ST 15: Optical Fiber Communication - Judiciary Exams MCQ

Concept:

The numerical aperture of fiber optic cable is given by 

Where n1 and n2 are core and cladding refractive indices

Optical fiber communication:

The communication system that uses lightwave for data transmission .

Optical fiber communication is carried out at a very high frequency 10¹⁴ to 10¹⁵ Hz. 

UltraViolet wave operates at 10¹⁴ to 10¹⁷.

Visible light band operates at 1014 to 1015.

Infrared band operates at 1013 to 1014.

Hence correct option is "4".

Optical Fibre communication:

The type of communication in which information or data is transmitted in the form infrared wave through optical fibre cable.

Number of modes depend on V number.

V= 

a = Radius of core

N_A = Numerical aperture of optical fibre

Hence mode is directly proportional to 

Hence correct option is "2"

Graded index fiber:

• A graded-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the optical axis of the fiber.
• Due to parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays after total reflection follow a parabolic path.
• Due to the parabolic path, refocussing of the signal within the core is increased, which eventually increases the data rate.

40 GHz is the frequency limit of an optical fiber.

Additional Information

• Optical fibers are transparent fibers and act as a light pipe to transmit light between its two ends. They are made up of silicon dioxide.

• The total internal reflection occurs when the angle of incidence is greater than the critical angle.
• In optical fiber glasses of a high and lower index are assembled in the precise order.
• The light comes in from one end of the fiber and after thousands of successive internal reflections, the light reaches the opposite end of the fiber with almost zero loss. 

Advantages and disadvantages of step-index mono-mode optical fiber.

The correct answer is Communication.

Key Points

• Optical fiber is made up of plastic or drawing glass, it is used for the transmission of data using light pulses traveling along with a long fiber.
• There are generally three types of fiber optic cable commonly used: single-mode, multimode and plastic optical fiber.
• Fiber optics are used in Cable TV, Telephone, Computer Networking, Surgery and Dentistry, Lighting and decorations.
• Optical Fiber Communication is the technique of communication in which a signal is transmitted in the form of light and the optical fiber is used as a medium of transmitting.
• The signal transmitted in optical fiber is converted from the electrical signal into light and at the receiving end, it is converted back into the electrical signal from the light.

​Important Point
The advantages of optical fibers are as follow.

• The fiber optic cables have higher transmission bandwidth than metal cables.
• The amount of data transmission is higher in fiber optic cables.
• The power loss of optics fiber is very low and also helpful in long-distance transmissions.
• Fiber optic cables provided high security and cannot be tapped.
• Fiber optic cables are the most secure way for data transmission.
• Fiber optic cables are immune to electromagnetic interference.
• Fiber optic cables are not affected by electrical noise.​

Additional Information

• An electrical transformer is an electrical device that transfers electrical energy from one electrical circuit to another.
• A nuclear reactor is a device used to control nuclear chain reactions. They are used in nuclear plants to generate electricity.
• Nuclear energy, also called atomic energy, the energy that is released in significant amounts in processes that affect atomic nuclei, the dense cores of atoms. Renewable energy is a better alternative to nuclear energy.

Optical fibers are transparent fibers and act as a light pipe to transmit light between its two ends. They are made up of silicon dioxide.

Refractive index: The ratio of the speed of light in a vacuum to the speed of light in a medium is called the refractive index of that medium.

It is also called an absolute refractive index.

Critical angle: When a ray of light is going from a denser medium to a rare medium then the angle of incidence at which the refraction angle is 90° is called the critical angle.

The critical angle is given by:

Θ_C = critical angle = sin^-1(n₂/n₁)

Where n₂ is the refractive index of second medium in which light ray is going and n₁ is the refractive index of first medium from which light is going to second medium.

Concept:

Full Acceptance Angle = 2α

• The Numerical Aperture (NA) of fiber is defined as the sine of the largest angle an incident ray can have for total internal reflectance in the core. 
• Rays launched outside the angle specified by a fiber's NA will excite radiation modes of the fiber.
• A higher core index, with respect to the cladding, means a larger NA.
• However, increasing NA causes higher scattering loss from greater concentrations of dopant.
• Qualitatively, NA is a measure of the light-gathering ability of a fiber.
• It also indicates how easy it is to couple light into a fiber.

