Cavity Ring-down Spectroscopy Technique Research Articles

Advancement of Langmuir probe-based laser photo-detachment technique for negative ion density measurement in a high-power helicon plasma source.

In the pursuit of precise diagnostics for measuring negative ion density in a helicon plasma source (HPS), a new approach utilizing a radio frequency (RF) broadband transformer-based Langmuir probe is developed specifically for laser photo-detachment (LPD) analysis. This inductively coupled LPD technique is useful for high power RF systems in which capacitive RF noise is in the same scale as the pulsed photo-detachment signal. The signal acquired by this transformer-based probe is compared against the conventional Langmuir probe-based LPD technique, revealing a remarkable enhancement in signal fidelity through an improved signal-to-noise ratio (SNR) achieved by the RF broadband transformer methodology. In addition, the localized hydrogen negative ion density measurements obtained through this probe are harmoniously aligned with the line-averaged negative ion density derived from the cavity ringdown spectroscopy (CRDS) technique. These concurrence measurements highlight the RF broadband transformer-based approach's accuracy in capturing localized negative ion density during helicon mode operation in an HPS setup. Furthermore, the correlation of negative ion density values with RF input exhibits a consistent trend in tandem with background plasma density. Notably, both CRDS and LPD measurements ascertain negative ion densities ranging from ∼5 to 6×1016m-3 under an RF power of 500-700W and a pressure of 8 × 10-3mbar, all under the influence of a 55G axial magnetic field. These specific parameters represent the optimal operational configuration for effective negative ion production with the present experimental HPS setup. Due to its better SNR, the RF broadband transformer-based Langmuir probe emerges as a useful tool for LPD diagnostics, particularly in the presence of pervasive RF noise.

Dielectric metasurface-assisted cavity ring-down spectroscopy for thin-film circular dichroism analysis.

Chiral molecules show differences in their chemical and optical properties due to the different spatial arrangements of the atoms in the two enantiomers. A common way to optically differentiate them is to detect the disparity in the absorption of light by the two enantiomers, i.e. absorption circular dichroism (CD). However, the CD of typical molecules is very small, limiting the sensitivity of chiroptical analysis based on CD. Cavity ring-down spectroscopy (CRDS) is a well-known ultrasensitive absorption spectroscopic method for low-absorbing gas-phase samples because the multiple reflections of light in the cavity greatly increase the absorption path. By inserting a prism into the cavity, the optical mode undergoes total internal reflection (TIR) at the prism surface and the evanescent wave (EW) enables the absorption detection of condensed-phase samples within a very thin layer near the prism surface, called EW-CRDS. Here, we propose an ultrasensitive chiral absorption spectroscopy platform using dielectric metasurface-assisted EW-CRDS. We theoretically show that, upon linearly polarized and oblique incidence, the metasurface exhibits minimum scattering and absorption loss, introduces negligible polarization change, and locally converts the linearly polarized light into near fields with finite optical chirality, enabling CD detection with EW-CRDS that typically works with linearly polarized light. We evaluate the ring-down time in the presence of chiral molecules and determine the sensitivity of the cavity as a function of total absorption from the molecules. The findings open the avenue for the ultrasensitive thin film detection of chiral molecules using CRDS techniques.

Low-cost NO2 trace concentration measuring device using a broadband diode laser and a customized cavity

A low-cost NO2 trace concentration measuring device based on a cavity ring-down spectroscopy (CRDS) technique is presented. We theoretically and experimentally demonstrate that, if the nonresonant direct reflection is effectively isolated, an inexpensive broadband diode laser (LD) can excite multiple cavity modes and stabilize the cavity output signal with an optical feedback (OF) method. To improve the measuring accuracy for low concentrations of NO2 gases, we also design and machine a cavity with several mixing rooms. Several standard concentrations NO2 gases are used to test the apparatus. The results prove a great agreement between measured and theoretical values over the full range.

