OPTICAL FIBER 1
Dec 23, 2014
OPTICAL FIBER
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Basic principle Total Internal Reflection in Fiber
An optical fiber (or fibre) is a glass or plastic fiber
that carries light along its length.
Light is kept in the "core" of the optical fiber by total
internal reflection.
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What Makes The Light Stay in Fiber
Refraction The light waves spread out along its beam. Speed of light depend on the material used called
refractive index. Speed of light in the material = speed of light in the free
space/refractive index Lower refractive index higher speed
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The Light is Refracted
This end travels further than the other hand
Lower Refractive index Region
Higher Refractive index Region
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Refraction When a light ray encounters a boundary separating two
different media, part of the ray is reflected back into the first medium and the remainder is bent (or refracted) as it enters the second material. (Light entering an optical fiber bends in towards the center of the fiber – refraction)
Refraction
LED or LASER Source
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Reflection
Light inside an optical fiber bounces off the cladding - reflection
Reflection
LED or LASER Source
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Critical Angle If light inside an optical fiber strikes the cladding too steeply,
the light refracts into the cladding - determined by the critical angle. (There will come a time when, eventually, the angle of refraction reaches 90o and the light is refracted along the boundary between the two materials. The angle of incidence which results in this effect is called the critical angle).
Critical Angle
n1Sin X=n2Sin90o
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Angle of Incidence
Also incident angle Measured from perpendicular Exercise: Mark two more incident angles
Incident Angles
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Angle of Reflection
Also reflection angle Measured from perpendicular Exercise: Mark the other reflection angle
Reflection Angle
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Reflection
Thus light is perfectly reflected at an interface between two materials of different refractive index if:
The light is incident on the interface from the side of higher refractive index.
The angle θ is greater than a specific value called the “critical angle”.
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Angle of Refraction
Also refraction angle Measured from perpendicular Exercise: Mark the other refraction angle
Refraction Angle
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Angle Summary
Refraction Angle
Three important angles The reflection angle always equals the incident
angle
Reflection Angle
Incident Angles
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Refractive Index
n = c/v c = velocity of light in a vacuum v = velocity of light in a specific medium
light bends as it passes from one medium to another with a different index of refraction air, n is about 1 glass, n is about 1.4
Light bends in towards normal - lower n to higher n
Light bends away from normal - higher n to lower n
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Snell’s Law
The amount light is bent by refraction is given by Snell’s Law: n1sin1 = n2sin2
Light is always refracted into a fiber (although there will be a certain amount of Fresnel reflection)
Light can either bounce off the cladding (TIR) or refract into the cladding
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Snell’s Law
Normal
Incidence Angle(1)
Refraction Angle(2)
Lower Refractive index(n2)
Higher Refractive index(n1)Ray of light
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Critical Angle Calculation
The angle of incidence that produces an angle of refraction of 90° is the critical angle n1sin(qc) = n2sin(90°)
n1sin(qc) = n2
qc = sin-1(n2 /n1)
Light at incident anglesgreater than the criticalangle will reflect backinto the core
Critical Angle, c
n1 = Refractive index of the coren2 = Refractive index of the cladding
OPTICAL FIBER CONSTRUCTION
Core – thin glass center of the fiber where light travels.Cladding – outer optical material surrounding the core
Buffer Coating – plastic coating that protect the fiber.
OPTICAL FIBER
The core, and the lower-refractive-index cladding, are
typically made of high-quality silica glass, though they
can both be made of plastic as well.
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NA & ACCEPTANCE ANGLE DERIVATION
In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.”
optical fiber will only propagate light that enters the fiber within a certain cone, known as the acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle, θmax.
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When a light ray is incident from a medium of refractive index n to the core of index n1, Snell's law at medium-core
interface gives
Substituting for sin θr in Snell's law we get:
By squaring both sides
Thus,
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from where the formula given above follows.
NUMERICAL APERATURE IS
ACCEPTANCE ANGLE
θmax =
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Definition:- Acceptance angle:- Acceptance angle is defined as the maximum angle of
incidence at the interface of air medium and core medium for which the light ray enters into the core and travels along the interface of core and cladding.
Acceptance Cone:- There is an imaginary cone of acceptance with an
angle .The light that enters the fiber at angles within the acceptance cone are guided down the fiber core
Numerical aperture:- Numerical aperture is defined as the light gathering capacity
of an optical fiber and it is directly proportional to the acceptance angle.
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Classification of Optical Fiber
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Three common type of fiber in terms of the material used:
• Glass core with glass cladding –all glass or silica fiber
• Glass core with plastic cladding –plastic cladded/coated silica (PCS)
• Plastic core with plastic cladding – all plastic or polymer fiber
Plastic and Silica Fibers
BASED ON MODE OF PROPAGATION
Two main categories of optical fiber used in fiber optic communications are
multi-mode optical fiber
single-mode optical fiber.
