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We developed a cavity ringdown spectrometer by utilizing a step-scanning and dithering method for matching laser wavelengths to optical resonances of an optical cavity. Our approach is capable of working with two and more lasers for quasi-simultaneous measurements of multiple gas species. The developed system was tested with two lasers operating around 1654 nm and 1658 nm for spectral detections of 12CH4 and its isotope 13CH4 in air, respectively. The ringdown time of the empty cavity was about 340 µs. The achieved high detection sensitivity of a noise-equivalent absorption coefficient was 2.8 × 10−11 cm−1 Hz−1/2 or 1 × 10−11 cm−1 by averaging for 30 s. The uncertainty of the high precision determination of δ13CH4 in air is about 1.3‰. Such a system will be useful for future applications such as environmental monitoring.

A wide-range, calibration-free tunable diode laser spectrometer is established by combining wavelength modulation and direct absorption spectroscopy (WM-DAS) with continuous wave cavity ringdown spectroscopy (CW-CRDS). This spectrometer combines the benefits of absolute concentration measurements, wide range, and high speed, using WM-DAS with enhanced noise reduction in CW-CRDS. The accurate baseline ringdown time, &tau, 0, is calculated by the absorption peak (measured by WM-DAS) and the ringdown time containing gas absorption information (measured by CW-CRDS at the center wavelength of the spectral line). The gas concentration is obtained without measuring &tau, 0 in real time, thus, greatly improving the measuring speed. A WM-DAS/CW-CRDS spectrometer at 1.57 &mu, m for CO detection was assembled for experimental validation of the multiplexing scheme over a concentration ranging from 4 ppm to 1.09% (0.1 MPa, 298 K). The measured concentration of CO at 6374.406 cm&minus, 1 shows that the dynamic range of this tunable diode laser absorption spectrometer is extendable up to five orders of magnitude and the corresponding precision is improved. The measurement speed of this spectrometer can extend up to 10 ms, and the detection limit can reach 35 ppb within 25 s.

Due to their advantages of having a wide bandwidth, low cost, and being easy to obtain, traditional photodetectors (PDs) are being widely applied in measurements of transient signals. The spatial inhomogeneity of such PD temporal responses was measured directly to account for the PD spatial effect of decay rate due to poor alignment in continuous wave cavity ringdown spectroscopy (CW-CRDS) experiments. Based on the measurements of three PDs (i.e., model 1611 (Newport), model 1811 (Newport), and model PDA10CF-EC (Thorlabs)), all the temporal responses followed a tendency of declining first and then rising, and steady platforms existed for the last two PDs. Moreover, as we expected, the closer the PD center was, the faster the response. On the other hand, the initial shut-off amplitude generally reached a larger value for a faster temporal response. As a result, the spatial effect can strongly influence the spectral line shape and value, which will introduce more errors into the precise measurements of spectral parameters using the CRDS technique if this effect is not considered. The defined effective detection area (EDA) of the PDs, which was close to the active area given by manufacturers, was the key parameter that should be paid more attention by researchers. Therefore, the PD should be aligned perfectly to make sure that the EDA covers the laser spot completely.

An optical resonator like a fiber ring (FR) or a whispering gallery mode (WGM) resonator with two couplers along its loop is referred to be in the add-drop configuration, in analogy with the add-drop multiplexer in telecom networks. Both for practical applications as well as in several fundamental studies involving high-Q resonators, this configuration is of great interest and the assessment of the intrinsic properties of the resonator and of its interaction with the coupling systems is extremely important. We developed an original method able to fully characterize high-Q resonators in an add-drop configuration. The method is based on the study of the two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We implemented the model describing the two CRD profiles from which a best fit process of the measured profiles allows deducing the key parameters. We successfully validated the model with an experiment based on a FR resonator of known characteristics. Finally, we fully characterized a high-Q, home-made, MgF2 WGM disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.

International audience; An original method able to fully characterize high-Q resonators in an add-drop configuration has been implemented. The method is based on the study of two cavity ringdown (CRD) signals, which are produced at the transmission and drop ports by wavelength sweeping a resonance in a time interval comparable with the photon cavity lifetime. All the resonator parameters can be assessed with a single set of simultaneous measurements. We first developed a model describing the two CRD output signals and a fitting program able to deduce the key parameters from the measured profiles. We successfully validated the model with an experiment based on a fiber ring resonator of known characteristics. Finally, we characterized a high-Q, home-made, MgF2 whispering gallery mode disk resonator in the add-drop configuration, assessing its intrinsic and coupling parameters.

