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Dense Wavelength Division Multiplexing Engineering Essay

In a practical point to pint optical network (DWDM) Dense Wavelength Division Multiplexing is used to combine multiples of signals having different wavelengths to transmit them through common medium. Years ago the Time Division Multiplexing (TDM) is used as multiplexing technique instead of using DWDM.

In order to transmit optical signal for a long haul communication, it is required the amplification in between the communication path due to the channel impairments. Typically this amplification is done by using (Erbium Doped Fibre Amplifier) EDFA. The major reasons that the EDFA is widely used because of reduce down the cost of the system. Another advantage is it s capable of amplifying multiple wavelengths simultaneously & this brought system capacity to increase rather than increasing bit rate.

Since the amplification is done for the signal being transmitted, there are other factors that limit the transmission of signal known as fibre nonlinearities. This include polarization related effects & Four Wave Mixing (FWM). The FWM is a phenomenon that three light signals at different wavelengths interact in the fibre to create a fourth light signal at a wavelength that may overlap with one of the light signals [6].

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Following lab session is done using Optisystem simulation tool which is based on several experiments such as EDFA characteristics, FWM & Polarization Mode Dispersion (PMD).For the WDM performance measurement experiment, the investigation is done at several different wavelengths across the C band in order to measure the performance of the optical network. For the PMD it is analysed for both good & bad fibre configuration by varying input power, bit rate & varying length of fibre.

In the case of FWM experiment it is measured the FWM components & its power levels for different transmitted power & analyse the variation of the FWM components for different dispersion status. In the EDFA experiment it is evaluated that how the EDFA is characterise for the different power levels & how the optical signal to noise ratio is varied according to that power levels.

2 Theory & Background Information.

Categories Of Emissions

There are two main emissions can be identified as stimulated emission & spontaneous emission. The atom is absorbed energy and jump to the next energy level. Then after sometimes it releases energy & jump back to the steady state by releasing energy spontaneously since it is not stable on other levels other than steady state. In the stimulated emission, it requires energy from outside rather than spontaneous emission. This can only occur for incoming photons that have photon energy close to the energy of the laser transition [1]. The emission then goes into the same way as the incoming photon. Following figure illustrate the operation of stimulated emission.

Operation Of Erbium Doped Fibre Amplifier

EDFA consist of fibre with a silica core & it is doped with ionized erbium atoms. The wave length selective coupler is used in order for pump signal to mix with input signal (wavelength 1550) at a wavelength of 980nm & 1480nm.At the output end ,the residual pump is required to split pump signal & inputted amplified signal. Further an isolator is used input or output of any amplifier to prevent from reflections in to the amplifier. Following shows a simple illustration of EDFA.

Fig[02]An Erbium Doped Fibre Amplifier[2]

Advantages of EDFA for optical communication.

i. Makes all polarization independent form all fibre devices.

ii. Easy to couple lights in & outs.

iii. Simplicity of device.

iv. No cross talk generation when amplifying a signal.

v. High power semiconductor pump lasers availability.

Four Wave Mixing (FWM)

Simply this is occurs due to nonlinear nature of the refractive index of the optical fibre itself. This is known as third order distortion phenomena which generate third order harmonics by adding or subtracting the wavelengths of signals together. As an example when four signals with different wavelengths are attended to the fibre, it will appear with added new FWM components.

The FWM can be described as following equation (N3-N2) where N is the number of signals.

Fig [03]: Generation of new frequency components via four-wave mixing [3]

There are two major factors that affecting the magnitude of the FWM components known as channel spacing & dispersion. When the channel spacing is getting low, it has higher probability to mixing signals due to interference. The dispersion can be crucial factor because for the higher dispersion, signals are less affected to FWM rather than for the lower dispersion.

Polarisation Mode Dispersion (PMD).

PMD is a source of pulse bordering due to phase delay between input polarization states. This can become the key restrictive factor for optical communication at higher transmission rates [4].

Since light signal is electromagnetic wave, it has two orthogonal polarization modes known as electric field & magnetic field (See Fig [04]). When light signal is transmitting through optical fibre, two polarization states will be introduced small index of refraction differences due to geometrically asymmetric of the fibre core as a result of thermal & mechanical stress introduced. This is called birefringence .

The birefringence property leads to differential group delay (DGD) by slowing one mode to travel than the other.[5] Following figure clearly illustrate occurrence of DGD due to PMD effect in single mode fibre.

3 Simulation Procedure.

Procedure 01

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EDFA characteristic.

Following components arrangement is to characterized EDFA by observing the output from Optical Spectral Amplifier (OSA) by double clicking on it.

Figure [ 05]: EDFA basic setup

Further continuous wave (CW) laser is connected to the same simulation instead of optical null. Then the selection of wavelength is done for the CW laser as 1550nm.Then initially provide a power for the laser as -30dBm to operate & reduce it to -20dBm in 2dBm steps in order to measure the power outputs & OSNR values of the system.

