Design and Analysis of a Gas Sensor Based on a Ring Multi-Wavelength Fiber Laser in Near Infrared

Numerical model of a gas sensor based on the ring multi-wavelength fiber laser is discussed to detect gas such as methane (CH₄), which has multi-line absorption in near infrared. The schematic of the gas sensor is given, and its detecting principle of multiple windows is demonstrated in detail. The detecting methods based on the differential method and the second harmonic wave method in the sensor are analyzed, and the optimal information syncretions are used to process the information of the detected gas. The analyzing results show the sensor has advantages over the traditional sensor, and can be expanded to detect other gas such as acetylene (C₂H₄).     

Synthesis and characterization of platinum nano sized particles by laser ablation in C<Subscript>2</Subscript>H<Subscript>6</Subscript>O<Subscript>2</Subscript> solution

Platinum nano sized particles (Pt NPs) are superior catalysts for many intentions, such as glucose sensors, cancer therapy, gas sensors, etc. Here, Pt NPs were produced by pulsed laser ablation in C₂H₆O₂ solution using Q-switched Nd:YAG laser, for the first time. Then, the influence of the laser fluence during synthesis of them was investigated; and they were characterized by UV–vis spectroscopy, TEM, FE-SEM, XRD, FT-IR, and Raman spectroscopy. The results showed that with increasing laser fluence, the mean particle size of the spherical NPs enhanced. Meanwhile, they had a polycrystalline cubic structure. Correspondingly, the plasmon peak position of generated NPs in the absorption spectra shifted from 257 to 266 nm, with a rise of laser fluence. The IR and Raman spectroscopy was used to achieve the information about the surface state of Pt NPs. We propose that the optimum adjusted laser fluence is an important factor to increase the ablation efficiency.     

Multi-species laser absorption sensors for in situ monitoring of syngas composition

Tunable diode laser absorption spectroscopy sensors for detection of CO, CO₂, CH₄ and H₂O at elevated pressures in mixtures of synthesis gas (syngas: products of coal and/or biomass gasification) were developed and tested. Wavelength modulation spectroscopy (WMS) with 1f-normalized 2f detection was employed. Fiber-coupled DFB diode lasers operating at 2325, 2017, 2290 and 1352 nm were used for simultaneously measuring CO, CO₂, CH₄ and H₂O, respectively. Criteria for the selection of transitions were developed, and transitions were selected to optimize the signal and minimize interference from other species. For quantitative WMS measurements, the collision-broadening coefficients of the selected transitions were determined for collisions with possible syngas components, namely CO, CO₂, CH₄, H₂O, N₂ and H₂. Sample measurements were performed for each species in gas cells at a temperature of 25 °C up to pressures of 20 atm. To validate the sensor performance, the composition of synthetic syngas was determined by the absorption sensor and compared with the known values. A method of estimating the lower heating value and Wobbe index of the syngas mixture from these measurements was also demonstrated.     

Continuous wave,distributed feedback diode laser based sensor for trace-gas detection of ethane

The development of a continuous wave (CW), thermoelectrically cooled (TEC), distributed feedback (DFB) laser diode based spectroscopic trace-gas sensor for ultra-sensitive and selective ethane (C₂H₆) concentration measurements is reported. The sensor platform used tunable diode laser absorption spectroscopy (TDLAS) based on a 2f wavelength modulation (WM) detection technique. TDLAS was performed with a 100 m optical path length astigmatic Herriott cell. For an interference free C₂H₆ absorption line located at 2976.8 cm⁻¹ a 1σ minimum detection limit of 240 pptv (part per trillion by volume) with a 1 second lock-in amplifier time constant was achieved. In addition, reliable and long-term sensor performance was obtained when operating the sensor in an absorption line locked mode.     

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.     

