Resolution Fourier Transform Spectrometer Research Articles

On-chip interrogator based on Fourier transform spectroscopy.

In this paper, the design and the characterization of a novel interrogator based on integrated Fourier transform (FT) spectroscopy is presented. To the best of our knowledge, this is the first integrated FT spectrometer used for the interrogation of photonic sensors. It consists of a planar spatial heterodyne spectrometer, which is implemented using an array of Mach-Zehnder interferometers (MZIs) with different optical path differences. Each MZI employs a 3×3 multi-mode interferometer, allowing the retrieval of the complex Fourier coefficients. We derive a system of non-linear equations whose solution, which is obtained numerically from Newton's method, gives the modulation of the sensor's resonances as a function of time. By taking one of the sensors as a reference, to which no external excitation is applied and its temperature is kept constant, about 92% of the thermal induced phase drift of the integrated MZIs has been compensated. The minimum modulation amplitude that is obtained experimentally is 400 fm, which is more than two orders of magnitude smaller than the FT spectrometer resolution.

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.

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.

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.

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.

Measurements of H2O broadening coefficients of infrared methane lines

H2O-broadening and shifting coefficients of 76 ro-vibrational transitions of methane in the mid-and near-infrared regions were measured for the first time. For this, eight spectra of methane diluted in water vapor were recorded with a high resolution Fourier Transform spectrometer for pressures ranging from 20 to 80Torr and at 323 and 367K. Line broadening and shifting coefficients were retrieved from the measured spectra through fits using Voigt profiles. Values at room temperature (296K) were then deduced and compared with those of dry air. The results show that H2O-broadenings of methane lines are, on average, 34% larger than those for dry air.

End-face reflected LiNbO3 waveguide based stationary miniature Fourier transform spectrometer with two-fold enhanced spectral resolution

Miniature Fourier transform spectrometer (FTS) has attracted considerable interest because of its important application in spaceborne spectroscopy and as a portable analytical tool for rapid on-site chemical/biochemical detection. In a previous paper, a stationary miniature FTS constructed with an electro-optic (EO) modulator of a LiNbO3 (LN) waveguide Mach-Zehnder interferometer (MZI) containing push-pull electrodes was demonstrated. This stationary miniature FTS is operated in the near-infrared region with either nonlinear or linear scanning of the modulating voltage. The simple and mirrorless structure renders the device compact, vibration resistant, and cost-effective. However, the spectral resolution of the proposed prototype FTS was not satisfactory due to the limited optical pathlength difference (OPD), thereby limiting the device application. To improve its spectral resolution, the factors affecting the spectral resolution of the LN waveguide-based FTS is investigated in this paper. Findings show that the spectral resolution is inversely proportional to the maximum OPD, which is proportional to the length of the EO modulating region. A simple method for two-fold enhancement of the spectral resolution of the FTS is proposed based on the end-face reflection in LN waveguide interferometer. With the end-face reflection geometry the guided mode can propagate back and forth in the LN waveguide, making the mode passing through the EO modulating region twice and consequently leading to two times enhancement of the OPD. Therefore, the end-face reflection geometry enables to double the maximum OPD of the interferometer without increasing the device size and thus to offer the device a two-fold enhanced spectral resolution according to the equation for FTS resolution. Two prototypes of FTS with and without the end-face reflection structure are prepared using the same commercial LN waveguide EO modulator. The spectral resolutions in terms of the full-width at half maximum (FWHM) at different wavelengths for the two prototypes of FTS are measured using a series of distributed feedback lasers. The FWHM measured at a specific wavelength with the end-face reflection structure is half as large as that obtained without the end-face reflection structure. Experimental results are in excellent agreement with the theoretical data, demonstrating the applicability of the end-face reflection method to the spectral resolution enhancement.

Electron interactions with positively and negatively multiply charged biomolecular clusters

Interactions of positively and negatively multiply charged biomolecular clusters with low-energy electrons, from ∼ 0 up to 50 eV of electron energy, were investigated in a high resolution Fourier-Transform Ion Cyclotron Resonance mass spectrometer equipped with an electrospray ionisation source. Electron-induced dissociation reactions of these clusters depend on the energy of the electrons, the size and the charge state of the cluster. The positively charged clusters [Mn+2H]2+ of zwitterionic betaines, M = (CH3)2XCH2CO2 (X = NCH3 and S), do capture an electron in the low electron energy region (< 10 eV). At higher electron energies neutral evaporation from the cluster becomes competitive with Coulomb explosion. In addition, a series of singly charged fragments arise from bond cleavage reactions, including decarboxylation and CH3 group transfer, due to the access of electronic excited states of the precursor ions. These fragmentation reactions depend on the type of betaine (X = NCH3 or S). For the negative dianionic clusters of tryptophan [Trp9-2H]2−, the important channel at low electron energies is loss of a neutral. Coulomb explosion competes from 19.8 eV and dominates at high electron energies. A small amount of [Trp2–H–NH3]− is observed at 21.8 eV.

