Term Drift Research Articles

Stable diode lasers for hydrogen precision spectroscopy

We report on an external cavity diode laser at 972 nm stabilized to a mid-plane mounted Fabry-Perot (FP) resonator with a finesse of 400000. The 0.5 Hz optical beat note line width between two similar lasers (Allan deviation 2 × 10 −15 ) is limited by thermal noise properties of two independent FP res- onators. The long term drift of the FP resonator and mirror substrates made from Ultra-Low-Expansion glass (ULE) is small and can be well predicted on time intervals up to many hours if the resonator is stabilized at the zero thermal expansion temperature Tc. Using a Peltier element in a vacuum chamber for tem- perature stabilization allows stabilization of the FP cavity to Tc which is usually below the room temperature. Beat note measurements with a femtosecond opti- cal frequency comb referenced to a H-maser during 15 hours have shown a well defined linear drift of the FP resonance frequency of about 60 mHz/s with residual frequency excursions of less than ±20 Hz.

Recent changes in solar outputs and the global mean surface temperature. III. Analysis of contributions to global mean air surface temperature rise

A multivariate fit to the variation in global mean surface air temperature anomaly over the past half century is presented. The fit procedure allows for the effect of response time on the waveform, amplitude and lag of each radiative forcing input, and each is allowed to have its own time constant. It is shown that the contribution of solar variability to the temperature trend since 1987 is small and downward; the best estimate is −1.3% and the 2 σ confidence level sets the uncertainty range of −0.7 to −1.9%. The result is the same if one quantifies the solar variation using galactic cosmic ray fluxes (for which the analysis can be extended back to 1953) or the most accurate total solar irradiance data composite. The rise in the global mean air surface temperatures is predominantly associated with a linear increase that represents the combined effects of changes in anthropogenic well-mixed greenhouse gases and aerosols, although, in recent decades, there is also a considerable contribution by a relative lack of major volcanic eruptions. The best estimate is that the anthropogenic factors contribute 75% of the rise since 1987, with an uncertainty range (set by the 2 σ confidence level using an AR(1) noise model) of 49–160%; thus, the uncertainty is large, but we can state that at least half of the temperature trend comes from the linear term and that this term could explain the entire rise. The results are consistent with the intergovernmental panel on climate change (IPCC) estimates of the changes in radiative forcing (given for 1961–1995) and are here combined with those estimates to find the response times, equilibrium climate sensitivities and pertinent heat capacities (i.e. the depth into the oceans to which a given radiative forcing variation penetrates) of the quasi-periodic (decadal-scale) input forcing variations. As shown by previous studies, the decadal-scale variations do not penetrate as deeply into the oceans as the longer term drifts and have shorter response times. Hence, conclusions about the response to century-scale forcing changes (and hence the associated equilibrium climate sensitivity and the temperature rise commitment) cannot be made from studies of the response to shorter period forcing changes.

The Calibration and Data Products of GALEX

We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases have identical pipeline calibrations that are significantly improved over the GR1 data release. GALEX continues to survey the sky in the Far Ultraviolet (FUV, ~154 nm) and Near Ultraviolet (NUV, ~232 nm) bands, providing simultaneous imaging with a pair of photon counting, microchannel plate, delay line readout detectors. These 1.25 degree field-of-view detectors are well-suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 mAB in the FUV and NUV, respectively. We have detected a long term drift of order 1% FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" RMS in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.

Shocks in dense clouds

Aims. A new multi-fluid approach to the dust dynamics in transverse shocks in dense clouds is presented with the aim of modelling the dust processing in C- and J-type shocks. Methods. We have augmented an existing steady-state shock code to include the effects of an MRN size distribution of grain cores with icy mantles. The dust charge distribution and its evolution is considered in detail and included in the ionization balance. The 2-D grain dynamics are determined, including the effects of grain inertia and charge fluctuations, paying particular attention to the gyration of the charged grains around the magnetic field lines and the feedback of the ionization state on grain dynamics. Results. We find that the critical velocity for C shocks increases with the gas density but that it is only weakly dependent on a high abundance of PAHs and on the photodetachment of electrons by secondary photons induced by cosmic-rays. The detailed dust dynamics in C shocks is shown to comprise two distinct phases: 1) a short gyration phase followed by 2) a long term drift phase. In J shocks only the first gyration phase is present. In C shocks propagating through molecular clouds (n H = 10 4 cm -3 ), large grains (»100 A) remain coupled to the magnetic field during the second phase. However, a high abundance of PAHs can lead to a shortage of electrons in the gas and the decoupling of large grains in the shock tail. Large grains are decoupled from the magnetic field all through the C shock in high density clouds (n H = 10 6 cm -3 ). In all C shocks small grains (≃100 A) remain strongly coupled to the magnetic field, whereas very small grains («100 A) are subject to stochastic dynamics. As long as they are charged very small grains remain strongly coupled to the magnetic field but tend to couple to the neutral gas everytime they become neutral. We have investigated the effects of an electric field along the shock direction in C shocks and find that it does not significantly modify the relative velocities between grains. The derived grain dynamics can be used to study dust processing in C and J shocks in dense clouds through the effects of gas-grain and grain-grain collisions.

