Wavelength Modulation Spectroscopy Measurements Research Articles

Cost-Effective Quasi-Parallel Sensing Instrumentation for Industrial Chemical Species Tomography

Chemical species tomography (CST) has been widely applied for the imaging of critical gas-phase parameters in industrial processes. To acquire high-fidelity images, CST is typically implemented by the line-of-sight wavelength modulation spectroscopy measurements from multiple laser beams. In this article, we present a novel quasi-parallel sensing technique and electronic circuits for industrial CST. Although the acquisition and processing of these multiple beams using a fully parallel data acquisition and signal processing system can achieve maximized temporal response in CST, it leads to a highly complex and power-consuming instrumentation with electronics-caused inconsistency between the sampled beams, in addition to a significant burden on data transfer infrastructure. To address these issues, the digitization and demodulation of the multibeam signals in the proposed quasi-parallel sensing technique are multiplexed over the high-frequency modulation within a wavelength scan. Our development not only maintains the temporal response of the fully parallel sensing scheme but also facilitates the cost-effective implementation of industrial CST with very low complexity and reduced load on data transfer compared with the fully parallel sensing technique. The proposed technique was analytically proofed and then numerically examined by noise-contaminated CST simulations. Finally, the designed electronics was experimentally validated using a lab-scale CST system with 32 laser beams.

Dual-beam antiphase method to improve the WMS measurement limit in long-distance methane detection.

Wavelength modulation spectroscopy (WMS) with second harmonic detection is an extremely effective technique to detect gases in site applications. However, the significant levels of nonlinear effects in a system give rise to high background signals that either limit detection sensitivity or distort the harmonic signals. This paper outlines the theory of WMS-involved background signals and focuses on the elimination of undesirable effects in the background. A real-time, long-distance methane sensor using a tunable diode laser near 1653.7 nm is developed to continuously monitor methane by using a variable optical attenuator to suppress the background. Trace methane detection experiments verify that the minimum detection limit of the system can be increased by 47.5 times compared to the traditional WMS method.

Demonstration of non-absorbing interference rejection using wavelength modulation spectroscopy in high-pressure shock tubes

We experimentally demonstrate the non-absorbing interference rejection capabilities of wavelength modulation spectroscopy (WMS) speciation in shock tube experiments by directly comparing WMS measurements against direct-absorption spectroscopy (DA) measurements. The improved capability is demonstrated by probing the P(20) transition of the CO fundamental band using a quantum cascade laser in shock-heated mixtures of CO and N $$_2$$ across a wide range of pressures between 3.5 and 18 atm. In the WMS measurements, the second harmonic (2f) served as the detection signal, while the first harmonic (1f) provided normalization to counteract intensity drift and fluctuations. These perturbations occur in shock tubes because of significant beam-steering noise and imperfect optical alignment when experiments are conducted at elevated pressures. The WMS detection system was evaluated at reflected shock pressures of 3.5 atm, 8.5 atm, and 18 atm, demonstrating improvement in signal-to-noise ratio over concurrent DA measurements. To the authors’ knowledge, this work represents the first direct experimental quantification of the intensity-fluctuation rejection capabilities of a WMS-based TDLAS sensor at high pressures.

Direct absorption spectroscopy baseline fitting for blended absorption features.

A method for determining the gas concentration from blended absorption features using direct absorption spectroscopy is developed. Blended spectra occur when multiple absorption lines are in close proximity in a laser scan and/or when pressures begin to increase. The blended-feature baseline fitting method estimates the baseline by coupling measured data with simulated fractional transmission at the peaks between absorption features. The method is first validated with synthetically created data. The method is then demonstrated using the absorption lines of CO2(P41e,P42f,P34e,P43e) and H2O in the ν1+ν3 combination band (3683.5 cm-1 to 3686.5 cm-1) in a heated static test cell. Results from the fitting method and wavelength modulation spectroscopy measurements agree within 1%. The method is finally applied to measurements of flue gas emissions from a practical combustion system (wood boiler) and compared to measurements from a hand-held gas analyzer. The mean percent difference of the blended-feature baseline fitting and the wavelength modulation spectroscopy is 2.8% and 5.8% for XCO2 and XH2O, respectively, indicating that this method of direct absorption data analysis can be used for spectra with moderate feature blending, which is useful for scan ranges with multiple closely spaced absorption lines and moderate pressures.

