Wavelength Uncertainty Research Articles

Design and development of a portable tuneable radiation source from UV to IR for in situ calibration of radiometers measuring atmospheric aerosol properties.

Abstract We present the results of development of a portable tuneable radiation source (TuPS II) covering the broad spectral range from 300 nm up to 1700 nm. It was developed to provide SI-traceable in-situ calibration of bandpass functions of radiometers measuring both sky radiance and sky irradiance in the surface-based remote sensing networks for column integrated atmospheric aerosol properties. The TuPS II provides quasi-monochromatic radiation with spectral bandwidth smaller than 0.2 nm and central wavelength uncertainty lower than 0.05 nm for a specified number of aerosol wavelength bands ranging from to 300 nm to 1700 nm. In its design it represents an evolution with extended spectral capabilities of the TuPS (Smid, M. et al., 2021) that was successfully used to characterize Dobson spectrophotometers in the UV spectral range.

Diffuse interstellar bands in Gaia DR3 RVS spectra

Diffuse interstellar bands (DIBs) are weak and broad interstellar absorption features in astronomical spectra that originate from unknown molecules. To measure DIBs in spectra of late-type stars more accurately and more efficiently, we developed a random forest model to isolate the DIB features from the stellar components. We applied this method to 780 thousand spectra collected by the Gaia Radial Velocity Spectrometer (RVS) that were published in the third data release (DR3). After subtracting the stellar components, we modeled the DIB at 8621 Å (λ8621) with a Gaussian function and the DIB around 8648 Å (λ8648) with a Lorentzian function. After quality control, we selected 7619 reliable measurements for DIB λ8621. The equivalent width (EW) of DIB λ8621 presented a moderate linear correlation with dust reddening, which was consistent with our previous measurements in Gaia DR3 and the newly focused product release. The rest-frame wavelength of DIB λ8621 was updated as λ0 = 8623.141 ± 0.030 Å in vacuum, corresponding to 8620.766 Å in air, which was determined by 77 DIB measurements toward the Galactic anticenter. The mean uncertainty of the fit central wave-length of these 77 measurements is 0.256 Å. With the peak-finding method and a coarse analysis, DIB λ8621 was found to correlate better with the neutral hydrogen than with the molecular hydrogen (represented by 12CO J = (1−0) emission). We also obtained 179 reliable measurements of DIB λ8648 in the RVS spectra of individual stars for the first time, further confirming this very broad DIB feature. Its EW and central wavelength presented a linear relation with those of DIB λ8621. A rough estimation of λ0 for DIB λ8648 was 8646.31 Å in vacuum, corresponding to 8643.93 Å in air, assuming that the carriers of λ8621 and λ8648 are comoving. Finally, we confirmed the impact of stellar residuals on the DIB measurements in Gaia DR3, which led to a distortion of the DIB profile and a shift of the center (≲0.5 Å), but the EW was consistent with our new measurements. With our measurements and analyses, we propose that the approach based on machine learning can be widely applied to measure DIBs in numerous spectra from spectroscopic surveys.

High-Resolution Optical Fiber Salinity Sensor With Self-Referenced Parallel Fabry–Perot Fiber Microcavity

A high-resolution salinity sensor based on a self-referenced parallel Fabry–Perot fiber microcavity is proposed. The sensor is fabricated by inserting a short piece of exposed-core microstructure fiber (ECF) between multimode fiber (MMF) and single-mode fiber (SMF). Due to the unique exposed-core fiber structure and the specially designed SMF-MMF-ECF-SMF structure, two prominent parallel Fabry–Perot interferometers (FPIs) are constructed. Besides, a self-referenced differential phase-demodulation technology is proposed to eliminate the wavelength uncertainty of the spectra measurement devices. Enhanced demodulation system stability and accuracy are achieved by analyzing the phase shift responses of the reflection spectrum. A high salinity-phase sensitivity of 17.36 deg/ ‰ in the range from 0 ‰ to 35 ‰ is obtained, and the corresponding refractive index (RI) sensitivity is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${1.04} \times {10} ^{{5}}$ </tex-math></inline-formula> deg/RIU. The salinity resolution and RI resolution are as low as 0.0058 ‰ and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${9.6} \times {10} ^{-{7}}$ </tex-math></inline-formula> RIU, respectively, which gives a great potential for harsh environmental monitoring and label-free biochemical analysis.

