Optical Fiber Lecture

8/2/2019 Optical Fiber Lecture

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1. OPTICAL FIBER TYPES


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A. Optical Fiber Construction


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Protections:

lacquer, silicone or acrylate coating applied outside the cladding to seal and

preserve the fibers strength -protect fiber from moisture to protect from

stress corrosions (static fatigue) caused by high

humidity


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-buffer jacket provides cable additional protectionagainst abrasion and shock

used steel, fiberglass, plastic, flame-retardantpolyvinyl chloride (FR-PVC), Kevlar yarn, andpaper

-materials commonly used to strengthen and protectfibers from abrasion and environmental stress


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strength member increases the tensile strength of the over-all cable

assembly


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polyurethane jacket over-all protection


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3 essential types of optical fibers:

1. Plastic core and cladding more flexible and more rugged than glass easier to install can better withstand stress less expensive weigh approximately 60% less than glass higher attenuation characteristics and do not

propagate light as efficiently as glass


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2. Glass core with plastic cladding (PCS fiber [plastic-clad silica])

lesser attenuation than plastic fibers slightly better than plastic core & cladding less affected by radiation more immune to

external interference


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3. Glass core and glass cladding (called SCS [silica-clad silica])

best propagation characteristics easier to terminate than PCS fibers least rugged more susceptible to increases

in attenuation when exposed to radiation.


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B. Cable Configurations

Loose tube construction each fiber is contained in a protective tube Inside the tube a polyurethane compound encapsules the fiber and preventsthe intrusion of water

Stress corrosion or static fatigue can result if the glass fiber is exposed tolong periods of high humidity. Silicon dioxide crystals interact with the moisture and cause bonds to break

down, casuing spontaneous fractures to from over prolonged period


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o Constrained fiber more protection

o Sorrounding the fiber are a primary and a secondary buffer comprised of o Kevlar yarn which Increases the tensile strength of the cable and providesprotection from external mechanical influences that could cause fiberbreakage or excessive otpical attenuation.

o Polyurethane prevents moisture from coming into contact with the fiber core


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Multiple strand cable configuration includes steel central member and alayer of Mylar tape wrap to increase the cables tensile strength.


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Telephone cable


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Plastic-silica cable


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Materials used to strengthen and protect fibers:

steel fiberglass

plastic FR-PVC (Flame-retardant polyvinyl chloride) Kevlar yarn paper


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2. Light Propagation


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A. Physics of Light


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Energy absorbed or emitted (photon)


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b. Optical Power

Photometry science of measuring only light waves that are visible to the human eye light intensity is described in terms of luminous flux density, lumens/A Radiometry measures light throughout the entire electromagnetic spectrum measures optical power as the rate at which electromagnetic waves

transfer light energy -generally used with light sources with output powers ranging from tens of

micrwatts to more than 100 milliwatts optical power flow of light energy past a given point in a specified time - sometimes called radiant flux ( )


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C. Velocity of Propagation

Refraction of light wavesrefract towards the normal (imaginary line drawn perpendicular to the interface

of the two materials at the point of incidence) from less dense to more dense material

refract away from the normal from more dense to less dense material


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violet wavelengths are refracted the most while the redis the least

prismatic refraction spectral separation of white lightrainbow occurrence as water droplets acting as prism

refractive index the ratio of the velocity of

propagation of a light ray in free space to the velocityof propagation of light ray in a given material


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Critical Angle minimum angle of incidence atwhich a light ray may strike the interface of two media and result in an angle of refractionof or greater.

light ray must travel from a medium of higherrefractive index to a lower refractive index (i.e.

glass into cladding)


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3. Optical Fiber Configuration


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a. Single-Mode Step-Index OpticalFiber


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Advantages: i. minimum dispersion - less loss ii. higher bandwidth higher information rate Disadvantages: i. Difficult to couple light into and out ii. need highly directive light source such as

laser iii. expensive and difficult to manufacture


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b. Multimode Step-Index Optical Fiber

similar to single-mode step-index except that the center core is much larger with the multimode configuration has large light to fiber aperture allows more external light to

enter the cable Advantages: i. inexpensive and simple to manufacture ii. easier to couple light into and out Disadvantages: i. more distortion - more losses ii. less bandwidth less information rate


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c. Multimode Graded-Index OpticalFiber

central core has nonuniform refractive index denser at the center continuous bending of light light rays traveling in the outermost area of the fiber travel

a greater distance than the rays traveling near the center(velocity is inversely proportional to refractive index)

Advantages and disadvantages: i. easier to couple light compared to single-mode step-

index but difficult compared to multimode step-index ii. more loss than single-mode but less loss than

multimode step-index


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Power losses reduce the : systems bandwidth information transmission rate efficiency over-all system capacity


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L

A = cable power loss


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Kinds of Losses:

1. Absorption Loss analogous to power dissipation in copper cables Caused by impurities in the fiber that absorb the light and convert it

to heat

Causes of Absorption Loss: Ultraviolet absorption caused by valence electrons in the silica

material from which fibers are manufactured (ionization) Infrared absorption photons of light are absorbed by the atoms of

the glass core molecules Ion resonance absorption caused by (water molecules

trapped in the glass during manufacture) ions in the material as wellas iron, copper and chromium molecules.


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2. Material, or Rayleigh Scattering Losses


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2. Material, or Rayleigh Scattering Losses caused by the submicroscopic irregularities which

are impurities. Spreads out the light rays when it strikes these

impurities


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3. Chromatic, or Wavelength Dispersion due to the different velocity that each wavelength

travel light do not arrive at the far end of the

fiber at the same time can be eliminated using a monochromatic light

source such as an injection laser diode (ILD) occurs only in fibers with a single mode of

transmission


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4. Radiation Loss caused by small bends and kinks in the fiber two types of bends:

microbends occurs as a result in the differences in thethermal contraction rates between the core and thecladding material

constant-radius bends caused by excessive pressure

and tension and generally occur when fibers are bentduring handling or installation


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5. Modal Dispersion sometimes called pulse spreading caused by the difference in the propagation times of light rays that take

different paths down a fiber occur only in multimode fibers reduced using graded-index fibers

Dispersion is expressed in: BLP Bandwidth Length Product (linewidth) Or BDP Bandwidth Distance Product

indicates what signal frequencies can be propagated through a given distance

of fiber cable as the distance of the fiber increases, the bandwidth decreases


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6. Coupling Losses - caused by imperfect physical connections - can occur at light source-to-fiber

connections, fiber-to-fiber connections andfiber-to-photodetector connections.

- caused by lateral misalignment, gapmisalignment, angular misalignment andimperfect surface finishes


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Optical Fiber Lecture

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