Tunable Semiconductor Diode Laser Research Articles

Development of gas fire detection system using tunable diode laser absorption spectroscopy

The conventional fire detection methods mainly produce an alarm through detecting the changes in smoke concentration, flame radiation, heat and other physical parameters in the environment, but are unable to provide an early warning of a fire emergency. We have designed a gas fire detection system with a high detection sensitivity and high selectivity using the tunable semiconductor diode laser as a light source and combining wavelength modulation and harmonic detection technology. This system can invert the second harmonic signal obtained to obtain the concentration of carbon monoxide gas (a fire characteristic gas) so as to provide an early warning of fire. We reduce the system offset noise and the background noise generated due to the laser interference by deducting the system background spectrum lines from the second harmonic signal. This can also eliminate the interference of other gas spectral lines to a large extent. We detected the concentration of the carbon monoxide gas generated in smoldering sandalwood fire and open beech wood fire with the homemade fire simulator, and tested the lowest detectable limit of system. The test results show that the lowest detectable limit can reach 5×10-6; the system can maintain stable operation for a long period of time and can automatically trigger a water mist fire extinguishing system, which can fully meet the needs of early fire warning.

A laser-induced fluorescence instrument for aircraft measurements of sulfur dioxide in the upper troposphere and lower stratosphere

Abstract. This work describes the development and testing of a new instrument for in situ measurements of sulfur dioxide (SO2) on airborne platforms in the upper troposphere and lower stratosphere (UT–LS). The instrument is based on the laser-induced fluorescence technique and uses the fifth harmonic of a tunable fiber-amplified semiconductor diode laser system at 1084.5 nm to excite SO2 at 216.9 nm. Sensitivity and background checks are achieved in flight by additions of SO2 calibration gas and zero air, respectively. Aircraft demonstration was performed during the NASA Volcano-Plume Investigation Readiness and Gas-Phase and Aerosol Sulfur (VIRGAS) experiment, which was a series of flights using the NASA WB-57F during October 2015 based at Ellington Field and Harlingen, Texas. During these flights, the instrument successfully measured SO2 in the UT–LS at background (non-volcanic) conditions with a precision of 2 ppt at 10 s and an overall uncertainty determined primarily by instrument drifts of ±(16 % + 0.9 ppt).

A simple scanning semiconductor diode laser source and its application in wavelength modulation spectroscopy around 825 nm

A very simple and inexpensive tunable semiconductor diode laser controller is designed for stable operation of the diode laser. The diode laser controller is stable within +/−8 μA and +/−10 mK, respectively. The noise spectrum of the current controller is studied by FFT analysis. We have used our home-made diode laser system in a tunable diode laser absorption spectrometer (TDLAS) to probe weak overtone transitions of water vapour molecule. The diode laser wavelength is coarsely tuned by changing the operating temperature to probe (2, 1, 1)←(0, 0, 0) band overtone transitions of water vapour within 818–835 nm. To demonstrate line shape study, seven transitions are scanned by ramping the drive current of the diode laser (at constant operating temperature) under different perturber (laboratory air) pressures within the sample cell. A balanced detector and a lock-in amplifier are used for phase sensitive detection purpose. Small current modulation amplitude, balanced detection and proper adjustment of the lock-in amplifier help to obtain a S/ N ratio ranging from ∼100 to ∼7 using a small sample path length of 1.5 m. Experimentally obtained derivative spectrum is numerically integrated to reveal the original line shape and fitted with a nonlinear least squares fitting program to extract air broadening coefficients and line strength parameters. The spectroscopic line parameters are compared with the results from HITRAN database.

