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.     

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.     

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.     

Measurement of the wavelength modulation indices with selective reflection spectroscopy

The wavelength modulation indices are measured based on harmonic amplitude ratio of 4famp/6famp (4famp and 6famp are the 4- and 6-th harmonic central peak amplitudes correspondingly) with the Doppler-free selective reflection modulation spectroscopy. The experiments for the 6S1/2(F = 4) → 6P3/2(F' = 5) transition of cesium D2 line with 30-MHz linewidth were carried out. The 4f- and 6f-harmonic signals were detected with two digital lock-in amplifiers separately. The maximum error for modulation indices measurement was ±0.1 within the range of m from 3 to 6. The non-linear modulation behaviour of an external cavity diode laser induced by voltage tuning was studied with this method. The method for modulation indices measurement does not require a solid etalon as usual for measuring the wavelength modulation depth and the absorption linewidth correspondingly.     

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.     

Broadening-independent demodulation for wavelength modulation spectroscopy based on even-order harmonics

The line broadening of gas absorption spectrum is a significant but unknown factor in traditional demodulation of wavelength modulation spectroscopy, which is a kind of tunable diode laser absorption spectroscopy in weak absorption cases. In this paper, another signal demodulation method based on some simple algebraic operations with the amplitudes of the zero-, second-, and fourth-order harmonics was proposed. With this method the effect of spectrum broadening could be selfeliminated. That makes the amplitude of the demodulated signal is irrelevant to the broadening, and enables the direct and fast measurement without complex simulations and curve fittings in traditional method.     

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.     

宽波长范围窄线宽波长连续可调的光纤激光器

研制了一种宽波长范围窄线宽波长连续可调的光纤激光器,激光器基于环形腔结构,采用980nm的半导体激光器作为泵浦源,掺铒光纤作为增益介质,使用一段未泵浦的保偏掺铒光纤作为饱和吸收体压窄线宽。使用光纤法布里-珀罗滤波器作为选频器件,通过调节光纤其驱动电压实现光纤激光器的波长扫描。利用激光器的相干长度与干涉仪干涉条纹的关系动态测量激光器的光谱线宽,最终得到了扫描范围为1515.1～1588.6nm,线宽小于0.025nm的波长连续可调的光谱输出。     

Calibration-free wavelength modulation spectroscopy for gas concentration measurements under low-absorbance conditions

<正>We derive the expressions of the first and second harmonic signals on the basis of absorption spectral and lock-in theories,and determine the gas concentration according to the ratio of second and first harmonic signals.It is found that the X and Y components of the harmonic signals are influenced by the phase shift between the detection and reference signal,and the phase shift can be any value in a range from 0 to 2π,which is different from the results obtained previously.Meanwhile,an additional item caused by the residual amplitude modulation will make a great contribution to the second harmonic signal,and may not be neglected under low absorbance conditions.Theoretical analysis indicates that subtracting back-ground signal from the second harmonic signal can remove the influence of this item,and can improve the measurement accuracy of gas concentration.On this basis,we select the transition of CO₂ at 6527.64 cm^-1 to analyse the approximation errors during the derivation by numerical simulation and then measure the CO₂ concentration under low absorbance conditions,with absorbance varying from l%o to 6‰.     

On the wavelength modulation spectroscopy of exciton lines and interference fringes

The lineshape of the wavelength modulation spectra is discussed for the case of strong modulation, when wavelength changes are comparable to the singularity width. Such a procedure is proposed in order to increase the sensitivity of the interference measurements under the weak contrast conditions.     

Novel discretely tunable narrow linewidth fiber laser with uniform wavelength spacing

A novel configuration of the tunable fiber laser with uniform wavelength spacing in dense wavelength division multiplexing (DWDM) application is proposed. The ring type tunable fiber laser consists of an all-fiber comb filter which determines the wavelength spacing, and a piece of adjustable fiber grating to select the discrete lasing wavelength for WDM application. The proposed all-fiber ring type tunable laser has potential application in the DWDM and other optical systems due to its advantages such as narrow linewidth, easy tuning, uniform wavelength interval, etc..     

