Performance analysis is carried out to evaluate the effect of XPM on a dispersion managed 20Gb/s optical WDM transmission system using either On-Off Keying (OOK) or Differential Phase Shift Keying (DPSK) modulation, in the presence of GVD, SPM, and ASE noise in this chapter. It is found that to achieve a BER of 10-9 at a distance of 160 km, a 1.0 dB XPM power penalty is incurred for input channel power of 3 dBm in OOK transmission and 7 dBm in DPSK. The power penalty increases with input channel powers, but it is inversely proportional and exhibits oscillations with respect to channel separation. The oscillation is evenly spaced for DPSK but not for OOK and suggests the presence of optimum separation values. XPM penalty decreases when high dispersion fiber is used, and it also increases linearly with increasing dispersion slope. Small residual dispersion can reduce the penalty of nonlinear effects.
TopSystem Model
The system model used for the analysis is depicted schematically in Figure 1. It consists of two or more channels, one of which is operating at a wavelength of 1548 nm and the others are each separated by a small channel spacing of Δλ. The channels are multiplexed together using an ideal WDM multiplexer (MUX). For analysis, the signal sources (Tx) are assumed to consist of an ideal laser source and a modulator for each channel. The channels are modulated from a pseudorandom bit stream to be either an OOK or a DPSK signal. The fiber used is a nonzero dispersion shifted fiber (NDSF) with small residual dispersion in the anomalous region. This is because zero dispersion fibers facilitate phase matching that introduces interchannel interference.
Figure 1. System block diagram (Abbou, Hiew, Chuah, Ong, & Abid, 2008)
An optical amplifier (AMP) is placed after the first section of fiber, at the midway stage and at the end of the transmission path just before the receiver. A wideband optical filter follows each amplifier. Lumped dispersion compensation is employed whereby a span of dispersion compensation fiber (DCF) is placed just before the final amplifier. Lumped dispersion compensation has been shown to reduce XPM-induced penalties by maintaining the incoherence between channels over most of the fiber length (Marhic, Kagi, Chiang, & Kazovsky, 1996). At the receiving side, the channels are demultiplexed by wavelength separation with a WDM demultiplexer (DEMUX). The received power used in our analysis is measured just after the WDM demultiplexer before demodulation or photodetection. Each channel is then fed to their respective demodulators (Demod) that are different for OOK or DPSK modulation.
For OOK demodulation, the channels are detected by separate envelope detectors, each consisting of a photodetector (Photo Det.), that is immediately followed by a low pass filter (LPF) to produce the baseband signal (Rx), shown in Figure 2. For DPSK demodulation, we use direct detection delay demodulation receivers. This is shown in Figure 3 using a two-filter model (Sahara, Kubota, & Nakazawa, 1996). The received signal is time delayed by one bit period (T). The original signal and the delayed signal are then summed and subtracted in two separate adders. These signals are detected directly with envelope detectors. At the end of the low pass filters are the baseband signal and its inverse respectively. These two signals are compared to make the bit decision.
Figure 2. OOK demodulator block diagram (Abbou, Hiew, Chuah, Ong, & Abid, 2008)
Figure 3. DPSK demodulator block diagram (Abbou, Hiew, Chuah, Ong, & Abid, 2008)