Richard W. Fox

Making optical atomic clocks more stable with 10 −16 -level laser stabilization

Scientists demonstrate a cavity-stabilized laser system with a reduced thermal noise floor, exhibiting a fractional frequency instability of 2 × 10−16. They use this system as a stable optical source in an ytterbium optical lattice clock to resolve an ultranarrow 1 Hz linewidth for the 518 THz clock transition. Consistent measurements with a clock instability of 5 × 10−16/√τ are reported.

Reactive uptake of NO3 on pure water and ionic solutions

The reactive uptake coefficients (γ) of NO 3 onto pure water and dilute solutions of NaCl, NaBr, and NaNO 2 were measured using a wetted-wall flow-tube setup combined with a long-path absorption cell for the detection of NO 3 . The measured γ values were in the range 1.5 × 10 -4 - 6 × 10 -3 , depending on the salt concentration in the water. By measuring γ as a function of salt concentration, HD l 0.5 for NO 3 in water was determined to be (1.9±0.4)×10 -3 M atm -1 cm s -0.5 at 273 K, assuming that the rate coefficient for the reaction of NO 3 with Cl - is 2.76×10 6 M -1 s -1 at 273 K. The Henrys law coefficient for NO 3 in water is estimated to be 0.6±0.3 M atm -1 , assuming that the diffusion coefficient of NO 3 in water is D l = (1.0±0.5)×10 -5 cm 2 s -1 . Uptake of NO 3 on pure water is interpreted as due to reaction of NO 3(aq) with H 2 O (l) to produce HNO 3 and OH in the liquid phase. Implications of these findings to the chemistry of NO 3 in the troposphere are also discussed.

Fiber-laser-based frequency comb with a tunable repetition rate.

A phase-locked, self-referenced frequency comb generated by a mode-locked fiber soliton laser with a tunable repetition rate is presented. The spacing of the frequency comb is set by the lasers repetition rate, which can be scanned from 49.3 MHz to 50.1 MHz while one tooth of the comb is held phase-locked to a stable RF source. This variable repetitionrate frequency comb should be useful for wavelength and length metrology, synchronization of different fiber laser-based frequency combs, and the generation of precise swept wavelength sources.

High-Accuracy Measurement of Atomic Polarizability in an Optical Lattice Clock

Presently, the Stark effect contributes the largest source of uncertainty in a ytterbium optical atomic clock through blackbody radiation. By employing an ultracold, trapped atomic ensemble and high stability optical clock, we characterize the quadratic Stark effect with unprecedented precision. We report the ytterbium optical clocks sensitivity to electric fields (such as blackbody radiation) as the differential static polarizability of the ground and excited clock levels α(clock) = 36.2612(7)  kHz (kV/cm)(-2). The clocks uncertainty due to room temperature blackbody radiation is reduced by an order of magnitude to 3×10(-17).

Optical probing of cold trapped atoms.

Transitions between excited states of laser-cooled and laser-trapped rubidium and cesium atoms are probed by use of fiber and diode lasers. High-resolution Doppler-free spectra are detected by observation of the absorption and fluorescence of light from the intermediate level of two-step cascade systems. The optical double-resonance spectra show Autler–Townes splitting in the weak probe limit and more complicated spectra for a strongly coupled three-level system.

1. Stabilizing diode lasers to high-finesse cavities

Publisher Summary This chapter discusses the issues involved in diode laser locking. The chapter describes in detail the various steps needed to lock the laser to a cavity resonance: (1) Derivetion of the error (locking) signal, (2) design of the electronic feedback circuitry, (3) initial locking of the laser, (4) adjustment of the feedback design, and (5) evaluation of the lock performance. The chapter illustrates this discussion by frequency locking an extended-cavity diode laser, reducing the linewidth to a few hertz relative to the cavity. The chapter concludes with an example in which the locking apparatus is modified for a cavity ring-down demonstration. Included are results showing the laser repetitively locking and unlocking to the cavity.

Direct comparison of two cold-atom-based optical frequency standards by using a femtosecond-laser comb

With a fiber-broadened, femtosecond-laser frequency comb, the 76-THz interval between two laser-cooled optical frequency standards was measured with a statistical uncertainty of 2x10(-13) in 5 s , to our knowledge the best short-term instability thus far reported for an optical frequency measurement. One standard is based on the calcium intercombination line at 657 nm, and the other, on the mercury ion electric-quadrupole transition at 282 nm. By linking this measurement to the known Ca frequency, we report a new frequency value for the Hg(+) clock transition with an improvement in accuracy of ~10(5) compared with its best previous measurement.

Absolute frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser

A Cr:forsterite laser-based frequency comb is stabilized simultaneously to two NIST frequency references. Several optical frequency reference frequencies are then measured from 1315 nm - 1620 nm, including methane lines near 1330 nm.

Wavelength references for interferometry in air

Cavity-mode wavelengths in air are determined by measuring a lasers frequency while it is locked to the mode in vacuum during a calibration step and subsequently correcting the mode wavelength for atmospheric pressure compression, temperature difference, and material aging. Using a Zerodur ring cavity, we demonstrate a repeatability of +/- 2 x 10(-8) (3sigma), with the wavelength accuracy limited to +/- 4 x 10(-8) by knowledge of the absolute helium gas temperature during the pressure calibration. Mirror cleaning perturbed the mode frequency by less than deltav/v approximately 3 x 10(-9), limited by temperature correction residuals.

High-resolution diode-laser spectroscopy of calcium

Saturated-absorption signals on the calcium 657 nm transition are observed by direct absorption using diode lasers and a high flux atomic-beam cell. Line-widths as narrow as 65 kHz are observed with a high signal-to-noise ratio. Prospects for using this system as a compact wavelength/frequency reference are considered.