CHAPTER-1

OPTICAL FIBER

1

CHAPTER 1

INTRODUCTION

1.1 OPTICAL FIBER

An optical fiber is a flexible, transparent fiber made of high quality extruded glass (silica) or

plastic, slightly thicker than a human hair. It can function as a waveguide to transmit light

between the two ends of the fiber. Power over Fiber (PoF) optic cables can also work to deliver

an electric current for low power electric devices. Optical fibers are defined as the glass that

transmits over long distances. Optical fibers are used to measure the physical parameters such as

temperature, strain and pressure etc. optical fibers can be used to make distributed measurements

over long distances and also it is not affected by electromagnetic interference.

Optical fibers typically include a transparent core surrounded by a transparent cladding

material with a lower index of refraction. Light is kept in the core by total internal reflection.

This causes the fiber to act as a waveguide. Fibers that support many propagation paths or

transverse modes are called multi-mode fibers (MMF), while those that only support a single

mode are called single-mode fibers (SMF). Multi-mode fibers generally have a wider core

diameter, and are used for short-distance communication links and for applications where high

power must be transmitted. Single-mode fibers are used for most communication links longer

than 1,000 meters (3,300 ft).

1.1.1 TYPES OF FIBER

There are three types of fiber optic cable commonly used:

1) Single mode

2) Multimode 

2

Fig 1.1 Structure of an optical fiber

1.1.1.1 Single Mode Fiber

Single mode fiber with a relatively narrow diameter, through which only one mode will

propagate. It carries higher bandwidth than multimode fiber, but requires a light source with a

narrow spectral width. This fiber also gives you a higher transmission rate and up to 50 times

more distance than multimode, but it also costs more. Single-mode fiber has a much smaller core

than multimode. The small core and single light-wave virtually eliminate any distortion that

could result from overlapping light pulses, providing the least signal attenuation and the highest

transmission speeds of any fiber cable type.

1.1.1.2 Multi-mode Fiber:

Multi-mode fiber has a little bit bigger diameter, with a common diameters in the 50 to

100 micron range for the light carry component. It gives us high bandwidth at high speeds over

medium distances. Light waves are dispersed into numerous paths, or modes, as they travel

through the cable's core typically 850 or 1300 nm. Typical multimode fiber core diameters are

50, 62.5, and 100 micrometers. However, in long cable runs, multiple paths of light can cause

signal distortion at the receiving end, resulting in an unclear and incomplete data transmission so

designers now call for single mode fiber in new applications using Gigabits and beyond.

1.1.2 ADVANTAGES OF OPTICAL FIBER

Fiber optic cables have a much greater bandwidth than metal cables. The amount of

information that can be transmitted per unit time of fibre over other transmission media is its

most significant advantage.  With the high performance single mode cable used by telephone

industries for long distance telecommunication. An optical fibre offers low power loss.  This

allows for longer transmission distances. Fibre optic cables are immune to electromagnetic

interference.  It can also be run in electrically noisy environments without concern as electrical

noise will not affect fibre. In comparison to copper, a fibre optic cable has nearly 4.5 times as

much capacity as the wire cable has and a cross sectional area that is 30 times less. Fibre optic

cables are much thinner and lighter than metal wires.  They also occupy less space with cables of

the same information capacity.  Lighter weight makes fibre easier to install. An optical fibre has

3

greater tensile strength than copper or steel fibres of the same diameter.  It is flexible, bends

easily and resists most corrosive elements that attack copper cable.

An optical fibre has greater tensile strength than copper or steel fibres of the same diameter.  It is

flexible, bends easily and resists most corrosive elements that attack copper cable.

1.2 FIBER OPTIC SENSORS

A fiber-optic sensor system consists of a fiber-optic cable connected to a remote sensor, or

amplifier. The sensor emits, receives, and converts the light energy into an electrical signal.

Fibers have many uses in remote sensing. Depending on the application, fiber may be used

because of its small size, and no electrical power is needed at the remote location. Many sensors

can be multiplexed along the length of a fiber by using light wavelength shift for each sensor. By

sensing the time delay as light passes along the fiber through each sensor. Time delay can be

determined using a device such as an optical time-domain reflectometer and wavelength shift can

be calculated using an instrument implementing optical frequency domain reflectometry. Fiber

optic sensors are also immune to electromagnetic interference, and do not conduct electricity so

they can be used in places where there is high voltage electricity or inflammable material such as

jet fuel. Fiber optic sensors can be designed to withstand high temperature.

