Simultaneous visualisation of transient species in flames by planar-laser-induced fluorescence using a single laser system

2 O (formaldehyde) have been visualised by two-dimensional laser-induced fluorescence applied to a rich methane–air flame of a Wolfhard–Parker burner. A novel arrangement of the frequency conversion crystals allows the simultaneous excitation of the mentioned species with a single dye laser system. Rearranging the crystals back to their standard position extends the wavelength range allowing the excitation of two additional species, OH and CHO, without the need to change the dye. Received: 6 August 1998/Revised version: 8 October 1998     

Generation of UV and VIS laser light by stimulated Raman scattering in H2, D2, and H2/He using a pulsed Nd:YAG laser at 355 nm

2 ), deuterium (D₂), and mixtures of hydrogen and helium (H₂/He), versus Raman gas pressure and input pump energy of the pulsed Nd:YAG laser at 355 nm, are reported. Photon conversion efficiencies of 50% and 27% are achieved at the first Stokes lines (S₁) in H₂ and D₂, respectively. As a result, ultraviolet and visible laser light (274–503 nm) was generated with energies ranging from a few mJ up to several tens of mJ. Received: 5 January 1998/Revised version: 3 June 1998     

Supersonic velocimetry in a shock tube using laser enhanced ionisation and planar laser induced fluorescence

 A novel flow-tagging technique is presented which was employed to measure gas velocities in the free stream of a shock tube. This method is based on the laser spectroscopic techniques of Laser-Enhanced Ionisation (LEI) and Laser-Induced Fluorescence (LIF). The flow in the shock tube is seeded with small amounts of sodium, and LEI is used to produce a substantial depletion of neutral sodium atom concentration in a well-defined region of the flow, by using two wavelength-resonance excitation and subsequent collisional ionisation. At a specific time delay, single-laser-pulse planar LIF is utilised to produce a two-dimensional (2-D) inverse image of the depleted tagged region downstream of the flow. By measuring the displacement of the tagged region, free stream velocities in a shock tube were determined. Large variations in the concentration of sodium seeded into the flow were observed and even in the presence of these large variations accurate free-stream velocity measurements were obtained. The experimentally determined value for velocity compares very well with the predicted velocity. Received: 25 March 1996/Revised version: 8 July 1996     

Experimental study with a Fourier set-up of the image transfer from a laser beam to a forward or backward stimulated Raman scattered beam

Received: 13 July 1998 / Revised version: 8 October 1998 / Published online: 24 February 1999     

Simultaneous measurement of pure-optical and thermo-optical nonlinearities induced by high-repetition-rate, femtosecond laser pulses: application to CS2

A simple experimental technique is presented capable of separating the contribution of purely optical Kerr effects from that of thermo-optical effects in the nonlinear response of materials under high-repetition-rate laser irradiation. The technique has been realized by combining the single-beam Z-scan method with the single-beam thermal lens measurement method. We demonstrate this technique by analysing the nonlinear response at 770 nm of CS₂ which exhibits cumulative thermal effects when irradiated by very intense femtosecond laser pulses at a 76-MHz repetition rate. Received: 3 November 1998 / Revised version: 4 January 1999 / Published online: 2 June 1999     

Wavelength-stable, narrow-spectral-width oscillation of an AlGaInP diode laser coupled to a BaTiO3:Co stimulated photorefractive backscattering phase conjugator

3 ) stimulated photorefractive backscattering (SPBS) phase conjugator. The SPBS process was successfully achieved by the combination of the short-wavelength diode laser and the highly doped photorefractive BaTiO₃:Co crystal. The spectral width of the diode laser is reduced to 7.2 pm because of the narrow spectral bandwidth of the gratings [with wave number of K=2k(2k gratings)] formed in the SPBS phase conjugator, compared with the transmission grating formed in the conventional cat conjugator, which was used previously. The SPBS phase conjugator was successfully used to suppress self-frequency scanning, and wavelength-stable oscillation of the SPBS phase-conjugator-coupled diode laser was achieved. Received: 4 November 1996/Revised version: 12 February 1997     

Time-resolved spectroscopy measurements of a titanium plasma induced by nanosecond and femtosecond lasers