High resolution polymer/air double-cavity Fabry–Perot fiber temperature sensor based on exposed core microstructured fiber

This study proposes a high-resolution optical fiber Fabry–Perot (FP) temperature sensor and it is based on an exposed-core microstructured optical fiber (ECF) coated by a ultraviolet curing polymer adhesive. Then, a small piece of multimode fiber (MMF) with large core diameter is spliced in front of ECF to expand the input light beam, and the parallel polymer/air double-cavity FP in such single-mode fiber (SMF)-MMF-ECF-SMF structure is constructed. Additionally, by employing a mobilized demodulation module, the interference signal is analyzed by phase demodulation method. The experimental results show that the temperature sensitivity of the sensor is 30.8 ℃−1 and the resolution is up to 1.6 × 10−4 ℃ in a range of 20 ℃–50 ℃, which can achieve high resolution temperature measurement. Furthermore, to alleviate the wavelength uncertainty of the demodulation device, a double-cavity self-reference differential phase modulation method is explored in the proposed parallel double-cavity FP. It shows that the stability could be improved to five times when system temperature is unstable, which offers an alternative method to further improve the temperature sensing resolution.

Resolving the scalability challenge of wavelength locking for multiple micro-rings via pipelined time-division-multiplexing control.

Micro-ring resonator (MRR) is a key photonic device that has a wide range of applications but suffers from wavelength uncertainties. For almost all practical applications, a wavelength controller is required for each MRR. The wavelength controller is usually much larger than the MRR. With more complicated control algorithms, the controller size becomes even larger. Equipping each MRR with a wavelength controller will not be scalable. We propose a pipelined time-division-multiplexing (PTDM) control scheme that achieves high scalability while maintaining good loop bandwidth by exploiting the speed mismatch between the heater and the controller. To verify this proposed scheme, a hybrid integrated controller supporting four MRRs is designed. Measurement results show that it achieves a sine tracking speed of about 15 nm/s while achieving a locking accuracy of 7 pm and a tuning range of 9 nm.

Approach based on χ2 to phasing segmented-mirror telescopes using multiple wavelengths: data reduction, wavelength selection, capture range.

The design of extremely large segmented-mirror telescopes (ELTs) calls for their segments to be phased, and a number of different interferometric techniques have been proposed in the literature for this purpose. To achieve the required large capture ranges for ELTs (typically a few microns), most of these techniques use measurements at multiple wavelengths, similar to what is done in standard distance-measuring laser interferometry. In this paper, we present a χ2-based framework not tied to a specific phasing technique to obtain intersegment edge heights from multiwavelength data. We show how this scheme can be used to select an optimal set of wavelengths that maximizes the capture range while achieving the very low error rates required for ELT phasing. We take into account the wavelength uncertainty associated with the finite-width interference filters used in on-sky phasing. Finally, we present data from the Keck 2 telescope that illustrate the method for three-wavelength phasing.

A Fuzzy Approach to LPFG-Based Optical Sensor Processing and Interrogation

In this work, we report on a new interrogation method for long-period fiber-grating (LPFG) sensors. This type of sensor has been gaining increasing attention due its several interesting practical and metrological advantages. However, the LPFG optical signal is complex and could be difficult to process. We present a novel approach to the LPFG resonant wavelength detection and tracking using a multifilter LPFG interrogator. Multifilter LPFG interrogators present a great solution for LPFG sensor interrogation because there is no need for a priori knowledge concerning the LPFG sensor spectrum. We present a novel source-compensating preprocessing and a fuzzy inference system (FIS) to track the LPFG resonant wavelength. Our proposal was compared to two baseline models: a simple linear regression and an artificial neural network. Our results showed the new approach could reduce resonant wavelength uncertainty by almost three times comparing to results found in the literature.

