Optical-feedback cavity-enhanced absorption spectroscopy with a quantum cascade laser

Optical-feedback cavity-enhanced absorption spectroscopy is demonstrated in the mid-IR by using a quantum cascade laser (emitting at 4.46 μm). The laser linewidth reduction and frequency locking by selective optical feedback from the resonant cavity field turns out to be particularly advantageous in this spectral range: It allows strong cavity transmission, which compensates for low light sensitivity, especially when using room-temperature detectors. We obtain a noise equivalent absorption coefficient of 3 × 10(-9)/cm for 1 s averaging of spectra composed by 100 independent points. At 4.46 μm, this yields a detection limit of 35 parts in 10(12) by volume for N(2)O at 50 mbar, corresponding to 4 × 10(7) molecules/cm(3), or still to 1 fmol in the sample volume.     

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum-cascade laser yields the lowest formaldehyde detection limit

We report on the first application of Optical Feedback-Cavity Enhanced Absorption Spectroscopy to formaldehyde trace gas analysis at mid-infrared wavelengths. A continuous-wave room-temperature, distributed-feedback quantum cascade laser emitting around 1,769 cm^?1 has been successfully coupled to an optical cavity with finesse 10,000 in an OF-CEAS spectrometer operating on the ν₂ fundamental absorption band of formaldehyde. This compact setup (easily transportable) is able to monitor H₂CO at ambient concentrations within few seconds, presently limited by the sample exchange rate. The minimum detectable absorption is 1.6 × 10^?9 cm^?1 for a single laser scan (100 ms, 100 data points), with a detectable H₂CO mixing ratio of 60 pptv at 10 Hz. The corresponding detection limit at 1 Hz is 5 × 10^?10 cm^?1, with a normalized figure of merit of 5 × 10^?11cm $^{-1}/\sqrt{\rm Hz}$ (100 data points recorded in each spectrum taken at 10 Hz rate). A preliminary Allan variance analysis shows white noise averaging down to a minimum detection limit of 5 pptv at an optimal integration time of 10 s, which is significantly better than previous results based on multi-pass or cavity-enhanced tunable QCL absorption spectroscopy.     

Optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) in a ring cavity

Optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) has been demonstrated by coupling a distributed feedback diode laser to a ring cavity. Frequency-selected light decaying from the ring cavity is retro-reflected, inducing a counter-propagating intra-cavity beam, and providing optical feedback to the laser. At specific laser-to-cavity distances, all cavity mode frequencies return to the diode laser with the same phase, allowing spectra to be accumulated across the range of frequencies of the current-tuned laser. OF-CEAS has been used to measure very weak oxygen isotopologue (¹⁶O¹⁸O and ¹⁶O¹⁷O) absorptions in ambient air at wavelengths near 762 nm using the electric-dipole forbidden O₂ A-band. A bandwidth reduced minimum detectable absorption coefficient of 2.2×10⁻⁹ cm⁻¹ Hz^−1/2 is demonstrated.     

Simultaneous cavity-enhanced and cavity ringdown absorption spectroscopy using optical feedback

We present a scheme of optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) including a fast optical switch to produce cavity ringdown spectra (OF-CRDS) simultaneously. This also works as a dynamically adjustable variable attenuator allowing to compensate for reduced signal levels in correspondence with absorption lines. For this, an acousto-optic deflector is used in a double-pass configuration to eliminate the single-pass frequency shift, which is incompatible with optical feedback. This is probably the most effective device providing the required fast response and the high extinction ratio necessary to perform clean ringdown measurements. The resulting direct comparison of OF-CEAS and OF-CRDS shows that these produce almost equivalent spectral data, with 0.3 % maximal difference at the top of an absorption line having a signal-to-noise ratio (S/N) of 3,300. OF-CEAS is largely winning on the short-term noise level while OF-CRDS appears to be more immune from interference fringes, delivering cleaner spectra after longer averaging.     

Optimisation of laser linewidth and cavity alignment in off-axis cavity-enhanced absorption spectroscopy

Laser linewidth affects baseline mode structured variations and hence measurement absorption sensitivity in off-axis cavity enhanced absorption spectroscopy with a continuous-wave tunable laser and a stable optical cavity formed by two high reflectivity mirrors. Cavity transmittances have been calculated for various laser linewidths and different optical beam re-entrant conditions for the cavity when overlapping of the optical beams occurs on the cavity mirrors after a finite number of beam round trips within the cavity. It is shown that in order to achieve maximum absorption sensitivity both a specific laser linewidth and specific arrangement of the optical cavity have to be selected and defined using the proposed approach.     