Forecasting natural gas prices using cointegration technique

AbstractThis paper uses Augmented Dickey‐Fuller and Phillips‐Perron technique for determining whether individual crude oil prices (West Texas Intermediate, Brent, Japan crude cocktail) and natural gas prices‐ Henry Hub (HH), National Balancing Point (NBP), European and Japanese liquefied natural gas (LNG) prices are stationary or non‐stationary. It then applies Johansen and Juselius cointegration technique for establishing long‐run correlation between respective oil prices and natural gas prices. The paper concludes that all individual series pertaining to oil and natural gas prices are non‐stationary and indeed having long‐run relationship, despite short term drift. Ordinary least square method was used to forecast individual natural gas prices in various markets, assuming of course, that historical relationship continues to hold with respective oil prices throughout the forecasting period. Natural gas prices in each of the markets are expected to be stronger during 2005–25 as compared to respective historical average prices showing the tightness of the market. The mean NBP and HH forecast during 2005–25 are expected to be 92 and 84 per cent stronger than the historical average, whilst LNG prices in Japan continue to exhibit stronger trends during the forecast period as compared to rest of the markets in Europe and North America ‐ showing greater dependency of imports and security of supply considerations.

Thermopile radiometer signal conditioning for surface atmospheric radiation measurements

A highly stable microvolt amplifier for use with atmospheric broadband thermopile radiometers is described. The amplifier has a nominal gain of 500, for bipolar input signals in the range ±10mV from a floating source. The noise level at the input is less than 5μV (at 100kΩ input impedance), permitting instantaneous diffuse solar radiation measurements to 0.5Wm−2 resolution with 12bit analog to digital conversion. The temperature stability of gain is better than 5ppm∕°C (−4to20°C). Averaged over a decade of use, the long term drift of the amplifier gain is less than ∼0.02%∕yr. As well as radiometers measuring solar and terrestrial radiations, the amplifier has also been successfully used with low level signals from thermocouples and ground heat flux plates.

Templated xerogels as platforms for biomolecule-less biomolecule sensors

We report on a new sensor strategy that we have termed protein imprinted xerogels with integrated emission sites (PIXIES). The PIXIES platform is completely self-contained, and it achieves analyte recognition without a biorecognition element (e.g., antibody). The PIXIES relies upon sol–gel-derived xerogels, molecular imprinting, and the selective installation of a luminescent reporter molecule directly within the molecularly imprint site. In operation the templated xerogel selectively recognizes the target analyte, the analyte binds to the template site, and binding causes a change in the physicochemical properties within the template site that are sensed and reported by the luminescent probe molecule. We report the PIXIES analytical figures of merit for and compare these results to a standard ELISA. For human interleukin-1 the PIXIES-based sensor elements exhibited the following analytical figures of merit: (i) ∼2 pg/mL detection limits; (ii) <2 min response times; (iii) >85 selectivity; (iv) <6% R.S.D. long term drift over 16 weeks of ambient storage; (v) >95% reversibility after more than 25 cycles; and (vi) >85% recoveries on spiked samples.

Zeeman effect spectroscopically locked Cs diode laser system for atomic physics

We present a diode laser system for Cs that uses a dichroic atomic vapor laser lock (DAVLL) to obtain stable single frequency operation around 852 nm from a high power quantum well index guided laser diode, &amp;gt;170 mW (JDS Uniphase SDL-5401-G1 and SDL-5421-G1). The laser diode is locked to an external cavity formed by a diffraction grating in the Littrow configuration. Passive stabilization of the laser system resulted in a laser drift of &amp;lt;20 MHz/min over a ∼10 min time scale. Over a 24 h period, the free running laser drift was less than 300 MHz. The diode laser and the external cavity temperature are actively stabilized independently so that temperature may be used to tune the cavity and/or the laser operating wavelength. The DAVLL lock actively stabilizes the external cavity to which the laser is locked by feeding back to piezoelectric transducers upon which the grating is mounted. Rapid fluctuations of the laser system are compensated for by feeding back a high frequency component (&amp;gt;1 kHz) of the error signal generated by the DAVLL signal to the laser current control. The instantaneous linewidth of the system was measured to be 200 kHz. The effective linewidth was 500 kHz root-mean-square while the long term drift was &amp;lt;1.5 MHz/h when the laser system was locked to the DAVLL signal. These results were obtained in a standard laboratory environment without temperature stabilizing the DAVLL spectrometer.

