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