Laser-induced fluorescence study of a xenon Hall thruster

0→6p[3/2]2(3P2-1D2) transition at 823.2 nm and the xenon-ion 5d[3]7/2→6p[2]5/2 0(4D7/2-4P5/2) transition is used to measure plasma parameters in the plume of a laboratory-model xenon Hall thruster. The Hall discharge operates nominally at 62 V, 4.2 A, and 3.2 mg s-1 xenon flow, with an overall thruster power of 320 W. A tunable semiconductor diode laser and an Ar+-pumped dye laser are used to probe the respective excited-state transitions. Axial velocity measurements are made at a number of axial and radial locations up to 4.5 cm downstream of the thruster-exit plane and under a variety of thruster operating conditions. Neutral velocities from 100 m s-1 to 400 m s-1 and ion velocities as high as 12 km s-1 are calculated from measured Doppler shifts. The charge-exchange phenomenon evidently does not significantly affect the xenon neutrals. The spectral-line shapes of the ion indicate a spread in ion energies through a non-Maxwellian distribution of axial velocities. Neutral kinetic temperatures of 500 (±200) K are observed under standard operating conditions. Zeeman and Stark effects on the spectral-line shapes, from the thruster’s magnetic and electric fields, are not substantial. The measured line center of the ion transition is 16521.23 (±0.02) cm-1.

Laser-induced fluorescence measurements of resonance broadening in xenon.

Resonance broadening in atomic xenon has been studied by using laser-induced fluorescence of two excited-state electronic transitions. The impact theory result for resonance broadening is verified to be accurate and the onset of the breakdown of the theory is chronicled. The spectral broadening of the 6s[3/2${]}_{1}^{\mathit{o}}$${(}^{3}$${\mathit{P}}_{1}$)\ensuremath{\rightarrow}6p[1/2${]}_{0}$ (828-nm) transition by the resonant interaction of its lower level with ground-state xenon was investigated for densities of 4\ifmmode\times\else\texttimes\fi{}${10}^{21}$--2\ifmmode\times\else\texttimes\fi{}${10}^{24}$ atoms ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$. The predominantly Doppler-broadened 6s[3/2${]}_{2}^{\mathit{o}}$${(}^{3}$${\mathit{P}}_{2}$)\ensuremath{\rightarrow}6p[3/2${]}_{2}$ (823-nm) transition was analyzed for similar conditions, primarily to establish the kinetic temperature of the neutral xenon. Tunable narrow-linewidth semiconductor diode lasers were used to probe the excited states, which were produced by a direct-current glow discharge. The intricate hyperfine structure of the measured profiles was taken into account by summing constituent Voigt profiles to construct model line shapes. The 6s[3/2${]}_{1}^{\mathit{o}}$\ensuremath{\rightarrow}6p[1/2${]}_{0}$ broadening constant at low densities and at 311 K was found to be 6.04(\ifmmode\pm\else\textpm\fi{}0.66)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}21}$ MHz ${\mathrm{m}}^{3}$ ${\mathrm{atom}}^{\mathrm{\ensuremath{-}}1}$. This result is in agreement with the value predicted by the impact theory for resonance broadening. The 6s[3/2${]}_{1}^{\mathit{o}}$\ensuremath{\rightarrow}6p[1/2${]}_{0}$ Lorentzian width extrapolated to zero density, 43\ifmmode\pm\else\textpm\fi{}15 MHz, is in accord with the natural linewidth. At densities above 6\ifmmode\times\else\texttimes\fi{}${10}^{23}$ atoms ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$ the recorded line shapes of this transition exhibited an asymmetry, providing clear evidence of the breakdown of the impact approximation. The measured Lorentzian broadening for the 6s[3/2${]}_{2}^{\mathit{o}}$\ensuremath{\rightarrow}6p[3/2${]}_{2}$ transition was 8.3(\ifmmode\pm\else\textpm\fi{}1.8)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}22}$ MHz ${\mathrm{m}}^{3}$ ${\mathrm{atom}}^{\mathrm{\ensuremath{-}}1}$ at 311 K. \textcopyright{} 1996 The American Physical Society.

