Multi-species detection using multi-mode absorption spectroscopy (MUMAS)

The detection of multiple species using a single laser and single detector employing multi-mode absorption spectroscopy (MUMAS) is reported. An in-house constructed, diode-pumped, Er:Yb:glass micro-laser operating at 1,565 nm with 10 modes separated by 18 GHz was used to record MUMAS signals in a gas mixture containing C₂H₂, N₂O and CO. The components of the mixture were detected simultaneously by identifying multiple transitions in each of the species. By using temperature- and pressure-dependent modelled spectral fits to the data, partial pressures of each species in the mixture were determined with an uncertainty of ±2 %.     

Multi-mode absorption spectroscopy

A method of absorption spectroscopy is reported that uses a comb of laser modes that is frequency tuned over the mode spacing. Absorption of individual modes in a gaseous absorber is detected by reduction in the total transmitted power as the modes are tuned yielding a signature characteristic of the absorbing species. Analysis of the signal provides information on the species concentrations, temperature and pressure. The potential of the method for rapid detection of multiple transitions with high resolution and wide spectral range is demonstrated using a multi-mode diode laser to detect multiple transitions in the A-band of molecular oxygen around 760 nm. Application of the technique to gas-phase thermometry with a precision of ±2 % is reported.     

Sensitivity enhancement of off-axis ICOS using wavelength modulation

In this paper, we analyze the sensitivity enhancement attainable by combining a wavelength modulation (WM) technique to integrated cavity output spectroscopy (ICOS), pointing out how the spectrometer??s parameters and the acquisition strategy affect the detection noise in both techniques. We point out that WM-ICOS is mainly limited by the slow scan rate that it requires, compared to regular ICOS. Nevertheless, according to our analysis, WM can still appreciably improve the SNR of an ultrasensitive ICOS system, if the cavity transmission is so low that the detector noise is not negligible. In light of these considerations, we directly compare the performance of ICOS and WM-ICOS in a high sensitivity ambient-air methane detection experiment, finding a good agreement with the theoretical influence of the various spectrometer parameters.     

The study of wavelength modulation off-axis integrated cavity output spectroscopy in the case of Lorentzian absorption profile

According to the property of optical cavity and the principle of signals and systems, the mathematical derivations for wavelength modulation off-axis integrated cavity output spectroscopy (WM-OA-ICOS) are presented. And based on the analytical method, the function expression of WM-OA-ICOS signal from Lorentzian absorption profile is obtained. To validate the analyses and deductions, some special cases in 2f-harmonic detection of WM-OA-ICOS are studied and experimented with a built compact WM-OA-ICOS apparatus. It is found that the measured results can coincide with the deduced conclusions. The works can provide some references for the application of WM-OA-ICOS in the case of Lorentzian absorption profile.     

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.     

Natural logarithm wavelength modulation spectroscopy

Natural logarithm wavelength modulation spectroscopy(ln-WMS) is demonstrated in this Letter. Unlike the conventional wavelength modulation spectroscopy(WMS)-2 f technique, it is a linear method even for large absorbance, which is the core advantage of ln-WMS. The treating method used in ln-WMS is to take the natural logarithm of the transmitted intensity. In order to determine the proper demodulation phase, the η-seeking algorithm is introduced, which minimizes the absolute value of the first harmonic within the non-absorbing region. Subsequently, the second harmonic of the absorption signal is extracted by setting the demodulating phase as 2η. To illustrate the validity of ln-WMS, it was applied to water vapor experimentally. The result shows that even if the absorbance(base-e) is between 1.60 and 6.26, the linearity between ln-WMS-2 f and volume fraction is still established. For comparison, measurement with conventional WMS-2 f was also done, whose response no longer kept linearity. The η values retrieved in continuous measurements and the residuals were shown so as to evaluate the performance of the η-seeking algorithm. Time consumed by this algorithm was roughly 0.28 s per measurement. As an alternative WMS strategy, ln-WMS has a wide range of potential applications, especially where the absorbance is large or varies over a wide area.     