1.2.1 TYPES OF OPTICAL SENSORS

There are two types of optical sensors are commonly used

1. Intrinsic sensors

2. Extrinsic sensors

4

Fig 1.2 structure of an fiber optic sensors

1.2.1.1 Intrinsic sensors

Optical fibers can be used as sensors to measure strain, temperature, pressure and other

quantities by modifying a fiber so that the quantity to be measured modulates the intensity,

phase, polarization, wavelength or transit time of light in the fiber. Sensors that vary the intensity

of light are the simplest, since only a simple source and detector are required. A particularly

useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed

sensing over very large distances. Temperature can be measured by using a fiber that has

evanescent loss that varies with temperature, or by analyzing the Raman scattering of the optical

fiber. Electrical voltage can be sensed by nonlinear optical effects in specially-doped fiber,

which alter the polarization of light as a function of voltage or electric field. Angle measurement

sensors can be based on the Sagnac effect. Special fibers like long-period fiber grating (LPG)

optical fibers can be used for direction recognition. Optical fibers are used as hydrophones for

seismic and sonar applications. Optical fibers can be made into interferometric sensors such as

fiber optic gyroscopes, which are used in the Boeing 767 and in some car models (for navigation

purposes). They are also used to make hydrogen sensors. Fiber-optic sensors have been

developed to measure co-located temperature and strain simultaneously with very high accuracy

using fiber Bragg gratings.This is particularly useful when acquiring information from small

complex structures. Brillouin scattering effects can be used to detect strain and temperature over

larger distances (20–30 kilometers).

5

1.2.1.2 Extrinsic sensors

Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to

transmit modulated light from either a non-fiber optical sensor, or an electronic sensor connected

to an optical transmitter. A major benefit of extrinsic sensors is their ability to reach places

which are otherwise inaccessible. An example is the measurement of temperature inside aircraft

jet engines by using a fiber to transmit radiation into a radiation pyrometer located outside the

engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of

electrical transformers, where the extreme electromagnetic fields present make other

measurement techniques impossible. Extrinsic fiber optic sensors provide excellent protection of

measurement signals against noise corruption. Unfortunately, many conventional sensors

produce electrical output which must be converted into an optical signal for use with fiber. For

example, in the case of a platinum resistance thermometer, the temperature changes are

translated into resistance changes. The PRT must therefore have an electrical power supply. The

modulated voltage level at the output of the PRT can then be injected into the optical fiber via

the usual type of transmitter. This complicates the measurement process and means that low-

voltage power cables must be routed to the transducer. Extrinsic sensors are used to measure

vibration, rotation, displacement, velocity, acceleration, torque, and twisting.

1.3 ADVANTAGES OF FIBER OPTIC SENSORS

Optical sensors generally, and especially fiber optic ones, have technical and economic

advantages. The biocompatibility, reliability and non intrusive nature of fiber optics combined

with the possibility of making small, simple interfaces between the sensors and the points of

measurement make optical fiber sensor very suitable for detection, measurement and, in some

cases, procedures with biomedical variables. They can withstand, relatively, high temperature to

the high fusion of the optical fiber. The sizes and nature of optical fibers, small, light-weight

optical transducer can be made. The sensitivity, dynamic range and resolution can potentially be

much greater than conventional sensors.

6

CHAPTER-2

FIBER OPTIC TEMPERATURE SENSOR

7

CHAPTER 2

2.1 FIBER OPTIC TEMPERATURE SENSOR

There are many types of temperature sensors are used to measure the temperature

values. The sensors are Thermometer, Thermocoupler, Thermistors and Thermostats. But the

temperature values are not efficient. For a example the sensors are easily affected by

electromagnetic interference and cannot be used in harsh environments. The fiber optic

temperature sensors have no such limitations. The fiber optic temperature sensors are used to

harsh environments and also there is no electromagnetic interference. Optical fibers many feature

than that of the excellent sensors. The optical fiber does not need external electrical power

supply. The sensors are not affected by magnetic noise induced by lighting and power cables.

2.2 TYPES OF FIBER OPTIC TEMPERATURE SENSOR

2.2.1 A simple fiber optic temperature sensor

A simple but useful fiber optic temperature sensor is shown in figure 2.1. The basic idea is

to terminate the fiber in a capsule at a reflecting surface. On the other side of the surface, there is

an air bubble. This bubble will change dimensions with temperature, thus allowing the

thermosensitive cladding of the fiber to expand or contract. Consequently, the numerical aperture

and the attenuation of light in the fiber will change with temperature.

