Theoretical detection limit of saturated absorption cavity ring-down spectroscopy (SCAR) and two-photon absorption cavity ring-down spectroscopy

Giusfredi et al. (Phys Rev Lett 104, 110801, 2010), have developed a new approach to cavity ring-down spectroscopy where a saturable sample absorption is determined simultaneously with the cavity loss, providing immunity to changes in cavity loss, thereby allowing for lower analyte detection limit. This paper presents an error analysis that provides predictions of the ultimate sensitivity limits that can be realized with this detection method. In particular, the sensitivity is strongly dependent upon the initial degree of saturation of the sample, and optimal values for this are determined both for photon detector and shot-noise-limited detection of both inhomogeneous and homogeneous broadened spectroscopic lines. Also presented are sensitivity limits expected for two-photon absorption spectroscopy determined by cavity ring-down spectroscopy.     

Spectral line-shapes investigation with Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectroscopy

A review of recent experiments involving a newly developed Pound-Drever-Hall-locked frequency-stabilized cavity ring-down spectroscopy (PDH-locked FS-CRDS) system is presented. By comparison to standard FS-CRDS, the PDH lock of the probe laser to the ring-down cavity optimized coupling into the cavity, thus increasing the ring-down signal acquisition rate nearly 300-fold to 14 kHz and reducing the noise-equivalent absorption coefficient by more than an order of magnitude to 7 × 10^?11 cm^?1. We discuss how averaging approximately 1000 spectra yielded a signal-to-noise ratio of 220000. We also discuss how the spectrum frequency axis was linked to an optical frequency comb, thus enabling absolute frequency measurements of molecular optical transitions at sub-MHz levels. Applications of the spectrometer to molecular line-shape studies are also presented. For these investigations, we use semi-classical line-shape models that consider the influence of Dicke narrowing as well as the speed dependence of the pressure broadening and shifting to fit spectra. We show that the improved precision and spectrum fidelity of the spectrometer enable precise determinations of line-shape parameters. We also discuss the importance of line-shape analysis with regard to the development of new spectroscopic databases as well as in the optical determination of the Boltzmann constant.     

Intracavity laser absorption spectroscopy and cavity ring-down spectroscopy in low-pressure flames

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) have been used for measurement of the NH₂-radical spectrum near 643 nm. NH₂ was obtained in low-pressure methane/air flat flames doped with minor amounts of ammonia (as low as 0.023%). The NH₂ concentration was measured both by CRDS and ICLAS in the same conditions. This enables us to compare the practical sensitivity of the two methods. Both methods were also used for measurements in a sooting acetylene/air flame (ϕ = 2.6). The comparative advantages of the methods and their complementarities are discussed.     

高灵敏度光谱测量技术:腔振铃吸收光谱

腔振铃激光吸收光谱技术是近年快速发展的一项新颖的光谱技术,它不仅检测灵敏度高,而且结构简单,不需要高昂的光谱设备,特别适合于测量弱吸收物质,包括气体、固体、液体等稳态粒子和金属化合物、自由基、团簇等瞬态粒子.本文在介绍其基本原理的基础上,介绍了使用脉冲激光与连续激光光源的2种技术方案.     

Quantum-noise-limited optical frequency comb spectroscopy

We achieve a quantum-noise-limited absorption sensitivity of 1.7×10(-12) cm(-1) per spectral element at 400 s of acquisition time with cavity-enhanced frequency comb spectroscopy, the highest demonstrated for a comb-based technique. The system comprises a frequency comb locked to a high-finesse cavity and a fast-scanning Fourier transform spectrometer with an ultralow-noise autobalancing detector. Spectra with a signal-to-noise ratio above 1000 and a resolution of 380 MHz are acquired within a few seconds. The measured absorption line shapes are in excellent agreement with theoretical predictions.     

High-precision pressure shifting measurement technique using frequency-stabilized cavity ring-down spectroscopy

We describe a high-precision method for measuring pressure shifting of absorption lines. The technique involves the acquisition of high-resolution spectra using a cavity ring-down spectrometer whose length is continuously locked to a frequency-stabilized reference laser over a range of sample pressures. We discuss a relatively large correction arising from the pressure-dependence of dispersion in the cavity modes, and we demonstrate pressure shifting measurements in air for transitions in the ¹⁶O₂A-band. Pressure shifts in the range -0.011 to are reported. We measured relative positions of line centers to within 70 kHz and determined pressure shifting coefficients over a 5 kPa pressure range with relative uncertainties approximately equal to 1.0%, which constitutes a five-fold improvement over previous measurements.     

