Frequency-temperature compensation in Ti/sup 3+/ and Ti/sup 4+/ 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 Ti/sup 3+/ ions. This technique offers significant advantages over other methods. A second sapphire resonator had the Ti/sup 3+/ ions converted to Ti/sup 4+/ ions. The method left an unknown residual concentration of Ti/sup 3+/ ions. We found that temperature compensation (turning) points existed for various modes in the Ti/sup 3+/ doped sample from about 76 K (in H modes) down to 27 K (in E modes). Whereas the turning points for the Ti/sup 4+/ doped sample were detected between 9 and 35 K. The mechanism of compensation in both cases was due to the paramagnetic susceptibility of Ti/sup 3+/ ions. For the Ti/sup 3+/ 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/sub 12,1,/spl delta// mode. H modes increased also but to no more than 40 million. At 50 K in the Ti/sup 4+/ doped sample the highest Q factor was 16 million in the E/sub 12,1,/spl delta// 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.