UNIT 1

What are the various elements of an optical communication system? Explain each element in brief?

Ans: Optical Fiber Communication System:

The figure 1.1 shows a block schematic of the different elements in an optical fiber communication system. The carrier is modulated using analog information signal. The variation of light emitting from the optical source is a continuous signal. The information source provides an electrical signal to the transmitter. The transmitter comprises electrical stage. The electrical stage (circuits) drives an optical source. The optical source output is a light which is intensity modulated by the information. The optical source converts the electrical signal into an optical signal. The source may be either semiconductor laser or Light Emitting Diode (LED). The intensity modulated light signal is coupled to fiber. The fiber which is made up of a glass acts as a channel between the transmitter and receiver.

At the receiver the optical signal is detected by the optical detectors such as PIN diode and Avalanche photodiode. Sometimes photo transistors and photo conductors are used for converting an optical signal into electrical signal. The electrical signal is again processed and given to the transducer to get the original information.

Give the block diagram of a digital optical communication system and explain the function of each block?

Ans: Digital Fiber optical Communication System

Figure shows a schematic of a typical digital optic fiber link. The input is given as digital signal from the information source and it is encoded for optical transmission in the encoder. The encoder, encodes or modulates the digital signal as in the case of simple communication system where we are using a message signal in which the signal is in analog form, but here the signal is in digital form which is encoded i.e., modulated in the encoder. The laser drive circuit directly modulates the intensity of semiconductor laser with the encoded digital signal. Hence a digital optical signal is launched into the optical fiber cable. At the receiver we have to decode the digital optical signal for which we are using another Avalanche Photo Diode (APD) as detector. The avalanche photo diode detector is followed by a front-end amplifier and equalizer or filter to provide gain as well as linear signal processing and noise bandwidth reductions. Then the signal is passed through the decoder to get original digital information which is transmitted

Distinguish between optical fiber communication system and conventional communication system? And List out the advantageous and disadvantage of optical fiber communication?

Ans:

Optical Fiber Communication System

Conventional Communication System

1. Requires a bandwidth of 1013 to 1016 Hz.

1. Requires a bandwidth of 500 MHz

2. Light weight

2. Heavier in weight.

3. Immune to R.F. interference

3. Needs external shielding.

4. Electrical isolation.

4. Exhibits earthing problems.

5. Low loss of about 0.2 dB/km.

5. Loss of about 10dB/km.

6. Secure signal propagation.

6. Signal can be tapped easily.

7. Due to increased bandwidth higher data

7. Low data rates compared to optical fiber.

Advantageous Of Optical Fibers Communication:

1. Information bandwidth is more.

1. Optical fibers are small in size and light weighted.

1. Optical fibers are more immune to ambient electrical noise, electromagnetic interference.

1. Cross talk and internal noise are eliminated in optical fibers.

1. There is no risk of short circuit in optical fibers.

1. Optical fibers can be used for wide range of temperature.

1. A single fiber can be used to send many signals of different wavelengths using Wavelengths Division Multiplexing (WDM).

1. Optical fibers are generally glass which is made up of sand and hence they are cheaper than copper cables.

1. Optical fibers are having less transmission loss and hence less number of repeaters are used.

1. Optical fibers are more reliable and easy to maintain.

Disadvantageous Of Optical Fibers Communication:

1. Attenuation offered by the optical fibers depends upon the material by which it is made.

1. Complex electronic circuitry is required at transmitter and receiver.

1. The coupling of optical fibers is difficult.

2. Skilled labors are required to maintain the optical fiber communication.

2. Separated power supply is required for electronic repeaters at different stages.

Compare the advantages and disadvantages of guided optical communication lines with that of microwave systems?

Ans:

Optical Communication System

Microwave System

1. Uses glass optical fibers or plastic optical fibers for transmission.

1. Uses co-axial cable or microwave waveguides for transmission.

2. Low weight, hence large transmission distance or same weight of microwave link.

2. Heavier than optical fibers.

3. Large bandwidth of range 1013 to l016Hz.

3. Bandwidth is lesser in the range of 108 to 1010Hz.

4.Electrically isolated, hence no shielding is required.