8

Fig 2.1 A simple fiber optic temperature sensor

2.2.2 Interferometric temperature sensors

Fiber optic interferometric techniques are applicable for measuring almost any physical

quantity. Since interferometry yields high resolution measurements of length, fiber optic

techniques have been developed where a measurand is converted into a change of displacement.

Temperature is one such measurand, and for anyone in need of high-precision temperature

measurements, fiber optic interferometer is an important alternative. Fiber optic versions of all

the well-known interferometers are available: Michelson, Fabry-Perot, Sagnac and, perhaps most

common, the Mach-Zender interferometer.

2.2.2.1 Fiber optic Fabry-Perot temperature sensor

Fig 2.2 Principle of a dual fiber-optic Fabry-Perot sensor

They use an LED as a (low-coherence) light source. The advantages of using a low-coherence

source in comparison with a semiconductor laser are, among others, that there is no need to

compensate for thermally induced shifts in the emission wavelength of the laser, that we are not

limited to select device for single-mode operation, and of course that LEDs are cheaper than

lasers. These advantages will have to be paid for with a somewhat smaller sensitivity. The sensor

uses two Fabry-Perot interferometers(FPI), one for sensing and one for reference. Similar sensors

using other types of interferometers also exist. Light from the LED is modulated by reflection,

first by the sensing FPI and then by the reference FPI. The power output is proportional to

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(1 + 0.5cos ∆p) where ∆p is the phase shift due to the perturbation applied to the sensor

interferometer. Measurements show that the phase shift is linearly dependent on temperature in a

range of 26 to 108 degrees Celsius.

Mach-Zehnder Interferometer Sensors

The Mach–Zehnder interferometer is a device used to determine the relative phase shift

variations between two collimated beams derived by splitting light from a single source.

Fig 2.3 principle of

mach-zender interferometer

The Mach–Zehnder interferometer is frequently used in the fields of aerodynamics, plasma

physics and heat transfer to measure pressure, density, and temperature changes in gases. In this

figure, we imagine analyzing a candle flame. Either output image may be monitored. The Mach–

Zehnder interferometer is a highly configurable instrument. The Mach–Zehnder interferometer is

a highly configurable instrument. . If it is decided to produce fringes in white light, then, since

white light has a limited coherence length, on the order of micrometers, great care must be taken

to simultaneously equalize the optical paths over all wavelengths or no fringes will be visible. As

seen in Fig 2.4, a compensating cell made of the same type of glass as the test cell (so as to have

equal optical dispersion) would be placed in the path of the reference beam to match the test cell.

Note also the precise orientation of the beam splitters. The reflecting surfaces of the beam

splitters would be oriented so that the test and reference beams pass through an equal amount of

10

glass. In this orientation, the test and reference beams each experience two front-surface

reflections, resulting in the same number of phase inversions. The result is that light traveling an

equal optical path length in the test and reference beams produces a white light fringe of

constructive interference.

Figure 2.4 Localized fringes result when an extended source is used in a Mach-Zehnder

interferometer.

Collimated sources result in a nonlocalized fringe pattern. Localized fringes result when

an extended source is used. In Fig. 2.4, the fringes can be adjusted so that they are localized in

any desired plane. In most cases, the fringes would be adjusted to lie in the same plane as the test

object, so that fringes and test object can be photographed together. The Mach–Zehnder

interferometer's relatively large and freely accessible working space, and its flexibility in

locating the fringes has made it the interferometer of choice for visualizing flow in wind tunnels

and for flow visualization studies in general. It is frequently used in the fields of aerodynamics,

plasma physics and heat transfer to measure pressure, density, and temperature changes in gases.

Mach–Zehnder interferometers are used in electro-optic modulators, electronic devices used in

various fibre-optic communications applications. Mach-Zehnder modulators are incorporated in

monolithic integrated circuits and offer well-behaved, high-bandwidth electro-optic amplitude

and phase responses over a multiple GHz frequency range. The versatility of the Mach–Zehnder

configuration has led to its being used in a wide range of fundamental research topics in quantum

mechanics, including studies on counterfactual definiteness, quantum entanglement, quantum

computation, quantum cryptography, quantum logic, Elitzur-Vaidman bomb tester, the quantum

eraser experiment, the quantum Zeno effect, and neutron diffraction.

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2.3 APPLICATIONS OF FIBER OPTIC TEMPERATURE SENSOR

Fiber optic thermo meters have invaluable measuring temperature in basic metals and

glass productions as well as in the initial hot forming process for materials. High temperature

processing operation in cement, refractory and chemical industries often use fiber optic

temperature sensing. Fiber optics are also used in fusion, sputtering, and crystal growth process

in the semiconductor company.

