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A novel scheme to generate coherent and frequency-lock optical subcarriers is proposed and experimentally demonstrated. 53 subcarriers with 12.5 GHz spacing and less than 10 dB power fluctuation are generated by cascading two stages of phase modulators, which are driven by 25 and 12.5 GHz clock sources at different modulation depths, respectively. The modulation index and phase difference of two stage phase modulation are theoretically and experimentally investigated. The flattening spectrum of the optical subcarriers is achieved by optimizing the clock phase relationship among the RF driving sources at both stages.