Effects of UV treatment on the properties of ultra-thin indium tin oxide films during growth and after deposition by cavity ring-down spectroscopy

In this work, an ultra-sensitive optical absorption technique based on Cavity Ring-Down Spectroscopy (CRDS) was employed to study the effects of UV treatment on the optical properties of ultra-thin indium tin oxide (ITO) films. The ITO films were submitted to UV treatment either after the deposition process or in-situ during the thin-film growth process. Different flow rates of oxygen in the vacuum chamber during film growth were also investigated. An ITO-coated glass substrate inserted in the CRDS cavity at a Brewster’s angle provided a ring-down time of about 1.6 µs, which enabled measurements of optical absorption loss as small as 3 × 10−6. To compare the effects of the UV film treatment, the CRDS technique was employed to measure the extinction coefficient for samples coated with and without the UV treatment. While the optical absorption data was being collected, the electrical resistivity was also simultaneously monitored. The post-deposition UV treatment was found to improve the optical transparency and the electrical performance of ITO film; the optical extinction coefficient of the ultra-thin ITO film is shown to decrease by about 24%. The in-situ UV treatment during growth is also shown to consistently increase the optical transparency of the ultra-thin ITO films and providing outstanding optical performance especially for high flow rates of oxygen during film growth. The electrical resistivity for oxygen flow rates in the range 0.6 - 1.4 sccm is also improved by the in-situ UV treatment, however it shows a sharp increase for oxygen flow rates beyond 1.4 sccm. The CRDS platform is demonstrated here to provide a highly accurate and sensitive methodology for measurement of minute optical absorption losses in ultra-thin films that typically cannot be precisely measured using other conventional spectrophotometric techniques.

Spectroscopic trace gas detection in air-based gas mixtures: Some methods and applications for breath analysis and environmental monitoring

Laser absorption spectroscopy as a powerful tool for detecting trace gases has been widely used in the monitoring of atmospheric greenhouse gases, pollutions, and respiration processes, including human breath analysis. The detection is based on the light absorption when it propagates through a medium. Most inorganic and organic molecules have characteristic absorption lines in the mid-infrared (mid-IR), which correspond to fundamental vibrational modes and in the near-IR (first overtones) presenting their absorption fingerprints. Here, we summarize the recent developments of the three techniques, namely, wavelength modulation spectroscopy (WMS), cavity ringdown spectroscopy (CRDS), and frequency comb spectroscopy (FCS), and describe their main features as well as possible applications, illustrated by recent experimental results. Emphasis is made on methane detection as applied to breath analysis and atmospheric monitoring. For the WMS technique, we consider local measurements with a multipass cell and also kilometer long open-path configurations for the near-IR and mid-IR spectral regions. The results of measurements of methane in exhaled breath with the CRDS technique in the near-IR are presented for a group of subjects of different ages. We consider various schemes of the FCS that enable fast broadband detection, including direct spectroscopy, dual FCS, and Vernier FCS, and review numerous applications of this approach that revolutionized the field of absorption spectroscopy. The current trends and possible future developments and applications are also discussed.

Infrared cavity ring-down spectroscopy for detecting non-small cell lung cancer in exhaled breath

Early diagnosis of lung cancer greatly improves the likelihood of survival and remission, but limitations in existing technologies like low-dose computed tomography have prevented the implementation of widespread screening programs. Breath-based solutions that seek disease biomarkers in exhaled volatile organic compound (VOC) profiles show promise as affordable, accessible and non-invasive alternatives to traditional imaging. In this pilot work, we present a lung cancer detection framework using cavity ring-down spectroscopy (CRDS), an effective and practical laser absorption spectroscopy technique that has the ability to advance breath screening into clinical reality. The main aims of this work were to (1) test the utility of infrared CRDS breath profiles for discriminating non-small cell lung cancer (NSCLC) patients from controls, (2) compare models with VOCs as predictors to those with patterns from the CRDS spectra (breathprints) as predictors, and (3) present a robust approach for identifying relevant disease biomarkers. First, based on a proposed learning curve technique that estimated the limits of a model’s performance at multiple sample sizes (10–158), the CRDS-based models developed in this work were found to achieve classification performance comparable or superior to like mass spectroscopy and sensor-based systems. Second, using 158 collected samples (62 NSCLC subjects and 96 controls), the accuracy range for the VOC-based model was 65.19%–85.44% (51.61%–66.13% sensitivity and 73.96%–97.92% specificity), depending on the employed cross-validation technique. The model based on breathprint predictors generally performed better, with accuracy ranging from 71.52%–86.08% (58.06%–82.26% sensitivity and 80.21%–88.54% specificity). Lastly, using a protocol based on consensus feature selection, three VOCs (isopropanol, dimethyl sulfide, and butyric acid) and two breathprint features (from a local binary pattern transformation of the spectra) were identified as possible NSCLC biomarkers. This research demonstrates the potential of infrared CRDS breath profiles and the developed early-stage classification techniques for lung cancer biomarker detection and screening.

