Tunable Diode Laser Sensor Research Articles

Real-time, in situ measurement of H2O generated during in situ combustion tests using 1f-normalized wavelength modulation spectroscopy with second harmonic detection.

The development and deployment of a real-time, in situ, non-invasive sensor to monitor the concentration of H 2 O during in situ combustion (ISC) experiments with a heavy-crude oil is described. A real-time sensor to monitor the gas-phase products from ISC can support the study of the kinetics of the complex chemical reactive system in ISC. The mole fraction of H 2 O was measured using tunable diode laser (TDL) absorption spectroscopy coupled with 1f-normalized wavelength modulation spectroscopy (WMS) and 2f detection. The WMS 2f/1f strategy was used to enhance sensitivity with effective noise rejection, particularly suitable when characterizing the water vapor evolved from oil-water emulsions. H 2 O was measured at 3934.10c m -1 from the fundamental band v 3. That transition was selected using the HITRAN database to increase the line strength and minimize interference from neighbor compounds. Measurements of H 2 O concentration were conducted at ambient temperature and pressure using a reference cell (H 2 O=2% at 98.6kPa) to validate the sensor architecture under controlled laboratory environments. The TDL sensor was also successfully validated during real ISC experiments involving heavy-crude oil. Validation and combustion experiments showed the potential of the TDL-based sensor for non-invasive, real-time, in situ measurements of gas-phase species in conditions similar to those of laboratory-scale experimental ISC tests.

A sensor based on high-sensitivity multi-pass resonant photoacoustic spectroscopy for detection of hydrogen sulfide

A high-sensitive tunable diode laser sensor based on photoacoustic spectroscopy and multi-pass absorption spectroscopy is developed for hydrogen sulfide (H2S) detection using a target absorption line at 6287.005 cm−1. Two spherical concave mirrors are utilized as the reflectors to make the laser beam pass through the photoacoustic cell for 24 times. The wavelength modulation spectroscopy technique with second harmonic detection (WMS-2f) method is employed to reduce external interference and increase the detection sensitivity. With the enhancement of the effective laser power in the multi-pass PA cell, the photoacoustic 2f signal of the target H2S absorption line has a ∼ 8 times improvement compared to the sensor system without multi-pass cell. Based on the results of Allan variance analysis, the minimum detection concentration of the system could reach 0.68 ppm with a 10-s average time and it can be further lowered down to ∼ 0.1 ppm when a 1-minute averaging time is used. The linear responsivity of the multi-pass PA spectroscopy sensor is estimated to be ∼ 0.00112 mV/ppm by linear fitting for a continuous flow gas measurement results. The excellent performance validates the potential of the sensor for field application.

OH radical measurements in combustion environments using wavelength modulation spectroscopy and dual-frequency comb spectroscopy near 1491 nm

Hydroxyl radical (OH) is a key intermediate reactive species during combustion processes relevant to power production, transportation, and manufacturing. We demonstrate an OH sensor based on in situ laser absorption spectroscopy for deployment in industrial conditions. The sensor relies on telecommunications-fiber-coupled, tunable-diode-laser absorption spectroscopy of an OH transition near 1491 nm. By employing wavelength modulation spectroscopy, the sensor is capable of in situ, quantitative detection of OH down to mole fraction values of 10−5 over a 75-cm pathlength. To increase the accuracy of the OH sensor, we perform the first dual-comb spectroscopy measurement above a flame and use the results to create an absorption database of water vapor transitions from 1489.2 to 1492.5 nm at temperatures up to 2165 K. The database is included in the analysis procedure for the tunable diode laser sensor to account for the water vapor absorption that overlaps with the OH absorption. The utility of the laser sensor is demonstrated by characterizing the concentration of OH radical above a catalytic combustor under different operating conditions.

Development of a tunable diode laser sensor for CO concentration analysis at laboratory-scale conditions for in situ combustion tests of heavy crude oils.

