High power solid-state sapphire Whispering Gallery mode maser

We present new results on a cryogenic solid-state maser frequency standard, which relies on the excitation of Whispering Gallery (WG) modes within a doped monocrystalline sapphire resonator (alpha-Al₂O₃). Included substitutively within the highest purity HEMEX-grade sapphire crystal lattice are Fe²⁺ impurities at a concentration of parts per million, an unavoidable result of the manufacturing process. Mass conversion of Fe²⁺ to Fe³⁺ ions was achieved by thermally annealing the sapphire in air. Above-threshold maser oscillation was then excited in the resonator at zero applied DC magnetic field by pumping high-Q WG modes coincident in frequency with the Electron Spin Resonance (ESR) energy levels of the Fe³⁺ spin population. A two stage annealing process was undertaken, resulting in an improvement of six orders of magnitude in output power over the previous best implementation of this scheme, and giving an output seven orders of magnitude more powerful than a typical commercial hydrogen maser. We estimate the limit on the frequency stability due to the Schawlow-Townes fundamental thermal noise limit on the order of 1 times 10^-17/radic(tau).