Artificial absorption creation for more accurate tunable diode laser absorption spectroscopy measurement

A novel strategy for more accurate tunable diode laser absorption spectroscopy (TDLAS) measurement is presented. This method is immune to non-absorption transmission losses, and allows dead zone removal for ultra-low concentration detection, and reference point selection at atmospheric pressure. The method adjusts laser emission and creates artificial absorption peaks according to requirements. By creating an artificial absorption peak next to the real absorption zone, calibration is not necessary. The developed method can be applied to not only wavelength modulation spectroscopy (WMS) but also direct absorption (DA). In WMS, the method does not need two harmonic signals, resulting in higher reliability, better performance, and no electro-optical gain uncertainty. At the same time, non-absorption transmission losses effect is suppressed from 70% to 0.425% with DA and from 70% to 0.225% with WMS method. When the artificial absorption peak coincides with the real one, the dead zone of measurement can be removed to give a lower detection limit, and water vapor still can be detected when concentration is lower than 0.2ppm in our experiment. Reference point selection uncertainty with the DA method, especially when the signal-to-noise ratio is low and absorption line is broad, can also be facilitated. And the uncertainty of reference point selection is improved from 6% to 0.8% by measuring reference point amplitude. The method is demonstrated and validated by WMS and DA measurements of water vapor (1atm, 296K, 1368.597nm). The measurement results obtained using the new method reveal its promise in TDLAS.

Large amplitude wavelength modulation spectroscopy for sensitive measurements of broad absorbers.

We demonstrate large amplitude wavelength modulation spectroscopy (WMS) with a MEMS-tunable vertical cavity surface-emitting laser (MEMS-VCSEL) to measure high-density gases. WMS enables sensitive measurements of gas phase thermodynamic properties in harsh environments, but has been limited to moderate pressure and density conditions because of the narrow tuning range of traditional DFB lasers. The MEMS-tunable laser is able to rapidly modulate across the broadened features of high-density gas mixtures to produce the harmonic signals in the detected light intensity typical of WMS. We illustrate the technique on high-pressure mixtures of CO2 in air that are 2.5 times higher density than previously published WMS measurements (equivalent to greater than 255 atm at 1500 K). We develop a WMS model that accounts for nonlinear tuning of the laser to enable extraction of thermodynamic properties from measured data. The agreement of the measured data and model suggests that this technique could be used now for calibrated measurements of gas concentration, and in the future for calibration-free operation with further high-pressure absorption model development and laser tuning characterization.

Remote ambient methane monitoring using fiber-optically coupled optical sensors

A tunable diode laser absorption spectroscopy system, employing a 2f wavelength modulation spectroscopy measurement scheme, was developed for remote monitoring of ambient methane fluctuations by way of fiber-optically connected all-optical sensors. Detection of fugitive methane emissions demands a measurement precision less than 2.0 ppmv and a lower detection limit less than ~1.7 ppmv methane in air. To determine the optimum base system configuration, the influence of the laser driving signal frequency and amplitude was characterized to strike a balance between measurement precision and system sensitivity. In addition, relative to the basic system configuration, a 50 and 96 % reduction in measurement deviation was achieved by way of polarization scrambling and thermal stabilization of critical optical components, respectively. For methane concentrations between 2.0 and 50.0 ppmv in air, the laboratory-based system achieved a measured precision of 1.36 ppmv and a lower detection limit of 1.56 ppmv using a 6.0-m single-mode optical fiber and an averaging time of 1 s. The long-term system stability and system performance were analyzed using datasets acquired 4 and 12 months after the initial system calibration, yielding a difference in measured precision within the uncertainties of the calibration gas mixture. Finally, it was determined that fiber length between individual remote optical sensors can lead to a varying measurement bias, which implies that length-specific calibrations for each remote optical sensor may be required for a field implementation.

