A CO.sub.2 waveguide laser frequency modulation (FM) system which applies the Fourier series components of a triangular drive waveform selectively to the tuning PZT at one end of the waveguide and the FM PZT at the other end to achieve an improvement over the waveform than can be achieved by applying the fundamental and its harmonics to the FM PZT alone. The system applies the fundamental component to the tuning PZT and the odd harmonics to the FM PZT. The frequency-separated drive signals are applied in proper phase to the PZT's at each end of the waveguide via waveform synthesizer, frequency splitter and amplifiers. The waveform synthesizer includes a ROM containing digitized values of segments of the waveform amplitudes and a digital-to-analog converter for converting the digital ROM data to an analog signal.