A tunable diode laser-based sensor is reported to monitor carbon monoxide (CO) concentration under conditions similar to those of laboratory-scale tests used to characterize the behavior of heavy crude oil during in situ combustion (ISC). The sensor uses a DFB diode laser operating over the spectral range of the rotational transition R(11) of the first overtone, wherein simulations of spectral absorption bands for CO, CO2, and H2O showed minimal spectral interference. The absorption spectra were calculated using the HiTran 2008 database at temperatures varying between 150°C and 800°C, pressures from 1 to 5atm, and the typical concentration of major species in ISC characterization experiments. The measurements of CO concentration were conducted at ambient temperature and pressure in a static glass cell of borosilicate, with a path length of 3.81cm, to validate the CO sensor architecture under controlled laboratory environments. The calibration curve for CO obtained by quantifying the optical density at the line center of R(11) for a molar-concentration range between 0.7% and 3.4% demonstrated a linear response (coefficient of determination of 0.9986). Experiments at 4atm and 600K were carried out in a custom-designed optical chamber with two optical wedged sapphire windows (2°) to avoid the etalon effect. CO-absorption spectra were validated by a comparative study with the HiTran database, while a calibration-free methodology using scanned-wavelength direct absorption was investigated for future characterizing experiments involving ISC. Signal to noise ratios (SNRs) were above 40 in the optical chamber and higher than 14 in the glass cell. The TDL sensor was also successfully validated during real ISC experiments involving Colombian heavy crude oil. The calibration and oxidation experiments showed the potential of TDL-based sensors in the region of 2.3μm for non-invasive, real-time, and in situ measurements of carbon monoxide generated at similar conditions to those of lab-scale experimental ISC tests.

An open-path tunable diode laser absorption spectrometer for detection of carbon dioxide at the Bonanza Creek Long-Term Ecological Research Site near Fairbanks, Alaska

We have developed a low-power, open-path, near-infrared (NIR) tunable diode laser sensor for the measurement of near ground-level concentrations of greenhouse gases. Here, we report on instrument design, characterization, and initial measurements of carbon dioxide concentrations during deployment to a thermokarst collapse scar bog near Fairbanks, AK (USA). The optics “launch-box” portion of the instrument couples radiation from an NIR, distributed feedback diode laser operating near 1572 nm with a visible laser for alignment purposes. The outgoing beam is directed through a 3.2-mm hole in a parabolic mirror and the launch-box is oriented using a two axis, altitude-azimuth telescope mount such that the beam strikes a retroreflector target at a set distance downfield. The beam then retraces the path back to the launch-box where the light is collected on the surface of the parabolic mirror and focused onto a multimode fiber that transfers the radiation to an InGaAs detector. Sweeps over a ~1.6 cm−1 spectral region were collected at a rate of 500 scans per second and were typically stored as 10 s sweep averages. These averaged sweeps could be individually spectrally fit for CO2 concentration or averaged into a single spectrum for fitting (after correction for slight frequency drift). Field data reported here was averaged for 2.5 min and was found to follow trends in diurnal cycles of CO2 concentration cycles reported by sensors located nearby in the field site.

Subsonic In-Flight Temperature and Pressure Measurements Using a Scramjet Inlet Flow Sensor

The performance of a scramjet inlet sensor based on tunable diode laser absorption spectroscopy of oxygen in the A-band near 760 nm during a low-altitude, subsonic flight test is discussed. Though the scramjet did not reach the targeted hypersonic flight regime due to the malfunction of the first-stage booster, analysis of the spectral absorption data acquired during the descent flight below 3 km successfully determined pressure and temperature in the inlet and demonstrated that the system was capable of producing an adequate signal-to-noise ratio to allow temperature, pressure, and velocity measurements under the expected hypersonic flight conditions at altitudes up to at least 30 km. This is the first flight-tested tunable diode laser sensor for a scramjet inlet flow, and the robustness and stability of the sensor design in dynamic load conditions were made evident by the quality of the data recorded after a very unstable and high- launch scenario.

