06-11-2012, 01:39 PM
WAVELENGTH CONVERTERS
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Wavelength converters are devices which changes the
wavelength of an input signal.
Wavelength converters are useful components in WDM networks
for three major reasons.
1. Convert the input wavelength to a wavelength that is suitable
for use within the network
2.Improve utilization of the available wavelengths on the network
links.
3.Used at the boundaries between different networks.
CLASSIFICATION
Wavelength converters are classified into the following based on the
range of wavelengths they can handle at their input and output
Fixed input -fixed output wavelength converter.
Fixed input -variable output wavelength converter.
Variable input -fixed output wavelength converter.
Variable input - variable output wavelength converter.
ELECTRO-OPTIC CONVERSION
In electro-optic conversion an input light signal is converted into
an electrical signal, regenerated and re-transmitted at a different
wavelength using a tunable laser.
Variable input -variable output wavelength converter.
4R process
1. Reamplification
2. Reshaping
3. Retiming
4. Reallocation of wavelengths
CROSS-GAIN MODULATION
An intensity modulated input signal and a continues wave probe
signal are applied to SOA.
The intensity modulated data signal, produces variation in carrier
density within the SOA which produces inverted gain modulation in
SOA medium.
These gain modulations can be imprinted onto a continuous wave
signal called probe signal .Thus the probe signal acquires an
inverse copy of the data.
Since the wavelength of probe signal is different from pump signal,
wavelength conversion is obtained where data has been shifted
from pump signal wavelength to a probe signal wavelength.
CROSS-PHASE MODULATION
The intensity modulated data signal, produces variation in carrier
density within the SOA.
The carrier density variation gives rise to a change in the
refractive index which is proportional to amplitude of the input
signal and inversely proportional to carrier density variation
These refractive index variation produce a phase modulation
when a continues wave probe signal is coupled to SOA.
The changes in phase are converted to changes in amplitude by
Mach-Zehnder interferometer.
A great advantage of this approach is that the output from the
wavelength converter is not inverted.
OPTICAL CROSS CONNECTS
Optical cross connects (OXC) are essentially switches which
connect any of the input ports to any of the output ports. It is an N
× N optical switch.
In the hybrid version of optical switches, first converts optical signal
to electrical signals, do switching electronically and then reconvert
the electrical signals to optical signals.
An OXC is an all optical switch which work entirely at photonic
level.
Cost of OXCs are high, hence hybrid switches are used commonly.
In large bandwidth applications, OXCs are more effective.
ACH-ZEHNDER INTERFEROMETRIC CROSSCONNECT
When light is input at Port 1, it is split evenly between both arms of the
interferometer. If there is no voltage applied the light arrives at the second3
dB coupler are in phase (since the arms of the interferometer are equal in
length). There is now constructive interference and light which entered at
Port 1 will now exit at Port 3. Likewise light entering at Port 4 will be output at
Port 2. This is the “cross” state.
When a voltage is applied across the electrodes the RI in each waveguide is
changed. The voltage is arranged to be such that the change in RI results in
the light on the two arms arriving at the second coupler is out of phase with
each other. There is now destructive interference and light which entered at
Port 1 will now exit at Port 2. Light entering at Port 4 will now leave at Port 3