Spectral calibration for electron beam ion trap and precision measurement of M1 transition wavelength in Ar&lt;sup&gt;13+&lt;/sup&gt;

The precise measurement of the transition wavelength of the fine structure of highly charged ions can not only test basic physical theories including the quantum electrodynamics effect and the electronic correlation effect but also provide key atomic data for astrophysics and fusion plasma physics. Furthermore, highly charged ions are considered as a potential candidate for optical clocks with extremely ultra-high precision. In this work, a new spectral calibration system is built in a high-temperature superconducting electron beam ion trap (SH-HtscEBIT) in the Institute of Modern Physics, Fudan University, and the uncertainty of its spectrum wavelength measurement is evaluated by combining internal and external calibrations. The minimum wavelength uncertainty caused by the new spectral calibration system in the visible light band reaches 0.002 nm. On this basis, the precise measurement of 2s&lt;sup&gt;2&lt;/sup&gt;2p &lt;sup&gt;2&lt;/sup&gt;P&lt;sub&gt;1/2&lt;/sub&gt;－&lt;sup&gt;2&lt;/sup&gt;P&lt;sub&gt;3/2&lt;/sub&gt; M1 transition wavelength for boron-like Ar&lt;sup&gt;13+&lt;/sup&gt; is performed at the SH-HtscEBIT by utilizing the new calibration system. The experimentally measured transition wavelength is (441.2567 ± 0.0026) nm. It is currently the experimental result with the highest measurement accuracy of spectroscopy of highly charged ions at the SH-HtscEBIT, which lays the foundation for the precise measurement of the hyperfine splitting and isotope shift of highly charged ions in the future experiments.

Experimental methods for laboratory measurements of helium spectral line broadening in white dwarf photospheres

White Dwarf (WD) stars are the most common stellar remnant in the universe. WDs usually have a hydrogen or helium atmosphere, and helium WD (called DB) spectra can be used to solve outstanding problems in stellar and galactic evolution. DB origins, which are still a mystery, must be known to solve these problems. DB masses are crucial for discriminating between different proposed DB evolutionary hypotheses. Current DB mass determination methods deliver conflicting results. The spectroscopic mass determination method relies on line broadening models that have not been validated at DB atmosphere conditions. We performed helium benchmark experiments using the White Dwarf Photosphere Experiment (WDPE) platform at Sandia National Laboratories' Z-machine that aims to study He line broadening at DB conditions. Using hydrogen/helium mixture plasmas allows investigating the importance of He Stark and van der Waals broadening simultaneously. Accurate experimental data reduction methods are essential to test these line-broadening theories. In this paper, we present data calibration methods for these benchmark He line shape experiments. We give a detailed account of data processing, spectral power calibrations, and instrument broadening measurements. Uncertainties for each data calibration step are also derived. We demonstrate that our experiments meet all benchmark experiment accuracy requirements: WDPE wavelength uncertainties are &amp;lt;1 Å, spectral powers can be determined to within 15%, densities are accurate at the 20% level, and instrumental broadening can be measured with 20% accuracy. Fulfilling these stringent requirements enables WDPE experimental data to provide physically meaningful conclusions about line broadening at DB conditions.

Intelligent Robot Obstacle Avoidance Perception Method Using a High-Precision Laser Distance Measuring System

Laser distance measuring is increasingly used in large-scale and real-time scanning measurement, including three-dimensional map construction, size measurement of large-scale buildings, and real-time surface information acquisition. Therefore, there are high-precision and high-efficiency requirements for the laser distance measuring system. Based on the phase laser distance measuring, a laser parallel distance measuring system is proposed as per the frequency division multiplexer principle. The system uses multi-channel modulation signals with different frequencies to drive multiple lasers to emit light intensity in parallel, and then the light wave is measured according to the change of the modulation signal. The single-channel modulation signal adopts the multi-scale composite wave to eliminate the uncertainty of the whole wavelength by using the combination of two scales and distance measurement. The multiple echo signals after diffuse reflection of the measured target are mixed and received by the same photodetector and go through signal amplification, clutter filtering, etc., and then are sent to the mixing unit together with the reference signal, followed by down-frequency processing, with the effective signal obtained through the low-frequency filter. In the experiment, the laser distance measuring system is used for obstacle avoidance control of mobile robots, and fuzzy control is used to design the corresponding obstacle avoidance controller. The robot lateral deviation and heading angle are used as the input of the fuzzy controller, and the track variation is the output. The test verifies that the obstacle avoidance control is effective, that is, the laser distance measuring system designed in this research can be used for the obstacle avoidance control of mobile robots.

