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We realized a UV laser spectrometer at 253.7 nm for Doppler broadening thermometry on the 1 S 0 − 3 P 1 intercombination line in mercury vapors. Our setup is based on the two-stage duplication of a 1014.8 nm diode laser in a fiber-coupled periodically poled lithium niobate waveguide crystal and a beta-barium borate crystal in enhancement cavity, and we exploit injection locking of a 507.4 nm diode laser to boost the available optical power after the first duplication. Our setup addresses spectroscopic features that allow the thermodynamic temperature determination of the atomic sample from the absorption profile with 10 − 6 accuracy. The realized UV laser source has 1 × 10 − 4 relative intensity stability, Gaussian shape, and over 10 GHz mode-hop-free tunable range. These features are crucial for the practical realization of the kelvin in the new International System of Units through a spectroscopic technique.

Absolute measurements of water vapor densities have been carried out using a new concept of frequency-stabilized cavity ring-down spectroscopy. Our scheme is based on the use of a pair of phase-locked extended cavity diode lasers, emitting at 1.39 $\mu$m, one of them being locked to a self-referenced optical frequency comb synthesizer. An intrinsically stable high-finesse optical cavity that tracks the laser frequency scans has been used. High quality, comb-calibrated, absorption spectra have been recorded in a certified H$_2$O/N$_2$ gas mixture at different gas pressures, in coincidence with the 2$_{1,2}→$1$1,1$transitionoftheH$2^{18}$O $\nu_1$+$\nu_3$ band. Water vapor mole fractions have been determined with a statistical uncertainty of 0.6 \%. Systematic deviations have been identified and carefully quantified, thus leading to an overall uncertainty of 0.8 \%.

Modulation transfer spectroscopy is used to demonstrate absolute frequency stabilization of an 8.6-µm-wavelength quantum cascade laser against a sub-Doppler absorption of the C H F 3 molecule. The obtained spectral emission properties are thoroughly characterized through a self-referenced optical frequency comb, stabilized against either a GPS-disciplined Rb clock or a 1.54-µm Er-fiber laser locked to a high-finesse ultra-low-expansion optical cavity. Fractional long-term stability and accuracy at a level of 4 × 10 − 12 (at 100 s) and 3 × 10 − 10 , respectively, are demonstrated, along with an emission linewidth as narrow as 10 kHz for observation times of 0.1 s.

We report on complementary tests and measurements regarding our recent determination of the Boltzmann constant, kB, by means of Doppler broadening thermometry, also providing additional information as compared to previous articles. A revised uncertainty budget is illustrated, including some new components that were ignored in previous spectroscopic experiments, and better quantifying other components that were estimated to be negligible. In particular, we consider the relativistic Doppler effect, the perturbation caused by the finite bandwidth of the detection system and the influence of the spontaneous emission content of the probe laser. These new components do not increase the global uncertainty which still amounts to 24 ppm. Our value for the Boltzmann constant is 1.380 631 (33) × 10−23 J K−1, which is the best determination reported so far by using an optical method.

Line intensities for carbon dioxide are measured with a novel spectroscopic approach, assisted by an optical frequency comb synthesizer for frequency calibration purposes. The main feature of the spectrometer consists in the exploitation of optical feedback from a V-shaped high-finesse optical resonator to effectively narrow a distributed feedback diode laser at the wavelength of 2 m. Lasergas interaction takes place inside an isothermal cell, which is placed on the transmission from the cavity. High quality, self-calibrated, absorption spectra are observed in pure CO2 samples at different gas pressures, in coincidence with three lines of the R-branch of the nu1 + 2nu2 + nu3 band. Line intensities are determined using a global fitting approach in which a manifold of spectra are simultaneously analyzed across the range of pressures between 5 and 100 Torr, sharing a restricted number of unknown parameters. Various sources of uncertainty have been identified and carefully quantified, thus leading to an overall uncertainty ranging between 0.17% and 0.23%. The measured values are in a very good agreement with recent ab initio predictions.

A method of performing comb-assisted spectroscopy in the 2- and 4-mu m wavelength regions is deeply discussed and applied to the spectroscopic characterization of absorption lines of a CO2 gas sample. The method relies on the use of a sum-frequency-generation scheme to relate the frequency of the probing laser to a frequency comb provided by an Er:fiber femtosecond oscillator. Controlled scans of the comb repetition frequency allow highly accurate and reproducible acquisition of the absorption spectra, which are used for subsequent retrieval of the underlying spectroscopic parameters.

