Frequency chirp linearization for ultraflat optical frequency comb generation based on group velocity dispersion.

We propose to insert group velocity dispersion between cascaded phase and amplitude modulation for ultraflat optical frequency comb (OFC) generation. With the dispersion, the sinusoidally varied chirp of the continuous wave light induced by phase modulation becomes linear within a relatively wide time interval. This is useful to improve the flatness of the generated OFC by directly cascaded phase and amplitude modulation. Simulation shows a flat comb of 37 tones within 0.88 dB power variation when the modulation index of the phase modulation reaches 20. An ultraflat comb generator with 10 GHz frequency spacing is also demonstrated. The flatness of the 15 tones around the center wavelength has been improved to 0.98 dB.