A compact NIR fiber-optic diode laser spectrometer for CO and CO(2): analysis of observed 2f wavelength modulation spectroscopy line shapes

A compact fiber-optic diode laser spectrometer for the measurement of CO and CO(2) gas concentrations in the near infrared around 1580 nm is described. By use of a balanced receiver to suppress diode laser intensity noise a sensitivity of 6.4 x 10(-7) at 1 Hz system bandwidth was achieved. At a reduced pressure of 80 hPa this equals to a detection limit of 5.1 ppm CO and 9.1 ppm CO(2) with 1m absorption path length. The observed line shapes of the 2f wavelength modulation spectroscopy (WMS) scheme are analyzed theoretically and experimentally. Accurate measurements of magnitude and phase of the diode laser frequency and intensity modulation responses were found critically for modeling the observed line shapes. In situ measurements of gas dissociation processes inside of a medium-power carbon dioxide laser are presented as an application example.     

Tunable single-frequency diode laser at wavelength lambda = 1.645 microm for methane concentration measurements

This paper deals with the development of a novel single-frequency tunable diode laser with fiber-optic output for gas-analysis applications. The approach we propose is a convenient, simple and cheap solution for spectroscopy of single absorption lines of any gases having absorption bands in the optical fiber transparency window (0.7 microm/1.7 microm). The presence of fiber-optic output is an additional advantage for remote sensing applications. The laser operation is demonstrated as applied to R7 line of 2 nu(3) methane absorption band at lambda = 1.645 microm. The mode-hop-free tuning range of 35 GHz (1.2 cm(-1)) has been achieved.     

Tunable single-frequency diode laser at wavelength λ=1.65 μm for methane concentration measurements

This paper deals with the development of a novel single-frequency tunable diode laser with fiber-optic output for gas-analysis applications. The approach we propose is a convenient, simple and cheap solution for spectroscopy of single absorption lines of any gases having absorption bands in the optical fiber transparency window (0.7 microm/1.7 microm). The presence of fiber-optic output is an additional advantage for remote sensing applications. The laser operation is demonstrated as applied to R7 line of 2 nu(3) methane absorption band at lambda = 1.645 microm. The mode-hop-free tuning range of 35 GHz (1.2 cm(-1)) has been achieved.     

Laser diode photoacoustic detection in the infrared and near infrared spectral ranges

A new technique for high resolution photoacoustic detection based on application of laser diodes has been developed. This method was tested and compared using identical photoacoustic instrumentation (cell and microphone) to study gas absorption in three different spectral regions, namely: the infrared range near 2100 cm(-1), CO and OCS fundamental band absorption; the ranges near 4200 and 4350 cm(-1), CH4, NH3 and N2O overtone and combination band absorption; the near infrared range near 6500 cm(-1), CO, CO2 and NH3 overtone absorption. Several types of diode laser operating at room temperature or at liquid nitrogen temperature were compared. The optimum gas pressures for the maximum sensitivity of the photoacoustic signals were found and the detection limits were estimated for all of the gases studied. The best sensitivity was achieved for NH3 at 100 ppbv. The sensitivity of the developed system was tested on detection of traces of NH3 and CO2 gases from car exhaust.     

Ethylene spectroscopy using a quasi-room-temperature quantum cascade laser

Spectroscopic measurements on ethylene were performed using a quasi-room-temperature quantum cascade (QC) laser operated in pulsed mode in the 10.3 microm range. Using transmission spectroscopy, a broadening of the ethylene absorption spectrum was observed with increasing laser pulse duration, due to an increase of the laser linewidth. This linewidth was determined from the measured absorption spectra, showing a value of 0.04 cm(-1) for a 20 ns pulse duration and an enhancement coefficient of 6.5 x 10(-3) cm(-1) per ns in the 20-50 ns pulse length range. Photoacoustic (PA) detection of ethylene was also performed using the QC laser and a resonant PA cell, with a detection limit of 60 ppm.     