Simulation-based comparison of noise effects in wavelength modulation spectroscopy and direct absorption TDLAS

A simulative investigation of noise effects in wavelength modulation spectroscopy (WMS) and direct absorption diode laser absorption spectroscopy is presented. Special attention is paid to the impact of quantization noise of the analog-to-digital conversion (ADC) of the photodetector signal in the two detection schemes with the goal of estimating the necessary ADC resolution for each technique. With laser relative intensity noise (RIN), photodetector shot noise and thermal amplifier noise included, the strategies used for noise reduction in direct and wavelength modulation spectroscopy are compared by simulating two respective systems. Results show that because of the combined effects of dithering by RIN and signal averaging, the resolutions required for the direct absorption setup are only slightly higher than for the WMS setup. Only for small contributions of RIN an increase in resolution will significantly improve signal quality in the direct scheme.     

波长调制-直接吸收光谱(WM-DAS)在线监测大气CO浓度

波长调制-直接吸收光谱(WM-DAS)同时具有直接吸收光谱(DAS)可测量吸收率函数和波长调制光谱(WMS)高信噪比的优点,本文首先采用WM-DAS光谱,在50 cm光程和室温低压下,CO分子近红外4300.7 cm^-1谱线吸收率检测限低至4×10^-7(200 s);然后结合120 m长光程Herriott池,在室温大气压下,吸收率函数拟合残差标准差达到5.1×10^-5(1 s).最后利用长光程WM-DAS测量系统,对不同浓度(体积分数为0.44×10^-6—9.6×10^-6)CO进行了动态测量,并将其与腔衰荡光谱(CRDS)进行比较;实验结果表明:本文采用的长光程WM-DAS与CRDS方法测量结果相同,其中长光程WM-DAS系统CO浓度检测限低至0.9×10^-9(200 s),系统简单且测量速度远快于CRDS.与此同时,利用建立的长光程WM-DAS测量系统连续监测1个月时间内大气痕量CO浓度及其变化趋势,测量结果与中国环境监测总站测量结题高度一致.     

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.     

Software-switching between direct absorption and wavelength modulation spectroscopy for the investigation of ADC resolution requirements

We present a tunable diode laser spectrometer that can be software-switched between a wavelength modulation spectroscopy (WMS) operation mode and the direct absorption spectroscopy (DAS) technique on a per-scan basis. The new setup allows a direct comparison between the two techniques under equal hardware and system environment conditions. In this work, we compare the impact of analog-to-digital converter resolution for operation in WMS and DAS mode. Example concentration evaluations show that in the DAS approach, relative absorptions smaller than the least significant bit can be measured with scan averaging and FIR-filtering applied. With full resolution, both techniques show similar performance in our setup, with a slight advantage for the WMS implementation.     

非标定波长调制吸收光谱气体测量研究

为消除可调谐激光调制吸收光谱气体测量技术对于标定过程的依赖,研究了二次谐波信号的非标定波长调制气体测量方法.通过对测量的二次谐波线型进行分析,给出相同工况下二次谐波模拟信号,并利用测量与模拟二次谐波信号进行线性拟合直接计算气体浓度.实验室内采用非标定波长调制气体测量方法,利用 6336.24 cm^-1处特征吸收谱线对10 cm长气体吸收池内的CO₂进行了测量.结果表明,非标定波长调制气体测量方法可适应各种不同条件,适合于现场气体在线测量.当调制系数在1.8—3. 关键词： 波长调制 二次谐波 吸收光谱 半导体激光器     

Absorption linewidth inversion with wavelength modulation spectroscopy

For absorption linewidth inversion with wavelength modulation spectroscopy(WMS), an optimized WMS spectral line fitting method was demonstrated to infer absorption linewidth effectively, and the analytical expressions for relationships between Lorentzian linewidth and the separations of first harmonic peak-to-valley and second harmonic zero-crossing were deduced. The transition of CO_2 centered at 4991.25 cm~(-1) was used to verify the optimized spectral fitting method and the analytical expressions. Results showed that the optimized spectra fitting method was able to infer absorption accurately and compute more than 10 times faster than the commonly used numerical fitting procedure. The second harmonic zero-crossing separation method calculated an even 6 orders faster than the spectra fitting without losing any accuracy for Lorentzian dominated cases. Additionally, linewidth calculated through second harmonic zero-crossing was preferred for much smaller error than the first harmonic peak-to-valley separation method. The presented analytical expressions can also be used in on-line optical sensing applications, electron paramagnetic resonance, and further theoretical characterization of absorption lineshape.     

