Summary notes on Optical fibers

8/7/2019 Summary notes on Optical fibers

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Summary notes on Optical fibers

A LIGHT WAVE is a form of energy that is moved by wave motion.

WAVE MOTION is defined as a recurring disturbance advancing throughspace with or without the use of a physical medium.

SCIENTIFIC EXPERIMENTS seem to show that light is composed of tinyparticles, while other experiments indicate that light is made up ofwaves. Today, physicists have come to accept a theory concerning lightthat is a combination of particle (ray) theory and wave (mode) theory.

TRANSVERSE WAVE MOTION describes the up and down wave motion that is atright angle (transverse) to the outward motion of the waves.

LIGHT RAYS , when they encounter any substance, are either transmitted,refracted, reflected, or absorbed.

REFLECTION occurs when a wave strikes an object and bounces back(toward the source). The wave that moves from the source to the objectis called the incident wave , and the wave that moves away from theobject is called the reflected wave .

The LAW OF REFLECTION states that the angle of incidence is equal tothe angle of reflection.


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REFRACTION occurs when a wave traveling through two different mediumspasses through the boundary of the mediums and bends toward or awayfrom the normal .

The RAY THEORY and the MODE THEORY describe how light energy istransmitted along an optical fiber.

The INDEX OF REFRACTION is the basic optical material property thatmeasures the speed of light in an optical medium.

SNELL'S LAW OF REFRACTION describes the relationship between theincident and the refracted rays when light rays encounter the boundarybetween two different transparent materials.

TOTAL INTERNAL REFLECTION occurs when light rays are totally reflectedat the boundary between two different transparent materials. The angleat which total internal reflection occurs is called the critical angleof incidence.

The CORE, CLADDING, and COATING or BUFFER are the three basic parts ofan optical fiber.


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The RAY THEORY describes how light rays propagate along an opticalfiber. MERIDIONAL RAYS pass through the axis of the optical fiber. SKEW RAYS propagate through an optical fiber without passing through itsaxis.

BOUND RAYS propagate through an optical fiber core by total internal

reflection. UNBOUND RAYS refract out of the fiber core into thecladding and are eventually lost.

The ACCEPTANCE ANGLE is the maximum angle to the axis of the fiber thatlight entering the fiber is bound or propagated.

The light ray incident on the fiber core must be within the acceptancecone defined by the acceptance angle to be propagated along an opticalfiber.

NUMERICAL APERTURE (NA) is a measurement of the ability of an opticalfiber to capture light.

The MODE THEORY uses electromagnetic wave behavior to describe thepropagation of light along an optical fiber. A set of guidedelectromagnetic waves are called the modes of the fiber.

MODES traveling in an optical fiber are said to be transverse. Modesare described by their electric, E(x,y,z), and magnetic, H(x,y,z),fields. The electric field and magnetic field are at right angles toeach other.


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NORMALIZED FREQUENCY determines how many modes a fiber can support. Thenumber of modes is represented by the normalized frequency constant.

SINGLE MODE and MULTIMODE

FIBERS are classified by the number of modes that propagate along the

optical fiber. Single mode fibers propagate only one mode because thecore size approaches the operational wavelength. Multimode fibers canpropagate over 100 modes depending on the core size and numericalaperture.

ATTENUATION is the loss of optical power as light travels along anoptical fiber. Attenuation in an optical fiber is caused by absorption,scattering, and bending losses.

DISPERSION spreads the optical pulse as it travels along the fiber.Dispersion limits how fast information is transferred.

ABSORPTION is the conversion of optical power into another energy form,such as heat. INTRINSIC

ABSORPTION is caused by basic fiber-material properties. EXTRINSICABSORPTION is caused by impurities introduced into the fiber material.

SILICA FIBERS are predominately used in fiber optic communications.They have low intrinsic material absorption at the wavelengths ofoperation.

The WAVELENGTH OF OPERATION in fiber optics is between 700 nm and 1600nm. The wavelength of operation is between the ultraviolet (below 400nm) and infrared (above 2000 nm) intrinsic absorption regions.

EXTRINSIC ABSORPTION occurs when impurities, such as hydroxyl ions(OH - ), are introduced into the fiber. OH - absorption peaks define threeregions or windows of preferred operation. The first window is centeredat 850 nm. The second window is centered at 1300 nm. The third windowis centered at 1550 nm.


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Q.39 What are the main causes of absorption in optical fiber?Q.40 Silica (pure glass) fibers are used because of their low intrinsicmaterial absorption at the wavelengths of operation. This wavelength ofoperation is between two intrinsic absorption regions. What are thesetwo regions called? What are the wavelengths of operation for these tworegions?Q.41 Extrinsic (OH - ) absorption peaks define three regions or windows ofpreferred operation. List the three windows of operation.

Q.42 What is the main loss mechanism between the ultraviolet andinfrared absorption regions?Q.43 Scattering losses are caused by the interaction of light withdensity fluctuations within a fiber. What are the two scatteringmechanisms called when the size of the density fluctuations is (a)greater than and (b) less than one-tenth of the operating wavelength?

Q.44 Microbend loss is caused by microscopic bends of the fiber axis.List three sources of microbend loss.Q.45 How is fiber sensitivity to bending losses reduced?

Q.46 Name the two types of intramodal, or chromatic, dispersion.Q.47 Which dispersion mechanism (material or waveguide) is a functionof the size of the fiber's core relative to the wavelength ofoperation?

Q.48 Modes of a light pulse that enter the fiber at one time exit thefiber at different times. This condition causes the light pulse tospread. What is this condition called?

ANSWERS TO QUESTIONS Q1. THROUGH Q48.

A1. Photons.

A2. Transverse-wave motion.A3. Light waves are either transmitted, refracted, reflected, orabsorbed.A4. Transparent.A5. Opaque.A6. The law of reflection states that the angle of incidence is equalto the angle of reflection.A7. When the wave is nearly parallel to the reflecting surface.A8. When the wave is perpendicular to the reflecting surface.A9. The law of reflection.A10. Depends on the bending caused by the velocity difference of thewave traveling through different mediums.A11. Transmitted.A12. Diffusion.A13. The ray theory and the mode theory.A14. The index of refraction.A15. Light will travel faster in an optical material that is lessdense.A16. Part of the light ray is reflected back into the glass and part ofthe light ray is refracted (bent) as it enters the air.A17. Total internal reflection occurs when the angle of refractionapproaches 90 degrees. This condition occurs when the angle ofincidence increases to the point where no refraction is possible.


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A18. Critical angle of incidence.A19. Core, cladding, and coating or buffer.A20. Core.A21. The ray theory.A22. Total internal reflection.

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Summary notes on Optical fibers