High-resolution Fourier-transform Spectrometer Research Articles

FT-UV emission spectroscopy of the [formula omitted] progression bands of the [formula omitted] system of [formula omitted]C[formula omitted]O[formula omitted]

We report an experimental investigation of the 0−ν′′ progression bands of the First-Negative (B2Σ+−X2Σ+) system of 12C16O+. The ultraviolet wavelength emission spectrum of CO+ was generated using an air-cooled stainless steel hollow-cathode discharge lamp filled with a mixture of CO2 and Ne. The study includes four bands, i.e. 0−0, 0−1, 0−2 and 0−3, observed in emission with a high-resolution Fourier-transform spectrometer in the 38500–46000 cm−1 region. In total, 541 ro-vibronic transitions have been measured with an absolute accuracy of 0.005–0.010 cm−1. The experimental data were analyzed using the PGOPHER program, which resulted in a set of high accuracy molecular constants and ro-vibronic level positions for the X2Σ+, ν=0−3 and B2Σ+, ν=0 levels. The current values of the band origins have been compared with historical data, including the most recent results obtained by Ventura et al. (2020) and the discrepancies pointed out by the authors were explained.

Br I spectral line measurements in the range 2000–6000 cm−1: Part I

The present study provides data on the spectral line measurements of atomic bromine (Br I) in the 2000–6000 cm−1 range in the infrared spectral region by using a high-resolution Fourier Transform Spectrometer. Electrodeless discharge lamps (EDL) of bromine were prepared and used as light sources while InSb was utilized as the light detector. A total of 210 spectral lines were observed with two different resolutions of 0.004 and 0.006 cm−1, of which 166 lines are reported for the first time. New center of gravity wavenumber values are reported for all the observed lines.

The Problem of CO2 Reabsorption in Emission Spectra

The authors present a detailed explanation for selective reabsorption by the CO2 ν3-band when passing its optical emission through an atmospheric-pressure air column (60 cm long, 410 ppm CO2) connecting a microwave-discharge cell and the entrance window of a high-resolution Fourier Transform spectrometer. The CO2 band shapes are explained with a two-temperature model of emission and foreground absorption. Selective CO2 atmospheric reabsorption is a common effect in optical emission measurements. It primarily affects the fundamental infrared bands, which comprise the main part of the missing laboratory emission and must be taken into account in (exo)planetary atmospheric models.

High Resolution Fourier Transform Spectrometer for Ground-Based Verification of Greenhouse Gases Satellites

Satellite remote sensing is currently the best monitoring means to obtain global carbon source and sink data. The United States, Japan, China and other countries are vigorously developing spaceborne detection technology. However, the important factors that restrict the application of greenhouse gas satellite remote sensing technology include the limited accuracy of data products. How to improve the retrieval level of greenhouse gas payloads is a problem that needs to be solved urgently. One effective way to improve data quality is to carry out satellite ground synchronous authenticity verification and system error correction. This paper mainly aims at the shortcomings of the existing TCCON and the portable verification equipment EM27/SUN, and develops a High-Resolution Fourier Transform Spectrometer (HRFTS) based on dynamic collimation technology. Through the gas absorption method and the band scanning method of the hyperspectral monochromatic light source, the instrument’s absorption spectrum measurement capability and the Instrument Line Shape (ILS) are demonstrated. The instrument’s spectral resolution is consistent with the on-orbit greenhouse gas satellite load, reaching 0.26 cm−1. For the interference data obtained by the spectrometer, spectral restoration processing, data quality control and inversion algorithm optimization were carried out to solve the problems of baseline correction, spectral fine registration, and environmental parameter profile reconstruction, and cross comparison experiments with EM27/SUN were carried out simultaneously. Finally, for the gases monitoring instrument (GMI) of the GF5-02 satellite launched on 7 September 2021, the first satellite ground synchronization verification experiment with high space-time matching was carried out. The results showed that the CO2 column concentration deviation of the satellite ground synchronization inversion was about 1.5 ppm, and the CH4 column concentration deviation was about 11.3 ppb, which verified the on-orbit detection accuracy of the GMI, and laid a foundation for the subsequent satellite inversion algorithm optimization and systematic error correction.

Hollow-core fiber delivery of broadband mid-infrared light for remote spectroscopy.