Small molecules exhibit characteristic ro-vibrational transitions in the near- and mid-infrared spectral regions, which are strongly influenced by isotopic substitution. This gift of nature has made it possible to use laser spectroscopy for the accurate analysis of the isotopic composition of gaseous samples. Nowadays, laser spectroscopy is clearly recognized as a valid alternative to isotope ratio mass spectrometry. Laser-based instruments are leaving the research laboratory stage and are being used by a growing number of isotope researchers for significant advances in their own field of research. In this review article, we discuss the current status and new frontiers of research on high-sensitivity and high-precision laser spectroscopy for isotope ratio analyses. Although many of our comments will be generally applicable to laser isotope ratio analyses in molecules of environmental importance, this paper concerns itself primarily with water and carbon dioxide, two molecules that were studied extensively in our respective laboratories. A complete coverage of the field is practically not feasible in the space constraints of this issue, and in any case doomed to fail, considering the large body of work that has appeared ever since the review by Kerstel in 2004 (Handbook of Stable Isotope Analytical Techniques, Chapt. 34, pp. 759–787).

Frequency fluctuations of an optical frequency standard at 1.39 µm have been measured by means of a highly-sensitive optical frequency discriminator based on the fringe-side transmission of a high finesse optical resonator. Built on a Zerodur spacer, the optical resonator exhibits a finesse of 5500 and a cavity-mode width of about 120 kHz. The optical frequency standard consists of an extended-cavity diode laser that is tightly stabilized against the center of a sub-Doppler H(2) (18)O line, this latter being detected by means of noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. The emission linewidth has been carefully determined from the frequency-noise power spectral density by using a rather simple approximation, known as β-line approach, as well as the exact method based on the autocorrelation function of the laser light field. It turns out that the linewidth of the optical frequency standard amounts to about 7 kHz (full width at half maximum) for an observation time of 1 ms. Compared to the free-running laser, the measured width corresponds to a line narrowing by a factor of ~220.

This paper reports on the development of a compact analyzer for ammonia monitoring in air, based on a distributed feedback diode laser at 2.005 μm. A dual-beam long-path technique was combined with wavelength modulation detection of absorption in order to reach a detection limit of about 25 ppb, with a 0.2-Hz equivalent noise bandwidth. Retrieval of NH3 concentration was accomplished through a careful spectra analysis procedure based on the formalism of Fourier expansion of the 2nd harmonic signals, also taking into account residual-amplitude-modulation effects. The system was tested on certified gas-mixtures and revealed a good performance in terms of accuracy and reproducibility. Particularly, the short-term precision was found to be about 1 ‰, for NH3 mixing ratios of 10 ppm. Finally, the possibility to make use of the analyzer for measurements of ammonia fluxes from soils is discussed.

We demonstrate the feasibility of the accurate and simultaneous measurement of the H-2/H-1, O-17/O-16, and O-18/O-16 isotope ratios in water vapor by means of tunable diode laser spectroscopy. The absorptions are due to the nu(1) + nu(3) combination band, observed using a room temperature, distributed feedback (DFB) diode laser at 1.39 mum. The precision of the instrument is approximately 3, 1, and 0.5% for the H-2, O-17, and O-18 isotope ratios, respectively, and is at present limited by residual optical feedback to the laser. The signal-to-noise, however, is superior to that obtained in a similar experiment using a color center laser at 2.7 mum. Replacing the current laser with a better unit, we are confident that a precision well below 1parts per thousand is attainable for all three isotope ratios. The diode laser apparatus is ideally suited for applications demanding a reliable, cheap, and/or portable instrument, such as the biomedical doubly labeled water method and atmospheric sensing. (C) 2002 Elsevier Science B.V. All rights reserved.

We report on a diode-laser-based spectrometer, operating at a wavelength of 2 μm, which enables for simultaneous measurements of CO2 and H2O concentrations in gaseous mixtures with high precision and accuracy. We implemented the very simple approach of a direct measurement of absorption in a multiple reflection cell with a pathlength of 50 m. Gas concentration is simply retrieved from the integrated absorbance, without using calibration cells. The spectrometer is equipped with single-mode optical fibers in order to probe remote environments. The possible application in volcanic areas will be discussed. We also investigated the possibility to measure the isotopic ratio 13CO2/12CO2, which is another relevant parameter for monitoring volcanic activity.

