Optical Fibers - Civil Engineering (CE) PDF Download

Optical Fibers
Optical fibers are transparent Dielectrics, which guide light over long distances with the help of successive Total Internal Reflections.

Optical Fibers - Civil Engineering (CE)

It consists of three regions
(1) The innermost Light guiding region called Core.
(2) The middle region-covering core made of material similar to Core is called Cladding. The RI of Cladding is less than that of Core.
(3) The outermost protecting layer for Core and Cladding from moisture, crushing and chemical reaction etc., is called Sheath. The Optical Fibers are either made as a single fiber or a flexible bundle or Cables. A Bundle fiber is a number of fibers in single jacket.

Principle of Optical fibres:
It is based on the principle of Total Internal reflection. Consider a ray of light passing from denser medium to rarer medium. As the angle of incidence increases the angle of refraction also increases. For a particular angle of incidence called Critical Angle the refracted ray just grazes the interface (Angle of refraction is 900 ). If the angle of incidence is greater than Critical Angle then the ray reflected back to the denser medium. This phenomenon is called Total Internal Reflection.

Optical Fibers - Civil Engineering (CE)

Propagation of light through fiber(Optical fiber as a light guide): 
The main function of the Optical fiber is to accept maximum light and transmit the same with minimum attenuation. The incident light enters the core and strikes the interface of the Core and Cladding at large angles as shown in fig. Since the Cladding has lower RI than Core the light suffers multiple Total Internal Reflections. This is possible since by geometry the angle of incidence at the interface is greater than the Critical angle. Since the Total internal reflection is the reflection at the rarer medium there is no energy loss. Entire energy is transmitted through the fiber. The propagation continues even if the fiber is bent but not too sharply. Since the fiber guides light it is called as fiber light guide or fiber waveguide.

Optical Fibers - Civil Engineering (CE)

Angle of Acceptance and Numerical Aperature
Light gathering ability of a fiber depends on two facts.
(1) Core size.
(2) Numerical Aperture.

Numerical aperture of fiber is determined by Acceptance angle. Let a light beam enter at an angle “i” to the axis of the fiber (On the launching face). The ray gets refracted at an angle “r” and strikes the Core Cladding interface at an angle θ. If θ > θc (Critical angle) for the system T I R takes place. It means as long as θ is greater than Critical angle light stays within the fiber.

In order to maintain θ to be greater than Critical angle, the angle of incidence relative to Axis of the fiber (At the launching face) should not be greater than a value θ0. The ray corresponding to θ0 can be used to describe a Conical surface. The cone formed is called Acceptance cone and the angle θ0 is called Acceptance angle. The sine of acceptance angle (ie. sin θ0 ) is called Numerical aperture.

Optical Fibers - Civil Engineering (CE)

Expression for Numerical Aperture Let n0, n1 and n2 be the Refractive indices of Surrounding, Core and Cladding respectively. Also RI of Core is Greater than RI of Cladding (i.e n1 > n2). By applying Snell’s law for the Launching face of Fiber

Optical Fibers - Civil Engineering (CE)

By applying Snell’s law at ‘B’

n1 sin(90 − θ1) = n2 sin(90)

Where (90 − θ1) is Critical angle

Optical Fibers - Civil Engineering (CE)

Substituting for sin θ1 in (6.1)

Optical Fibers - Civil Engineering (CE)

for air n0 = 1

Optical Fibers - Civil Engineering (CE)

If ‘i’ is the angle of incidence at the launching face then the condition for propagation of light through the optical fiber is

Optical Fibers - Civil Engineering (CE)

sine of the angle of incidence must be less than or equal to numerical aperture Fractional RI Change(∆): 
It is the ratio of RI difference between Core and Cladding to the RI of core

Optical Fibers - Civil Engineering (CE)

Relation between NA and ∆

Optical Fibers - Civil Engineering (CE)

Optical Fibers - Civil Engineering (CE)

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FAQs on Optical Fibers - Civil Engineering (CE)

1. What are optical fibers made of?
Ans. Optical fibers are made of a core, which is typically made of high-purity silica glass, surrounded by a cladding layer. These materials are chosen for their ability to efficiently transmit light signals.
2. How do optical fibers transmit information?
Ans. Optical fibers transmit information through the principle of total internal reflection. Light signals, usually in the form of laser or LED-generated pulses, bounce off the inner walls of the fiber, continuously reflecting and propagating through the core. This allows for the transmission of information in the form of light signals.
3. What advantages do optical fibers offer over traditional copper cables?
Ans. Optical fibers offer several advantages over traditional copper cables. They have a much higher bandwidth, allowing for the transmission of large amounts of data over long distances. They are also immune to electromagnetic interference, which can degrade the quality of signals in copper cables. Additionally, optical fibers are thinner, lighter, and more flexible, making them easier to install and maintain.
4. Are there any limitations to the use of optical fibers?
Ans. While optical fibers have numerous benefits, there are also some limitations to their use. One limitation is the initial cost of installation, as optical fiber networks require specialized equipment and expertise. Another limitation is the fragility of the fibers, which can be damaged if bent too tightly or subjected to excessive tension. Lastly, optical fibers may experience signal loss over very long distances, requiring the use of repeaters to amplify the signal.
5. What are some common applications of optical fibers?
Ans. Optical fibers are widely used in telecommunications, serving as the backbone of the internet and long-distance communication networks. They are also used in medical imaging and sensing applications, such as endoscopes and fiber optic sensors. Additionally, optical fibers are employed in high-speed data transmission for industries like aerospace, defense, and transportation.
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