12

CHAPTER-3

TIME DIVISION MULTIPLEXING (TDM)

13

CHAPTER 3

TIME DIVISION MULTIPLEXING (TDM)

3.1 INTRODUCTION

Time-division multiplexing (TDM) is a method of transmitting and receiving independent

signals over a common signal path by means of synchronized switches at each end of the

transmission line so that each signal appears on the line only a fraction of time in an alternating

pattern. Time-division multiplexing is used primarily for digital signals, but may be applied in

analog multiplexing in which two or more signals or bit streams are transferred appearing

simultaneously as sub-channels in one communication channel, but are physically taking turns

on the channel. The time domain is divided into several recurrent time slots of fixed length, one

for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during time

slot 1, sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-

channel plus a synchronization channel and sometimes error correction channel before the

synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts

all over again with a new frame, starting with the second sample, byte or data block from sub-

channel 1, etc.Systems based on TDM techniques rely upon a synchronized clock frequency and

timing to separate the multiplexed channels.

TDM is comprised of two major categories: TDM and synchronous time division multiplexing (sync

TDM). TDM is used for long distance communication links and bears heavy data traffic loads from end

users. Sync TDM is used for high speed transmission. During each time slot a TDM frame (or data

packet) is created as a sample of the signal of a given sub-channel; the frame also consists of a

synchronization channel and sometimes an error correction channel. After the first sample of the

given sub-channel (along with its associated and newly created error correction and

synchronization channels) are taken, the process is repeated for a second sample when a second

frame is created, then repeated for a third frame, etc; and the frames are interleaved one after the

other. When the time slot has expired, the process is repeated for the next sub-channel.

Fig 3.1 schematic of optical time division multiplexing

14

The time division multiplexing circuit are used to adds a different time delay for different

units. In traditional TDM systems, the allocation of bandwidth is quantum-based, i.e., a fixed

number of time-slots. The time delay circuit is used for different units and in different time slots.

The TDM circuit is run based on the round robin scheduling. The outputs sources are given to

the input of the TDM circuit.

3.2 Round robin scheduling

Round robin scheduling are generally used time slices are assigned to each process in equal

portions and in circular order, handling all processes without priority (also known as cyclic

executive). Round-robin scheduling is simple, easy to implement, and starvation-free. Round-

robin scheduling can also be applied to other scheduling problems, such as data packet

scheduling in computer networks. The scheduling are based on two types,

1) Process scheduling

2) Network packet scheduling

3.2.1 Process scheduling

The schedule processes fairly, a round-robin scheduler generally employs time-sharing,

giving each job a time slot or quantum (its allowance of CPU time), and interrupting the job if it

is not completed by then. The job is resumed next time a time slot is assigned to that process. In

the absence of time-sharing, or if the quanta were large relative to the sizes of the jobs, a process

that produced large jobs would be favoured over other processes.

For example, if the time slot is 100 milliseconds, and job1 takes a total time of 250 ms to

complete, the round-robin scheduler will suspend the job after 100 ms and give other jobs their

time on the CPU. Once the other jobs have had their equal share (100 ms each), job1 will get

another allocation of CPU time and the cycle will repeat. This process continues until the job

finishes and needs no more time on the CPU.

Job1 = Total time to complete 250 ms (quantum 100 ms).

1. First allocation = 100 ms.

2. Second allocation = 100 ms.

3. Third allocation = 100 ms but job1 self-terminates after 50 ms.

4. Total CPU time of job1 = 250 ms

15

Another approach is to divide all processes into an equal number of timing quanta such that

the quantum size is proportional to the size of the process. Hence, all processes end at the

same time.

3.2.2 Network packet based scheduling

In packet switching and other statistical multiplexing, round-robin scheduling can be used

as an alternative to first-come first-served queuing. A multiplexer, switch, or router that provides

round-robin scheduling has a separate queue for every data flow, where a data flow may be

identified by its source and destination address. The algorithm lets every active data flow that has

data packets in the queue to take turns in transferring packets on a shared channel in a

periodically repeated order. The scheduling is work-conserving, meaning that if one flow is out

of packets, the next data flow will take its place. Hence, the scheduling tries to prevent link

resources from going unused.

Round-robin scheduling results in max-min fairness if the data packets are equally sized,

since the data flow that has waited the longest time is given scheduling priority. It may not be

desirable if the size of the data packets varies widely from one job to another. A user that

produces large packets would be favored over other users. In that case fair queuing would be

preferable.