We report a proof-of-principle demonstration of intracavity pump–probe, cavity ringdown (CRD) detection in a three-mirror, traveling-wave cavity. With cavity-enhanced pump power and probe absorption path length, the technique is a generally applicable, high-sensitivity, high-selectivity detection method. In our experiments, the pump radiation is switched off during every other probe ringdown, which allows uncorrelated measurements of analyte and background cavity decay rates. The net, two-color signal from the difference between the pump-on and pump-off decay rates is immune to empty-CRD drifts and spectral overlaps from non-target molecular transitions. The immunity to the ringdown drifts allows longer signal-averaging and, thus, higher detection sensitivity. The ability to compensate for the background absorption enhances the detection selectivity in spectrally congested regions. Our technique is well-suited for trace-detection in the mid-IR region, where pump–probe schemes based on strong rovibrational transitions can be applied. In this work, two-color CRD detection is implemented on a ladder-type, three-level system based on the N2O, ν3 = 1 ← 0, P(19) (pump) and ν3 = 2 ← 1, R(18) (probe), rovibrational transitions. By frequency-locking two-quantum cascade lasers to the p-polarization (pump, Finesse = 5280) and s-polarization (probe, Finesse = 67 700) cavity modes, we achieve high intracavity pump power (36 W) and high probe ringdown rates (>2 kHz). The observed two-color spectra are simulated by a density-matrix, three-level system model that is solved under the constraints of the cavity resonance conditions. In addition to its background compensation capability, experimental flexibility in the selection of pump–probe schemes and signal insensitivity to intracavity laser power are further features that enhance the utility of our technique for mid-IR trace-detection.

International audience; Nowadays, narrow linewidth lasers are unavoidable tools for fundamental and applied physics. One promising approach to generate coherent laser emission is based on stimulated Brillouin scattering (SBS) optical nonlinearity. Implementing SBS in an optical cavity gives rise to coherent emission with impressive performances in terms of noise and linewidth. Cascading the SBS process can even, under given conditions, improves the coherent emission specifications. To achieve such performances it is crucial to determine the Brillouin gain coefficient. Usual pump-probe methods are carried out in single pass waveguides. The probe intensity is proportional to e gBP in L eff / A eff where P in is the incident pump power, A eff the effective mode area and L eff the usual effective interaction length. To observe significant probe amplification one needs either long waveguides or strong pump power to reach the SBS threshold. We propose a method, based on the cavity ringdown spectroscopy, to directly characterize the Brillouin gain coefficient inside a resonator. This configuration allows to extract Brillouin parameters from the light recirculation with both a reduced waveguide length and input pump power when compared to usual methods.

International audience; We report a technique based upon the cavity ringdown method that enables to characterize the Brillouin gain coefficient directly in a laser cavity. Material gain, optical cavity parameters and lasing properties can be extracted from measurements whithin a single experiment.

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.

The plasma plume induced by dual-pulse laser ablation of a titanium tar- get in vacuum was analyzed by the technique of cavity ringdown spectroscopy (CRDS). Large Doppler-splitting of the absorption spectral lines was observed which is due to increase of the velocity components parallel to the optical axis and specific features of the CRDS measurements. Vertical velocity component, the particle number density and plasma volume also shows increase compared to the single-pulse laser ablation. The forward convolution best fit of absorption lineshapes was used to extract parameters describing dual-pulse laser ablation plasma plume.

The laser-induced plasma plume of a Ti target in vacuum is probed by the technique of cavity ringdown spectroscopy. A model is developed to perform a forward convolution of atomic absorption line profile measurements. The model accounts for laser-induced plasma characteristics such as anisotropy of the plume and velocity distributions of the ablated particles as well as of the cavity ringdown features such as geometry and time selectivity. The absorption lineshapes of atomic transitions are calculated and discussed for given sets of parameters. Calculated line profiles are fitted to experimental line profiles obtained from nanosecond-laser ablation of the target and provide data about the plume dynamics.

Detection of mid-infrared light (λ in range of 2–5 μm) is traditionally limited by noisy and slow detectors. Upconversion of infrared light to near-infrared wavelengths enables fast and sensitive mid-infrared (MIR) detection using regular Si detectors or photomultiplier tubes (PMT). Efficient continuous wave (CW) upconversion has been introduced in spectral imaging [1] and spectrometer applications [2]. We apply upconversion to cavity ringdown spectroscopy CRDS by using a pulsed supercontinuum light source at 3 μm wavelength region. Intracavity upconversion utilizes sum frequency generation (SFG) between 1064 nm Nd:YVO4 laser field and MIR wavelengths resulting detection at 780 nm region.