Diode laser atomic absorption spectrometry

The paper reviews the past 11 years of literature on the application of diode lasers in atomic absorption spectrometry with graphite furnaces (GF), plasmas and flames as atomizers. Experimental arrangements and techniques for powerful absorption measurements as well as the theoretical background are covered. The analytical possibilities of high-resolution spectroscopy, including Doppler-free techniques for isotope selective measurements and isotope dilution analysis are discussed and various applications of element-selective detection by diode laser atomic absorption in combination with separation techniques, such as liquid (LC) and gas chromatography (GC), and with laser ablation of solid samples, are presented.     

Optimisation of photoacoustic resonant cells with commercial microphones for diode laser gas detection

The theoretical and experimental study of the differential Helmholtz resonant (DHR) cell sensitivity under variation of the total gas pressure is made for various commercial microphones. Near-infrared lasers (room-temperature diode lasers) were used to measure the response of DHR cell versus pressure of the absorbing gas and frequency of the laser radiation modulation. Several molecular absorbers like H2O, CH4, mixed with molecular buffer gases were used to investigate the behavior of the photoacoustic (PA) signal characteristics with a DHR cell. The experimental data are compared with the results of computer simulation. The minimal detectable concentrations of gases were determined for the DHR cell for each commercial microphone.     

A novel single frequency stabilized Fabry-Perot laser diode at 1590 nm for gas sensing

A novel single frequency stabilized Fabry-Perot (SFP) laser diode with an emission wavelength of lambda = 1590 nm for H2S gas sensing is reported. Sculpting of the multi-mode spectral distribution of a FP laser to achieve single frequency emission is carried out using post growth photolitographic processing of the device. The resulting longitudinal-mode controlled FP laser has a stabilized single frequency emission with a side mode suppression ratio (SMSR) of 40 dB. The application of this device to spectroscopic based H2S sensing is demonstrated by targeting absorption lines in the wavelength range 1588 < or = lambda < or = 1591 nm. Using wavelength modulation spectroscopy (WMS), a low detection limit of 120 ppm x m x Hz(-1/2) was estimated while targeting the absorption line at 1590.08 nm. These initial results demonstrate the potential of the stabilized FP laser diode at this wavelength as a tunable, single frequency source for spectroscopic based gas sensing.     

Real-time monitoring of Yb vapor density using an extended cavity violet diode laser

Based on laser absorption spectroscopy (LAS), we developed a vapor density monitor for controlling the vaporization rate of Yb using a tunable diode laser. The laser source consisted of an extended cavity violet diode laser which has an emission wavelength of 398.8 nm coincident with the Yb absorption transition line, 6s(2) 1S(0)-6s6p 1P(1). The light emitted from the diode laser was transmitted across an atomic vapor column generated by heating the Yb metal, while the laser frequency was scanned across the atomic transition line. By comparing the amount of incident light to the amount of light transmitted after the light passed through the vapor column, the vapor density was determined using the Beer's law. From the experimental results, we demonstrated that the diode-laser-based LAS operated successfully for the real-time monitoring of the Yb vapor density.     

Near-infrared (NIR) to red and green up-conversion emission from silica sol-gel thin films made with La(0.45)Yb(0.50)Er(0.05)F(3) nanoparticles, hetero-looping-enhanced energy transfer (Hetero-LEET): a new up-conversion process

Bright green and red luminescence has been generated with a 980 nm diode laser from silica sol-gel thin films made with La0.45Yb0.50Er0.05F3 nanoparticles through a newly described hetero-looping-enhanced energy-transfer (hetero-LEET) up-conversion process, which exhibits a power dependence similar to that of a photon avalanche (PA). The hetero-LEET mechanism is potentially more efficient than PA, ground-state absorption/excited-state absorption (GSA/ESA), and energy-transfer (ETU) mechanisms because it combines resonant ground-state absorption with a looping or feedback process.     