Nanosecond time-resolved infrared spectroscopy: a comparative view of spectrometers and their applications in organometallic chemistry

Three time-resolved infrared (TRIR) spectrometers are described. These instruments are based on (i) a line-tunable CO laser (2000–1550 cm–1), (ii) a continuously tunable semiconductor diode laser (approximately 2250–1900 cm–1) and (iii) a modified and adapted dispersive IR spectrometer with Nernst glower (or globar) which, in principle, could cover all of the mid-IR region. The spectrometers differ in their inherent signal-to-noise ratio and their response. Although the ultimate performance can be achieved with the CO laser, the instruments are complementary, each with its own advantages. The performances of the three TRIR spectrometers were compared in a number of applications in organometallic photochemistry, including the IR detection of molecules in electronically excited states, the kinetic behaviour of intermediates in the reaction of (η5-C5H5)M(CO)4 compounds (M = V, Nb and Ta), an IR study of the photochemical reaction of CpMn(CO)3 within polyethylene film and solvent interactions in supercritical xenon solution and the detection of W(CO)5Xe.

Variable time delay of microwave signals using high dispersion fibre

A programmable time delay of 0–5 ns has been demonstrated for a 3 GHz signal using a tunable semiconductor diode laser and 1 km of a new high-dispersion fibre.

Analysis of the Q branch of the 3 ν3 overtone of UF 6: The implied structure of the nν3 ladder

Spectra of the 3 ν 3 region of 238UF 6 near 1875.5 cm −1 have been obtained at 256 K with path lengths up to 400 m using tunable semiconductor diode lasers. Seventeen features have been assigned to 27 Q-branch fine-structure levels, and five molecular parameters have been derived. Two of these refer to vibration-rotation interaction and agree well with earlier values determined from the ν 3 fundamental band. The remaining three constants are pure vibrational. They, along with information from the ν 3 fundamental, determine the excited vibrational structures of the nν 3 overtones, to the second order of approximation in the formalism of Hecht. These structures are consistent with strong localization of the three individual components of the triply degenerate ν 3 mode of vibration. Comparisons are made with the two-photon spectrum of Harzer, Schweizer, and Selter, and with the bond-stretching model of Halonen and Child.

Laser atomic absorption spectroscopy applying semiconductor diode lasers

The application of tunable single mode semiconductor diode lasers in atomic absorption spectroscopy is discussed in general. The use of several diode lasers, periodical modulation of the laser powers and Fourier analysis of the absorption signals allow background-corrected multi-element atomic absorption spectroscopy with extended dynamic range and internal standardization. This is demonstrated by the simultaneous determination of rubidium and barium in aqueous solutions with a commercial graphite tube atomizer.

Optical piston in rubidium

We report observation of an optical piston due to the effect of light induced drift in a rubidium-argon mixture. A tunable semiconductor diode laser is used for diagnostics of rubidium density and isotropic ratio. Sign and magnitude of the light induced drift effect depend directly on fine and hyperfine structure of rubidium.

Absolute line intensity measurements in the ν2 bands of HDO and D 2O using a tunable diode laser spectrometer

Absolute line intensities for several individual vibration-rotation transitions in the ν 2 bands of HDO and D 2O have been determined at room temperature from laboratory spectra recorded with a tunable diode laser spectrometer. Two tunable semiconductor diode lasers operating in the 1250- to 1350- and 1060- to 1140-cm −1 spectral regions have been used in the recording of the data. Comparisons are made with previously published results where appropriate.

