Optical Fiber

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Transcript of Optical Fiber

Optical Fiber Communications

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Fiber Optics Fiber optics uses light to send information(data). More formally, fiber optics is the branch of optical technology concerned with the transmission of radiant power (light energy) through fibers. Light frequencies used in fiber optic systems are 100,000 to 400,000 GHz.2

Brief History of Fiber Opticsexperimented with an apparatus he called a photophone. The photophone was a device constructed from mirrors and selenium detectors that transmitted sound waves over a beam of light.3

In 1880, Alexander Graham Bell

In 1930, John Logie Baird, an English scientist and Clarence W. Hansell, an American scientist, was granted patents for scanning and transmitting television images through uncoated cables.4

In 1951, Abraham C.S. van Heel of Holland and Harold H. Hopkins and Narinder S. Kapany of England experimented with light transmission through bundles of fibers. Their studies led to the development of the flexible fiberscope, which used extensively in the medical field.5

In 1956, Kapany coined the termed fiber optics.

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In 1958, Charles H. Townes, an American, and Arthur L. Schawlow, a Canadian, wrote a paper describing how it was possible to use stimulated emission for amplifying light waves (laser) as well as microwaves (maser).7

In 1960, Theodore H. Maiman, a scientist built the first optical maser.8

In 1967, Charles K. Kao and George A. Bockham proposed using cladded fiber cables.

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FIBER OPTIC DATA LINKS To convert an electrical input signal to anoptical signal To send the optical signal over an optical fiber To convert the optical signal back to an electrical signal

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Fiber Optic Data LinkOptical Transmitter

Input

A/D Interface

Voltage-tocurrent Converter

Light

Source

Source-tofiber interface

Optical Fiber

Fiber-tolight detector interface

Light

Detector

Current-tocurrent converter

A/D Interface

Output

Optical Receiver12

Fiber Optic Cable The cable consists of one or more glass fibers,which act as waveguides for the optical signal. Fiber optic cable is similar to electrical cable in its construction, but provides special protection for the optical fiber within. For systems requiring transmission over distances of many kilometers, or where two or more fiber optic cables must be joined together, an optical splice is commonly used.13

The Optical Receiver The receiver converts the optical signalback into a replica of the original electrical signal. The detector of the optical signal is either a PIN-type photodiode or PINavalancheavalanche-type photodiode.

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The Optical Transmitter The transmitter converts an electricalanalog or digital signal into a corresponding optical signal. The source of the optical signal can be either a light emitting diode, or a solid- state laser soliddiode. The most popular wavelengths of operation for optical transmitters are 850, 1300, or 1550 nanometers15

Types of Optical Fiber1. Plastic core and cladding 2. Glass core with plastic cladding (PCS) 3. Glass core and glass cladding (SCS)

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Modes of Propagation Single mode there is only one path forlight to take down the cableCladding

Multimode if there is more than onepathCladding

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Index ProfilesA graphical representation of the value of the refractive index across the fiber

StepStep-index fiber it has a central core with a uniform refractive index. The core is surrounded by an outside cladding with a uniform refractive index less than that of the central core GradeGrade-index fiber has no cladding, and the refractive index of the core is nonuniform; it is highest at the center and decreases gradually toward the outer edge18

Optical Fiber ConfigurationSingleStep1. Single-Mode Step-Index Fiber has a central core that is sufficiently small so that there is essentially one path that light takes as it propagates down the cable Multimode Step-Index Fiber similar to the singleStepsinglemode configuration except that the core is much larger. larger. This type of fiber has a large light-to-fiber light-toaperture, and consequently, allows more light to enter the cable. cable. Multimode Graded-Index it is characterized by a Gradedcentral core that has a refractive index that is nonnonuniform. uniform. Light is propagated down this type of fiber through refraction. refraction.19