4. Prone to electrical disturbances and hence, shielding for reducing RE interference.

5. Low loss of 0.2dB/km.

5. A considerable loss of 5 dB/km.

6. Large spacing between repeaters about 1 in 300 km.

6. Spacing distance between repeaters is less, is suitable only for short distance if waveguides are used.

7. Because large bandwidth, higher data rate of the order of terabits per second.

7. Data rates of mega bits per second can be obtained.

8. Message security is obtained.

8. Signal can be tapped easily.

9. No cross talk, hence many fiber communication channels can be packed inside one single cable.

9. If shielding is not done properly, cross talk is introduced.

Disadvantages

1. Expensive transmitter and receiver.

1. Simple and less expensive transmitter and receiver.

2. Difficult coupling.

2. Easy coupling.

3. Power transmission depends upon the quantum efficiency of light source (LED or LASER).

3. Output power is directly coupled to the transmission line.

4. Unable to excite the terminal device directly.

4. Able to operate the terminal device directly.

Write in detail about ray optics?

Ans: Ray optics is used for representing the mechanism of a ray which propagates through an ideal multimode step index optical waveguide. There are two types of rays, the skew rays and meridional rays which propagate through a fiber.

The path of meridional can be tracked very easily as they are confined to a single plane. Meridional are described in two classes. They are,

1. Bound rays

1. Unbound rays

Bound rays are those rays which are trapped in a core and they move along the fiber whereas unbound rays are those rays which get refracted out of the fiber.

Skew rays are those rays which follow helical path but they are not confined to a single plane.

We know that skew rays are not confined to a particular plane so they cannot be tracked easily.

Analyzing the meridional rays is sufficient for the purpose of result, rather than skew rays, because skew rays lead to greater power loss.

Now coming to ray theory, we need to consider meridional rays. Representation of meridional rays is given below.

From the medium of refractive index 'n' which is at an angle 0with respect to fiber axis, the light enters the fiber core. If the light strikes at such an angle then it gets reflected internally and the meridional ray moves in a zig zag path along the fiber core, passing through the axis of the guide. Now by using Snell's law the minimum angle min supports total internal reflection for meridional ray is given by

If the ray strikes the core-cladding interface at an angle less than min then they get refracted out of the core and they will be lost from the cladding.

By applying Snells law to the air-fiber face boundaries, we get max

nsin max = n1 sin c = (n n )1/2

Where c = /2 0 (From the figure)

So, the rays whose entrance angle 0 is less than the max will be reflected back in to core cladding interface.

Numerical aperture for a step index is given by the formula N.A = n sin max

(n n )1/2 = n12

An optical fiber has a NA of 0.20 and a cladding refractive index of 1.59 Determine

0. The acceptance angle for the fiber in water which has a refractive index of 1.33

0. Critical angle at the core cladding interface.

Ans:

Given NA = 0.2 n1 =1.59

(i) The acceptance by the water is Refractive index for water n =1.33 NA = n sin a

a= sin-1(NA/n) = sin-1(0.2/1.59) = 8.640

Therefore the acceptance angle is = 8.640

(ii) Critical angle at core cladding interface is

We know that

NA= (n12-n22)1/2

We know that,

NA = 0.2 and n1=1.59 0.2 = (1.592-n22)1/2

0.447= (1.592- n22)

n22 = 2.081

c= n sin-1(n2/ n1) = 1.33 sin-1(1.44/ 1.59) = 86.330

Define an optical fiber. Explain in detail different types of optical fibers giving neat sketches?

Ans: A dielectric waveguide that operates at optical frequencies is known as optical fiber. It is generally available in cylindrical form.

Fiber Types:

There are two fiber types

0. Step index fiber

0. Graded index fiber.

1. Step Index Fiber

Step index fiber is further divided in two types,

1. Single mode step index fiber

1. Multi mode step index fiber.

Single mode step index fiber is shown below,

The typical dimension of core is 8 to 12 m and cladding is 125 m.