Self-calibration methodology by normalized intensity for wavelength modulation spectroscopy measurement

A methodology of self-calibration for concentration measurement based on wavelength modulation absorption spectroscopy has been developed by normalized fixed intensity. Experimental results show that the simple self-calibration method not only effectively improves the calibration accuracy, but also greatly improves stabilization and reliability as compared with the popular method called self-calibration by 1f normalized. In addition, the proposed system do not need any additional equipment when comparing with the use of traditional wavelength modulation absorption spectrometer. The standard deviation of a concentration measurement with different optical intensity aimed on the detector has been found to be below 1.0%. The dependence of the concentration assessment on the laser intensity fluctuation has also been investigated, which shows that the method of self-calibration could be applied in the field, specially where the dust is easily splattered on the windows of the detector.

Hypersonic Scramjet Testing via Diode Laser Absorption in a Reflected Shock Tunnel

A wavelength-multiplexed two-color tunable diode laser absorption spectroscopy sensor probing transitions near was developed to measure temperature and column density simultaneously across three lines of sight in a ground-based model scramjet combustor. High-enthalpy scramjet conditions equivalent to Mach 10 flight were generated with a reflected shock tunnel. The sensor hardware development and optical engineering to overcome noisy combustor conditions, including short test time, beam steering, and mechanical vibration, are discussed. A new scanned-wavelength-modulation spectroscopy technique was used to acquire the complete spectral lineshape while maintaining the high signal-to-noise ratio characteristic of wavelength-modulation spectroscopy measurements. Two combusting flow experiments were conducted, and the results compared with steady-state computational fluid dynamics calculations, with both the simulations and the measurements finding formation of from combustion during the test time. This work represents the first use of wavelength-modulation spectroscopy in a reflected shock tunnel and the first use of scanned-wavelength-modulation spectroscopy in a scramjet combustor.

301 可搬型排気計測装置に適用する小型多重反射セルの構築(大気・水環境評価・改善技術(1))

The multi-pass optical cell is an important tool for high-sensitive laser absorption spectroscopy. For portable emission measurement systems (PEMS), which are useful for on-board measurements of trace-gas emitted from automobiles, a compact design is essential. Therefore, we have constructed a compact multi-pass optical cell based on a pair of cylindrical mirrors. To investigate the performance of the measurements system using the new cell, such as stability and response, 2f wavelength modulation spectroscopy measurements of molecular CO_2 were performed. We also demonstrated the continuous measurements of CO_2 in automobile exhaust and ambient air.

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.

A 3 µm difference frequency laser source for probing hydrocarbon plasmas

The practicality of a compact solid-state laser-based difference frequency generation system is demonstrated as a tool for probing hydrocarbon-based plasmas. The laser light of a cw Nd : YAG operating at 1064 nm and one of two distributed feedback diode lasers operating at 1560 and 1620 nm were mixed in a periodically poled lithium niobate crystal producing mid-infrared radiation at 3.35 and 3.1 µm for the detection of CH4 and C2H6 at the first wavelength and C2H2 and C2H4 at the latter. The radiation was used to probe a rf capacitively coupled CH4 plasma for a matrix of conditions, varying power (<180 W) and pressure (<1 Torr) in both direct absorption spectroscopy and wavelength modulation spectroscopy (WMS) experiments to achieve relatively high sensitivities while retaining accurate spectral information in the form of linewidths. A minimum detectable absorption coefficient αmin of 2 × 10−5 cm−1 was achieved for direct absorption spectroscopy which corresponds to a minimum detectable density for CH4 of 1.7 × 1012 cm−3 and an αmin value of 2 × 10−6 cm−1 was achieved for WMS measurements on C2H2. The depletion of CH4 was measured to increase from 23% at 40 W to 40% at 180 W at a pressure of 0.84 Torr, and the depletion increased to 56% as the pressure decreased to 0.45 Torr.