• In fiber-optic communication, a diode laser is used for transmitting light signals into the fiber, and a photodiode is used for receiving the light signal.
• A photodiode is a device that converts a light signal to an electrical signal
• A photodiode is operated in the reverse-biased mode

Some important photodiodes are:

• PN photodiode
• PIN photodiode
• Avalanche photodiode

Notes:

PIN DIODE has three regions:

• P-type layer
• Intrinsic Layer
• N-Type layer
• The depletion region that exists between p and n regions is large. The layer between p and n regions includes no charge carriers
• In the reverse bias, as the depletion region of the diode has no charge carriers it works as an insulator
• The depletion region exists within the PIN diode but if the PIN diode is forward biased then carriers come into the depletion region and there will be a flow of current.

More from LMS

For a coaxial cable, the propagation constant is expressed as:

This can be written as:

Γ = α(ω) + β(ω)

R is the resistance per unit length

L is the inductance per unit length

G is the conductance per unit length

C is the capacitance per unit length, ω is the angular frequency = 2πf.

α(ω) and β(ω) are the attenuation and phase constants.

∴ The attenuation per unit length in a coaxial cable increases with an increase in frequency.

Concept:

• An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber.
• The Optical Time Domain Reflectometer (OTDR) is useful for testing the integrity of fiber optic cables.
• OTDR is used to verify splice loss, find faults, and measure the length of optical communication.
• The instrument emits short laser pulses, e.g. with pulse durations of some tens of nanoseconds and peak power of a few hundred milliwatts, as can be obtained with a single-mode laser diode.
• The pulses are injected through a launch cable into the tested optical fiber.
• Any reflected light is extracted with a directional fiber coupler and sent to a fast and sensitive photodetector.
• The photodetector allows for the time-resolved measurement of the reflected optical power and thus of the return loss.
• One can find the propagation losses and also find whether the loss coefficient is constant along with the fire or changes locally. 

Step-index fiber:

1. The refractive index of the core is uniform throughout and undergoes on abrupt change at the core-cladding boundary.

2. The path of light propagation is zig-zag in a manner

Additional Information

Optical fibres are transparent fibres and act as a light pipe to transmit light between its two ends. They are made up of silicon dioxide.

Refracting index is changing between the cladding and the core in optical fibre

Graded-index fiber:

1. The refractive index of the core is made to vary gradually such that it is maximum at the center of the core.

2. The path of light is helical in manner.

This type of fiber optics works when the wavelength is much smaller than the core radius.

So, the refractive index of the core remains constant for step-index fiber.

Important Point

Difference between step-index and graded-index fiber.

Fibers that carry more than one mode are called multimode fibers. There are two types of multimode fibers:

1) Step Index

2) Graded Index

A comparison for step and graded fibers are respectively shown below:

We observe that in graded-index multimode optical fiber, the refractive index of the core is maximum at the fiber axis and decreases gradually towards the cladding.

Important Notes:

The multimode step-index multimode fiber suffers from Modal dispersion.

• Rays of light enter the fiber with different angles to the fiber axis. The limit is the fiber’s acceptance angle.
• Rays that enter with a shallow angle travel a more direct path and arrive sooner than those that enter at steeper angles.
• This arrival of different modes of light at different times is called modal dispersion.

• Single-mode step indexed fibers are widely used for wideband communications are preferred for long-distance communication.
• Single-mode step-index fiber is used to eliminate modal dispersion during optical communication.
• In this fiber, a light ray can travel on only one path so minimum refraction takes place hence, no pulse spreading permits high pulse repetition rates.

Advantages of single-mode fiber:

1) Low signal loss

2) No modal dispersion

3) Does not suffer from modal dispersion

4) Can be used for higher bandwidth applications

5) Long-distance applications

6) Cable TV ends

7) High speed local and wide area network

Important Point

Single-mode means the fiber enables one type of light mode to be propagated at a time. This is explained with the help of the following diagram:

Single-mode fiber core diameter is much smaller than multimode fiber.

