We investigate the transmission performance of N × 40 Gbps carrier-suppressed return-to-zero (CS-RZ) and duobinary CS-RZ (DCS-RZ) modulated wavelength division multiplexed (WDM) signals over the standard single-mode fiber (SSMF) based on non-ideal Mach-Zehnder (MZ) modulators through numerical simulations. In addition to that, the impact on receiver margin related to the residual chirp due to an asymmetry ratio of modulators as well as the effect of the applied chirp of modulators has been studied. As the asymmetry ratio of modulators is increased, dispersion penalties are increased asymmetrically at around the zero dispersion wavelength for the CS-RZ format. The DCS-RZ modulation format has a symmetric behavior due to different characteristics of the residual chirp of modulators. The receiver margin for the DCS-RZ modulation format is larger than the CS-RZ modulation format above the asymmetry ratio of 0.82 of modulators (extinction ratio of 20 dB) with the optimal ΔλDCF. By controlling the negative/positive applied chirp of the first/second modulator, dispersion penalties can be reduced for both the CS-RZ and DCS-RZ formats. In the DCS-RZ format compared to the CS-RZ format, the effect of the applied chirp of the first modulator is more dominant than that of the second modulator. The simulation results show that the receiver margin is limited by the asymmetry ratio of modulators as well as a deviated wavelength from the zero dispersion wavelength of dispersion compensating fibers (DCF) in order to be fully post-compensated. Dispersion penalties can be reduced with appropriate chirp parameters of two non-ideal modulators.

We have theoretically investigated 8 x 10-Gb/s wavelength-division multiplexing (WDM) signal transmission characteristics based on semiconductor optical amplifiers (SOAs) with equalized gain using discrete Raman amplification (DRA). Gain equalization and low noise figures have been obtained by adjusting the backward Raman pumping power and wavelength at a dispersion compensating fiber (DCF) for each span. Bit-error-rate characteristics were calculated for 8 x 10-Gb/s WDM signal transmission over 6 x 40-km single-mode fiber (SMF) + DCF links with gain-equalized SOAs using DRAs at DCF. Approximately a 2.5-dB improvement of the receiver sensitivity was achieved by using SOAs and DRAs with optimized Raman pumping. One can easily upgrade the transmission length of a link based on SOAs with an appropriate backward pump laser at each DCF.

We investigate the performance limits and the impact on the system margin of 10-Gb/s duobinary modulations due to reducing the bandwidths of single-arm Mach-Zehnder (MZ) modulators in a 10-Gb/s duobinary transmitter through numerical simulation. In addition to that, the individual and combined effects of dc-bias offset ratio to switching voltage and residual chirp due to the asymmetry of modulators on the system performance have been studied using reduced bandwidth single-arm MZ modulators. Our results suggest that with a single-arm MZ modulator of bandwidth reduced to 5 GHz and a duobinary filter of 2.6-GHz bandwidth, the sensitivity penalty can be maintained within 2 dB over a 200-km span of uncompensated single-mode fiber referred to that at 0 km of ideal duobinary transmitters. In this case, the allowable range of the residual-chirp parameter is to be from -0.1 to +0.1 with dc-bias offset ratio of less than /spl plusmn/2.5%.

We develop a numerical model for the transmission performance of bidirectional wavelength-division-multiplexed (WDM) transmissions. Using the numerical model, wavelength-interleaved 10-Gb/s bidirectional WDM systems are analyzed by considering the impairment of the system performance due to Rayleigh backscattering (RB). We also calculate the power penalty and the crosstalk due to RB from the analytical model, and then compare to the results obtained from the numerical model. The analytical model can provide similar results obtained from the numerical model. Both the numerical and analytical models suggest that the total isolation of filters must be more than 19.5 dB for 320-km transmission while keeping the power penalty of less than 2 dB.

Optically preamplified receiver performance according to the vestigial sideband (VSB) filtering has been numerically investigated for 40-Gb/s optical signals modulated with nonreturn-to-zero, duobinary nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed RZ, and duobinary carrier-suppressed RZ formats. The VSB filtering enables the spectral widths of NRZ, duobinary NRZ, and RZ signals to be reduced without severe power penalties at the receiver. On the other hand, carrier-suppressed RZ and duobinary carrier-suppressed RZ signals have no large advantages over VSB filtering because of the characteristics of their signals. Our results suggest that RZ signals are the most suitable modulation format for VSB filtering, without considering the filter loss, because of the tolerance of the intersymbol interference and a large spectral width. However, duobinary NRZ signals are the most suitable modulation format for VSB filtering, considering the filter loss, because of their narrow spectral width.