Dense Wavelength Division Multiplexing

INTRODUCTION

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 – NETWORKS

 

            The general architectural forms that have been most commonly used in WDM networks are wavelength routing network and broadcast – and – select network.

Fig(3)

 

                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 n×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 n² 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.

                                                                                      Fig(4)

 

            In figure ( 4 ) the wavelength to go from input S¹ to output port R3 is λ2. 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 n² interconnection fibers between the WDM  stages.

Fig(5)

 

                The second major architectural type is the broadcast  - and – 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  n×n interconnection.

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