Diode Laser Wavelength Modulation Spectroscopy Research Articles

Microbiological total viable count detection based on tunable diode laser wavelength-modulation spectroscopy

Total viable counts (TVC) are vitally critical to health management, and the quantitative testing of TVC is an essential part of the microbiological testing process. This work demonstrates the rapid and accurate measurement of microbiological TVC using a wavelength-modulated tunable diode laser based on absorption spectroscopy (WM-TDLAS). To detect CO2 in sealed culture bottles, a tunable distributed feedback (DFB) laser with a central wavelength of 2004 nm was employed as the light source. Experiments with Salmonella show that the technique can generate effective second harmonic signals, allowing the detection limit to be 71.8 ppm. Simultaneously, the optimal parameters for measuring the growth curve of Salmonella at various concentrations were determined to establish the standard curve used to calculate the TVC. When compared to the conventional method ISO 4833:2013, the new detection method illustrates good correlation (R2 = 0.98), and the results can be obtained within the detection range of 1–8 log10(CFU/mL) in 4–12 h, saving at least 36 h. Thus, it was shown that WM-TDLAS is a user-friendly, non-invasive technique that allows rapid and accurate assessment of TVC.

The Trace Gas Monitoring Method Based on Diode Laser Wavelength-Modulation Spectroscopy Technology for the Detection of Clinical Blood Infection

It is important to monitor and evaluate the growth of microorganisms in order to accurately judge the situation of blood microbial infection. In this paper, diode laser wavelength modulation spectroscopy (DLWMS) technology is used to design a set of low-cost, high sensitivity, fast dynamic responses and a non-invasive trace gas measurement system, which can quickly and accurately assess the concentration of carbon dioxide (CO2) produced by blood microbial reproduction. The measurement principle and spectral processing algorithm of DLWMS are introduced first. The automatic and rapid detection of CO2 is realized through a self-designed optical system. By using the system to detect blood infection, the accuracy of the technology was verified. Therefore, it also indicates that DLWMS CO2 monitoring is a highly sensitive, fast-response and non-invasive technology, which can accurately and quickly determine blood infection and meet the clinical application requirements of human septicemia, bacteremia and other diseases.

Characterization of the Buoyant Jet above a Catalytic Combustor Using Wavelength Modulation Spectroscopy

ABSTRACT Combustion-based processing of polymer films and other materials often requires temporal and spatial uniformity of the combustor. We characterize the temperature and H2O mole fraction of the buoyant jet above a rectangular iron-chromium catalytic combustor using diode-laser wavelength modulation spectroscopy (WMS). The sensor is first validated in a tube furnace under known conditions. It is then used to characterize the temporal and spatial variation of the flow exiting the combustor. The H2O mole fraction measurements incorporate a new pathlength correction approach that uses computational fluid dynamics to account for the changes in jet width. We observe 3% temporal variation of temperature over 60 s and 7% spatial variation along the length of the burner. Vertical profiles indicate that additional combustion is likely occurring above the catalytic surface under certain operating conditions. This type of characterization can be used for optimizing the combustor for a variety of catalytic combustion applications.

Automated rapid blood culture sensor system based on diode laser wavelength-modulation spectroscopy for microbial growth analysis

An automated rapid blood culture sensor system was developed to detect CO2 concentration levels in blood culture bottles and thereby monitoring microbial growth. The sensor system is based on diode laser wavelength-modulation spectroscopy (DLWMS), which offers low cost, high sensitivity, fast response as well as label-free and non-intrusive detection. The optical gas sensing module was based on a careful design to realize automated and rapid CO2 detection with the assistance of control and storage elements. A 2-month field test was carried out at the Jinhua Guangfu Hospital, Jinhua, China for the new DLWMS based sensor system, which exhibits similar performance to a commercial BacT/ALERT 3D automated blood culture system in terms of accuracy but with significantly reduced detection time.

Measurement of CO2 Concentration and Temperature in an Aero Engine Exhaust Plume Using Wavelength Modulation Spectroscopy

In this paper, the 2f/1f tunable diode laser wavelength modulation spectroscopy is used for the simultaneous measurement of concentration and temperature of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the exhaust plume of an aero engine. The suitability of the R48 spectral feature of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> at 1997.2 nm is discussed for this project and for further application of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> tomographic imaging on large-scale aero-engines. To ensure accurate recovery of gas parameters at the high exhaust temperatures, a full spectral characterization of the absorption feature is presented using a direct spectroscopy. The 2f/1f method is validated in the laboratory for controlled gas mixtures and temperatures to recover concentration and temperature. Good agreement with the actual temperature and concentration values is demonstrated. Finally, single path measurements are presented for an aero engine exhaust showing good correlation with the measured engine conditions.

Determination of Equilibrium Constants for the Reaction between Acetone and HO2 Using Infrared Kinetic Spectroscopy

