Broadened and Flattened Optical Frequency Comb Based on Directly Modulated DFB Semiconductor Laser

A simple scheme for generating a broadened and flattened optical frequency comb (OFC) based on a directly modulated distributed feedback (DFB) semiconductor laser cascaded with electro-optic modulators is proposed. The initial OFC is generated by a gain-switched DFB semiconductor laser. The number of comb lines is further expanded by cascading a phase modulator and the flatness performance is optimized by adjusting the power of the radio frequency (RF) and bias voltage of the cascaded Mach–Zehnder modulator. Moreover, the spacing of comb lines can be flexibly tuned by varying the frequency of the RF signal, and an OFC with seven spectral lines within a 4.72 dB power variation is demonstrated. The 17-line OFC with a frequency spacing of 12.5 GHz and a flatness of 3 dB is further applied to an optical transmission system, which enables error-free transmission of a non-return-to-zero data signal. The proposed scheme is expected to find applications in flexible-grid optical transmission systems.