Laser lineshape effects on cavity-enhanced absorption spectroscopy signals

The quantitative effects of laser lineshape on signals from cavity-enhanced absorption spectroscopy (CEAS) and integrated cavity output spectroscopy (ICOS) experiments are examined. The governing equations for CEAS signals including the laser lineshape are derived. Approximations under which the laser lineshape may be neglected or replaced with an effective lineshape are presented. It is shown that the laser lineshape effects may be parameterized with two dimensionless variables: the laser linewidth normalized by the absorption linewidth, and the peak sample absorbance normalized by the mirror loss. In terms of the dimensionless variables, we simulate CEAS and ICOS signals and the absorbances inferred from them. The simulation results provide a useful tool for CEAS and ICOS practitioners to gauge the importance of laser lineshape effects in specific experiments. Simulations are performed for the four combinations of Gaussian and Lorentzian lineshapes for the laser and the absorption. PACS 42.62.Fi; 78.40.-q; 32.70.Jz     

Open-path trace gas detection of ammonia based on cavity-enhanced absorption spectroscopy

A compact open-path optical ammonia detector is developed. A tunable external-cavity diode laser operating at 1.5 μm is used to probe absorptions of ammonia via the cavity-enhanced absorption (CEA) technique. The detector is tested in a climate chamber. The sensitivity and linearity of this system are studied for ammonia and water at atmospheric pressure. A cluster of closely spaced rovibrational overtone and combination band transitions, observed as one broad absorption feature, is used for the detection of ammonia. On these molecular transitions a detection limit of 100 ppb (1 s) is determined. The ammonia measurements are calibrated independently with a chemiluminescence monitor. Compared to other optical open-path detection methods in the 1–2 μm region, the present result shows an improved sensitivity for contactless ammonia detection by over one order of magnitude. Using the same set-up, a detection limit of 100 ppm (1 s) is determined for the detection of water at atmospheric pressure. Received: 19 January 2000 / Revised version: 6 March 2000 / Published online: 7 June 2000     

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEM_oo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm⁻¹) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10⁻¹⁰ cm⁻¹ for 1 s averaging over scans, with a dynamic range of four orders of magnitude.     

Sub-ppb NO2 detection by optical feedback cavity-enhanced absorption spectroscopy with a blue diode laser

We report preliminary results on the first application of optical feedback cavity-enhanced absorption spectroscopy with a blue (411 nm) extended cavity diode laser (ECDL) for NO₂ detection. While this technique was originally developed to operate with distributed feedback diode lasers in the near infrared, it is here extended to ECDLs and applied to the blue spectral region. With a simple and compact optical setup, we demonstrate from the baseline noise a minimum detectable NO₂ concentration of 6×10⁹ molecules/cm³ for a single laser scan (70 ms), which extrapolated under atmospheric conditions corresponds to 200 pptv. Signal averaging should allow further lowering of this limit. Observed absorption spectra display more structure than previous spectra obtained at lower resolution by Fourier-transform spectroscopy at the same wavelength. PACS 07.88.+y; 42.55.Px; 42.62.Fi     

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.     

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.     

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.     

Off-axis continuous-wave cavity-enhanced absorption spectroscopy of narrow-band and broadband absorbers using red diode lasers

We present an application of continuous-wave (cw) cavity-enhanced absorption spectroscopy (CEAS) with off-axis alignment geometry of the cavity and with time integration of the cavity output intensity for detection of narrow-band and broadband absorbers using single-mode red diode lasers at λ=687.1 nm and λ=662 nm, respectively. Off-axis cw CEAS was applied to kinetic studies of the nitrate radical using a broadband absorption line at λ=662 nm. A rate constant for the reaction between the nitrate radical and E-but-2-eneof (3.78±0.17)×10^-13 cm³ molecule^-1 s^-1 was measured using a discharge-flow system. A nitrate-radical noise-equivalent (1σ≡ root-mean-square variation of the signal) detection sensitivity of 5.5×10⁹ molecule cm^-3 was achieved in a flow tube with a diameter of 4 cm and for a mirror reflectivity of ∼99.9% and a lock-in amplifier time constant of 3 s. In this case, a noise-equivalent fractional absorption per one optical pass of 1.6×10^-6 was demonstrated at a detection bandwidth of 1 Hz. A wavelength-modulation technique (modulation frequency of 10 kHz) in conjunction with off-axis cw CEAS has also been used for recording 1f- and 2f-harmonic spectra of the ^RR(15) absorption of the b¹Σ_g ⁺-X³Σ_g ^- (1,0) band of molecular oxygen at =14553.947 cm^-1. Noise-equivalent fractional absorptions per one optical pass of 1.35×10^-5, 6.9×10^-7 and 1.9×10^-6 were obtained for direct detection of the time-integrated cavity output intensity, 1f- and 2f-harmonic detection, respectively, with a mirror reflectivity of ∼99.8%, a cavity length of 0.22 m and a detection bandwidth of 1 Hz. Received: 24 June 2002 / Revised version: 12 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: vlk@physchem.ox.ac.uk     

