Optics and Electromagnetics
Optics and Electromagnetics
Optics and Electromagnetics
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Refraction & Total Internal Reflection
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Total Internal Reflection
n1 sin1 n2 sin 2
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Refractive index
A ray of light being refracted in a plastic block.
In optics the refractive index or index of refraction n of an optical
medium is a dimensionless number that describes how light, or any
other radiation, propagates through that medium. It is defined as
n= c/v
Where, c is the speed of light, in vacuum and
v is the phase velocity of light in the medium.
The refractive index determines how much light is bent, or
refracted, when entering a material. 4
Optical Fiber & Communications System
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Optical Fiber
Optical fiber is made from thin strands of either glass or plastic.
Often, two or more fibers are enclosed in the same cable for increased
It is also easier to build a full-duplex system using two fibers, one for
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An optical fiber is essentially a waveguide for light
The index of refraction of the cladding is less than that of the core, causing
rays of light leaving the core to be refracted back into the core
A light-emitting diode (LED) or laser diode (LD) can be used for the source
• Advantages of optical fiber include:
Greater bandwidth than copper, Lower loss, Immunity to crosstalk
No electrical hazard
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Numerical Aperture
The numerical aperture of the fiber is closely related to the critical angle and is
often used in the specification for optical fiber and the components that work with
it
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Modes and Materials
Since optical fiber is a waveguide, light can propagate in a number of modes.
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Types of Fiber
Both types of fiber described earlier are known as step-index fibers because the index of refraction
changes radially between the core and the cladding.
Graded-index fiber is a compromise multimode fiber, but the index of refraction gradually
decreases away from the center of the core.
Graded-index fiber has less dispersion than a multimode step-index fiber.
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Dispersion
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
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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
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Types of Losses
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Fiber-Optic Cables
There are two basic types of fiber-optic cable
– The difference is whether the fiber is free to move inside a tube with a
diameter much larger than the fiber or is inside a relatively tight-fitting jacket
They are referred to as loose-tube and tight-buffer cables
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Fiber-Optic Cable Construction
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Splices and Connectors
In fiber-optic systems, the losses from splices and connections can be more than in the
cable itself
Losses result from:
– Axial or angular misalignment
– Air gaps between the fibers
– Rough surfaces at the ends of the fibers
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Fiber-Optic Connectors
Coupling the fiber to sources and detectors creates losses as well, especially when it
involves mismatches in numerical aperture or in the size of optical fibers
Good connections are more critical with single-mode fiber, due to its smaller
diameter and numerical aperture
A splice is a permanent connection and a connector is removable
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Optical Couplers and Switches
As with coaxial cable and
microwave waveguides, it is
possible to build power splitters
and directional couplers for fiber-
optic systems
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Coupler Construction
Optical couplers can be made in many different ways:
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Optical Switches and Relays
Occasionally, it is necessary to switch optical
signals from one fiber to another.
The simplest type of optical switch moves
fibers so that an input fiber can be positioned
next to the appropriate output fiber.
Another approach is direct the incoming light
into a prism, which reflects it into the outgoing
fiber. By moving the prism, the light can be
switched between different output fibers.
Lenses are necessary with this approach to
avoid excessive loss of light.
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Optical Emitters
Optical emitters operate on the idea that electromagnetic energy can
only appear in a discrete amount known as a quantum. These quanta
are called photons when the energy is radiated
Energy in one photon varies directly with the frequency
– Light-Emitting Diodes
– Laser Diodes
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Light-Emitting Diodes
• An LED is form of junction diode that is operated with forward bias
• Instead of generating heat at the PN junction, light is generated and passes through an opening or lens
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Laser Diodes
Laser diodes generate coherent, intense light of a very narrow
bandwidth.
Laser diodes are constructed much like LEDs but operate at higher
current levels.
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Laser Diode Construction
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Optical Detectors
• The most common optical detector used with fiber-optic systems is
the PIN diode.
• The PIN diode is operated in the reverse-bias mode.
• As a photodetector, the PIN diode takes advantage of its wide depletion region, in which electrons can create
electron-hole pairs.
• The low junction capacitance of the PIN diode allows for very fast switching.
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Avalanche Photodiode
The avalanche photodiode (APD) is also operated in the reverse-
bias mode
The creation of electron-hole pairs due to the absorption of a photon
of incoming light may set off avalanche breakdown, creating up to
100 more pairs
This multiplying effect gives an APD very high sensitivity
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Optical fiber vs Copper
Challenges: Connectivity and Interoperability
Newer technologies involving higher datarates, smaller form factors,
higher port densities, pluggability, and parallel links are showing an
increased need to focus on connectivity and interoperability issues
Transceiver Fiber Optic Cable
Transceiver
Electrical
Connector Electrical
Optical Optical Optical Connector
Optical
Port Connector Connector Port
A failure anywhere along this link will cause the entire link to fail
Starin measurement by using optical
fibrers
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Microbending & Macrobending
Power Losses in Optical Fibres
By severely bending an optical fibre will incur an optical power loss,
measure of deformation.
In a sense, one could visualize changes in the length of the fiber resulting
fiber and another signal coming through a similar fiber that contains
Thus a comparison between ref. signal and signal from fiber with
intensity detection.
detection.
A GRIN fiber that is periodically microbent attenuates light
transmitted through it.
GRIN fibers embedded in composites undergo microbending when
transverse stress is applied and therefore detect applied stress on the
composite.
•But step index fibers are less sensitive to applied stress relatively as in
fig …..
Macrobending:
One way to observe the effect of bending losses
is to use a Visual Fault Locator (VFL) where the
visible red light glow seen at the point of bend
indicates light escaping from the fibre core,
passing through the cladding and fibre coatings
before escaping into the environment.
Macrobending:
time” mode where a severe induced bend causes a noticeable drop in the
The leaked signal is not visible to the naked eye at the point of bending
as networks and optical testing instruments operate in the infra red (heat)
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Crack detection by using optical fibre
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