Dissemination of UTC(NIST) over 20 km of commercial optical fiber with active phase stabilization.

We demonstrate the transfer of a cesium frequency standard steered to UTC(NIST) over 20 km of dark telecom optical fiber. Our dissemination scheme uses an active stabilization technique with a phase-locked voltage-controlled oscillator. Out-of-loop characterization of the optical fiber link performance is done with dual-fiber and single-fiber transfer schemes. We observe a fractional frequency instability of 1.5 × 10-12 and 2 × 10-15 at averaging intervals of 1 s and 105 s, respectively, for the link. Both schemes are sufficient to transfer the cesium clock reference without degrading the signal, with nearly an order of magnitude lower fractional frequency instability than the cesium clocks over all time scales. The simplicity of the two-fiber technique may be useful in future long-distance applications where higher stability requirements are not paramount, as it avoids technical complications involved with the single-fiber scheme.

Ramsey Spectroscopy of the 2S_{1/2} Hyperfine Interval in Atomic Hydrogen.

The 2S_{1/2} hyperfine interval in atomic hydrogen was measured using Ramsey spectroscopy with a thermal beam cooled to cryogenic temperatures. The measured value is 177 556 838.87(85) Hz, which represents the most precise determination of this interval to date. The 1S_{1/2} hyperfine interval f(1S_{1/2}) and the 2S_{1/2} hyperfine interval f(2S_{1/2}) can be combined to give the quantity D_{21}=8f(2S_{1/2})-f(1S_{1/2}), which mostly eliminates uncertainty due to nuclear structure effects and is well described by bound-state quantum electrodynamics. Using the value of f(2S_{1/2}) from this work gives a value of D_{21}^{expt}=48 959.2(6.8) Hz, which is in agreement with the theoretical value of D_{21}^{Theory}=48 954.1(2.3) Hz.

Measurement of the 2S_{1/2}-8D_{5/2} Transition in Hydrogen.

We present a measurement of the hydrogen 2S_{1/2}-8D_{5/2} transition performed with a cryogenic atomic beam. The measured resonance frequency is ν=770649561570.9(2.0) kHz, which corresponds to a relative uncertainty of 2.6×10^{-12}. Combining our result with the most recent measurement of the 1S-2S transition, we find a proton radius of r_{p}=0.8584(51) fm and a Rydberg constant of R_{∞}=10973731.568332(52) m^{-1}. This result has a combined 3.1σ disagreement with the Committee on Data for Science and Technology (CODATA) 2018 recommended value.

Highly coherent, watt-level deep-UV radiation via a frequency-quadrupled Yb-fiber laser system.

We demonstrate a 1.4 W continuous-wave (CW) laser at 243.1 nm. The radiation is generated through frequency quadrupling the output of a ytterbium-doped fiber amplifier system. which produces >10 W of CW power at 972.5 nm. We demonstrate absolute frequency control by locking the laser to an optical frequency comb and exciting the 1S-2S transition in atomic hydrogen. This frequency-stabilized, high-power deep-UV laser is of significant interest for precision spectroscopy of simple and exotic atoms, two-photon laser cooling of hydrogen, and Raman spectroscopy.

Cavity-enhanced deep ultraviolet laser for two-photon cooling of atomic hydrogen.

We demonstrate a 650 mW 243 nm continuous-wave laser coupled to a linear optical enhancement cavity. The enhancement cavity can maintain >30 W of intracavity power for 1 h of continuous operation without degradation. This system has sufficient power for a demonstration of two-photon laser cooling of hydrogen and may be useful for experiments on other simple two-body atomic systems.

Lyman-α source for laser cooling antihydrogen.

We present a Lyman-α laser developed for cooling trapped antihydrogen. The system is based on a pulsed Ti:sapphire laser operating at 729 nm that is frequency doubled using an LBO crystal and then frequency tripled in a Kr/Ar gas cell. After frequency conversion, this system produces up to 5.7 µW of average power at the Lyman-α wavelength. This laser is part of the ATRAP experiment at the antiproton decelerator in CERN.

Reduced phase noise in an erbium frequency comb via intensity noise suppression.

We present a coherent erbium fiber frequency comb that achieves low phase noise operation through the active suppression of amplitude fluctuations within the laser oscillator. The amplitude noise servo has a bandwidth of 550 kHz and is achieved by current actuation of the laser pump diode. This servo reduces the integrated phase noise of the carrier envelope offset frequency of the comb, fceo, due to the strong coupling of amplitude and phase noise in the laser oscillator. Additionally, we use a composite error signal that utilizes information from both the amplitude noise and the fceo error signal to actuate the pump diode current, which further increases the coherence of the comb. With this locking scheme, the integrated phase noise on fceo is measured to be 270 mrad from 10 Hz to 1.5 MHz, indicating 93% of the optical carrier power is in the coherent signal. A simultaneous phase lock to a narrow-linewidth continuous-wave laser is achieved by actuating on the cavity length, and shows an integrated phase noise of 44 mrad.

