Two-dimensional quenching lifetime measurement of OH:A 2 Σ +(υ′ = 1) and NO:A 2 Σ(υ′ = 0) in atmospheric-pressure flames

Planar laser-induced fluorescence is currently widely applied to research on combustion. However, conventional approaches for semi-quantitative measurement could not provide the satisfactory solution and the problem due to collisional quenching remains to be solved. This paper describes the two-dimensional (2D) quenching-time-constant measurement of OH:A ^{2 +}( = 1) and NO:A ² ( = 0) in flame cross sections at atmospheric pressure. These measurements involve 1–2 ns decay time of the excited state using a high-speed image intensifier and a tunable laser with a pulse duration of 3 ns at FWHM. The correlation factors of the exponential fits for the fluorescence decays after the laser pulse were larger than 0.999 in all experiments presented in this study. Furthermore, the measured Stern-Volmer plots of the quenching rate at 1.0, 1/2, 1/3, and 1/4 atm pressure using the same experimental apparatus was confirmed to have a linear relationship for both OH and NO, showing that the 2D decay-time measurements of 1–2 ns have been successful. For NO:A ² ( = 0), the obtained quenching rates inside the inner cone and in the outer flame in the NO-seeded methane-air Bunsen flame were 8.7 × 10⁸ and 7.8 × 10⁸ s^–1, respectively, and for OH:A ² ( = 1), the obtained quenching rate mapping in the outer flame was around 5.6 × 10⁸ s^–1 in the methane-air flame.     

Wide-range tunable sub-millimetre cyclotron resonance laser

The results of investigation of cyclotron type stimulated emission processes inp-Ge in strong crossedE B fields are presented. Using the optimumE,B field orientation the spectral range of continuous tuning of the emission line frequency was increased substantially. A tunable sub-millimetre laser with the tuning rangev = 25 to 95cm^–1 ( = 105 to 400m), output power 10 to 100mW and emission linewidthv 0.2cm^–1 is described.     

Degenerate four-wave mixing in nitrogen dioxide: Application to combustion diagnostics

Single shot degenerate four wave mixing (DFWM) images of the distribution of nitrogen dioxide (NO₂) doped into a propane/air flame at concentrations of the order of 7000 ppm have been obtained. These images indicate the relative concentration of NO₂ in different parts of the flame with an estimated spatial resolution of 150 m.Initial experiments were performed using NO₂ in a glass cell with nitrogen buffer gas. DFWM signals were generated using both the frequency doubled output of a pulsed ND:YAG laser and the tunable blue output of an excimer pumped dye laser. The signal was investigated as a function of laser power, NO₂ concentration and buffer gas pressure. In addition, spectra of NO₂ in the region 450 to 480 nm were obtained.Signals were then sought in both a cold air/NO₂ gas flow and an ignited mixture of propane and air seeded with NO₂, using a DFWM imaging geometry. The resulting images from the flame demonstrate the disappearance of the NO₂ molecules in the flame interaction zone.This work was done when previously employed by AEA Technology at Harwell     

Temperature measurement using degenerate four-wave mixing with non-saturating laser powers

We report the use of Degenerate Four-Wave Mixing (DFWM) in the OHA ^{2 +} X ² ( = 0 = 0) band for temperature determination in a propane/air flame using laser powers below the saturation level. We show that at these low power levels the dependence of the signal on the dipole moment for the transition has to be established before meaningful temperature data can be obtained. This presents a paradox in that the temperature has to be known before the form of the dependence on the dipole moment can be determined. Solutions to this paradox are presented. We also show that absorption of the laser beams in this OH band system cannot be neglected and that failure to correctly account for the absorption leads to a large over estimate of the flame temperature. Furthermore, we show that the accuracy of the absorption-corrected temperature is critically dependent on the accuracy with which the measurement position within the flame is known. Finally, the temperature calculated from DFWM spectra using the correct dipole moment power and absorption is compared to the temperatures obtained using Laser-Induced Fluorescence (LIF) and Coherent Anti-Stokes Raman Spectroscopy (CARS).     

