Photodiodes LDR Fiber Optic Sensors

Finals Part 3 Flashcards by chxngy Unknown

The term Fiber Optics
was coined by

N.S.
Kapany

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What year was the term Fiber Optics coined by N.S. Kapany?

1956

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Where was the term Fiber Optics coined by N.S. Kapany?

At Imperial College of
Science and
Technology, London in
their image
transmitting device
“Flexible fiberscope”

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uses light as
the information carrier.

optical communications system

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Since light propagation
through Earth’s atmosphere is difficult and often
impractical, __________ are used
to contain and guide the light, similar to how
electromagnetic waves are guided through
metallic transmission media.

glass or plastic fiber cables

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Optical fibers are thin strands of

pure glass

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Optical fibers carry data over

long distances

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What is the classification of speed for optical fibers?

very high speed

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Optical fibers can be _____ or ______

bent or twisted

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Sources of Light

Light Emitting Diodes (LEDs)
LASER

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What is replacing copper?

Optical fiber

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the carrier of information for optical fiber that has properties that can be modulated: Amplitude, Phase, Frequency, and State
of polarization

Light

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What are the properties that can be modulated when it comes to utilizing optical fiber?

Amplitude
Phase
Frequency
State of polarization

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Detectors

Photodiodes
LDR
Fiber Optic Sensors

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ratio of
speed of light in vacuum to speed of
light in medium

*Index of refraction of material

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bending of light as
it travels from one media to another

Refraction of light:

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3 practical types of optical fiber configurations

single-mode step-index
multimode step index
multimode graded index

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are the dominant fibers used in today’s telecommunications and data networking industries

Single-mode step-index fibers

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Single-mode step-index fibers has a central core that is significantly ________ than any of the multimode cables.

smaller in
diameter

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similar to the single-mode step-index fibers except
the center core is much larger with the multimode
configuration.

Multimode step-index fibers

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Multimode step-index fibers has a ______ and, consequently, allows more external light to
enter the cable

large light-to-fiber aperture

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characterized by a central core with a nonuniform
refractive index.

Multimode graded-index optical fibers

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The cable density in Multimode graded-index optical fibers is ____________ and _________ toward the outer
edge.

maximum at
the center and decreases gradually

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Light rays propagate down Multimode graded-index optical fibers through ________ rather than reflection

refraction

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Predominant Losses in Optical Fiber Cables

* Absorption loss * Material, or Rayleigh, scattering losses * Chromatic, or wavelength, dispersion * Radiation losses * Modal dispersion * Coupling losses

3 factors of absorption losses in optical fibers:

* Ultraviolet absorption * Infrared absorption * Ion resonance absorption

analogous to power dissipation in copper cables; impurities in the fiber absorb the light and convert it to heat

Absorption losses

caused by valence electrons in the silica material from which fibers are manufactured.

Ultraviolet absorption

In ultraviolet absorption, the light ionizes the valence electrons into ________. The ionization is equivalent to a _________ and, consequently, contributes to the ________ of the fiber.

conduction loss in the total light field transmission losses

result of photons of light that are absorbed by the atoms of the glass core molecules.

Infrared absorption

in Infrared absorption, The absorbed photons are converted to random ________ typical of heating.

mechanical vibrations

is caused by -OH ions in the material. The source of the -OH ions is water molecules that have been trapped in the glass during the manufacturing process

Ion resonance absorption

molecules that cause ion absorption.

Iron, copper, and chromium molecules

A type of signal loss in optical fibers caused by microscopic irregularities formed during the fiber manufacturing process. These irregularities scatter light in multiple directions, with some light escaping through the cladding, resulting in a loss of signal power.

Rayleigh Scattering Loss

A type of distortion in optical fibers where different wavelengths of light travel at different speeds, causing light signals from an LED to arrive at different times and leading to signal spreading and degradation.

Chromatic Distortion or wavelength dispersion

caused mainly by small bends and kinks in the fiber.

Radiation losses

2 types of bends in radiation losses

microbending constant-radius bends

occurs as a result of differences in the thermal contraction rates between the core and the cladding material.

microbending

a miniature bend or geometric imperfection along the axis of the fiber and represents a discontinuity in the fiber where Rayleigh scattering can occur

microbend

are caused by excessive pressure and tension and generally occur when fibers are bent during handling or installation.

Constant-radius bends

caused by the difference in the propagation times of light rays that take different paths down a fiber.

Modal dispersion (Pulse Spreading)

modal dispersion can occur only in

multimode fibers.

Modal dispersion can be reduced considerably by using ________ and almost entirely eliminated by using _______

graded index fibers single-mode step-index fibers.

caused by imperfect physical connections. In fiber cables, these losses can occur at any of the following three types of optical junctions

Coupling losses

three types of optical junctions

light source-to-fiber connections, fiber-to-fiber connections, and fiber-to-photodetector connections

ratio of the velocity of propagation of a light ray in free space to the velocity of propagation of a light ray in a given material.

Refractive Index

Formula for refractive index

n = c / v Where n = refractive index (unitless) c = speed of light (3 x 10^8 m/s) v = speed of light in given material (m/s)

The angle of incidence is the angle at which the propagating ray strikes the interface with respect to the normal, and the angle of refraction is the angle formed between the propagating ray and the normal after the ray has entered the second medium.

Snell’s Law

formula for snell's law

n1sinΘ1 = n2sinΘ2 where n1 = refractive index of material 1 (unitless) n2 = refractive index of material 2 (unitless) Θ1 = angle of incidence (degrees) Θ2 = angle of refraction (degrees)

Critical angle: Angle of incidence at which angle of refraction =

90 degrees`

Speed of light changes as it across the boundary of two media

Refraction of light

memorize typical indexes of refraction

see powerpoint

closely related to acceptance angle and is the figure of merit commonly used to measure the magnitude of the acceptance angle.

Numerical aperture (NA)

numerical aperture is used to describe the ___________ (i.e., the ability to couple light into the cable from an external source).

light-gathering or light-collecting ability of an optical fiber

The larger the magnitude of the _________, the greater the amount of _______ the fiber will accept.

numerical aperture external light

Formula of Numerical Aperture

NA = sqrt[ (n1)^2 - (n2)^2 ] where NA = numerical aperture (unitless) n1 = refractive index of glass fiber core (unitless) n2 = refractive index of quartz fiber cladding (unitless)

called the acceptance angle or acceptance cone half-angle

θin(max)

θin(max) defines the maximum angle in which external light rays may strike the _____________ and still propagate down the fiber.

air/glass interface

Rotating the acceptance angle around the fiber core axis describes the _______________

acceptance cone of the fiber input.

formula for acceptance angle

θin(max) = sin-1[ (sqrt( (n1)^2 - (n2)^2) / no ] where θin(max) = acceptance angle (degrees) no = refractive index of air n1 = refractive index of glass fiber core n2 = refractive index of quartz fiber cladding

often called attenuation and results in a reduction in the power of the light wave as it travels down the cable.

Power loss in an optical fiber cable

Formula for Power Loss

A(dB) = 10log[Pout / Pin] where A(dB) = total reduction in power level, attenuation (unitless) Pout = cable output power (watts) Pin= cable input power (watts)

Optical Power Formula (Watts)

P = Pt x 10^[(-A*l)/10] where P = measured power level (watts) Pt = transmitted power level (watts) A = cable power loss (dB/km) l = cable length (km)

Optical Power Formula (dB)

P(dBm) = Pin(dBm) - A(dB/km)*l(km) where P = measured power level (dBm) Pin = transmit power (dBm) A = cable power loss (dB/km) l = cable length (km)

Finals Part 3 Flashcards

(64 cards)