Titanium doped cryogenic sapphire resonator oscillators

Residual paramagnetic impurities are present in high quality sapphire crystal. This phenomenon was exploited as paramagnetic spin compensation at liquid helium temperature to build high Q cryogenic sapphire resonator oscillators (CSRO). In this paper we present the characterization of intentionally Ti³⁺ and Ti⁴⁺ doped sapphire resonator oscillators. For the Ti³⁺ resonator we used the 12.7 GHz mode operating at 34K. This resonator exhibited a frequency stability of order 2 times 10^-3 for 8s < tau < 20s with the use of a Gifford-McMahon cryocooler as a cold source. This corresponds to state-of-the-art line splitting of the order of 10 ^-7. A second experiment with a pulse tube cryocooler shows an improvement in the short term frequency stability. The Ti⁴⁺ resonator oscillator at 13.8 GHz exhibited a frequency stability of 7 times 10^-14 at tau = 16s