Thermometry of an oxy-acetylene flame using multiplex degenerate four-wave mixing of C2

Thermometry of an oxy-acetylene flame using multiplex Degenerate Four-Wave Mixing (DFWM) of C₂ is demonstrated. More than 100 rotational transitions in thed ³ _g a ³ _u (0,0) Swan band of C₂ could be recorded simultaneously by use of a pulsed, broad bandwidth modeless laser. Temperatures were inferred by fitting temperature-dependent synthetic spectra of single- or multiple-shot averaged spectra. The strength and reliability of recorded signals together with the large number of rotational lines observed suggest that multiplex DFWM is a promising technique for minor species detection and for temporally resolved temperature measurements in luminous environments. Factors influencing the accuracy and precision of single-shot thermometry using the technique are discussed.     

Simultaneous detection of atomic and molecular hydrogen using a tunable ArF excimer laser

We report preliminary experiments which demonstrate the simultaneous detection of both atomic and molecular hydrogen using a single tunable ArF laser. The tunable ArF laser was modified to lase simultaneously at two wavelengths by the addition of a second grating to the oscillator stage as in earlier work by Ketterle et al. for KrF operation. To our knowledge, this is the first demonstration of dual-wavelength capability with a tunable ArF excimer. The H atom diagnostic utilized tunable ArF excimer laser output at 193.29 nm which was Raman shifted in D₂ (first Stokes) to generate 205.14 nm radiation which then excited the H atom to its (3s, 3d) states via a two-photon transition. Fluorescence was detected at 656 nm on the (3s, 3d) 2p transition. The second wavelength from the tunable excimer was used to excite several rovibronic LIF transitions of hydrogen molecule via two-photon absorption on the E ¹ _g ⁺ X ¹ _g ⁺ (2,0) band. Fluorescence occurs at 750 nm and 830 nm on the (E ¹ _g ⁺ B ¹ _g ⁺ ) transitions.     

High-power diode-cladding-pumped Tm<Superscript>3+</Superscript>, Ho<Superscript>3+</Superscript>-doped silica fibre laser

The fundamental characteristics of a continuous-wave high-power diode-pumped Tm³⁺, Ho³⁺-doped double-clad silica fibre laser are presented. A maximum output power of 5.2 W was measured and was generated at a slope efficiency of 42 (44)% with respect to the launched (absorbed) pump power. At the optimum length of 7 m (_effL=2.9, where _eff is the effective absorption coefficient of the fibre and L is the fibre length), the fibre laser output was measured to have a centre wavelength of 2105 nm and a line width of 20 nm. The centre wavelength of the emission was tunable over a 32-nm extent when 0.68<_effL<3.28 or for a 6.2-m change in L. PACS 42.55.Wd; 42.55.Xi; 42.60.Lh; 42.60.Pk     

A tunable light source in the 370 nm range based on an optically stabilized,frequency-doubled semiconductor laser

A tunable harmonic output power of 18 W at a wavelength of =370 nm is obtained by resonance-enhanced frequency doubling of an optically-stabilized semiconductor laser. A commercially available AlGaAs laser diode which emits a maximum power of 10 mW at =740 nm is operated in an extended-cavity configuration. Dispersion prisms are used in the extended cavity to obtain longitudinal-mode selection with low loss of optical power. The output is focussed into an optically isolated high-finesse ring resonator which contains a LiIO₃ crystal for second-harmonic generation. One potential application of this laser source is the optical excitation and laser cooling of ytterbium in an ion trap. In a related demonstration experiment, the frequency-doubled diode laser is applied to excite the =369.5 nm ² S _1/2-² P _1/2 transition of ytterbium ions in a hollow-cathode discharge.     

Degenerate four-wave-mixing spectroscopy in NaH

We performed DFWM spectroscopy on X ^–1+–A ¹⁺ transitions in NaH produced in an indirect photochemical reaction between Na(3p) and H₂ and detected v=1, 2 and 3 ground state vibrational levels of NaH molecules, whereas with resonance enhanced CARS, we observed v=0 levels only. This different sensitivity can be explained by considering the Franck-Condon-factors and the relevant damping coefficients for the corresponding transitions in the NaH molecule. Time resolved DFWM spectroscopy showed that NaH(v=1) molecules effectively live much longer than Na(3p) atoms which merely follow the laser excitation pulse.     

Two-photon resonance degenerate four-wave mixing in a three-level polar molecule (model calculations)

The third-order non-linear susceptibility, ⁽³⁾(; , , –), describing the phase conjugation by degenerate four-wave mixing (DFWM) is calculated for a three-level polar system under a two-photon resonance (TPR) condition. The part of the susceptibility expressed only by the transitional dipole moments and the part resulting from the permanent dipole moments are distinguished. The spectra of ⁽³⁾(; , , –), as well as the DFWM power-reflection and power-transmission spectra near to the TPR, are modelled for polar molecules. The effect of enhanced reflectivity of TPR-DFWM is obtained numerically.     

