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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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