We integrated a rigid optical trap into a tunable pulsed cavity ringdown spectroscopy (OT-CRDS) system to characterize the extinction of single airborne particles in the UV spectral region (306-315 nm). Single solid particles from a multi-walled carbon nanotube (MWCNT), Bermuda grass smut spore, carbon microsphere, and blackened polyethylene microsphere were trapped in air based on the photophoretic force. The improved OT-CRDS system was highly sensitive and able to resolve extinctions of single particles from different materials and sizes at a given wavelength. Further, we successfully manipulated the number of particles, e.g., 1, 2 or more particles, in the trap and measured their distinguishable extinctions using the OT-CRDS. We also show that the particle size and extinction have a good linear correlation from the measurements of 24 single MWCNT particles. Material- and wavelength-dependent extinctions of the four types of airborne particles were also characterized. Results reveal that single airborne particles regardless of their differences in material and size, due to their heterogeneous morphology, have individual-particle dependent extinctions and that dependence can be resolved and characterized using the OT-CRDS technique.

An extremely sensitive and simple fiber loop-cavity ringdown spectroscopy (FL-CRDS) setup has been designed based on a turn-key nanosecond pulse laser source operating at 1535 nm. The system sensitivity is demonstrated to be approximately 0.01 dB after extracting the characteristic macrobend loss curve of a standard single mode fiber (SMF-28). The experiment demonstrated that the oscillatory behavior in the rapid loss due to the increasing curvature could be seen for single turn bare fibers with radii of curvature from 10–20 mm. Since the wavelength of laser lies in the range of the conventional window, known as C-band, the demonstrated scheme may suggest the use of these techniques in versatile applications where quick results are necessary without the system complexity.

Plasma-cavity ringdown spectroscopy is a powerful absorption technique for analytical measurement. It combines the inherent advantages of high sensitivity, absolute measurement, and relative insensitivity to light source intensity fluctuations of the cavity ringdown technique with use of plasma as an atomization/ionization source. In this review, we briefly describe the background and principles of plasma-cavity ringdown spectroscopy(CRDS) technology, the instrumental components, and various applications. The significant developments of the plasma sources, lasers, and cavity optics are illustrated. Analytical applications of plasma-CRDS for elemental detection and isotopic measurement in atomic spectrometry are outlined in this review. Plasma-CRDS is shown to have a promising future for various analytical applications, while some further efforts are still needed in fields such as cavity design, plasma source design, instrumental improvement and integration, as well as potential applications in radical and molecular measurements.

Cavity ringdown spectroscopy (CRDS) in the linear absorption regime is a well-established method for sensitive trace gas detection, but only a few studies have addressed quantitative measurements in the presence of a saturated sample. In fact, saturation is usually avoided in order to escape from the required complex modeling of the saturation process that depends on the characteristics of the absorbing species, its interaction with the surrounding gas as well as on the temporal and spectral characteristics of the cavity excitation. Conversely, the novel saturated-absorption cavity ringdown spectroscopy approach (SCAR/Sat-CRDS) takes advantage of sample saturation in order to allow one to extract both the gas absorption and the empty cavity loss rates from a single ringdown event. Using a new continuous-wave infrared CRD spectrometer equipped with a tunable narrow-bandwidth high-power OPO laser system and a 18 bit digitizer, the transient dynamics of absorption saturation and the working limits of the Sat-CRDS approach in terms of its ability to extract reliable trace gas concentrations have been experimentally studied in this work. Using a strong methane transition as a test case, the excitation power P0 and saturation power PS have been systematically varied to explore a wide range of saturation regimes. At pressures 5 μbarp2 mbar, the saturation intensity revealed a nearly linear pressure dependence showing that non-collisional processes contribute to the overall relaxation. A ratio of P0/PS ≈ 15 turned out to be optimal with working limits of 5P0/PS300. Moreover, the ratio of the absorption and empty cavity loss rates, γg/γc, has been varied to test the dynamic range of the method. At γgγc, a pronounced coupling between the two parameters has been observed. Finally, a standard error analysis was performed revealing that the Sat-CRDS approach holds its advantages over conventional CRDS implementations in particular when the attainable ultimate detection sensitivity is limited by uncertainties in the empty cavity ringdown constant.

For the exploration of gas hydrate resources by measuring the dissolved methane concentration in seawater, a continuous-wave cavity ringdown spectroscopy (CW-CRDS) experimental setup was constructed for trace methane detection. A current-modulation method, rather than a cavity-modulation method using an optical switch and a piezoelectric transducer, was employed to realize the cavity excitation and shutoff. Such a current-modulation method enabled the improvement of the experimental setup construction and stability, and the system size and stability are critical for a sensor to be deployed underwater. Ringdown data acquisition and processing were performed, followed by an evaluation of the experimental setup stability and sensitivity. The obtained results demonstrate that great errors are introduced when a large fitting window is selected if the analog-to-digital converter has an insufficient resolution. The ringdown spectrum of methane corresponding to the 2v3 band R(4) branch was captured, and the methane concentration in lab air was determined to be 2.06 ppm. Further experiments for evaluating the quantitative ability of this CW-CRDS experimental setup are underway from which a high-sensitivity methane sensor that can be combined with a degassing system is expected.