The application of a raman-shifted tunable KrF excimer laser for laser-induced fluorescence combustion diagnostics

Four wavelength extensions have been investigated by stimulated Raman scattering in hydrogen or deuterium gas of the 248 nm fundamental output of a narrow-band tunable KrF excimer laser. They have been used to acquire laser-induced fluorescence spectra of NO and OH in flames at atmospheric pressure. NO is detected in relatively high rotational states within the -band electronic system at 225 nm. OH was excited at 291.5 nm in the (1–0), at 313 nm in the (1–1) and at 268.5 nm in the predissociative (3–1) band of the ²–² electronic band system, respectively. Prospects of 2D imaging for concentration and temperature measurements in flames using these wavelength extensions are discussed.     

Tunable CW laser oscillation of NdP5O14 at 1.3 μm

Low-threshold tunable cw laser oscillation is achieved at the⁴ F _3/2⁴ I _13/2 transition of NdP₅O₁₄. Continuous tunign over 1.8% of the central wavelenght is possible with 55 mW of absorbed pump power. At gain line maximum near 1.32 , the output power exceeded 1 mW. Pump threshold as low as 8 mW was observed. Reliable cw operation of the 1.3 m NdP₅O₁₄ laser pumped by a 15 mW AlGaAs laser diode is demonstrated.     

Relationship between temporal coherence and laser parameters in a two-longitudinal-mode HeNe laser

The relationship between the temporal coherenceg ⁽¹⁾ () and laser parameters - the output relative intensity (I ₁/I ₂) and the frequency width _h of the longitudinal mode in a two-longitudinal-mode HeNe laser - is studied experimentally. The experimental results show that both the relative output intensity and the frequency width of the longitudinal mode have a great influence on the temporal coherenceg ⁽¹⁾ (). Moreover, the spectral line-shape function of each longitudinal mode in a two-longitudinal mode HeNe laser is a Lorentzian function, with a frequency width of about 18.2 MHz. A model of the power spectrum with Gaussian amplitude distribution and equal frequency width of the longitudinal mode in a multi-longitudinal-mode gas laser is employed in the theoretical analysis. Good agreement between theory and experiment is obtained.     

Method of Measuring the Spectral Shape of the Radiation Line of a Three-Micron Long-Path Laser Gas Analyzer

A new method to experimentally determine the spectral shape of the radiation line of a tunable long-path absorption laser gas analyzer operating in a three-micron range ( = 2.5–4.5 m) on the basis of a difference-frequency generator is suggested. The method is based on measurement of the transmission function of a gas medium upon modulation of its optical thickness or pressure. Specific vibrational–rotational transitions and also optimal pressures of CO₂, N₂O, and CH₄ are recommended for carrying out experiments.     

CW coherent VUV radiation generated by resonant sum-frequency mixing in metal vapors

Resonant third-order sum-frequency mixing of continuous-wave visible and ultraviolet laser light is investigated in the metal vapors of Sr, Ca, Mg, and Zn. The laser radiation (with the frequencies ₁, ₂, and ₃) is generated by single-mode dye- and argonion lasers. The third-order nonlinear susceptibility is enhanced by tuning the laser frequencies 2₁ or ₁+₂ to a two-photon resonance. The wavelength of the generated coherent VUV radiation (with the frequency _VUV=3₁, _VUV=2₁+₂ or _VUV=₁+₂+₃) is tunable in spectral regions between 133 and 217 nm. For most conversion processes an additional enhancement of the nonlinearity is provided by an autoionizing level or by a highly excited state close to E=_VUV. With laser powers of 0.2–3 W the power of the generated sum frequencies is in the range of 5×10⁵–6×10¹⁰ photons s^–1 (0.5 pW–60 nW).     

