Evolution of the UWA solid nitrogen dual mode sapphire oscillator, JULIA

Low temperature, high precision sapphire oscillators rely on a turning point in mode frequency-temperature dependence at around 10 K. This, along with sapphire´s extremely low dielectric losses at microwave frequencies, allows fractional frequency stabilities on the order of 10^-15. At higher temperatures, the lack of a turning point makes a single mode oscillator very sensitive to temperature fluctuations. By exciting two quasi-orthogonal whispering gallery (WG) modes, a turning point can be found in their beat frequency. A novel temperature control technique has been used to maintain the sapphire at the turning point temperature and a fractional frequency instability of 4.3×10^-14 at 1 s integration time, dropping to 3.5×10^-14 at 30 seconds integration time, has been measured.