Frequency-temperature compensation in Ti3+ and Ti4+ doped sapphire whispering gallery mode resonators

A new method of compensating the frequency-temperature dependence of high-Q monolithic sapphire dielectric resonators near liquid nitrogen temperature is presented. This is achieved by doping monocrystalline sapphire with Ti³⁺ ions. This technique offers significant advantages over other methods. A second sapphire resonator had the Ti ³⁺ ions converted to Ti⁴⁺ ions. The method left an unknown residual concentration of Ti³⁺ ions. We found that temperature compensation (turning) points existed for various modes in the Ti³⁺ doped sample from about 76 K (in H modes) down to 27 K (in E modes). Whereas the turning points for the Ti⁴⁺ doped sample were detected between 9 and 35 K. The mechanism of compensation in both cases was due to the paramagnetic susceptibility of Ti³⁺ ions. For the Ti³⁺ doped sample at 50 K the Q factor was highest in E modes at 4 million, whereas the H modes were about half a million or less. On cooling to 15 K the Q factor rapidly increased and at 4.2 K reached 1.7 billion in the E_12,1,δ mode. H modes increased also but to no more than 40 million. At 50 K in the Ti ⁴⁺ doped sample the highest Q factor was 16 million in the E _12,1,δ mode, and reached 190 million at 5 K. The highest Q factors observed in H modes were about 5 million at 50 K rising to a little more than 50 million at 5 K