We investigate the use of an arbitrary waveform generator to phase-modulate a laser source and externally broaden its linewidth. Through nonlinear optimization in a computer, we find modulation signals that produce top-hat-shaped optical spectra of discrete lines with highest total power within a limited bandwidth and limited peak spectral power density. The required modulation bandwidth is comparable to the targeted optical bandwidth. Such spectra are attractive for suppressing stimulated Brillouin scattering in optical fiber. Experimentally, we generate 15 lines in a 0.5 GHz optical linewidth. However, the method can also be used to generate other optical spectra.