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SingleSingle-Mode Step-Index Fiber StepAdvantages: There is minimum dispersion. Because all rays propagating down the fiber take approximately the same path, they take approximately the same amount of time to travel down the cable. Because of the high accuracy in reproducing transmitted pulses at the receive end, larger bandwidths and higher information transmission rates are possible with single- mode step-index fibers singlestepthan with other types of fiber. Disadvantages: Because the central core is very small, it is difficult to couple light into and out of this type of fiber. The source-to-fiber aperture is the source-tosmallest of all the fiber types. A highly directive light source such as laser is required. It is expensive and difficult to manufacture.20

Multimode Step-Index Fiber StepAdvantages: Inexpensive and easy to manufacture. It is easy to couple light into and out; they have a relatively high large source-to-fiber aperture. source-toDisadvantages: Light rays take many different paths down the fiber, which results in large differences in their propagation times. Because of this, rays traveling down this type of fiber have a tendency to spread out. The bandwidth and rate of information transfer possible with this type of cable are less than the other types.21

Single Mode Step Index

Index Profile

Multimode Step Index

Multimode Graded Index

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Acceptance Angle & Acceptance Cone The acceptance angle (or the acceptancecone half angle) defines the maximum angle in which external light rays may strike the air/fiber interface and still propagate down the fiber with a response that is no greater than 10 dB down from the peak value. Rotating the acceptance value. angle around the fiber axis describes the acceptance cone of the fiber input. input.24

Maximum Acceptance Angle =

Optical Fiber Acceptance Cone Acceptance Angle

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Numerical ApertureFor a step-index fiber: And NA = Sin (Acceptance Angle) NA =

For a Graded-Index: NA = sin (Critical Angle) The acceptance angle of a fiber is expressed in terms of numerical aperture. The numerical aperture (NA) is defined as the sine of one half of the acceptance angle of the fiber. It is a figure of merit that is used to describe the light-gathering or light-collecting ability of the optical fiber. The larger the magnitude of NA, the greater the amount of light accepted by the fiber from the external light source. Typical NA values are 0.1 to 0.4 which correspond to acceptance angles of 11 degrees to 46 degrees. Optical fibers will only transmit light that enters at an angle that is equal to or less than the 26 acceptance angle for the particular fiber.

Attenuation in Optical Fibers

L = the length of fiber in kilometers Therefore the unit of attenuation is expressed as dB/km

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Losses in the Optical Fiber Absorption Losses Material or Rayleigh Scattering Losses Chromatic or Wavelength Dispersion Radiation Losses Modal Dispersion Coupling Losses

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Absorption Lossesto power dissipation in copper cables; impurities in the fiber absorb the light and convert it to heat. Absorption in optical fibers is explained by three factors: Imperfections in the atomic structure of the fiber material The intrinsic or basic fiber-material properties The extrinsic (presence of impurities) fiber-material properties29

Absorption loss in an optical fiber is analogous

Absorption Essentially, there are three factors thatcontribute to the absorption losses in optical fibers: ultraviolet absorption, infrared absorption, ion resonance absorption.

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Ultraviolet Absorption Is caused by valence electrons in the silicamaterial from which fibers are manufactured. Light ionizes the valence electrons into conduction. The ionization is equivalent to a loss in the total light field and, consequently contributes to the transmission losses of the fiber.31

Infrared Absorption Is a result of photons of light that areabsorbed by the atoms of the glass core molecules. The absorbed photons are converted to random mechanical vibrations typical of heating.

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Ion Resonance Absorption Is caused by OH- ions in the material. OH The source of the OH- ions is water OHmolecules that have been trapped in the glass during the manufacturing process. Ion absorption is also caused by iron, copper, and chromium molecules.

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Material or Rayleigh Scattering Lossessubmicroscopic irregularities developed in the fiber during the manufacturing process. When light rays are propagating down a fiber strike one of these impurities, they are diffracted. Diffraction causes the light to disperse or spread out in many directions. Some of the diffracted light continues down the fiber and some of it escapes through the cladding. The light rays that escape represent a loss in the light power. This is called Rayleigh scattering loss.34

This type of losses in the fiber is caused by

Chromatic or Wavelength Dispersionthat emits light spontaneously such as the LED. Each wavelength within the composite light signal travels at a different velocity. Thus arriving at the receiver end at different times. This result