In this letter we report on an all optical-fiber approach to the generation of ultra-low noise microwave signals. We make use of two erbium fiber mode-locked lasers phase locked to a common ultra stable laser source to generate an 11.55 GHz signal with an unprecedented relative phase noise of -111 dBc/Hz at 1 Hz from the carrier. The residual frequency instability of the microwave signals derived from the two optical frequency combs is below 2.3x10 -16 at 1s and about 4x10 -19 at 6.5x10 4 s (in 5Hz Bandwidth, three days continuous operation).
We report the first observation of above-threshold maser oscillation in a whispering-gallery(WG)mode resonator, whose quasi-transverse-magnetic, 17 th -azimuthal-order WG mode, at a frequency of approx. 12.038 GHz, with a loaded Q of several hundred million, is supported on a cylinder of mono-crystalline sapphire. An electron spin resonance (ESR) associated with Fe 3+ ions, that are substitutively included within the sapphire at a concentration of a few parts per billion, coincides in frequency with that of the (considerably narrower) WG mode. By applying a c.w. 'pump' to the resonator at a frequency of approx. 31.34 GHz, with no applied d.c. magnetic field, the WG ('signal') mode is energized through a three-level maser scheme. Preliminary measurements demonstrate a frequency stability (Allan deviation) of a few times 10 −14 for sampling intervals up to 100 s.
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