2) For single-mode fiber, the B.W ranges from 50 to 100 GHz/km

Multimode fibers:

Fibers that carry more than one mode are called multimode fibers. There are two types of multimode fibers:

1) Step Index

2) Graded Index

The comparison of the refractive index profile for step and graded fibers are respectively shown as:

The multimode step-index multimode fiber suffers from Modal dispersion.

• Rays of light enter the fiber with different angles to the fiber axis. The limit is the fiber’s acceptance angle.
• Rays that enter with a shallow angle travel a more direct path and arrive sooner than those that enter at steeper angles.
• This arrival of different modes of light at different times is called modal dispersion.

Scattering:

• Scattering is the loss of signal caused by the diffusion of a light beam, where the diffusion itself is caused by microscopic variations in the transmission medium
• Scattering typically happens when a light signal hits an impurity in the fibre

Dispersion:

• Dispersion is defined as pulse spreading in an optical fibre
• As a pulse of light propagates through a fibre, elements such as numerical aperture, core diameter, refractive index profile, wavelength, and laser linewidth cause the pulse to broaden

Attenuation:

• The attenuation of an optical fibre measures the amount of light lost between input and output
• Total attenuation is the sum of all losses
• For a given fibre, these losses are wavelength-dependent

Inter-symbol Interference:

• This is the effect of dispersion on optical fibre
• Intersymbol interference occurs when the pulse spreading caused by dispersion causes the output pulses of a system to overlap

The principle of operation of fibre-optic cable is Reflection.

Key Points

• In an optical fiber, the information is passed through light, which must not escape outside of it.
• This phenomenon of confining the light inside the optical fiber is termed as Total internal reflection.
• For this, the construction and material used to ensure that the total internal reflection of light takes place to prevent the escape of it.

Principle:

• When light travels from a high refractive index medium to a low-refractive-index medium, it is refracted away from the normal as shown:

• At a certain angle θ_i, there is no refracted wave and the wave is totally internally reflected  (θ_r = 90°).
• This angle is called a critical angle.
• Inside an optical fiber, we have a high refractive index core (n₁) and low refractive index cladding (n₂).
• This results in the propagation of waves inside a fiber through total internal reflection phenomenon.

Optical Fiber:

• In an optical fiber, the information is passed through light, which must not escape outside of it.
• This phenomenon of confining the light inside the optical fiber is termed Total internal reflection.
• For this, the construction and material used to ensure that the total internal reflection of light takes place to prevent the escape of it.

• In any type of optical fiber, the refractive index of the core is always greater than the refractive index of the cladding.
• It is this property of core and cladding which makes light propagate inside the fiber. 
• Thus, In a step-index optical fiber refractive index of the core is higher than the cladding

Principle:

When light travels from a high refractive index medium to a low-refractive-index medium, it is refracted away from the normal as shown:

• At a certain angle θ_i, there is no refracted wave and the wave is totally internally reflected  (θ_r = 90°).
• This angle is called a critical angle.
• Inside an optical fiber, we have a high refractive index core (n₁) and low refractive index cladding (n₂).
• This results in the propagation of waves inside a fiber through total internal reflection phenomenon.

Key Points

 Optical fiber refers to the technology that transmits information as light pulses along with a glass or plastic fiber.

An optical fiber consists of three concentric elements,

Core:
The inner light-carrying member with a high index of refraction.

Cladding:
The middle layer, which serves to confine the light to the core.  It has a lower index of refraction.

Buffer:

The outer layer, which serves as a "shock absorber" to protect the core and cladding from damage.  The coating usually comprises one or more coats of plastic material to protect the fiber from the physical environment.  Sometimes metallic sheaths are added to the coating for further physical protection.

The outer jacket provides one last layer of protection and also adds strength to the fiber. The jacket is typically colored to help the user determine what type of optical fiber is in the cable.

Hence the correct answer is Jacket.