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We report the generation of fifty 12.5-GHz-spaced optical carriers with high power flatness and stability by using a single-side band (SSB) modulator-based recirculating frequency shifter (RFS). The peak-to-peak power difference and the RMS power difference of the generated carriers are 2.5 and 0.3 dB, respectively. We also experimentally investigate the impact of implementation imperfections on the flatness of the generated carriers. The important factors that contribute to the flatness of the generated carriers are found to be the amplitude balance of two inphase (I) and quadrature (Q) drive signals for the SSB modulator, the time misalignment between the I and Q signals, the accuracy of the /2 phase bias of the SSB modulator and the stability of the polarization alignment in the RFS. By carefully controlling all the factors mentioned earlier, we obtain stable operation of 50 frequency-locked carriers; showing that the SSB modulator-based RFS is a promising technique for future terabit per second multicarrier transmission.