Zero-field Fe/sup 3+/: sapphire whispering-gallery-mode solid-state maser oscillator

We report the realization of a new type of solid-state maser oscillator whose output frequency is controlled by a whispering-gallery (WG) electromagnetic mode. Here, the maser oscillator can be regarded as a free-running loop oscillator whose component loop resonator and sustaining amplifier are consolidated within the same compact object, namely a cylinder of Fe/sup 3+/-doped mono-crystalline sapphire. This cylinder supports a 17/sup th/-azimuthal-order quasi-transverse-magnetic WG mode at approx. 12.04 GHz, with a loaded (ESR-saturated) Q of several hundred million. The WG mode's frequency lies within the line-width of the electron spin resonance (ESR) associated with transitions between the lowest two paramagnetic levels of Fe/sup 3+/ ions, that are included within the sapphire's lattice at a substitutive concentration of approx. one part per billion. By applying an auxiliary microwave 'pump' at a frequency of approx. 31.3 GHz to the resonator, a population inversion is achieved across this 'signal' ESR transition with no applied d.c. magnetic bias field. Despite the extremely low concentration of Fe/sup 3+/ ions present, continuous above-threshold maser oscillation is achieved on the 12.04-GHz whispering-gallery mode due to its high Q. The power of the maser oscillator's signal output exceeds -50 dBm, some 10/sup 4/ times greater than that provided by an active hydrogen maser. In a highly preliminary experiment, the oscillator's frequency stability at 30 seconds was measured to be 2.5 /spl times/ 10/sup -14/.