An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy(TDLAS)is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region.A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback(DFB)tunable diode lasers is designed to detect CH_4 and H_2S in leakage of natural gas.A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m.High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection.The minimum detection limits of 1.1 ppm·m for CH_4 and 15 ppm·m for H_2S are demonstrated with a total optical path of 120 m.The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection,the detectable optical path of the system can achieve 1-2 km.The sensor is suitable for natural gas leakage monitoring application.     

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     

Photoacoustic spectroscopy detection and extraction of discharge feature gases in transformer oil based on 1.5 μ tunable fiber laser

Information from the analysis of gasses dissolved in insulating oils is valuable for early a transformer maintenance. By means of dissolved gas analysis (DGA), it is possible to distinguish faults such as partial discharge (corona), overheating (pyrolysis) and arcing in a great variety of oil-filled equipment. Tunable fiber laser-based second harmonic photoacoustic spectroscopy offers a fast and good-noise-immunity technique for the quantitative analysis of trace gases in transformer oil. In this work, the discharge feature gases, such as C₂H₂, CH₄, CO₂ and H₂O, were measured with a tunable laser photoacoustic spectrometer at the 1530.3709 nm transition line, as a typical application of precise measurement of multi-gas, a proposed BSS model based on overcomplete ICA basis and five-point-sampling method based on a created weight-truncation-constraint equation was used to remove noise so that several fault gases can be extracted with a higher detection accuracy and a method detection limit. Experiment shows that within 0.15 nm band near 1530.3709 nm, the four-gas contents have been detected and extracted and the detection accuracy has been improved from available scanning spacing of 0.03 nm to available extracting spacing of 0.0011 nm. At room temperature and atmospheric pressure, this can achieve simultaneous detection for multiple feature gases in discharged transformer oil using laser source with a limited waveband.     

Photoacoustic gas sensing with a commercial external cavity-quantum cascade laser at 10.5 μm

We report the implementation of a commercial external cavity-quantum cascade laser emitting at 10.5 μm in a photoacoustic spectrometer. This spectrometer enables measurements on broad spectral range up to 60 cm⁻¹ which means that spectra of complex molecules can be recorded as well as a whole absorption band of a small molecule. The wide tuning range of the source of this photoacoustic spectrometer demonstrates the possibility to detect small and complex molecules such as carbon dioxide and butane.     

Self-broadening coefficients and positions of acetylene around 1.533 μm studied by high-resolution diode laser absorption spectrometry

The self-broadening coefficients of acetylene at room temperature have been measured for 10 lines in the P branch of the bands of ¹²C₂H₂ and ¹³C¹²CH₂ near 1.533 μm, using a high resolution tunable diode laser spectrometer developed for the Martian space mission PHOBOS-Grunt. The collisional widths are obtained by fitting each recorded line with the Voigt profile as well as the Rautian profile accounting for the collisional Dicke narrowing effect. The standard Voigt model provides slightly smaller broadening coefficients than the Rautian model. Our data are thoroughly compared to the main atmospheric molecule database HITRAN and previous values in various bands of acetylene. Moreover, it is worth noting that a large number of new transitions not listed in the latest HITRAN08 were measured and identified for the first time.     

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.     

Fiber-distributed multi-channel open-path H_2S sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser based gas detectors are now being used in a wide variety of applications for safety and environmental interest. A fiber-distributed multi-channel open-path H2S sensor based on tunable diode laser absorption spectroscopy (TDLAS) is developed, the laser used is a telecommunication near infrared distributed feed-back (DFB) tunable diode laser, combining with wavelength modulation spectroscopy technology, a detection limit of 20 ppm·m is demonstrated. Multi-channel detection is achieved by combining optical fiber technique. An on-board reference cell provides on-line sensor calibration and almost maintenance-free operation. The sensor is suitable for large area field H2S monitoring application.     

