Study on the radiation and self-absorption characteristics of plasma under various background gases.

The self-absorption effect is a primary factor responsible for the decline in the precision of quantitative analysis techniques using plasma emission spectroscopy, such as laser-induced breakdown spectroscopy (LIBS). In this study, based on the thermal ablation and hydrodynamics models, the radiation characteristics and self-absorption of laser-induced plasmas under different background gases were theoretically simulated and experimentally verified to investigate ways of weakening the self-absorption effect in plasma. The results reveal that the plasma temperature and density increase with higher molecular weight and pressure of the background gas, leading to stronger species emission line intensity. To reduce the self-absorption effect in the later stages of plasma evolution, we can decrease the gas pressure or substitute the background gas with a lower molecular weight. As the excitation energy of the species increases, the impact of the background gas type on the spectral line intensity becomes more pronounced. Moreover, we accurately calculated the optically thin moments under various conditions using theoretical models, which are consistent with the experimental results. From the temporal evolution of the doublet intensity ratio of species, it is deduced that the optically thin moment appears later with higher molecular weight and pressure of the background gas and lower upper energy of the species. This theoretical research is essential in selecting the appropriate background gas type and pressure and doublets in self-absorption-free LIBS (SAF-LIBS) experiments to weaken the self-absorption effect.

Highly sensitive photoacoustic multicomponent gas sensor for SF6 decomposition online monitoring.

A ppb-level photoacoustic multicomponent gas sensor system for sulfur hexafluoride (SF6) decomposition detection was developed by the use of two near-infrared (NIR) diode lasers and an ultraviolet (UV) solid-state laser. A telecommunication fiber amplifier module was used to boost up the excitation optical power from the two NIR lasers. A dual-channel high-Q photoacoustic cell (PAC) was designed for the simultaneous detection of CO, H2S, and SO2 in SF6 buffer gas by means of a time division multiplexing (TDM) method. Feasibility and performance of the multicomponent sensor was evaluated, resulting in minimum detection limits of 435 ppbv, 89 ppbv, and 115 ppbv for CO, H2S, and SO2 detection at atmospheric pressure.

A novel methodology to directly pre-determine the relative wavelength response of DFB laser in wavelength modulation spectroscopy.

A novel methodology to directly pre-determine the relative wavelength response (RWR) of a DFB laser, in terms of a combined current linearly scanned wavelength response and current modulated wavelength response (CMWR), in wavelength modulation spectroscopy (WMS) is presented. It is shown that the assessed RWR can be used to mimic the measured response with standard deviation of discriminations that are below 3.4 × 10-3cm-1 under a variety of conditions. It is also shown that its performance supersedes two commonly used assessment models of the CMWR but is slightly worse than that of the third model, however with the benefit of solely using a single fitting parameter (the concentration) instead of more. When the novel method is applied to the assessment of CO2 concentration in a Herriot-type multipass cell by using the technique of calibration-free WMS, the results show that there is virtually no difference compared to that by use of the best of the other methods. It is concluded that the novel method is more robust and simplifies the retrieval process of gas concentration.

Calculation model of dense spot pattern multi-pass cells based on a spherical mirror aberration.

We report a novel calculation model for dense spot pattern multi-pass cells consisting of two common identical spherical mirrors. A modified ABCD matrix without the paraxial approximation was developed to describe the ray propagation between two spherical mirrors and the reflection on the mirror surfaces. The intrinsic aberration from the spherical curvature creates a set of intricate variants with respect to a standard Herriot circle spot pattern. A series of detailed numerical simulations are implemented to verify that the input and output beams remain the same and, hence, retrace the same ray pattern. The set of exotic spot patterns obtained with a high fill factor improves the utilization efficiency of the mirror surfaces and produces a longer total optical path length with a low mirror cost.

Highly sensitive and selective CO sensor using a 2.33 µm diode laser and wavelength modulation spectroscopy.