Reference wavelengths of Si ii, C ii, Fe i, and Ni ii for quasar absorption spectroscopy.

Wavelengths of absorption lines in the spectra of galaxies along the line of sight to distant quasars can be used to probe the variablility of the fine structure constant, α, at high redshifts, provided that the laboratory wavelengths are known to better than 6 parts in 108, corresponding to a radial velocity of ≈20 ms-1. For several lines of Si ii, C ii, Fe i, and Ni ii, previously published wavelengths are inadequate for this purpose. Improved wavelengths for these lines were derived by re-analysing archival Fourier transform (FT) spectra of iron hollow cathode lamps (HCL), a silicon carbide Penning discharge lamp, and with new spectra of nickel HCLs. By reoptimizing the energy levels of Fe i, the absolute uncertainty of 13 resonance lines has been reduced by over a factor of 2. A similar analysis for Si ii gives improved values for 45 lines with wavelength uncertainties over an order of magnitude scer than previous measurements. Improved wavelengths for eight lines of Ni ii were measured and Ritz wavelengths from optimized energy levels determined for an additional three lines at shorter wavelengths. Three lines of C ii near 135 nm were observed using FT spectroscopy and the wavelengths confirm previous measurements.

Calibrating spectrometers for measurements of the spectral irradiance caused by solar radiation

Measuring the spectral irradiance of solar radiation is required in many fields of science and technology. In this work, we present an in-depth discussion of the measuring procedure and required corrections for such measurements. We also describe our measurement uncertainty analysis, which is based on a Monte-Carlo procedure in accordance with the Guide to the expression of uncertainty in measurement (JCGM, Paris, 2008). For this purpose, fifteen uncertainty sources are identified, analyzed and described analytically. As a specific application example, we describe the instrumentation and procedure for determining the spectral irradiance of a solar simulator at the ISO/IEC 17 025 accredited solar cell calibration laboratory ISFH CalTeC and the corresponding measurement uncertainty analysis. Moreover, we provide a Python implementation for this calculation along with the paper. We show that for state-of-the-art instrumentation, significant uncertainty contributions arise from the reference lamp (primary calibration standard), stray light and signal-to-noise ratio. If sharp spectral features are present (which is common, e.g. for Xenon lamps), spectral bandwidth and wavelength uncertainty also contribute significantly to the overall uncertainty.

Analysis of deviations in wavelength measurements with a diffraction grating monochromator with respect to a certified reference spectral lamp

This paper shows the analysis of the deviations in the wavelength measurements made with a DK-480 monochromator of diffraction grating respect to the values reported for a certified calibration spectral lamp. The radiation emitted by the calibration lamp was dispersed using diffraction gratings of 300, 1200 and 2400 grooves/mm provided in the monochromator. The emitted spectrum contains neutral Mercury and neutral Argon lines, which was registered with a photomultiplier tube coupled to an amplifier circuit. In this study it was applied the method of least squares to the results of the spectral measurements, and it was obtained for each diffraction grating, the linear model or linear fitting of the comparison with respect to the values of the wavelengths of the reference lamp. With the general model of the propagation of the uncertainty, the uncertainty function was obtained for each diffraction grating, in order to estimate the uncertainty of wavelengths interpolated in each spectral range, which are useful for measuring wavelengths of any other spectral source.

Constraining the magnetic field on white dwarf surfaces; Zeeman effects and fine structure constant variation

ABSTRACT White dwarf (WD) atmospheres are subjected to gravitational potentials around 105 times larger than occur on Earth. They provide a unique environment in which to search for any possible variation in fundamental physics in the presence of strong gravitational fields. However, a sufficiently strong magnetic field will alter absorption line profiles and introduce additional uncertainties in measurements of the fine structure constant. Estimating the magnetic field strength is thus essential in this context. Here, we model the absorption profiles of a large number of atomic transitions in the WD photosphere, including first-order Zeeman effects in the line profiles, varying the magnetic field as a free parameter. We apply the method to a high signal-to-noise, high-resolution, far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectrum of the WD G191−B2B. The method yields a sensitive upper limit on its magnetic field of B &amp;lt; 2300 G at the 3σ level. Using this upper limit, we find that the potential impact of quadratic Zeeman shifts on measurements of the fine structure constant in G191−B2B is 4 orders of magnitude below laboratory wavelength uncertainties.