基于波长调制光谱技术的线宽测量理论及其应用研究

气体吸收谱线的线宽主要包括碰撞作用引起的洛伦兹线宽和分子热运动引起的高斯线宽, 是可调谐二极管激光吸收光谱技术(TDLAS)的重要参数. 本项研究在弱吸收条件下, 通过对波长调制法中二次与四次谐波峰值比进行理论分析和仿真计算, 发现无论洛伦兹和高斯线宽如何变化, 二次和四次谐波峰值比具有恒过不动点的特征. 本文基于该不动点提出了一种线宽在线测量的方法, 并以CO₂分子6982.0678 cm^-1 吸收谱线为例进行实验验证. 实验结果表明, 该方法可以精确测量线宽, 进而根据测量得到的线宽确定气体分压和总压, 可有效地提高TDLAS技术在工业现场中的测量精度.     

A pressure-calibration method of wavelength modulation spectroscopy in sealed microbial growth environment

A new pressure-calibration method for calibrating the reduction of second harmonic(2 f) amplitude caused by pressure broadening effect in sealed microbial growth environment is present. The new method combines with linewidth compensation and modulation depth compensation and makes the 2 f amplitude accurately retrieve metabolic CO_2 in microbial growth. In order to verify the method, a simulation experiment is developed, in which the increasing CO_2 concentration leads to the increasing pressure. Comparing with the relation between the traditional 2 f amplitude and gas concentration,there is a monotonous relation between the calibrated 2 f amplitude and CO_2 concentration, particularly, a linear relation is present when the CO_2 concentration is replaced with the CO_2 particle number. In terms of microbial measurement,the growth of Escherichia coli is measured, and the culture bottle is sealed during the microbial growth process. The experimental results show that, comparing to the microbial growth retrieved by traditional 2 f amplitude, the calibrated 2 f amplitude can accurately retrieve microbial growth in sealed environment.     

Multi-mode absorption spectroscopy, MUMAS, using wavelength modulation and cavity enhancement techniques

Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band $b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-}Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band b¹\varSigma _g⁺? X³\varSigma _g^-b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-} of molecular oxygen at 760 nm. Direct MUMAS and WMS-MUMAS signals were recorded using a White cell for air and pure oxygen for pressures in the range 0 to 1 bar. CEAS-MUMAS signals were recorded with and without WMS in an open enhancement cavity containing laboratory air. Enhancement of the signal-to-noise ratio has been obtained demonstrating the potential for increased detection sensitivity for gas-sensing applications of MUMAS.     

Optical pathlength saturation signatures in wavelength modulation spectroscopy signals of atmospheric molecular oxygen

We discuss experimental and theoretical results of absorption features of the oxygen A-band transitions in the optically thick regime when synchronous detection at higher harmonics (N≥2), using wavelength modulation spectroscopy (WMS), is performed. We show that the absorption saturation effects demonstrate a distinctive feature which results in suppression of the line-center lobes of the harmonic signals. These effects depend on the optical pathlength as well as on the modulation index. The rich structure of WMS signals, especially at higher detection orders, is central to the technique’s advantages in resolving spectra congested with highly disparate oscillator-strength lines. The effect of pathlength saturation on this structure is investigated.     

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.     

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.     

Surface-state linewidth from scanning tunnelling spectroscopy

The STM can be used to investigate lifetimes of hot holes in a surface state at low temperatures. We analysed dI/dV data from Ag(111) using detailed tunnelling calculations and a simple model and found an electron self-energy of Σ=4.9±0.6 meV. The corresponding lifetime τ= 67±8 fs is considerably higher than those determined by angle-resolved photoemission, although it remains below theoretical predictions. Spatially resolved dI/dV spectra reveal that the lifetime decreases drastically in proximity to defects such as surface steps. Received: 22 March 1999 / Accepted: 25 June 1999 / Published online: 16 September 1999