Frequency-stabilized cavity ring-down spectroscopy measurements of carbon dioxide isotopic ratios

Carbon dioxide (CO₂) isotopic ratios on samples of pure CO₂ were measured in the 1.6 μm wavelength region using the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) technique. We present CO₂ absorption spectra with peak signal-to-noise ratios as high as 28,000:1. Measured single-spectrum signal-to-noise ratios were as high as 8900:1, 10,000:1, and 1700:1 for ¹³C/¹²C, ¹⁸O/¹⁶O, and ¹⁷O/¹⁶O, respectively. In addition, we demonstrate the importance of utilizing the Galatry line profile in the spectrum analysis. The use of the Voigt line profile, which neglects the observed collisional narrowing, leads to large systematic errors which are transition-dependent and vary with temperature and pressure. While the relatively low intensities of CO₂ transitions near λ=1.6 μm make this spectral region non-optimal, the sensitivity and stability of FS-CRDS enabled measurement precision of pure CO₂ samples which are comparable to those of other optical techniques which operate at far more propitious wavelengths. These results indicate that a FS-CRDS spectrometer designed to probe CO₂ bands near wavelengths of 2.0 μm or 4.3 μm could achieve significantly improved precision over the present instrument and likely be competitive with mass spectrometric methods.     

Using a DS-DBR laser for widely tunable near-infrared cavity ring-down spectroscopy

Studies into the suitability of a novel, widely tunable telecom L-band (1,563–1,613 nm) digital supermode distributed Bragg reflector (DS-DBR) laser for cavity ring-down spectroscopy (CRDS) are presented. The spectrometer comprised of a 36.6?cm long linear cavity with ring-down times varying between 19–26 μs across the 50 nm DS-DBR wavelength range due to changes in the cavity mirror reflectivities with wavelength. The potential of such a broadband, high-resolution CRD spectrometer was illustrated by investigating several transitions of CO₂ in air, a 5 % calibrated mixture and breath samples. Allan variance measurements at a single wavelength indicated an optimal minimum detectable absorption coefficient (α _min) of 3 × 10^?10 cm^?1 over 20 s.     

Molecular oxygen detection in low pressure flames using cavity ring-down spectroscopy

Cavity ring down spectroscopy is used for measurement of the concentration profiles of oxygen in the low pressure (30 Torr) methane/nitrogen/oxygen flames. Three different equivalence ratios are used: 0.8, 1.0 and 1.17. Molecular oxygen concentration is monitored via rotational spectrum of b¹ Σ _g ⁺←X³ Σ _g ^- (v^′=0-v^′′=0) transition, also known as atmospheric A band, located near 750 nm. The P(15)P(15) line is used for concentration measurements. The sensitivity reached is 2.2×10^-8 cm^-1. The concentration profiles are in a good agreement with the ones calculated using GRI-3.0 mechanism. PACS 33.20.Kf; 33.70.Fd; 42.60.Da     

Direct detection of acetylene in air by continuous wave cavity ring-down spectroscopy

Diode laser-based continuous wave cavity ring-down spectroscopy (cw-CRDS) in the near-infrared region has been used to measure the mixing ratio of acetylene (C₂H₂) in ambient air. Detection limits of 120 parts per trillion by volume (pptv) for 20 min and 340 pptv for 70 s acquisition time were achieved without sample pre-concentration, measuring on a C₂H₂ absorption line at 6565.620 cm^?1 (～1523 nm). Several indoor and outdoor air samples were collected at different locations in the Helsinki metropolitan area and analyzed using static-cell measurements. In addition, flow measurements of indoor and outdoor air have been performed continuously over several days with a time resolution of down to one minute. Baseline acetylene levels in the range of 0.4 to 3 parts per billion by volume (ppbv), with a maximum around midday and a minimum during the night, were measured. Sudden high mixing ratios of up to 60 ppbv were observed in outdoor air during daytime on a minute time scale. In general, the indoor mixing ratios were found to be higher than those in outdoor air. The acetylene levels correlated with the ambient CO levels and with outdoor temperature.     