A scanned-wavelength-modulation absorption-spectroscopy sensor for temperature and H2O in low-pressure flames

The design and demonstration of a tunable diode laser sensor for in situ temperature and water (H2O) measurements in laboratory flames is presented. A newly-designed probe consisting of two single-crystal sapphire rods is used to guide 2.9 μm laser light across the core of a flame while avoiding absorption by water outside of the flame. The sensor probes two H2O transitions near 2.9 μm using a scanned-wavelength-modulation absorption-spectroscopy spectral-fitting technique, which enables measurements without calibration or knowledge of the mixture collisional-broadening coefficient. To demonstrate the sensor, temperature and water mole fraction measurements were acquired in stoichiometric, burner-stabilized flames of methane/oxygen/argon at pressures of 25 Torr and 60 Torr. Typical total uncertainties in the temperature and water mole fraction measurements were ±50 K and ±0.016, respectively. This sensor is simple, robust and accurate and enables improved chemical kinetics studies of low-pressure flames. Improvements to reduce temperature and H2O precision to ±20 K and ±0.008, respectively, are discussed.

Spatially Resolved Water Measurements in a Scramjet Combustor Using Diode Laser Absorption

A two-color tunable diode laser sensor for detection of absorption near was used to simultaneously determine temperature, column density, and velocity in a direct-connect scramjet combustor. Axial and transverse spatial resolutions were achieved by translating the measurement line of sight across and along the flowpath with measurements spanning six axial planes and 24 measurement lines of sight in each plane. Results are presented for a uniform noncombusting -seeded case as well as for air combustion at equivalence ratios of 0.17 and 0.46. A combination of wavelength-modulation spectroscopy and scanned-wavelength direct absorption were used to mitigate nonuniform gas conditions along the measurement line of sight present in the combustion cases. Comparisons of tunable diode laser absorption spectroscopy measurements with computational fluid dynamics simulations are included and illustrate the use of line-of-sight tunable diode laser absorption spectroscopy to validate computational efforts. Additional comparisons with coherent anti-Stokes Raman scattering measurements in the same model scramjet corroborate many key findings from tunable diode laser absorption spectroscopy.

Sensor for headspace pressure and H2O concentration measurements in closed vials by tunable diode laser absorption spectroscopy

The concentration of H2O and the pressure in the headspace of vials are simultaneously measured by a tunable diode laser sensor based on absorption spectroscopy techniques. The 7168.437cm−1 spectral line of H2O is chosen as the sensing transition for its strong absorption strength and being reasonably far away from its neighboring molecular transitions. In order to prevent interference absorption by ambient water vapor in the room air, a difference between the measured signal and the referenced signal is used to calculate the pressure and H2O concentration in the headspace of vials, eliminating the need for inert gas purges and calibration with known gas. The validation of the sensor is conducted in a static vial, yielding an accuracy of 1.23% for pressure and 3.81% for H2O concentration. The sensitivity of the sensor is estimated to be about 2.5Torr for pressure and 400ppm for H2O concentration over a 3cm absorption path length respectively. Accurate measurements for commercial freeze-dried products demonstrate the in-line applications of the sensor for the pharmaceutical industry.

Wavelength-modulation spectroscopy near 2.5 μm for H2O and temperature in high-pressure and -temperature gases

The design and validation of a tunable diode laser (TDL) sensor for temperature and H2O in high-pressure and -temperature gases are presented. High-fidelity measurements are enabled through the use of: (1) strong H2O fundamental-band absorption near 2.5 μm, (2) calibration-free first-harmonic-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f), (3) an experimentally derived and validated spectroscopic database, and (4) a new approach to selecting the optimal wavelength and modulation depth of each laser. This sensor uses two TDLs near 2,474 and 2,482 nm that were fiber coupled in free space and frequency multiplexed to enable measurements along a single line-of-sight. The lasers were modulated at 35 and 45.5 kHz, respectively, to achieve a sensor bandwidth of 4.5 kHz. This sensor was validated in a shock tube at temperatures and pressures ranging from 1,000 to 2,700 K and 8 to 50 bar. There the sensor resolved transients and recovered the known steady-state temperature and H2O mole fraction with a precision of 3.2 and 2.6 % RMS, respectively.

