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 (??-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 2 stage annealing process was undertaken for a sapphire resonator with exceptionally low Fe³⁺ concentration, resulting in an improvement of 6 orders of magnitude in output power for this particular crystal, and exceeding the previous best implementation of our scheme in another crystal by nearly 20 dB. This represents an output signal 7 orders of magnitude more powerful than a typical commercial hydrogen maser. At this power level, we estimate a limit on the frequency stability of order 1 ?? 10^-17/???? due to the Schawlow-Townes fundamental thermal noise limit.