Off-axis re-entrant cavity ring-down spectroscopy with a mid-infrared continuous-wave optical parametric oscillator

We demonstrate an off-axis cavity ring-down spectroscopy system, which uses a mid-infrared continuous-wave optical parametric oscillator as a light source. Off-axis injection with re-entrant configuration of the ring-down cavity is used to achieve high spectral resolution while maintaining high measurement speed. This makes the setup suitable for sensitive molecular spectroscopy in the mid-infrared region, particularly for studies that require high temporal resolution. Formaldehyde (H₂CO) absorption spectrum at 3.4 µm is measured using the off-axis re-entrant cavity ring-down spectrometer.     

Detection of chemical warfare agents based on quantum cascade laser cavity ring-down spectroscopy

Chemical warfare agents(CWAs) are recognized as serious threats of terrorist acts against the civilian population.Minimizing the impact of these threats requires early detection of the presence of CWAs.Cavity ring-down spectroscopy(CRDS) is an exquisitely sensitive technique for the detection of trace gaseous species.In this letter,the CRDS technique is employed using a pulsed quantum cascade laser for the detection of dimethyl methylphosphonate(DMMP).A limit of DMMP detection of approximately 77 ppb is achieved.The best achievable sensitivity that corresponds to noise-equivalent absorption is approximately 2×10-7cm 1.     

Real-time measurement of nitrogen dioxide in vehicle exhaust gas by mid-infrared cavity ring-down spectroscopy

The application of pulsed cavity ring-down spectroscopy (CRDS) was demonstrated for the measurement of nitrogen dioxide (NO₂) in automotive exhaust gas. The transition of the ν ₃ vibrational band assigned to the antisymmetric stretching mode of NO₂ was probed with a thermoelectrically cooled, pulsed, mid-infrared, distributed feedback, quantum cascade laser (QCL) at 6.13 μm. The measurement of NO₂ in the exhaust gas from two diesel vehicles equipped with different aftertreatment devices was demonstrated using a CRDS-based NO₂ sensor, which employs a HEPA filter and a membrane gas dryer to remove interference from water as well as particulates in the exhaust gas. Stable and sensitive measurement of NO₂ in the exhaust gas was achieved for more than 30 minutes with a time resolution of 1 s.     

Investigation on an evanescent wave fiber-optic absorption sensor based on fiber loop cavity ring-down spectroscopy

An improved ring-down measurement principle for optical waveguides is presented. Fiber loop ring-down spectroscopy allows for measurement of minute optical losses in high-finesse fiber-optic cavities and immunity to the fluctuation of laser source. The evanescent wave absorption losses dependent on the absorption and the refractive index of ambient solution have been theoretically analyzed. The complex refractive index is introduced into our model and extinction coefficient can be calculated accurately through finite element analysis by setting the boundaries of the fiber and the ambient conditions. Using this method, the refractive index of environment can be taken into consideration. Our principle is validated by the highly-sensitive measurement of evanescent wave absorption loss. By chemically processing the surface of sensing segment along an extending ring-down cavity, the concentration of small volume Diethyl Sulphoxide solution where the etched fiber immersed into has been successfully measured and discussed.     

Widely tunable diffraction limited 1000 mW external cavity diode laser in Littman/Metcalf configuration for cavity ring-down spectroscopy

We report on recent progress on external cavity diode lasers (ECDL) using a new concept of a Littman/Metcalf configuration. Within this concept one facet of the diode laser chip is used for coupling to a high quality Littman/Metcalf resonator whereas the other side of the diode laser chip emits the output beam. The alignment of the external resonator is independent from the alignment of the output beam and there is no need for any compromise in the alignment. This results in an improved behavior of the external resonator with the benefit of a drastic increase in power and single mode tuning.We investigated this light source for high resolution spectroscopy in the field of cw-cavity ring-down spectroscopy (CRDS). The monitoring of environmental and medical gases from vehicles or human breath requires a suitable radiation source in the mid-infrared (MIR) between 3 and 5 μm that is frequency stable and can be widely tuned. Since this wavelength cannot be reached via direct emitting room temperature semiconductor lasers, additional techniques like difference frequency generation (DFG) are essential. Tunable difference frequency generation relies on high power, small linewidth, fast tunable, robust laser diode sources with excellent beam quality.With our new compact, alignment-insensitive and robust ECDL concept, we achieved an output power of 1000 mW and an almost Gaussian shaped beam quality (M²<1.2). The coupling efficiency for optical waveguides as well as single mode fibers exceeds 70%. The wavelength is widely tunable within the tuning range of 20 nm via remote control. This laser system operates longitudinally in single mode with a mode-hop free tuning range of more than 150 GHz without current compensation and a side-mode-suppression better than 50 dB. This concept is currently realized within the wavelength regime between 750 and 1080 nm.Our high powered Littman/Metcalf laser system was part of a MIR-light source which utilizes DFG in periodically poled lithium niobate (PPLN) crystals. At the wavelength of 3.3 μm we were able to achieve a high-resolution absorption spectrum of water with four different isotoplogues of H₂O components. This application clearly demonstrates the suitability of this laser for high-precision measurements. PACS 07.57.Ty; 42.55.Px; 42.62.Fi     