Relativistic Modeling of Ultra-Short Electron Pulse Propagation

The ultrafast electron microscopy, electron diffraction, electron crystallography, and nanocrystallography methods opened the possibility of studying the coherent structural dynamics of matter. The time resolution of the ultrafast electron microscopy and electron diffraction methods determined by the duration of electron pulses is the key parameter of experimental setups. This paper treats electron pulse dynamics in the field free drift region specifically for applications in atomic imaging. The electron beam is modeled as a system of particles (N) with N = 1000 and N = 10 000 electrons. The beam propagates for a certain period of time (1–4 ns); during its propagation, electron distribution parameters (over coordinates and velocities) are calculated to characterize the temporal profile and uncertainty in the electron wavelength at the sample. The results of applying relativistic dynamic equations show that nonrelativistic results are satisfactorily applicable (with 15 per cent or better accuracy) for modeling short electron pulse elongation and broadening at 30 keV and lower energies. However, the results of such modeling may be significantly in error for intermediate energies (300 keV), and for the fast relativistic beams (3 MeV) they become completely wrong. The relative reduction in Coulomb repulsion effects at higher energies is known, however; we give a comprehensive treatment that allows a quantitative picture. Using high-energy electron pulses results in almost complete elimination of the repulsive Coulomb effect. Dispersion of electron velocities becomes much lower at higher energies. For 3 MeV electrons, electron pulse duration as well as its radius does not noticeably change even after traveling for 4 ns (1.2 m). Even at 300 keV, the pulse duration increase is negligible until 1 ns (0.2 m). A simple mean-field model suggested in [13] has been extended to arbitrarily fast relativistic electron pulses with good correspondence to direct dynamic modeling.

Experimental Demonstration of Low-Uncertainty Calibration Methods for Bragg Grating Interrogators.

In this paper we propose and demonstrate two alternative methods for the high-precision calibration of fiber Bragg grating (FBG) interrogators. The first method is based on the direct comparison between the wavelength measurements of the interrogator under test and a calibrated wavemeter, while analyzing a simulated symmetric Bragg grating constructed by a tunable filter and a fiber mirror. This first method is applicable to most commercial systems but presents an uncertainty limited by the spectral width and the wavelength stability of the tunable filter. The second method consists in measuring multiple reference absorption lines of calibrated absorption gas cells. This second method presents lower uncertainties, limited only by the optical resolution of the interrogator and the wavelength uncertainty of the reference cell absorption lines. However, it imposes more restrictive requirements on the interrogator software. Both methods were experimentally demonstrated by calibrating multiple commercial systems, reaching uncertainties down to 0.63 pm at a central wavelength of 1550 nm.

High resolution EUV spectroscopy of xenon ions with a compact electron beam ion trap

We performed high resolution extreme ultraviolet (EUV) spectroscopy measurements of highly charged xenon ions with a compact electron beam ion trap. The spectra were recorded with a flat-field grazing incidence spectrometer while varying the electron beam energy between 200 and 890eV. We measured the wavelengths for several lines of Rh-like Xe9+ - Cd-like Xe6+ and Cu-like Xe25+- Se-like Xe20+ in the range of 150–200Å with an uncertainty of 0.05Å. Previously, most of these lines have been reported from EBITs with a wavelength uncertainty of 0.2Å. Additionally, based on the electron beam energy dependence of the observed spectra we tentatively identified three new lines, which were reported as unidentified lines in the previous studies.