NMR at 23.5 T by a Resistive Magnet in NIMS

Homogeneity and stability of a Bitter-type resistive magnet of National Institute for Material Science (NIMS) have been investigated at fields up to 23.5 T by means of NMR. The field homogeneity is 100 ppm over a 5 mm DSV. The field instability observed at 23.5 T contains, at least, two contributions: 1) 50 Hz fluctuations with an amplitude of 100 ppm, 2) long term drift at the rate of 50 ppm/hr. The magnet power supply may have a prime responsibility for the field fluctuations. We need an improvement of the power supply system to achieve a high resolution solid-state NMR measurement which may require stability of 10 ppm/hr. Possible applications of such a high field NMR may cover a wide range of inorganic materials including catalyzes, which consist of quadrupolar nuclei like Al, B, Ca, Cu, O, Ti, etc. We can expect that the spectra of quadrupolar nuclei would be simplified remarkably by such a high field magnet.

Conservation of properties in a free-surface model

In height coordinate ocean models, natural conservation of tracers (temperature, salinity or any passive tracer) requires that the thickness of the surface cell varies with the free-surface displacement, leading to a non-linear free-surface formulation (NLFS). However, NLFS does not guarantee exact conservation unless special care is taken in the implementation, and in particular the time stepping scheme, as pointed out by Griffies et al. (Monthly Weather Rev. 129 (2001) 1081). This paper presents a general method to implement a NLFS in a conservative way, using an implicit free surface formulation. Details are provided for two tracer time stepping schemes, both second order in time and space: a two time-level scheme, such as Lax–Wendroff scheme, guarantees exact tracer conservation; a three time-level scheme such as the Adams–Bashforth II requires further adaptations to achieve exact local conservation and accurate global conservation preventing long term drift of the model tracer content. No compromise is required between local and global conservation since the method accurately conserves any tracer. In addition to the commonly used backward time stepping, the implicit free surface formulation also offers the option of a Crank–Nickelson time stepping which conserves the energy. The methods are tested in idealized experiments designed to emphasize problems of tracer and energy conservation. The tests show the ability of the NLFS method to conserve tracers, in contrast to the linear free-surface formulation. At test of energy conservation reveals that free-surface backward time-stepping strongly damps the solution. In contrast, Crank–Nickelson time stepping exactly conserves energy in the pure linear case and confirms the NLFS improvement relative to the linear free-surface when momentum advection is included.

Linear Langevin equation with time-dependent drift and multiplicative noise term: exact study

We investigate a linear Langevin equation with mulitplicative noise term and time-dependent drift. This system extends the system analyzed by Lillo and Mantegna [Phys. Rev. E 61 (2000) R4675]. We calculate the average of every moments and the variance of the system. We show that the system, with a particular choice of γ(t), can describe different processes like diffusive, subdiffusive and superdiffusive. Other kinds of processes can also been obtained. We also show that the multiplicative noise term has the effect of splitting the peak of transition probability P(x,t|x′,t′), at a fixed time, into two peaks in x coordinate.

Three years experiment with the same tin oxide sensor arrays for the identification of malodorous sources in the environment

The paper discusses the ability of recognition models to classify malodours in the environment 3 years after their calibration, with the same sources releasing odours. Two methodologies applicable either in the lab or in the field are used. The effect of the long term drift of the sensors is highlighted on the results of principal component analysis (PCA) and of discriminant function analysis (DFA). The paper examines also the applicability of some drift counteraction methods proposed in the literature. Handled with some care, a correction based on the drift direction in the principal components subspace should be applicable to the classification of real odorous sources in the field.

Basic Properties of Stationary First-Order Autoregressive Processes and Random Walks

This paper compares the basic properties of stationary autoregressive processes and random walks with special regard to their implications to unit root testing. In particular, it aims to answer three basic but important questions. Firstly: 'What do a constant term (drift) and a linear time trend mean in stationary first-order autoregressive equations and in random walks?'. Secondly: 'Can bounded series be modelled by random walk processes?'. Thirdly: 'Is there any difference between having a unit root in the level or in the logarithm of an economic time series?'.

Long term laser frequency control for applications in atomicphysics

An improvement to the saturated absorption technique of long termstabilization of lasers is reported. This new method enables stabilization to the centreof saturated absorption features regardless of whether the peaks are symmetricor asymmetric. Another feature of the locking system is its ability to stabilizethe laser at precisely known small detunings from the centre of the absorptionline. The locking system has been applied to both dye laser and titaniumsapphire laser systems. The long term drift for each was measured to be smallerthan the laser jitter bandwidth of about 1 MHz for periods of hours, and no modehops were recorded over several weeks of operation.

