We demonstrated a synchronous control technique for external optical modulation in a format of continuous-phase frequency-shift keying (CPFSK) at 10 Gb/s. In this method, the FSK signal in the upper- or lower-sideband state synchronously shifts to the other state at the timing when their phases are the same. We investigated the accuracy of the timing control required for the synchronous control. Experimental results show that the allowable timing misalignment to keep power penalty of the receiver sensitivity less than 1 dB was more than 25 ps, 25% of each bit period.
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