The rotational temperatures of Silicon hydride isotopomer lines in sunspot umbrae

The published sunspot spectrum obtained with National Solar Observatory/Kitt Peak laboratory’s high signal to noise ratio telescope and high resolution Fourier Transform Spectrometer were used for the study. A search was made for the prominent lines of the (0, 0) and (1, 1) A 2Δ − X 2Π for Silicon hydride isotopomers which lie between 23500 cm −1 and 24500 cm −1. The presence of lines from the (0, 0) and (1, 1) A 2Δ − X 2Π transition of SiH molecule coincided with the sunspot umbral lines suggest that Silicon hydride appears to be a non-negligible component of sunspot umbrae. However, the presence of A 2Δ − X 2Π (0, 0) and (1, 1) bands of SiD was found to be doubtful because of the lack of number of well identified lines in sunspot umbral spectra. Equivalent widths have been measured for well-resolved lines and, thereby, the rotational temperatures have been estimated for the band systems for which the presence is confirmed.

A detailed comparison of spectral line intensities with plane and hollow cathodes in a Grimm type glow discharge source

The hollow cathode discharge was investigated and compared with the plane cathode discharge in a Grimm-type source studying a large number of lines with iron and titanium as cathode materials in argon plasma. The high resolution Fourier Transform Spectrometer (FTS) at Imperial College (IC) was used for recording spectra. The intensity changes for atomic and ionic lines of iron, titanium and argon are discussed, whereby clear trends in intensity increase and reduction according to the excitation energies of the emission lines are identified. Differences in line profiles in spectra using the hollow and plane cathodes were detected. The comparison between the hollow (10–15 mm deep) and shallow (2 mm deep) cathodes gives additional information about the depth influence on the hollow cathode effect. The extended study presented in this paper is a further step towards a better understanding of the discharge processes in the glow discharge.

The BΠ1 state of NaCs: High resolution laser induced fluorescence spectroscopy and potential construction

The lowest (1)Pi state of the NaCs molecule, the B(1)(1)Pi state, was studied using a dye laser for inducing fluorescence that was resolved by a high resolution Fourier-transform spectrometer. The presence of argon buffer gas yielded rich rotational relaxation spectra allowing to enlarge the data set for the B(1)(1)Pi state, to obtain Lambda-splittings and to reveal numerous local perturbations. 543 weakly perturbed energy levels for rotational quantum numbers from J(')=5 to 168 and vibrational quantum numbers from v(')=0 to 25, which cover about 87% of the potential well depth, were used for a direct pointwise fit of the potential energy curve applying the inverted perturbation approach method. The resulting potential reproduces the term values for v(')=0-7 with an experimental accuracy of about 0.01-0.02 cm(-1), whereas for v(')=8-25 the deviations increase due to the perturbations, going to the order of 1 cm(-1); an extrapolation is made to the dissociation asymptote.

Carbon dioxide column abundances at the Wisconsin Tall Tower site

We have developed an automated observatory for measuring atmospheric column abundances of CO2 and O2 using near‐infrared spectra of the Sun obtained with a high spectral resolution Fourier Transform Spectrometer (FTS). This is the first dedicated laboratory in a new network of ground‐based observatories named the Total Carbon Column Observing Network. This network will be used for carbon cycle studies and validation of spaceborne column measurements of greenhouse gases. The observatory was assembled in Pasadena, California, and then permanently deployed to northern Wisconsin during May 2004. It is located in the heavily forested Chequamegon National Forest at the WLEF Tall Tower site, 12 km east of Park Falls, Wisconsin. Under clear sky conditions, ∼0.1% measurement precision is demonstrated for the retrieved column CO2 abundances. During the Intercontinental Chemical Transport Experiment–North America and CO2 Boundary Layer Regional Airborne Experiment campaigns in summer 2004, the DC‐8 and King Air aircraft recorded eight in situ CO2 profiles over the WLEF site. Comparison of the integrated aircraft profiles and CO2 column abundances shows a small bias (∼2%) but an excellent correlation.

Infra‐red FTS measurements of CH4, N2O, O3, HNO3, HCl, CFC‐11 and CFC‐12 from the MANTRA balloon campaign

The Middle Atmosphere Nitrogen TRend Assessment (MANTRA) campaign is intended to address the question of whether possible changes in the mid‐latitude nitrogen budget can account for discrepancies between predicted and observed ozone loss at these latitudes. In addition, MANTRA seeks to establish consistency between old and new measurement techniques. We report here the results of infra‐red spectral measurements made with a high resolution Fourier Transform Spectrometer (FTS) during the August 1998 MANTRA flight. Vertical mixing ratio profiles of methane, nitrous oxide, ozone, nitric acid, hydrogen chloride, CFC‐11 and CFC‐12 are presented. The error estimates on the retrieved values are larger than typical for this type of measurement, due mostly to instrument problems that occurred during flight. In addition, the CFC values are larger than expected. Possible reasons for this are discussed.