If guaranteed or differentiated quality of service is offered, and not only best-effort

communication, deficit round-robin (DRR) scheduling, weighted round-robin (WRR)

scheduling, or weighted fair queuing (WFQ) may be considered.

In multiple-access networks, where several terminals are connected to a shared physical

medium, round-robin scheduling may be provided by token passing channel access schemes such

as token ring, or by polling or resource reservation from a central control station.

In a centralized wireless packet radio network, where many stations share one frequency

channel, a scheduling algorithm in a central base station may reserve time slots for the mobile

stations in a round-robin fashion and provide fairness. However, if link adaptation is used, it will

take a much longer time to transmit a certain amount of data to "expensive" users than to others

since the channel conditions differ. It would be more efficient to wait with the transmission until

the channel conditions are improved, or at least to give scheduling priority to less expensive

users. Round-robin scheduling does not utilize this. Higher throughput and system spectrum

efficiency may be achieved by channel-dependent scheduling, for example a proportionally fair

16

algorithm, or maximum throughput scheduling. Note that the latter is characterized by

undesirable scheduling starvation. This type of scheduling is one of the very basic algorithms for

Operating Systems in computers which can be implemented through circular queue Data

Structure.

3.3 TYPES OF TDM

The TDM circuit are divided into two types. They are

1) Synchronous TDM

2) Asynchronous TDM

3.3.1 Synchronous TDM

Fig 3.2 synchronous TDM

Synchronous TDM works by the multiplexor giving exactly the same amount of time to

each device connected to it. This time slice is allocated even if a device has nothing to transmit.

This is wasteful in that there will be many times when allocated time slots are not being used.

Therefore, the use of Synchronous TDM does not guarantee maximum line usage and efficiency.

3.3.2 Asynchronous TDM

Fig 3.3 Asynchnronous TDM

17

Asynchronous TDM is a more flexible method of TDM. With Asynchronous TDM the length of

time allocated is not fixed for each device but time is given to devices that have data to transmit.

This version of TDM works by tagging each frame with an identification number to note which

device it belongs to. This may require more processing by the multiplexor and take longer,

however, the time saved by efficient and effective bandwidth utilization makes it worthwhile.

Asynchronous TDM allows more devices than there is physical bandwidth for. This type of

TDM is used in Asynchronous Transfer Mode (ATM) networks.

18

CHAPTER-4

WAVELENGTH DIVISION MULTIPLEXING (WDM)

19

CHAPTER 4

WAVELENGTH DIVISION MULTIPLEXING (WDM)

4.1 Introduction to WDM

Wavelength Division Multiplexing (WDM) involves the transmission of a number of

different peak wavelength optical signals in parallel on a single optical fiber. The WDM standard

developed by the International Telecommunication Union (ITU) specifies channel spacing’s in

terms of frequency. WDM is a method of combining multiple services on a single fiber specified

by ITU-TG.692. Many different wavelengths can be sent along a fiber simultaneously in the

1300-1600 nm spectrum. This is achieved through WDM. WDM is nothing but N independent

optically formatted information streams each transmitted at a different wavelength and combined

with optical multiplexer and sent over the same fiber. The wavelength in WDM must be properly

spaced to avoid inter channel interference. The wdm demux are used to demultiplexes a user

define number of wdm sigal channels. The delayed signal is demultiplexed with wdm demux.

The wdm channel frequency is calculated by using line width and frequency of input source. The

line width is in the range of mhz and default frequency is 193.1 thz. . Improvements in optical

fibers and narrowband lasers which lead to the birth of Dense WDM (DWDM) and Coarse

Wavelength Division Multiplexing (CWDM).

4.2 TYPES OF WDM

There are two main types of Wavelength Division Multiplexing

Coarse Wavelength Division Multiplexing (CWDM).

Dense Wavelength Division Multiplexing (DWDM).

4.2.1Coarse Wavelength Division Multiplexing

Coarse wavelength division multiplexing (CWDM) is a method of combining multiple

signals on laser beams at various wavelengths for transmission along fiber optic cables, such that

the number of channels is fewer than in Dense Wavelength Division Multiplexing (DWDM) but

20

more than in standard wavelength division multiplexing (WDM). CWDM systems have channels

at wavelengths spaced 20 nanometers (nm) apart, compared with 0.4 nm spacing for DWDM.