In step index fiber, the refractive index of the core is uniform and at the cladding boundary, it undergoes a step change.

In single mode step index fiber, there is only one mode of propagation. The multimode step index fiber is shown below,

In multimode step index fiber, hundreds of modes are present.

The typical dimension of core is 50 to 200 m and cladding is 125 to 400 m. Multimode fiber has several advantages, which includes, the transmitting the light directly in to fiber using LED.

Graded Index Fiber

Graded index fiber also contains single mode and multimode. The multimode graded index fiber is shown below,

In graded index fiber, the refractive index of the core is made to vary as a function of radial distance taken from the center of the fiber.

The dimension of its core is 50 to 100 m and cladding is 125 to 140 m.

In both cases (step index and graded index) multimode has several advantages. When compared with single mode, however, multimode has a drawback, that is, it suffers from inter model dispersion.

Compare the fiber structure and numerical aperture in step index and graded index fiber?

Ans: Fiber structure:

A fiber consists of a single solid dielectric cylinder of radius V and refractive index n{ called as core of the fiber. The core is surrounded by a solid dielectric cladding with refractive index n2 that is less than n1 The variation of material composition of core give rise to the two commonly used fiber types (i). If the refractive index of the core is uniform throughout and undergoes an abrupt change at the cladding boundary then such a fiber is called step index fiber (ii). If the core refractive index gradually varies along the radial distance from the centre of the fiber and becomes equal to the refractive index of the cladding at the boundary, then such a fiber is called graded-index fiber.

The step-index and graded-index fibers are further divided into single mode and multimode fibers The core radius in single mode fiber is very small hence only one mode of propagation is possible and laser diode is-required to launch the light beam m the fiber. Multimode fibers has larger core radius and hence supports many hundreds of modes of propagation. Due to larger core radius a CED is sufficient to launch the light beam into fiber making it less expensive than single mode fibers. But multi mode fibers suffer from inter model dispersion.

Numerical Aperture:

There are two types of rays that can propagate through fiber, they are meridional rays and skew rays. Meridional rays are confined to the meridian planes of fiber which contains core axis whereas skew rays are not confined to a single plane, but instead tend to follow a helical path along the fiber. To obtain the general condition of ray propagation through fiber meridional rays are considered.

(i) Stepindex Fiber

Consider a step index fiber with core radius a and refractive index n1 and with a cladding of refractive index n2 which is lower than n1, then we can say

n2 = n1(1- )

Where 'A' is called the core-cladding index difference, when a light ray enters the fiber core from a medium of refractive index at an angle and strikes the core-cladding boundary at a normal

angle such that it results m total internal reflection. Then the angle should not be less min than given by Snells law,

Sin min= n2/ n1

By applying Snell's law to air-fiber face boundary and using equation (1) it can be related to maximum entrance angle max given by,

n sin imax = n1sin c = (n12-n22)1/2 where c = /2

Therefore for step index the numerical aperture is given by,

NA = n sin imax = (n12-n22)1/2 = n12

(ii) Graded-Index Fiber

For a graded index fiber the refractive index difference is given by,

is approximately equal in both step-index fiber and graded index fiber.

Numerical aperture of graded index fiber is a function of position across, the case end face, whereas, NA is step-index is constant across the core. The light incident on the fiber core at position r will propagate through fiber only if it is within the local numerical aperture of the fiber at that position given by,

Where, r is the radial distance from the centered the fiber V is the radius of core a is dimensionless parameter defining the shape of index profile and NA(0) is axial numerical aperture defined as,

NA(0) = (n2(0) - n )1/2

from centre to core-cladding boundary i.e., at centre NA is equal to that of step index and gradually reduces until it becomes zero at the core-cladding boundary.

Give three applications of optical fiber in instrumentation and explain them with necessary figure?

Ans: Optical fibers are used as sensing-elements (sensors) in instrumentation applications. Since, they have the advantage of efficient telemetry and control communication they can also work in electrically harsh environments and are free from EM interference.