High-quality in situ observations of aerosol particle optical properties, namely extinction, scattering, and absorption, provide important information needed to constrain the role of aerosols in the climate system. This paper outlines the design and performance of an aircraft instrument utilizing cavity ringdown spectroscopy for the measurement of aerosol extinction. The 8-channel cavity ringdown spectrometer measures extinction at multiple wavelengths (405, 532, and 662 nm) and at multiple relative humidities (e.g., 10%, 70%, and 95%). Key performance characteristics include a 1-s detection limit better than 0.1 Mm−1, accuracy of 10 Mm−1. Laboratory and field data demonstrate that the 1-s precision is limited by the statistics of aerosol particles in the laser beam rather than the precision of the extinction measurement per se. The measurement precision improves with averaging to 5% at 60 s for extinction leve...

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10−7 cm−1 in a compact 1-cm3 ringdown cavity volume.

Single-mode optical fibers can be used to couple remotely located open-path ringdown cavities to a single central control module. This enables laser-spectroscopic sensing of atmospheric species at trace levels in agricultural and environmental media.

We report on a compact trace nitrogen dioxide (NO2) detector based on cavity ringdown spectroscopy (CRDS), using a passively locked external cavity diode laser (PLEC-DL) in the visible wavelength region. Tailoring the spatial mode matching conditions of an antireflection (AR)-coated diode laser with a high finesse external cavity for effective optical feedback into the DL allows for stable resonant enhancement in the external cavity without any electric feedback apparatus. The external cavity having a finesse in excess of 1.4×105 acts as a ringdown cavity with an effective optical volume (i.e., detection volume) of less than 1 cm3. We successfully demonstrated a noise equivalent absorption loss of 1.4×10-8 cm-1, corresponding to a detection limit of 40 ppb NO2 concentration with a very compact configuration. For differential absorption CRDS, we realized a dual wavelength-locked PLEC-DL in an external cavity simultaneously. Measurements of various NO2 concentrations in N2 based on differential absorption CRDS were performed.

Continuous-wave (cw) cavity-ringdown (CRD) spectroscopy provides a highly sensitive way to measure optical absorption by observing the decay rate of light from a high-finesse optical cavity containing the sample of interest (usually gas-phase molecules). In rapidly swept cw-CRD spectroscopy, optical build-up and subsequent ringdown decay are initiated by rapidly sweeping the cavity length or the wavelength of the monochromatic tunable cw laser radiation, thereby establishing and interrupting optical resonance between the laser light and the longitudinal-mode frequencies of the cavity. We review the experimental methodology and applications of this technique, indicating its advantages and prospects for spectroscopic sensing.

Acetone in human breath gas has been established as a biomarker for type-1 diabetes (T1D). Non-invasive breath gas analysis for diabetes diagnostics using breath acetone as a biomarker depends on the availability of a portable, fast-response and sensitive acetone detection device. So far such an instrument is not available yet. We report on the first acetone detection device using cavity ringdown spectroscopy and evaluate its performance prior to breath gas testing. Taking advantage of the coincidence of the largest absorption cross-section of acetone in the UV being at 266 nm with the availability of the palm-size 266 nm laser source, we construct a portable ringdown acetone detection device. The high quality single mode laser beam allows us to simplify the optical design for coupling the laser beam into the cavity, which consists of a pair of high reflectivity mirrors (R = 99.87%) separated by 45 cm. The ringdown signal observed by a miniature detector is processed by a laptop computer. The instrument performance on sensitivity, response time and sampling methods is evaluated using acetone diluted in deionized water in the concentration range of 6 drops l−1 to 0.5 drop l−1. The device operating with two different sampling methods is investigated and shows different measurement sensitivities and response times. The minimum detectable concentration of acetone sample solutions is 0.5 drop l−1 or 7.9 µg l−1, which corresponds to a gas concentration of 0.49 ppmv derived from the measured absorbance (based on 1-σ). The typical measuring time in the second sampling method, including the times for the sample introduction, the purge of the gas cell and the data processing, is less than 50 s. Since the average concentration of acetone in healthy human breath gas is approximately 0.79–1.4 ppmv, this instrument has sufficient sensitivity to detect breath acetone with elevated concentrations. Issues in future developments towards an instrument suitable for breath gas testing are discussed.