We implemented an optical frequency standard based on noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) at 1.39 μm. The emission frequency of an extended-cavity diode laser was actively stabilized against the center of the 4(4,1)→4(4,0) transition of the H(2)(18)O ν1+ν3 band, under optical saturation conditions. The nonlinear regime of laser-gas interaction was reached by using an optical cavity with a finesse of about 8700. By filling it with an 18O-enriched water sample at a pressure of a few Pa, the Lamb dip could be observed with a full width at half-maximum of about 2 MHz. Absolute frequency stabilization was obtained by locking the cavity resonance to the center of the sub-Doppler signal, which was provided by the NICE-OHMS technique under the dispersion regime of operation. An Allan deviation analysis demonstrated a relative frequency stability of ∼5×10(-13) for an integration time of 1 s. For longer integration times, the flicker frequency noise floor set the stability at the level of 4×10(-14).

We report on a narrow-linewidth distributed-feedback quantum cascade laser at 8.6 mu m that is optical-feedback locked to a high-finesse V-shaped cavity. The spectral purity of the quantum cascade laser is fully characterized using a high-sensitivity optical frequency discriminator, leading to a 1 ms linewidth of less than 4 kHz and a minimum laser frequency noise spectral density as low as 0.01 Hz(2)/Hz for Fourier frequencies larger than 100 kHz. The cumulative standard deviation of the laser intensity is better than 0.1% over an integration bandwidth from 2 Hz to 100 MHz. (C) 2014 Optical Society of America

We report on our recent determination of the Boltzmann constant, kB, by means of Doppler broadening thermometry. This relatively new method of primary gas thermometry was implemented by using a pair of offset-frequency locked extended-cavity diode lasers at 1.39 μm, to probe a particular vibration–rotation transition of the H218O molecule. Adopting a rather sophisticated and extremely refined line shape model in the spectral analysis procedure, we were able to determine the Doppler width from high-quality absorption spectra with unprecedented accuracy. Our spectroscopic determination of kB exhibits a combined (type A plus type B) uncertainty of 24 parts over 106. The complete uncertainty budget is presented and discussed.

The combination of frequency combs and quantum cascade lasers is opening new opportunities in the field of precision spectroscopy in the mid-infrared. Frequency combs allow quantum cascade lasers to be referenced to a highly repeatable, precise and absolute frequency axis. Repeatability is a key feature to obtain high quality measurements of absorption profiles and thus accurate determination of spectroscopic parameters, while absolute frequency calibration makes it possible the comparison of spectroscopic data acquired in different laboratories and at different times, as well as comparison with theoretical predictions or existing databases. This work reviews some of the main results achieved at 4.3 μm by investigation of a manifold of rovibrational lines of CO 2 . Spectroscopic parameters such as line-centre frequencies, line intensity factors, pressure shift and pressure broadening coefficients are retrieved with an unprecedented quality from the metrological point of view.

We propose a method for the measurements of the O-17/O-16 isotope amount ratio in water, based upon the use of a pair of offset-frequency locked extended-cavity diode lasers at 1.39 mu m. This method enables one to acquire absorption spectra with an extremely high fidelity, exploiting the highly accurate, absolute, and repeatable frequency axis. One of the two lasers, namely the so-called slave laser, is continuously scanned across a pair of (H2O)-O-16 and (H2O)-O-17 lines at 7183.5 cm(-1) and it interacts with a water vapor sample inside a multiple reflections cell, thus producing absorption spectra with a signal-to-noise ratio of the order of 4000 for a detection bandwidth of 1 kHz. The determination of the isotope amount ratio is performed through a careful analysis of the acquired spectra, by using semiclassical line profiles. In this respect, the influence of the choice of the line shape model is investigated. The experimental reproducibility of the spectrometer has been carefully assessed by means of an Allan variance analysis. Finally, the application of the Kalman filtering technique has shown that a precision of 0.6% can be achieved, from repeated spectral acquisitions over a time span of 6000 s.