Methyl Iodide Photodissociation at 193 nm: The I((2)P(1/2)) Quantum Yield

Methyl iodide photolysis at 193 nm has been studied through probing the I((2)P(1/2)-(2)P(3/2)) transition in the atomic iodine photofragment using diode laser spectroscopy. The I((2)P(1/2)) quantum yield has been determined through two different diode laser techniques and then compared. Frequency-modulated diode laser based absorption spectroscopy was used to extract nascent Doppler lineshapes from which an I((2)P(1/2)) quantum yield of unity is inferred. However when diode laser gain/absorption measurements were made, an I((2)P(1/2)) quantum yield of 0.68 ± 0.04 was found. The reason for this discrepancy is shown to lie in the diode laser gain/absorption method. Molecular iodine is found to be formed during the experiment via atomic iodine recombination and then in turn dissociates to produce both I((2)P(1/2)) and I((2)P(3/2)), thus distorting the returned quantum yield. This conclusion is supported both by the reduction of the I((2)P(1/2)) quantum yield with number of photolysis laser shots when measured using this technique and by the presence of fluoresence which is shown to have excited-state lifetimes and quenching rates that are consistent with those previously measured for the D and D' states of molecular iodine.     

Two-photon induced photodecarbonylation reaction of cyclopropenones

Irradiation of cyclopropenones (1a-c) with 800 nm pulses of ultrafast laser results in a photodecarbonylation reaction via nonresonant two-photon absorption of light.     

Application of laser photoacoustic spectroscopy for the detection of water vapor near 1.38 μm

Laser photoacoustic (PA) spectroscopy is applied for the determination of the trace content of water vapor using a differential Helmholtz resonant (DHR) cell and a narrow bandwidth diode laser operating near 1380 nm. The PA spectroscopy revealed a rich absorption spectra in this wavelength region and the observed result was compared with the HITRAN database. A multipass optical system was also developed by adopting one aluminum-coated flat mirror with a small uncoated spot for the laser introduction to the detection chamber and one aluminum-coated concave mirror. The multipass optical system enabled the enhancement of the PA signal up to eight times when compared to the single pass case. The calibration curve was plotted by measuring PA signals for various pressures of the water vapor. The sensitivity of the PA detection system is estimated as 7.3×10¹² molecules cm⁻³ with a signal-to-noise ratio (SNR) of 1.     

Diode laser emission linewidth determination: application to H2O line profile studies in the 5 and 1.4 microm regions

In order to study absorption line profiles using the stabilized diode laser spectrometer of Laboratoire de Physique Moléculaire et Applications (LPMA), a reliable determination of the emission line shape of different diodes laser is needed. In the near infrared region (1.39 and 1.66 microm) we used Distributed Feed Back diode lasers which operate around room temperature and in the middle infrared (5 and 8 microm) we used lead salt diode lasers cooled in a helium closed cycle cryostat or in a liquid nitrogen dewar. Some results obtained in H2O line profile studies in the 1.39 and 5 microm regions are presented as examples demonstrating how absorption line profile measurements can lead to erroneous values of the spectroscopic parameters when the contribution of the diode laser emission line width is neglected.     

Pressure-induced shift and broadening of acetylene lines in the region 6580-6600 cm-1

Highly accurate measurements of pressure shift and broadening parameters of acetylene absorption lines in the region 6580-6600 cm-1 have been performed by tunable diode laser spectroscopy (TDLS). For these purposes the three channel spectrometer with distributed-feedback diode laser, operated at 1.53 microm was used. The laser is generating pulses of 4-10 ms duration at a repetition frequency of 40 Hz. A temperature-stabilization system, using a thermoelectric cooling unit affords a temperature stability of the order of 10(-4)K in the temperature range from -15 to +50 degrees C. A three channels acquisition system ensured simultaneous real time recording of the sample gas absorption spectrum and of two spectral calibration signals (Fabry-Perot fringes and low-pressure reference lines). We have measured the pressure-induced self-shift and broadening coefficients for six lines of the R-branch in the nu1+nu3 rotation-vibration band of acetylene 12C2H2. The self-shift coefficients have been determined for these lines in the wide pressure region. A non-linear behavior of the pressure dependence of the shift was observed. The temperature exponent n of pressure-induced broadening and shift are reported.     