Preliminary results of an aircraft system based on near-IR diode lasers for continuous measurements of the concentration of methane, carbon dioxide, water and its isotopes

The Federal Agency for Hydrometeorology of the Russian Federation created the flying laboratory on board the passenger airplane Yak-42D for geophysical monitoring of the environment, including aircraft measurements of vertical concentrations of greenhouse gases in the troposphere. Within the limits of this project, General Physics Institute of the Russian Academy of Science developed airborne tunable diode laser spectrometer (TDLS) on the basis of diode lasers of a near-IR range for measurement of the altitude profiles of CO₂, CH₄, H₂O and its isotopes. TDLS complex was integrated aboard in standard 19-in. rack. Air samples, taken over an aircraft on the pipeline, were injected into the optical cell. Using the system of inflow and heating, the air was set laminar with a flowrate of 0.2?l/s at a reduced pressure of 100?mbar for detecting narrow absorption lines of water vapor isotopes. For registration of the absorption spectra and for the measurement of greenhouse gas concentrations in online mode, modulation-correlation technique was used. Diode laser spectrometer output data were transferred to the airborne central computer. Sensitivity of TDLS measurements was 20?C30?ppm for water, 3?C4?ppm for CO₂ and 20?C25?ppb for CH₄. Time of one-unit measurement is about 30?ms.     

The absorbance enhancement investigation of multiple gas absorption lines based on ICFRL

The absorbance enhancement of multiple gas absorption lines is investigated by simulations and experiments in this paper. The relationship between cavity loss and gas absorbance of different absorption lines is simulated based on rate equation. For the purpose of using simple arithmetic mean instead of weighted arithmetic mean in calculating gas concentration by multiple absorption lines, the absorbance equality method is proposed and simulated. To verify the simulations, some experiments were carried out based on the intra-cavity fiber ring laser (ICFRL) gas sensing system. The experimental results of absorbance enhancement extremely match to the simulation results. More than 40 C₂H₂ absorption lines can be observed and a minimum detectable C₂H₂ concentration of 200 ppm is acquired according to the experiments.     

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν ₁+ν ₂+ν ₃ and 2ν ₁+ν ₂ combination bands around 1.58 μm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cavity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H₂S sensing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spectroscopy (WMS). The set-up sensitivity has been calculated as 1.73×10⁻⁸ cm⁻¹ s^1/2, and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Furthermore, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolecular potentials governing the collision process; the H₂S–H₂S dimer well depth is estimated to be 7.06±0.09 kJ mol⁻¹.     

Application of a continuous-wave tunable erbium-doped fiber laser to molecular spectroscopy in the near infrared

Development of a continuous-wave tunable fiber laser-based spectrometer for applied spectroscopy is reported. Wide wavelength tunability of an erbium-doped fiber laser (EDFL) was investigated in the near-infrared region of 1543–1601 nm. Continuous mode-hop free fine frequency tuning has been accomplished by temperature tuning in conjunction with mechanical tuning. The overall spectroscopic performance of the EDFL was evaluated in terms of frequency tunability along with its suitability for molecular spectroscopy. High-resolution absorption spectra of acetylene (C₂H₂) were recorded near 1544 nm with a minimum measurable absorption coefficient of about 3.5×10^-7 cm^-1/Hz^1/2 for direct absorption spectroscopy associated with a 100-m long multipass cell. Detections of C₂H₂ at different concentration levels were performed as well with high dynamic detection range varying from 100% purity to sub ppmv using cavity ring down spectroscopy. A 3σ-detection-limited minimum detectable concentration (MDC) of 400 ppbv has been obtained by using the transition line P_e(22) of the ν₁+ν₃+ν₅ ¹(Π_g)-ν₅ ¹(Π_u) hot band near 1543.92 nm with a detection bandwidth of 2.3 Hz. This corresponds to a minimum detectable absorption coefficient of 6.6×10^-11 cm^-1/Hz^1/2. The sensitivity limit could be further improved by almost one order of magnitude (down to ∼60 ppbv) by use of the P_e(27) line of the ν₁+ν₃(Σ_u ⁺)-0(Σ_g ⁺)combination band near 1543.68 nm. PACS 42.55.Wd; 42.62.Fi; 07.57.Ty; 07.88.+y     