This allows the use of low-cost, uncooled lasers for CWDM. In a typical CWDM system, laser

emissions occur on eight channels at eight defined wavelengths 1610 nm, 1590 nm, 1570 nm,

1550 nm, 1530 nm, 1510 nm, 1490 nm, and 1470 nm. But up to 18 different channels are

allowed, with wavelengths ranging down to 1270 nm. The energy from the laser in a CWDM

system is spread out over a larger range of wavelengths than is the energy from the lasers in a

DWDM system. The tolerance (extent of wavelength imprecision or variability) in a CWDM

laser is up to ± 3 nm, whereas in a DWDM laser the tolerance is much tighter. Because of the use

of lasers with lower precision, a CWDM system is less expensive and consumes less power than

a DWDM system. However, the maximum realizable distance between nodes is smaller with

CWDM. The total CWDM optical span to somewhere near 60 km for a 2.5 Gb/s signal, is

suitable for use in metropolitan applications. The relaxed optical frequency stabilization

requirements allow the associated costs of CWDM to approach those of non-WDM optical

components. CWDM is also used in cable television networks. The wavelength spacing for

CWDM is shown in figure 4.1.

Fig 4.1

Wavelength spacing for CWDM

4.2.2 Dense Wavelength Division Multiplexing

The demand for bandwidth has increased in an extremely fast pace over the past few

years and is likely to continue to do so for the near future. To be able to meet the customer’s

needs, the only solution is to increase their channel capacity at the lowest cost possible. There are

three totally di erent approaches: Laying new fibres, increasing the data rate and using severalff

wavelengths simultaneously in one fibre. Laying new fibre is most often not a valid solution, as

21

the costs are too high. Increasing the data rate is basically possible, but there are limits to the

speed one can achieve and hardware gets highly expensive for fast data rates. For most cases,

dense wavelength division multiplexing (DWDM) seems to be a promising solution. In DWDM,

several di erent wavelengths are transmitted over one single mode fibre at the same time. Theff

channel spacing is below 200 GHz, and the number of channels is greater than 32. DWDM

system is explained in figure 4.2.

Using several wavelengths at once dense wavelength division multiplexing circumvents

these problems, as the data rate stays the same as before and no new fibers are needed.

Fig 4.2 DWDM system

DWDM also has other benefits: It is highly scalable and can be built in a modular way.

This allows telecommunication providers to install just the amount of hardware they actually

need an upgrade their networks slowly, when needed.

DWDM systems have their capability of being compatible to existing hardware, being

modular and having the ability of saving a lot of costly equipment if designed properly.

The key components for WDM systems are the multiplexers and the demultiplexers. They

allow the combining of several wavelengths into one fibre and the separation of those

wavelengths at the end of the fibre.

4.3 FEATURES OF DWDM

4.3.1 Capacity Upgradation

22

The application of WDM is to upgrade the capacity of existing point to point fiber

optic transmission links.Each channel supports gigabits per second. Wavelength division

multiplexing increases the capacity.

4.3.2 Transparency

In WDM, each optical channel can carry any transmission format, any type of

information, analog or digital can be sent simultaneously over the same fiber.

4.3.3 Wavelength routing

Wavelength routed networks use the actual wavelength of a signal as the intermediate or

final address. Wavelength sensitive optical routing devices are used in wavelength routing.

Wavelength is used as another dimension.

4.3.4 Wavelength switching

Wavelength switching architectures allow reconfiguration of the optical layers.

Wavelength Spacing for 100 GHz is shown in figure 4.3.

Fig 4.3 Wavelength spacing for 100GHz DWDM system

23

CHAPTER-5

BLOCKDIAFRAM AND DESCRIPTION

CHAPTER 5

24

BLOCK DIAGRAM AND DESCRIPTION

5.1 BLOCK DIAGRAM

In this block diagram consists of five units. They are optical source, time division

multiplexing unit (TDM), wavelength division multiplexing(WDM), optical sensing unit(OSU),

receiver.

Fig 5.1 Block diagram

5.2 BLOCK DESCRIPTION

5.2.1 Laser source

In an optical source the laser signal are used as a input. Here laser signal are used

to generates a continuous wave signal. The output of the optical signal given to the TDM circuit.

5.2.2 TIME DIVISION MULTIPLEXING (TDM)

25

The TDM circuit is used to adds a time delay to the optical input. In our project the time

delay circuit is used to different time slots for different units to identify the temperature

sensing.

5.2.3 WAVELENGTH DIVISION MULTIPLEXING (WDM)

It is used to demultiplexes a user-define number of WDM signal channels. The delayed

signal is demultiplexed with WDM demux circuit. The demultiplexed output is given to the

optical sensing element.