Application of laser photoacoustic spectroscopy for the detection of water vapor near 1.38 μm

Laser photoacoustic (PA) spectroscopy is applied for the determination of the trace content of water vapor using a differential Helmholtz resonant (DHR) cell and a narrow bandwidth diode laser operating near 1380 nm. The PA spectroscopy revealed a rich absorption spectra in this wavelength region and the observed result was compared with the HITRAN database. A multipass optical system was also developed by adopting one aluminum-coated flat mirror with a small uncoated spot for the laser introduction to the detection chamber and one aluminum-coated concave mirror. The multipass optical system enabled the enhancement of the PA signal up to eight times when compared to the single pass case. The calibration curve was plotted by measuring PA signals for various pressures of the water vapor. The sensitivity of the PA detection system is estimated as 7.3×10¹² molecules cm⁻³ with a signal-to-noise ratio (SNR) of 1.     

Gas hydrates in low water content gases: Experimental measurements and modelling using the CPA equation of state

In this communication, new experimental data are reported for the water content of methane and two synthetic gas mixtures in equilibrium with hydrates at pressures range from 5 to 40 MPa and temperature down to 251.65 K. The measurements have been made on equilibrated samples taken from a high-pressure variable volume hydrate cell using a new analyser based upon tuneable diode laser absorption spectroscopy (TDLAS) technology. A statistical thermodynamic approach, with the Cubic-Plus-Association equation of state, is employed to model the phase equilibria. The hydrate-forming conditions are modelled by the solid solution theory of van der Waals and Platteeuw. The thermodynamic model was used to predict the water content of methane and synthetic gases in equilibrium with gas hydrates.     

Narrow and broad band diode laser absorption spectrometry—concepts,limitations and applications

Implementation concepts as well as the fundamental aspects concerning the analytical capability of diode laser spectrometry with respect to narrow and broad band absorption are discussed. The applicability is illustrated by means of the element-selective analysis of flames or plasmas and the molecular analysis of liquids or turbid media. While in narrow band absorption one diode laser and different modulation techniques can be applied to obtain very low detection limits, two diode lasers and a double-beam scheme should be used when the absorption bands are very broad. The two-laser, double-beam method is demonstrated by means of absorption measurements in a turbid medium and by concentration analyses of liquid samples applying the surface plasmon resonances technique.     

Concentration measurements of ozone in the 1200-300ppbv range: an intercomparison between the BNM ultraviolet standard and infrared methods

Simultaneous ultraviolet (UV) and infrared (IR) measurements of ozone concentration in air in the 1200-300 ppbv range have been performed using the ultraviolet absorption in the Hartley band at 0.2537 microm and the infrared absorption of a doublet at 9.507 microm in the nu(3) vibration-rotation band. Infrared concentration measurements were achieved using the tunable diode laser spectrometer of LPMA in Paris with interferometric control of the emitted wavelength while the UV concentration measurements were performed with the 49PS Megatec ozone generator of the Bureau National de Metrologie (BNM). The simultaneous recording of spectra of a reference cell filled with pure distilled ozone and of a low concentration mixture inside a long absorbing path Herriott cell allows to carry out infrared concentration measurements with an accuracy of the same order as the ultraviolet ones and provides the instrumental parameters of the spectrometer corresponding to each concentration measurement, which reduces systematic errors. Within the respective absolute uncertainties proper to the two techniques, no systematic discrepancy was evidenced between the IR and the UV measurements. The ozone ultraviolet absorption coefficient value determined by Hearn (308.3 +/- 4 cm(-1)atm(-1)) and used by the BNM and the National Institute of Standards and Technology (NIST) is confirmed by the present work.     

Effect of broadband amplified spontaneous emission on absorption measurements in phase-shift off-axis cavity enhanced absorption spectroscopy

Phase-shift cavity enhanced absorption spectroscopy has been described for the case of an off-axis alignment of an optical cavity formed by high reflectivity mirrors. The effect of laser broadband amplified spontaneous emission (ASE) from a DFB diode laser (1605 nm) has been analysed for absorption measurements extracted from the phase-shift of the intensity modulated beam. A simple model has been proposed for extraction of actual absorption spectra and linestrengths from the phase and amplitude measurements. It has been shown that omitting the ASE contribution to the cavity output light intensity may lead to an overestimation of absorption linestrength.