title: High-power solid-state sapphire whispering gallery mode maser.
abstract: 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 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 x 10(-17)/square root(tau) due to the Schawlow-Townes fundamental thermal noise limit.
authors.affiliation: School of Physics, The University of Western Australia, Crawley WA, Australia. creedon@physics.uwa.edu.au
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Creedon
authors.forename: Karim
authors.initials: K
authors.lastname: Benmessaï
authors.forename: Michael E
authors.initials: ME
authors.lastname: Tobar
authors.forename: John G
authors.initials: JG
authors.lastname: Hartnett
authors.forename: Pierre-Yves
authors.initials: PY
authors.lastname: Bourgeois
authors.forename: Yann
authors.initials: Y
authors.lastname: Kersale
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Le Floch
authors.forename: Vincent
authors.initials: V
authors.lastname: Giordano