We demonstrate ultralow phase noise 10-GHz microwave signal generation from afree-running mode-locked Er-fiber laser with -142 and -157 dBc/Hzsingle-sideband absolute phase noise at 10- and 100-kHz offset frequencies,respectively. The absolute RMS timing jitter is 1.5 fs when integrated from1-kHz to 5-GHz (Nyquist frequency) offset frequency. In the 10-kHz to 10-MHzintegration bandwidth typically used for microwave generators, the RMSintegrated jitter is 0.49 fs. The Er-fiber laser is operated in thestretched-pulse regime at close-to-zero dispersion to minimize the phase noiseof extracted microwaves. In order to suppress the excess phase noise in theoptical-to-electronic conversion process, we synchronize a low-noisevoltage-controlled oscillator to the fiber laser using a fiberSagnac-loop-based optical-microwave phase detector.
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