A ppm-level CO sensor based on a 2f wavelength modulation spectroscopy (2f-WMS) technique was developed for the application of SF6 decomposition analysis in an electric power system. A detailed investigation of the optimum target line selection was carried out to avoid spectral interference from high purity SF6 in a wide wavelength range. A diode laser emitting at 2.33 µm and a 14.5-m multipass gas cell (MGC) was employed to target the R(6) line of the CO first overtone band and increase the optical path, respectively, thus resulting in a minimum detection sensitivity of 1 ppm. A Levenberg-Marquardt nonlinear least-squares fit algorithm makes full use of the information from all data points of the 2f spectrum and as a result, a measurement precision of ~40 ppb was achieved with a data update rate of 0.6 s. The sensor performance was also evaluated in terms of the gas flow rate, stability, and linearity. The results showed that the best operating condition with a precision of 6 ppb can be achieved by increasing the gas flow rate to the value that matches the optimum averaging time of 48 s.

Development and performance evaluation of self-absorption-free laser-induced breakdown spectroscopy for directly capturing optically thin spectral line and realizing accurate chemical composition measurements.

A novel self-absorption-free laser-induced breakdown spectroscopy (SAF-LIBS) technique is proposed to directly capture the optically thin spectral line by matching the measured doublet atomic lines intensity ratios with the theoretical one. To realize the experimental SAF-LIBS, the integration time, the fiber collection angle, and the delay time are optimized. The optically thin conditions are validated by comparing the linearity of Boltzmann plots with the traditional self-absorption (SA) correction method and evaluating the SA coefficients. The applicability and limitation of SAF-LIBS on element concentration and laser energy are also discussed. Univariate quantitative analysis results show that, compared with ordinary LIBS, the average absolute error of aluminum concentration has been reduced by an order of magnitude, which proves that this SAF-LIBS technique is qualified to realize accurate chemical composition measurements.

Homogeneous-material-based calibration method for correcting laser-induced breakdown spectroscopy measurement-error bias in the case of dust pollution.

A calibration method based on homogeneous material for correcting laser-induced breakdown spectroscopy (LIBS) measurement-error bias in the case of dust pollution under laboratory conditions is proposed. The measured plasma spectra of the sample can be corrected by measuring the spectral integral of the homogeneous material. Thus, we can effectively minimize the dust pollution effect on LIBS and guarantee its precision. Results show that the mean absolute errors of CaO, MgO, Fe2O3, Al2O3, and SiO2 in cement samples are decreased notably from 1.02%, 0.06%, 0.15%, 0.57%, and 0.80% to 0.41%, 0.02%, 0.04%, 0.35%, and 0.39%, respectively. Combination of this calibration method with the traditional optical dustproof methods will significantly extend the LIBS equipment maintenance cycle and make preliminary preparations for the next practical industrial application.

Scattered light modulation cancellation method for sub-ppb-level NO2 detection in a LD-excited QEPAS system.

A sub-ppb-level nitrogen dioxide (NO2) QEPAS sensor is developed by use of a cost-effective wide stripe laser diode (LD) emitting at 450 nm and a novel background noise suppression method called scattered light modulation cancellation method (SL-MOCAM). The SL-MOCAM is a variant of modulation spectroscopy using two light sources: excitation and balance light sources. The background noise caused by the stray light of the excitation light sources can be eliminated by exposing the QEPAS spectrophone to the modulated balance light. The noise in the LD-excited QEPAS system is investigated in detail and the results shows that > ~90% background noise can be effectively eliminated by the SL-MOCAM. For NO2 detection, a 1σ detection limit of ~60 ppb is achieved for 1 s integration time and the detection limit can be improved to 0.6 ppb with an integration time of 360 s. Moreover, the SLMOCAM shows a remote working ability in the preliminary investigation.

Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser.

