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

Abstract

We present the characterization of 8 − 12 GHz whispering gallery mode resonators machined in high-quality sapphire crystals elaborated with different growth techniques. These microwave resonators are inttended to constitute the reference frequency of ultra-stable Cryogenic Sapphire Oscillators. We conducted systematic tests near 4K on these crystals to determine the unloaded Qfactor and the turnover temperature for whispering gallery modes in the 8-12 GHz frequency range. These characterizations show that high quality sapphire crystals elaborated 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.

Frequency synthesis from Cryogenic Sapphire Oscillator

“…The development of Cryogenic Sapphire Oscillators based on whispering gallery mode sapphire resonator has allowed an improvement of 1 to 2 orders of magnitude in terms of frequency stability, between one and several thousand seconds of integration time, compared to the hydrogen maser [4], [5]. The state-of-the-art of such a source in terms of frequency stability is around 6.10 at 1s of integration time [5]- [9]. Other technologies are able to produce ultra-low-noise microwave signals, like optical-comb-based frequency division of a cavity-stabilized laser [11]- [13].…”

Frequency Stability Measurement of Cryogenic Sapphire Oscillators With a Multichannel Tracking DDS and the Two-Sample Covariance

“…This is due to the presence of paramagnetic Cr 3+ , Fe 3+ and Mo 3+ impurities. The growth process provides the right amount of such impurities, with very similar results over at least two growth technologies and manufacturers [28]. The WGH 15,0,0 mode of a cylinder of 54 mm diameter and 30 mm height resonates close to 10 GHz.…”

Magnetic sensitivity of the Microwave Cryogenic Sapphire Oscillator

“…The figure 1 represents the immediate environment of the resonator thermally linked to the cryocooler second stage through soft copper braids. We initially selected a sapphire crystal elaborated with the Kyropoulos method 13 . This resonator, designed as PST-17-03, presents an unloaded Q-factor of 0.9 × 10 9 measured at its turnover temperature T 0 = 9.01 K. This relative high value of T 0 departs from our preceeding realizations, for which T 0 was always found below 7 K. Nevertheless, 9 K is still compatible with the cryocooler operation and, providing the short term stability is limited by the intrinsic noise of the control electronics, the Q-factor is sufficient to reach an Allan deviation (ADEV) of 1 × 10 −15 at τ = 1 s 11 .…”