We describe a new continuous wave cavity ringdown spectroscopy (cw-CRDS) approach using an extended cavity diode laser (ECDL) optically self-locked to a high finesse cavity including an intracavity glass plate under the Brewster angle. Low noise, mode-by-mode absorption spectra are recorded at a high acquisition rate (laser frequency scan greater than 400 GHz/s) and covering four orders of magnitude in absorption coefficient. Sampling spectra with the fixed high finesse cavity frequency comb provides high precision frequency markers. An original scheme for the laser beam shut-down, based on signal shape analysis and the diode laser injection current control, is presented. This scheme avoids any supplementary switching device. To retrieve ringdown processing at a kilohertz rate several exponential decay fit algorithms are compared. Performances of this new scheme are demonstrated with the observation of very weak lines of the oxygen B-band around 680 nm. Atmospheric spectra of isolated lines averaged for less than 10 s show a baseline noise of 5×10-10 cm-1 and a single point minimum detectable absorption loss over a one-second measurement interval of 2×10-10 cm/\(\sqrt{\text{Hz}}\) is obtained.

International audience; The time domain coupled-mode theory (CMT) is applied to the analysis of the dynamic of third-order optical nonlinear effects in high-finesse whispering-gallery-mode (WGM) resonators. We show that this model is well adapted to the analysis of cavity-ringdown spectroscopy signal under modal-coupling due to Rayleigh backscattering in linear and nonlinear regimes. The experiments are carried out in silica WGM microspheres. Considering thermal and Kerr effects, CMT simulations are in good agreement, with experimental results for input power up to about 1 mW. For well-known optical materials such as silica, this experimental data analysis method can be used to measure the quality factor, the coupling regime, and the mode volume of high-finesse WGM. Furthermore, this technique could be developed to infer linear and nonlinear properties of high-finesse coated WGM microspheres.

A high performance continuous-wave cavity ringdown spectrometer using a rapidly swept cavity has been applied to investigate weak absorption of CO2 line near 1064 nm. In the experimental setup, offset locking of the Nd:YAG laser to other iodine-stabilized Nd:YAG laser provides a frequency accuracy better than 20 kHz. The ringdown time was obtained by fitting the experimental ringdown data to a theoretically obtained ringdown curve. The absorption coefficient and absolute center wavelength are determined with accuracy better than two orders of magnitude with respect to the previous observations. Our experimental setup yields high performance in a relatively simple, low cost, and compact system that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment.

This article describes the measurement and rotational analysis of several overtone vibrational bands of H 13 C 13 CH in the region between 12 100 cm −1 and 12 265 cm −1 using cavity ringdown spectroscopy. The recorded spectra contain three perpendicular bands and one parallel band. We present rotational parameters and vibrational term values for seven different vibrational states. In the experimental technique employed, the laser source is frequency stabilised to a transmission maximum of a high-finesse ringdown cavity. After each exponential decay, the laser re-acquires the lock and maintains it until the next ringdown is commenced. Some improvements have been made to the initial version of the experimental setup and these are discussed.

We perform a theoretical analysis on the transient dynamics of absorption saturation in pulsed cavity ringdown spectroscopy. Based on a coupled rate equation approach, modelling of the dynamic saturation is carried out to account for time-evolving intracavity photon density and absorbing population. Master equations are derived in terms of normalized system parameters, which enables one to systematically study the non-exponential feature of saturated cavity ringdown signals. Erroneous quantification of sample absorbance by the conventional ringdown time analysis is numerically simulated, and the saturated spectral feature showing a Lamb dip is obtained for a Doppler broadened sample. Finally a novel numerical recipe is proposed to handle saturated ringdown signals and retrieve unsaturated spectra, allowing relevant absorption parameters without degrading the measurement signal-to-noise ratio.

Cavity ringdown (CRD) absorption spectroscopy enables spectroscopic sensing of gases with a high sensitivity and accuracy. Instrumental improvements result in a new high-performance continuous-wave (cw) CRD spectrometer using a rapidly-swept cavity of simple design. It employs efficient data-acquisition procedures, high-reflectivity mirrors, a low-adsorption flow cell, and various compact fibre-optical components in a single-ended transmitter-receiver configuration suitable for remote sensing. Baseline noise levels in our latest cw-CRD experiments yield a competitive noise-equivalent absorption limit of ∼5×10-10 cm-1Hz-1/2, independent of whatever molecules are to be detected. Measurements in the near-infrared wavelength range of 1.51–1.56 μm yield sub-ppmv (i.e., ppbv or better) sensitivity in the gas phase for several representative molecules (notably CO2, CO, H2O, NH3, C2H2, and other hydrocarbons). By measuring spectroscopic features in the 1.525 μm band of C2H2 gas, we realise detection limits of 19 nTorr (2.5×10-11 atm) of neat C2H2 (Doppler-limited at low pressure) and 0.37 ppbv of C2H2 in air (pressure-broadened at 1 atm). Our cw-CRD spectrometer is a high-performance sensor in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment.