Measurement and analysis of the infrared-active stretching fundamental (ν3) of UF6

High-resolution spectra of the infrared-active stretching fundamental ν3 of 238UF6 have been obtained between 620.6 and 633.5 cm−1 using tunable semiconductor diode lasers. Interference from hot bands was suppressed by cooling the UF6 in a supersonic expansion, and useful monomer concentrations were produced with effective temperatures of <100 K. Portions of the band from P(77) to R(66) are illustrated. All transitions from the vibrational ground state have been assigned, and the Q branch has been fully analyzed. A total of 43 line frequencies and 110 frequency differences extending in J to P(77), Q(91), and R(67) has been used to fit seven spectroscopic constants. The ground- and excited-state values of the rotational constant B could be individually determined, and the U–F bond length in the ground vibrational state is r0=1.9962±0.0007 Å. The Q branch of 235UF6 has also been analyzed and the 235UF6–238UF6 ν3 isotope shift measured to be 0.603 79±0.000 17 cm−1. The isotope shift and the Coriolis constant ζ3 have been used to refine the general quadratic intramolecular force field of UF6, and the Cartesian displacement coordinates of both infrared-active fundamentals are illustrated and compared with those of SF6.

Infrared–microwave double resonance spectroscopy of the SiF4 ν3 fundamental using a tunable diode laser

Pure rotational spectrum of silicon tetrafluoride SiF4 is investigated in the ν3 vibrational state by infrared–microwave double resonance. Frequency tunable semiconductor diode lasers are used as an infrared pumping radiation source. 106 radiofrequency transitions among the tetrahedral fine structure splittings and 60 microwave transitions among the various Coriolis sublevels are measured with an accuracy of a few tens of kilohertz. Seven scalar and four tensor constants of the excited state are determined accurately. A part of the badly congested infrared Q-branch spectrum is analyzed on the basis of assignment by double resonance. Three ground-state constants B0, D0, and D044 are determined.

ChemInform Abstract: DIODE LASER SPECTROSCOPY OF THE ν9 BAND OF ETHYL CHLORIDE

Abstract The vibration-rotation spectrum of the ν9 band of C2H5Cl was recorded using a Nicolet 7199 Fourier Transform Infrared Spectrometer; the Q-branch, R(5)-R(6) multiplets, and P(21)-P(22) multiplets were investigated using a tunable semiconductor diode laser spectrometer. Constants derived from these assignments are (cm−1) ν9 = 973.8379(2), A9 = 1.039516(3), B9 = 0.183081(7), C9 = 0.164505(8). The line which coincides with the CO2 laser 10R(6), which has been employed in multiphoton dissociation studies, is tentatively assigned to the 203 18-213 19 transition. In addition, a very strong coincidence of transitions 199 11-199 10 and 199 10-199 11 with the CO2 10R(16) line is found, which can provide a good candidate for laser chemistry and infrared double resonance spectroscopy.

Diode laser spectroscopy of the ν9 band of ethyl chloride

The vibration-rotation spectrum of the ν 9 band of C 2H 5Cl was recorded using a Nicolet 7199 Fourier Transform Infrared Spectrometer; the Q-branch, R(5)- R(6) multiplets, and P(21)- P(22) multiplets were investigated using a tunable semiconductor diode laser spectrometer. Constants derived from these assignments are (cm −1) ν 9 = 973.8379(2), A 9 = 1.039516(3), B 9 = 0.183081(7), C 9 = 0.164505(8). The line which coincides with the CO 2 laser 10 R(6), which has been employed in multiphoton dissociation studies, is tentatively assigned to the 20 3 18-21 3 19 transition. In addition, a very strong coincidence of transitions 19 9 11-19 9 10 and 19 9 10-19 9 11 with the CO 2 10 R(16) line is found, which can provide a good candidate for laser chemistry and infrared double resonance spectroscopy.

Infrared radio frequency double resonance spectroscopy of the 13CF4 ν3 fundamental using a tunable diode laser

Pure rotational spectrum of 13CF4 was investigated in the ν3 vibrational state by infrared–radio-frequency double resonance. A frequency tunable semiconductor diode laser was used as an infrared pumping radiation source. About 100 radio-frequency transitions among the ν3 tetrahedral fine structure splittings were measured with the accuracy of a few tens of kHz. The observed spectrum showed that the ν3 state of 13CF4, in particular the R = J+1 highest Coriolis sublevel, was largely perturbed by 2ν4 which was about 15 cm−1 above ν3. Seven tensor constants were determined tentatively from the transitions measured in the R = J and J−1 sublevels.

