ObjectivesUnderstand the importance of fiber-optic

technologies in the information societyIdentify the fundamental components of a fiber-optic

cableUnderstand the principles by which light travels

within a fiber-optic cable, including refraction and total internal reflection

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Background on Optical Communications

Fiber-optic communicationTrapping light inside an optical fiberCan carry any form of informationFiber is an optical medium, which means it is capable

of transmitting lightBased on total internal reflection (TIR)

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Tyndall’s Experiment

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Structure of Fiber-Optic Cables

CoreCladdingCoating

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General Structure of Fiber-Optic Cables

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General Structure of Fiber-Optic Cables (continued)

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Structure of Fiber-Optic Cables – CladdingCylindrical material made of glass or specialized

plasticCentral portion of the fiberLight signal carrying the information travels

through the core The diameter of the core can range from a couple

of micrometers (µm-one millionth of a meter) to a couple of millimeters (mm-one thousandth of a meter)

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Structure of Fiber-Optic Cables – Jacket

Surrounds the cladding Insulates and protects the fiber from physical damage

and environmental effects, such as moisture, that might interfere with the inner workings of the cable

Usually made of opaque plastic or another type of material

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How Light Travels Through FiberTIR is the basis of fiber-optic communicationTIR may be considered to be an extreme case of

refractionWhen a light ray strikes a boundary of two materials

with different RIs, it bends, or in other terms, refracts to an extent that depends on the ratio of the RIs of the two materials

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Fiber-Optic Communication Systems

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• Pulses of electricity corresponding to the bits arrive at the input transducer – A device that converts one form of energy to

another

• Devices similar to light bulbs are used as optical transducers at the input of fiber-optic cables to convert electricity into light– Light emitting diodes (LED)

– Laser diodes (LD)

Fiber-Optic Communication Systems (continued)

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• Electrical signals arriving at the input of optoelectronic devices are used to modulate the light source

• The modulated optical signal is emitted by the source and coupled into the cable

• Once the light is trapped inside the cable, it travels to the other end, where it is demodulated and an output transducer – Photodiode (PD) or phototransistor

Fiber Optic Data Links

Light Used In Fiber OpticsFiber optic systems transmit using

infrared light, invisible to the human eye, because it goes further in the optical fiber at those wavelengths.

Wavelength-Division Multiplexing

RefractionThe phenomenon that causes a spoon inside a clear glass of water to appear shorter and bent to an observer looking from the outside

Light rays that strike the water/air boundary bend to create an image that is shorter than the actual height of the spoon in the water, because water has a higher RI than air

TIR is the phenomenon that makes the side of an aquarium act as a mirror when viewed at an appropriate angle

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Refractive Index

Refractive index of an optical medium = Speed of light in a vacuum (300,000,000 meters per second)/speed of light in the optical medium

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Refractive Index (continued)

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Total Internal Reflection

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Total Internal Reflection (continued)

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Total Internal Reflection (continued)

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Types of Fiber-Optic Cables

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• Based on mode of propagation• Single-mode fiber• Multi-mode fiber

• Based on refractive indices (physical construction)• Step index fiber• Graded index fiber

Multi-mode (Step-index), Graded Index, Single Mode

Cross sectional views ( should be circles*)Multi-mode Graded Index Single Mode

125m

~10m~80m

Accurate alignment less neededfor splicing. Higher loss. Majortime dispersion of short opticalpulses due to different geometricpaths. Less used today, buthistorically important.

Accurate alignment less neededfor splicing. Higher loss. Reduceddispersion due to lower wave speedin central rays, higher wave speed(lower index) in outer part of core.Used for “last mile” and service drops,with single mode reserved for long runs.

Accurate alignment neededfor splicing. Best low loss.Most widely used fiber typefor long spans.

*non-circularity of images is an artifact of computer artwork software.

Step Index Fiber

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• A fiber-optic cable with a uniform refractive index throughout its core is classified as a step index fiber

• Both single-mode and multi-mode

Graded Index Fiber

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• A cable whose core refractive index is non-uniform and varies gradually is classified as a graded index fiber

• The value of the RI of a graded index fiber is highest in the center of the core and gradually diminishes towards the cladding

Classification based on Modes of propagation

Two types of modes exist in both, the step- and the graded-index fibers:

Single-mode: As the name implies, a single-mode fiber sustains only one mode of propagation.

Multimode: Multimode fibers contain many hundreds of modes.

Single Mode and Multimode

Single Mode Step-indexThis fiber has a core of constant refractive index

but its diameter is only 8-12 µm. This narrow core limits the propagation of

waveguide modes to a single one as the angle required for the higher modes would not be achievable in this configuration.

The step in refractive index is small (about 1%) but this is sufficient to produce the total internal reflection required to confine the wave to the core of the fiber.

Cladding

Cladding

Core

Core

Multimode Step-indexThe fiber has a core of constant refractive

index surrounded by a cladding of lower refractive index

Step-index fiber is so called because the refractive index of the fiber “steps” up as we move from the cladding to the core of the fiber.

The diameter of the core is about 50-100 µm and this is wide enough to allow several different waveguide modes to propagate down the fiber.

Multimode Step-index Fiber

Multimode Graded-indexThe core is similar in size to that of

multimode step-index fiberHowever, in this case the refraction index is

higher at the centre and decreases outwards in a specified non-linear fashion toward the lower refractive index of the cladding.

Multimode Graded-indexThis graded refractive index profile causes

the modes of higher orders which travel at higher velocities to be reflected in the outer portion of the core and thus travel a longer distance than the lower order modes which are slower but are confined to the inner portion of the core.

Graded-Index Fiber

Multimode Graded-index Fiber

Graded-index Fiber

Overview of the different fibers

Fiber-Optic Technology: Benefits/Drawbacks

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• High transmission rate• Immunity to electromagnetic interference• Low attenuation• High security• Small weight and size• Low power consumption• High installation cost• Difficulty in splicing