Monolithic two‐section GaInAsP/InP lasers are shown to operate in a single longitudinal mode under high‐speed pulsed current modulation. The length of the emitted monomode light pulses is less than 500 ps. The suppression of secondary modes is described in a rate equation model that is generally useful for a variety of coupled‐cavity configurations. It is found that 10% increase in cavity loss for the unwanted modes is sufficient to provide 17 dB suppression.