The optical fiber sensor system modulates a light beam either directly or indirectly by the parameters like temperature, pressure, displacement, strain etc. Modulation is done in the modulation zone of the optical fiber sensor system as shown in figure 9.1. The light beam is modulated in any of its parameters, which includes optical intensity, phase, polarization, wavelength and spectral distribution.

(i) Optical Fluid Level Detector

Figure (9.2) shows the functioning of a simple optical fluid level detector. It contains an optical source, optical detector, optical dipstick and fluid. The optical dipstick is formed by glass (with refractive index 1) and fluid has a refractive index 2. The refractive index of fluid is greater than refractive index of optical dipstick (1>2). When the fluid does not touch the optical dipstick the light beam from optical source passes through the glass as shown in figure 9.2(a). When the fluid touches the chamfered end, total internal reflection halts and the light is transmitted into the fluid as shown in figure 9.2(b). As a result, an indication of the fluid level is acquired at the optical detector.

1. Optical Displacement Detector

This is also implemented as extrinsic device. The received light ray is modulated by intensity. The reflected light from the target is received and the intensity of received light is proportional to distance/displacement of target. Thus, displacement is measured.

(iii) Optical Fiber Flow Meter

This is implemented as intrinsic device, where the flow rate itself causes the modulation of light.

A multimode fiber is placed along the cross-section of flow pipe, so that liquid flow pass the fiber. Presence of fiber causes turbulence in the liquid flow as a result fiber oscillates and frequency of oscillation is directly proportional to flow rate. This oscillation gives a modulated light at the receiver. Thus, flow rate is measured

A single Mode step index fiber has a core diameter of 7m and core refractive index of 1.49.Estimate the shortest wavelength of light which allows single mode operation when the refractive index difference for the fiber is 1%?

Ans; Given that

For a single mode step index fiber, n1 = 1.49

2a = 7m => a = 3.5 m = 0.01

We have

n2 = n1 (1- )

1.49(1-0.01)

1.4751 Therefore n2 =1.48

OPTICAL FIBRE SYSTEM

An optical fiber (or optical fiber) is a flexible, transparent fiber made of extruded glass (silica) or plastic, slightly thicker than a human hair. It can function as a waveguide, or light pipe, to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of optical fibers is known as fiber optics.

Optical fibers are widely used in fiber-optic communications, where they permit transmission over longer distances and at higher bandwidths (data rates) than wire cables. Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so that they may be used to carry images, thus allowing viewing in confined spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.

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).

How a Fiber Optic Communication Works?

Unlike copper wire based transmission where the transmission entirely depends on electrical signals passing through the cable, the fiber optics transmission involves transmission of signals in the form of light from one point to the other. Furthermore, a fiber optic communication network consists of transmitting and receiving circuitry, a light source and detector devices like the ones shown in the figure.

When the input data, in the form of electrical signals, is given to the transmitter circuitry, it converts them into light signal with the help of a light source. This source is of LED whose amplitude, frequency and phases must remain stable and free from fluctuation in order to have efficient transmission. The light beam from the source is carried by a fiber optic cable to the destination circuitry wherein the information is transmitted back to the electrical signal by a receiver circuit.

The Receiver circuit consists of a photo detector along with an appropriate electronic circuit, which is capable of measuring magnitude, frequency and phase of the optic field. This type of communication uses the wave lengths near to the infrared band that are just above the visible range. Both LED and Laser can be used as light sources based on the application.

Basic Elements of a Fiber Optic Communication System

There are three main basic elements of fiber optic communication system. They are

Compact Light Source

Low loss Optical Fiber

Photo Detector

Accessories like connectors, switches, couplers, multiplexing devices, amplifiers and splices are also essential elements in this communication system.

1. Compact Light Source

Laser Diodes

Depending on the applications like local area networks and the long haul communication systems, the light source requirements vary. The requirements of the sources include power, speed, spectral line width, noise, ruggedness, cost, temperature, and so on. Two components are used as light sources: light emitting diodes (LEDs) and laser diodes.

