Performance Analysis of Four Wave Mixing Effect in WDM Systems

Abstract

The nonlinear effects degrade the system performance. Because nonlinear effects tend to manifest themselves when optical power is very high, they become important in DWDM. Four-wave mixing (FWM) is one of the dominating degradation effects in wavelength-division-multiplexing (WDM) systems with dense channel spacing and low chromatic dispersion on the fiber. If in a WDM system the channels are equally spaced, the new waves generated by FWM will fall at channel frequencies and, thus, will give rise to crosstalk. Four-wave mixing (FWM) is a parametric process in which different frequencies interact and by frequency mixing generate new spectral components.

The four wave mixing effect on bit error rate, Q-factor, output spectrums and eye diagrams at different channel spacing is investigated. The design, implementation and performance analysis of four wave mixing in optical communication system for different values of spacing between input channels has been done. The comparison of four wave mixing effect at various values of channel spacing revealed that 75 GHz spacing has the edge over 6.25 GHz spacing in optical communication system. According to the results, spacing of 75 GHz has the lowest BER and better system performance. Hence, the higher spacing values between the input channels is recommended for long distance transmission without four wave mixing. The graphs of BER, Q-factor and eye opening show that higher channel spacing gives the best performance as compared to lower channel spacing. Hence, it is concluded that higher channel spacing is best suitable to be employed in the optical communication systems. But much higher channel spacing is blocked by bandwidth constraints so channel spacing must be optimized.

The design and performance analysis of four wave mixing effect on changing various components in the system is also presented. Components involve different data sources like pn-sequence generator with different bit rate; modulator drivers like NRZ and RZ raised cosine, NRZ and RZ rectangular, RZ supergaussian, RZ soliton; modulators like linear amplitude, linear electroabsorption and optical phase modulator; laser with different power values. The extent of four wave mixing is considered for each and every component separately. The comparison of four wave mixing with different components revealed that changing the bit rate of data source doesn’t affect the four wave mixing at the output. Changing the modulator drivers least affects the four wave mixing. It is observed that NRZ raised cosine modulator driver gives the least four wave mixing. On changing the modulator, the four wave mixing is largely effected. The linear amplitude modulator gives the best system performance and the optical phase modulator gives the worst performance among all the modulators. Hence, it is concluded that linear amplitude modulator is best suitable to be employed in the optical communication system minimizing the four wave mixing effect. The simulation results revealed that NRZ raised cosine modulator driver and linear amplitude modulator gives the best performance in terms of four wave mixing in optical communication system. Also, the eye opening and the Q-factor increases on increasing the laser power. Moreover, the bit error rate is minimum when the power level of laser is maximum.

The investigation of four wave mixing effect with different number of channels at various channel spacings has also been done. All the input channels are spaced evenly at various values like 6.25 GHz, 12.5 GHz, 25 GHz, 40 GHz, 50 GHz with the different number of channels at the input i.e. with 2, 4, 6, 8, 12 input channels. Analyzing the effect of four wave mixing for each channel spacing with these number of channels in terms of eye diagrams, BER, eye opening and Q-factor, it has been observed that on increasing the number of input channels/users, the interference increases and thus, the four wave mixing effect also increases. The eye opening decreases as the number of channels increases. Increasing the number of channels causes the Q-factor to decrease. Moreover, as the number of channels increases, the BER also increases. The simulation results revealed that the less number of users at input cause less four wave mixing but in today’s technology, it is important for the circuit to handle wavelength division multiplexing.

Thus, the thesis presents the design and performance analysis of four wave mixing effect on bit error rate, Q-factor, output spectrums and eye opening with varying parameters.