Active fiber-based retroreflector providing phase-retracing anti-parallel laser beams for precision spectroscopy.

We present an active fiber-based retroreflector providing high quality phase-retracing anti-parallel Gaussian laser beams for precision spectroscopy of Doppler sensitive transitions. Our design is well-suited for a number of applications where implementing optical cavities is technically challenging and corner cubes fail to match the demanded requirements, most importantly retracing wavefronts and preservation of the laser polarization. To illustrate the performance of the system, we use it for spectroscopy of the 2S-4P transition in atomic hydrogen and demonstrate an average suppression of the first order Doppler shift to 4 parts in 106 of the full collinear shift. This high degree of cancellation combined with our cryogenic source of hydrogen atoms in the metastable 2S state is sufficient to enable determinations of the Rydberg constant and the proton charge radius with competitive uncertainties. Advantages over the usual Doppler cancellation based on corner cube type retroreflectors are discussed as well as an alternative method using a high finesse cavity.

Megawatt-scale average-power ultrashort pulses in an enhancement cavity.

We investigate power scaling of ultrashort-pulse enhancement cavities. We propose a model for the sensitivity of a cavity design to thermal deformations of the mirrors due to the high circulating powers. Using this model and optimized cavity mirrors, we demonstrate 400 kW of average power with 250 fs pulses and 670 kW with 10 ps pulses at a central wavelength of 1040 nm and a repetition rate of 250 MHz. These results represent an average power improvement of one order of magnitude compared to state-of-the-art systems with similar pulse durations and will thus benefit numerous applications such as the further scaling of tabletop sources of hard x rays (via Thomson scattering of relativistic electrons) and of soft x rays (via high harmonic generation).

Power optimization of XUV frequency combs for spectroscopy applications [Invited].

We address technical impediments to the generation of high-photon flux XUV frequency combs through cavity-enhanced high harmonic generation. These difficulties arise from mirror damage, cavity nonlinearity, the intracavity plasma generated during the HHG process, and imperfect phase-matching. By eliminating or minimizing each of these effects we have developed a system capable of generating > 200 µW and delivering ~20 µW of average power for each spectrally separated harmonic (wavelengths ranging from 50 nm - 120 nm), to actual comb-based spectroscopy experiments.

Broadband phase noise suppression in a Yb-fiber frequency comb.

We report a simple technique to suppress high-frequency phase noise of a Yb-based fiber optical frequency comb using an active intensity noise servo. Out-of-loop measurements of the phase noise using an optical heterodyne beat with a cw laser show suppression of phase noise by ≥7 dB out to Fourier frequencies of 100 kHz with a unity-gain crossing of ∼700 kHz. These results are enabled by the strong correlation between the intensity and phase noise of the laser. Detailed measurements of intensity and phase noise spectra, as well as transfer functions, reveal that the dominant phase and intensity noise contribution above ∼100 kHz is due to amplified spontaneous emission or other quantum noise sources.

Extreme nonlinear optics in a femtosecond enhancement cavity.

Intrinsic to the process of high-order harmonic generation is the creation of plasma and the resulting spatiotemporal distortions of the driving laser pulse. Inside a high-finesse cavity where the driver pulse and gas medium are reused, this can lead to optical bistability of the cavity-plasma system, accumulated self-phase modulation of the intracavity pulse, and coupling to higher-order cavity modes. We present an experimental and theoretical study of these effects and discuss their implications for power scaling of intracavity high-order harmonic generation and extreme ultraviolet frequency combs.

Cryogenic atomic hydrogen beam apparatus with velocity characterization.

Precision spectroscopy of hydrogen often relies on effusive thermal atomic beams, and the uncertainty in the velocity distribution of these beams can introduce systematic errors and complicate lineshape models. Here, we present an apparatus capable of high signal-to-noise studies of these velocity distributions at cryogenic temperatures for both ground state (1S) and metastable (2S) hydrogen using a simple time-of-flight technique. We also investigate how the cryogenic nozzle geometry affects these results.

Increased phosphorylation of myosin light chain kinase after an increase in cyclic AMP in intact smooth muscle.

The role of cyclic adenosine monophosphate-mediated phosphorylation of myosin light chain kinase in relaxing smooth muscle was examined. The kinase was immunoprecipitated from tissue extracts and the phosphate content was determined. The addition of forskolin to resting or methacholine-contracted muscles resulted in an increase in myosin light chain kinase phosphorylation of myosin light chain kinase is one of the reactions in the process by which cyclic adenosine monophosphate causes relaxation of smooth muscle.

Detection of NAD:arginine ADPribosyltransferases in animal tissues using 125I-labeled 1-(p-hydroxyphenyl) 2-guanidinoethane as ADPribose acceptor.