Biodegradability screening of soil amendments through coupling of wavelength-scanned cavity ring-down spectroscopy to multiple dynamic chambers

A system was developed for the automatic measurements of 13CO? efflux to determine biodegradation of extra carbon amendments to soils. The system combines wavelength-scanned cavity ring down laser spectroscopy (WS-CRDS) with the open-dynamic chamber (ODC) method. The WS-CRDS instrument and a batch of 24 ODC are coupled via microprocessor-controlled valves. Determination of the biodegradation requires a known δ13C value and the applied mass of the carbon compounds, and the biodegradation is calculated based on the 13CO? mixing ratio (ppm) sampled from the headspace of the chambers. The WS-CRDS system provided accurate detection based on parallel samples of three standard gases (13CO? of 2, 11 and 22 ppm) that were measured simultaneously by isotope ratio mass spectrometry (linear regression R2 = 0.99). Repeated checking with the same standards showed that the WS-CRDS system showed no drift over seven months.The applicability of the ODC was checked against the closed static chamber (CSC) method using the rapid biodegradation of cane sugar-δ13C-labeled through C4 photosynthesis. There was no significant difference between the results from 7-min ODC and 120-min CSC measurements. Further, a test using samples of either cane sugar (C4) or beetroot sugar (C3) mixed into standard soil proved the target functionality of the system, which is to identify the biodegradation of carbon sources with significantly different isotopic signatures.     

Suppression of ring-down in noise spectroscopy

Suppression of systematic noise artifacts which complicate the understanding of broadline NMR spectra remains a difficult problem. Many strategies for canceling noise have evolved; many appear designed for very specific hardware implementations. In this paper we address the problem of artifact suppression in noise-based (stochastic) NMR, where low power pulses applied at high duty cycle are used to probe the spectral frequencies found in an NMR or NQR experiment. While typical peak powers are reduced by four to six orders of magnitude as compared to conventional NMR experiments, this power reduction corresponds only to an approximate halving of the idealized electronic ring-down in a tuned circuit; where other systematic sources of noise contribute, the recovery time advantage of noise spectroscopy may be larger or smaller. We suggest a simple experimental modification which exploits the linear response properties of nuclear spins in the presence of small rotations to demonstrate how ring-down—from whatever source—can be substantially eliminated as a problem in noise spectroscopy. This should provide substantial improvements in quantitation and lineshape measurements in NMR spectra of many solid state systems.     

Quantification of stable minor species in confined flames by cavity ring-down spectroscopy: application to NO

Cavity ring-down spectroscopy (CRDS) is used to measure the NO mole fraction formed in the burnt gases of low-pressure premixed flames. It is shown that the line-of-sight absorption is greatly increased by the contribution of the NO molecules surrounding the burner. This contribution has been quantified by developing a mathematical procedure taking into account the spatial and spectral features of the CRDS measurement. Calculations have been undertaken in the general case of a stable species not consumed in the flame. The most sensitive parameter is the temperature both in the flame and outside the flame. Simulations allow the selection of the best spectroscopic transitions for a given flame (i.e. a given temperature profile), ensuring the weakest influence of the inaccuracy affecting the temperature determination. High quantum states belonging to the A–X (0–1) band of NO have been found to be the most valuable and have led to a NO mole fraction determination with an accuracy of ±13%. NO absorption in the flame was completely masked using the A–X (0–0) band. Finally, the prompt-NO mole fraction formed in a methane/air flame stabilized at 33 Torr is obtained by combining CRDS and laser induced fluorescence techniques. Received: 12 October / Revised version: 1 February 2002 / Published online: 14 March 2002