Scanned-wavelength-modulation spectroscopy near 2.5 μm for H2O and temperature in a hydrocarbon-fueled scramjet combustor

The design and demonstration of a two-color tunable diode laser sensor for measurements of temperature and H2O in an ethylene-fueled model scramjet combustor are presented. This sensor probes multiple H2O transitions in the fundamental vibration bands near 2.5 μm that are up to 20 times stronger than those used by previous near-infrared H2O sensors. In addition, two design measures enabled high-fidelity measurements in the nonuniform flow field. (1) A recently developed calibration-free scanned-wavelength-modulation spectroscopy spectral-fitting strategy was used to infer the integrated absorbance of each transition without a priori knowledge of the absorption lineshape and (2) transitions with strengths that scale near-linearly with temperature were used to accurately determine the H2O column density and the H2O-weighted path-averaged temperature from the integrated absorbance of two transitions.

Improving the Stability of Tunable Diode Laser Sensor for Natural Gas Leakage Monitoring

Tunable diode laser absorption spectroscopy (TDLAS) is a method of using a tunable diode laser scanning one of absorption lines of trace gas, this technique has advantage of high sensitivity, fast response, multi-component and non-contact measurement. We had already developed a TDLAS sensor for monitoring natural gas leakage during the natural gas production and the process of gathering and transportation in the natural gas station, but in the practical application we found that the TDLAS sensor was influenced by visibility changes. In order to overcome this interference and improve sensor stability, we present an automatic control circuit to process photoelectric signal for eliminating interference of visibility changes. Long-term monitoring results show that the concentration of CH4 are regular and stable during the natural gas production and transmission, therefore the application of the automatic gain circuit improved the stability of the TDLAS sensor.

Tunable diode laser absorption sensor for the simultaneous measurement of water film thickness, liquid- and vapor-phase temperature

A four-wavelength near-infrared (NIR) tunable diode laser sensor has been developed for the simultaneous measurement of liquid water film thickness, liquid-phase temperature and vapor-phase temperature above the film. This work is an important improvement of a three-wavelength concept previously introduced by Yang et al. (Appl. Phys. B 99:385, 2010), which measured the film thickness in environments with known temperature only. In the new sensor, an optimized combination of four wavelengths is chosen based on a sensitivity analysis with regard to the temperature dependence of the liquid water absorption cross section around 1.4 μm. The temperature of liquid water and the film thickness are calculated from absorbance ratios taken at three wavelength positions assessing the broad-band spectral signature of liquid water. The vapor-phase temperature is determined from the absorbance ratio of two lasers rapidly tuned across two narrow-band gas-phase water absorption transitions. The performance of the sensor was demonstrated in a calibration cell providing liquid layers of variable thickness and temperature with uncertainties smaller than 5% for thickness measurements and 1.5% for liquid-phase temperatures, respectively. Experiments are also presented for time-resolved thickness and temperature measurements of evaporating water films on a quartz plate.

