Experimental and analytical evaluation of dispersion effects that control intersymbol interference in gigabit, single-mode-fiber, digital systems is described. Both direct modulation of distributed-feedback (DFB) lasers and external modulation by Mach-Zehnder modulators are studied. The experiment uses sinusoidal modulation of the light at a microwave frequency, e.g. 5 GHz, and the subsequent measurement of signal gain or loss in a dispersive fiber. For DFB lasers, the transmission measurements are used to derive the transient and adiabatic components of chirp. It is shown that, for external modulation at 5 GHz, the 1.55- mu m optical signals suffer relatively little dispersion loss for transmission distances up to 60 km. Analytical solutions for waveform distortion are given for external modulation and show good agreement with measurements.