Wavelength Offsets Research Articles

Assessment and correction of radiometric calibration for the medium resolution spectral imager on FY-3C

As a keystone instrument on the Fengyun-3C (FY-3C) satellite, the medium resolution spectral imager (MERSI) provides global coverage of top-of-atmosphere radiances for a broad range of scientific studies of the earth system. Based on the onboard measurements of the blackbody (BB) and deep space view (SV), the modified radiometric calibration equation was first given for the MERSI thermal emission band. The accuracy of the radiometric calibration was evaluated by comparing the average irradiance corresponding to the spectral response function and the irradiance corresponding to the wavelength, whose relative error was <0.40 % . Moreover, the BB-derived brightness temperature TR calculated by the BB irradiance shows a systematic error relative to the BB equivalent temperature TE obtained by the voltage code values of seven platinum resistors, and the value of TR was lower 0.25 K than that of TE. To further verify the validity of the calibration coefficient, the influences of the BB temperature, wavelength, and emissivity offsets on the derived irradiance and temperature of the earth were analyzed in detail. For the BB temperature changing by 0.10 K, the absolute values of the earth irradiance together with its equivalent temperature were 0.01 Wm − 2 sr − 1 μm − 1 and 0.10 K, respectively, whose relative errors were about 0.16% and 0.40%. When the BB wavelength and emissivity changing by 0.1 μm and 0.005, the variations of the earth irradiance were 0.02 and 0.04 Wm − 2 sr − 1 μm − 1, respectively, whereas the relative errors of the equivalent temperature were 0.07% and 0.11%. These works provided an important theoretical guidance for the satellite data assimilations along with the quantitative remote sensing applications.

Initial In-Flight Spectral Calibration of the Near-Infrared Spectra Acquired by the MarSCoDe Onboard the Zhurong Rover

The Zhurong rover successfully landed in southern Utopia Planitia as part of the Tianwen-1 mission on 15 May 2021. One of the objectives of the Mars Surface Composition Detector (MarSCoDe) onboard the rover is to investigate mineral compositions on the Martian surface by measuring the near-infrared reflectance spectra. Before conducting spectral interpretation, in-flight data calibration is a crucial step due to the significant differences between the laboratory and the Martian environment. The work of the MarSCoDe spectrometer is based on an acousto-optic tunable filter (AOTF). The temperature variation of the AOTF could induce wavelength offset, making mineral identification uncertain. We first analyzed the viewing geometry of the spectral measurements of the calibration targets on the Mars rover according to its attitude to identify the anomalous data. The wavelength offsets were then determined by fitting the absorption positions of CO2 at ~1400 and ~2000 nm, representing the primary composition of the Martian atmosphere. The results showed 2–8 nm wavelength offsets, which correlated well with AOTF temperatures. The artifacts were removed in the wavelength-corrected reflectance spectra, which is critical to identify the material types on Mars, especially water-related minerals.

Effect of light quality and ionising radiation on morphological and nutraceutical traits of sprouts for astronauts’ diet

Sprouts are nutritious food, easy to produce even in extra-terrestrial platforms, where the exposure to ionising radiation can alter their morpho-anatomical traits and phytochemical content. The aim of this study was to evaluate whether sprout production under specific light wavelengths can mitigate the negative effects of radiation and/or stimulate the induction of hormesis. Germinated seeds, with actively proliferating cells, of mung bean were irradiated with increasing X-ray doses (0–20 Gy) and then incubated in controlled conditions under four different light regimes: dark (D), white light (W), red light (R), red-blue light (RB). Morpho-anatomical development of the sprouts was investigated through light-microscopy and their content of flavonoids and isoflavones was quantified by HPLC. Two significant conclusions emerged: 1) RB wavelength induces hormesis by stimulating the production of antioxidant compounds; 2) R wavelength offsets the harmful effects of radiation on morpho-anatomical traits, even at the highest X-ray dose.

Flow over seal whiskers: Importance of geometric features for force and frequency response.