A methodology for calibration-free wavelength modulation spectroscopy (CF-WMS) that is based upon an extensive empirical description of the wavelength-modulation frequency response (WMFR) of DFB laser is presented. An assessment of the WMFR of a DFB laser by the use of an etalon confirms that it consists of two parts: a 1st harmonic component with an amplitude that is linear with the sweep and a nonlinear 2nd harmonic component with a constant amplitude. Simulations show that, among the various factors that affect the line shape of a background-subtracted peak-normalized 2f signal, such as concentration, phase shifts between intensity modulation and frequency modulation, and WMFR, only the last factor has a decisive impact. Based on this and to avoid the impractical use of an etalon, a novel method to pre-determine the parameters of the WMFR by fitting to a background-subtracted peak-normalized 2f signal has been developed. The accuracy of the new scheme to determine the WMFR is demonstrated and compared with that of conventional methods in CF-WMS by detection of trace acetylene. The results show that the new method provides a four times smaller fitting error than the conventional methods and retrieves concentration more accurately.

Single-tube on-beam quartz-enhanced photoacoustic spectroscopy.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) with a single-tube acoustic microresonator (AmR) inserted between the prongs of a custom quartz tuning fork (QTF) was developed, investigated, and optimized experimentally. Due to the high acoustic coupling efficiency between the AmR and the QTF, the single-tube on-beam QEPAS spectrophone configuration improves the detection sensitivity by 2 orders of magnitude compared to a bare QTF. This approach significantly reduces the spectrophone size with respect to the traditional on-beam spectrophone configuration, thereby facilitating the laser beam alignment. A 1σ normalized noise equivalent absorption coefficient of 1.21×10(-8) cm(-1)·W/√Hz was obtained for dry CO2 detection at normal atmospheric pressure.

Impact of Humidity on Quartz-Enhanced Photoacoustic Spectroscopy Based CO Detection Using a Near-IR Telecommunication Diode Laser.

A near-IR CO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) is evaluated using humidified nitrogen samples. Relaxation processes in the CO-N2-H2O system are investigated. A simple kinetic model is used to predict the sensor performance at different gas pressures. The results show that CO has a ~3 and ~5 times slower relaxation time constant than CH4 and HCN, respectively, under dry conditions. However, with the presence of water, its relaxation time constant can be improved by three orders of magnitude. The experimentally determined normalized detection sensitivity for CO in humid gas is 1.556 × 10(-8) W ⋅ cm (-1)/Hz(1/2).

Intensity-Stabilized Fast-Scanned Direct Absorption Spectroscopy Instrumentation Based on a Distributed Feedback Laser with Detection Sensitivity down to 4 × 10-6.

A novel, intensity-stabilized, fast-scanned, direct absorption spectroscopy (IS-FS-DAS) instrumentation, based on a distributed feedback (DFB) diode laser, is developed. A fiber-coupled polarization rotator and a fiber-coupled polarizer are used to stabilize the intensity of the laser, which significantly reduces its relative intensity noise (RIN). The influence of white noise is reduced by fast scanning over the spectral feature (at 1 kHz), followed by averaging. By combining these two noise-reducing techniques, it is demonstrated that direct absorption spectroscopy (DAS) can be swiftly performed down to a limit of detection (LOD) (1σ) of 4 × 10-6, which opens up a number of new applications.

[Development of a Laser On-Line Cement Raw Material Analysis Equipment].

In engineering construction, cement quality directly affects the safety of construction projects. So it is necessary that we use qualified cement in the engineering structure. It is of great signification that a method detects cement raw material rapidly to adjust the mixture ratio of raw ores to ensure the cement quality. Traditional detection method needs sampling, sample preparation and test, etc. With many procedures, the test results are seriously lagged behind the production process. This paper introduces a set of online analysis equipment to determinate elemental composition of cement powder timely based on laser induced breakdown spectroscopy. This equipment is composed of a LIBS detection system and a pneumatic system. The equipment can achieve the real-time measurement for it needn't sample preparation. Thus, it can guide cement raw material proportioning in time. In this paper, we have quantitatively analyzed the main components of Al2O3, CaO, Fe2O3, MgO and SiO2 in the cement raw materials using the full spectrum normalization method as well as the support vector machine. The corresponding maximum absolute errors were 0.34%, 0.35%, 0.07%, 0.14%, and 0.55%, respectively. Results showed that the measurement results of the newly developed LIBS equipment are in accord with those of the conventional chemical method. Furthermore, the measurement precision is in line with X-Ray fluorescence spectrometry. It is confirmed that the LIBS technique could be a prospect method for determination of elemental composition in the cement production industries.

