The impact of the signal pulse width and the optical filter bandwidth on the performance of both RZ and NRZ On-Off Keying (OOK) Optical Time Division Multiplexing (OTDM)-Wavelength Division Multiplexing (WDM) systems are studied in this chapter. Using polynomial fitting, an approximated expression for the optimal signal pulse duty cycle as a function of the spectral density SD and Optical Signal to Noise Ratio (OSNR) is provided. Further, it is found that the bit rate per WDM channel does not affect the optimum signal pulse duty cycle. As the spectral density SD increases, DCopt increases, reducing the signal spectral width to compensate for the reduced the WDM channel frequency spacing ?f. For increasing OSNR, DCopt increases slightly, especially at higher SD. The authors found that ideal NRZ performs better than optimized RZ at high SD but worse at low SD.
TopThe 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 OOK signal, with assumed forward error coding (FEC) overhead of 7%, yielding a total bit rate of 10.664 Gb/s and is given by
(11.1)Figure 1. System block diagram for OOK parameter optimization
where
an is the pseudorandom bit sequence (PRBS) of length
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. The total time multiplexed signal bit period
TR =
T/
M while the bit rate
BR = 1/
TR =
M/
T. For RZ modulation, the pulse shape is 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. For NRZ modulation, the pulse shape is ideally square:
p(
t) = rect(
t(
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 is second-order Gaussian and the electrical filter is fifth-order Bessel. Shot and electrical thermal noise are neglected since optical noise is dominant. The OTDM demultiplexing window is almost ideal (square): fifth-order Gaussian with a width of 0.6(T/M). The electrical filter bandwidth, EBW = 2OBW, where OBW is the optical filter bandwidth, because high EBWs are optimal when shot and thermal noises are neglected (Bosco, Carena, Curri, Gaudino, & Poggiolini, 2002).