A simple, economic diode laser based cavity ringdown system for trace-gas applications in the petrochemical industry is presented. As acetylene (C2H2) is sometimes present as an interfering contaminant in the gas flow of ethylene (ethene, C2H4) in a polyethylene production process, an on-line monitoring of such traces is essential. We investigated C2H2–C2H4 mixtures in a gas-flow configuration in real time. The experimental setup consists of a near-infrared external cavity diode laser with an output power of a few mW, standard telecommunication fibers and a home-made gas cell providing a user-friendly cavity alignment. A noise-equivalent detection sensitivity of 4.5×10-8 cm-1 Hz-1/2 was achieved, corresponding to a detection limit of 20 ppbV C2H2 in synthetic air at 100 mbar. In an actual C2H2–C2H4 gas-flow measurement the minimum detectable concentration of C2H2 added to the C2H4 gas stream (which may already contain an unknown C2H2 contamination) increased to 160 ppbV. Moreover, stepwise C2H2 concentration increments of 500 ppbV were resolved with a 1-min time resolution and an excellent linear relationship between the absorption coefficient and the concentration was found.

A novel instrument, based on cavity-ringdown spectroscopy (CRDS), has been developed for trace gas detection. The new instrument utilizes a widely tunable optical parametric oscillator (OPO), which incorporates a zinc–germanium–phosphide (ZGP) crystal that is pumped at 2.8 μm by a 25-Hz Er,Cr:YSGG laser. The resultant mid-IR beam profile is nearly Gaussian, with energies exceeding 200 μJ/pulse between 6 and 8 μm, corresponding to a quantum conversion efficiency of approximately 35%. Vapor-phase mid-infrared spectra of common explosives (TNT, TATP, RDX, PETN and Tetryl) were acquired using the CRDS technique. Parts-per-billion concentration levels were readily detected with no sample preconcentration. A collection/flash-heating sequence was implemented in order to enhance detection limits for ambient air sampling. Detection limits as low as 75 ppt for TNT are expected, with similar concentration levels for the other explosives.

A simple and reliable method is presented for optimizing the mode matching of a laser beam to the high-finesse cavity used in pulsed cavity ringdown spectroscopy (CRDS). The method is based on minimizing the excitation of higher-order transverse cavity modes through monitoring the non-degenerate transverse mode beating which becomes visible with induced cavity asymmetry caused by slight misalignment. No additional instrument is required other than a pinhole aperture, thus this method can be applied for CRDS experiments in the whole wavelength range. Measurements of the CRDS absorption spectrum of acetylene (C2H2) near 571 nm demonstrate that the mode-matching optimization improves the sensitivity of pulsed CRDS.

A novel approach to cavity ringdown spectroscopy uses a continuous-wave laser and a rapidly swept optical cavity to shift the frequency of optical radiation stored in the cavity. This frequency-shifted radiation from the ringdown cavity is then combined with incident laser radiation to generate optical heterodyne signals, simply and efficiently. A noise-limited absorbance sensitivity of 3×10 −9 cm −1 is realised, using ∼35 μW of single-mode radiation from a 1.53 μm tunable diode laser. The resonance properties of a swept optical cavity simplify this heterodyne-detected technique by avoiding the customary need for a fast optical switch or for wavelength-locking of cavity length.

We theoretically investigate the spectral measurement errors that are apt to occur in continuous wave (CW) cavity ringdown (CRD) techniques suffering intracavity Doppler shifts. In the typical CW CRD scheme based on a cavity sweep operation for the resonant coupling of a probe laser into a ringdown cavity, the intracavity probe light detunes gradually over time, carrying time-dependent loss information of an absorption feature. Frequency-drifting ringdown signals are theoretically modelled and found to result in erroneous absorption spectra that exhibit the frequency shift of absorption profiles as well as linewidth broadening.

A high resolution cavity ringdown spectrometer (CRDS) has been constructed using a 1.5 μm continuous-wave external-cavity tunable diode laser, a mode-matched near-confocal ringdown cavity, and 2 cm pulsed slit jet. Without signal averaging, the RMS noise in the absorption signal is 1.7 × 10 −9 cm −1 . The rotationally resolved overtone spectrum of the OH( ν 1 ) + CH( ν 3 ) stretch combination band of methanol between 6510 and 6550 cm −1 has been observed for J ′ =0–8 and K ′ =0–3 at sub-Doppler resolution. In total, 418 lines are assigned and global fits yield molecular torsion–rotation parameters for the upper state. Four K ′ -localized perturbations are analyzed and the pattern of residuals is discussed.

