scholarly journals Intrinsic Spectral Resolution Limitations of QEPAS Sensors for Fast and Broad Wavelength Tuning

Sensors ◽  
2020 ◽  
Vol 20 (17) ◽  
pp. 4725 ◽  
Author(s):  
Jesper B. Christensen ◽  
Lasse Høgstedt ◽  
Søren M. M. Friis ◽  
Jui-Yu Lai ◽  
Ming-Hsien Chou ◽  
...  
Keyword(s):  
Spectral Resolution ◽  
Signal To Noise Ratio ◽  
Quartz Tuning Fork ◽  
Wavelength Tuning ◽  
Trace Gas ◽  
Gas Mixture ◽  
Signal To Noise ◽  
Tuning Rate ◽  
Scan Rate ◽  
Noise Ratio

Quartz-enhanced photoacoustic sensing is a promising method for low-concentration trace-gas monitoring due to the resonant signal enhancement provided by a high-Q quartz tuning fork. However, quartz-enhanced photoacoustic spectroscopy (QEPAS) is associated with a relatively slow acoustic decay, which results in a reduced spectral resolution and signal-to-noise ratio as the wavelength tuning rate is increased. In this work, we investigate the influence of wavelength scan rate on the spectral resolution and signal-to-noise ratio of QEPAS sensors. We demonstrate the acquisition of photoacoustic spectra from 3.1 μm to 3.6 μm using a tunable mid-infrared optical parametric oscillator. The spectra are attained using wavelength scan rates differing by more than two orders of magnitude (from 0.3 nm s−1 to 96 nm s−1). With this variation in scan rate, the spectral resolution is found to change from 2.5 cm−1 to 9 cm−1. The investigated gas samples are methane (in nitrogen) and a gas mixture consisting of methane, water, and ethanol. For the gas mixture, the reduced spectral resolution at fast scan rates significantly complicates the quantification of constituent gas concentrations.

scholarly journals Analysis of ACAM Data for Trace Gas Retrievals during the 2011 DISCOVER-AQ Campaign

2015 ◽  
Vol 2015 ◽  
pp. 1-7 ◽  
Author(s):  
C. Liu ◽  
X. Liu ◽  
M. G. Kowalewski ◽  
S. J. Janz ◽  
G. González Abad ◽  
...  
Keyword(s):  
Spectral Resolution ◽  
Spectral Region ◽  
Total Ozone ◽  
Signal To Noise Ratio ◽  
Trace Gas ◽  
System Throughput ◽  
Signal To Noise ◽  
Ozone Profile ◽  
Blue Spectral Region ◽  
Large Wavelength

To improve the trace gas retrieval from Airborne Compact Atmospheric Mapper (ACAM) during the DSICOVER-AQ campaigns, we characterize the signal to noise ratio (SNR) of the ACAM measurement. From the standard deviations of the fitting residuals, the SNRs of ACAM nadir measurements are estimated to vary from ~300 at 310 nm to ~1000 in the blue spectral region; the zenith data are noisier due to reduced levels of illumination and lower system throughput and also show many more pixels with abrupt anomalous values; therefore, a new method is developed to derive a solar irradiance reference at the top of the atmosphere (TOA) from average nadir measurements, at instrument spectral resolution and including instrument calibration characteristics. Using this reference can significantly reduce fitting residuals and improve the retrievals. This approach derives an absolute reference for direct fitting algorithms involving radiative transfer calculations and thus can be applied to both aircraft and ground-based measurements. The comparison of ACAM radiance with simulations using coincident ozonesonde and OMI data shows large wavelength-dependent biases in ACAM data, varying from ~−19% at 310 nm to 5% at 360 nm. Correcting ACAM radiance in direct-fitting based ozone profile algorithm significantly improves the consistency with OMI total ozone.

Rocket Experiment by Electronic Camera for Studying the Metallic Discontinuities in the Ultraviolet Spectrum of ‘A’ Stars

1971 ◽  
Vol 41 ◽  
pp. 361-362
Author(s):  
M. Combes
Keyword(s):  
Spectral Resolution ◽  
High Efficiency ◽  
Signal To Noise Ratio ◽  
Absolute Calibration ◽  
List Type ◽  
Signal To Noise ◽  
Concave Mirror ◽  
Rocket Experiment ◽  
Solar Blind ◽  
Noise Ratio

