Infrared laser-absorption sensing for combustion gases

Tunable diode laser spectroscopy as a technique for combustion diagnostics

Diode laser based absorption spectroscopy (DLAS) is widely used for gas detection in variety of applications across the energy, petrochemical and mining industries. Recent developments in near and mid infrared diode lasers have improved the sensitivity of gas measurement based on high resolution spectra of target species. The availability of near-infrared diodes that can operate at room temperature has expanded the application of spectroscopy technique in hand-held gas detection devices. DLAS in conjunction with optical fibres has also been applied for the distributed sensing and monitoring of various gases in real-time basis. This paper, after introducing methane characteristics and semiconductor diode lasers, comprehensively reviews development in spectroscopic methane sensing techniques in accordance with methane absorption lines in the near infrared spectrum.

A review of developments in near infrared methane detection based on tunable diode laser

Tunable diode laser absorption spectroscopy (TDLAS), as a noninvasive spectroscopic method, permits high-resolution, high-sensitivity, fast, in situ absorption measurements of atomic and molecular species and narrow spectral features in gaseous, solid, and liquid phases. Advances in new diode laser sources and laser spectroscopic techniques generally have triggered an increasing application of TDLAS in various disciplines (for example, atmospheric environmental monitoring, chemical analysis, industrial process control, medical diagnostics and combustion monitoring, etc.) over the last four decades. This article reviews some important developments in TDLAS, from its basic principles as a spectroscopic tool to the demonstration of gas absorption measurements, emphasizing signal enhancement and noise reduction techniques developed for improving current TDLAS performance.

A Review of Signal Enhancement and Noise Reduction Techniques for Tunable Diode Laser Absorption Spectroscopy

Breath analysis can be a valuable, noninvasive tool for the clinical diagnosis of a number of pathological conditions. The detection of ammonia in exhaled breath is of particular interest for it has been linked to kidney malfunction and peptic ulcers. Pulsed cavity ringdown spectroscopy in the mid-IR region has developed into a sensitive analytical technique for trace gas analysis. A gas analyzer based on a pulsed mid-IR quantum cascade laser operating near 970 cm(-1) has been developed for the detection of ammonia levels in breath. We report a sensitivity of approximately 50 parts per billion with a 20 s time resolution for ammonia detection in breath with this system. The challenges and possible solutions for the quantification of ammonia in human breath by the described technique are discussed.

Pulsed quantum cascade laser-based cavity ring-down spectroscopy for ammonia detection in breath

Frequency tuning of long-wavelength VCSELs.

A pulsed distributed feedback quantum cascade laser operating near 970 cm-1 (10.3 μm) was coupled with the technique of cavity ring-down spectroscopy, as described here for the first time. The newly constructed set-up was tested by recording three relatively weak rotational lines of the 1000→0001 vibrational band of CO2 in the range from 966.75 cm-1 to 971.5 cm-1. The CO2 lines were recorded by measuring the decay time of a CO2 - N2 mixture flowing through an open sample tube placed between the cavity ring-down mirrors. The quantum cascade laser frequency was tuned at a rate of ~ 0.071 cm-1/K by changing the heat sink temperature in the range between -20 and 50 °C. The first results demonstrated the applicability and high sensitivity of the cavity ring-down spectroscopy - pulsed quantum cascade laser combination and encouraged us to extend our research to the study and detection of ammonia. We demonstrated that a detection limit of ammonia of ~ 25 ppbv can be attained with the current set-up. Basic instrument performance and optimization of the experimental parameters for sensitivity improvement are discussed.

Cavity ring-down spectroscopy with a pulsed distributed feedback quantum cascade laser

A method of gas phase chemical analysis using direct absorption spectroscopy is implemented with long-wavelength vertical-cavity surface-emitting lasers (VCSELs) operating near 1577 nm. The method is based on a linear dependence of widths of collisionally broadened absorption lines on gas pressure. It is shown that the absolute gas concentrations in multicomponent gas mixtures can be extracted from the line widths of all compounds measured simultaneously. The concentrations of both absorbing and nonabsorbing compounds are extracted from the results of simultaneous measurements of peak absorption and line width of the absorbing compound. Long-wavelength VCSELs with a buried tunnel junction (Vertilas, Germany) are used, for the first time, for multispecies and trace gas detection. Continuous single-mode tuning of the VCSELs up to 30 cm –1 is achieved with temperature and injection current varied in the range 0 to 50 °C and 1.3 to 6.5 mA, respectively. A fractional absorption of ~10 –4 (600 ppm of CO 2 in air) is measured with a single-beam spectroscope. The method described can be used for laser chemical analysis of gas mixtures with relatively high concentrations of target compounds and for open-path trace gas sensing. Compact sensors based on long-wavelength VCSELs can be developed for environmental and industrial gas monitoring.

Gas phase chemical analysis using long-wavelength vertical-cavity surface-emitting lasers

We used direct absorption spectroscopy for characterization of long-wavelength VCSELs (VERTILAS, Germany). Gas mixture CO:CO2=1:1 (total pressure 0.1 - 0.5 bar) provided in the tuning ranges of lasers a multitude of absorption lines spaced by intervals from 1 to 60 GHz that allowed the absolute calibration of laser scans. We measured tuning rates of the VCSELs in 0-50 °C temperature interval in dependence on injection current and modulation frequency varied from 0 up to 6.5 mA and 500 kHz respectively. The continuous tuning range of the 1577.5-nm VCSEL with single-mode output of up to 1.5 mW was found to be of 9.61 nm (38.62 cm-1). The temperature tuning rates for all VCSELs under study were in the range between -0.4 and -0.5 cm-1/ °C. The current tuning rates, varied from laser to laser between -1.9 and -2.2 cm-1/mA at low injection currents and modulation frequencies, were found to be twice as big for all lasers at high injection currents and decreased by factor 2 and 5 at modulation frequencies of 300 and 500 kHz respectively. A phase shift between the amplitude and frequency modulations was measured by fine tuning VCSEL with a DC injection current to display the end of a laser scan in respect to a local maximum in laser power. The phase shift changed from 0 to -0.3 rad with modulation frequency raised from 500 Hz to 500 kHz. The results of phase shift measurements have been compared with those obtained recently for DFB lasers with different methods.

Long-wavelength VCSELs for applications in absorption spectroscopy: tuning rates and modulation performances

A method for range-resolved gas sensing using path-integrated optical systems is presented. The method involves dividing an absorption path into several measurement segments and extracting the gas concentration in each segment from two path-integrated measurements. We implemented the method with tunable lasers (a 1389-nm VCSEL and a 10.9-μm pulsed quantum cascade laser) and a group of retro reflectors (RRs) distributed along absorption paths. Using a rotating mirror with the VCSEL configuration, we could scan a group of seven tape RRs spaced by 10 cm in ∼ 9 ms to extract an H2O concentration profile. Reduced H2O concentrations were recorded in the segments purged with dry air. Hollow corner cube RRs were used in the quantum cascade laser configuration at distances up to 1.1 km from the laser. Two RRs placed at 66 m and 125 m from the laser allowed us to determine H2O concentrations in both segments. The RRs returns were separated due to the different round trip travel time of the 200-ns laser pulse. Novel instruments for range-resolved remote sensing in the atmosphere can be developed for a variety of applications, including monitoring the fluxes of atmospheric pollutants and controlling air quality in populated areas.

Range-resolved gas concentration measurements using tunable semiconductor lasers

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