The complex undulated geometry of seal whiskers has been shown to substantially modify the turbulent structures directly downstream, resulting in a reduction of hydrodynamic forces as well as modified vortex-induced-vibration response when compared with smooth whiskers. Although the unique hydrodynamic response has been well documented, an understanding of the fluid flow effects from each geometric feature remains incomplete. In this computational investigation, nondimensional geometric parameters of the seal whisker morphology are defined in terms of their hydrodynamic relevance, such that wavelength, aspect ratio, undulation amplitudes, symmetry and undulation off-set can be varied independently of one another. A two-factor fractional factorial design of experiments procedure is used to create 16 unique geometries, each of which dramatically amplifies or attenuates the geometric parameters compared with the baseline model. The flow over each unique topography is computed with a large-eddy simulation at a Reynolds number of 500 with respect to the mean whisker thickness and the effects on force and frequency are recorded. The results determine the specific fluid flow impact of each geometric feature which will inform both biologists and engineers who seek to understand the impact of whisker morphology or lay out a framework for biomimetic design of undulated structures.

Atmospheric absorption ratio algorithm for airborne short-wave infrared hyperspectral imagery spectral calibration based on carbon dioxide and water vapor

In view of the fact that the hyperspectral imager can simultaneously obtain the target’s geometric and spectral characteristics, it has been widely applied in many research fields. However, due to the changes in the laboratory and the on-board environment, center wavelengths of the on-board spectral response function (SRF) need to be recalibrated to ensure high-precision measurement of the target’s spectral characteristics retrieved by hyperspectral imager. In this paper, an on-board spectral calibration method for short-wave hyperspectral imager based on constructing differential function between the simulated and the actual absorption ratios is introduced in detail, and the center wavelength offsets of the on-board SRF during the operation process can be monitored through the statistics of the best spectral smoothing position of each frame of the flight image. Moreover, the on-board offsets of SRF in different spectral channels is studied in this paper. Experiments show that the deviation of SRF of on-board hyperspectral imager is not linear, and the three-sigma confidence interval is ±0.38 nm. The method is beneficial to improve the practical application ability of short-wave hyperspectral imagers.

Error analysis of on-orbit thermal emission band radiometric calibration for FY3C/MERSI

The Chinese medium resolution spectral imager (MERSI) carried on the Fengyun-3C (FY-3C) satellite can realize the high-precision global measurement of the cloud, aerosol, land surface, sea surface, and low-level water vapor. The MERSI is described conceptually in this paper, and the radiometric calibrations of its thermal emission bands by the on-board blackbody (BB) were given by two different algorithms. In order to evaluate the stability of the instrument’s performance, the measured irradiance after the dark current correction was compared with the theoretical value calculated from the BB temperature by Planck function. The results showed that the algorithm of linear fitting to the BB measurements averaged over sampling points was an optimal choice for the radiometric calibration, and that the calibration coefficients determined by this method could reflect the pixel difference while maintaining a relative stability between each frame. Using the perturbation method, the influences of the BB temperature, wavelength, and emissivity offsets on the calibration coefficients were analyzed and discussed through theoretical deduction and computer simulation for improving the calibration accuracy of the instrument.

On-board spectral calibration algorithm for an airborne hyperspectral imager and elimination of the effect of the atmospheric underlying surface.

As an image-spectrum merging technology, the hyperspectral sensor has become an important part in remote sensing. The spectral calibration results of the hyperspectral sensor measured in the laboratory should be refined when applied to the on-board imaging spectrometer due to variations between the laboratory and actual flight environments. The larger the spectrum offset of the spectral response function (SRF), the worse the retrieval accuracy of the reflective characteristics of the targets. Therefore, an on-board spectral calibration algorithm based on standard diffuse boards with a gradient of reflectivity is proposed in this paper. By constructing the differential function of radiation transfer coefficients, the center wavelength offsets of SRF can be solved. In addition, a back-propagation neural network has been established to eliminate the effect of the atmospheric underlying surface and improve theaccuracy of on-board spectral calibration. The three-sigma confidence interval of the on-board spectral precision is $ \pm 0.23\,\,{\rm nm}$±0.23nm (uncertainty $ \lt {0.05}$<0.05 spectral pixels). The algorithm can be applied to a general hyperspectral sensor.