Procedure 02

WDM Network performance experiment.

In this experiment it is evaluate the performance of the WDM system using different wavelengths within the C band. As the following figure [07] describes the WDM system which includes four transmitters in order to generate four different wavelengths. The AWG multiplexer or demultiplexer is used to separate or combine signals with different wavelengths together. Then after it is either fed in to the fibre or fed in to the receiver. Since WDM Mux contains several inputs it is important to add optical null in order to prevent from unwanted signals at the outputs.

The EDFA is placed next to the fibre in order to amplify the input signal. The Optical Spectrum Analyzer (OSA) is to measure & compare the two spectrums of transmitted signal with amplified signal from 2 optical links. At the receiving end the BER analyzer is placed in order to measure different eye patterns with different BER values.

Open the file WDM amplified system.osd from the folder in desktop called optisystem lab. Then run the simulation by clicking calculate (play) button. Then the outputs are observed by two OSA s for the four different wavelengths & measure different SNR of the second fibre. Then after measured different eye patterns & BER values form BER analyzer.

Procedure 03

Four Wave Mixing (FWM)

Following experiment (Figure [08]) was done for FWM using four different wavelengths each spaced of 50GHz. The fiber length is 100 km & EDFA is added end of the fiber.

The components arrangement is simulated & observed the output from OSA s.

Then, increased the input power by 3dB steps in each transmitter & observed the characteristics of FWM components at each stage & compare the power relationship of outputs at different power levels.

In order to determine the behaviour of FWM over dispersion, change the dispersion parameter from 16.75 ps/nm/km to 11.75 ps/nm/km & observed the changed in the spectrum from OSA.

Procedure 04

Polarisation Mode Dispersion (PMD)

In here it is analyzed the effect of polarization mode dispersion for various bit rates, various distances & various transmitted power for good fiber & for the bad fiber.

For the good fiber, the PMD coefficient is set to 0.5 ps/vkm by double clicking on the fiber & selecting PMD tab. Then set the fiber length to 20km.As the first step, change the transmitter power by keeping distance 20km since the BER is achieving 10-9. Then increased the bit rate until sensitivity penalty reaches 1dB.

4. Simulations Results & Analysis.

EDFA characteristic experiment.

i. Output from OSA for Optical Null.

As the above figure shown the ASE (Amplified Spontaneous Emission) which includes different power levels of different wavelengths.

ii. Output from OSA for CW laser at a wavelength of 1550nm.

Input power (dBm)

Output power (dBm) Gain (dB) OSNR (dB)

-30 13.088 43.088 44.8113

-28 13.272 41.272 46.733

-26 13.532 39.232 48.0431

-24 13.887 37.887 50.4525

-22 14.333 36.333 51.1252

-20 14.849 34.849 52.1273

Following figures illustrate the spectrum of laser signal has been transmitted for different transmitted powers. The spectrum of laser is varied within the ASE noise area, because of its spectral width. Since practical transmitters don t transmit one unique wavelength due to the spectral width, it is unable to transmit one wavelength at all the time. The highest power component consist in the spectrum is the wavelength of transmitted signal. The Optical Signal to Noise Ratio (OSNR) is measured using pointers A & B as following figure [11].

When investigating above figures [11], [12] & table [01] it is clearly seen that the OSNR value is gradually increasing when increasing transmitting power. In other word, for the higher power, the EDFA is working accurately & provide better performance.

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WDM Network performance experiment.

i. Out puts from BER analyzers for different wavelengths

When analyzing different outputs from BER analyzers for different wavelengths, it was clearly seen that the BER value is gradually increasing from 3.19 x10-44 to 8.737 x10-35 when moving the wavelength from 1560.61 nm to 1550.12 nm.

Furthermore for the wavelength of 1541.35 nm & 1532.68 nm, the BER values are 3.75 x10-60 & 2.772 x10-40 respectively.

ii. Output power levels of the transmitted signals from OSA connected to the second fibre.

Received Signals power measurements.

Signal Wavelength(nm) Signal Power(dBm) Noise Power(dBm) SNR (dBm)

1560.61 -15.9596 -41.4185 25.4589

1550.12 -13.1176 -38.2213 25.1037

1541.35 -9.32841 -38.2213 28.8929

1532.68 -3.5261 -27.8009 24.2748

The above table [02] shows the output power & noise power of each signal received by OSA connected to second fibre. The SNR is calculated using each signal & noise powers.

Four Wave Mixing (FWM) Experiment.

i. Four dense wavelengths at the fibre input and output.