InGaAsP/InP photodetectors targeting on 1.06 μm wavelength detection

InP-based InGaAsP photodetectors targeting on 1.06 μm wavelength detection have been grown by gas source molecular beam epitaxy and demonstrated. For the detector with 200 μm mesa diameter, the dark current at 10 mV reverse bias and R₀A are 8.89 pA (2.2 × 10⁻⁸ A/cm²) and 3.9 × 10⁵ Ω cm² at room temperature. The responsivity and detectivity of the InGaAsP detector are 0.30 A/W and 1.45 × 10¹² cm Hz^1/2 W⁻¹ at 1.06 μm wavelength. Comparing to the reference In_0.53Ga_0.47As detector, the dark current of this InGaAsP detector is about 570 times lower and the detectivity is more than ten times higher, which agrees well with the theoretical estimation.     

Welding of polymers using a 2 μm thulium fiber laser

Absorber-free transmission and butt-welding of different polymers were performed using thulium fiber laser radiation at the wavelength 2 μm. The relations between the laser process conditions and the dimensions and quality of the seam were investigated by means of optical and phase-contrast microscopy. Mechanical properties of the weld joints were studied in tensile strength tests. Laser-welded polyethylene samples revealed a tensile strength of greater than 80% of the bulk material strength. Transmission welding of different polymer combinations featured the formation of different joint classes depending on the spectral properties. The experiments demonstrate new application areas of mid-IR fiber laser sources for materials processing.     

Atmospheric N_2O gas detection based on an inter-band cascade laser around 3.939 μm

N_2O is a significant atmospheric greenhouse gas that contributes to global warming and climate change. In this work,the high sensitivity detection of atmospheric N_2O is achieved using wavelength modulation spectroscopy(WMS) with an inter-band cascade laser operating around 3.939 μm. A Lab VIEW-based software signal generator and software lock-in amplifiers are designed to simplify the system. In order to eliminate the interference from water vapor, the detection was performed at a pressure of 0.1 atm(1 atm = 1.01325×10~5 Pa) and a drying tube was added to the system. To improve the system performance for long term detection, a novel frequency locking method and 2 f/1 f calibration-free method were employed to lock the laser frequency and calibrate the power fluctuations, respectively. The Allan deviation analysis of the results indicates a detection limit of ～ 20 ppb(1 ppb = 1.81205 μg/m~3) for a 1 s integration time, and the optimal detection limit is ～ 5 ppb for a 40-s integration time.     

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.     

A laser system providing tunable, narrow-band radiation from 35 nm to 2 μm

We describe a laser system that readily provides radiation tunable from 2 μm in the infra-red to 35 nm in the extreme ultraviolet spectral range. The broad spectral range is covered through a range of non-linear processes such as Raman shifting and high-order harmonic generation. Pulses with duration of tens of picoseconds are obtained. The relative bandwidth of the radiation is δλ/λ=10^-4, comparable with what can be achieved by using high-resolution monochromators at state-of-the-art synchrotron beamlines. We discuss different methods for characterising the radiation in this wide wavelength regime. We also discuss the capabilities of the system from the measured parameters. Received: 12 December 2000 / Revised version: 8 March 2001 / Published online: 27 April 2001     

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.     

GaSb-based mid-infrared 2–5 μm laser diodes

Laser diodes emitting at room temperature in continuous wave regime (CW) in the mid-infrared (2–5 μm spectral domain) are needed for applications such as high sensitivity gas analysis by tunable diode laser absorption spectroscopy (TDLAS) and environmental monitoring. Such semiconductor devices do not exist today, with the exception of type-I GaInAsSb/AlGaAsSb quantum well laser diodes which show excellent room temperature performance, but only in the 2.0–2.6 μm wavelength range. Beyond 2.6 μm, type-II GaInAsSb/GaSb QW lasers, type-III ‘W’ InAs/GaInSb lasers, and interband quantum cascade lasers employing the InAs/Ga(In)Sb/AlSb system, all based on GaSb substrate, are competitive technologies to reach the goal of room temperature CW operation. These different technologies are discussed in this paper. To cite this article: A. Joullié, P. Christol, C. R. Physique 4 (2003).     