Assessment of GHG Interactions in the Vicinity of the Municipal Waste Landfill Site—Case Study

Landfills have been identified as one of the major sources of greenhouse gas (GHG) emissions and as a contributor to climate change. Landfill facilities exhibit considerable spatial and temporal variability of both methane (CH4) and carbon dioxide (CO2) rates. The present work aimed to evaluate the spatial distribution of CH4 and CO2 and their δ13C isotopic composition originating from a municipal landfill site, to identify its contribution to the local GHG budget and the potential impact on the air quality of the immediate surroundings in a short-term response to environmental conditions. The objective was met by performing direct measurements of atmospheric CO2 and CH4 at the selected monitoring points on the surface and applying a binary mixing model for the determination of carbon isotopic ratios in the vicinity of the municipal waste landfill site. Air samples were collected and analysed for isotopic composition using flask sampling with a Picarro G2201-I Cavity Ring-Down Spectroscopy (CRDS) technique. Kriging and Inverse distance weighting (IDW) methods were used to evaluate the values at unsampled locations and to map the excess of GHGs emitted from the landfill surface. The large off-site dispersion of methane from the landfill site at a 500 m distance was identified during field measurements using isotopic data. The mean δ13C of the landfill biogas emitted to the surrounded atmosphere was −53.9 ± 2.2‰, which corresponded well to the microbial degradation processes during acetate fermentation in the waste deposits. The calculated isotopic compositions of CO2 (δ13C = −18.64 ± 1.75‰) indicate the domination of biogenic carbon reduction by vegetation surrounding the landfill. Finally, amounts of methane escaping into the air can be limited by the appropriate landfill management practices (faster covers active quarter through separation layer), and CH4 reduction can be achieved by sealing the cover on the leachate tank.

Comparison of isotope ratio mass spectrometry and cavity ring-down spectroscopy procedures and precision of the doubly labeled water method in different physiological specimens.

This study determines if saliva collection procedures for the doubly labeled water (DLW) method, used for measuring total energy expenditure (TEE), are comparable to urine and plasma collection. Both the cavity ring-down spectroscopy (CRDS) and isotope ratio mass spectrometry (IRMS) analyses techniques are compared. Saliva specimens were collected from participants for the DLW method. The specimens were collected under different conditions: after consumption of tap water, after chewing gum, and during exposure to conditions of high and low relative humidity. The isotopes in saliva were compared with simultaneous plasma and urine collection. TEE calculated from saliva and analyzed using CRDS was compared to that of plasma analyzed using IRMS. The within-individual variances were not significantly different between the saliva specimens (0.4‰) and plasma (0.3‰). After the oral dose of DLW, the saliva specimens displayed a shorter equilibration time to urine. When participants consumed 500 mL of tap water, the enrichment of saliva specimens reached a new plateau value faster than urine. Saliva collection exposed to high ambient humidity conditions was slightly less enriched as compared to low-humidity conditions while urine enrichment was unaffected. In contrast, whereas the within-individual effects of gum chewing during saliva collection on 18 O were unaffected, the abundance of 2 H in saliva was slightly lower after chewing the gum. The within-individual difference between TEE calculated from saliva and that calculated from plasma analyzed using IRMS did not differ from zero, and the standard deviation was not different from that predicted by a propagation of error analysis based on analytical error alone. Our findings support using saliva specimens for the DLW method. The analysis of plasma and urine, however, requires reducing the memory effect resulting from contaminants. Also, it should be performed in a manner that minimizes exposure to air where specimens may be exposed to evaporation or contamination from water vapor during sampling.

