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 Fe³⁺-doped mono-crystalline sapphire. This cylinder supports a 17^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 Fe³⁺ 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 Fe³⁺ 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 10⁴ 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 times 10^-14