Detection of propane using tunable diode laser spectroscopy at 3.37 μm

We report Tunable Diode Laser Spectroscopy measurements of propane using a recently developed 3.37 μm GaInAsSb/AlGaInAsSb DFB laser. We have demonstrated that Wavelength Modulation Spectroscopy can be utilized to enhance a sharp feature in the broader propane spectrum around 3370.4 nm. A minimum detectable concentration of 30 ppb×m was obtained at a response time of 0.5 s. This corresponded to a minimum detectable absorption of 8 × 10^?5 which is why an improvement of the sensitivity by an order of magnitude is possible using this laser and a more optimized optical setup.     

Laser diode photoacoustic and FTIR laser spectroscopy of formaldehyde in the 2.3 μm and 3.5 μm spectral range

The room temperature operating GaInAsSb/AlGaAsSb based diode laser and 66 K InAsSb/InAsSbP laser diode both operating in spectral range of formaldehyde absorption 4350-4361 cm⁻¹ and 2821-2823 cm⁻¹ have been characterized and compared. Very precise arrangement of laser absorption together with high resolution Fourier transform technique was tested. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) diluted by nitrogen for the strongest absorption line of the ν₃ + ν₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of ν₁, ν₅)_.     

Temperature and current coefficients of lasing wavelength in tunable diode laser spectroscopy

The factors determining temperature and current coefficients of lasing wavelength are investigated and discussed under monitoring CO₂-gas absorption spectra. The diffusion rate of Joule heating at the active layer to the surrounding region is observed by monitoring the change in the junction voltage, which is a function of temperature and the wavelength (frequency) deviation under sinusoidal current modulation. Based on the experimental results, the time interval of monitoring the wavelength after changing the ambient temperature or injected current (scanning rate) has to be constant at least to eliminate the monitoring error induced by the deviation of lasing wavelength, though the temperature and current coefficients of lasing wavelength differ with the rate.     

Simultaneous CO concentration and temperature measurements using tunable diode laser absorption spectroscopy near 2.3 μm

Simultaneous measurements of carbon monoxide (CO) mole fraction and temperature using tunable diode laser absorption spectroscopy (TDLAS) near 2.3 μm are reported. The measurement method uses ro-vibrational transitions [R(27): v″ = 1 → v′ = 3] and [R(6): v″ = 0 → v′ = 2] in the first overtone band of CO near 2.3 μm (~4,278 cm^?1). The measurements were performed in the post flame environment of fuel rich premixed ethylene–air flames with a N₂ co-flow, stabilized over a water cooled McKenna burner. Non-uniformity in the temperature and CO mole fraction, along the absorption line of sight, in the mixing layer of the co-flow, was considered during data analysis. The TDLAS based temperature measurements (±80 K) were in good agreement with those obtained using N₂ vibrational coherent anti-Stokes Raman scattering (±20 K), and the CO mole fraction measurements were in good agreement with the equilibrium values, for equivalence ratios lower than 1.8. A signal to noise ratio of 45 was achieved at an equivalence ratio of 1 for a CO concentration of 0.8 % at 1,854 K.     

Influence of laser intensity in second-harmonic detection with tunable diode laser multi-pass absorption spectroscopy

Tunable diode laser absorption spectroscopy （TDLAS） has been widely employed in atmospheric trace gases detection. The ratio of the second-harmonic signal to the intensity of laser beam incident to the multi-pass cell is proved to be proportional to the product of the path length and the gas concentration under any condition. A new calibration method based on this relation in TDLAS system for the measurement of trace gas concentration is proposed for the first time. The detection limit and the sensitivity of the system are below 110 and 31ppbv （parts-per-billion in volume）, respectively.     

Simultaneous measurement of liquid water film thickness and vapor temperature using near-infrared tunable diode laser spectroscopy

A fiber-based multiplexed tunable diode-laser absorption sensor with three near-infrared distributed-feedback diode lasers at ∼1.4 μm is used for simultaneous nonintrusive measurements of liquid water film thickness and vapor-phase temperature. Water film thicknesses are derived from broad-band absorption determined at two fixed wavelengths while gas-phase temperature above the film is obtained via two-line thermometry using the fast wavelength tuning with line-integrating absorption. Probing the liquid film at two wavelengths with significantly different liquid-phase absorption cross sections allows discriminating against additional signal losses due to surface fowling, reflection, and beam steering. The technique is demonstrated for liquid layers of defined thicknesses and in time-resolved measurements of evaporating films.     