High sensitivity Faraday rotation spectrometer for hydroxyl radical detection at 2.8 μm

A distributed feedback diode laser based Faraday rotation spectroscopy (FRS) instrument was developed for detection of hydroxyl free radical (OH) at 2.8???m. Fast wavelength sweeping method was implemented and the instrument performances were compared with point-by-point wavelength tuning method. The fast sweeping operation mode showed the same short-term minimum detection limit and improved immunity to baseline drift. The effects of strong diamagnetic H₂O vapor absorption on FRS detection of paramagnetic OH were investigated. We demonstrated that in the case of strong H₂O vapor absorption, the magnitude of the FRS signal for OH might be affected due to changes in the received optical power. The effects of higher laser intensity on the FRS detection sensitivity were also studied experimentally.     

Diagnostics of a Compact Discharge-Pumped XeCl Laser with BCl3 Halogen Donor

The results of a comprehensive study of a compact UV-preionized XeCl laser are presented. The subjects of this study were: discharge voltage and current measurements, dye laser probing of the active medium, and the mass spectrometry of gas mixture degradation products. It is shown that the gas lifetime was significantly improved when the laser was operated with BCl₃ as a halogen donor instead of commonly used HCl. By the dye laser absorption and gain probing, the temporal and spatial dependences of the densities for several plasma components, Ne*, Xe*, Xe⁺*, Cl^–, XeCl* and of ground state boron atoms were measured. Some aspects of plasma kinetics for uniform and constricted phases of the discharge are discussed. By the mass spectrometry of gas mixture degradation products on long-term operation of the laser device several gaseous (N₂, O₂, CO₂, H₂O, C₂H₄) and solid (NiCl₂, H₃BO₃) products were detected in the laser chamber. NH₄Cl was determined to be a stable fraction of the deposits on optics surfaces. The reasons for the improvement of gas lifetime for BCl₃-containing gas mixtures are discussed.     

A photonic crystal fiber sensor based on differential optical absorption spectroscopy for mixed gases detection

A photonic crystal fiber sensor based on differential optical absorption spectroscopy for mixed gas detection is presented. In such sensor, hollow core photonic crystal fiber is utilized as gas cell and the feasibility for gas detection is verified by experiment. The components concentration of mixed gas NH₃ and C₂H₂ are measured and the detection sensitivity is 143 ppmv.     

Self- and foreign-broadening and shift coefficients for C2H2 lines at 1.54 μm

In this article we present experimental data for line parameters measurements of several transition lines of overtone bands of C₂H₂ at 1540 nm. The measurements were done with a spectrometer based on a semiconductor diode laser and a direct absorption spectroscopy scheme. Broadening and shift coefficients have been measured for the pure gas or in mixtures with N₂, O₂ and CO₂ as perturbing gas. Received 6 November 2000 and Received in final form 13 March 2001     

Measurement of the coefficients of H<Subscript>2</Subscript>O absorption line shifts by intracavity laser spectroscopy

The results of measurement of H ₂ O absorption line shifts induced by argon pressure are discussed. The measurements were made using an intracavity spectrometer based on a Nd laser. The estimations of possible half-widths and H ₂ O line shifts in the range 1μm are reported, and the measurement procedure for the line shift coefficients by intracavity laser spectroscopy is developed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 72–77, October, 2007.     

Absorption coefficients for hydrogen—III. calculated pressure-induced H2-H vibrational absorption in the fundamental region

The coefficient for absorption during H₂-H collisions was calculated for temperatures from 2000 to 7000 K and wave numbers between 100 and 40 000 cm^-1 for LTE. Because only transitions with Δν = 1 were considered, the absorption was near the fundamental at 4161 cm^-1 in the infrared. The model included mechanical anharmonicity, vibration-rotation interaction, excited vibrational states, and transverse components of the dipole moment. At 5500 cm^-1, 3750° K, and 1 atm pressure, inclusion of H₂-H vibrational absorption increased the absorption of pure hydrogen gas 48 per cent. An approximate formula for the H²-H absorption coefficient is given for rapid calculation.     

