Study of Fe3+-sapphire maser above 4K

The Whispering Gallery Mode Maser Oscillator (WHIGMO) uses paramagnetic Fe³⁺ ions in HEMEX sapphire at low temperature, and has been described in many previous publications [1-7]. The fractional frequency instability has been demonstrated to be as low as σ_y(1s<;τ<;100s) = 10^-14. This instability is normally measured at the frequency-temperature turnover point of the sapphire where the effects of temperature fluctuations on frequency are nullified to first order [8-9]. Usually the temperature is determined by the relative concentrations of several paramagnetic ions such as Cr3+, Mo3+, and Ti3+. In this paper, we show that Fe3+ ions have a strong effect on the turnover point. We compare the behavior of two crystals - “Pinocchio” (concentration of active Fe3+ ions = 10ppb) and “Geppetto” (100ppb). We also report the observation of an upper limit for maser operation above 30K. This limit is explained using a Boltzmann distribution for the active ion, and we show that the populations of the energy levels are so close at this temperature that the effect of the pump at 31 GHz used to create the maser at 12 GHz is ineffective.