Detection of acetylene impurities in ethylene and polyethylene manufacturing processes using tunable diode laser spectroscopy in the 3-��m range

Using recently developed GaInAsSb/AlGaInAsSb DFB lasers, tunable diode laser spectroscopy (TDLS) has been extended into the 3-??m wavelength region for the detection of acetylene impurities in hydrocarbon compounds encountered in ethylene manufacturing. Measurements of acetylene in pure polymer grade ethylene and in a gas mixture of ethylene and ethane typical of the process stream around a hydrogenation reactor have been performed. Using a procedure incorporating subtraction of a hydrocarbon background spectrum a detection limit of 5?ppb?m was achieved under ordinary laboratory conditions. Under forced temperature cycling conditions, the detection limit deteriorated to 180 ppb?m, due to temperature drift caused by optical interferences generated by reflections in the laser TO8 can.     

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     

Quartz enhanced photoacoustic spectroscopy with a 3.38 μm antimonide distributed feedback laser

A system for gas sensing based on the quartz-enhanced photoacoustic spectroscopy technique has been developed. It makes use of a quantum well distributed feedback (DFB) laser diode emitting at 3.38 μm. This laser emits near room temperature in the continuous wave regime. A spectrophone, consisting of a quartz tuning fork and two steel microresonators were used. Second derivative wavelength modulation detection is used to perform low concentration measurements. The sensitivity and the linearity of the Quartz enhanced photoacoustic spectroscopy (QEPAS) sensor were studied. A normalized noise equivalent absorption coefficient of 4.06×10(-9) cm(-1)·W/Hz(1/2) was achieved.     

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.     

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.     

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.     

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.     

Dual-beam laser heterodyne spectrometer: Ethylene absorption spectrum in the 10 μm range

A laser heterodyne spectrometer for the 10 m region has been built. A 5 MHz apparatus function is obtained. The improvement of this spectrometer using a dual-beam technique is described. The folding effects in heterodyne spectroscopy are discussed. The recorded lineshapes are explained. An ethylene spectrum is shown as an example.     

Frequency stabilization of diode laser to 1.637 μm based on the methane absorption line

The frequency of an external-cavity diode laser has been stabilized to 1.637 μm by using the reference of absorption lines of methane. The method can be applied to wavelength division multiplexed optical communication, fiber-optic sensing systems, as well as the high-sensitivity detection of methane. The derivative-like error signal yielded by frequency modulation and phase sensitivity detection technology is inputted into the PI feedback loop circuit in order to stabilize the frequency to the line center. After stabilization, the frequency fluctuation of diode laser is held within 5.6 MHz, and the root of Allan variance of error signal reaches a minimum of 1.66×10-10 for an average time of 10 s.     

Determination of the dielectric constant of YBa2Cu3O7−δ single crystals in the 10 μm spectral range by SEW phase spectroscopy

Phase spectroscopy of surface electromagnetic waves was used to determine the complex dielectric constant of YBa₂Cu₃O_7−δ single crystals in the spectral range 930–1090 cm⁻¹. The value obtained (ε=−45+90i) permits one to determine, in the case when the Drude model is valid, the plasma frequency (∼15000 cm⁻¹) and the collisional frequency of free carriers (∼2000 cm⁻¹). The plasma edge in the reflection spectrum is observed at higher frequencies (∼25000 cm⁻¹). Possible reasons for the discrepancy are discussed.     

Tunable diode laser measurements of absolute linestrengths in the HNO3 ν2 band near 5.8 μm

Absolute linestrengths of selected lines in the υ₂ band of HNO₃ have been measured using a tunable diode laser spectrometer operating in a sweep integration mode. The direct measurement technique has been employed to obtain line intensities at 296 K for 22 isolated lines in the 1720–1725 cm^-1 region. The reported linestrengths have estimated uncertainties of 4%, a significant portion of this uncertainty arising from spectral interference from hot band transitions. From these linestrength measurements, an integrated band intensity of 1375 cm^-2-atm^-1 at 296 K is inferred.     

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.     

3.5 μm optical communication experiment at 100 Mbits−1 using a PbCdS diode laser

A mid-infrared optical communication experiment using a lead-salt diode laser emitting at 3.5 m is reported. Bit-error rate measurements have been performed for pulse code modulated data at 100 Mbits^–1 using a fluoride glass fibre as a transmission channel. A bit-error rate better than 10^–10 was achieved at a signal-to-noise ratio of 20 dB.S. T. Eng has a joint appointment with the Jet Propulsion Laboratory, Pasadena, California 91109, USA.     

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.     

Thermal Lensing, Laser Performance at 1.06 μm and 0.53 μm of a NYAB Laser End pumped by a Diode laser array

1　Ｉｎｔｒｏｄｕｃｔｉｏｎ　　Ｓｅｌｆ ｆｒｅｑｕｅｎｃｙ ｄｏｕｂｌｉｎｇｌａｓｅｒｃｒｙｓｔａｌｓ,ｗｈｉｃｈｃｏｍｂｉｎｅｔｗｏｄｉｆｆｅｒｅｎｔｆｕｎｃｔｉｏｎｓｏｆｌａｓｅｒｅｍｉｓｓｉｏｎａｎｄｆｒｅｑｕｅｎｃｙｃｏｎｖｅｒｓｉｏｎ ,ｐｒｏｖｉｄｅａｓｉｍｐｌｅｗａｙｏｆｇｅｎｅｒａｔｉｎｇｃｏｈｅｒｅｎｔｖｉｓｉｂｌｅｒａｄｉａｔｉｏｎ .Ａｍｏｎｇｔｈｅｖａｒｉｏｕｓｓｅｌｆ ｆｒｅｑｕｅｎｃｙ ｄｏｕｂｌｉｎｇｃｒｙｓｔａｌｓ ,ｎｅｏｄｙｍｉｕｍｙｔｔｒｉｕｍａｌｕｍｉｎｕｍｂｏｒａ…     

CO<Subscript>2</Subscript> laser oscillating in the range of 16 μm

The ability of a CO₂ laser to oscillate in the range of 16 (14) μm at room temperature was investigated experimentally and theoretically. The output energy per pulse was ~60 mJ at peak power of ~50 kW. It was necessary to minimize not only harmful losses but also useful ones in both channels 00⁰1–02⁰0 and 02⁰0–0110 and to increase the input energy, i.e., the density of free electrons in the discharge, in order to increase the peak power and energy of 16-μm radiation. The highest values of peak power and energy of radiation were reached at different pressures of the active mixture. The rotational bottleneck effect limiting the peak power and energy of oscillation was important at rather low pressures of the active medium. Oscillation at the R12 line is more preferable than that at the P12 line for use as 9.6-μm dumping radiation.     

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 and water concentration measurements in combustion gases using a DFB diode laser at 1.4 μm

A sensor using a single DFB diode laser at 1.4 μm based on wavelength modulation spectroscopy for the measurements of the gas temperature and the H₂O concentration in combustion gases is developed. A line pair of H₂O absorption transitions located at 7085.251 and 7085.876 cm^?1 is selected based on some design rules. The 1f normalized 2f method is used to remove the need for the calibration and to correct for the transmission variation due to beam steering, mechanical misalignments, soot, and window fouling. The precision for the temperature and H₂O concentration measurements are 1.05 and 2.10% in a controlled static cell, respectively. Burner experiments demonstrate the ability of the system for in situ measurements.