The outcomes of a specific study on line shape deviations from the usual Voigt profile, in the near-IR spectrum of water, will be discussed. Particularly, a great attention is paid to the role of speed-dependence of collisional broadening. From the experimental point of view, high spectral fidelity has been reached by using the technique of offset-frequency locking between two extended-cavity diode lasers. In such a system, the master laser is frequency stabilized against a narrow sub-Doppler molecular line, while the slave laser is actively controlled so that its emission frequency maintains a given offset with respect to the frequency of the master. This method allows one to perform highly accurate and reproducible frequency scans of the slave laser around a given vibration-rotation line. I will present recent findings on the speed dependence of collision parameters, influencing the shape of H218O absorption lines. Finally, I will discuss the implications for the spectroscopic determination of the Boltzmann constant.

We report on the absolute frequency measurement of an extended-cavity diode laser stabilized against a Doppler-free vibration–rotation transition of the H2 18O isotopologue near 1.384 μm. Absolute determination is performed by comparing the water-stabilized laser frequency with respect to a GPS-disciplined Rb microwave standard by means of a self-referenced fiber-based optical frequency comb. The line center frequency of the 22,1→22,0 transition of the H2 18O ν1+ν3 combination band is found to be 216 519 045 955(13) kHz.

This work presents a very simple yet effective way to obtain direct referencing of a quantum-cascade-laser at 4.3 μm to a near-IR frequency-comb. Precise tuning of the comb repetition-rate allows the quantum-cascade-laser to be scanned across absorption lines of a CO2 gaseous sample and line profiles to be acquired with extreme reproducibility and accuracy. By averaging over 50 acquisitions, line-centre frequencies are retrieved with an uncertainty of 30 kHz in a linear interaction regime. The extension of this methodology to other lines and molecules, by the use of widely tunable extended-cavity quantum-cascade-lasers, paves the way to a wide availability of high-quality and traceable spectroscopic data in the most crucial region for molecular detection and interrogation.

We describe a continuous-wave diode laser spectrometer for water-vapour precision spectroscopy at 1.38 μm. The spectrometer is based upon the use of a simple scheme for offset-frequency locking of a pair of extended-cavity diode lasers that allows to achieve unprecedented accuracy and reproducibility levels in measuring molecular absorption. When locked to the master laser with an offset frequency of 1.5 GHz, the slave laser exhibits residual frequency fluctuations of 1 kHz over a time interval of 25 minutes, for a 1-s integration time. The slave laser could be continuously tuned up to 3 GHz, the scan showing relative deviations from linearity below the 10{-6} level. Simultaneously, a capture range of the order of 1 GHz was obtained. Quantitative spectroscopy was also demonstrated by accurately determining relevant spectroscopic parameters for the 22,1→22,0line of the H2(18)O v1+v3 band at 1384.6008 nm.

An intensity-stabilized diode laser absorption spectrometer was developed and used to perform a highly accurate study of the line shape of CO(2) absorption lines, in the spectral region around 5000 cm(-1), belonging to the nu(1) + 2nu(2)(0) + nu(3) combination band, at a temperature of 296.00 K. Standard and complex semiclassical models, including Dicke narrowing and speed-dependent broadening effects, were applied, tested, and compared in the pressure range between 0.7 and 4 kPa, in order to single out the model best reproducing the absorption profile and, hence, the physical situation of self-colliding CO(2) molecules. Line intensity factors and self-broadening coefficients were determined. The 1-sigma overall accuracy of our determinations is at a level of 0.1%, which is, to our knowledge, the highest ever reached.

An intensity-stabilized laser absorption spectrometer, which incorporates a mirror-extended cavity diode laser, a temperature-stabilized gas cell, and a Michelson interferometer, was developed and applied to a highly accurate investigation of line intensity factors within the nu(1)+2nu(2) (0)+nu(3) combination band of carbon dioxide, around 2 microm wavelength, at a temperature of 296.0 K. This relatively complex apparatus enables one to observe the absorption line shape with high precision and accuracy in such a way that it is possible to retrieve the integrated absorbance with a relative uncertainty better than 0.1%. The absorption spectra were interpolated with the uncorrelated strong-collision model of Rautian and Sobel'man in order to take into account Dicke narrowing effects, thus obtaining an agreement at a level of a few parts per 10(-5). We report line strength values for the R(2)-R(18) transitions with an unprecedented level of accuracy, in the range between 0.1% and 0.15%. Finally, we discuss the possibility of providing a first experimental test of the theoretical model for molecular line strengths based on the Herman-Wallis expansion.