Optical fiber coupled light emitting diode based absorbance detector with a reflective flow cell

We present a versatile, optical fiber coupled light emitting diode (LED) light source based flow-through optical absorbance detector. The LED source is readily changeable. Optical fibers are used to carry light from the electronics/display unit to a reflective flow-through cell and back. The cell can thus be located remotely from the electronics unit and the umbilical connection is not susceptible to electrical noise. The noise level of this detector with LEDs of different emission maxima were observed to be in the range of 3–20 μAU under actual use conditions, with a maximum short term drift of 4 μAU/min after the initial warm-up period. When the analyte absorbance is well matched with the source emission characteristics, the detector response is linear with concentration over at least two orders of magnitude. The liquid flow path through the cell is linear with a large exit aperture such that bubbles are not trapped in the optical path. The optical arrangement is such that the incident light crosses the liquid flow orthogonally and is reflected back by a rear mirror to the receiver fiber. This arrangement reduces the refractive index sensitivity by an order of magnitude relative to conventional Z-path flow cells.

Noise, drift, and calibration in optical flux monitoring for MBE

Sources of error in optical flux monitoring (OFM) for MBE are identified and measured. The primary challenge in measuring atomic flux by OFM is to reduce the effects of signal drift during the measurement. To reduce long term drift we have (1) used an additional reference beam to measure the transparency of the optical path and (2) chopped the atomic beam at low frequencies (2 Hz) with the effusion cell shutters and performed phase-sensitive detection on the absorption signal. With the chopped beam technique, we have essentially eliminated long term drift from our measurements. We have also measured the errors introduced by changes in the relationship between absorption and growth rate that occur as the effusion cell is depleted.

Seasonal global water mass budget and mean sea level variations

Analysis of TOPEX/Poseidon satellite altimeter data indicates that the global mean sea level variation has a clear seasonal signal with an amplitude of about 2 to 3 mm, along with a long term drift. This seasonal variation is associated with mass redistribution within the global hydrological cycle plus steric thermal contributions. We investigate seasonal variations of water vapor in the atmosphere and water storage on land using both assimilated atmospheric models and climatological data, and to estimate the corresponding global mean sea level changes. The predicted seasonal global mean sea level changes are then compared with the seasonal variabilities observed by TOPEX/Poseidon altimetry data, after the latter are corrected for the steric effect using a simplified thermal model derived from the NOAA World Ocean Atlas 1994. The good agreement in both amplitude and phase indicates that the T/P altimeter may provide key information for the global water mass budget by placing observational constraints on the mass budget variations predicted by global atmospheric and hydrological models.

Measurement of package-induced stress and thermal zero shift in transfer molded silicon piezoresistive pressure sensors

The package-induced stress in a transfer molded piezoresistive pressure sensor has been measured by monitoring the individual piezoresistors in the measuring bridge. Based on measurements on six different samples, the induced difference between longitudinal and transversal stress in the sensor diaphragm has been measured as MPa at room temperature. The induced stress creates thermal zero shift and long term drift of the sensor signal. By covering the sensor chip with a low modulus glob top before molding, the stress and thermal zero shift may be considerably reduced.

Improved spectral properties of diode lasers

We report on several complementary methods for improving the stability of a free running diode laser. We were primarily concerned with: (a) the construction of a stable power supply to reduce the main frequency and rf noise, and (b) long term drift of the laser frequency, for which we have developed a lock-in amplifier to be combined with a simple proportional-integrating circuit. While we have kept everything simple and inexpensive to construct, the stability is comparable with that of far more expensive, commercially available systems. Development and testing of our stabilization scheme was done on the distributed Bragg reflector diode lasers for use in a range of atomic collision and atom optics experiments, but it is applicable to all diode lasers, and can be used in a large range of atomic physics experiments.

Improved MBE-grown GaAs using a novel, high-capacity Ga effusion cell

A novel PBN crucible has allowed the design of a new high-capacity Ga source incorporating a unique heat shielding cap. The extra heat shielding at the front of the source allows hot lip operation with considerably lower power than for previous designs, thus completely eliminating Ga droplets without an inordinately high heat load. Compared with another high-capacity, low-flux transient cell (an EPI 125DF), the new source operated with significantly less power and produced lower defect densities. Specifically, defect densities of 22–27 defects/cm 2 for ∼ 1 μ m thick GaAs layers have been obtained with the new source, compared with 88–92 defects/cm 2 from the 125DF. In addition, an 18.7 μm thick GaAs layer grown with the new cell had only 277 defects/cm 2 . The new source also produced GaAs with low background doping (< 2 × 10 13 /cm 3 ), low total trap density (< 2.0 × 10 12 /cm 3 ), and good uniformity over a 3 in diameter wafer (center-to-edge decrease in GaAs thickness of ∼ 2.2%). It also exhibited low flux transients (< 2%) and low-long term drift.