Fiber-amplifier-enhanced QEPAS sensor for simultaneous trace gas detection of NH3 and H2S.

A selective and sensitive quartz enhanced photoacoustic spectroscopy (QEPAS) sensor, employing an erbium-doped fiber amplifier (EDFA), and a distributed feedback (DFB) laser operating at 1582 nm was demonstrated for simultaneous detection of ammonia (NH3) and hydrogen sulfide (H2S). Two interference-free absorption lines located at 6322.45 cm(-1) and 6328.88 cm(-1) for NH3 and H2S detection, respectively, were identified. The sensor was optimized in terms of current modulation depth for both of the two target gases. An electrical modulation cancellation unit was equipped to suppress the background noise caused by the stray light. An Allan-Werle variance analysis was performed to investigate the long-term performance of the fiber-amplifier-enhanced QEPAS sensor. Benefitting from the high power boosted by the EDFA, a detection sensitivity (1σ) of 52 parts per billion by volume (ppbv) and 17 ppbv for NH3 and H2S, respectively, were achieved with a 132 s data acquisition time at atmospheric pressure and room temperature.

[High-Sensitive Carbon Dioxide Detection Using Quartz-Enhanced Photoacoustic Spectroscopy with a 2.0 µm Distributed Feedback Laser].

A carbon dioxide (CO2) sensor is developed using quartz enhanced photoacoustic spectroscopy (QEPAS) with a 2.0 µm distributed feedback diode laser. The detection is based on a 2f wavelength-modulation spectroscopy approach by dithering and scanning the laser current. The laser modulation depth is optimized at normal atmosphere pressure and room temperature. The influence of the H2O presence in the sample gas mixture on the CO2 sensor performance is also investigated. The results show that, with 1% CO2 concentration, the H2O in the concentration ranges of 0 to 0.2% has an effect on the CO2 signal amplitude and phase, and the largest amplitude difference is ~2.1 times. When the H2O concentration is over 0.2%, the CO2 signal amplitude is saturated and remains steady. Atmospheric CO2 concentration is well measured using the optimal sensor parameters. Benefiting from the strong absorption line intensity at 4989.97 cm(-1), a detection limit of 19 ppm (1σ, 300 ms averaging time) is achieved, which corresponds to a normalized noise equivalent absorption coefficient of 4.71 × 10(-9) cm(-1) · W · Hz(-1/2).

Double acoustic microresonator quartz-enhanced photoacoustic spectroscopy.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) based on double acoustic microresonators (AmRs) is developed and experimentally investigated. The double AmR spectrophone configuration exhibits a strong acoustic coupling between the AmR and the quartz tuning fork, which results in a ∼5 ms fast response time. Moreover, the double AmRs provide two independent detection channels that allow optical signal addition or cancellation from different optical wavelengths and facilitate rapid multigas sensing measurements, thereby avoiding laser beam combination.

[The research on residual amplitude modulation characteristics in fiber frequency modulation spectroscopy].

Frequency modulation spectroscopy (FMS) not only can be used to simultaneously measure the absorption and dispersion of atoms and molecules, but is the key technology of the noise immunity cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS). The optical devices or the instability of output light polarization of the laser source will induce the residual amplitude modulation (RAM) in the FMS. RAM greatly limits the FMS technology application in trace gas detection, so the research on the RAM characteristics in the FMS has very important significance. Firstly, the lineshape of FMS without absorption was analyzed, and the impact factors on the RAM were acquired, then the influence of input and output polarization direction and electro-optical modulation (EOM) temperature was measured, respectively. They all have linear relationship with the RAM. The results verify the theoretical analysis and provide the basis for reducing the RAM and other related working.