This paper reports on a compact analyzer for ammonia monitoring in the air based on near-infrared wavelength-modulation diode-laser absorption spectroscopy. Particular attention is devoted to the problem of residual amplitude modulation (RAM), which is responsible for the occurrence of the line profiles' distortions. The RAM effects are eliminated by means of an intensity-control feedback loop based on an acoustooptic modulator. This offers the advantage of simplifying enormously the spectral-analysis procedure, which, otherwise, would require a considerable amount of calculation if the RAM distortions were considered

We report what we believe to be the first absolute frequency measurement performed using a quantum-cascade laser (QCL) referenced to an optical frequency comb synthesizer (OFCS). A QCL at 4.43 microm has been used for producing near-infrared radiation at 858 nm by means of sum-frequency generation with a Nd:YAG source in a periodically poled lithium niobate nonlinear crystal. The absolute frequency of the QCL source has been measured by detecting the beat note between the sum frequency and a diode laser at the same wavelength, while both the Nd:YAG and the diode laser were referenced to the OFCS. Doppler-broadened line profiles of (13)CO(2) molecular transitions have been recorded with such an absolute frequency reference.

International audience; We report on the first application of extended–wavelength DFB diode lasers to Cavity–Enhanced Absorption Spectroscopy in-situ trace measurements on geothermal gases. The emission from the most active fumarole at the Solfatara volcano near Naples (Italy) was probed for the presence of CO and CH4. After passing through a gas dryer and cooler, the volcanic gas flow (98% CO2) was analysed in real time for the concentration of these species, whose relatively strong absorption lines could be monitored simultaneously by a single Distributed Feed–Back (DFB) GaSb–based diode laser emitting around 2.33 μm (4300 cm−1) at room temperature. The concentrations were found to be about 3 ppm and 75 ppm, respectively, while actual detection limits for these molecules are around 1 ppb. We discuss the possibility of detecting other species of interest for volcanic emission monitoring

Near-infrared laser spectroscopy is used to measure the 13C/12C isotope abundance ratio in gas phase carbon dioxide. The spectrometer, developed expressly for field applications, is based on a 2 μm distributed feedback diode laser in combination with sensitive wavelength modulation detection. It is characterized by a simplified optical layout, in which a single detector and associated electronics are used to probe absorptions of a pair of 13CO2 and 12CO2 lines, simultaneously in a sample, as well as a reference gas. For a careful investigation of the achievable precision and accuracy levels, we carried out a variety of laboratory tests on CO2 samples with different isotopic compositions, calibrated with respect to the international standard material by means of isotope ratio mass spectrometry. The 1-σ accuracy of the 13CO2/12CO2 determinations, reported in the so-called δ notation, is about 0.5‰ (including both statistical and systematic errors), for δ-values in the range from -30‰ to +20‰. We show that the major source of systematic errors is a consequence of the non-linearity of the Lambert–Beer absorption law, and can be corrected for to a very high degree of accuracy.

Recent progress in telecom-based optoelectronic devices and low power laser sources has provided new powerful tools for in-situ monitoring of volcanic areas in real-time and for long periods, with unprecedented accuracy and resolution. Such techniques represent a great improvement if compared to the traditionally used "in-situ" sampling techniques such as gas chromatography and mass spectrometry which result often impractical or dangerous, and do not allow temporally continuous monitoring Optical communication diode lasers, which emit visible and IR radiation with mW power, operating on single mode at room temperature, are ideal sources to develop high sensitivity gas analysers based on absorption spectroscopy techniques. Their possible use in conjunction with fiber components is particularly advantageous in the perspective of developing networks for simultaneous monitoring of many geochemical and geophysical parameters. We report on the field tests of a novel Portable Diode Laser Spectrometer (PDLS), operating at a wavelength of 1.99μm and based on direct absorption spectroscopy, for high precision, accurate and real-time, simultaneous measurements of CO 2 and H 2 O concentrations. We used an open-path configuration, positioning the detection bench, equipped with a a Herriott cell especially designed for measurements in hazardous environment, at different sites of the fumarolic areas. With such a configuration we can take advantage of the long optical path-length allowed by the multiple-pass cell (20.3m). The measured CO 2 and H 2 O concentrations vs. time are shown. This work was supported by GNV-INGV, in the framework of the project "Development of spectroscopic integrated system for remote and continuous monitoring of volcanic gases".