High-resolution multi-species spectroscopy is achieved by delivering broadband3-4-μmmid-infrared light through a 4.5-meter-long silica-based hollow-core optical fiber. Absorptions from H37Cl, H35Cl, H2O and CH4 present in the gas within the fiber core are observed, and the corresponding gas concentrations are obtained to 5-ppb precision using a high-resolution Fourier-transform spectrometer and a full-spectrum multi-species fitting algorithm. We show that by fully fitting the narrow absorption features of these light molecules their contributions can be nulled, enabling further spectroscopy of C3H6O and C3H8O contained in a Herriott cell after the fiber. As a demonstration of the potential to extend fiber-delivered broadband mid-infrared spectroscopy to significant distances, we present a high-resolution characterization of the transmission of a 63-meter length of hollow-core fiber, fully fitting the input and output spectra to obtain the intra-fiber gas concentrations. We show that, despite the fiber not having been purged, useful spectroscopic windows are still preserved which have the potential to enable hydrocarbon spectroscopy at the distal end of fibers with lengths of tens or even hundreds of meters.

Quantitative fs-TALIF in high-pressure NRP discharges: calibration using VUV absorption spectroscopy

This work presents a femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF) diagnostic for measuring ground-state atomic nitrogen in nanosecond repetitively pulsed (NRP) discharges. Absolute atom density is obtained from the TALIF signal via a novel calibration technique based on one-photon direct absorption measurements performed in a low-pressure DC discharge. The Vacuum Ultraviolet (VUV) measurements were done at the Soleil synchrotron facility using the high-resolution Fourier-transform spectrometer (minimum linewidth Δ = 0.08 cm−1). The main goal of this work was to develop a quench-free diagnostic technique, which would allow measurements at elevated pressures with high spatial and temporal resolution. Here fs-TALIF measurements of N(4S) are demonstrated in the NRP post-discharge between 1–500 μs after the nanosecond high-voltage pulse. A maximum number density of N-atoms of 5 × 1016 cm−3 was measured at 1 μs after the pulse when the discharge was operated at 1 bar in pure nitrogen. This corresponds to a dissociation fraction of ∼0.1%. The fs-TALIF technique at high laser intensity regime (>1 TW cm−2) calibrated using VUV absorption was compared with the fs-TALIF at low laser intensity regime (<100 MW cm−2) calibrated via the well-established non-saturated TALIF technique using krypton as an etalon gas. It was found that the two measurements of N(4S) in the NRP post-discharge agree within a factor of 3. Importantly, the limit of detection of the fs-TALIF at high laser intensity regime was determined to be . This is approximately one order of magnitude better than previously reported by ns-TALIF in low-pressure discharges.

Design and characteristics of a cavity-enhanced Fourier-transform spectrometer based on a supercontinuum source.

We report the in-house fabrication of a high-resolution Fourier-transform spectrometer (FTS) for the spectroscopy of molecules in the gas phase at resolutions down to 0.002 cm-1 working in the spectral range from 5880 cm-1 (1.7 μm) to 15 380 cm-1 (650 nm). The FTS employs a supercontinuum as a broadband light source and a He:Ne laser with a homemade frequency-stabilization scheme as the spatial reference for the sampling of the interferogram on a constant optical path difference (OPD) grid. The sampling of the two lasers is performed at constant time intervals, and the resampling process is performed at the software level. The resampling of the interferogram on a constant OPD grid relies on cubic approximations of the He:Ne interference pattern to determine its zero-crossings. The use of an invariant in the sampling process allows us to perform on-the-fly data treatment. Both the hardware aspect and the data processing are described with, in each case, an original approach. We also report the successful coupling of the FTS with a high finesse optical cavity with effective mirror reflectivities of 99.76%, allowing us to reach sensitivities down to 6.5 × 10-8 cm-1 with a root-mean-square accuracy of 0.0017 cm-1 on the position of the Doppler-broadened transitions with a mean transition width of 0.046 cm-1 for spectra recorded at a spectral resolution of 0.015 cm-1. The sensitivity of the instrument per spectral element, once normalized, represents the best sensitivity reported in the literature for Fourier-transform incoherent broadband cavity-enhanced absorption spectroscopy with a supercontinuum light source.