Retrievals for the atmospheric chemistry experiment Fourier-transform spectrometer

SCISAT-1, also known as the Atmospheric Chemistry Experiment, is a satellite mission for remote sensing of the Earth's atmosphere, launched on 12 August 2003. The primary instrument on the satellite is a 0.02 cm(-1) resolution Fourier-transform spectrometer operating in the mid-IR (750-4400 cm(-1)). We describe the approach developed for the retrieval of atmospheric temperature and pressure from the troposphere to the lower thermosphere as well as the strategy for the retrievals of volume-mixing ratio profiles of atmospheric species.

Atmospheric Chemistry Experiment (ACE): Mission overview

SCISAT‐1, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission for remote sensing of the Earth's atmosphere. It was launched into low Earth circular orbit (altitude 650 km, inclination 74°) on 12 Aug. 2003. The primary ACE instrument is a high spectral resolution (0.02 cm−1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 μm (750–4400 cm−1). The satellite also features a dual spectrophotometer known as MAESTRO with wavelength coverage of 285–1030 nm and spectral resolution of 1–2 nm. A pair of filtered CMOS detector arrays records images of the Sun at 0.525 and 1.02 μm. Working primarily in solar occultation, the satellite provides altitude profile information (typically 10–100 km) for temperature, pressure, and the volume mixing ratios for several dozen molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S. This paper presents a mission overview and some of the first scientific results.

Branching Ratio Measurements for Ge I and Ge II

A high resolution Fourier-transform spectrometer was used to measure branching ratios for 5s-5p, 4d-6s, 6s-6p, 5s-6p, 4d-6p, 4d-7p transitions in neutral germanium and 4d-4f, 4p 2 -4f, 4p2 -5p, 5s-5p, 4p2 -4p, 4d-6p, 4p 2 -6p, 4d-5f, 4p2 -5f, 4d-7p, 4p2 -7p of singly ionized germanium. Measurements were performed with a hollow cathode as a light source. Spectral lines intensities were measured in a spectral range from 200 to 3500 nm. Absolute transition probabilities for some Ge II lines were obtained using experimental lifetime values for the 4s 2 4f 2F5/2, 2F7/2 and 482 5p 2p1/2, 2P32 levels. Our results for transitions in neutral germanium show that some infrared lines, so far overlooked in spectral analyses, give a strong contribution to Ge spectrum. Our transition probabilities obtained for Ge II lines are lower than all other experimental and theoretical results, in one case even by an order of magnitude.

Resolution and Signal to Noise Ratios in Fourier Transform Spectroscopy

Abstract The effect that resolution has on the calculated signal-to-noise ratio (SNR) of an emission line measured in the visible by a high resolution Fourier Transform Spectrometer is discussed. The resolution varies from a physically resolved profile (0.02 cm−1) to a shape dominated by the instrumental function (2 cm−1). The effects that incorporating inaccurate signal-to-noise ratios has on figures of merit for a FT-UV/VIS system is discussed.

Observation of characteristic, polarity-dependent, Doppler shifts from neutral species in the positive column of a discharge plasma

Doppler shifts of emissions from neutral species have been observed for the first time in a positive column of a dc glow discharge. For a detailed study, we have chosen emissions of H2 and its isotopomers in a hydrogen discharge using a high resolution Fourier-transform spectrometer. Experimetal evidence supports the conclusion that the observed shifts derive from momentum transfer during electron-molecule collisions that produce the emissive excited states. A theoretical model has been formulated to relate Doppler shifts to the drift velocity of electrons in the discharge as well as to the momentum transfer of electron-molecule collisions. Predictions based on the model are consistent with the experimental observations.

Accurate mirror movement design for interferometry

A highly accurate and precise mirror displacement system for a Michelson interferometer is described. The system uses gravity in place of mechanical methods to achieve movement over large distances. This modification simplifies one of the major design constraints in building a high resolution Fourier Transform Spectrometer for the visible, ultraviolet and vacuum ultraviolet.

Multiphonon absorption in InP

IR absorption measurements have been made using a high resolution Fourier-transform spectrometer on crystals of InP in the region of (400–4000)cm -1 at temperature between 11–300k. Very weak, new absorption bands, their absorption coefficients are of ∼10 -1cm -1, were observed at 996cm -1, 965cm -1, 932cm -1, 838cm -1, 776cm -1, 742cm -1, 718cm -1, 590cm -1, 558cm -1 and 538cm -1. The frequencies and temperature dependence of these weak bands indicate that they arise from three-phonon processes and appropriate assignments are given. We observed using IR absorption technique for the first time the boron contamination in LEC(B 2O 3)-CZ InP crystal. The content of boron contamination has been proved to be of ∼10 16cm -3.