Temperature sensing in shock-heated evaporating aerosol using wavelength-modulation absorption spectroscopy of CO2 near 2.7 µm

A tunable diode laser sensor with a detection bandwidth of 40 kHz is developed for measuring the time-varying gas temperature of CO2 during the evaporation of shock-heated hydrocarbon fuel aerosol. Normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f) is used to probe R(28) and P(70) transitions in the ν1 + ν3 combination band of CO2 near 2.7 µm. The fixed-center-wavelength WMS sensor was first validated in a shock tube with non-reactive CO2/Ar gas mixtures, yielding an accuracy of better than 1.5% over the entire range of 650–1500 K. The sensor was then evaluated in a well-controlled aerosol flow cell, demonstrating the potential for precise gas temperature measurement even when aerosol scattering attenuates more than 99% of the incident light. Applications of this sensor for accurate temperature measurement of evaporating n-dodecane aerosol were then performed in an aerosol shock tube. The time-resolved temperature variation due to the evaporation of fuel droplets was accurately captured without using an off-resonant laser to account for the extinction from droplet scattering. Measured temperatures confirmed the accuracy of the gasdynamic model used to calculate the pre- and post-evaporation shock conditions, as needed in shock tube studies on combustion chemistry.

Real-time equivalence ratio measurements in gas turbine combustors with a near-infrared diode laser sensor

A rapid (2kHz) near-infrared tunable diode laser (TDL) absorption sensor is developed for real-time measurements of methane concentration (and thus equivalence ratio) in gas turbine combustors. The non-intrusive sensor is based on fixed-wavelength laser absorption of methane near 1.65μm using a single fiber-coupled diode laser. This CH4 absorption wavelength is free of interference from the absorption of other species in air. Wavelength modulation spectroscopy (WMS) is combined with second-harmonic detection to improve the sensor sensitivity and accuracy. WMS-1f signal is used to normalize the 2f signal to remove the need for calibration and account for the laser transmission variations in gas turbine combustors. The near-IR TDL sensor is first calibrated in a static cell with CH4 and N2 mixture to determine the line-strength and find the laser setpoint. Test with a gas turbine combustor operating at atmospheric pressure is then conducted to demonstrate the sensor accuracy and response time. The TDL sensor is used to characterize equivalence ratio fluctuations in the combustor before the fuel/air mixtures enter the combustion chamber. The gain between heat release and equivalence ratio fluctuation is measured when the fuel flow rate is modulated at different levels. The combustion test results demonstrate the utility of the TDL sensor as a useful diagnostic tool to study flame characteristics and evaluate different combustor designs.

A sensor for measurements of temperature and water concentration using a single tunable diode laser near 1.4 μm

Gas temperature and water concentration are measured by a tunable diode laser sensor using a single laser near 1.4 μm. Temperature is determined from the ratio of absorption of a H 2O line pair near 7079.176 and 7079.855 cm −1, which are chosen using some selected criteria. The second harmonic detection of wavelength modulation spectroscopy (WMS-2 f) is employed to improve the sensor sensitivity and accuracy. Normalization of the 2 f signal with the 1 f signal magnitude is used to remove the need for calibration and correct for transmission variation due to beam steering, mechanical misalignments, soot, and windows fouling. An optimum laser modulation depth is selected to make the WMS-2 f signals of both lines as big as possible for the target test conditions. The influence of total pressure and water mole fraction is evaluated through theoretical simulations. The validation of the sensor is conducted in a controlled static cell, yielding an accuracy of 1.06% temperature and 1.9% for H 2O concentration. Burner experiments demonstrate the accuracy (1.2% for temperature and 3.2% for H 2O concentration) and potential utility of the system for combustion diagnosis.

Measurements of high-pressure CO2 absorption near 2.0 μm and implications on tunable diode laser sensor design

A tunable diode laser (TDL) is used to measure the absorption spectra of the R46 through R54 transitions of the 20012←00001 band of CO2 near 2.0 μm (5000 cm−1) at room temperature and pressures to 10 atm (densities to 9.2 amagat). Spectra are recorded using direct absorption spectroscopy and wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) in a mixture containing 11% CO2 in air. The direct absorption spectra are influenced by non-Lorentzian effects including finite-duration collisions which perturb far-wing absorption, and an empirical χ-function correction to the Voigt line shape is shown to greatly reduce error in the spectral model. WMS-2f spectra are shown to be at least a factor of four less-influenced by non-Lorentzian effects in this region, making this approach more resistant to errors in the far-wing line shape model and allowing a comparison between the spectral parameters of HITRAN and a new database which includes pressure-induced shift coefficients. The implications of these measurements on practical, high-pressure CO2 sensor design are discussed.

