OTDM-WDM Intensity Modulation (IM) systems employing optical amplification and Dispersion Management (DM) are analyzed. Interchannel nonlinear effects present the input power limit to system performance. Therefore, a lumped DM system performed better than a fully inline DM system through reduction of coherent channel interaction. With proper channel spacing, all demultiplexed OTDM channels and the multiplexed signal show the same performance.
TopSystem Block Diagram And Operation Model
The system block diagram is shown in Figure 1. Each transmitted channel employs RZ modulation, which consists of Gaussian pulses with full wave half maximum (FWHM) width of 8 ps. The OTDM channel bit rate is 10 Gb/s, which are multiplexed to form 40 Gb/s WDM channels. Both the OTDM and WDM multiplexing processes are assumed ideal. We simulate five adjacent WDM channels with equal channel spacing of Δf GHz, and the results are obtained from the middle channel (centered at 1550 nm). This is sufficient as almost all effects on a WDM channel are caused by its first two spectrally adjacent neighbors (Yu, Reimer, Grigoryan, & Menyuk, 2000). Each OTDM channel is encoded using different pseudo-random bit sequences (PRBS) of length 29-1 bits so that they are uncorrelated. The signal is transmitted across 18 spans of nonzero dispersion shifted fiber (NZ-DSF) of length 50 km each with dispersion D = 4 ps/nm/km, dispersion slope Sl = 0.1 ps/nm2/km, effective area Aeff = 55 μm2 and attenuation α = 0.2 dB/km. The dispersion compensating fiber (DCF) has D = -100 ps/nm/km, Sl = -2.5 ps/nm2/km, Aeff = 30 μm2 and α = 0.55 dB/km. The DCF is either used as periodic (inline) dispersion compensation placed after each NZ-DSF span (specified in terms of length, Linl) or as pre-dispersion compensation (length, Lpre) and post-dispersion compensation (length, Lpost).
Figure 1. System block diagram for OTDM-WDM propagation impairments study (Hiew, Abbou, Chuah, Majumder, & Hairul, 2004)
The DM scheme is ideal so that second and third order dispersion cancels out for all channels at the receiver. A second order, super-Gaussian optical bandpass filter with bandwidth BWwdm, is used for WDM demultiplexing as this is the shape of many common demultiplexers. OTDM demultiplexing is performed using an ideally square switching window with 92% duty cycle. All receivers are assumed ideal with no shot and electrical thermal noise, considering that the dominant noise in long-range systems is accumulated ASE. A five-pole, Bessel postdetection lowpass electric filter is used with single sided bandwidth BWele. Erbium doped fiber amplifiers (EDFA), with a population inversion factor nsp of 2, are placed after each fiber span. The ASE is modeled as circular complex additive white Gaussian noise (AWGN) with uniform random phase distribution and added onto the WDM signal after each amplifier along the transmission link. The signal and noise are propagated together by solving the nonlinear Schrödinger equation (NLSE) using the split-step Fourier method (SSFM). Single polarization is assumed for all signals and noise. Multiple runs are performed until the results converge.