Abstract
This paper analyzes the effects of laser linewidth on the performance of eight-channel dual-polarization quadrature phase-shift keying (DP-QPSK) dense wavelength division multiplexing (DWDM) transmission at 50 GHz channel spacing. The performance of differential encoding and Gray coding is compared with the system without any coding for the DP-QPSK DWDM system. It is shown that the differentially-encoded system performs better for lasers with linewidth (∆ν) greater than 0.04 MHz compared to the system with Gray coding and system without any coding. However, for lasers with linewidth lesser than 0.04 MHz, the three systems considered here perform equally and there is no need for differential or Gray coding in such a case. It is also shown that if we apply frequency offset estimation (FOE) before adaptive equalization (AE) in the digital coherent receiver then it further improve the performance of the differentially encoded system with lasers sources having linewidth even greater than 0.1 MHz.
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