Line strength and collisional broadening studies of hydrogen sulphide in the 1.58 μm region using diode laser spectroscopy

High resolution diode laser spectroscopy has been applied to the detection of hydrogen sulphide at ppm levels utilizing different transitions within the region of the ν ₁+ν ₂+ν ₃ and 2ν ₁+ν ₂ combination bands around 1.58 μm. Suitable lines in this spectral region have been identified, and absolute absorption cross sections have been determined through single-pass absorption spectroscopy and confirmed in the Doppler linewidth regime using cavity enhanced absorption spectroscopy (CEAS). The desire for a sensitive system potentially applicable to H₂S sensing at atmospheric pressure has led to an investigation on suitable transitions using wavelength modulation spectroscopy (WMS). The set-up sensitivity has been calculated as 1.73×10⁻⁸ cm⁻¹ s^1/2, and probing the strongest line at 1576.29 nm a minimum detectable concentration of 700 ppb under atmospheric conditions has been achieved. Furthermore, pressure broadening coefficients for a variety of buffer gasses have been measured and correlated to the intermolecular potentials governing the collision process; the H₂S–H₂S dimer well depth is estimated to be 7.06±0.09 kJ mol⁻¹.     

Doppler spectroscopy of NH3 and SF6 in the 10-μm range with a tunable AgGaS2 difference-frequency spectrometer

A continuous-wave mid-IR diode-laser-based, dual-cavity difference-frequency spectrometer employing silver thiogallate (AgGaS₂) as the down-conversion material, and producing only 30 nW of usable mid-IR output, is used to record Doppler spectra of NH₃ and SF₆ around 10.2 m and 10.5 m over the 90-GHz range. The tuning procedure of the spectrometer as well as a baseline noise cancellation technique based on analogue signal ratioing are described. The device can potentially be used for ultra-high-resolution saturation spectroscopy, pending an improvement of the down-conversion efficiency to 1-W power. PACS 42.65.Ky; 42.62.Fi; 42.70.Mp; 42.72.Ai; 42.60.Da; 33.20.Ea     

Infrared tunable diode laser spectroscopy of the Δv = 2 band of 7Li127I

The high-resolution infrared spectrum of the Δv = 2 transitions of ⁷LiI has been measured at temperatures near 1050 K. The observations include vibrational transitions ranging from 2-0 to 10-8. The data were fit to a set of Dunham potential constants complete through the a₆ term, from which the Dunham rovibrational constants were calculated. The band centers for the v = 1-0 and v = 2-0 transitions were determined to be 491.17398 ± 0.00026 cm⁻¹ and 976.73042 ± 0.00038 cm⁻¹, respectively.     

Carbon dioxide and ammonia detection using 2 μm diode laser based quartz-enhanced photoacoustic spectroscopy

Quartz-enhanced photoacoustic spectroscopy was employed for trace gas concentration measurements of CO₂ and NH₃ using a continuous wave thermoelectrically cooled, distributed feedback diode laser operating at 2 μm. A normalized noise equivalent absorption coefficient, NNEA(1σ)=1.4×10^-8 cm^-1W/ was obtained for CO₂ using the R18 line of the 2ν₁+ν₃ band at 4991.26 cm^-1. This corresponds to minimum detection limit (1σ) of 18 parts per million (ppm) for a 1 s lock-in time constant. The influence of the H₂O presence in the sample gas mixture on the CO₂ sensor performance was investigated. Ammonia detection was performed using the ^P P ₆(6)_S line of the ν₃+ν₄ band at 4986.99 cm^-1. A detection limit (1σ) of 3 ppm for NH₃ concentration with a 1 s lock-in time constant was achieved. This results in a normalized noise equivalent absorption of NNEA(1σ)=8.9×10^-9 cm^-1W/. PACS 82.80.Kq; 46.62.Fi; 42.55.Px     

Infrared diode laser spectroscopy of the ν2 fundamental band of SH3+

The high-resolution absorption spectrum of the ν₂ fundamental band of SH₃⁺ has been observed with the magnetic field modulation technique. The ions were generated in a hollow cathode discharge through a mixture of H₂ and H₂S. The molecular constants in the excited vibrational state are determined through a least-squares fit of the data to a standard symmetric top energy level expression. The anomalous sign of the centrifugal distortion constants is explained by the Coriolis interaction between the v₂ = 1 and v₄ = 1 states.     

Laser Stark and diode laser spectroscopy of the 10-μm bands of CH2NH: Determination of the dipole moment orientation

The complexities apparent in the laser Stark spectra obtained from the 10-μm band system of CH₂NH are shown to be due to the interplay of the a and b components of the Coriolis and Stark effect coupling. As a result it has been possible to determine the relative orientations of the a and b components of the permanent electric dipole moment. This is analogous to the determination of the relative signs of the dipole moment derivatives from the analysis of the intensity perturbations produced by Coriolis interaction.A detailed comparison has also been made of the uses of laser Stark, diode laser, and Fourier transform spectroscopy for studying heavily perturbed molecular spectra.     