The light emitting diodes are used for short distances and low data rate applications due to their low bandwidth and power capabilities. Two such LEDs structures include Surface and Edge Emitting Systems. The surface emitting diodes are simple in design and are reliable, but due to its broader line width and modulation frequency limitation edge emitting diode are mostly used. Edge emitting diodes have high power and narrower line width capabilities.

For longer distances and high data rate transmission, Laser Diodes are preferred due to its high power, high speed and narrower spectral line width characteristics. But these are inherently non-linear and more sensitive to temperature variations.

LED Vs Laser Diodes

Nowadays many improvements and advancements have made these sources more reliable. A few of such comparisons of these two sources are given below. Both these sources are modulated using either direct or external modulation techniques.

2. Low Loss Optical Fiber

Optical fiber is a cable, which is also known as cylindrical dielectric waveguide made of low loss material. An optical fiber also considers the parameters like the environment in which it is operating, the tensile strength, durability and rigidity. The Fiber optic cable is made of high quality extruded glass (si) or plastic, and it is flexible. The diameter of the fiber optic cable is in between 0.25 to 0.5mm (slightly thicker than a human hair).

Fiber Optic Cable consists of four parts.

Core

Cladding Buffer

Jacket

Core

The core of a fiber cable is a cylinder of plastic that runs all along the fiber cables length, and offers protection by cladding. The diameter of the core depends on the application used. Due to internal reflection, the light travelling within the core reflects from the core, the cladding boundary. The core cross section needs to be a circular one for most of the applications.

Cladding

Cladding is an outer optical material that protects the core. The main function of the cladding is that it reflects the light back into the core. When light enters through the core (dense material) into the cladding(less dense material), it changes its angle, and then reflects back to the core.

Buffer

The main function of the buffer is to protect the fiber from damage and thousands of optical fibers arranged in hundreds of optical cables. These bundles are protected by the cables outer covering that is called jacket.

JACKET

Fiber optic cables jackets are available in different colors that can easily make us recognize the exact color of the cable we are dealing with. The color yellow clearly signifies a single mode cable, and orange color indicates multimode.

2 Types of Optical Fibers

Single-Mode Fibers: Single mode fibers are used to transmit one signal per fiber; these fibers are used in telephone and television sets. Single mode fibers have small cores.

Multi-Mode Fibers: Multimode fibers are used to transmit many signals per fiber; these signals are used in computer and local area networks that have larger cores.

3. Photo Detectors

The purpose of photo detectors is to convert the light signal back to an electrical signal. Two types of photo detectors are mainly used for optical receiver in optical communication system: PN photo diode and avalanche photo diode. Depending on the applications wavelengths, the material composition of these devices vary. These materials include silicon, germanium, InGaAs, etc.

Basic optical laws

Refraction of light

As a light ray passes from one transparent medium to another, it changes direction; this phenomenon is called refraction of light. How much that light ray changes its direction depends on the refractive index of the mediums.

Refractive Index

Refractive index is the speed of light in a vacuum (abbreviated c, c=299,792.458km/second) divided by the speed of light in a material (abbreviated v). Refractive index measures how much a material refracts light. Refractive index of a material, abbreviated as n, is defined as

n=c/v

Snells Law

In 1621, a Dutch physicist named Will ebrord Snell derived the relationship between the different angles of light as it passes from one transparent medium to another. When light passes from one transparent material to another, it bends according to Snell's law which is defined as:

n1sin( 1) = n2sin( 2)

where:

n1 is the refractive index of the medium the light is leaving 1 is the incident angle between the light beam and the normal (normal is 90 to the interface between two materials)

n2 is the refractive index of the material the light is entering 2 is the refractive angle between the light ray and the normal

Note:

For the case of 1 = 0 (i.e., a ray perpendicular to the interface) the solution is 2 = 0 regardless of the values of n1 and n2. That means a ray entering a medium perpendicular to the surface is never bent.