Assessment of urban CO2 budget: Anthropogenic and biogenic inputs

Understanding combinations of biogenic and anthropogenic inputs, and dynamics of carbon dioxide (CO2) fluxes in the urban environment is critical to investigate feedback between carbon cycle and climate system. The main objectives of this study are to analyse the role and interactions of anthropogenic and biogenic CO2 emission sources and sinks in the total carbon cycle within city limits, determine processes controlling urban ecosystem-atmosphere gas exchange under varying environmental conditions with a high temporal and spatial resolution.The research addressing this issue was carried out in Wroclaw, the fourth largest city in Poland, throughout the year from July 2017 to August 2018. Chamber measurements of surface-atmosphere CO2 and CO2 net fluxes across various types of land cover (grassland, city park and arable land) provided the basis for assessing biogenic component of the urban C budget. In particular, the approach applied Picarro CRDS (cavity ring-down spectroscopy) technique with model G2201-I. Anthropogenic C fluxes were reported based on local statistical datasets and existing emission inventories, as well as the Low Emission Economy Plan from the Wroclaw urban area.The findings showed that biogenic fluxes, over the course of one year, accounted for about 16.3% (RESP = 0.68 Mton CO2·yr−1) of the total city carbon emissions for Wroclaw, being entirely compensated by local vegetation C uptake (GEE = −0.89 Mt. CO2·yr−1). Compared to fluxes given by the previously reported emission inventory, the net biogenic CO2 sink (NEE) of 0.201 Mt. CO2·yr−1 was substantially greater, which indicates that the biosphere in Wroclaw is a net biogenic C sink. The average annual C budget in Wroclaw, obtained from the investigation of sectoral emission trends including those related to energy, road traffic, industrial processes, and other fluxes (human respiration), reaches about 3.87 kg C·m−2·yr−1.

Molecular transition frequencies of CO2 near 1.6 µm with kHz-level uncertainties

We present measurements of molecular transition frequencies based on the comb-locked cavity ring-down spectroscopy technique, reporting vacuum transition frequencies of Doppler-broadened 12C16O2 in the 1.6 μm region for the (30012) ← (00001) and (30013) ← (00001) bands with an average combined standard uncertainty of ≈ 1 kHz. A global four-state model was fit to these data and literature values to provide spectroscopic parameters and a best-case fit precision of 4 kHz. We identified and quantified an interaction between the (30012) and (33301) states which was manifest as an observed Fermi-resonance-type perturbation in the (30012) ← (00001) band. This interaction was accounted for in the global fit, substantially reducing systematic uncertainties in the spectroscopic parameters of the (30012) state and altering many predicted transition frequencies in the (30012) ← (00001) band by 1 MHz or more relative to literature values. The accurate transition frequencies reported here may be considered as SI-traceable reference data for diverse applications including remote sensing and telecommunications.

GIS-Based Approach to Spatio-Temporal Interpolation of Atmospheric CO2 Concentrations in Limited Monitoring Dataset

Understanding the magnitude and distribution of the mixes of the near-ground carbon dioxide (CO2) components spatially (related to the surface characteristics) and temporally (over seasonal timescales) is critical to evaluating present and future climate impacts. Thus, the application of in situ measurement approaches, combined with the spatial interpolation methods, will help to explore variations in source contribution to the total CO2 mixing ratios in the urban atmosphere. This study presents the spatial characteristic and temporal trend of atmospheric CO2 levels observed within the city of Wroclaw, Poland for the July 2017–August 2018 period. The seasonal variability of atmospheric CO2 around the city was directly measured at the selected sites using flask sampling with a Picarro G2201-I Cavity Ring-Down Spectroscopy (CRDS) technique. The current work aimed at determining the accuracy of the interpolation techniques and adjusting the interpolation parameters for estimating the magnitude of CO2 time series/seasonal variability in terms of limited observations during the vegetation and non-vegetation periods. The objective was to evaluate how different interpolation methods will affect the assessment of air pollutant levels in the urban environment and identify the optimal sampling strategy. The study discusses the schemes for optimization of the interpolation results that may be adopted in areas where no observations are available, which is based on the kriging error predictions for an appropriate spatial density of measurement locations. Finally, the interpolation results were extended regarding the average prediction bias by exploring additional experimental configurations and introducing the limitation of the future sampling strategy on the seasonal representation of the CO2 levels in the urban area.

