Theoretical Evaluation Of WDM Subsystem Communications Essay

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With increasing demand of consumers for more bandwidth we tend to use different transmission techniques with increasing performance in terms of bandwidth. Techniques include different forms of multiplexing. The main forms of multiplexing used are time division multiplexing (TDM), Frequency division multiplexing (FDM) and Wavelength division multiplexing (WDM). Each technique employed increases the transmission capability of the medium by some factor.

Developments in optical fibre technology gave origins to optical networks. Application of WDM on an optical network increases the capacity of the existing fibres.WDM employs different light wavelengths to transmit data parallel-by-bit or serial-by-character. It is a very crucial component of optical networks that will allow the transmission of data simultaneously. Currently available systems support up to about 100 wavelengths per fibre, enabling a single fibre to carry several hundred gigabits of information. Let's evaluate the WDM subsystem for performance factors based on mathematical calculations and theoretical evaluation.

Introduction to WDM:

The basic concept of this system is to split the light into different wavelengths and use it as a transmission media to transmit data through same channel thereby increasing the overall transmission capacity of the channel.

This system uses signals of different wavelengths and multiplexes them using multiplexer on the transmission side and demultiplexer to get individual signals on the receivers end.

Wavelength Division Multiplexer is a device that combines optical signals from multiple different single-wavelength end devices onto a single fiber. Wavelength Division Multiplexer carries two to four wavelengths per fiber. Typically, the same device can also perform the reverse process with the same WDM techniques: de-compose the data stream with multiple wavelengths into multiple single wavelength data streams, a process call de-multiplexing. Therefore, it is very often a Wavelength Division Multiplexer and Demultiplexer is in the same box

Wavelength Division Multiplexing (WDM) is a fibre-optic transmission technique. It involves the process of multiplexing many different wavelength signals onto a single fibre. So each fibre has a set of parallel optical channels, each using slightly different light wavelengths. From the instrumentation point of view optical fibre have some integral benefits over conventional information carriers like copper wires.

Advantages such as: ~

  • Wide signal bandwidth
  • Electrical isolation
  • No cross talk
  • Interference immunity
  • Light weight
  • Low volume cabling

The following diagram shows the working of a WDM subsystem

{Source: Fiber optic communication by Joseph C. Palais fifth edition }

Where ds1, ds2, ds3....dsn are the different data signals to be transmitted. dl1, dl2, dl3.... dln are the multiplexed signals that are transmitted over the optical fibre and dr1, dr2, dr3, dr4.....drn are the received signals on the receivers end after demultiplexing.


ds1=dr1, ds2=dr2, ds3=dr3, ds4=dr4....dsn=drn

The two main entities of this system are multiplexer and demultiplexer.

WDM as a process:

The operation of this system is mainly based on transmitting different data signals simultaneously over a single optical fibre. First the data to be transmitted are coupled with the desired wavelength of light and this process is carried out for different wavelengths based on requirement of the system. These multiple wavelengths of light are then subjected to multiplexer which combines all the signals and transmits them over to the optical fibre.On the receivers side combined signal is subjected to demultiplexer which divides the combined signal into individual signals.

Optical fibre has very high bandwidth rough calculations show

  • Each wavelength runs at a minimum data rate of 2.5Gbits/s or even higher
  • If light is split into 16 wavelengths
  • 162.5=40Gbits/s on a single fibre
  • Consider having around 50 fibres in a single cable
  • 5040=2000Gbits/s on a single cable

And these values vary on the transmission capability of different wavelengths which may reach up to 100Gbits/s under laboratory conditions

Claiming a world-best speed record for optical data transmission, researchers at Siemens have transmitted 7 Tbits/s over a single optical fibre using DWDM (Dense Wavelength Division Multiplexing) telephone calls or a billion pages of typed data per second. The demonstration was conducted by Siemens' Information and Communication Networks Group at the company's Advanced Optical Networks Laboratories. The researchers simultaneously transmitted 176 channels of 40-Gbit/s data over a 50-km fibre optic cable. The 40-Gbit/s channels, in turn, were produced by time-division multiplexing (TDM) using a prototype of TransXpress FOX, a multiplexer and regenerator system developed by Siemens.


Description of the subsystem and the model:

Block diagram for WDM system

The basic components we use in our present wavelength division multiplexing are Directional couplers; Etalon filters also known as Fabry-Perot interferometer each of individual wavelength ratings, Non linear Optical Loop Mirror, Optical Receiver

The output of first device or component would be the input for the next one. Like the WDM signal with N different wave length channels running with signals each of n bps will be the input for directional coupler and the output here would be demultilplexed into N different signals in each N ports. This output of directional coupler through N ports will be the input for the N Etalon filters with each receiving only one laser beam of a particular wavelength and all the other wavelengths are reflected back which may be received by any one of the n etalon filters and the out put laser beam of a particular wavelength will be given as input for non linear optical loop mirror for non linear transmission or switching characteristics using Kerr effects and the output of this NOLM will be the input for optical receiver which receives the optical signal from NOLM and amplifies the signal and the required out electronic data signal of n bps is obtained.

Directional Couplers:

Directional coupler is optical device used for coupling optical power between two strands of fibre optic material in a given direction of propagation. They are used for dividing optical power from one input fibre to usually two or three output fibres. It can also be used to combine power from two or more input fibres into one output fibre.

There are two types of fibres multimode and single mode fibres with core width of (about 60 microns) and (10-15microns) respectively. Coupling is relatively difficult in small core fibres. There are two types of directional couplers mainly fused directional couplers and polished directional couplers.

Some characteristics of directional couplers

1) Coupling Factor:

A primary aspect of directional couplers is coupling factor which fluctuates with frequency, and it is not a constant. This classifies the quantity of power coupled from core to the other waveguide and can be represented using dB. The coupled power will be smaller, so it must be represented in negative figure. Typically the negative sign is ignored. In coupling frequently used values are 10 and 20 dB except high power system may use 40dB coupling. .

Coupling Factor =10 log (P4/P1) = 10 log (P3/P2) dB Input VSWR (Voltage Standing Wave Ratio): It is the VSWR 'seen' glancing into some port, through all the other finished in corresponding loads.

Isolation: It is a variation in signal points in DB between the input port and isolated port while remain output ports will be terminated by matched loads. It is also defines the isolation between output ports.

Isolation 1, 4 = 10 log (P4/P1) dB

Isolation 2, 3 = 10 log (P3/P2) dB

In reflection measurements, coupling factor is less important than directivity and SWR, since both the forward and reverse coupling elements are usually identical, and so the variation of coupling factors match versus frequency.

2) Loss:

In an ideal directional coupler, the main line loss from port 1 to port 2 (P1 - P2) due to power coupled to the coupled output port is

Insertion loss in dB = 10 log [1- (p3 / p1) ]

Etalon Filters:

Etalons, also known as Fabry-Perot (FB) etalons is the main entity in the demultiplexer which is used to filter a mixed wavelength signal. Etalons are widely used in telecommunications, lasers and spectroscopy to control and measure the wavelengths of light.Etalon is a mirrored cavity with two reflecting surfaces which is used to attain a narrow laser of spreading wavelength. The outer surface of etalon is slightly bent at an angle to eliminate interference patterns that may occur from reflection.

If the transmitted beams are in phase, and this corresponds to a high-transmission peak of the etalon the constructive interference occurs. Destructive interference occurs when the transmitted waves are out of phase and this corresponds to minimum transmission.

A typical etalon transmission function is: