The technology of combining a number of optical wavelengths and then transmitting the same through a single fibre is called wavelength division multiplexing (WDM). Conceptually, the technology is similar to that of frequency division multiplexing (FDM) used in analogue transmission. Dense wavelength division multiplexing (DWDM) is anew born multiplexing technology in the fibre optic transmission, bringing about a revolution in the bit rate carrying capacity over a single fibre. The emergent of DWDM system is one of the important phenomena in development of optic fibre transmission. This article gives introduction of DWDM technology.
There has always been a technological talent to fulfill the constant need to extent the capacity of communication channel and DWDM (Dense Wavelength Division Multiplexing) has dramatically brought about an explosive enlargement of the capacity of fiber network, solving the problem of increasing traffic demand most economically.
DWDM is a technique that makes possible transmission of multiple discrete wavelengths carrying data rate as high as fiber plant allows over a single fiber unidirectionally or bidirectionally.
It is an advanced type of WDM in which the optical channels are more closely spaced than WDM.
PRINCIPLE OF DWDM TECHNOLOGY
In normal optical link there is one optical source at transmitting end and one photo detector at receiving end. Signals from different light sources use separate and unique assigned fiber for transmission of signal. As the spectral bandwidth of the laser source is very narrow, this type of transmission makes use of only a small portion of the entire optical band and remaining portion of the band is not used. In DWDM technology, the different light sources are first converted to pre-assigned wavelength according to the DWDM standards and then combined in such a manner that they occupy different portion of the available optical band. In between the two optical signals suitable guard band is also left, so that there is no interference from adjacent channels. Thus DWDM technology makes use of the entire optical bandwidth.
DWDM FUNCTIONAL SCHEMATIC
The system performs the following main functions.
Generating the signal: The source, the solid state laser, must provide stable light within the specific, narrow band width that carries the digital data, modulated as an analog signal.
Combining the signals: Modern DWDM systems employ multiplexers to combine the signal. There is some inherent loss associated with multiplexing and demultiplexing. These loss is dependent upon the number of challenge but can be mitigated with optical amplifiers, which boost all the wavelengths at once with out electrical conversion.
Transmitting the signals: The effect s of cross talk and optical signal degradation or loss must be reckoned with in fiber optic transmission. These affects can be minimized by controlling variables such as channel spacings, wavelength tolerance, and laser power levels. Over a transmission link, the signal may need to be optically amplified.
Separating the signals: At the receiving end, the multiplexed signals must be separated out. Although this task would appear to be simply the opposite of combining the signals.
Receiving the signals: The demultiplexed signals is received by photo detectors.
WDM ARCHITECTURE TYPES â€œ NETWORKS
The general architectural forms that have been most commonly used in WDM networks are wavelength routing network and broadcast â€œ and â€œ select network.
Wavelength routing networks are composed of one or more wavelength selective elements and have the property that the signals takes through the network is uniquely determined by the wavelength of the signal and port through which it enters the network.
So, for example, in figure (3 ) an nÃƒâ€”n network is shown in which n receivers through a network consist of several WDM elements by tuning to a selected wavelength the signal from a given laser can be rated as a selected output port on the network. Since there are n inputs and n output one might expect nÃ‚Â² wavelength would be required to form a complete interconnection. It turn out, how ever, that it can always be arranged so that with only n wavelengths, n inputs can be interconnected with n output in a completely non interfering way.
In figure ( 4 ) the wavelength to go from input SÃ‚Â¹ to output port R3 is ÃŽÂ»2. it is possible to address each output port uniquely by choice of wavelength and no out port can receive any given wave length from more than one input. This is extendible to any size network with n wavelengths but it does require nÃ‚Â² interconnection fibers between the WDM stages.
The second major architectural type is the broadcast - and â€œ select network illustrated in figure (5 ). In this network, all inputs are combined in a star coupler and broadcast to all output. Several different possibilities exist depending on whether the input laser, the output receivers, or both are made tunable. If the input lasers are tunable and output receivers are tuned to fixed wavelength, the architecture is basically a space-division switch in function. The properties of this network are that it uses wavelength addressing of the output port, but that with only a single wavelength selectable at each output, only point to point connection are possible and multicast connection can not be achieved.
If the output receivers are made tunable but the input lasers are tuned to fixed unique wavelength, this architecture supports multicast connection. This is achieved by arranging to have more than one receivers tuned the same source wavelength at the same time. Output port exists in this mode and is exacerbated by multicast function. If both the transmitters and receivers are made tunable ,the possibility exists for reducing the number of wavelength required but the result that there are not enough wavelength available to support simultaneous nÃƒâ€”n interconnection.