Impact of Residual Water Vapor on the Simultaneous Measurements of Trace CH4 and N2O in Air with Cavity Ring-Down Spectroscopy

Methane (CH4) and nitrous oxide (N2O) are among the most important atmospheric greenhouse gases. A gas sensor based on a tunable 7.6 μm continuous-wave external-cavity mode-hop-free (EC-MHF) quantum cascade laser (from 1290 to 1350 cm−1) cavity ring-down spectroscopy (CRDS) technique was developed for the simultaneous detection of CH4 and N2O in ambient air with water vapor (H2O) mostly removed via molecular sieve drying to minimize the impact of H2O on the simultaneous measurements. Still, due to the broad and strong absorption spectrum of H2O in the entire mid-infrared (mid-IR) spectral range, residual H2O in the dried ambient air due to incomplete drying and leakage, if not properly accounted for, could cause a significant influence on the measurement accuracy of the simultaneous CH4 and N2O detection. In this paper, the impact of residual H2O on the simultaneous CH4 and N2O measurements were analyzed by comparing the CH4 and N2O concentrations determined from the measured spectrum in the spectral range from 1311 to 1312.1 cm−1 via simultaneous CH4 and N2O measurements and that determined from the measured spectrum in the spectral range from 1311 to 1313 cm−1 via simultaneous CH4, N2O, and H2O measurements. The measured dependence of CH4 and N2O concentration errors on the simultaneously determined H2O concentration indicated that the residual H2O caused an under-estimation of CH4 concentration and over-estimation of N2O concentration. The H2O induced CH4 and N2O concentration errors were approximately linearly proportional to the residual H2O concentration. For the measurement of air flowing at 3 L per min, the residual H2O concentration was stabilized to approximately 14 ppmv, and the corresponding H2O induced errors were −1.3 ppbv for CH4 and 3.7 ppbv for N2O, respectively.

Validation of spectroscopic data in the 1.27 µm spectral region by comparisons with ground-based atmospheric measurements

Various spectroscopic data for absorption lines due to the magnetic dipole transitions of the a1Δg−X3Σg−(0−0) band of O2 centered at 1.27 µm are tested by comparison with high-resolution ground-based atmospheric measurements recorded by Fourier Transform Spectrometers at Park Falls and Caltech (USA). This band is of importance for atmospheric remote sensing since it will be used (together with the O2 A-band near 760 nm) by the passive short wave infrared spectrometer onboard the MicroCarb satellite mission (i.e. this band includes the B4 band of MicroCarb, from about 7800 cm−1 to 7912 cm−1) for the determination of surface pressure and atmospheric aerosols. Spectroscopic data of the HITRAN2016 and GEISA2015 databases as well as those from recent laboratory studies are here used in a radiative transfer code to simulate atmospheric transmissions under the conditions of the measurements. Comparisons are made for different solar zenith angles and for the whole B4 spectral range considered by MicroCarb. Spectroscopic data of water vapor are also tested by considering both relatively dry and humid atmospheric conditions. Averaging the “calculated-observed” residuals over numerous recordings made for close values of the solar zenith angle and humidity enables reduction of the uncertainties due to the radiometric noise of the instrument and to the imperfect description of the atmospheric state. This enables the detection of systematic differences in the spectral residuals caused by small changes in spectroscopic data. The results show that the spectroscopic parameters in the HITRAN2016 and GEISA2015 databases lead to large residuals while data of two recent laboratory studies, obtained from spectra measured with the cavity ring-down spectroscopy technique using the speed-dependent Nelkin-Ghatak profile lead to much better agreement with atmospheric measurements. Significant residuals are noted for water vapor absorption lines simulated using parameters provided by both the HITRAN and GEISA databases.