1.Ultraviolet spectra (1400–1800 Å) of Ap, Am and normal A stars are needed by F. Praderie, R. Bonnet and R. Cayrel.The spectral resolution has to be nearly 1 Å. Accurate relative photometry (5%) and absolute calibration (30–50%) are required.A rocket experiment, proposed to ESRO by M. Combes and P. Felenbok is planned for launch in 1972.2.As neutral silicon and magnesium are very efficient ultra-violet absorbents, A stars ultraviolet fluxes are very faint (Praderie, 1968).Then a very luminous optical set-up and a high efficiency receiver have to be used. A 30 cm in diameter concave objective grating is associated with a Lallemand electronic camera. The grating (2000 //mm; //l) is holographically made (Labeyrie, 1969). The electronic camera is electrostatically focussed. A semi-transparent solar-blind CsL photocathode is used (Carruthers, 1966).3.A little mirror, placed against the grating and forming a direct view of the sky, permits to establish an absolute wavelength scale.During the fly, before and after stellar observations, a little concave mirror mounted into the opening side-door is used to form on the photocathode a spectrum of a Deuterium calibrated lamp. Two photomultipliers, one on each side of the electronic camera, control the lamp stability.The complete mounting is calibrated in the laboratory using a thermopile as reference, before the launch and after the recovery of the waterproof payload.4.The chosen stars are the brightest Ap and Am stars: α Dra (Ap; mv = 3.64; equivalent type A 0) and α2 Lib (Am; mv = 2.75; equivalent type A3-A7).It seems to be possible to obtain spectra (1400-1800 Å) of the Ap star with a spectral resolution of 1 Å and a signal to noise ratio better than 40. But at a pinch one may accept a resolution of 2 Å and a signal to noise ratio of 15 for the shortest range of the Ap star spectrum.

scholarly journals Vertical wind profiling from troposphere to the lower mesosphere based on high resolution heterodyne near-infrared spectroradiometry

2019 ◽  
Author(s):  
Alexander V. Rodin ◽  
Dmitry V. Churbanov ◽  
Sergei G. Zenevich ◽  
Artem Yu. Klimchuk ◽  
Vladimir M. Semenov ◽  
...  
Keyword(s):  
Absorption Line ◽  
Line Profile ◽  
Spectral Resolution ◽  
Near Infrared ◽  
Signal To Noise Ratio ◽  
Rotational Line ◽  
Co2 Absorption ◽  
Signal To Noise ◽  
Overtone Band ◽  
Noise Ratio

Abstract. We propose a new technique of remote wind measurements based on Doppler analysis of a CO2 absorption line in the 1.605 μm overtone band measured in the direct Sun observation geometry. Heterodyne spectroradiometric measurements of the solar radiation passed through the atmosphere provides an unprecedented spectral resolution up to λ/δλ ~ 107–108 with a signal-to-noise ratio more than 100. The shape of the individual rotational line profile provides unambiguous relationship between offset from the line centre and altitude where a respective part of the line profile is formed. Therefore, an inverse problem may be posed in order to retrieve vertical distribution of wind, with retrievals vertical resolution compromised by a spectral resolution and signal-to-noise ratio of the measurements. A close coincidence between measured and synthetic absorption line is reached, with retrieved wind profile between the surface and 50 km being in a good agreement with reanalysis models. This method may pose an alternative to widely employed lidar and radar techniques.

scholarly journals Comparison of 2-Hydroxyglutarate Detection With sLASER and MEGA-sLASER at 7T

2021 ◽  
Vol 12 ◽  
Author(s):  
Zahra Shams ◽  
Wybe J. M. van der Kemp ◽  
Uzay Emir ◽  
Jan Willem Dankbaar ◽  
Tom J. Snijders ◽  
...  
Keyword(s):  
Spectral Resolution ◽  
Mr Spectroscopy ◽  
Signal To Noise Ratio ◽  
Difference Spectroscopy ◽  
Signal To Noise ◽  
Spectral Editing ◽  
Specificity And Sensitivity ◽  
Noise Ratio