5.2.4 OPTICAL SENSING UNIT (OSU)

The optical sensing unit are used to sense the temperature measurement. The optical

fiber are acts as a sensing element. It simulates temperature dependance effect.

5.2.5 RECEIVER UNIT

This unit consists of avalanche photo diode and power meter. The avalanche photo

diode are used to detect the erbium doped fiber output. Power meter are used to get a output

from photo detector.

26

CHAPTER- 6

SYSTEM MODELLING

27

CHAPTER 6

SYSTEM MODELLING

6.1 OPTISYSTEM

OptiSystem is an innovative optical communication system simulation package which was

explored by optiwave company in order to meet the academic requirement of the system

designers, optical communications engineers, researchers.

It integrates design, test and optimize all types of broadband optical network physical

layer functions such as virtual optical connection. From the long-distance communication

systems to LANS (Local area network) and MAN (Wireless local area network), it can be well

used. It has a huge database of active and passive components, including power, wavelength, loss

and other related parameters. Parameters allow the user to scan and optimization of device-

specific technical parameters on the system performance.

Opti-System has powerful simulation environment and real components and systems of

classification definitions. OptiSystem allows for the design automation of virtually any type of

optical link in the physical layer, and the analysis of a broad spectrum of optical networks, from

long-haul systems to MANs and LANs. OptiSystem’s wide range of applications

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6.1.1 PROJECT LAYOUT

Project layout window

Fig 6.1 Optisystem GUI

The Main layout work area is initially set to 3000 X 2000 units. This is not a fixed size and

can be changed to suit the needs of different systems projects. There are several ways to change

the size of the layout.

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6.2 SIMULATION LAYOUT

Fig 6.1.1 Simulation layout to sense the temperature

30

The set-up shown in Figure 6.2 can be obtained by choosing the desired components

from the component library. The component can be taken by double clicking the component on

the library and then place that component on the design area. Place the desired components in an

order and the join them by the fibers. Choose the desired parameters of the different components

by clicking right on the component. After making the set-up we have to check whether there is

any error in choosing the parameters or the system. After checking the system, data can be

displayed through the data display.

6.2.1 CW Laser

This model implements a simplified continuous wave (CW) laser. Laser

phase noise is taken into account by generating a Lorentzian emission line shape whose FWHM

is specified by the parameters. It generates continuous wave optical signal.

Frequency: set the value of frequency is 193.1THZ

Power: set the value of power is 20dBm

Line width: set the value of line width is 10MHZ

Noise Bandwidth: set the value of noise bandwidth is 0 THZ

Noise Threshold: set the value of noise threshold is -100dB

Noise Dynamic: set the value of noise dynamic is 3 dB

6.2.2 Time delay

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Delay is an audio effect which records an input signal to an audio storage

medium, and then plays it back after a period of time. The delayed signal may either be played

back multiple times, or played back into the recording again, to create the sound of a repeating,

decaying echo.

Time Delay: set the value time delay in the range of s

6.2.3 WDM Demux

In fiber-optic communications, wavelength-division multiplexing (WDM) is a

technology which multiplexes a number of optical carrier signals onto a single optical fiber by

using different wavelengths of laser light. This technique enables bidirectional communications

over one strand of fiber, as well as multiplication of capacity.

The term wavelength-division multiplexing is commonly applied to an optical carrier

(which is typically described by its wavelength), whereas frequency-division multiplexing

typically applies to a radio carrier (which is more often described by frequency). Since

wavelength and frequency are tied together through a simple directly inverse relationship, in

which the product of frequency and wavelength equals c (the propagation speed of light), the two

terms actually describe the same concept.

Number of Outputs Ports: set the number of ports value

Bandwidth [Hz] or[GHz] or[THz] or[nm]: set the bandwidth range in GHZ

Frequency: set the frequency range in THZ

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6.2.4 Erbium Doped Fiber

This component simulates a bidirectional Erbium doped fiber considering ESA,

Rayleigh scattering, ion-ion interactions, and temperature dependence effects. The component

solves numerically the rate and propagation equations in the steady-state case, assuming a two-

level Erbium system for an inhomogeneous and homogeneous approach.

The following parameters are discussed are:

Length(L): set the length range in meter

Temperature : set the temperature range in Celsius

6.2.5 GROUND

6.2.6 OPTICAL NULL

Generates a zero-value optical signal.

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6.2.7 AVALANCHE PHOTO DIODE

An avalanche photodiode (APD) is a highly sensitive semiconductor electronic

device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of

as photodetectors that provide a built-in first stage of gain through avalanche multiplication.

