Opt. Lett. 36, 3260–3262 (2011)

Microwave signals with low phase noise are important for applications such as large-scale high-precision remote synchronization and long-baseline interferometry. It has been shown that an ultralow phase noise of −104 dBc Hz−1 at an offset of 1 Hz from a 10 GHz carrier can be generated by a Ti:sapphire-based optical frequency divider (OFD). Achieving this noise level using an erbium-doped fibre-based OFD would allow large-scale pulse distribution at telecommunications wavelengths. Scott Diddams and co-workers in the USA have now presented a scheme that is capable of producing 10 GHz microwave signals with absolute phase noise below −100 dBc Hz−1 at an offset of 1 Hz, limited by the optical frequency reference. For frequencies of >10 kHz, the phase noise is shot-noise-limited to −145 dBc Hz−1. The key component of the scheme is a 200 MHz erbium-doped fibre mode-locked laser with a high-speed intracavity electro-optic phase modulator and low intrinsic relative intensity noise. The team achieved a phase noise that is equal to or better than the 10 GHz phase noise from cryogenic microwave oscillators and is more than 40 dB lower than 10 GHz room-temperature oscillators at an offset frequency of 1 Hz.