Cavity ring-down spectroscopy and its applications to environmental, chemical and biomedical systems

In recent times, the need for high-sensitive detection has grown drastically as more and more applications of molecular sensing are being explored. Cavity ring-down spectroscopy (CRDS) is one of the new age and robust techniques which has revolutionized the field of molecular detection, particularly gas-phase analysis. In this short review, we have first provided a brief introduction to the working principles and system requirements of CRDS. Subsequently, we have described the various applications of CRDS in detail, including environmental monitoring, biomedical analysis specifically human breath analysis and reaction chemistry. We have also focused on the importance of isotope analysis and their accurate measurements using CRDS and its variants. Furthermore, we described the use of CRDS in the condensed phase through evanescent wave (EW) coupled CRDS and discussed its applications in the investigation of interfacial kinetics, thin-film and biological detection. We also outlined certain variants of CRDS to give a glimpse of the different techniques which have emerged with CRDS. Hence, in this mini-review, we aimed to illustrate the use of the CRDS technique in various interdisciplinary fields. SYNOPSIS The review provides an overview of the cavity ring-down spectroscopy (CRDS) technique and its variants. The applications of CRDS in environmental monitoring, biomedical diagnostics, reaction chemistry and isotope analysis for both the gas phase and the condensed phase molecules have been discussed.

Development of an in situ analysis system for methane dissolved in seawater based on cavity ringdown spectroscopy.

This paper reports the development of a compact in situ real-time concentration analysis system for methane dissolved in seawater by using a continuous-wave cavity ringdown spectroscopy (CRDS) technique. The miniaturized design of the system, including optical resonance cavity and control and data acquisition-analysis electronics, has a cylindrical dimension of 550 mm in length and 100 mm in diameter. Ringdown signal generation, data acquisition and storage, current driver, and temperature controller of the diode laser are all integrated in the miniaturized system circuits, with an electrical power consumption of less than 12 W. Fitting algorithms of the ringdown signal and spectral line are implemented in a digital signal processor, which is the main control chip of the system circuit. The detection sensitivity for methane concentration can reach 0.4 ppbv with an approximate averaging time of 240 s (or 4 min). Comparing the system's measurement of ambient air against a high-quality commercial CRDS instrument has demonstrated a good agreement in results. In addition, as a "proof of concept" for measuring dissolved methane, the developed instrument was tested in an actual underwater environment. The results showed the potential of this miniaturized portable instrument for in situ gas sensing applications.

High-resolution investigation of temperature and pressure-induced spectroscopic parameters of 13C-isotopomer of CH4 in the ν4 band using cavity ring-down spectroscopy

Measurements of high-resolution spectroscopic line parameters in the ν4 fundamental band of 13CH4 isotopomer of methane (CH4) occurring at 7.5 µm mid-IR spectral region are important for the studies of remote sensing of the Earth and planetary atmospheres at different conditions. Here, a continuous wave (cw) external-cavity quantum cascade laser (EC-QCL) coupled with cavity ring-down spectroscopy (CRDS) technique has been employed to probe the ν4 (F2) fundamental band of 13CH4 isotopomer. We recorded the high-resolution CRD spectra of 22 interference-free transition lines in the P, Q and R-branches of 13CH4 and subsequently determined the symmetry-specific line parameters such as line-intensity, pressure broadening coefficients along with temperature dependence of broadening coefficients of the transition lines. Our study thus provides more accurate experimental evidences of the high-resolution spectral features of 13CH4 isotopomer with rotational symmetry (A, E, F) for the ν4 (F2) band at wavelengths near 7.5 µm for a range of temperatures.

A Prototype Evanescent Wave-Coupled Cavity Ring-down Spectrometer for Probing Real-Time Aggregation Kinetics of Gold and Silver Nanoparticles.