From a functional standpoint, they can be regarded as the semiconductor analog to

photomultipliers. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs

show an internal current gain effect (around 100) due to impact ionization (avalanche effect).

However, some silicon APDs employ alternative doping and beveling techniques compared to

traditional APDs that allow greater voltage to be applied (> 1500 V) before breakdown is

reached and hence a greater operating gain (> 1000).

Gain : set the gain range in 3

Responsivity : set the resposivity range in 1A/W

Ionization Ratio : set the ionization range in 0.9

Dark current : set the dark current range in 10Na

6.2.8 Optical power meter

This visualizer allows the user to calculate and display the average power of optical

signals. It can also calculate the power for polarizations X and Y

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6.2.9 Electrical power meter

This visualize allows the user to calculate and display the average power of electrical signals. It

can also calculate the AC and DC power.

35

CHAPTER-7

SIMULATION RESULTS AND DISCUSSION

36

CHAPTER 7

SIMULATION RESULTS AND DISCUSSIONS

7.1 SIMULATION LAYOUT

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Fig 7.1 Simulation layout

The simulation layout for sensing the temperature is shown in the figure 7.1. The laser source is

used as a input source is give continuous wave optical signal. The default frequency of laser

source is 193.1THZ.and the line width is in the range of MHz .and power value is 20dB.The

output of laser source is given to the time delay circuit. It adds a time delay. The time delay

output is given to theWDM DEMUX. The WDM DEMUX channel value is calculated by using

line width and frequency of the input source.

Here erbium doped fiber act as a sensing element, it is used to measure the temperature. In

normal considerations the fiber will be sensed but in the opti-system tool there are no

possibilities. That’s why here erbium doped fiber is used to sense the temperature. Here erbium

doped fiber has acts as a different temperature range.

The above diagram is one unit. For a more number of units the sensing area will be large by

using of the hybrid TDM/WDM circuit. Here the fiber is not affected by electromagnetic

interference, and also it is used in harsh environments. The optical fiber does not need external

power supply. The photo detector is used to detect the output of the erbium doped fiber

output.The output power is measured by electrical power meter.

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Fig 7.2 Output window

OUTPUTS

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TEMPERATURE ( C) POWER (WATT)

30 82.605

30.5 82.602

31 82.600

31.5 82.596

32 82.595

32.5 82.591

33 82.590

33.5 82.587

Table 1: temperature(degree Celsius) vs power(watts)

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TEMPERATURE ( C) POWER (WATT)

120 82.255

120.5 82.254

121 82.252

121.5 82.251

122 82.249

122.5 82.248

123 82.245

123.5 82.243

Table 2: temperature(degree Celsius) vs power(watts)

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TEMPERATURE ( C) POWER (WATT)

180 82.099

180.5 82.096

181 82.096

181.5 82.095

182 82.094

182.5 82.093

183 82.091

183.5 82.091

Table 3: temperature(degree Celsius) vs power(watts)

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The temperature (in Celsius) will be increased power(watts) will be reduced. This is used to

determine where the temperature will be changed. The avalanche photo detector is used to detect

the temperature ranges.

FUTURE WORK

Fig 7.3 simulation diagram with phase shift

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In the figure 7.3 phase shift and mach-zender interferometer are used. The input source is a

laser source. In the laser source the default frequency value is 193.1 THz. The ouput of the laser

source are given to the time delay circuit. The time delay circuits are adds different time delay.

The output is given to the WDM demux. The WDM demux circuit demultiplexes the time delay

signal and the WDM channel range is calculated by line width and frequency of the laser source.

The fork is used to get a duplicate channel. The one WDM output is given to the mach Zender

interferometer. The other WDM output is given to the optical fiber and phase shift. Phase shift

and temperature are linearly dependent. Mach zender interferometer is used to splitted

collimated beams into a single light source. The phase shift value is given by using temperature.

So the output range is more efficient. If the material can be changed for example different fibers

the phase shift value will be changed due to the temperature.

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CONCLUSION

CONCLUSION

45

In this project “FIBER OPTIC SENSORS BASED ON HYBRID TDM/WDM

TECHNIQUE”, a solution of sense the temperature is proposed by using of the hybrid

TDM/WDM network. The results indicate that the system can sense a temperature over a long

distance by using time delay unit and WDM demux unit. The time delay unit adds a different

time delay and a WDM demux is used to produce a different frequency channel. Each of the

frequency channels is given to the optical sensing unit. So the system will be sense the

temperature over a long distance. The system is flexible. Therefore this system has a very good

application project.

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REFERENCES

REFERENCES

47

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