Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators

Abstract

Two nominally identical ultra-stable cryogenic microwave oscillators are compared. Each incorporates a dielectric-sapphire resonator cooled to near 6 K in an ultra-low vibration cryostat using a low-vibration pulse-tube cryocooler. The phase noise for a single oscillator is measured at -105 dBc/Hz at 1 Hz offset on the 11.2 GHz carrier. The oscillator fractional frequency stability is characterized in terms of Allan deviation by 5.3 × 10 −16 τ −1/2 + 9 × 10 −17 for integration times 0.1 s < τ < 1000 s and is limited by a flicker frequency noise floor below 1 × 10 −16 . This result is better than any other microwave source even those generated from an optical comb phase-locked to a room temperature ultra-stable optical cavity. PACS numbers: 77.90.+k, 06.30.Ft, 07.20.Mc The time and frequency standards community is ever advancing the state-of-the-art in both stability and accuracy of atomic clocks [1]. With the development of atomic fountain clocks it was found that a more stable microwave interrogation oscillator was required than was available from an ultra-stable quartz oscillator to avoid the Dick effect [2][3][4][5]. The latter results from the fact that most atomic fountain clocks have significant dead time in their interrogation cycle, simply due to the fact that they are pulsed devices where one has to wait until the detection process is finished before the next cloud of cold atoms is launched [6]. This results in the introduction of the phase noise of the local reference oscillator, from which the microwave cavity frequency is derived, to the short-term stability of the fountain clock. The problem was overcome through the use of cryogenic sapphire oscillators with significantly much better short-term frequency stability [7] and consequently it has resulted in atomic fountain clocks reaching a performance limited only by quantum projection noise [8].These microwave oscillators based on a dielectricsapphire resonator cryogenically cooled to nearly 6 K are in use in several standards labs and have been found to be useful for various applications [9][10][11][12]. The oscillators [13][14][15], until recently, have relied on large dewars of liquid helium to cool the resonator with a few exceptions [16][17][18][19] where a cryocooler was used. However the latter had their short-term performance compromised by vibrations from the cryocooler itself. More recently low vibration pulse-tube cryocoolers and specially designed cryostats [20] have enabled cryogenic oscillators as equally stable [21-25] yet without being limited by the noise from the cryocooler.The present design is currently state-of-the-art. A silver-plated copper cavity encloses a 51 mm diameter * Electronic address: john.hartnett@uwa.edu.au and 30 mm high HEMEX grade sapphire cylindrical resonator and is the same as previously reported [13] (see Fig. 1 therein) but the thermal design for housing it in the cryostat is significantly different [24]. See Fig. 1. Also long coaxial cables connecting the cryogenic resonator to the room temperature loop o...

Resonator with Ultrahigh Length Stability as a Probe for Equivalence-Principle-Violating Physics

“…and 4 × 10 −19 /s [10] and the lowest value reported was −1.9 × 10 −20 /s during a 9-day long interval [11]. For a sapphire cryogenic optical resonator [12] a mean drift smaller than 6.4 × 10 −19 /s was measured [13].…”

Drift-Compensated Low-Noise Frequency Synthesis Based on a cryoCSO for the KRISS-F1

“…Recently, we introduced a cryogenic sapphire oscillator (CSO) using an ultralow vibration pulsetube cryocooler (cryoCSO) developed by Hartnett's group [9]. The stability of this cryoCSO was measured using another cryoCSO before shipment to the KRISS laboratory, as described in [10] (curve 4 in Fig. 6).…”

Autonomous cryogenic sapphire oscillators employing low vibration pulse-tube cryocoolers at NMIJ

“…In order to overcome these problems, a low vibration cryostat has been implemented using a pulse-tube-cryocooler [5], [6]. Cryogenic sapphire oscillators have been converted to use this technology (hereafter, designated 'cryoCSO') and as a result their frequency stability has been measured to be better than that of liquid-helium-cooled versions [6], [7]. At NMIJ, two liquid-heliumcooled CSOs, which had been operated since 2005, were converted using pulse-tube cryo-refrigerators and low vibration cryostats and their resulting performance evaluated.…”