Using the Differential Phase Q (DP-Q) and the traditional Q factor, performance comparison of RZ-OOK and RZ-DPSK in dense OTDM-WDM systems is obtained in this chapter. When signal pulse widths and optical filter bandwidths are optimized, there is no upper limit to WDM channel bit rate (BR) in the purely linear back-to-back configuration. Here, RZ-DPSK performed increasingly better than RZ-OOK in higher spectral density with Q gain increasing from 3 dB to 5 dB. In the nonlinear point-to-point configuration, higher BR leads to increased performance penalties for both RZ-DPSK and RZ-OOK, while RZ-DPSK still outperforms RZ-OOK by up to 4 dB. The results obtained correlate with conventional results, indicating the potential of the DP-Q as a performance evaluation tool in numerical simulations.
TopSystem Model
The system block diagram for both back-to-back and point-to-point configuration is shown in Figure 1. Each OTDM channel consists of a 9.953 Gb/s signal, with assumed forward error coding (FEC) overhead of 7%, yielding a total bit rate of 10.664 Gb/s. It is given by
(13.1)Figure 1. System block diagram for comparisons between RZ-OOK and RZ-DPSK (Abbou, Chuah, Hiew, & Abid, 2008)
where
for RZ-OOK or
for RZ-DPSK,
N = (2
15)/
M bits and
M is the number of OTDM channels,
T is the 10.664 Gb/s signal bit period and
p(
t) is the pulse shape assumed to be chirp-free Gaussian:
p(
t) = exp(-0.5(
t/
T0)
2) where
T0 =
tFWHM/1.665 and
tFWHM is the full wave half maximum (FWHM) pulse width. The total time multiplexed signal bit period
TR =
T/
M while the bit rate
BR = 1/
TR =
M/
T.
Five time-multiplexed (M x 10.664 Gb/s) signals are WDM multiplexed with frequency spacing ∆f and centered at 1549 nm. Results are obtained from the centre channel where WDM crosstalk is balanced (Yu, Reimer, Grigoryan, & Menyuk, 2000). The optical filter used is a second-order Gaussian bandpass filter with bandwidth, OBW. Shot and electrical thermal noise are neglected since optical noise is dominant. The postdetection electrical filter is a fifth-order Bessel lowpass filter with bandwidth, EBW = 2OBW, because high EBWs are optimal when shot and thermal noises are neglected (Bosco, Carena, Curri, Gaudino, & Poggiolini, 2002). For RZ-OOK, BER estimation is carried out after the postdetection filter while for RZ-DPSK it is performed right after the optical filter with no balanced detection performed. This is because the DP-Q operates on the optical signal and implicitly assumes that it is followed by an ideal balanced detector Hiew, Abbou, Chuah, Majumder, & Hairul, 2004).