Characterisation of the potential of frequency modulation and optical feedback locking for cavity-enhanced absorption spectroscopy

A combination of optical feedback self-locking of a continuous-wave distributed feedback diode laser to a V-shaped high finesse cavity, laser phase modulation at a frequency equal to the free spectral range of the V-cavity and detection of the transmitted laser beam at this high modulation frequency is described for the possible application in cavity-enhanced absorption spectroscopy. In order to estimate the noise level of an absorbance baseline, the triplet of frequency modulated light, i.e. the central laser frequency and the two sidebands, were transmitted through both the V-cavity in open air and a 1.5-cm long optical cell placed behind the cavity output mirror and filled with acetylene (C₂H₂) at low pressure. The performance of the setup was evaluated from the measured relative intensity noise on the cavity output (normalised by the bandwidth) and the frequency modulation absorption signals induced by C₂H₂ absorption in the 1.5-cm cell. From these data, we estimate that the noise-equivalent absorption sensitivity of 2.1 × 10^?11 cm^?1 Hz^?1/2—by a factor of 11.7 above the shot-noise limit—can be achieved for C₂H₂ absorption spectra extracted from the heterodyne beat signals recorded at the transmission maxima intensity peaks of the successive TEM₀₀ resonances.     

In situ measurements of atmospheric NO2 using incoherent broadband cavity-enhanced absorption spectroscopy with a blue light-emitting diode

We describe the application of incoherent broadband cavity-enhanced absorption spectroscopy(IBBCEAS) for in situ measurements of atmospheric NO2 using a blue light-emitting diode.The mirror reflectivity is determined by the transmitted intensity variation through the cavity caused by Rayleigh scattering.Concentrations of atmospheric NO2(1 to 35 ppbv) during the seven-day period are retrieved from the absorption spectra.The IBBCEAS measurement data are compared with those of a commercial long path differential optical absorption spectroscopy.The linear regression has a correlation coefficient and a slope     

Optical feedback cavity-enhanced absorption spectroscopy for in situ measurements of the ratio 13C:12C in CO2

We report on the design and laboratory performance of a portable infrared absorption spectrometer for the measurement of the isotopic ratio ¹³C:¹²C in CO₂. The design relies on optical feedback cavity-enhanced absorption spectroscopy in the 2 μm spectral region to achieve optimal performance at ambient CO₂ concentrations. The prototype instrument measures δ¹³C, relative to a standard calibration bottle, with a precision of ±0.7‰ for a 20-s integration time and with an automatic recalibration every 6 min. The absolute accuracy obtained is 0.9‰. The principal performance limitations are discussed along with improvements currently being implemented for the second generation instrument. PACS  42.62.Fi; 07.57.Ty; 33.20.Ea     

Three-level depletion by cavity ringdown absorption spectroscopy: proof of concept

Theoretical quantitative considerations as well as experimental data are presented based on absorption population depletion coupled with cavity ringdown spectroscopy. The absorbing number densities inside the cavity are determined by numerical integration of the coupled rate equations. The number of photons involved in absorption, cavity losses due to mirror reflectivity and stimulated emission are taken into account. The principle is to monitor a first transition by cavity ringdown spectroscopy while a second transition, with a state in common, is resonantly excited by the decaying radiation of different frequency also trapped inside the optical cavity. A numerical example is given for atomic lines of neon and the measurements carried out in a supersonic slit-jet expansion discharge demonstrate the feasibility of the technique. The technique is also proven to work with two resonant transitions of C₂. Translational velocity of the jet modifying the rate equations is included in the model.     

Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy: Current status and future potential

As a result of a combination of an external cavity and modulation techniques, noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is one of the most sensitive absorption techniques, capable of reaching close-to-shot-noise sensitivities, down to 5×10^-13 fractional absorption at 1 s averaging. Due to its ability to provide sub-Doppler signals from weak molecular overtone transitions, the technique was first developed for frequency standard applications. It has since then also found use in fields of molecular spectroscopy of weak overtone transitions and trace gas detection. This paper describes the principles and the unique properties of NICE-OHMS. The historical background, the contributions of various groups, as well as the performance and present status of the technique are reviewed. Recent progress is highlighted and the future potential of the technique for trace species detection is discussed. PACS  33.80.-b; 07.57.-c; 42.62.Fi