Ultra-low vibration pulse-tube cryocooler stabilized cryogenic sapphire oscillator with 10^-16 fractional frequency stability

A low maintenance long-term operational cryogenic sapphire oscillator has been implemented at 11.2 GHz using an ultra-low-vibration cryostat and pulse-tube cryocooler. It is currently the world's most stable microwave oscillator employing a cryocooler. Its performance is explained in terms of temperature and frequency stability. The phase noise and the Allan deviation of frequency fluctuations have been evaluated by comparing it to an ultra-stable liquid-helium cooled cryogenic sapphire oscillator in the same laboratory. Assuming both contribute equally, the Allan deviation evaluated for the cryocooled oscillator is sigma_y = 1 x 10^-15 tau^-1/2 for integration times 1 < tau < 10 s with a minimum sigma_y = 3.9 x 10^-16 at tau = 20 s. The long term frequency drift is less than 5 x 10^-14/day. From the measured power spectral density of phase fluctuations the single side band phase noise can be represented by L_phi(f) = 10^-14.0/f^4+10^-11.6/f^3+10^-10.0/f^2+10^-10.2/f+ 10^-11.0 for Fourier frequencies 10^-3<f<10^3 Hz in the single oscillator. As a result L_phi approx -97.5 dBc/Hz at 1 Hz offset from the carrier.Comment: 8 pages, 10 figures, presented at European Frequency and Time Forum, ESTEC, Noordwijk, Netherland, April 11-16th 2010 accepted in IEEE Trans. on Micro. Theory & Technique

Ultra-low-phase-noise cryocooled microwave dielectric-sapphire-resonator oscillators with 1 x 10^-16 frequency instability

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 x 10^-16 tau^-1/2 + 9 x 10^-17 for integration times 0.1 s < tau < 1000 s and is limited by a flicker frequency noise floor below 1 x 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.Comment: 4 pages, 5 figure

Cryogenic Sapphire Oscillator using a low-vibration design pulse-tube cryocooler: First results

A Cryogenic Sapphire Oscillator has been implemented at 11.2 GHz using a low-vibration design pulse-tube cryocooler. Compared with a state-of-the-art liquid helium cooled CSO in the same laboratory, the square root Allan variance of their combined fractional frequency instability is $\sigma_y = 1.4 \times 10^{-15}\tau^{-1/2}$ for integration times $1 < \tau < 10$ s, dominated by white frequency noise. The minimum $\sigma_y = 5.3 \times 10^{-16}$ for the two oscillators was reached at $\tau = 20$ s. Assuming equal contributions from both CSOs, the single oscillator phase noise $S_{\phi} \approx -96 \; dB \; rad^2/Hz$ at 1 Hz offset from the carrier.Comment: 5 pages, 5 figures, accepted in IEEE Trans on Ultrasonics, Ferroelectrics and Frequency Contro

Hyperparametric effects in a whispering-gallery mode rutile dielectric resonator at liquid helium temperatures

We report the first observation of low power drive level sensitivity, hyperparametric amplification, and single-mode hyperparametric oscillations in a dielectric rutile whispering-gallery mode resonator at 4.2 K. The latter gives rise to a comb of sidebands at 19.756 GHz. Whereas, most frequency combs in the literature have been observed in optical systems using an ensemble of equally spaced modes in microresonators or fibers, the present work represents generation of a frequency comb using only a single-mode. The experimental observations are explained by an additional 1/2 degree-of-freedom originating from an intrinsic material nonlinearity at optical frequencies, which affects the microwave properties due to the extremely low loss of rutile. Using a model based on lumped circuits, we demonstrate that the resonance between the photonic and material 1/2 degree-of-freedom, is responsible for the hyperparametric energy transfer in the system.Comment: 9 pages, 10 figure

Frequency stability and phase noise of an improved X-band cryocooled sapphire oscillator

A previously implemented Cryogenic Sapphire Oscillator (CSO) based on a commercial cryocooler has been modified and its frequency stability and phase noise re-measured against a nominally similar liquid helium cooled CSO in the same laboratory. Assuming both contribute equally, their frequency stability and phase noise have been evaluated. We report for the oscillator a minimum Allan deviation of 3.9 x 10⁻¹⁶ at 20 s, a long-term frequency drift less than 1 x 10⁻¹⁴/day, and a measured single sideband phase noise of -97 dBc/Hz at 1 Hz offset from the carrier. The stated performance of the cryocooled CSO is adequate for it to be deployed at a local VLBI site for comparison against the hydrogen maser which is the current reference standard.4 page(s

State-of-the-art cryocooled sapphire oscillators

Two nominally identical ultra-low vibration cryocooled microwave sapphire oscillators are compared. The phase noise of a single oscillator was measured to be -105 dBc/Hz at 1 Hz offset on the 11.2 GHz carrier and is characterized by white frequency noise. The Allan deviation at 1s integration time, with drift removed, is 5.8 x 10⁻¹⁶. Between 0.1 and 100 s, the frequency stability is white frequency noise limited with a minimum of 1.6 x 10⁻¹⁶ around 100 s. The long-term stability of the oscillator degrades due to a sensitivity to diurnal room temperature changes. In the absence of this effect the oscillator has potential to reach a stability below 10⁻¹⁶.2 page(s

Ultra-Stable very-low phase-noise signal source for very long baseline interferometry using a cryocooled sapphire oscillator

The design and implementation of a novel frequency synthesizer based on low phase-noise digital dividers and a direct digital synthesizer is presented. The synthesis produces two low noise accurate tunable signals at 10 and 100 MHz. We report the measured residual phase noise and frequency stability of the syn thesizer and estimate the total frequency stability, which can be expected from the synthesizer seeded with a signal near 11.2 GHz from an ultra-stable cryocooled sapphire oscillator (cryoCSO). The synthesizer residual single-sideband phase noise, at 1-Hz offset, on 10and 100-MHz signals was -135 and -130 dBc/Hz, respectively. The frequency stability contributions of these two sig nals was σy = 9 × 10⁻¹⁵ and σy = 2.2 × 10⁻¹⁵, respectively, at 1-s integration time. The Allan deviation of the total fractional frequency noise on the 10- and 100-MHz signals derived from the synthesizer with the cry oCSO may be estimated, respectively, as σy ≈ 3.6 × 10⁻¹⁵ T-1/2 + 4 × 10⁻¹⁶ and σy ≈ s 5.2 × 10⁻² × 10⁻¹⁶ T-1/2 + 3 × 10⁻¹⁶, respectively, for 1 ≤ T <; 10⁴s. We also calculate the coherence function (a figure of merit for very long baseline interferometry in radio astronomy) for observation frequencies of 100, 230, and 345 GHz, when using the cry oCSO and a hydrogen maser. The results show that the cryoCSO offers a significant advantage at frequencies above 100 GHz.9 page(s