Authentication of apple juice categories based on multivariate analysis of the synchronous fluorescence spectra

The usage of synchronous fluorescence as a means for discrimination between the commercial apple juice categories was studied. We recorded total synchronous fluorescence spectra (TSFS) of commercial apple juices belonging to the two categories: those produced directly - not from concentrate (NFC), and those reconstituted from concentrate (FC).An exploratory study of the spectra using the principal component analysis (PCA) revealed differentiation between the juices of the two categories based on their fluorescence.Partial least squares discriminant analysis (PLS-DA) was used for the development of the classification models. The analysis was performed for the unfolded TSFS (uTSFS) and individual synchronous fluorescence spectra (SFS) measured at particular emission-excitation wavelength offsets (Δλ), from 10 to 160 nm with 10 nm step. The best discrimination results were obtained for the model using uTSFS in the range of Δλ = 30–160 nm, with cross-validation and external validation error rates of 0.05 and 0.06, respectively. Models based on selected individual SFS also showed similarly good predictive ability, with cross-validation and external validation error rates of 0.08 and 0.05, respectively, for the SFS measured at Δλ = 70 and 90 nm. The analysis of significant variables using selectivity ratio (SR) suggests that the fluorescence of non-enzymatic browning products may significantly contribute to the differentiation of FC and NFC juices.

Dye-Specific Wavelength Offsets to Resolve Spectrally Overlapping and Co-Localized Two-Photon Induced Fluorescence.

The fundamentally important fluorescent imaging has one major limitation, resolution of over three dyes. This limitation is in part due to the overlap of the broad emission profile of each of the emitters used in fluorescence detection. The overlapping emission contaminates each emitter's detection channel, referred to as cross-talk. To reduce fluorescence cross-talk for two photon applications, we present an innovative Two-Photon-Dye-Specific Excitation-Emission Offset (TP-DSO) method. TP-DSO selectively detects each dye by synchronously scanning the excitation and emission wavelengths at defined wavelength offsets. This technique advances multicolor analysis significantly by resolving dyes with highly overlapping spectral profiles. We identified three benefits: reduced excitation spectral bandwidth, reduced emission cross-talk between colocalized emitters with closely overlapping fluorescence, and validated use of thin-film variable optical emission filters for tuning the bandwidth and center wavelength. TP-DSO will advance multicolor analysis for many applications.

Corrections to “Impact of the Quantum Well Gain-to-Cavity Etalon Wavelength Offset on the High Temperature Performance of High Bit Rate 980-nm VCSELs” [Aug 14 613-621

In the above paper (ibid., vol. 50, no. 8, pp. 613-621, Aug. 2014), the sixth author was misidentified. His name should have read James A. Lott in the byline and in his biography.

Experimental characterization of phase tuning using fine wavelength offsets in a tunable complex-coefficient optical tapped-delay-line

We use fine-detuning of pump wavelengths to adjust the tap phases in a complex-coefficient optical tapped-delay-line that utilizes conversion/dispersion-based delays and nonlinear wave mixing. Full 2π phase tuning is demonstrated by detuning the frequency of laser pumps by <20 GHz, which shows close agreement with theory.

Combining Cryogenic Fiber Optic Probes with Commercial Spectrofluorimeters for the Synchronous Fluorescence Shpol'skii Spectroscopy of High Molecular Weight Polycyclic Aromatic Hydrocarbons

Cryogenic fiber optic probes are combined for the first time with a commercial spectrofluorometer for Shpol'skii spectroscopy measurements at liquid nitrogen (77 K) and liquid helium (4.2 K) temperatures. Accurate and reproducible acquisition of fluorescence spectra and signal intensities is demonstrated with three well known Shpol'skii systems, namely, anthracene/heptane, pyrene/hexane, and benzo[a]pyrene/octane. The ability to adjust the excitation and emission bandpass of the spectrofluorimeter to reach both site-resolution and analytically valuable signal-to-noise ratios was illustrated with benzo[a]pyrene in n-octane. The analytical potential of 4.2 K synchronous fluorescence Shpol'skii spectroscopy for the analysis of high molecular weight-polycyclic aromatic hydrocarbons was then explored for the first time. The judicious optimization of wavelength offsets permitted the successful determination of dibenzo[a,l]pyrene, dibenzo[a,e]pyrene, dibenzo[a,h]pyrene, dibenzo[a,i]pyrene, and naphtho[2,3-a]pyrene without previous chromatographic separation from a soil extract with complex matrix composition. The simplicity of the experimental procedure, the competitive analytical figures of merit, and the selectivity of analysis turn 4.2 K synchronous fluorescence Shpol'skii spectroscopy into a valuable alternative for screening isomers of high molecular weight polycyclic aromatic hydrocarbons in environmental samples.

