28-11-2012, 05:28 PM
OPTICAL NETWORK ELEMENTS
OPTICAL NETWORK ELEMENTS.ppt (Size: 5.33 MB / Downloads: 45)
DWDM technology
is more cost effective
more flexible for up gradation of channel capacity,
add/drop of channels,
re-routing
traffic distribution,
supports all types of network topology
protection systems
synchronization
TRANSPONDER (TP):
is interface between STM-n wide pulse optical signal and MUX/DEMUX equipments.
optical signal may be co-located or coming from different physical mediums, different protocols and traffic types.
converts the wide pulse signal into a narrow wavelength (spot or colored frequency) of the order of nano-meter (nm) with spacing of 1.6 nm; sending to MUX.
In the recieve direction, colored output from the DEMUX is converted to wide pulse optical signal.
output power level is +1 to –3 dbm in both directions.
The conversion is optical to Electrical and Electrical to optical (O to E & E to O) in 2R or 3R method.
In 2R, regeneration and re-shaping are done while in 3R, regeneration, re-shaping and re-timing are performed.
TP may be wavelength colour and bit rate dependent or tunable for both (costly and not used).
in 2R, any bit rate, PDH, STM-4 or STM-16 may be the channel rate.
The unit has limitation with receiver sensitivity and overload point.
MULTIPLEXER (MUX) AND DEMULTIPLEXER (De-MUX)
DWDM systems send signals from several stations over a single fiber, they must include some means to combine the incoming signals.
This is done with a Multiplexer, which takes optical wavelengths from multiple fibers and converges them into a beam.
At the receiving end the system must be able to separate out the transmitted wavelengths of the light beam so that they can be discretely detected.
Demultiplexer separates the received beam into its wavelength components and couples them into individual fibers.
Multiplexers and Demultiplexers can be either passive or active in design.
Passive design uses Prism, diffraction gratings, or filters
while active design combine passive devices with tunable filters,.
OPTICAL AMPLIFIERS (OAs)
were used as repeaters in simplex mode.
One fiber was used in send path and second fiber was used in return path.
the latest optical amplifiers will operate in two directions at the same time.
We can even use the same wavelength in two directions provided two different bit rates are employed.
A single fiber can, therefore be used for duplex operation.
The optical amplifiers must also have sufficient bandwidth to pass a range of signals operating at different wavelengths.
For example, an SLA with a spectral bandwidth of say, 40 nm, can handle about different ten optical signals.
Semiconductor Amplifiers :
basic travelling wave semiconductor laser amplifier (TWSLA) consists of a Fabry-Perot laser structure with facet reflectivities of less than 10-3 to ensure optimal gain, bandwidth, insensitivity to variations in bias current and input polarization.
TE and TM components refer to the output powers in the transverse electric and magnetic modes of the semiconductor wave-guide.
possess a degree of sensitivity to input polarization
for some 3 dB variation of the mean gain.
careful device design or the use of an automatic gain control system can reduce this effect to be negligible in a system's context.
Semiconductor optical amplifiers (SOAs)
are essentially laser diodes, without end mirrors, which have fiber attached to both ends.
They amplify any optical signal that comes from either fiber and transmit an amplified version of the signal out of the second fiber.
SOAs are typically constructed in a small package, and they work for 1310 nm and 1550 nm systems.
they transmit bidirectionally, making the reduced size of the device an advantage over regenerators of EDFAs.
the drawbacks to SOAs include high-coupling loss, polarization dependence, and a higher noise figure.
Erbium Doped Fiber Amplifiers
Optical fibres can also achieve direct amplification of a light signal without converting into its electrical equivalent by doping with a special material called erbium.
This amplifier depends on material gain defined by the erbium doping of a relatively short section (1-100 m) of silica fibre pumped with optical rather than an electrical energy.
can amplify a weak light signal if pumped with a laser. Since these optical fibre amplifiers can amplify light signals without any complicated equipment,
they are useful for non-repeated long distance communications such as transoceanic undersea optical cables and for the multicasting of general optical subscriber systems or optical CATV systems.
OADM
An optical filter is used to isolate, or drop the desired wavelength from multiple wavelengths arriving on a fibre.
Once a wavelength is dropped another channel employing the same wavelength can be added, or inserted on to the fiber as it leaves OADM.
A simple ADM has only 4 inputs and outputs channels, each with 4 wavelengths.
In OADM ,wavelengths might be amplified ,equalized or further processed.
OADM arranges the wavelengths from input fiber to output fiber by using optical cross connect.