High-resolution spectroscopy and analysis of the ν3 bands of ClNO (nitrosyl chloride) around 30 µm

The ν3 fundamental bands of 35ClNO and 37ClNO and the 2ν3-ν3 band of 35ClNO around 30 µm have been recorded using a high-resolution Fourier-transform spectrometer. The line-by-line analysis of the spectrum provides improved rotational and centrifugal distortion constants for the ν3 fundamental band of 35ClNO, and the first determination of these constants together with accurate band centers for the ν3 band of 37ClNO and for the 2ν3-ν3 band of 35ClNO. The band centers are 331.970996(28) cm−1 for the ν3 fundamental band of 35ClNO, 327.944735(78) cm−1 for the ν3 fundamental band of 37ClNO, and 329.962453(92) cm−1 for the 2ν3-ν3 band of 35ClNO.

Comparison of XCH4 Derived from g-b FTS and GOSAT and Evaluation Using Aircraft In-Situ Observations over TCCON Site

It is evident that evaluating the measurement of greenhouse gases (GHGs) obtained from multi-platform instruments against accurate and precise instrument such as aircraft in-situ is very essential when using remote sensing GHGs results for source/sink estimations with inverse modeling. The results of the inverse models are very sensitive even to small biases in the data (Rayner and O’Brien 2001). In this work, we have evaluated ground-based high resolution Fourier Transform Spectrometer (g-b FTS) and the Greenhouse gases Observing SATellite (GOSAT) column-averaged dry air mole fraction of methane (XCH4) through aircraft in-situ observations over Anmyeondo station (36.538o N, 126.331o E, 30 m above sea level). The impact of the spatial coincidence criteria was assessed by comparing GOSAT data against g-b FTS. We noticed there was no any systematic difference based on the given coincidence criteria. GOSAT exhibited a bias ranging from 0.10 to 3.37 ppb, with the standard deviation from 4.92 to 12.54 ppb, against g-b FTS with the spatial coincidence criteria of ±1, ±3, ±5 degrees of latitude and longitude and ± 1 h time window. Data observed during ascent and descent of the aircraft is considered as vertical profiles within an altitude range of 0.2 to a maximum of 9.0 km so that some assumptions were applied for the construction of the profiles below 0.2 and above 9.0 km. In addition, the suitability of aircraft data for evaluation of remote sensing instruments was confirmed based on the assessment of uncertainties. The spatial coincidence criteria is ±1o latitude and ± 2o longitude and for temporal difference is ±1 h of the satellite observation overpass time were applied, whereas g-b FTS data are the mean values measured within ±30 min of the aircraft observation time. Furthermore, the sensitivity differences of the instruments were taken into account. With respect to aircraft, the g-b FTS data were biased by −0.19 ± 0.69%, while GOSAT data were biased by −0.42 ± 0.84%. These results confirm that both g-b FTS and GOSAT are consistent aircraft observations and assure the reliability of the datasets for inverse estimate of CH4.

Validation of Column-Averaged Dry-Air Mole Fraction of CO2 Retrieved from OCO-2 Using Ground-Based FTS Measurements

In order to correctly use the column-averaged atmospheric CO2 dry-air mole fraction (XCO2) data in the CO2 flux studies, XCO2 measurements retrieved from the Orbiting Carbon Observatory-2 (OCO-2) in 2015 were compared with those obtained from the global ground-based high-resolution Fourier Transform Spectrometer (FTS) participating in the Total Carbon Column Observing Network (TCCON). The XCO2 retrieved from three observing modes adopted by OCO-2, i.e., nadir, target, and glint, were separately validated by the FTS measurements at up to eight TCCON stations located in different areas. These comparisons show that OCO-2 glint mode yields the best qualitative estimations of CO2 concentration among the three operational approaches. The overall results regarding the glint mode show no obvious systematic biases. These facts may indicate that the glint concept is appropriate for not only oceans but also land regions. Negative systematic biases in nadir and target modes have been found at most TCCON sites. The standard deviations of XCO2 retrieved from target and nadir modes within the observation period are simi-lar, and larger than those from glint mode. We also used the FTS site in Beijing, China, to assess the OCO-2 XCO2 in 2016. This site is located in a typical urban area, which has been absent in previous studies. Overall, OCO-2 XCO2 agrees well with that from FTS at this site. Such a study will benefit the validation of the newly launched TanSat products in China.

