Optical receivers used in WDM networks are often preceeded by nn optical amplifier to ensure good receiver sensitivity. In this letter, we present an experimental investigation of homowavelength crosstalk in optically preamplified systems using an IM/DD transmission scheme. As a light source, a DFB laser is modulated either directly or externally by an electrical generator. Performance comparisons for both modulation schemes are given to show the influences of noise contribution of amplified spontaneous emission (ASE) noise. (C) 1999 John Wiley & Sons, Inc.