Tests of Sapphire Crystals Produced with Different Growth Processes for Ultra-stable Microwave Oscillators

TL;DR

This study compares sapphire crystals produced by different growth methods to identify which yields the best performance for ultra-stable microwave oscillators at cryogenic temperatures.

Contribution

It provides a systematic characterization of sapphire crystals from various growth processes for use in ultra-stable cryogenic microwave oscillators.

Findings

High-quality sapphire crystals from Heat Exchange and Kyropoulos methods achieve fractional frequency stability better than 1x10^-15.

Systematic tests at 4 K determine optimal whispering gallery modes for oscillator stability.

Both growth techniques produce suitable crystals for ultra-stable frequency references.

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 intended to constitute the reference frequency of ultra-stable Cryogenic Sapphire Oscillators. We conducted systematic tests near 4 K on these crystals to determine the unloaded Q-factor 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 1x10-15 for 1 s to 10.000 s integration times.