[Research on locking of the output power of pulsed laser in laser-induced breakdown spectroscopy].

Laser-induced breakdown spectroscopy (LIBS) as a rapid spectral analysis technology shows the outstanding application foreground and research value in coal quality on-line monitoring. In the practical application of this technology, the pulsed laser power fluctuation leads to the worse performance of long term stability, so a closed-loop feedback pulsed laser power locking device is set up, using laser power feedback signal to control and lock Nd:YAG laser output power. The laser power locking experiments are investigated in the same pre-set value with different splitting ratios, the different laser output power with the same proportion and the long time running modes. The results show that the beam split ration has little impact to the stability of the laser power, and the smaller split ration leads to the faster stabilization. This device can keep the output power of the pulsed laser being locked in the pre-set range for a long-term running, RSD values decrease from 2.4% of free-running to 1.1%.

[Experimental investigations of cavity ring-down spectroscopy under different threshold circuit design].

In the present paper, the authors simply describe the principle of cavity ring-down spectroscopy (CRDS) measurement technology and introduce a threshold circuit, based on 555 timer IC (integrated circuit), applied to CRDS. And we verify the feasibility of the circuit. By means of changing the input resistor and input capacity, we analyzee the influence of threshold circuit characteristics on single ring-down event, and found that the greater the input capacitance, and the smaller the input impedance of circuit, the more serious the distortion of measurement of ring-down curve, especially when the resistance is less than 50 omega the ring-down time produces very big change. Then we collected and analyzed the C2H2 absorption signal at 6 531.7805 cm(-1) with different input resistance and input capacitance, and fitted the absorption line with Voigt profile. We found that the change of the input capacitance and resistance caused the distortion of fitting curve. Finally We give the optimal design of threshold circuit: the input impedance of the threshold circuit should be great, minimum of 100 omega, the capacitive reactance should be as small as possible, preferably below 1 nf, at the same time it should be ensured that the threshold of the circuit time constant is much smaller than ring-down time. The experiment research on CRDS technique application has important reference value to threshold circuit design.

[Theoretical analysis of saturated spectral line shape of noise immune cavity enhanced optical heterodyne molecular spectroscopy].

In the present paper a theoretical description of the line shapes of frequency modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) under saturated conditions is presented. Expressions for both Doppler broadened and Doppler free frequency modulated NICE-OHMS signals at their absorption and dispersion phases are given. The modulation frequency, saturation parameter and frequency modulation index are set to 384 MHz, 10 and 0.2, respectively in the simulation. Based on the simulated line shape curves, in the absorption detection phase there are four sub Doppler structures existing in the spectral envelope of absorption signals of two sidebands, and in the dispersion phase there are five sub-Doppler structures existing in the spectral envelope of dispersion signals of carrier and two sidebands. The sub-Doppler structure originates from the fact that two counter propagating waves interact with the same group of molecules and one can act as a detecting beam and the other can act as a pumping beam. Therefore the sub-Doppler structure can appear at laser carrier and sidebands frequency and their middle frequencies of carrier and sidebands. Since NICE-OHMS absorption signal is only related to the absorption of sidebands the small saturation parameters of two sidebands can not saturate the Doppler broadened signal too much Although the NICE-OHMS dispersion signal includes the dispersion of carrier and two sidebands, the amplitude of Doppler broadened signal does not have so much influence by saturation except for the Doppler free signal. Therefore NICE-OHMS technique is a good candidate for the high sensitive detection of gas medium but with high selectivity under the condition of low pressure. Finally the dependences of NICE-OHMS line shape on the detection phase and saturation parameter are simulated and analyzed, the results of which nicely agree with the existing experimental results. The researches on NICE-OHMS line shapes will help explain the phenomena of related experiment.