DWDM SYSTEM ARCHITECTURE
A typical 8-channel DWDM system block is shown in the figure ( ). The main components are,
- VOA (variable optical attenuator)
- MUX (multiplexer)
- DE MUX (de multiplexer)
- OPTICAL FIBRE AMPLIFIERS
-Erbium-dropped fiber optic amplifier
- Optical add-drop multiplexer (OADM)
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.
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).
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.
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.
It is two stage (EDFA) amplifier consisting of pre-amplifier and booster amplifier. With out two stages it is not possible to amplify the signal up to 33dB on EDFA principle avoiding large ASE (amplifier spontaneous emission) noise.
This amplifier along is used at the terminal to interface the DEMUX and line for receiving coming from the distant station. Hence the attenuated line signal is amplified to a level of 3dBm to 10dBm before entering into DEMUX unit.
OPTICAL SUPERVISORY CHANNEL (OSC)
The function of transmission of additional data at a separate wavelength of lower optical power with out any optical safety provision, accompanied with and independent of the main optical traffic signal, is performed by this OSC.The OSC helps management to control and monitor the optical line devices. The management for fault location, configuration, performance and security.
OPTICAL ADD/DROP MULTIPLEXER (OADM)
Adding or dropping of channels at optical level is possible by using optical add/drop multiplexer module. It is a unidirectional module with facility for dropping or adding optical channel of specific wavelength. The dropping and adding of the optical wavelength this performed with fixed optical filters. With the help of OADM module it is possible to insert or drop maximum for optical wave lengths at any intermediate stations.
Fig(9 ):optical add\drop MUX
Depending upon the design, pre and post optical amplifiers may or may not be present in OADM.There are two types of OADM.The first type is fixed device that is physically configured to add/drop pre defined wavelengths. The second type is reconfigurable and capable of dynamically selecting the wave length to be added or dropped.
ADVANTAGES OF DWDM TECHNOLOGY
Â¢ The capacity of transmission media can be upgraded easily by using DWDM technology. The capacity of existing DWDM system can be upgraded by deploying higher channel capacity system.Thus, The need of laying new fibers for increasing capacity of transmission media is avoided.
Â¢ Bit rate transparency: in DWDM system, optical channels can carry any transmission format. thus the different wavelengths from different systems can be transmitted simultaneously and independently over the same fiber without need for a common ATM,Gigabit Ethernet etc over a common layer. Thus DWDM system can transport any type of optical signal.
Â¢ Quick deployment: The DWDM technology is, generally, deployed using existing fibers. The time required for laying new fiber is much more as compared to equipment deployment time.hence, the deployment of dwdm systems can be done quickly.
Â¢ Economical: The DWDM system is cheaper as compared to overall cost of laying new fiber for increasing transmission capacity. In DWDM system, one optical amplifier is used for amplification of all the channels, hence per channel cost is drastically reduced as compared to providing regenerator for individual channels in SDH network.
Â¢ Wavelength routing: In DWDM system, by using wavelength sensitive optical routing devices, it is possible to route any wavelength to any station. Thus it is possible to use wavelength as other dimension, in addition to time and space in designing transmission network.
Â¢ Wavelength switching: In DWDM system, wavelength switching can be accomplished by using OADM, optical cross connect and wavelength converters.thus, it is possible to reconfigure the optical layer using wavelength switched architecture.
PROTECTION INDWDM SYSTEM: DWDM link can be designed to provide either path switched protection (two fibre working) or bi-directional line switched protection (four fibre working). The equipment protection can also be provided by using additional set of equipment .the protection facility is not available in the equipment being deployed in telecom network. In case of falure, the protection system of SDH ring will take care of the fault.
REQUIREMENT OF FIBRE
There are two categories of optical fibres namely mono mode and multi mode. The mono mode fibre is used for long haul transmission and it is of following three types.
1. Non-dispersion shifted fibre(NDSF)
2. Dispersion shifted fibre(DSF)
3. Non-zero dispersion shifted fibre(NZDSF)
To optimize the performance of fibre in L (1625nm) and C (1550nm) bands, it was designed in such a way that dispersion was very low at 1310nm(S band) and this type of fibre is called NDSF.Later anew type of fibre was developed in which the zero dispersion was shifted to 1550nm region called DSF.But due to the non linear effect the DSF is not suitable for DWDM. NZDSFis designed in such away that the dispersion is low at 1550nm but not zero.
The demand of bandwidth is increasing day by day, especially for data traffic. Service providers are required to provide the bandwidth dynamically and in shortest possible time. This can only be done by DWDM. In future advanced DWDM components will be available. Thus, it will be possible to manage the optical signal dynamically, which will allow more flexibility to the service providers.
1. Shri H. Saha, Shri Nural Anowar, DWDM System & Testing TELECOMMUNICATION March â€œApril 2002
2. P.K. Pandy, Dense Wave Length Division Multiplexing TELECOMMUNICATION November â€œDecember 2002