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/sup -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/spl times/10/sup -14/ at 1 s integration time, dropping to 3.5/spl times/10/sup -14/ at 30 seconds integration time, has been measured.