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Single-mode fiber Carries light pulses along single path
Multimode fiber Many pulses of light generated by LED travel at different
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Based on the index profile
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The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-
index fiber
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Step Index Fibers
A step-index fiber has a central core with a uniform
refractive index. An outside cladding that also has a uniform
refractive index surrounds the core;
however, the refractive index of the cladding is less than
that of the central core.
The refractive index profile may be defined as
n(r) = n1 r < a (core) n2 r ≥ a (cladding)
GRADED-INDEX
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding.
This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary.
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33Figure.2.6
(a)
(b)
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multimode step-index fiber the reflective walls of the fiber move the light pulses to
the receiver multimode graded-index fiber
acts to refract the light toward the center of the fiber by variations in the density
single mode fiber the light is guided down the center of an extremely
narrow core
Figure 2.10 Two types of fiber: (Top) step index fiber; (Bottom) Graded index fiber
Attenuation
Definition: a loss of signal strength in a lightwave, electrical or radio signal usually related to the distance the signal must travel.
Attenuation is caused by: Absorption Scattering Radiative loss
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Losses
Losses in optical fiber result from attenuation in the material itself and from scattering, which causes some light to strike the cladding at less than the critical angle
Bending the optical fiber too sharply can also cause losses by causing some of the light to meet the cladding at less than the critical angle
Losses vary greatly depending upon the type of fiber Plastic fiber may have losses of several hundred dB
per kilometer Graded-index multimode glass fiber has a loss of
about 2–4 dB per kilometer
Single-mode fiber has a loss of 0.4 dB/km or less37
Macrobending Loss: The curvature of the bend is much larger than fiber
diameter. Lightwave suffers sever loss due to radiation of the evanescent field in the cladding region. As the radius of the curvature decreases, the loss increases exponentially until it reaches at a certain critical radius. For any radius a bit smaller than this point, the losses suddenly becomes extremely large. Higher order modes radiate away faster than lower order modes.
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Microbending Loss
Microbending Loss: microscopic bends of the fiber axis that can arise when the fibers are incorporated into cables. The power is dissipated through the microbended fiber, because of the repetitive coupling of energy between guided modes & the leaky or radiation modes in the fiber.
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Dispersion
The phenomenon in an optical fibre whereby light photons arrive at a distant point in different phase than they entered the fibre.
Dispersion causes receive signal distortion that ultimately limits the bandwidth and usable length of the fiBer cable
The two main causes of dispersion are:
Material (Chromatic) dispersion
Waveguide dispersion
Intermodal delay (in multimode fibres)
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Dispersion in fiber optics results from the fact that in multimode propagation, the signal travels faster in some modes than it would in others
Single-mode fibers are relatively free from dispersion except for intramodal dispersion
Graded-index fibers reduce dispersion by taking advantage of higher-order modes
One form of intramodal dispersion is called material dispersion because it depends upon the material of the core
Another form of dispersion is called waveguide dispersion Dispersion increases with the bandwidth of the light source
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Advantages of Optical Fibre
Thinner Less Expensive Higher Carrying
Capacity Less Signal
Degradation& Digital Signals
Light Signals Non-Flammable Light Weight
Advantages of fiber optics
Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber
Immunity to Noise - Immune to electromagnetic interference (EMI).
Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline.
High Security - Impossible to “tap into.”
Advantages of fiber optics
Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation)
Reliability - More resilient than copper in extreme environmental conditions.
Size - Lighter and more compact than copper. Flexibility - Unlike impure, brittle glass, fiber is
physically very flexible.
Fiber Optic Advantages greater capacity (bandwidth up
to 2 Gbps, or more)
smaller size and lighter weight
lower attenuation
immunity to environmental
interference
highly secure due to tap
difficulty and lack of signal
radiation
Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult.
Disadvantages of fiber optics
Areas of Application
Telecommunications Local Area Networks Cable TV CCTV Optical Fiber Sensors
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Formula Summary
Index of Refraction
Snell’s Law
Critical Angle
Acceptance Angle
Numerical Aperture
v
cn
2211 sinsin nn
1
21sinn
nc
22
21
1sin nn
22
21sin nnNA
STUDENTS YOU CAN ALSO REFER IT……
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http://hank.uoregon.edu/experiments/Dispersion-in-Optical-Fiber/Unit_1.6%20(2).pdfhttp://www1.ceit.es/asignaturas/comuopticas/pdf/chapter4.pdf
http://course.ee.ust.hk/elec342/notes/Lecture%206_attenuation%20and%20dispersion.pdf
1 Engineering Physics by H Aruldhas, PHI India 2 Engineering Physics by B K Pandey , S. Chaturvedi, Cengage Learning 3Resnick, Halliday and Krane, Physics part I and II, 5th Edition John Wiely 4Engineering Physics by S.CHAND5Engineering Physics by G VIJIYAKUMARI