Considering the spectrums of figure [15] a & b , it is clearly seen that the FWM components are added to the transmitted signals by adding or subtracting wavelengths of different signals are transmitted. Also the powers of the transmitted signals are increased when it reached to the end of the fibre.

ii. Spectrum at the output of the EDFA & highest FWM components.

Above figure [16] illustrate the highest FWM components which are coming out from the EDFA. Those values are recorded as follows,

Signal Wavelength(nm) Power (dBm)

1550.68 -24.5647

1550.92 -24.9559

1551.07 -27.825

iii. Increase the power of all four transmitters by 3dB until it reaches 19dB.

When analyzing the FWM products, both for 16dBm & 19dBm transmitting power, it can be clearly seen that the received signals has affected for the FWM for all cases. In the case of 13dBm power input, power of the highest FWM components pointed in Fig [16] is much lower than the FWM components in the case of 16dBm.When Increasing the transmitted power further to 19dBm, the highest FWM components related to Fig [16] is increasing the power level than above two cases as shown in following table,

FWM Wavelength

(nm) FWM Power (dBm)

13dBm FWM Power (dBm)

For 16dBm FWM Power (dBm)

For 19dBm

1550.68 -24.5647 -19.8192 -15.0518

1550.92 -24.9559 -18.443 -9.9239

1551.07 -27.825 -20.9952 -15.3476

iv. FWM products output from OSA1 for Dispersion parameter of 16.75 ps/nm/km.

v. FWM products output from OSA1 for Dispersion parameter of 11.75 ps/nm/km

FWM Wavelength

(nm) FWM Power (dBm)

13dBm FWM Power (dBm)

For 16dBm FWM Power (dBm)

For 19dBm

1548.93 -53.7325 -41.4916 -29.8427

1550.68 -45.9375 -36.5669 -21.1738

1550.92 -46.0793 -36.8261 -25.5082

In above six figures [18.a,b,c] & [19.a,b,c] the effect of FWM occurrence is gradually increasing its power level when increasing transmitting power from 13dBm until it reaches 19dBm.Theoritically the mixing efficiency is inversely proportional to the fibre dispersion. So that it is getting stronger at the zero-dispersion point. Since the dispersion parameter changed from 16.75 ps/nm/km to 11.75 ps/nm/km. the increment of power level of FWM components can be recorded as follows.

FWM Wavelength

(nm) FWM Power (dBm)

13dBm FWM Power (dBm)

For 16dBm FWM Power (dBm)

For 19dBm

1548.93 -50.6955 -42.4098 -29.4157

1550.68 -43.3169 -34.001 -20.8073

1550.92 -43.5852 -33.0118 -24.3953

Polarisation Mode Dispersion (PMD).

i. Power at the receiver optical input in order to get BER of 10-9 .

This experiment was done only by varying power of the transmitter keeping distance & bit rate constant as 20km & 10Gbps respectively. The recordings are as follows.

Power(dB) BER

20 9.35 x 10-21

19 2.8 x10-14

18 6.4 x 10-10

18.219469 1 x10-10

18.4 1.8 x10-11

18.5 7.09 x10-12

By analysing the table [ ] it is identified that the maximum power can be transmitted in order to keep bit error rate to 10-9. Since the power value 18.219469dB is much closer to the BER value of 1x 10- 9. It can be said that maximum power is around 18.2dB.

ii. Maximum bit rate allowed for the distance of 20km.

As the above figures illustrated the maximum bit rate can achieve for the 20km long optical fibre is 52.5Gbps in order to get minimum BER of 10-9. This is achieved under the attenuation of 42.5dB.

5. Conclusion and discussion

When taking the performance factor of the network into consideration as a function of OSNR & BER for different wavelength of C band, the maximum OSNR which corresponds to wavelength of 1541.35 nm has the minimum BER value. In addition to that the maximum BER which corresponds to a wavelength of 1550.12 nm has the minimum SNR value. From that performance figure it can be conclude that for the lower SNR, BER is achieving higher values. This analysis is further known as system error performance curve.

The bandwidth of EDFA available for amplification is relay within the range of 1540nm 1560nm. Then the bandwidth available is 20nm.

The gain of the EDFA is 38.783dBm.

The effects of putting optical fibre in a system cater several advantages such as higher bandwidth availability, good noise immunity, less electromagnetic interference, lower attenuation, high speed data transmission & inexpensive installation.

For the amplified system the FWM becomes significant issue. When increasing transmitting power the FWM components are increasing. For the decrement of dispersion factor, the FWM power levels are increased. FWM can only be mitigating by increasing the dispersion factor since it is inversely proportional to the mixing factor.

The PMD effect is causing performance degradation for the communication links since light signal having two polarization modes orthogonal to each other & receiving it with small deviation comparatively. For the long haul communication, the main limiting factor is dispersion & it degrades the transmission distance as well as the bitrates.

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