The issue of what is the best duty cycle in an intensity-modulated dir
ect-detection optical system keeps resurfacing. Here, this issue is ad
dressed from a transmission point of view for a wavelength division-mu
ltiplexed (WDM) system. It is shown that nonreturn-to-zero (NRZ) is mo
re robust to optical fiber transmission impairments for both already d
eployed systems and new systems in which the dispersion map is a desig
n parameter.