A new method of high resolution cavity ringdown spectroscopy (CRDS) was recently developed in our laboratory, where a narrow line, continuous wave (CW) single-frequency laser is used instead of a pulsed laser. Here, we will first discuss the main differences between the `traditional' pulsed CRDS and CW-CRDS. Then, we will describe our results exploiting the high intracavity power that can be achieved with CW-CRDS. Using a single-mode Ti:Sa laser, we obtained CRDS spectra where the excitation power of a single cavity mode is close to 20 W. In the virtually collisionless regime of a supersonic slit jet, we observed saturation in some of the weak rovibronic transitions of NO 2 around 796 nm, as evidenced by loss of absorption intensity and formation of Doppler-free Lamb dips. In addition, in coincidence with absorption by these near infrared transitions, an appreciable fluorescence signal was detected in the visible range. According to our interpretation, this fluorescence is from NO 2 levels excited by two photons in a stepwise incoherent process, with a strongly allowed second step. Since the fluorescence spectrum has the same lineshapes as the CRDS absorption spectrum, it seems that the first transition step is the one limiting the overall two-step process. In addition, we also observed very narrow fluorescence features, not coincident with any absorption feature. These must be coherent (non-stepwise), Doppler-free, two-photon transitions. Interesting new questions arise from these preliminary data, and we believe that more measurements of this kind will provide new information about the rovibronic states of NO 2 in the dissociation region.

We report in this letter the first application of Cavity Ringdown Spectroscopy (CRDS) to the quantitative detection of negative ions—H−-ions in a magnetic multipole source in this specific case. The absorption connected with the photodetachment process is measured. A pulsed (τ ≈ 15 ns) green (λ = 532 nm) laser (Nd:YAG with SHG) beam is radiated into a high-Q cavity containing the discharge plasma. The temporal behaviour of the decaying pulse at the cavity exit yields the density of absorbing H−-ions on the basis of the known (broadband) photodetachment cross-section. The densities determined by CRDS agree well with earlier measurements obtained with a multipass scheme and with the alternative method which consists in the detection with a Langmuir probe of the electrons released in the photodetachment process.

Spectroscopic sensing of gases can be performed with high sensitivity and photometric precision by cavity ringdown (CRD) absorption spectroscopy. Our cavity ringdown spectrometer incorporates continuous-wave (cw) tunable diode lasers, fibre-optic coupling and standard photonics and optical telecommunications components. It comprises a rapidly swept optical cavity in a single-ended optical heterodyne transmitter–receiver configuration, enabling optical absorption of gases to be recorded either as single-frequency scanned spectra or as simultaneous, multi-wavelength tailored spectra. By measuring weak near-infrared rovibrational spectra of carbon dioxide gas (CO2), with high resolution in the vicinity of 1.53 μm, we have realised a noise-limited absorption sensitivity of 2.5×10-9 cm-1 Hz-1/2. Analytical sensitivity limits (both actual and projected) and prospective gas-diagnostic applications are discussed. Our approach to cw-CRD spectroscopy offers high performance in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medical, agricultural, industrial and environmental situations.

We report the design and performance of a tunable, pulsed high resolution mid infrared cavity ringdown spectrometer. Stimulated Raman scattering in H2/D2 is used to downconvert the output of a SLM Alexandrite ring laser (720–800 nm) to the mid infrared (3–8 μm). The infrared frequency bandwidth was determined to be 90±5 MHz from measurements of Doppler broadened OCS transitions at 5 μm. The minimum detectable per pass fractional absorption is 1 ppm. We observe a frequency dependent ringdown cavity transmission of ±5 ppm due to spatial variations of the mirror reflectivity. The υ6 band of linear C9 formed by laser ablation of graphite in a He molecular beam was measured, showing a factor of 2 improvement in sensitivity relative to previous IR diode laser experiments. Based on calculated IR intensities, the number density of C9 in the molecular beam is 1.3*1011 molec/cm3 and the minimum detectable density is 1*109 molec/cm3. We expect this spectrometer to be a powerful tool for the study of transient specie...

Laser-based cavity ringdown techniques have demonstrated ultrahigh sensitivities for trace gas detection in the optical and infrared wavelength regions. We have investigated the applicability of the cavity ringdown technique in the millimeter wave region, which is rich in the rotational spectra of molecules. The millimeter-wave system uses a tunable Fabry–Perot cavity that is excited by a continuous-wave, phase-locked source at the W band; a fast PIN diode switch that turns off the excitation after the cavity is tuned to resonance; and a diode detector that records the resonance decay. Proof of concept has been established by measuring the ringdown times with absorbing materials in the cavity and comparing them with theoretical prediction.

Cavity-enhanced absorption techniques show considerable promise for applications in which a high detection sensitivity is required while maintaining a small probed volume of liquid sample. Conventional two-mirror cavity ringdown or cavity-enhanced absorption spectroscopy appears to provide a simple general 'benchtop' platform for measurements on flow cells and microfluidic chips with suitable optical properties, and recent progress in developing integrated cavities on-chip hints at the potential for future miniaturised self-contained microfluidic devices. © 2014 Springer-Verlag Berlin Heidelberg.

