Three kinds of clock recovery circuit for 40-Gbit/s optical RZ signals are fabricated and their characteristics are compared. The three schemes comprise a phase lock loop (PLL) including a voltage-controlled oscillator, an injection locking, and a logic operation by T-flip-flop. The injection-locking scheme is then applied for 80-Gbit/s signals, where the input optical signal is converted into a 40-GHz signal by an optical modulator driven by the output of the locking circuit. Characteristics as hold-in rage, frequency response, and phase noise are measured and compared among the schemes. The properties of the PLL and the injection locking circuit were relatively similar and those of the TFF circuit were uncharacteristic compared with others. Performance of the circuits was confirmed by error free transmission and by stable phase noise over 24 hours. Phase noise transfer properties of the three schemes are briefly discussed.
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