The impact of a post-OTDM-demultiplexing optical filter on the performance of dense On-Off Keying (OOK) Optical Time Division Multiplexing (OTDM)-Wavelength Division Multiplexing (WDM) systems is studied in this chapter. For Return-to-Zero (RZ) modulation, it was found that the additional filter working in a double-tier filter configuration did not offer any significant improvements to performance when the signal pulse width is optimized. Improvements generally increase only when the signal pulse width deviates from its optimal value and only for low spectral densities. For ideal Non-Return-to-Zero (NRZ) modulation, however, significant improvements of around 1 dB are obtained using the double-tier configuration over a large range of spectral densities.
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 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
(14.1)Figure 1. System block diagram for studying post-OTDM-demultiplexing filter (Hiew, Abbou, Chuah, Majumder, & Hairul, 2005)
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 rate
BR =
M x 10.664 Gb/s =
M/
T. For RZ modulation, the pulse shape is chirp-free Gaussian (
m=1)
p(
t) = exp[-0.5(
t/
T0)
2m]
(14.2) where
T0 =
tFWHM/
k,
tFWHM is the full wave half maximum (FWHM) pulse width and
k = 2(exp[-0.3665/(2
m)]). For NRZ modulation, the pulse shape is ideally square
p(
t) = rect[
t(
M/
T)]
(14.3) where rect[] is the rectangular function. 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).