High-resolution vibration-rotation spectroscopy of fluoroform- d

The vibration-rotation spectrum of the ν 2 and ν 5 fundamentals of CDF 3 have been recorded using a Nicolet 7199 Fourier transform infrared spectrometer; in addition the Q branch and several subbands of each of these transitions have been investigated using a tunable semiconductor diode laser spectrometer. The Q branch and the K structure in several P( J) and R( J) subbands of ν 2, and in several Q branches of ν 5, are resolved and assigned for the first time. Constants derived for these bands are (in cm −1) ν 2 = 1111.1823 6, B 2 = 0.32928 2, A 2 = 0.18872 2, α 2 B = 16.44 5 × 10 −4, α 2 B − α 2 A = 12.43 5 × 10 −4, D 0 j = 3.7 3 × 10 −7, D 2 J = 4.8 3 × 10 −7; ν 5 = 975.39 1, B 5 = 0.33062, A 5 = 0.18887, α 5 B = 2.83 1 × 10 −4, α 5 A = 2.4 3 × 10 −4, ζ 5 = 0.736, D 5 J − D 0 J = 1.2 2 × 10 −8. Some of these constants are nearly 100 times more precise than those reported in previous work.

Double resonance and high resolution infrared spectroscopy of the CF4 ν3 fundamental. II. Infrared–microwave double resonance and infrared Q-branch spectrum

Pure rotational transitions between the v3 = 1 Coriolis sublevels of CF4 were observed by infrared–microwave double resonance. A frequency tunable semiconductor diode laser was used as a pumping radiation source. 117 microwave transitions in the JJ+1–JJ, J+1J+1–JJ, JJ–J−1J, JJ–JJ−1, and J+1J–JJ branches were measured within the accuracy of a few tens of kHz. Transitions in the last two branches showed a large systematic frequency shift caused by perturbations fom the 2ν4 state. The observed pure rotational spectrum was analyzed including the previously measured radio frequency transitions. About 200 infrared ν3 Q-branch absorption lines, which were assigned by infrared–radio frequency double resonance, were also analyzed. Eight tensor and nine scalar constants were tentatively determined, including the three ground state constants B0, D0, and D0(4q4).

Diode laser spectroscopy of the ν6 band of 12CH 3I

The vibration-rotation spectrum of the ν 6 fundamental of methyl iodide has been recorded in the 824 to 862 cm −1 region by using a tunable semiconductor diode laser spectrometer. The rotational analysis performed for six Q branches p Q( J,3), p Q( J,4), p Q( J,5), p Q( J,6), p Q( J,7), and q Q( J,8) led to accurate values for several molecular constants. The nuclear quadrupole splitting arising from the spin of iodine has been observed very clearly in the low J transitions and for various K values.

High-resolution spectroscopy of the 16-μm bending fundamental of CF 4

The 16-μm bending fundamentals ( ν 4) of 12CF 4, 13CF 4, and 14CF 4 have been observed at Doppler-limited resolution using a tunable PbSnSe semiconductor diode laser. The tetrahedral splittings of the rotational manifolds have been observed in all three branches, and in particular the dense and partially overlapping transitions in the Q branches have been resolved and assigned. A least-squares fit of the Hamiltonian, including off-diagonal terms, yielded five scalar and three tensor spectroscopic constants for each of the three isotopes. From these constants the upper-state rotational constant B 4 and the Coriolis constant ζ 4 have been calculated, together with some of the other molecular constants. An absorption feature at about 0.18 cm −1 to the red of the main Q branch of each isotopic species has been identified as the Q branch of ( ν 2 + ν 4) − ν 2, which is the transition that lases when CF 4 is pumped by a CO 2 laser at 9.4 μm (i.e., in ν 2 + ν 4).