Multi-harmonic detection in wavelength modulation spectroscopy systems

Two multi-harmonic detection methods for wavelength modulation spectroscopy (WMS) systems are presented and compared. The two possibilities discussed in this paper are: simultaneous curve fitting of multiple harmonic spectra, and reconstruction of the transmission from harmonic coefficients. The optimum number of harmonics is four and 25 harmonics, respectively. Compared with standard single-harmonic curve fitting, the methods give about a factor of 3 better performance than standard second-harmonic curve fitting. Concluding, multi-harmonic detection is better than single-harmonic detection and should be used if the system bandwidth is high enough to allow for proper detection of the higher harmonics.     

Multi-mode absorption spectroscopy of oxygen for measurement of concentration, temperature and pressure

The use of multi-mode absorption spectroscopy (MUMAS) to detect multiple transitions in the A-band b¹Σ_g ⁺-X³Σ_g ^- of molecular oxygen is reported. The modelling of MUMAS signatures and the procedure for fitting such model signatures to experimental data obtained using a multi-mode diode laser is described. The technique is shown to allow accurate and precise measurement of concentration, temperature over the range 300 to 500 K and of pressure over the range 0.2 to 1 bar. Extension of the technique to other ranges of temperature and pressure and to other species is discussed. PACS 42.62.Fi; 33.20.Kf     

利用离轴腔增强吸收光谱技术探测实际大气中的二氧化碳

以1.573μm窄线宽可调谐半导体激光器作光源,结合高精细度光学谐振腔的离轴腔增强吸收光谱技术,选择CO2在6 357.311 6cm-1的吸收谱线,对实际大气CO2分子进行了测量。为了得到更准确的有效方程,对谐振腔吸收程长的标定方法进行了研究,给出了一种简单、实用的标定方法。实验结果表明,高精密度光学谐振腔的有效吸收程长为～1 195.73m,测得实际大气CO2的浓度为～388.3ppm(S/N≈22),最小可探测浓度为17.65ppm。将波长调制技术与OA-CEAS技术结合后,最终将CO2分子的最小可探测浓度提高到0.36ppm(S/N≈1 064)。     

Characterization of lineshape structure by wavelength modulation spectroscopy

Wavelength Modulation Spectroscopy (WMS) is performed in the oxygen A-band at different harmonics for different modulation indices and optical pathlengths. The experimental data is compared to two models with different absorption profiles: one assuming a Lorentzian lineshape function and the other with a Voigt lineshape function. We show that the larger amount of structure in higher harmonic signals provides greater sensitivity to the type of lineshape profile utilized for modeling. A useful new feature explored in this work is optical path length saturation in WMS that was described for the first time in a recent paper from our group. We discuss the role of such saturation and how it can be utilized as a diagnostic to probe lineshape. We also address the effect of the ever present modulation broadening. Results of experiments in which sets of nine scans (direct absorption, 1f, 2f,??,8f; i.e., detection harmonic N=0,1,2,3,??,8) were made simultaneously are described and discussed. Finally, the role that increased structure plays??as a result of increasing order of detection, N, as well as from the modification of the signal profile with increasing optical thickness??is outlined from the perspective of classical information theory.     

High frequency wavelength modulation spectroscopy with diode lasers

High frequency wavelength modulation spectroscopy with diode lasers is accomplished by dithering the drive current at RF frequencies as high as 250 MHz. This technique is useful for fast and sensitive detection of absorption lines in the near-and mid-infrared spectral regions. Detection of 300 MHz wide spectral features corresponding to 1% changes in transmission is accomplished in time intervals as short as 500 ns. A potential application is for fast reading of information contained in frequency domain optical memories based upon photochemical hole burning.     

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.