Wavelength modulation and cavity enhanced absorption spectroscopy using ∼1.9 μm radiation produced by difference frequency generation with a MgO doped PPLN crystal

We have demonstrated the production of ∼1.9 μm near-infrared radiation by using difference frequency generation within a 5% MgO doped PPLN crystal by coupling ∼735 nm radiation from a tunable external cavity diode laser with relatively high powered 532 nm radiation from both Nd:YVO3 and Nd:YAG lasers. The radiation produced is of low power, ∼15 μW, and was used in conjunction with the sensitivity enhancing techniques of wavelength modulation spectroscopy (WMS) and cavity enhanced absorption spectroscopy (CEAS). Experiments were carried out on rotationally resolved transitions in the combination bands of NH3 and CO2 in the 1.9 μm region. An αmin value of 3.6×10−6 cm−1 Hz−1/2 was achieved for WMS measurements on CO2. A comparable αmin value of 2.2×10−6 cm−1 Hz−1/2 was achieved for NH3 using CEAS. The low NIR power indicates that despite the level of MgO doping quoted for the crystal, under prolonged exposure photorefractive damage has occurred.

Wavelength modulation spectroscopy with a widely tunable InP-based 23 μm vertical-cavity surface-emitting laser

We report on the successful application of recently developed 2.3 microm InP-based vertical-cavity surface-emitting lasers with a buried tunnel junction in a wavelength modulation spectroscopy measurement for carbon monoxide (CO) detection. The electrically pumped devices operate at room temperature under cw operation with stable single-mode emission and should allow for parts in 10(6) (ppm) level resolution measurement of CO with a standard optical setup.

Extension of wavelength-modulation spectroscopy to large modulation depth for diode laser absorption measurements in high-pressure gases

Tunable diode laser absorption measurements at high pressures by use of wavelength-modulation spectroscopy (WMS) require large modulation depths for optimum detection of molecular absorption spectra blended by collisional broadening or dense spacing of the rovibrational transitions. Diode lasers have a large and nonlinear intensity modulation when the wavelength is modulated over a large range by injection-current tuning. In addition to this intensity modulation, other laser performance parameters are measured, including the phase shift between the frequency modulation and the intensity modulation. Following published theory, these parameters are incorporated into an improved model of the WMS signal. The influence of these nonideal laser effects is investigated by means of wavelength-scanned WMS measurements as a function of bath gas pressure on rovibrational transitions of water vapor near 1388 nm. Lock-in detection of the magnitude of the 2f signal is performed to remove the dependence on detection phase. We find good agreement between measurements and the improved model developed for the 2f component of the WMS signal. The effects of the nonideal performance parameters of commercial diode lasers are especially important away from the line center of discrete spectra, and these contributions become more pronounced for 2f signals with the large modulation depths needed for WMS at elevated pressures.

Time-resolved wavelength modulation spectroscopy measurements of HO_2 kinetics

High-frequency wavelength modulation spectroscopy (WMS) has been applied to the detection of the hydroperoxyl radical (HO2 ) in a laser photolysis and long-path absorption pump-probe kinetics reactor with a near-infrared distributed feedback diode laser. The HO2 is formed by the 355-nm photolysis of Cl2 in the presence of CH3 OH and O2 and monitored by a phase-sensitive detection of the second-harmonic (2f ) signal in the 2?1 band with a 1.5- ?m diode laser directly modulated at 5 MHz. The measured 2f WMS signal is calibrated by direct absorption and converted to an absolute number density with the known absorption line strength of the HO2 line at 6625.80cm-1 . The utility of time-resolved WMS as a second-order kinetics probe is demonstrated through the measurement of the HO2 self-reaction rate constant at 295 K.