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Multiplexing In Optical Fiber Communication

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Multiplexing

• Multiplexing– a process where multiple message signals or data streams

are combined into one signal over a shared medium.• Types

– Time division multiplexing– Frequency division multiplexing

• Optically– Time division multiplexing– Wavelength division multiplexing

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Higher data rate. ; Low loss ; Longer distanceLess weight/size ; Interference freeElectrical isolation ; Security

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Option To Fulfill the demand

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Time Division Multiplexing

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Wavelength Division Multiplexing

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Continued …

• The technology of combining a number of separate optical channels on a single fiber is known as WDM.

• It is based on the fact that the light of different wavelength doesn’t interfere and can be separated efficiently.

• It is similar to FDM.• Different data channels are carried by different wavelengths.

• 1997 Bell Labs– 100 beams, each at 10 Gbps

• Systems of 160 channels of 10 Gbps now available

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• The WDM transmission is performed in 2nd & 3rd windows of the wavelength.

• Let we have a source of λ wavelength and ∆λ spectral width, then BW of the WDM system is

Continued …

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• In 1310 nm range with spectral width 80 nm the available BW is 14 THz.

• In 1550 nm range with spectral width 120 nm the available BW is 30 THz.

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Channel Spacing

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• By using number of light sources and allotting some guard band between wavelength of two source, a good quality transmission can be done with WDM.

Continued …

Transmitters

DWDM Multiplexer

Power Amp

Line Amp

Line Amp

Receive Preamp

200 km

DWDM DeMultiplexer

Receivers

Optical fibre

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• At transmitter, there are many independently modulated light sources.

• Each emitting at a unique wavelength.• The Multiplexers are required to combine these optical

signals and couple them onto a single fiber.• Multiplexer should provide a low-loss path to each

optical source to fiber.

• At the receiver a Demultiplexer is needed to separate the optical signals into appropriate detection channel.

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• Wavelength multiplexer types include– Fiber Couplers– Fiber gratings

• Wavelength demultiplexer types include– Grating Demultiplexer– Tunable filter

Grating Multiplexer Demultiplex

erGrating

Grin Rod Lens

Fibres

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• GRIN lenses are short cylindrical lengths of glass with a graded refractive index.

• The index decreases with the square of the distance from the axis of the lens.

• In this they are very similar to graded index fibre but they are usually quite a lot thicker (1-2 mm)..

• GRIN lenses operate by internal refraction as light passes through material of varying refractive indices.

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• A diffraction grating is a device that reflects or refracts light by an amount varying according to the wavelength.

• It consists of a very closely spaced set of parallel lines or grooves made in a mirror surface of a solid material.

• A grating can be formed in almost any material where we vary the optical properties (such as refractive index) in a regular way with a period close to the wavelength.

• Reflective gratings are wavelength-selective filters.• In optical communications they are used for splitting and/or

combining optical signals in WDM systems

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WDM MUX/DMUX