Properties of sapphire crystals elaborated with different grow technics for microwave ultra-stable oscillator applications

Abstract

State-of-the-art microwave ultra-stable oscillators are currently based on sapphire resonator operated in whispering gallery modes in the range 5 - 12 GHz. Indeed the best near carrier phase noise is achieved with commercial systems incorporating a room temperature sapphire reference associated with a sophisticated electronics degenerating the noise of the sustaining oscillator stage [1]. On the other part, relative frequency instabilities better than few 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> are achieved with laboratory Cryogenic Sapphire Oscillator (CSO) in which the sapphire crystal is cooled into a large liquid Helium dewar and maintained at its turnover temperature (between 5-8 K) [2]. More reccenlty, we demonstrated an original and reliable technology incorporating a pulse-tube cooler instead of a bath cryostat thus eliminating the need for regular supplies and manual transferring of liquid helium [3]. The advent of reliable and cryocooled (CSO) open the possibility to implement such an ultra-stable reference not only in metrological laboratories with liquid helium facilities but also in remote sites like base stations for space navigation, VBLI antenna sites, ... This technology is today available through a newly created business unit: ULISS® [4]. To get such a type of high performances, the heart of the system, i.e. the Sapphire resonator, is made from a high purity monocrystal elaborated with a sophisticated grow method able to produce large sapphire boule exempt of structural defect. Nevertheless presence of paramagnetic impurities in small concentration (1 ppm or less typically) is required to obtain a turnover temperature near the liquid helium temperature. In this paper we present the comparison of sapphire resonators machined from monocrystal elaborated with two different grow methods. For each crystal, Q-factor and thermal sensitivity have been measured at low temperature for some whispering galery modes.