Dense Wavelength Division Multiplexing

DWDM SYSTEM ARCHITECTURE

A typical 8-channel DWDM system block is shown in the figure ( ). The main components are,

 TP (transponders)

 VOA (variable optical attenuator)

 MUX (multiplexer)

 DE MUX (de multiplexer)

 OPTICAL FIBRE AMPLIFIERS

• Erbium-dropped fiber optic amplifier

• Booster amplifier

• Pre-amplifier

• Line amplifier

 Optical add-drop multiplexer (OADM)

fig(6)

 

TRANSPONDERS

 

                This unit interfacing wide pulse optical signal and MUX/DMUX equipment. It converts the wide pulse signal into a narrow wavelength of the order of  1.6nm, sending to MUX.

 

            In the reverse direction, coloured output from DMUX is converted to wide pulse optical signal.

The transponders are of two types namely transmit transponders and receive transponders. The function of transmit transponder is to convert the incoming  optical signal into pre-defined optical wavelength. The transponder (transmit) first converts the optical signal to an electrical signal and performs reshaping, retiming and  retransmitting functions, also called 3R functions. The electrical signal is then used to drive the laser, which generates the optical signals having optical wavelength. The output from the all transponders (transmits) is fed to combiner  in order to combine all optical channels  in optical domain. In receive transponder, reverse process takes place.

 

                Individual wavelengths are first split from the combined optical signal with the help of  splitter and then fed to individual receive transponders, which convert the optical signal to electrical, thus 3R function and finally convert the signal back to the optical. Thus the individual channels are obtained. As the output of the transponder is factory set to a particular wavelength, each optical channel requires unique transponder.

 

VARIABLE OPTICAL ATTENUATOR (VOA)

 

               

                This is a passive network like pre-emphasis required to adjust for uniform distribution of signal level over EDFA band so that individual channel optical output power of MUX unit remain  same irrespective of the number of channels being loaded in the system.

 

COMBINER & SPLITTER

 

            The DWDM system transmits several optical signals over a single fiber. All the signals are combined at the transmission end and again split at receiving end. The combining is done by combiner, also called multiplexer and splitting is cone by splitter,  also called  demultiplexer.

fig(7)

                The combiner and splitter can be either passive or active devices. Passive devices are based on prisms, diffraction gratings or filters, whereas the active  devices are combination of  passive devices and tunable filters.

 

                Multiplexer is an optical device and converges all the colour rays to combine on one point  to make  a  broadband pulse. Here in 8-channel systems, the 8 colour rays from 8 TPs  are connected  to the appropriate  input ports of the MUX  and the common  single port is the output connected to the  (Booster Amplifier).

 

                DEMUX performs the reverse function of MUX. By this unit, the received  beam  is separated  into its wavelength (colour) components coupling  them in appropriate  ports  to individual fiber. This DEMUX output may be fed to TP.

 

 

 

OPTICAL FIBRE AMPLIFIER

 

                In DWDM technology optical amplifiers are used instead of electrical amplifier. Thus pulse shaping and retiming functions are not done at repeater stations. The deployment of electrical amplifier in DWDM system is complex and expensive, hence optical amplifiers are used. The erbium doped fiber amplifier widely used in DWDM system. Depending upon the use of amplifier in the network it is called booster amplifier, line amplifier, preamplifiers etc.

 

 

ERBIUM DOPED FIBRE AMPLIFIER (EDFA)

 

                EDFAs are widely used in DWDM system for amplification of optical signals. Erbium is a rare earth element and emits light around 1550 nm region when it is exited. Thus it is most suited for DWDM operations as DWDM also makes use of 1550nm window. The block diagram of EDFA is shown in fig(8).

                        Fig(8)

1&6:tap couplers          3:wavelength coupler             7:pump laser

            2&5:isolaters                4:erbium-doped  fibre

It consists of doped fiber (10 to 50mlong), one or more pump lasers, a passive wavelength coupler, optical isolators and tap couplers. The tap couplers are wavelength insensitive with typical splitting ratio ranging from 99:1 to 95:5.They are generally used on both sides of the amplifier to compare the incoming signal with the amplified output. The optical isolators prevent the amplified signal from reflecting back into the device; otherwise it could increase the amplifier noise and decrease the efficiency.

 

                The weak optical signal enters the Erbium doped fibre, into which light is injected using pump laser. The injected light stimulates the Erbium atom to release the stored energy as additional light at 1550nm.This process continues and amplification of the signal takes place. The pump power is usually injected from the same direction as that of the signal flow. This is known as co-directional pumping. It is also possible to inject the pump power in opposite of the signal flow which is known as counter-directional pumping. It is also possible to use dual pump scheme, which results doubling of the gain of amplifier. The counter-directional pumping allows higher gain where as co-directional pumping give better noise performance.

 

                The requirement of low noise is a key factor in selecting the EDFA, because noise is also amplified along with the signal. The effect of noise is cumulative and cannot be filtered out. Therefore signal to noise ratio is an ultimate limiting factor that limits the total number of amplifiers that can be used in the concatenation

                Depending upon the gain, EDFAs are classified into following three categories.

                         I.      For long haul application.

                      II.      For very long haul application.

                   III.      For ultra long haul application.

 

                For long haul applications, amplifiers are deployed after every 80kms of sections length and maximum permissible fibre lose in one section is 22dB.For very long haul applications, amplifiers are deployed after 120kms of section length and maximum permissible fibre lose in a section is 33dB.In ultra long haul applications, line amplifiers are not used and a maximum permissible lose in a section in 44dB and it can cover upto 160km of distance.

 

BOOSTER AMPLIFIER

 

                It is basically an EDFA amplifier which boost the entire wide band optical signal coming from the out put of MUX.Here the total output power booster amplifier is constant irrespective of the number of channels being loaded to the system. Line is connected to the amplifier for transmission of signal to the distant end supporting the optical safety operation.

 

LINE (OFC MEDIA)

 

            This is the optical fiber media over which the DWDM signal travel. Attenuation and dispersion are the main limitation factors determining transmission distance and bit rate capacity etc.Normally 22dB and 33dB are taken as the line loses for hop length of long haul and very long haul system respectively. The very long haul line length can be 120kms with out repeater but with a number of repeaters cascaded, the length may be up to 600 kms which can further be increased up to 1200kms by using dispersion compensating module. However after such a distance it needs regeneration in electrical stage instead of repeater in optical stage only.

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