Application of high-resolution IR and microwave spectroscopies for investigation of the impurity composition of silicon tetrafluoride

The possibility of combined application of gas chromatography, high-resolution IR spectroscopy, and transient microwave gas spectroscopy (TMGS) for investigation of the impurity composition of silicon tetrafluoride is studied. Using high-resolution IR Fourier spectroscopy, the lines of a number of impurities are observed in the region from 4500 to 550 cm^?1. The absorption lines of some of the main well-known impurities in silicon tetrafluoride in the 2-mm wavelength range are analyzed. The advantages of the TMGS method for investigating the multicomponent SiF₄-impurities system are demonstrated. Using the TMGS method, the freons CHF₃, CH₂F₂, and CH₃F are experimentally detected in silicon tetrafluoride.     

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 %.     

A double-beam UV-laser differential absorption method for monitoring tropospheric trace gases

A method is described which uses the differential absorption of a narrow-band tunable ultra-violet laser for integrated measurements of tropospheric trace gas concentrations over a path length of 12.8 km, at near ground level. A XeCl excimer laser serves as a reference beam to compensate for turbulence-induced intensity losses. After suitable statistical averaging the method is capable of detecting differential absorptions as low as 10^-4, enabling the simultaneous detection of SO₂, CH₂O and of OH radicals around 308 nm.     

Impact of water vapor on 1.51 μm ammonia absorption features used in trace gas sensing applications

Water-vapor-induced pressure broadening is reported for two NH₃ absorption features at 6612.7 and 6596.4 cm⁻¹ that are exploitable for gas sensing applications at atmospheric pressure. Absorption spectra of different NH₃–H₂O–N₂ mixtures were measured at an elevated temperature of 70°C to enable high H₂O concentrations to be reached. Line parameters were determined from a fitting procedure. The significantly greater values obtained for the H₂O-broadening coefficients of the two lines compared to N₂-broadening leads to cross-sensitivity effects in NH₃ trace gas sensors based on spectroscopic techniques that are sensitive to the width of the analyzed absorption line, as is the case in a simple implementation of wavelength modulation spectroscopy or in photoacoustic spectroscopy. In such a case, cross-sensitivity results in inaccurate gas concentration retrieval when the composition of the diluting gas changes. H₂O represents a potentially significant cross-sensitivity source as its concentration may be subject to large variations, especially in high-temperature applications where concentrations up to a couple of tens of percent may be encountered. In contrast to interference which can be minimized by an appropriate choice of the analyzed transitions, cross-sensitivity affects the entire spectrum of the analyte and is thus unavoidable in the mentioned type of gas sensors.     

Mid-infrared heterodyne phase-sensitive dispersion spectroscopy in flame measurements

We present the first demonstration of heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for in situ, non-intrusive and quantitative CO₂ concentration measurements in flames. Dispersion spectroscopy retrieves gas properties by measuring the refractive index in the vicinity of a molecular resonance. The HPSDS scheme features a significant diagnostic advantage of the intrinsic immunity to laser power fluctuations caused by beam steering, thermal radiation and soot scattering in combustion environments, and thus no extra calibration process is required. In this work, we described the spectroscopic fundamentals for measuring heterodyne phase signals in flames. As a proof of principle, we used a mid-infrared interband cascade laser (ICL) near 4183?nm to exploit the strong CO₂ transitions in the R-branch of the v₃ fundamental band. The HPSDS signals of four CO₂ lines, R(76), R(78), R(80) and R(82), were measured in CH₄/air flames to obtain CO₂ concentrations at different equivalence ratios (Φ?=?0.8–1.2), yielding a good agreement with the simultaneous laser absorption measurements using the same ICL. With its immunity to laser power fluctuations verified experimentally, the HPSDS sensor was successfully implemented to measure CO₂ concentrations in C₂H₄/air sooting flames (Φ?=?1.78–2.38). Laser dispersion spectroscopy proves to be a promising and alternative diagnostic tool for combustion measurements.