Tests of Sapphire Crystals Manufactured With Different Growth Processes for Ultra-Stable Microwave Oscillators

We present the characterization of 8-12 GHz whispering gallery mode resonators machined in high-quality sapphire crystals manufactured with different growth techniques. These microwave resonators are intended to constitute the frequency reference of ultra-stable Cryogenic Sapphire Oscillators. We conducted systematic tests near 4 K to determine the unloaded Q-factor and the turnover temperature for whispering gallery modes in the 8-12 GHz frequency range. We have shown that high quality sapphire crystals manufactured with the Heat Exchange or the Kyropoulos growth technique are both suitable to meet a fractional frequency stability better than 1×10
^-15
for 1 s to 10,000 s integration times.