. |
| 4. | Taper length | : | 1-3 meter. |
| 5. | Total length | : | typical 3-10 meter. |
| 6. | Core material | : | pure synthetic silica. |
| 7. | Primary coating | : | Aluminium or acrylate. |
| 8. | Optional secondary coating | : | Epoxy acrylate. |
| 9. | Operating temperature | : | 196-400o C (Alocated). |
| 10. | Humidity : 100%. | : | 100%. |
| 11. | Radiation resistance | : | Good. |
| 12. | Power transmission | : | i. cw up to 100 kw/cm2 |
| ii. Pulse 145, 500 kw/cm2 |
2.2.11 Modes:
Light propagates inside a fiberonly 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 – < jc.
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 (2mm to 10mm)
Allows to propagates only one mode.
b) Multi-mode fiber (MMF):
1. Core diameter is relatively large (50 mm to 100 mm)
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
= ?d/l (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 Ohmle
Contacts
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 Lasing
Figure 2.8 (a) Energy diagram for Lasing action
Mirror Mirror
Amplifying Medium Optical
Output
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 Reflecting
Cutting 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· 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 µm.
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 50micron 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 jointingthe 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.