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     

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.     

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.     

Application of a broadband blue laser diode to trace NO2 detection using off-beam quartz-enhanced photoacoustic spectroscopy

We applied for the first time, to our knowledge, broadband off-beam quartz-enhanced photoacoustic spectroscopy (BB-OB-QEPAS) to trace NO2 detection using a broadband blue laser diode centered at 450?nm. A detection limit of 18?ppbv (parts in 10(9) by volume) for NO2 in N2 at atmospheric pressure was achieved with an average laser power of 7?mW at a 1?s integration time, which corresponds to a 1?σ normalized noise equivalent absorption coefficient of 4.1×10(-9) cm(-1)?W=Hz(1=2). An Allan variance analysis was performed to investigate the long-term stability of the BB-OB-QEPAS-based NO2 sensor.     

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.     

可调谐二极管激光吸收光谱二次谐波检测方法的研究

可调谐二极管激光吸收光谱技术(TDLAS)是利用二极管激光器的波长扫描和电流调谐特性来实现痕量气体吸收曲线二次谐波检测的一种新技术,具有高灵敏、高选择性、高精度等特点。理论分析和实验结果表明:利用二次谐波检测到的信号特征除了与被测气体的吸收特征有关外,还与波长扫描参数和电流调制参数有密切的关系。通过研究不同波长的扫描参数和在功率调制参数下的二次谐波曲线,分析了它们的波形特征和稳定性,以便寻求最佳的波长扫描参数和功率调制参数,从而使二次谐波曲线的稳定性和形状达到最佳。     

Effect of optical feedback noise on tunable diode laser spectroscopy

The effect of optical feedback noise of a laser diode on the stability of gas-absorption spectra was investigated by monitoring the optical absorption of CO₂ gas. Lasing longitudinal mode instability greatly degraded the shape of the absorption spectra, and coherence collapse in the laser diode greatly broadened the spectra. The correlation between the optical feedback noise of the laser diode and the shape of the gas-absorption spectra was clarified on the basis of these spectral behaviors. PACS 42.62.Fi; 42.55.Px; 42.68.Ca     

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.     

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.     

Cavity-enhanced absorption spectroscopy with a rapidly swept diode laser

Cavity-enhanced absorption spectroscopy is explained in terms of the transmission function of a rapidly swept interferometer, and the integrated transmission is shown to be proportional to the cavity ringdown time. The technique is demonstrated on the b¹Σ_g ⁺-X³Σ_g ^-  (1,0) band in molecular oxygen at 687 nm using a tunable diode laser and a relative-ly high-Q optical cavity (finesse ≈4000). A detection limit of 3×10^-8 cm^-1 s^1/2 is achieved for a 0.8 cm^-1 scanning range. Received: 24 June 2002 / Revised version: 5 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +44-1865/275410, E-mail: peverall@physchem.ox.ac.uk     

Influence of laser intensity in second-harmonic detection with tunable diode laser multi-pass absorption spectroscopy

Tunable diode laser absorption spectroscopy （TDLAS） has been widely employed in atmospheric trace gases detection. The ratio of the second-harmonic signal to the intensity of laser beam incident to the multi-pass cell is proved to be proportional to the product of the path length and the gas concentration under any condition. A new calibration method based on this relation in TDLAS system for the measurement of trace gas concentration is proposed for the first time. The detection limit and the sensitivity of the system are below 110 and 31ppbv （parts-per-billion in volume）, respectively.     

Approach to optical interference fringes reduction in diode laser absorption spectroscopy

The advantage of a new scheme for balanced detection has been investigated to reduce the influence of optical interference fringes when performing diode laser gas absorption spectroscopy employing lock-in amplifiers and pigtailed lasers. The influence of the fringes has been reduced by comparing the lock-in 2 f signal due to the gas sample with that of a reference beam. The frequency regions outside the absorption feature have been used to obtain information on the interference fringe impact on the signal of interest. We have demonstrated an efficient way to reduce the influence of such fringes by employing this technique combined with non-linear signal processing methods. The different steps of the algorithm are presented. In the experimental arrangement presented, a reduction of the optical interference fringes by about 10 times is achieved, as demonstrated in measurements on molecular oxygen around 761 nm. The new technique is compared with an analog technique for balanced detection and certain advantages of the computer algorithm are pointed out. In particular, the emerging field of gas spectroscopy in scattering solid media strongly benefits from the technique presented. PACS 42.55.Px; 39.30.+w; 42.25.Hz     