Measurements and modeling of absorption by CO2 + H2O mixtures in the spectral region beyond the CO2 ν3-band head

In this work, we measured the absorption by CO2 + H2O mixtures from 2400 to 2600 cm−1 which corresponds to the spectral region beyond the ν3 band head of CO2. Transmission spectra of CO2 mixed with water vapor were recorded with a high-resolution Fourier-transform spectrometer for various pressure, temperature and concentration conditions. The continuum absorption by CO2 due to the presence of water vapor was determined by subtracting from measured spectra the contribution of local lines of both species, that of the continuum of pure CO2 as well as of the self- and CO2-continua of water vapor induced by the H2O-H2O and H2O-CO2 interactions. The obtained results are in very good agreement with the unique previous measurement (in a narrower spectral range). They confirm that the H2O-continuum of CO2 is significantly larger than that observed for pure CO2. This continuum thus must be taken into account in radiative transfer calculations for media involving CO2 + H2O mixture. An empirical model, using sub-Lorentzian line shapes based on some temperature-dependent correction factors χ is proposed which enables an accurate description of the experimental results.

Controlled production of atomic oxygen and nitrogen in a pulsed radio-frequency atmospheric-pressure plasma

Radio-frequency driven atmospheric pressure plasmas are efficient sources for the production of reactive species at ambient pressure and close to room temperature. Pulsing the radio-frequency power input provides additional control over species production and gas temperature. Here, we demonstrate the controlled production of highly reactive atomic oxygen and nitrogen in a pulsed radio-frequency ( MHz) atmospheric-pressure plasma, operated with a small % air-like admixture (/ at ) through variations in the duty cycle. Absolute densities of atomic oxygen and nitrogen are determined through vacuum-ultraviolet absorption spectroscopy using the DESIRS beamline at the SOLEIL synchrotron coupled with a high resolution Fourier-transform spectrometer. The neutral-gas temperature is measured using nitrogen molecular optical emission spectroscopy. For a fixed applied-voltage amplitude (234 V), varying the pulse duty cycle from 10% to 100% at a fixed 10 kHz pulse frequency enables us to regulate the densities of atomic oxygen and nitrogen over the ranges of – and – , respectively. The corresponding 11 K increase in the neutral-gas temperature with increased duty cycle, up to a maximum of K, is relatively small. This additional degree of control, achieved through regulation of the pulse duty cycle and time-averaged power, could be of particular interest for prospective biomedical applications.

Hyperfine structure measurements of neutral iodine atom (127I) using Fourier Transform Spectrometry

We report the hyperfine Structure (hfs) splitting observations of neutral iodine atom (II) in the 6000 – 10,000 cm−1 near infrared spectral region. The measurements were carried out using a high-resolution Fourier Transform Spectrometer (FTS), where an electrodeless discharge lamp (EDL), excited using microwaves, was employed as the light source and InGaAs as the light detector. A specially designed setup was used to lower the plasma temperature of the medium so as to reduce the Doppler width and consequently to increase the spectral resolution of hfs components. A total of 183 lines with hfs splitting have been observed, out of which hfs in 53 spectral lines are reported for the first time. On the basis of hfs analysis, we derived the magnetic dipole and electric quadrupole coupling constants, A and B respectively for 30 even and 30 odd energy levels and are compared with the values available in the literature. New hfs values for 5 even and 4 odd levels are also reported here for the first time.

Laser and Fourier-transform spectroscopy of KCa

KCa was produced in a heat-pipe oven and its thermal emission spectrum around $8900\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ was recorded with a high-resolution Fourier-transform spectrometer. In addition, many selected transitions of this spectrum between deeply bound vibrational levels of the $X{(1)}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$ and ${(2)}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$ states were studied using laser excitation to facilitate the assignment of the lines. The ground state is described for ${v}^{\ensuremath{'}\ensuremath{'}}=0--5$ and the ${(2)}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$ state for ${v}^{\ensuremath{'}}=0\ensuremath{-}8$ with rotational levels up to 175. For both states, Dunham coefficients, spin-rotation parameters, and potential energy curves are derived.

Adding the s-Process Element Cerium to the APOGEE Survey: Identification and Characterization of Ce ii Lines in the H-band Spectral Window

Abstract Nine Ce ii lines have been identified and characterized within the spectral window observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey (between λ1.51 and 1.69 μm). At solar metallicities, cerium is an element that is produced predominantly as a result of the slow capture of neutrons (the s-process) during asymptotic giant branch stellar evolution. The Ce ii lines were identified using a combination of a high-resolution ( ) Fourier Transform Spectrometer (FTS) spectrum of α Boo and an APOGEE spectrum (R = 22,400) of a metal-poor, but s-process enriched, red giant (2M16011638-1201525). Laboratory oscillator strengths are not available for these lines. Astrophysical gf-values were derived using α Boo as a standard star, with the absolute cerium abundance in α Boo set by using optical Ce ii lines that have precise published laboratory gf-values. The near-infrared Ce ii lines identified here are also analyzed, as consistency checks, in a small number of bright red giants using archival FTS spectra, as well as a small sample of APOGEE red giants, including two members of the open cluster NGC 6819, two field stars, and seven metal-poor N- and Al-rich stars. The conclusion is that this set of Ce ii lines can be detected and analyzed in a large fraction of the APOGEE red giant sample and will be useful for probing chemical evolution of the s-process products in various populations of the Milky Way.