Traceable trans-scale heterodyne interferometer with subnanometer resolution

In order to realize the traceable trans-scale displacement measurements with high resolutions in the fields of fundamental scientific research and ultra-precision machining, we demonstrate a trans-scale heterodyne interferometer with a sub-nanometer resolution, through assembling a compact iodine-stabilized laser at 532 nm. Using modulation transfer spectroscopy, the green laser is traced back to the transition line R(56)32-O(a10), which is one of the recommended spectral lines for meter redefinition. The Allan standard deviation of the laser frequency is 1.310-12 within an average time of 1 s. Compared with most He-Ne lasers, the green laser has a short wavelength and good stability, which leads to a higher resolution. We use two acoustic-optic modulators driven by a two-channel acoustic-optic driver sharing the same crystal oscillator to separate input beams spatially. The frequency of one beam is shifted by 80 MHz and the other is shifted by 82 MHz, which results in a beat frequency of 2 MHz. As a result, the nonlinearity caused by source mixing substantially is reduced. The phase noises of the fibers and two acoustic-optic modulators are well compensated. In order to minimize the difficulty in adjusting the optical path and the error of the measurement, we integrate the interferometry components and design a monolithic prism. The optical resolution of the interferometer reaches to /4. The experiment is carried out in a vacuum environment to reduce the influence of the refractive index of air. High-precision phase measurement technology is used to improve the accuracy of the interferometer. The errors of the interferometer can be classified as random and systematic errors. Random errors include the error from the frequency instability of the laser and the error due to environmental effects. Systematic errors include the phase measurement error and the nonlinearity error. To verify the performance of the interferometer, these errors must be evaluated. In a span of 100 mm, the measurement uncertainties caused by laser wavelength uncertainty, the air refractive index uncertainty, the phase measurement uncertainty and the nonlinearity error are 3 pm, 300 pm, 6.3 pm and 118 pm, respectively. Finally, the performance evaluation shows that the combined uncertainty of the interferometer reaches 322 pm in a span of 100 mm, which is mainly due to the refractive index of air. The heterodyne interferometer meets the requirements for traceable trans-scale measurement with a sub-nanometer resolution, which can be widely used in instrument calibration, length standard making, and geometric measurement.

The Calibration Home Base for Imaging Spectrometers

The Calibration Home Base (CHB) is an optical laboratory designed for the calibration of imaging spectrometers for the VNIR/SWIR wavelength range. Radiometric, spectral and geometric calibration as well as the characterization of sensor signal dependency on polarization are realized in a precise and highly automated fashion. This allows to carry out a wide range of time consuming measurements in an ecient way. The implementation of ISO 9001 standards in all procedures ensures a traceable quality of results. Spectral measurements in the wavelength range 380–1000 nm are performed to a wavelength uncertainty of +- 0.1 nm, while an uncertainty of +-0.2 nm is reached in the wavelength range 1000 – 2500 nm. Geometric measurements are performed at increments of 1.7 µrad across track and 7.6 µrad along track. Radiometric measurements reach an absolute uncertainty of +-3% (k=1). Sensor artifacts, such as caused by stray light will be characterizable and correctable in the near future. For now, the CHB is suitable for the characterization of pushbroom sensors, spectrometers and cameras. However, it is planned to extend the CHBs capabilities in the near future such that snapshot hyperspectral imagers can be characterized as well. The calibration services of the CHB are open to third party customers from research institutes as well as industry.

A Solar Irradiance Climate Data Record

Abstract We present a new climate data record for total solar irradiance and solar spectral irradiance between 1610 and the present day with associated wavelength and time-dependent uncertainties and quarterly updates. The data record, which is part of the National Oceanic and Atmospheric Administration’s (NOAA) Climate Data Record (CDR) program, provides a robust, sustainable, and scientifically defensible record of solar irradiance that is of sufficient length, consistency, and continuity for use in studies of climate variability and climate change on multiple time scales and for user groups spanning climate modeling, remote sensing, and natural resource and renewable energy industries. The data record, jointly developed by the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP) and the Naval Research Laboratory (NRL), is constructed from solar irradiance models that determine the changes with respect to quiet sun conditions when facular brightening and sunspot darkening features are present on the solar disk where the magnitude of the changes in irradiance are determined from the linear regression of a proxy magnesium (Mg) II index and sunspot area indices against the approximately decade-long solar irradiance measurements of the Solar Radiation and Climate Experiment (SORCE). To promote long-term data usage and sharing for a broad range of users, the source code, the dataset itself, and supporting documentation are archived at NOAA’s National Centers for Environmental Information (NCEI). In the future, the dataset will also be available through the LASP Interactive Solar Irradiance Data Center (LISIRD) for user-specified time periods and spectral ranges of interest.