High-repetition-rate electro-optic frequency combs using cascaded silicon phase modulators.

We explore what we believe to be novel methods to improve the spacing and flatness of comb lines in an electro-optic frequency comb using cascaded silicon phase modulators. Our analysis extends to various factors influencing the comb spectral symmetry: phase modulation nonlinearity, PN-junction transient response, and waveguide group dispersion. We confirm the feasibility of harnessing these diverse effects to fine-tune the comb spectral shape to enhance uniformity when using cascaded phase modulators. The comb spacing in our experimental solution is adjustable and can be expanded to a maximum of 37.5 GHz. Targeting frequency spacing of 37.5 GHz and a flatness of 6 dB, we can achieve a seven-line frequency comb. This demonstration represents the highest repetition rates ever reported for silicon modulators. For instance, such a comb could carry a super-channel signal of up to 300 Gbaud (i.e., 75 Gbaud × 4) per comb line. Improving spectral uniformity extends comb frequency span, further enhancing the advantages of employing cascaded modulators in the generation of integrated electro-optic frequency combs.