Investigating the performance of a greenhouse gas observatory in Hefei, China

Abstract. A ground-based high-resolution Fourier transform spectrometer (FTS) station has been established in Hefei, China to remotely measure CO2, CO and other greenhouse gases based on near-infrared solar absorption spectra. Total column measurements of atmospheric CO2 and CO were successfully obtained from July 2014 to April 2016. The spectra collected with an InSb detector in the first year were compared with those collected by an InGaAs detector from July 2015, demonstrating that InGaAs spectra have better signal-to-noise ratios and rms of spectral fitting residuals relative to InSb spectra. Consequently, the measurement precision of the retrieved XCO2 and XCO for InGaAs spectra is superior to InSb spectra, with about 0.04 and 0.09 % for XCO2, and 1.07 and 2.00 % for XCO within clear-sky days respectively. Daily and monthly averages of column-averaged dry air mole fraction of CO2 show a clear seasonal cycle, while the daily and monthly averages of XCO displayed no seasonal variation. Also, we analysed the relationship of the anomalies of XCO and XCO2, found that the correlations are only observable for individual days, and the data under different prevailing wind conditions during the observations displayed weak correlation. The observations based on the high-resolution FTS were also compared with the temporally coinciding measurements taken with a low-resolution solar FTS instrument, the EM27/SUN. Ratioing the daily averaged XCO2 of EM27 and FTS gives an overall calibration factor of 0.996 ± 0.001. We also compared ground-based observations from the Tsukuba TCCON station with our observations, the results showing that the variation in phase and seasonal amplitude of XCO2 are similar to our results, but the variation of XCO in Tsukuba is quite different from our data in Hefei. To further evaluate our retrieved data, we made use of satellite measurements. The direct comparison of our observations with the Greenhouse Gases Observing Satellite (GOSAT) data shows good agreement of daily median XCO2, with a bias of −0.52 ppm and standard deviation of 1.63 ppm. The correlation coefficient (R2) is 0.79 for daily median XCO2 between our FTS and GOSAT observations. Daily median Orbiting Carbon Observatory 2 (OCO-2) data produce a positive bias of 0.81 ppm and standard deviation of 1.73 ppm relative to our ground-based data. Our daily median XCO2 also show strong correlation with OCO-2 data, with correlation coefficient (R2) of 0.83. Although there were a limited number of data during the observations due to instrument downtime and adverse weather, the results confirm the suitability of the observatory for ground-based long-term measurements of greenhouse gases with high precision and accuracy, and fulfil the requirements of the Total Carbon Column Observing Network (TCCON).

Super Lorentzian effects on the wings of self-broadened HCl and of HCl diluted in Ar

Super-Lorentzian effects in the troughs between HCl lines were observed long time ago [Varanasi et al., J Quant Rad Transfer, Vol. 12, pag. 857, 1972]. The observed spectral shape was then modelled by using an empirical law and there was no explanation about the mechanisms underlying these super-Lorentzian effects. In this work, new spectra of pure HCl and HCl diluted in Ar have been measured using a high resolution Fourier Transform spectrometer, for pressure from 6 to 10 bars. Spectra of pure HCl and HCl in Ar have been also computed using classical molecular dynamics simulations (CMDS). First comparisons between CMDS-calculated spectra and measured ones, for regions at the troughs between HCl lines, show that the observed super-Lorentzian behaviour is correctly reproduced by the calculations. These results thus open the paths for the determination of the origin of these super-Lorentzian effects.

