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Why Filters?

• Sometimes, it is necessary to select one

particular channel from the system for special

transmission

• In a WDM optical system each optical channelis characterized by an individual wavelength

• Therefore the capability of optical selection is

required which is achieved using optical filters

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What does a filter do?

• It allows only one wavelength to pass through

and blocks all other wavelengths

Optical filter selects optical signals at one wavelength (λ3)

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Why Optical Filters for dispersion

management?

• A shortcoming of DCFs is that a relatively longlength (> 5 km) is required to compensate theGVD acquired over 50 km of standard fiber.

•

This adds considerably to the link loss, especiallyin the case of long-haul applications.

• Schemes have been developed for dispersionmanagement

•Most are classified in the category of opticalequalizing filters.

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Dispersion management in long-haul fiber link using optical filters after each

amplifier.

Filters compensate for GVD and also reduce amplifier noise.

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Types of filters used

• Mach-Zehnder interferometer

• Fiber Bragg gratings

• Chirped fiber gratings

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• Optical filters can be made using an

interferometer

• It is sensitive to the frequency of the input

light

• Acts as an optical filter because of its

frequency-dependent transmission

characteristics

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Mach-Zehnder (MZ) filter

• A Mach–Zehnder (MZ) interferometer can also

act as an optical filter.

• A single MZ interferometer does not act as an

optical equalizer.• But a cascaded chain of several MZ

interferometers forms an excellent equalizing

filter

• Such filters have been fabricated in the form of aplanar lightwave circuit by using silica waveguides

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Mach Zehnder interferometer

A planar lightwave circuit made using of a chain of

Mach–Zehnder interferometers

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Operation

• Operation of the MZ filter understood from the

unfolded view shown• The device is designed such that the higher-

frequency components propagate in the longerarm of the MZ interferometers.

• As a result, they experience more delay than the

lower-frequency components taking the shorterroute.

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Fiber Bragg grating (FBG)•

Has a periodic variation of the refractive index(RI) in the core along the fiber length.

• Change of the core RI is formed by intensiveexposure to UV radiation using an interference

pattern• For a FBG the following equation holds

2Λneff = λB

Λ=grating period; neff = effective core RI;

λB = Bragg wavelength (center wavelength ofchannel to be reflected)

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Principle of operation

Index variation in fiber Bragg grating

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• A fiber Bragg grating acts as an optical filter

because of the existence of a stop band

(the frequency region in which most of theincident light is reflected back)

• The stop band is centered at the Bragg

wavelength λB

FBG as filter

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• The periodic nature of index variations

couples forward- and backward-propagating

waves at wavelengths close to the Bragg

wave-length• Thus, waves are reflected depending on their

frequency over a bandwidth determined by

the grating strength.

FBG as filter

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Spectral response

Spectral response on the reflection of a fibe Bragg grating

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Chirped Fiber Gratings (CFG)

• The optical period neff Λ in a chirped grating is

not constant but changes over its length

• Bragg wavelength (λB) also varies along the

grating length

• Hence, different frequency components of an

incident optical pulse are reflected at different

points

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Chirped Fiber Gratings (CFG)

• Stop band of CFG results from overlapping of

many mini stop bands (each shifted as Bragg

wavelength shifts along the grating)

• Low-frequency components of a pulse are

delayed more because of increasing optical

period (and the Bragg wavelength)

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Dispersion compensation by a linearly chirped fiber grating:

a) Index profile along grating lengthb) reflection of low and high frequencies at different

locations within the grating because of variations in the

Bragg wavelength.

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