John J. McFerran

Considerations on the measurement of the stability of oscillators with frequency counters

The most common time-domain measure of frequency stability, the Allan variance, is typically estimated using a frequency counter. Close examination of the operation of modern high-resolution frequency counters shows that they do not make measurements in the way commonly assumed. The consequence is that the results typically reported by many laboratories using these counters are not, in fact, the Allan variance, but a distorted representation. We elucidate the action of these counters by consideration of their operation in the Fourier domain, and demonstrate that the difference between the actual Allan variance and that delivered by these counters can be very significant for some types of oscillators. We also discuss ways to avoid, or account for, a distorted estimation of Allan variance

Neutral atom frequency reference in the deep ultraviolet with fractional uncertainty = 5.7×10(-15).

We present an assessment of the (6s) 1S0 ↔ (6s7s) 3P0 clock transition frequency in Hg with an uncertainty reduction of nearly three orders of magnitude and demonstrate an atomic quality factor, Q, of ∼10. The Hg atoms are confined in a vertical lattice trap with light at the newly determined magic wavelength of 362.5697±0.0011 nm and at a lattice depth of 20ER. The atoms are loaded from a single stage magneto-optical trap with cooling light at 253.7 nm. The high Q factor is obtained with an 80ms Rabi pulse at 265.6 nm. The frequency of the clock transition is found to be 1 128 575 290 808 162.0 ± 6.4 (sys.) ± 0.3 (stat.) Hz (fractional uncertainty = 5.7×10). Neither an atom number nor second order Zeeman dependence have yet to be detected. Only three laser wavelengths are used for the cooling, lattice trapping, probing and detection.

Noise properties of microwave signals synthesized with femtosecond lasers

The excess noise associated with the process of coherent optical-to-microwave frequency division was measured. This was accomplished by referencing two mode-locked Ti: sapphire lasers to the same stable CW laser and extracting microwave signals at the harmonics of pulse repetition rate. The spectral density of the excess phase noise was found to be close to -140dBc/Hz at 100Hz offset from a 10 GHz carrier

Sub-Doppler cooling of fermionic Hg isotopes in a magneto-optical trap

Laser cooling and trapping of neutral mercury is performed in a single-stage (1)S(0)↔(3)P(1) 3D magneto-optical trap. We give a detailed account of the atom cloud size and temperature for both bosonic ((200)Hg and (202)Hg) and fermionic ((199)Hg and (201)Hg) isotopes. The bosonic isotope temperatures are in close agreement with Doppler cooling theory, while temperatures obtained for the fermionic isotopes are lower, suggesting the presence of sub-Doppler cooling. A minimum temperature of 29±4 μK is achieved for (201)Hg.

Fractional frequency instability in the 10 −14 range with a thermal beam optical frequency reference

We demonstrate a means of increasing the signal-to-noise ratio in a Ramsey-Borde interferometer with spatially separated oscillatory fields on a thermal atomic beam. The S01↔P13 intercombination line in neutral Ca40 is used as a frequency discriminator, with an extended cavity diode laser at 423 nm probing the ground state population after a Ramsey-Borde sequence of 657 nm light-field interactions with the atoms. Evaluation of the instability of the Ca frequency reference is carried out by comparison with (i) a hydrogen-maser and (ii) a cryogenic sapphire oscillator. In the latter case the Ca reference exhibits a square-root Λ variance of 9.2×10−14 at 1 s and 2.0×10−14 at 64 s. This is an order-of-magnitude improvement for optical beam frequency references, to our knowledge. The shot noise of the readout fluorescence produces a limiting square-root Λ variance of 7×10−14/√τ, highlighting the potential for improvement. This work demonstrates the feasibility of a portable frequency reference in the optical domain with 10−14 range frequency instability.

Optical frequency synthesis from a cryogenic microwave sapphire oscillator

We demonstrate an optical frequency comb with fractional frequency instability of </=2x10(-14) at measurement times near 1 s, when the 10th harmonic of the comb spacing is controlled by a liquid helium cooled microwave sapphire oscillator. The frequency instability of the comb is estimated by comparing it to a cavity-stabilized optical oscillator. The less conventional approach of synthesizing low-noise optical signals from a microwave source is relevant when a laboratory has microwave sources with frequency stability superior to their optical counterparts. We describe the influence of high frequency environmental noise and how it impacts the phase-stabilized frequency comb performance at integration times less than 1 s.

Testing local position and fundamental constant invariance due to periodic gravitational and boost using long-term comparison of the SYRTE atomic fountains and H-masers

The frequencies of three separate Cs fountain clocks and one Rb fountain clock have been compared to various hydrogen masers to search for periodic changes correlated with the changing solar gravitational potential at the Earth and boost with respect to the cosmic microwave background rest frame. The data sets span over more than 8 yr. The main sources of long-term noise in such experiments are the offsets and linear drifts associated with the various H-masers. The drift can vary from nearly immeasurable to as high as

Échelle spectrograph calibration with a frequency comb based on a harmonically mode-locked fiber laser: a proposal

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Laser locking to the 199 Hg 1 S 0 − 3 P 0 clock transition with 5.4 × 10 −15 /✓τ fractional frequency instability

per day. To circumvent these effects, we apply a numerical derivative to the data, which significantly reduces the standard error when searching for periodic signals. We determine a standard error for the putative local position invariance coefficient with respect to gravity for a Cs-fountain H-maser comparison of

Ultraviolet laser spectroscopy of neutral mercury in a one-dimensional optical lattice

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