The reaction between the hydroperoxy radical, HO(2), and acetone may play an important role in acetone removal and the budget of HO(x) radicals in the upper troposphere. We measured the equilibrium constants of this reaction over the temperature range of 215-272 K at an overall pressure of 100 Torr using a flow tube apparatus and laser flash photolysis to produce HO(2). The HO(2) concentration was monitored as a function of time by near-IR diode laser wavelength modulation spectroscopy. The resulting [HO(2)] decay curves in the presence of acetone are characterized by an immediate decrease in initial [HO(2)] followed by subsequent decay. These curves are interpreted as a rapid (<100 μs) equilibrium reaction between acetone and the HO(2) radical that occurs on time scales faster than the time resolution of the apparatus, followed by subsequent reactions. This separation of time scales between the initial equilibrium and ensuing reactions enabled the determination of the equilibrium constant with values ranging from 4.0 × 10(-16) to 7.7 × 10(-18) cm(3) molecule(-1) for T = 215-272 K. Thermodynamic parameters for the reaction determined from a second-law fit of our van't Hoff plot were Δ(r)H°(245) = -35.4 ± 2.0 kJ mol(-1) and Δ(r)S°(245) = -88.2 ± 8.5 J mol(-1) K(-1). Recent ab initio calculations predict that the reaction proceeds through a prereactive hydrogen-bonded molecular complex (HO(2)-acetone) with subsequent isomerization to a hydroxy-peroxy radical, 2-hydroxyisopropylperoxy (2-HIPP). The calculations differ greatly in the energetics of the complex and the peroxy radical, as well as the transition state for isomerization, leading to significant differences in their predictions of the extent of this reaction at tropospheric temperatures. The current results are consistent with equilibrium formation of the hydrogen-bonded molecular complex on a short time scale (100 μs). Formation of the hydrogen-bonded complex will have a negligible impact on the atmosphere. However, the complex could subsequently isomerize to form the 2-HIPP radical on longer time scales. Further experimental studies are needed to assess the ultimate impact of the reaction of HO(2) and acetone on the atmosphere.

Elimination of residual amplitude modulation in tunable diode laser wavelength modulation spectroscopy using an optical fiber delay line

A new fiber-optic technique to eliminate residual amplitude modulation in tunable diode laser wavelength modulation spectroscopy is presented. The modulated laser output is split to pass in parallel through the gas measurement cell and an optical fiber delay line, with the modulation frequency / delay chosen to introduce a relative phase shift of pi between them. The two signals are balanced using a variable attenuator and recombined through a fiber coupler. In the absence of gas, the direct laser intensity modulation cancels, thereby eliminating the high background. The presence of gas induces a concentration-dependent imbalance at the coupler's output from which the absolute absorption profile is directly recovered with high accuracy using 1f detection.

Experimental research on the sensitivity of wavelength modulation by signal processing

We present a signal-processing system developed for enhancing the sensitivity of absorption measurement based on tunable diode laser wavelength modulation spectroscopy. A signal-to-noise ratio enhancement of an order of magnitude was observed when digital signal-processing algorithms were applied to wavelength modulation spectrometers in the near-infrared，at the same time a new spectra was found by observation of CO2 near 1.53 μm. These algorithms include digital low-pass filters, multiple averages, and a least squares fit. Digital signal processing has a practical advantage over other noise suppression techniques because it is easy to implement and adapt able to all experiments.

Mixture fraction measurements via WMS/ITAC in a laminar diffusion flame

A new approach to determine the mixture fraction at all position in a flame, using experimental measurements of as few as single species, is described. This approach combines measurements using diode laser wavelength modulation spectroscopy (WMS) and a method termed iterative temperature with assumed chemistry (ITAC). Combustion models used in ITAC include equilibrium and near-equilibrium chemistry and are based on the understanding of the important flow, fluid transport, and chemical properties of the system. The conserved scalar approach is used to model the flow-chemistry coupling and provides the state relationships to relate the reactants and products to mixture fraction in the case of equilibrium and near-equilibrium chemistry. In this work, the basic WMS/ITAC method is described and validated for a laminar diffusion flame in a Wolfhard-Parker burner. Methane concentration and temperature are measured and compared to published and ITAC results. There is good agreement between the direct measurements and ITAC results. These results agree well with published results, considering the differences in ambient pressure. Effects, including differential diffusion, radiative heat loss, and finite-rate chemistry, on the combustion models are addressed and found to be important for the simple laminar diffusion flame. Extension of WMS/ITAC to more complex combustion systems wich include turbulence and non-equilibrium chemistry are suggested.

HBr concentration and temperature measurements in a plasma etch reactor using diode laser absorption spectroscopy

In situ measurements of HBr concentrations and rotational temperatures were recorded in a 300 mm planar inductively coupled plasma (ICP) etch reactor using diode laser wavelength modulation spectroscopy. A pair of diode lasers operating near 1.95 and 2.00 μm were wavelength tuned over the R(7) and P(2) transitions of HBr (2–0 band), time-division multiplexed, and directed through an industrial wafer etch reactor. The rotational temperature (typically 435±8 K) was determined from the ratio of peak absorption signals and the HBr concentration was determined from the measured temperature and absorbance from a single line. The measured rotational temperature in the plasma was relatively independent of conditions studied. The estimated HBr dissociation fraction ranged from 25%–60%, depending on the ICP power applied, gas flow rate, and chamber pressure. Decreases in HBr concentration were detected 1 cm above the wafer surface during blank silicon wafer etching. The HBr dissociation fractions were measured before and after SF6 plasma clean with various focus rings made of quartz, alumina, and silicon carbide. The HBr dissociation fraction increased 17% with the quartz focus ring after the chamber was seasoned. The silicon carbide focus ring had essentially no influence on HBr concentration before and after chamber clean. The success of this work demonstrates the potential of diode lasers for in situ monitoring of the plasma etch process for real time control applications.

Quantitative species measurements in microgravity flames with near-IR diode lasers

Absolute concentrations of water vapor are measured in microgravity (µ-g), nonpremixed methane, and propane jet flames with diode-laser wavelength modulation spectroscopy. These experiments are performed in the 2.2-s µ-g drop facility at the NASA Lewis Research Center. Abel inversion methods are used to determine time-dependent radial profiles from eight line-of-sight projections across the flames. At all measured heights above the nozzle, water vapor spatial distributions in µ-g flames are much wider than their 1-g counterparts. Radial growth of the water signal continues throughout the drop, verifying earlier suggestions that a steady state is not reached during the duration of the test, despite a quasi-steady flame shape. Large amounts of water vapor are observed at larger radii, at odds with visual (video) observations and numerical predictions.