Generation of a 25-line flattened optical frequency comb and its coherent beating properties

A 25-line flattened optical frequency comb (OFC) is generated by cascading Mach-Zehnder modulators, and its coherent beating properties are investigated. The effect of bias voltages and the power of radio frequency signals on the flatness of the OFC, and the phase relationships between each comb line are theoretically deduced. In the experiment, a 25-line OFC with a flatness of less than 1 dB and an optical sideband suppression ratio of 12 dB is successfully demonstrated, and the agreement between simulation and experimental results supports the validity of the model for analysis of the flatness. The measured amplitude of harmonic signals by coherent beating of the generated 25-line OFC matches well with the theoretical prediction. A phase noise of -105.31dBc/Hz at 10 kHz frequency offset for the 12th harmonic signal almost without phase noise degradation proves the high coherence of the generated OFC lines.