125I-labeled 1-(p-hydroxyphenyl) 2-guanidinoethane (N-guanyltyramine), previously used to assay for the bacterial toxin choleragen (Mekalanos, J.J., Collier, R.J. and Romig, W.R. (1979) J. Biol. Chem. 254, 5849-5854) was utilized to identify NAD:arginine ADPribosyltransferases in animal tissues. The use of this radiolabelled ADPribose acceptor, rather than radiolabelled NAD, would bypass the problem posed by the almost ubiquitous presence of enzymes that degrade NAD. With a homogeneous ADPribosyltransferase from turkey erythrocytes, NAD and 125I-labeled guanyltyramine as ADPribose acceptor, formation of ADPribosyl 125I-guanyltyramine was linear with time and enzyme concentration. The product was indistinguishable on both thin-layer and high-performance liquid chromatography from that formed by choleragen. Using 125I-guanyltyramine, ADPribosyltransferase activity was also demonstrated in crude turkey erythrocyte cytosolic and membrane fractions. When rat liver was fractionated, apparent activity was detected primarily in the microsomes. The NAD-dependent product of the microsomal reaction was, however, distinguished from the turkey erythrocyte transferase product by thin-layer and DEAE-Sephadex chromatography; this product had a retention time identical to that of free 125I on high-performance liquid chromatography. In addition to NAD, the microsomal deiodinase activity was supported by NADH, NADP and NADPH. Phenyl boronate selectively bound ADPribosyl 125I-guanyltyramine and other metabolites of 125I-guanyltyramine which were formed by microsomes in a NAD-dependent process. These metabolites were distinguished from ADPribosyl 125I-guanyltyramine by high-performance liquid chromatography. These results indicate that in some cases, for example, turkey erythrocyte cytosolic and membrane fractions, 125I-guanyltyramine can be used to quantify ADPribosyltransferases in crude mixtures, whereas in others, for example, rat liver microsomes, high-performance liquid chromatographic analysis must be used to identify products.

Sjögren-Larsson syndrome: delta 5-and delta 6-fatty acid desaturases in skin fibroblasts.

Cultured skin fibroblasts from two patients with Sjögren-Larsson syndrome (SLS) and from a normal control were analyzed for trienoic and tetraenoic fatty acids. In addition, we assayed desaturation of [1-14C]linoleic acid in cells from four patients and four controls. There was no significant effect of the disease on the composition of polyunsaturated fatty acids or on the rate of linoleic acid desaturation in fibroblasts. The results indicate the presence of delta 5- and delta 6-fatty acid desaturases in cells from SLS patients.

Daytime alertness in relation to mood, performance, and nocturnal sleep in chronic insomniacs and noncomplaining sleepers.

Nocturnal sleep was recorded prior to daytime testing that included the Multiple Sleep Latency Test, profile of mood states, card sorting, and Stanford Sleepiness Scale in 138 volunteers with the complaint of chronic insomnia and 89 noncomplaining sleepers ("normals"). In both groups daytime sleep tendency had no significant linear correlation either with any Minnesota Multiphasic Personality Inventory scale or with tension/anxiety and other moods assessed in the morning. In normals, speed of card sorting but not subjective sleepiness tended to correlate with sleep tendency. Given that physiological sleepiness is the most predictable consequence of sleep deprivation in normals, it is particularly interesting that 14% of the insomniac group are chronic insomniacs with no measurable daytime sleep tendency. Despite this lack of sleep tendency during the day, their nocturnal sleep was just as poor as insomniacs with greater daytime sleep tendency. The lack of daytime sleepiness seen in this subgroup may reflect a basic pathophysiological aspect of their insomnia.

Low polarization dependent diffraction grating for wavelength demultimlexing.

A low polarization dependent, high diffraction efficiency grating for wavelength demultiplexer is proposed, manufactured by standard crystallographic etching of Si surface. Light is incident and diffracted inside the wafer, which is covered with reflecting metal. Optimized groove form results in a flat spectral response for TE and TM polarizations.

Studies of self-inactivation of bovine seminal fluid NAD glycohydrolase.

Bovine seminal fluid NAD glycohydrolase (NADase) was observed to be rapidly inactivated during catalytic hydrolysis of the substrate NAD. The first-order rate constant for the self-inactivation process was independent of enzyme concentration. The enzyme self-inactivation was a turnover-related process and the number of moles of NAD hydrolyzed required for inactivation was proportional to the enzyme concentration. A number of dinucleotides serving as substrates for the enzyme also promoted self-inactivation. The self-inactivation was an irreversible process having a different rate-limiting step from NAD hydrolysis and was not related to the reversible binding of products and substrate-competitive inhibitors. Modification of arginine residues of the enzyme resulted in the loss of NAD hydrolase activity with no differential effect on the self-inactivation process.

Practices and attitudes of postsecondary LD service providers in North America.

A nationwide survey of 510 postsecondary LD service providers was conducted to determine (a) the types of interventions utilized by service providers, and (b) their attitudes toward the needs of adults with learning disabilities. The results of this study revealed that an educationally diverse group of professionals have been given the responsibility of developing programs and providing direct instruction to individuals with LD. The proliferation of services found in this survey is analogous to a "menu" of options, with little or no philosophical base. Findings suggest that service providers should be encouraged to utilize interventions that lead to self-determination and independence for adults with LD.