In situ combustion measurements of H2O and temperature near 2.5 µm using tunable diode laser absorption

In situ combustion measurements of water vapor concentration and gas temperature were carried out with a new tunable diode laser sensor near 2.5 µm. Recent availability of room-temperature semiconductor diode lasers operating at longer wavelengths provides access to fundamental vibrational bands (ν1 and ν3) of H2O. These bands have stronger absorption line strength compared to the overtone (2ν1, 2ν3) and combination (ν1 + ν3) vibrational bands in the near-infrared region probed previously with telecommunication diode lasers. The absorption transitions of H2O vapor in the 2.5–3.0 µm region are systematically analyzed via spectral simulation, and optimal spectral line pairs are selected for combustion measurements in the temperature range of 1000–2500 K. Fundamental spectroscopic parameters (line strength, line position and line-broadening coefficients) of the selected transitions are determined via laboratory measurements in a heated cell. Absorption measurements of H2O concentration and temperature are then made in a laboratory flat-flame burner to illustrate the potential of this sensor for sensitive and accurate measurements in combustion gases with short optical path lengths.

Sensing and Control of Combustion Instabilities in Swirl-Stabilized Combustors Using Diode-Laser Absorption

** † ‡ Thermoacoustic instability and lean blowout (LBO) are monitored in propane/air flames in a swirl-stabilized combustor using a novel tunable diode laser (TDL) sensor for gas temperature using wavelength-scanned laser absorption of two neighboring water vapor transitions near 1.4 μm. Although the gas composition and temperature are not uniform along the sensor line-of-sight, temperature oscillations and fluctuations are clearly identified in the FFT power spectra of the time-resolved data. Detailed experiments are conducted to optimize the position of the sensor line-of-sight in the flame for thermoacoustic instability and LBO sensing. The output of the sensor is used to suppress the thermoacoustic oscillations in a phase-delay feedback control system, and acoustic modulation of the intake air flow suppresses the temperature oscillations by more than 7dB. The low-frequency temperature fluctuations measured by the TDL sensor increase sharply as the flame approaches LBO. The intensity of these low-frequency fluctuations is used to detect the proximity to LBO, and can be used as a control variable for active LBO suppression without knowing the LBO fuel/air ratio limit. The TDL sensor results are compared with traditional pressure and emission sensors, and the TDL sensor is found to offer several advantages including better spatial resolution and insensitivity to background acoustic noise and flame emissions.

Development of a tunable diode laser sensor for measurements of gas turbine exhaust temperature

A tunable diode laser (TDL) temperature sensor is designed, constructed, tested, and demonstrated in the exhaust of an industrial gas turbine. Temperature is determined from the ratio of the measured absorbance of two water vapor overtone transitions in the near infrared where telecommunication diode lasers are available. Design rules are developed to select the optimal pair of transitions for direct absorption measurements using spectral simulations by systematically examining the absorption strength, spectral isolation, and temperature sensitivity to maximize temperature accuracy in the core flow and minimize sensitivity to water vapor in the cold boundary layer. The contribution to temperature uncertainty from the spectroscopic database is evaluated and precise line-strength data are measured for the selected transitions. Gas-temperature measurements in a heated cell are used to verify the sensor accuracy (over the temperature range of 350 to 1000 K, ΔT∼2 K for the optimal line pair and ΔT∼5 K for an alternative line pair). Field measurements of exhaust-gas temperature in an industrial gas turbine demonstrate the practical utility of TDL sensing in harsh industrial environments.