Anisotropy of the nonlinear optical susceptibility χ(3) in polydiacetylene single crystals

The anisotropy of ⁽³⁾ in Poly-bis(p-Toluene Sulfonate) of 2.4-hexadiyne-1.6-diol (PTS) PolyDiAcetylene (PDA) single crystals is investigated by Degenerate Four-Wave Mixing (DFWM) at a wavelength of 720 nm. The symmetries of ⁽³⁾(–;, –,) identify the non-vanishing and independent components of the tensor. For the calculation of the ⁽³⁾ elements from the measured values, the anisotropy of both the linear optical index of refraction and the absorption coefficient is included in the coupled wave approach. A magnitude of 10^–10 esu is measured for the ⁽³⁾ component if all beam polarizations are parallel to the polymer chains. The least upper bound for all other measurable components is estimated to be 10^–12 esu. The results for the intensity dependence of the DFWM signal are explained in terms of a thermooptic effect. Measurements on the orientational dependence of the DFWM signal in the samples are carried out, and the influence of the anisotropy of the linear optical parameters on the measured curves is discussed. A method for the preparation of thin layered polymer single crystals of poly-bis (4-ButoxyCarbonyle-Methylene-Urethane) of 4.6-decadiyne-1.1-diol (4-BCMU) and the first DFWM measurements on these samples are presented.     

Laser collimation of an Fe atomic beam on a leaky transition

We report results of the first laser collimation of a thermal beam of Fe atoms on the leaky ⁵D₄ ⁵F₅ transition, with both parallel linear ^xx and crossed linear ^xy laser polarization configurations. The measured atomic beam divergence is compared to a rate-equation model and a quantum Monte Carlo model. The experimental values for the divergence are limited by the finite laser line width, which is comparable to the natural line width of the Fe atom. In general, flux decreases with higher intensities, showing the effect of the leaky transition. At the best beam collimation _RMS = 0.17 mrad, which is for a detuning of = – and a saturation parameter of s = 6, the flux decreased to approximately 70%. Highest flux was measured for a detuning of = –2 and s = 4, reaching 135% of the uncooled value. From our measurements we estimate the total leak rate to be 1/(240 ± 40), which is in good agreement with the literature value of 1/244. The crossed linear polarization configuration is the better choice, with a slightly better collimation but the same atomic beam flux. Plugging of the largest leak would increase the flux to at least 80% of the closed transition value, resulting in better contrast for atom lithography.     

Narrow-line diode laser system for laser cooling of strontium atoms on the intercombination transition

We report a diode laser system developed for narrow-line cooling and trapping on the ¹S₀–³P₁ intercombination transition of neutral strontium atoms. Doppler cooling on this spin-forbidden transition with a line width of /2=7.1 kHz enables us to achieve sub-K temperatures in a two-step cooling process. The required reduction of the laser line width to the kHz level was achieved by locking the laser to a tunable Fabry–Pérot cavity. The long-term drift (>0.1 s) of the reference cavity was compensated by employing the saturated absorption signal obtained from Sr vapor in a heat pipe of novel design. We demonstrate the potential of the system by performing spectroscopy of Sr atoms confined to the Lamb–Dicke regime in a one-dimensional optical lattice. PACS 32.80.Pj; 39.30.+w; 42.55.Px     

A new method to measure the wavelength of single-mode pulsed lasers with a scanning Michelson interferometer

We report on the possibility to measure the wave-lengths of pulsed single-mode lasers by means of a two-beam Michelson interferometer in motion [1,2]. The corner reflector moves with a nearly constant speed creating a path differenceL so thatL/C 1/, being the spectral width of the laser to be measured. The reference laser is a stabilized He-Ne (Spectra-Physics, model 117 A) to a precision of the order of two parts in 10⁹. The fringe pattern of the two beams (reference beam and measured beam) is sampled simultaneously with a repetition rate of 40 ms. With this new method, the frequency doubled injection-seeded Nd: YAG laser wavelength has been measured with an accuracy of the order of 1.5 in 10⁸3 × 10^–4 cm^–1 at 532nm.     

NaK 22Σ+ → 11Σ+ band optically pumped laser near 1.02 μm

Optically pumped laser emission has been observed on the NaK 2(A)¹⁺ 1(X)¹⁺ electronic state transition. The emission occurs between 1.015 and 1.035 m when a sodium-potassium heat-pipe oven is pumped with 695–745 nm pulsed dye laser radiation. The laser emission occurs on many ro-vibrational transitions without the use of cavity mirrors. However, the addition of a simple cavity increases both the number of observed lasing transitions and the amplitude of the emission on each line. We report our results for the dependence of the emission intensity on pump laser power, oven temperature, and buffer gas pressure.