Cavity ringdown (CRD) absorption spectroscopy provides high sensitivity and photometric precision in the measurement of weak optical absorption spectra. We report several innovations in CRD technique involving either pulsed or continuous-wave (cw) tunable coherent sources. In the former case, we use narrowband, injection-seeded optical parametric oscillator (OPO) devices based on quasi phase-matched nonlinear-optical media and a novel intensity-dip cavity control scheme; one such pulsed OPO is pumped by a compact, in expensive multimode laser but still delivers single-mode OPO signal output. The latter area concerns cw-CRD spectroscopy with a single-mode tunable diode laser (TDL) and a rapidly swept ringdown cavity, most recently enhanced by a novel optical heterodyne approach that yields a noise-limited absorption sensitivity of 3 × 10−9 cm−1. Such pulsed and cw CRD methods are used to record weak rovibrational spectra of carbon dioxide gas (CO2) with high resolution in the 1.5-μm near-infrared region. Emphasis is on high-performance spectroscopic systems with relatively simple, in expensive, compact instrumentation.

We demonstrate a cavity ringdown spectrometer that aAords a frequency resolution higher than the conventional limit imposed by the bandwidth of a pulsed laser source. A Fabry‐Perot etalon at the exit of a ringdown cavity disperses the frequency components of a broadband ringdown signal spatially, permitting a narrow-linewidth spectral measurement based on single-exponential decay signals. A proof of principle experiment is presented with the P(9) transition of the â50a 10 0 0 0 band of C2H2. This technique is expected to be useful for in situ monitoring molecular concentration. ” 2000 Elsevier Science B.V. All rights reserved.

We present a new variant of the Cavity Ringdown Spectroscopy (CRDS) that is able to scan across more than 1400 nm of spectral bandwidth around 6000 nm, acquiring and analysing more than 150,000 spectral datapoints in less than four seconds.

We present a new variant of the Cavity Ringdown Spectroscopy (CRDS) that is able to scan across more than 1400 nm of spectral bandwidth, acquiring and analysing more than 150,000 spectral datapoints in less than four seconds.

We report what we believe to be the first systematic study of Doppler-free, nonlinear absorption by use of cavity ringdown spectroscopy. We have developed a variant of cavity ringdown spectroscopy for the mid-infrared region between 9 and 11 microm, exploiting the intracavity power buildup that is possible with continuous-wave lasers. The infrared source consists of a continuous-wave CO2 laser with 1-mW tunable infrared sidebands that couple into a high-finesse stable resonator. We tune the sideband frequencies to observe a saturated, Doppler-free Lamb dip in the nu7, 11(1,10)-- 11(2,10) rovibrational transition of ethylene (C2H4). Power studies of the Lamb dip are presented to examine the intracavity effects of saturation on the Lamb-dip linewidth, the peak depth, and the broadband absorption.

We demonstrate a coupling scheme for cavity enhanced absorption spectroscopy that makes use of an intracavity acousto-optical modulator to actively switch light into (and out of) a resonator. This allows cavity ringdown spectroscopy (CRDS) to be implemented with broadband nonlaser light sources with spectral power densities of less than 30μW/nm. Although the acousto-optical element reduces the ultimate detection limit by introducing additional losses, it permits absorptivities to be measured with a high dynamic range, especially in lossy environments. Absorption measurements for the forbidden transition of gaseous oxygen in air at ∼760nm are presented using a low-coherence cw-superluminescent diode. The same setup was electronically configured to cover absorption losses from 1.8×10sup-8/supcmsup-1/supto 7.5% per roundtrip. This could be of interest in process analytical applications.

A trace gas sensor for the detection of nitrogen dioxide based on cavity ringdown spectroscopy (CRDS) and a continuous wave external cavity tunable quantum cascade laser operating at room temperature has been designed, and its features and performance characteristics are reported. By measuring the ringdown times of the cavity at different concentrations of NO(2), we report a sensitivity of 1.2 ppb for the detection of NO(2) in Zero Air.

Cavity ringdown spectroscopy is an efficient gas-sensing method, but improvement in measurement speed is required before this method can be applied to the analysis of fast phenomena. We present a new continuous-wave cavity ringdown design, involving fast tuning of the laser frequency and a rapidly swept optical cavity, to allow high-speed sensing with spectral resolution refinement. This approach, which provides a simple and versatile instrument, is investigated numerically and experimentally. By performing detection of a forbidden transition of molecular oxygen near 766 nm during a 2-ms single sweep of the laser frequency, we show that our system fulfils the requirements for probing rapid chemical processes.