Diode laser absorption measurements of CH(3)Cl and CH(4) near 1.65 microm

The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν₂ mode spectrum of methane (CH₄). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000 cm^-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH₄ ν₂ CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH₄ chemistry is demonstrated in a laboratory CH₄/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O₂), carbon dioxide (CO₂), and molecular hydrogen (H₂), along with CH₄. Fundamental physicochemical processes, such as H₂ production via CH₄ pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH₄ v₂ CRS thermometry, and we validate it against CO₂ CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH₄-rich environments, e.g., in plasma reactors for CH₄ pyrolysis and H₂ production.

Optical Methods of Methane Detection

Methane is the most frequently analyzed gas with different concentrations ranging from single ppm or ppb to 100%. There are a wide range of applications for gas sensors including urban uses, industrial uses, rural measurements, and environment monitoring. The most important applications include the measurement of anthropogenic greenhouse gases in the atmosphere and methane leak detection. In this review, we discuss common optical methods used for detecting methane such as non-dispersive infrared (NIR) technology, direct tunable diode spectroscopy (TDLS), cavity ring-down spectroscopy (CRDS), cavity-enhanced absorption spectroscopy (CEAS), lidar techniques, and laser photoacoustic spectroscopy. We also present our own designs of laser methane analyzers for various applications (DIAL, TDLS, NIR).

Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization

Laser absorption spectroscopy (LAS) is a promising diagnostic method capable of providing high-bandwidth, species-specific sensing, and highly quantitative measurements. This review aims at providing general guidelines from the perspective of LAS sensor system design for realizing quantitative species diagnostics in combustion-related environments. A brief overview of representative detection limits and bandwidths achieved in different measurement scenarios is first provided to understand measurement needs and identify design targets. Different measurement schemes including direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS), and their variations are discussed and compared in terms of advantages and limitations. Based on the analysis of the major sources of noise including electronic, optical, and environmental noises, strategies of noise reduction and design optimization are categorized and compared. This addresses various means of laser control parameter optimization and data processing algorithms such as baseline extraction, in situ laser characterization, and wavelet analysis. There is still a large gap between the current sensor capabilities and the demands of combustion and engine diagnostic research. This calls for a profound understanding of the underlying fundamentals of a LAS sensing system in terms of optics, spectroscopy, and signal processing.

Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: rapid spectral sensing of gas-phase molecules

A cavity ringdown spectrometer, based on a continuous-wave swept-frequency laser, enables efficient, rapid recording of wide-ranging absorption spectra as characteristic spectral signatures of airborne molecules. The rapidly swept laser frequency resonates with the longitudinal modes of the ringdown cavity, effectively sampling the absorption spectrum of an intracavity gas at intervals defined by the cavity's free spectral range and generating a full absorption spectrum within a single rapid sweep of the widely tunable laser frequency. We report a new analog detection scheme that registers a single data point for each buildup and ringdown decay event without logging details of the full signal waveform; this minimizes demand on digitizer speed and memory depth, reducing the time scale of data processing. This results in a compact, robust, easy-to-use instrument that offers fresh prospects for spectroscopic sensing of trace species in the atmosphere.

Measuring methane and its isotopes 12CH4, 13CH4, and CH3D on the surface of Mars with in situ laser spectroscopy

In light of the recent discovery of methane on Mars and its possible biological origin, a strategy is described for making in situ measurements of methane and its isotopes on the surface of Mars by laser spectroscopy in the 3.3-microm wavelength region. An instrument of reasonable mass (approximately 1 lb) and power (few watts) is capable of measuring mixing ratios down to 0.1 part per 10(9) by volume, a hundred times lower than recently reported observations. Making accurate measurements of 13CH4 and CH3D will be more difficult. For measuring delta13C to 10/1000 and deltaD to 50/1000, sample preconcentration will be required to approximately 3 parts per 10(6) by volume for delta13C and to approximately 40 parts per 10(6) by volume for deltaD. This need would be mitigated by the discovery of larger local abundances of methane near the source regions.

Signal processing and calibration procedures for in situ diode-laser absorption spectroscopy

Gas analyzers based on tunable diode-laser spectroscopy (TDLS) provide high sensitivity, fast response and highly specific in situ measurements of several atmospheric trace gases simultaneously. Under optimum conditions even a shot noise limited performance can be obtained. For field applications outside the laboratory practical limitations are important. At ambient mixing ratios below a few parts-per-billion spectrometers become more and more sensitive towards noise, interference, drift effects and background changes associated with low level signals. It is the purpose of this review to address some of the problems which are encountered at these low levels and to describe a signal processing strategy for trace gas monitoring and a concept for in situ system calibration applicable for tunable diode-laser spectroscopy. To meet the requirement of quality assurance for field measurements and monitoring applications, procedures to check the linearity according to International Standard Organization regulations are described and some measurements of calibration functions are presented and discussed.

Single diode laser sensor for wide-range H2O temperature measurements

A single diode laser absorption sensor (near 1477 nm) useful for simultaneous temperature and H2O concentration measurements is developed. The diode laser tunes approximately 1.2 cm(-1) over three H2O absorption transitions in each measurement. The line strengths of the transitions are measured over a temperature range from 468 to 977 K, based on high-resolution absorption measurements in a heated static cell. The results indicate that the selected transitions are suitable for sensitive temperature measurements in atmospheric pressure combustion systems using absorption line ratios. Comparing the results with HITRAN 96 data, it appears that these transitions will be sensitive over a wide range of temperatures (450-2000 K), suggesting applicability for combustion measurements.

