We report for the first time high‐speed (Gbit s−1), single mode, digital (on‐off) modulation of intracavity loss modulated 1.55 &mgr;m InGaAs/InP multiple quantum well distributed feedback lasers. The lasing spectrum, observed to be asymmetrically broadened under high‐speed modulation, is shown to result from large carrier density variations which accompany this switching scheme. We also show, for the first time, digitally current‐modulated lasers have broader linewidths than devices which use intracavity loss modulation.