Quantitative detection of singlet O2 by cavity-enhanced absorption

A method for the practical determination of the absolute concentration of single (a1delta(g)) oxygen is discussed. The method is based on sensitive off-axis integrated-cavity-output spectroscopy (ICOS). Off-axis ICOS allows narrowband, continuous-wave lasers to be used in conjunction with optical cavities to record sensitive absorption measurements. The details of the method as well as spectroscopic data confirming the first observation of the (1, 0) band of the b1sigma(g)(+) - a1delta(g) Noxon system are presented. The absolute transition probabilities for the b1sigma(g)(+) - a1delta(g) Noxon system, which are not known precisely from experiments, are determined by quantum chemistry theory.     

Sensitive detection of methane with a 1.65 μm diode laser by photoacoustic and absorption spectroscopy

Received: 18 September 1997/Revised version: 11 November 1997     

Atomic gallium laser spectroscopy with violet/blue diode lasers

We describe the operation of two GaN-based diode lasers for the laser spectroscopy of gallium at 403 nm and 417 nm. Their use in an external cavity configuration has enabled us to investigate of absorption spectroscopy in a gallium hollow cathode. We have analyzed the Doppler-broadened profiles, accounting for hyperfine and isotope structure and extracting both the temperature and density of the neutral atomic sample produced in the glow discharge. We have also built a setup to produce a thermal atomic beam of gallium. By using the GaN-based diode lasers, we have studied the laser-induced fluorescence and hyperfine-resolved spectra of gallium. PACS 42.55.Px; 42.60.-v; 32.30.-r; 03.75.Be     

Self-calibration wavelength modulation spectroscopy for acetylene detection based on tunable diode laser absorption spectroscopy

The expressions of the second harmonic(2f) signal are derived on the basis of absorption spectral and lock-in theories.A parametric study indicates that the phase shift between the intensity and wavelength modulation makes a great contribution to the 2f signal.A self-calibration wavelength modulation spectroscopy(WMS) method based on tunable diode laser absorption spectroscopy(TOLAS) is applied,combining the advantages of ambient pressure,temperature suppression,and phase-shift influences elimination.Species concentration is retrieved simultaneously from selected 2f signal pairs of measured and reference WMS-2f spectra.The absorption line of acetylene(C_2H_2) at 1530.36 nm near-infrared is selected to detect C_2H_2 concentrations in the range of 0-400 ppmv.System sensitivity,detection precision and limit are markedly improved,demonstrating that the self-calibration method has better detecting performance than the conventional WMS.     

High-sensitivity detection of NO2 using a 740 nm semiconductor diode laser

 An AlGaAs diode laser was used to detect NO₂ absorption lines belonging to the (0 0 0)–(2 13 1) vibrational band, within the X˜² A ₁ electronic ground state, at 739 nm. A simple absorption spectrometer based on wavelength-modulation spectroscopy with second-harmonic detection was developed. The minimum detectable pressure of pure NO₂ was 0.1 μbar with 2 m absorption path-length, corresponding to an absorbance of 10^-6. High-sensitivity detection of NO₂ was also performed in the presence of N₂ and air at different total pressures: The effects on the detection limit of our apparatus were accurately investigated. The minimum NO₂ concentration at 500 mbar of air was measured to be 2 ppm. Received: 11 June 1996 / Revised version: 11 October 1996     

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     

利用多轴差分吸收光谱技术反演对流层NO2

介绍了基于多轴差分吸收光谱技术(MAX-DOAS)反演对流层NO2的方法.利用差分吸收光谱技术(DOAS),扣除太阳夫琅和费结构及Ring效应的影响,拟合得到了大气中NO2的差分斜柱浓度dSCD,结合不同观测方向的测最结果分析得到了对流层大气中NO2的差分斜柱浓度(△SCID),结合辐射传输模型SCIATRAN计算得到了大气质量因子(AMF),并进一步计算得到了对流层NO2的垂直柱浓度(VCD)信息.为确保数据的准确性和可比性,将计算结果与长光程差分吸收光谱仪(LP-DOAS)的测量结果进行对比,二者具有较好的一致性,其相关系数R2分别为0.940 27和0.969 24.