Wavelength tuning and spectral properties of distributed feedback diode lasers with a short external optical cavity

We report on the wavelength tuning and spectral properties of distributed feedback (DFB) diode lasers operated with a plane external cavity (XC) mirror positioned as close as possible to the diode-laser front facet. These lasers generate single-frequency near IR radiation at wavelengths of 1392, 1580, 1602, and 1653 nm. A piezoelectric variation of the XC length provided continuous single-frequency tuning to as high as 19 GHz. A further benefit of XC DFB lasers is a residual amplitude modulation per gigahertz tuning of less than 10(-3). The XC feedback also suppresses residual side-mode oscillations to less than 60 dB. The laser's total intensity noise is close to the shot noise limit. The laser linewidth (measured in a beat note experiment) is less than 90 kHz within an acquisition time of 40 ms. The advantageous properties of XC DFB lasers for molecular spectroscopy are demonstrated by recording R(3) 2nu(3) overtone spectra of methane by single-scan single-pass absorption or frequency-modulation spectroscopy.

High-Resolution Measurements of HBr Transitions in the First Overtone Band Using Tunable Diode Lasers

High-resolution absorption lineshapes of the R(7) and P(2) transitions in the first overtone (v = 0-2) band of H(79)Br have been recorded at room temperature using a pair of distributed feedback diode lasers operating near 1.95 and 2.00 µm, respectively. Spectral line intensities and self-broadening coefficients were determined by fitting the measured spectra (for various pressures P = 10-100 Torr) with Voigt profiles and compared with values in the literature. Measured line intensities for the P(2) and R(7) transitions are approximately 11 and 16% higher than those listed in the HITRAN database, respectively. The measured self-broadening coefficient of the P(2) transition is approximately 14% lower than the value listed in HITRAN. Measurements of the P(2) lineshapes at low pressure (100 mTorr) were modeled with eight-line Gaussian profiles based on ground state (v = 0) hyperfine constants to include the effects of nuclear electric quadrupole interactions. Copyright 2000 Academic Press.

Spectral Intensity and Lineshape Measurements in the First Overtone Band of HF Using Tunable Diode Lasers

High-resolution absorption lineshapes for the P(3) and P(6) transitions of the first overtone (v = 2-0) band of HF at 296 K have been measured using a pair of distributed feedback diode lasers operating near 1.31 and 1.34 µm, respectively. Spectral line intensities and self-broadening parameters were determined by fitting the measured spectra with Voigt, Galatry, and Rautian lineshape models. Voigt profiles fit the low-pressure (<10 Torr) spectra of the P(3) transition reasonably well due to the relatively strong collisional broadening effect. Lineshape measurements of the P(6) transition (for pressures ranging from 5 to 60 Torr) show significant variation from the Voigt lineshape model due to velocity-changing collisions that effectively reduce the Doppler component of the spectral line. Lineshape models that include motional (Dicke) narrowing effects, Galatry (soft collision) and Rautian (hard collision) profiles yield significant improvements in the spectral lineshape fits compared with Voigt profiles. The collisional broadening coefficient (gamma) of the P(6) transition obtained from a Voigt fit is approximately 4% lower than those found with either Galatry or Rautian profile fits. The measured intensities and self-broadening coefficients are compared with values in the HITRAN database and previous measurements. Copyright 1999 Academic Press.

Diode laser absorption sensors for gas-dynamic and combustion flows

Recent advances in room-temperature, near-IR and visible diode laser sources for tele-communication, high-speed computer networks, and optical data storage applications are enabling a new generation of gas-dynamic and combustion-flow sensors based on laser absorption spectroscopy. In addition to conventional species concentration and density measurements, spectroscopic techniques for temperature, velocity, pressure and mass flux have been demonstrated in laboratory, industrial and technical flows. Combined with fibreoptic distribution networks and ultrasensitive detection strategies, compact and portable sensors are now appearing for a variety of applications. In many cases, the superior spectroscopic quality of the new laser sources compared with earlier cryogenic, mid-IR devices is allowing increased sensitivity of trace species measurements, high-precision spectroscopy of major gas constituents, and stable, autonomous measurement systems. The purpose of this article is to review recent progress in this field and suggest likely directions for future research and development. The various laser-source technologies are briefly reviewed as they relate to sensor applications. Basic theory for laser absorption measurements of gas-dynamic properties is reviewed and special detection strategies for the weak near-IR and visible absorption spectra are described. Typical sensor configurations are described and compared for various application scenarios, ranging from laboratory research to automated field and airborne packages. Recent applications of gas-dynamic sensors for air flows and fluxes of trace atmospheric species are presented. Applications of gas-dynamic and combustion sensors to research and development of high-speed flows aeropropulsion engines, and combustion emissions monitoring are presented in detail, along with emerging flow control systems based on these new sensors. Finally, technology in nonlinear frequency conversion, UV laser materials, room-temperature mid-IR materials and broadly tunable multisection devices is reviewed to suggest new sensor possibilities.