Achieving second order advantage with multi-way partial least squares and residual bi-linearization with total synchronous fluorescence data of monohydroxy–polycyclic aromatic hydrocarbons in urine samples

An attractive approach to handle matrix interference in samples of unknown composition is to generate second- or higher-order data formats and process them with appropriate chemometric algorithms. Several strategies exist to generate high-order data in fluorescence spectroscopy, including wavelength time matrices, excitation–emission matrices and time-resolved excitation–emission matrices. This article tackles a different aspect of generating high-order fluorescence data as it focuses on total synchronous fluorescence spectroscopy. This approach refers to recording synchronous fluorescence spectra at various wavelength offsets. Analogous to the concept of an excitation–emission data format, total synchronous data arrays fit into the category of second-order data. The main difference between them is the non-bilinear behavior of synchronous fluorescence data. Synchronous spectral profiles change with the wavelength offset used for sample excitation. The work presented here reports the first application of total synchronous fluorescence spectroscopy to the analysis of monohydroxy-polycyclic aromatic hydrocarbons in urine samples of unknown composition. Matrix interference is appropriately handled by processing the data either with unfolded-partial least squares and multi-way partial least squares, both followed by residual bi-linearization.

Application of ‘multivariate curve resolution alternating least square (MCR–ALS)’ analysis to extract pure component synchronous fluorescence spectra at various wavelength offsets from total synchronous fluorescence spectroscopy (TSFS) data set of dilute aqueous solutions of fluorophores

In the present study, the possibility of using the multivariate curve resolution alternating least square (MCR–ALS) analysis for the simultaneous extraction of the pure synchronous fluorescence spectra at various wavelength offsets (Δλ) for each fluorophore from the total synchronous fluorescence spectroscopy (TSFS) data set of the dilute aqueous mixtures of the three fluorophores, was explored. The present work was based on the assumption that unfolded TSFS data has a bilinear structure and therefore it can be subjected to MCR–ALS analysis. Three fluorophores, benzo[a]pyrene(BaP), perylene(PE), and pyrene(PY), were chosen. These three fluorophores show fluorescence at all the seven wavelength offsets (Δλ) used to create the TSFS data set. In addition, Raman scattering due to solvent molecules (i.e. water) also appear in the wavelength ranges where these fluorophores show fluorescence. These two factors make the simultaneous extraction of synchronous spectral profile at various Δλs from the TSFS data set relatively difficult. The appearance of the diagonal signals in the three‐dimensional landscapes of TSFS shows the presence of the Raman scattering. The Raman signal due to solvent molecules was found to influence the synchronous profile of a fluorophore to different extents at different Δλs. TSFS data set of dimension, sample×wavelength×Δλ, was unfolded along the first mode to obtain the unfolded TSFS data set. Pure synchronous spectral profiles at various Δλs were obtained for each fluorophore by performing the MCR–ALS analysis on the unfolded TSFS data. However Raman scattering signals could not be eliminated from the synchronous spectral profiles of the PE and PY. For the mitigation of Raman scattering from the calculated spectral profiles, TSFS data of solvent blank were subtracted from all the samples before performing the MCR–ALS analysis. The obtained spectral profiles of BaP, PE, and PY were found to match with their actual spectral profiles which verifies that unfolded TSFS data set has a bilinear structure. To test the strength of the present work, MCR–ALS analysis was also performed on the unfolded TSFS data set of 12 groundwater samples which were contaminated with the BaP‐ and PY‐spiked gasoline. The obtained results show that it is possible to monitor the presence of BaP and PY in groundwater samples.

Testing absolute plate reference frames and the implications for the generation of geodynamic mantle heterogeneity structure