We report on the development of a simple, linear optical cavity-based system combining evanescent wave (EW) with high-sensitive cavity ring-down spectroscopy (CRDS) technique using a diode laser at 644 nm and a right-angled prism for evanescent field generation on prism surface. We characterized the setup in detail and achieved an optimum ring-down time of 159.4 ns and a minimum absorption coefficient of αmin = 1.67 × 10-6 cm-1. We utilized this setup to investigate the salt-induced aggregation kinetics of gold (Au) and silver (Ag) nanoparticles (NPs) at the prism interface with high-sensitivity. We evaluated the extinction rates on the surface due to Au and Ag NPs aggregation and examined the variations due to their respective concentrations. To demonstrate the applicability of the developed EW-CRDS prototype setup to different molecular systems, we investigated the urease-bound aggregation kinetics of the Au and Ag NPs which has not been explored earlier by this linear cavity geometry. We finally illustrated the aggregation dynamics through surface imaging, thus demonstrating an alternative analytical approach to monitor interfacial phenomena using EW-CRDS technique.

Precision spectroscopy of HD at 1.38μm

The technique of frequency-stabilized comb-calibrated cavity ring-down spectroscopy was used to observe the weak R(1) transition of the deuturated molecular hydrogen (HD) first overtone band in the near infrared. Like molecular hydrogen, HD is a benchmark system to test quantum electrodynamics, looking for new physics beyond the standard model. The spectral line shape was measured with an extremely high fidelity at relatively low gas pressures. The use of a very refined line-shape model allowed us to retrieve the unperturbed line-center frequency of the R(1) line with a global uncertainty of about 100 kHz ($\ensuremath{\delta}\ensuremath{\nu}/\ensuremath{\nu}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$). Our value may solve the ambiguity that recently emerged for this line from a pair of sub-Doppler experiments. We also determined other parameters of interest, such as the line strength, the transition dipole moment, and the pressure-broadening coefficient.

Reflectivity measurement of fiber Bragg grating by cavity ring-down spectroscopy technique

A novel method to measure the reflectivity of fiber Bragg grating (FBG) is proposed and demonstrated experimentally. A kind of fiber Bragg grating FP(FBG-FP) cavity is constructed, which includes a couple of identical uniform FBG as the cavity mirrors linked by a piece of Er-doped fiber. The response of FBG-FP cavity to pulsed laser injection is demonstrated theoretically and experimentally, and the effect of the reflectivity of FBG to the output of FBG-FP cavity is discussed. The reflectivity of FBG is achieved by measuring the transmission loss of FBG-FP cavity with cavity ring-down spectroscopy technique. A general relationship between the reflectivity of FBG and ring-down time of FBG-FP cavity is derived. It’s demonstrated that the output loss of FBG-FP cavity increases as the reflectivity of FBG decreases, the stable light intensity in the cavity decreases fastly, and the ring-down time of the FBG-FP cavity decreases, the operating wavelength of FBG is a function of its temperature. The results demonstrate a new concept of the measurement of reflectivity of FBG and the technical feasibility.

Quantification of carbon dioxide and methane emissions in urban areas: source apportionment based on atmospheric observations

Anthropogenic emissions of carbon dioxide (CO2) and methane (CH4) in the atmosphere constitute an important component of the related carbon budget. The main source of anthropogenic CO2 is burning of fossil fuels, especially in densely populated areas. Similar emissions of CH4 are associated with the agricultural sector, coal mining, and other human activities, such as waste management and storage and natural gas networks supplying methane to large urban, industrial centers. We discuss several methods aimed at characterizing and quantifying atmospheric loads and fluxes of CO2 and CH4 in Krakow, the second largest city in Poland. The methods are based on atmospheric observations of mixing ratios as well as isotopic composition of the investigated gases. Atmospheric mixing ratios of CO2 and CH4 were measured using gas chromatography (GC) and cavity ring-down spectroscopy (CRDS). The isotopic composition of CO2 and CH4 was analyzed using isotope ratio mass spectrometry (IRMS), accelerator mass spectrometry (AMS), and CRDS techniques. These data, combined with auxiliary information characterizing the intensity of vertical mixing in the lower atmosphere (height of the nocturnal boundary layer [NBL] and atmospheric 222Rn concentration), were further used to quantify emission rates of CO2 and CH4 in the urban atmosphere of Krakow. These methods provide an efficient way of quantifying surface emissions of major greenhouse gases originating from distributed sources, thus complementing the widely used bottom-up methodology based on emission statistics.