The BER performances of a classical external optical Mach–Zehnder modulator and a passive all‐optical integrated device, an unbalanced Mach–Zehnder interferometer, are measured and analyzed theoretically. This analysis show better results for the passive all‐optical device than for the external modulator in the case of the laser diode's high enhancement factor. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 36: 496–498, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10801