Absolute reference frames are a means of describing the motion of plates on the surface of the Earth over time, relative to a fixed point or “frame.” Multiple models of absolute plate motion have been proposed for the Cretaceous–Tertiary period, however, estimating the robustness and limitations of each model remains a significant limitation for refining both regional and global models of plate motion as well as fully integrated and time dependent geodynamic models. Here, we use a novel approach to compare five models of absolute plate motion in terms of their consequences for forward modelled deep mantle structure since at least 140Ma. We show that the use of hotspots, either fixed or moving, or palaeomagnetics, with or without corrections for true-polar wander, leads to significant differences in palaeo-plate velocities of over 10cm/yr as well as differences in the location of palaeo-plate boundaries of up to 30° in longitude and latitude. Furthermore, we suggest that first order differences in forward predicted mantle structure between the models are due mostly to differences in palaeo-plate velocities, whereas variation in the location of plate boundaries may contribute to smaller wavelength offsets. We present a global comparison of the absolute reference frames in terms of mantle structure, which we have tomographically filtered to reflect the resolution of the seismic tomography model S20RTS. At very long wavelengths hotspot models best reproduce the mantle structure. However, when geometry and the match of smaller-scale subducted slab volumes are compared, a hybrid model based on moving hotspots after 100Ma and palaeomagnetic data before (with no corrections for true-polar wander), best reproduces the overall mantle structure of slab burial grounds, even though no single model fits best at all mantle depths. We find also that the published subduction reference frame tested here results in a modelled mantle structure that agrees well with S20RTS for depths >2500km, equivalent to subduction before the Cretaceous, but not for shallower depths. This indicates that a careful assimilation of hotspot, palaeomagnetic and seismic tomography data into future absolute plate motion models is required to derive a more robust subduction reference frame.

WAVELENGTH ACCURACY OF THE KECK HIRES SPECTROGRAPH AND MEASURING CHANGES IN THE FINE STRUCTURE CONSTANT

We report on an attempt to accurately wavelength calibrate four nights of data taken with the Keck HIRES spectrograph on QSO PHL957, for the purpose of determining whether the fine structure constant was different in the past. Using new software and techniques, we measured the redshifts of various Ni II, Fe II, Si II, etc. lines in a damped Lyalpha system at z = 2.309. Roughly half the data were taken through the Keck iodine cell which contains thousands of well calibrated iodine lines. Using these iodine exposures to calibrate the normal Th-Ar Keck data pipeline output, we found absolute wavelength offsets of 500 m s{sup -1} to 1000 m s{sup -1} with drifts of more than 500 m s{sup -1} over a single night, and drifts of nearly 2000 m s{sup -1} over several nights. These offsets correspond to an absolute redshift of uncertainty of about DELTAz approx 10{sup -5}(DELTAlambda approx 0.02 A), with daily drifts of around DELTAz approx 5 x 10{sup -6} (DELTAlambda approx 0.01 A), and multiday drifts of nearly DELTAz approx 2 x 10{sup -5}(approx0.04 A). The causes of the wavelength offsets are not known, but since claimed shifts in the fine structure constant wouldmore » result in velocity shifts of less than 100 m s{sup -1}, this level of systematic uncertainty may make it difficult to use Keck HIRES data to constrain the change in the fine structure constant. Using our calibrated data, we applied both our own fitting software and standard fitting software to measure DELTAalpha/alpha, but discovered that we could obtain results ranging from significant detection of either sign, to strong null limits, depending upon which sets of lines and which fitting method were used. We thus speculate that the discrepant results on DELTAalpha/alpha reported in the literature may be due to random fluctuations coming from underestimated systematic errors in wavelength calibration and fitting procedure.« less

Sticky pulses: two-color cross-mode-locked femtosecond operation of a single Ti:sapphire laser

We demonstrate mode-locked two-wavelength synchronized operation of a single cw-pumped Ti:sapphire laser. There is no active feedback or external cavity. The measured cross correlation is 105 fs. Cross mode locking is maintained for relative cavity length detunings as large as ±3 μm and wavelength offsets as large as 20 nm.

Crosstalk between intensity-modulated wavelength-division multiplexed signals in a semiconductor laser amplifier

The performance of a 1.5- mu m Fabry-Perot type laser amplifier having two input signals at different wavelengths is experimentally studied. Crosstalk between the two signals at the output of the amplifier is measured. The crosstalk is strongly dependent on the input powers and on the signal gain, and is found to be very sensitive to the relative wavelengths of the input signals. The crosstalk can be either positive or negative, depending on the wavelength offsets. A theoretical model taking gain saturation and wavelength shift of the amplifier spectrum into account is shown to explain the experimental results well. Using this model, some general aspects on crosstalk in semiconductor laser amplifiers are discussed. >

New laser lines from optically pumped CH 3OH: measurements and assignments

New CW FIR laser lines from optically pumped CH 3OH are reported. Experimental values for wavelength, polarization and pump offsets are given. Lines assignments are proposed. New experimental results and possible assignments are also given for previously reported lines.