High-power 3.8 μm tunable optical parametric oscillator based on PPMgO:CLN

The experimental results of a high-power 3.8 μm tunable laser are presented on a quasi-phase-matched single-resonated optical parametric oscillator in PPMgO:CLN pumped by a 1064 nm laser of an elliptical beam. Theoretical analyses of the PPMgO:CLN wavelength tuning are presented. The pump source was an acousto-optical Q-switched cw-diode-side-pumped Nd:YAG laser. The beam polarization matched the e-ee interaction in PPMgO:CLN. When the crystal was operated at 90 °C and the pump power was 150 W with a repetition rate of 10 kHz, average output power of 22.6 W at 3.86 μm and 63 W at 1.47 μm was obtained. The slope efficiency of the 3.86 μm laser with respect to the pump laser was 17.8%. The M² factors of the 3.86 μm laser were 1.74 and 4.86 in the parallel and perpendicular directions, respectively. The mid-IR wavelength tunability of 3.7-3.9 μm can be achieved by adjusting the temperature of a 29.2 μm period PPMgO:CLN crystal from 200 °C to 30 °C, which basically is accorded with the theoretic calculation.     

A novel discrete mode narrow linewidth laser diode for spectroscopic based gas sensing in the 1.5 μm region

The characteristics of a novel Discrete Mode laser diode are described with particular reference to its capability in spectroscopic based gas sensing. In this regard, its emission linewidth, Side Mode Suppression Ratio and tuneability are of particular note. The measured narrow linewidth of 550?kHz has applications with respect to spectrally narrow absorption features found, for example, in saturated absorption lines, in hyperfine structure of some rotational manifolds and in whispering gallery mode features associated with optical microspheres. Other notable measured characteristics show mode-hop-free temperature tuning of 11 nm at a linear tuning rate of 0.1 nm/°C and a Side Mode Suppression Ratio in the range 56 to 46 dB and a long term (72?hours) wavelength stability of 1?pm per day. In essence, this novel Discrete Mode laser displays excellent potential for diode laser based sensing applications in the near infrared.     

A 1550-nm linearly tunable continuous wave single-mode external cavity diode laser based on a single-cavity all-dielectric thin-film Fabry Pérot filter

A 1550-nm linearly tunable continuous wave (CW) single-mode external cavity diode laser (ECDL) based on a singlecavity all-dielectric thin-film Fabry-Pérot filter (s-AFPF) is proposed and realized in this paper. Its internal optical components as well as their operation mechanisms are introduced first, and then its longitudinal mode output characteristic is theoretically analyzed. Afterwards, we set up the experimental platform for the output characteristic measurement of this tunable ECDL; under different experimental conditions, we execute accurate and real-time measurements for the output central wavelength, output optical power, output longitudinal mode distribution, and the line-width of the tunable ECDL in its tuning process. By summing up the optimal experimental condition from the measured data, we obtain the optimal tunable ECDL relevant parameters: the tunable ECDL has a linear mode-hop-free wavelength tuning region of 1547.203 nm-1552.426 nm, a stable output optical power in the range of 40 μW-50 μW, and a stable output longitudinal mode distribution of a single longitudinal mode with a line-width in the range of 100 MHz-150 MHz. This tunable ECDL can be used in environmental gas monitoring, atomic and molecular laser spectroscopy research, precise measurements, and so on.     

A novel 2-μm pulsed fiber laser based on a supercontinuum source and its application to mid-infrared supercontinuum generation

A new method to achieve 2-μm pulsed fiber lasers based on a supercontinuum(SC) is demonstrated. The incident pump light is a pulsed SC which contains a pump light and a signal light at the same time. The initial signal of the seed laser is provided by the incident pump light and amplified in the cavity. Based on this, we obtain a 2-μm pulsed laser with pulse repetition rate of 50 kHz and pulse width of 2 ns from the Tm-doped fiber laser. This 2-μm pulsed laser is amplified by two stages of fiber amplifiers, then the amplified laser is used for mid-infrared(mid-IR) SC generation in a 10-m length of ZrF4–BaF2–LaF3–AlF3–NaF(ZBLAN) fiber. An all-fiber-integrated mid-IR SC with spectrum ranging from 1.8 μm to4.3 μm is achieved. The maximal average output power of the mid-IR SC from the ZBLAN fiber is 1.24 W(average output power beyond 2.5 μm is 340 mW), corresponding to an output efficiency of 6.6% with respect to the 790-nm pump power.