A novel diode laser spectrometer was developed using dual-wavelength multiplexing, ensuring ideal conditions for high-precision and simultaneous measurements of the H-2/H-1, O-17/O-16, and O-18/O-16 isotope ratios in water. A 1.4-mum diode laser probed a (HOH)-O-16/(HOH)-H-2 line pair near 7198 cm(-1), while a similar laser observed (HOH)-O-16, (HOH)-O-17, and (HOH)-O-18 rovibrational lines around 7183 cm(-1), or a (HOH)-O-16/(HOH)-O-18 line pair near 7200 cm(-1). The 1-sigma standard deviation is 0.2parts per thousand for O-18/O-16, and 0.5parts per thousand for the H-2/H-1 and O-17/O-16 isotope ratios. Preliminary experiments with repeated injections of a natural abundance sample point to an accuracy of about 1parts per thousand for all three isotope ratios in natural samples. (C) 2003 Optical Society of America.

Present optical techniques. which are considered to be relevant to monitor volcanoes, are reviewed. A short description of each technique is given, as well as the peculiar conditions of applicability. Recent experimental results taken at Solfatara crater with a fiber-coupled diode laser system are reported and discussed. To take into account the tremendous progress that is experiencing the field of new laser sources and optical materials, an overview to future laser devices, that may have a significant impact in the field of gas monitoring, is given. (C) 2001 Elsevier Science B.V. All rights reserved.

A high-finesse optical cavity was employed to perform highly sensitive spectroscopy of molecular oxygen at wavelengths near 763 nm. An equivalent absorption length of ∼1 km was obtained by a 26-cm-long optical cavity with a finesse of 6000. An extended cavity diode laser was frequency locked to the cavity, and pure absorption profiles were recovered by monitoring of the cavity transmission during continuous scans of the cavity resonance through O2 rotational lines, allowing a detailed investigation of the line shapes. Phase modulation of the laser at a frequency equal to the cavity free-spectral-range frequency was employed for detection of weak absorption signals inside the cavity. A minimum detectable absorption coefficient of 6.9×10-11 cm-1 Hz-1/2 was measured. Finally, a test of the symmetrization postulate in 16O nuclei was demonstrated.

We report construction and operation of a multiplexed laser spectrometer, based on fiber coupled Distributed Feed-Back (DFB) semiconductor diode lasers, for monitoring concentration of gases emitted from volcanoes. The main problems related to this particular application, as well as the way of operation of the apparatus are discussed. The first experimental data, recorded at the test site of Solfatara volcano, Naples, Italy are reported. To our knowledge, this is the first design of a small, low power consuming laser spectrometer for geochemical applications.

A distributed feedback diode laser emitting at 760 nm has been used to detect weak absorption lines of molecular oxygen. Using wavelength-modulation spectroscopy and second-harmonic detection at audio frequencies, a minimum detectable absorbance of 10 −6 is achieved, with an effective bandwidth of 1 Hz. Such sensitivity allows us to observe lines of the 16 O 18 O and 16 O 17 O molecules at atmospheric pressure, in a natural-abundance sample. Highly sensitive detection of NO 2 has also been performed; in this case, the laser source is an AlGaAs diode laser at 742 nm.

A visible diode laser emitting at 690 nm at room temperature has been frequency-stabilized using a simple scheme based on optical feedback from a diffraction grating. The possibilities offered by these lasers for high-resolution spectroscopy are demonstrated by recording the sub-Doppler signal of the 689 nm intercombination line of strontium and resolving the hyperfine structure of the close P63 R70 (8, 4) transitions of iodine.

The frequency of a DFB quantum cascade laser (QCL) emitting at 4.3 microm has been long-term stabilized to the Lamb-dip center of a CO2 ro-vibrational transition by means of first-derivative locking to the saturated absorption signal. Thanks to the non-linear sum-frequency generation (SFG) process with a fiber-amplified Nd:YAG laser, the QCL mid-infrared (IR) radiation has been linked to an optical frequency-comb synthesizer (OFCS) and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz.

Recent advances in infrared laser spectroscopy may enable scientists to make accurate, in situ, real-time measurements of the isotopic composition of gas species emitted from volcanoes.

An overview of novel laser techniques suitable for volcanic monitoring, based on different kinds of infrared laser sources, is presented. Their main advantages and drawbacks are discussed focusing on the achievable sensitivity and precision levels in analysis of gaseous species. Some of the most recent experimental results obtained in laboratory development as well as in field tests of home-built laser spectrometers are reported. New perspectives in optical devices aimed at geochemical and geophysical applications are also considered.