The CU mobile Solar Occultation Flux instrument: structure functions and emission rates of NH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;, NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and C&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;H&amp;lt;sub&amp;gt;6&amp;lt;/sub&amp;gt;

Abstract. We describe the University of Colorado mobile Solar Occultation Flux instrument (CU mobile SOF). The instrument consists of a digital mobile solar tracker that is coupled to a Fourier transform spectrometer (FTS) of 0.5 cm−1 resolution and a UV–visible spectrometer (UV–vis) of 0.55 nm resolution. The instrument is used to simultaneously measure the absorption of ammonia (NH3), ethane (C2H6) and nitrogen dioxide (NO2) along the direct solar beam from a moving laboratory. These direct-sun observations provide high photon flux and enable measurements of vertical column densities (VCDs) with geometric air mass factors, high temporal resolution of 2 s and spatial resolution of 5–19 m. It is shown that the instrument line shape (ILS) of the FTS is independent of the azimuth and elevation angle pointing of the solar tracker. Further, collocated measurements next to a high-resolution FTS at the National Center for Atmospheric Research (HR-NCAR-FTS) show that the CU mobile SOF measurements of NH3 and C2H6 are precise and accurate; the VCD error at high signal to noise ratio is 2–7 %. During the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) from 21 July to 3 September 2014 in Colorado, the CU mobile SOF instrument measured median (minimum, maximum) VCDs of 4.3 (0.5, 45) × 1016 molecules cm−2 NH3, 0.30 (0.06, 2.23) × 1016 molecules cm−2 NO2 and 3.5 (1.5, 7.7) × 1016 molecules cm−2 C2H6. All gases were detected in larger 95 % of the spectra recorded in urban, semi-polluted rural and remote rural areas of the Colorado Front Range. We calculate structure functions based on VCDs, which describe the variability of a gas column over distance, and find the largest variability for NH3. The structure functions suggest that currently available satellites resolve about 10 % of the observed NH3 and NO2 VCD variability in the study area. We further quantify the trace gas emission fluxes of NH3 and C2H6 and production rates of NO2 from concentrated animal feeding operations (CAFO) using the mass balance method, i.e., the closed-loop vector integral of the VCD times wind speed along the drive track. Excellent reproducibility is found for NH3 fluxes and also, to a lesser extent, NO2 production rates on 2 consecutive days; for C2H6 the fluxes are affected by variable upwind conditions. Average emission factors were 12.0 and 11.4 gNH3 h−1 head−1 at 30 °C for feedlots with a combined capacity for ∼ 54 000 cattle and a dairy farm of ∼ 7400 cattle; the pooled rate of 11.8 ± 2.0 gNH3 h−1 head−1 is compatible with the upper range of literature values. At this emission rate the NH3 source from cattle in Weld County, CO (535 766 cattle), could be underestimated by a factor of 2–10. CAFO soils are found to be a significant source of NOx. The NOx source accounts for ∼ 1.2 % of the N flux in NH3 and has the potential to add ∼ 10 % to the overall NOx emissions in Weld County and double the NOx source in remote areas. This potential of CAFO to influence ambient NOx concentrations on the regional scale is relevant because O3 formation is NOx sensitive in the Colorado Front Range. Emissions of NH3 and NOx are relevant for the photochemical O3 and secondary aerosol formation.

Algorithmic Enhancement of Spectral Resolution of a LiNbO3 Waveguide-Based Miniature Fourier Transform Spectrometer.

In a recent report we demonstrated a miniature static Fourier transform spectrometer (FTS) that was implemented with a LiNbO3 (LN) waveguide electro-optic modulator (EOM) combined with the dispersion relation between its half-wave voltage and wavelength. The FTS was verified to be able to measure laser wavelength and for low-resolution spectroscopy. In this report, we successfully applied the resolution enhancement algorithm to the FTS, resulting in at least a three-fold increase in its spectral resolution without causing obvious distortion of the measured spectra. The algorithm method used is based on an autoregressive (AR) model, singular value decomposition (SVD), and forward-backward linear prediction (FBLP). The combination of these methods allows the FTS to remain a small size but to possess good spectral resolution, effectively mitigating the conflict between the small size and high resolution of the device. This study opens the way to development of high-resolution miniature FTS.

Broadening of CO2 lines in the 4.3 μm region by H2O

Transmission spectra of CO2 highly diluted in water vapor have been recorded at 50 and 95°C for four pressures between 0.02 and 0.1atm using a high resolution Fourier Transform spectrometer. The collisional (Lorentz) widths of many lines of the ν3 band (and of some of the ν3+ν2−ν2 hot band) have been retrieved from each spectrum through fits using Voigt line shapes. Our result are about 4% lower than the values recommended in a previous study but they confirm the relative variations of the line broadening on the rotational quantum numbers. We also provide the first determination of H2O-induced line shifts of CO2 lines.