The above is also valid for light going from a dense (higher n) to a less dense (lower n) material; the symmetry of Snell's law shows that the same ray paths are applicable in opposite direction.

Total Internal Reflection

When a light ray crosses an interface into a medium with a higher refractive index, it bends towards the normal. Conversely, light traveling cross an interface from a higher refractive index medium to a lower refractive index medium will bend away from the normal.

This has an interesting implication: at some angle, known as the critical angle c, light traveling from a higher refractive index medium to a lower refractive index medium will be refracted at 90; in other words, refracted along the interface.

If the light hits the interface at any angle larger than this critical angle, it will not pass through to the second medium at all. Instead, all of it will be reflected back into the first medium, a process known as total internal reflection.

The critical angle can be calculated from Snell's law, putting in an angle of 90 for the angle of the refracted ray 2. This gives 1:

Since n2 = 90

So

sin(n 2) = 1

Then

c = 1 = arcsin(n2/n1)

For example, with light trying to emerge from glass with n1=1.5 into air (n2 =1), the critical angle c is arcsin(1/1.5), or 41.8.

For any angle of incidence larger than the critical angle, Snell's law will not be able to be solved for the angle of refraction, because it will show that the refracted angle has a sine larger than 1, which is not possible. In that case all the light is totally reflected off the interface, obeying the law of reflection.

What is Fiber Mode?

An optical fiber guides light waves in distinct patterns called modes. Mode describes the distribution of light energy across the fiber. The precise patterns depend on the wavelength of light transmitted and on the variation in refractive index that shapes the core. In essence, the variations in refractive index create boundary conditions that shape how light waves travel through the fiber, like the walls of a tunnel affect how sounds echo inside.

We can take a look at large-core step-index fibers. Light rays enter the fiber at a range of angles, and rays at different angles can all stably travel down the length of the fiber as long as they hit the core-cladding interface at an angle larger than critical angle. These rays are different modes.

Fibers that carry more than one mode at a specific light wavelength are called multimode fibers. Some fibers have very small diameter core that they can carry only one mode which travels as a straight line at the center of the core. These fibers are single mode fibers. This is illustrated in the following picture.

Optical Fiber Index Profile

Index profile is the refractive index distribution across the core and the cladding of a fiber. Some optical fiber has a step index profile, in which the core has one uniformly distributed index and the cladding has a lower uniformly distributed index. Other optical fiber has a graded index profile, in which refractive index varies gradually as a function of radial distance from the fiber center. Graded-index profiles include power-law index profiles and parabolic index profiles. The following figure shows some common types of index profiles for single mode and multimode fibers.

Multimode Fibers

As their name implies, multimode fibers propagate more than one mode. Multimode fibers can propagate over 100 modes. The number of modes propagated depends on the core size and numerical aperture (NA).

As the core size and NA increase, the number of modes increases. Typical values of fiber core size and NA are 50 to 100 micrometer and 0.20 to 0.29, respectively.

Single Mode Fibers

The core size of single mode fibers is small. The core size (diameter) is typically around 8 to 10 micrometers. A fiber core of this size allows only the fundamental or lowest order mode to propagate around a 1300 nanometer (nm) wavelength. Single mode fibers propagate only one mode, because the core size approaches the operational wavelength. The value of the normalized frequency parameter (V) relates core size with mode propagation.

In single mode fibers, V is less than or equal to 2.405. When V = 2.405, single mode fibers propagate the fundamental mode down the fiber core, while high order modes are lost in the cladding. For low V values ( 10,000 km

Iii) Operating wavelength: 1.45 to 1.62 m

Fifth generation

Fifth generation uses Roman amplification technique and optical solitiors.

i) Bit rate: 40 - 160 Gb/s

ii) Repeater spacing: 24000 km - 35000 km

iii) Operating wavelength: 1.53 to 1.57 m

Linearly Polarized Modes

The exact analysis of the modes of a fiber is mathematically very complex

Fortunately, the analysis may be simplified when using weakly guiding

approximation

weakly guiding approximation : refractive index difference