The onco-metabolite 2-hydroxyglutarate (2HG), a biomarker of IDH-mutant gliomas, can be detected with 1H MR spectroscopy (1H-MRS). Recent studies showed measurements of 2HG at 7T with substantial gain in signal to noise ratio (SNR) and spectral resolution, offering higher specificity and sensitivity for 2HG detection. In this study, we assessed the sensitivity of semi-localized by adiabatic selective refocusing (sLASER) and J-difference MEsher-GArwood-semi-LASER (MEGA-sLASER) for 2HG detection at 7T. We performed spectral editing at long TE using a TE-optimized sLASER sequence (110 ms) and J-difference spectroscopy using MEGA-sLASER (TE = 74ms) in phantoms with different 2HG concentrations to assess the sensitivity of 2HG detection. The robustness of the methods against B0 inhomogeneity was investigated. Moreover, the performance of these two techniques was evaluated in four patients with IDH1-mutated glioma. In contrary to MEGA-sLASER, sLASER was able to detect 2HG concentration as low as 0.5 mM. In case of a composite phantom containing 2HG with overlapping metabolites, MEGA-sLASER provided a clean 2HG signal with higher fitting reliability (lower %CRLB). The results demonstrate that sLASER is more robust against field inhomogeneities and experimental or motion-related artifacts which promotes to adopt sLASER in clinical implementations.

scholarly journals Strategies for the Efficient Estimation of Soil Moisture through Spectroscopy: Sensitive Wavelength Algorithm, Spectral Resampling and Signal-to-Noise Ratio Selection

2022 ◽  
Vol 12 (2) ◽  
pp. 826
Author(s):  
Jing Yuan ◽  
Bo Yu ◽  
Changxiang Yan ◽  
Junqiang Zhang ◽  
Ning Ding ◽  
...  
Keyword(s):  
Soil Moisture ◽  
Moisture Content ◽  
Spectral Resolution ◽  
Soil Moisture Content ◽  
Signal To Noise Ratio ◽  
Efficient Estimation ◽  
Estimation Accuracy ◽  
Transfer Model ◽  
Signal To Noise ◽  
Noise Ratio

It is found that the remote sensing parameters such as spectral range, spectral resolution and signal-to-noise ratio directly affect the estimation accuracy of soil moisture content. However, the lack of research on the relationship between the parameters and estimation accuracy restricts the prolongation of application. Therefore, this study took the demand for this application as the foothold for developing spectrometry. Firstly, a method based on sensitivity analysis of soil radiative transfer model-successive projection algorithm (SA-SPA) was proposed to select sensitive wavelengths. Then, the spectral resampling method was used to select the best spectral resolution in the corresponding sensitive wavelengths. Finally, the noise-free spectral data simulated by the soil radiative transfer model was added with Gaussian random noise to change the signal-to-noise ratio, so as to explore the influence of signal-to-noise ratio on the estimation accuracy. The research results show that the estimation accuracy obtained through the SA-SPA (RMSEP < 12.1 g kg−1) is generally superior to that from full-spectrum data (RMSEP < 14 g kg−1). At selected sensitive wavelengths, the best spectral resolution is 34 nm, and the applicable signal-to-noise ratio ranges from 150 to 350. This study provides technical support for the efficient estimation of soil moisture content and the development of spectrometry, which comprehensively considers the common influence of spectral range, spectral resolution and signal-to-noise ratio on the estimation accuracy of soil moisture content.

scholarly journals Vertical wind profiling from the troposphere to the lower mesosphere based on high-resolution heterodyne near-infrared spectroradiometry

2020 ◽  
Vol 13 (5) ◽  
pp. 2299-2308 ◽  
Author(s):  
Alexander V. Rodin ◽  
Dmitry V. Churbanov ◽  
Sergei G. Zenevich ◽  
Artem Y. Klimchuk ◽  
Vladimir M. Semenov ◽  
...  
Keyword(s):  
Absorption Line ◽  
Line Profile ◽  
Spectral Resolution ◽  
Near Infrared ◽  
Signal To Noise Ratio ◽  
Line Center ◽  
Rotational Line ◽  
Co2 Absorption ◽  
Signal To Noise ◽  
Noise Ratio

Abstract. We propose a new technique of remote wind measurements based on Doppler analysis of a CO2 absorption line in the 1.605 µm overtone band measured in the direct Sun observation geometry. Heterodyne spectroradiometric measurements of the solar radiation passing through the atmosphere provide an unprecedented spectral resolution up to λ/δλ∼6×107, with a signal-to-noise ratio of more than 100. The shape of the individual rotational line profile provides an unambiguous relationship between the offset from the line center and the altitude at which the respective part of the line profile is formed. Therefore, an inverse problem may be posed in order to retrieve the vertical distribution of wind because with retrievals the vertical resolution is compromised by a spectral resolution and the signal-to-noise ratio of the measurements. A close coincidence between the measured and synthetic absorption line is reached, with retrieved wind profiles between the surface and 50 km being in good agreement with reanalysis models. This method may pose an alternative to widely employed lidar and radar techniques.

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