A near-infrared acetylene detection system based on a 1.534 μm tunable diode laser and a miniature gas chamber

A near-infrared (NIR) dual-channel differential acetylene (C₂H₂) detection system was experimentally demonstrated based on tunable diode laser absorption spectroscopy (TDLAS) technique and wavelength modulation spectroscopy (WMS) technique. A distributed feedback (DFB) laser modulated by a self-developed driver around 1.534 μm is used as light source. A miniature gas chamber with 15 cm path length is adopted as absorption pool, and an orthogonal lock-in amplifier is developed to extract the second harmonic (2f) signal. Sufficient standard C₂H₂ samples with different concentrations were prepared, and detailed measurements were carried out to study the detection performance. A good linear relationship is observed between the amplitude of the 2f signal and C₂H₂ concentration within the range of 200–10,000 ppm, and the relative measurement error is less than 5% within the whole range. A long-term monitoring lasting for 20 h on a 1000 ppm C₂H₂ sample was carried out, and the maximum concentration fluctuation is less than 2%. Due to the capability of using long-distance and low-loss optical fiber, the gas-cell can be placed in the filed for remote monitoring, which enables the system to have good prospects in industrial field.     

Infrared diode laser spectroscopy of the ν3 band of the fluoromethyl radical,CH2F

The ν₃ (CF stretching) band of the CH₂F radical was observed with Doppler-limited resolution, by using infrared diode laser spectroscopy with Zeeman and discharge current modulation. The CH₂F radical was generated directly in a multiple reflection absorption cell by the electrical discharge in CH₂FCOOCH₃. The observed spectrum was analyzed to yield the band origin of 1170.4165(6) cm⁻¹ with one standard error in parentheses, in addition to the rotational constants, centrifugal distortion constants, and spin-rotation interaction constants in the v₃ = 1 state.     

T-shape microresonator-based quartz-enhanced photoacoustic spectroscopy for ambient methane monitoring using 3.38-μm antimonide-distributed feedback laser diode

This paper reports on the development of a gas sensor involving a newly available 3.38-μm distributed feedback laser in combination with a novel T-shape microresonator-based quartz-enhanced photoacoustic spectroscopy (T-mR QEPAS), capable of simultaneous monitoring of multi-species (such as CH₄, H₂CO, HCl, C₂H₄) using the same QEPAS spectrophone. As a first demonstration, monitoring of ambient methane (CH₄) was achieved at atmospheric pressure with a 1σ detection limit of 400 ppbv (parts per billion by volume) in an integration time of 10 s and a water vapor concentration of 1.15 vol% (11,500 ppm) in the atmosphere, which is very suitable for field measurement of CH₄ emission.     

Antimonide-based lasers and DFB laser diodes in the 2–2.7 μm wavelength range for absorption spectroscopy

We report on Fabry–Pérot semiconductor lasers and single frequency distributed feedback lasers based on GaInAsSb/AlGaAsSb quantum wells. The laser structures were grown by molecular beam epitaxy on GaSb substrates. The devices were etched either by wet process or by inductively coupled plasma (ICP) process. Electron-beam lithography was used to deposit a metal Bragg grating on each side of the laser ridge to fabricate the DFB lasers. The devices all operate in the continuous wave regime at room temperature with a single frequency emission above 2.6 μm and good tuning properties, making them well adapted to tunable diode laser absorption spectroscopy. PACS 42.55.Px; 42.62.Fi     

Temperature-dependent pressure broadened line shape measurements in the ν 1+ν 3 band of acetylene using a diode laser referenced to a frequency comb

Using an extended cavity diode laser referenced to a femtosecond frequency comb, the P(11) absorption line in the ?? ₁+?? ₃ combination band of the most abundant isotopologue of pure acetylene was studied at temperatures of 296, 240, 200, 175, 165, 160, 155, and 150?K to determine pressure-dependent line shape parameters at these temperatures. The laser emission profile, the instrumental resolution, is a Lorentz function characterized by a half width at half the maximum emission (HWHM) of 8.3×10^?6?cm^?1 (or 250?kHz) for these measurements. Six collision models were tested in fitting the experimental data: Voigt, speed-dependent Voigt, Rautian?CSobel??man, Galatry, and two Rautian?CGalatry hybrid models (with and without speed-dependence). Only the speed-dependent Voigt model was able to fit the data to the experimental noise level at all temperatures and for pressures between 3 and nearly 360?torr. The variations of the speed-dependent Voigt profile line shape parameters with temperature were also characterized, and this model accurately reproduces the observations over their entire range of temperature and pressure.     

Detection of HCl on the first and second overtone using semiconductor diode lasers at 1.7 μm and 1.2 μm

Sensitivity studies are also performed, to evaluate the minimum detectable concentration of HCl in air. Received: 7 August 1998/Revised version: 5 October 1998