The explosive growth in current data centric internet traffic requires increasing the number of optical channels and the data rate per channel in today's dense-wavelength division multiplexed (DWDM) optical core transmission networks. With the limited gain bandwidth of EDFAs, increasing the number of wavelength channels can be accomplished by reducing the spacing between adjacent channels. Although the current ITU-T grid specifies 100-GHz channel spacing, the future proof DWDM optical network technologies are aiming for 50-GHz or even 25-GHz between two successive channels. Again, the ITU-T has recently accepted 40Gb/s/channel as a prime candidate for the future optical transport networks. However, 40Gb/s technologies are yet to mature and there will be an immediate need for accommodating current lower bit rate systems to higher 40Gb/s systems. Motivated by these facts, we proposed a new scheme of transmitting 40Gb/s per channel data over 50-GHz spaced DWDM systems using today's available mature low cost technologies. The compact signal spectrum of the optical Duobinary coding makes it an excellent candidate for highly spectral-efficient transmission in ultra DWDM systems. It also provides large tolerance to chromatic dispersion that facilitates to achieve longer fiber transmission distance without any need for dispersion compensation. The spectral bandwidth of the 40Gb/s Optical Duobinary signal is about 40-GHz which can fit into the 50-GHz spaced DWDM systems. However, compared to 10Gb/s systems, the 40Gb/s systems require more lunching power to attain same optical signal-to-noise (OSNR) after same transmission distance, which in turn causes severe nonlinear effects for 40Gb/s systems. To reduce such nonlinearity effects on 40Gb/s channels and achieve better transmission performance in 50-GHz spaced DWDM systems, we propose a novel scheme by multiplexing two 20Gb/s optical Duobinary sub-channels using optical carrier suppression and separation (OCSS) technique.Fig. 1 shows the schematic of the proposed system. A dual-arm LN-MOD (LN-MOD1) is used to suppress 50-GHz spaced N continuous light wave (CW) channels (λ1, λ2, ¼λn) generated from N distributed feedback laser diodes (LD1, LD2, ¼LDn) respectively. An RF sinusoidal clock with frequency f0 and its complement are used to drive the LN-MOD1 which is biased at the minimal intensity point. At the output of the LN-MOD1, the original carrier of i-th wavelength channel λi (frequency Ωi), i = 1..n, is suppressed and two longitudinal modes at (Ωi - f0) and (Ωi + f0) are generated. The carrier suppressed (CS) signal at channel λi is fed into the respective OCSS-Duobinary transmitter, where the two longitudinal modes at (Ωi - f0) and (Ωi + f0) are separated using an optical filter (OF) and modulated by two 20Gbp/s Duobinary encoded data streams using two Mach-Zehnder modulators MZM1 and MZM2, respectively, before they are combined using an optical coupler (OC). With f0 equals 10-GHz, the separation between the two longitudinal modes is 20GHz and the spectral bandwidth after combining the two 20Gb/s optical Duobinary signals is 40-GHz.We evaluated the performance of our proposed scheme through simulation. The initial results show that the proposed scheme achieve almost double transmission distance compare to the regular single 40Gb/s/channel duobinary 50-GHz spaced DWDM system.

© 2006 Optical Society of America

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