Short pulses are generated by mode-locking techniques: amplitude modulation in time domain or frequency modulation in frequency domain. Direct Fourier synthesis of radiation from several single-frequency sources offers an opportunity to generate arbitrary waveforms. Here we report on a new technique of short-pulse synthesis in the Fourier domain. Instead of independent laser sources, we use a single multimode laser with retrieval of its individual cavity modes into a time sequence coherently combined in an external cavity. Combination of 20 consequent single-mode pulses has been performed, demonstrating a new way for arbitrary waveforms synthesis.