We have studied the plasma induced at the surface of a titanium target following irradiation with femtosecond and nanosecond laser pulses. Time-resolved imaging and spectroscopic measurements allowed us to evidence some features specific to the femtosecond-laser-induced plasma. In this ultrashort interaction regime, we could discriminate between three different velocity populations in the plasma expansion. Coulomb explosion firstly creates highly energetic Ti⁺ ions, which are followed by atomic neutral titanium and lastly by nanoscale titanium oxide clusters. Received: 28 September 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +33-1/6931-9996, E-mail: albert@ensta.fr     

Differentiation and subtraction of amplitude and phase images using a photorefractive novelty filter

In this article, we demonstrate the capability of a two-beam coupling photorefractive optical novelty filter of detecting changes in the amplitude or phase of optical images. These changes may either be continuous or discrete in time. The performance of the two-beam coupling novelty filter is investigated and expressions for the output contrast corresponding to phase and amplitude changes based on a novel, simple interference model of two-beam coupling are derived. These expressions are verified by experimental results on the novelty contrast, revealing that the amplitude contrast is not described correctly by the commonly accepted coupled-wave theory. The novelty filter was applied to the detection of temporally continuous phase changes provided by a gas flow and moving microscopic objects. A novel scheme for image subtraction is also demonstrated, showing the novelty filter’s ability to detect temporally discrete changes. Received: 14 November 1998 / Revised version: 18 January 1999 / Published online: 12 April 1999     

Cavity ring-down spectroscopy of CH and CD radicals in a diamond thin film chemical vapor deposition reactor

A mixture of H₂ and CH₄ is passed over a hot-wire tungsten filament in a diamond thin film chemical vapor deposition reactor. The resulting CH radicals are measured in absorption using cavity ring-down spectroscopy (CRDS). The concentration of the CH radicals increases as the filament is approached. The rotational temperature measurements indicate a large temperature discontinuity between the filament and the CH in the gas phase. The pathways for CH production were investigated by replacing H₂ by D₂ in the feed gas mixture, which resulted in the exclusive production of CD. From this observation it is concluded that rapid H/D isotope exchange dominates in the gas phase. Nonperiodic temporal oscillations in the CH concentration are observed when a rhenium filament is used in place of a tungsten filament. The oscillations are attributed to the nonperiodic changes in the amount of carbon at the filament surface. Received: 21 August 2000 / Accepted: 23 August 2000 / Published online: 23 May 2001     

Observation of intense stokes and anti-stokes lines in CH4 pumped by 355 nm of a Nd:YAG laser

The stimulated Raman scattering (SRS) and four-wave mixing (FWM) processes have been studied in detail in CH₄ gas pumped by the third-harmonic of Nd:YAG laser at =355 nm. The conversion efficiency and average output energy have been measured for the various Stokes and anti-Stokes lines at different experimental conditions. The threshold input energy at various CH₄ pressures has been calculated for the unguided and capillary waveguide Raman cells. The calculated values for the capillary case have been compared with the measured values at different CH₄ pressures and the two results are in good agreement.     

Pulsed laser deposition and in situ diagnostics of the process applied to shape-memory alloys

NiTi films deposited by pulsed laser ablation on Si/SiO₂ are shown to exhibit structural and functional properties related to the shape-memory effect. Film characterization suggests that relevant temperatures for the solid-to-solid transformation responsible for the shape memory are in substantial agreement with those of the bulk target material, demonstrating a good congruency of the deposition process. Besides the technological interest for this class of thin films, our findings point out the suitability of laser ablation for metal alloy deposition. An investigation based on in situ ion-mass spectroscopy and covariance mapping analysis allows us to determine the main vapor-phase processes leading to the formation of stoichiometric clusters expected to play a relevant role in assisting the growth of NiTi thin films. Received: 6 August 2001 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +39-50/2214-333, E-mail: fuso@df.unipi.it     

Measurements of picosecond laser induced fluorescence from gas phase 3-pentanone and acetone: Implications to combustion diagnostics

between 323 K and 473 K at pressures of air or nitrogen between 0.1 MPa and 1.0 MPa. Received: 9 May 1996/Revised version: 3 September 1996     

150-times phase conjugation by degenerate four-wave mixing in a continuous-wave Nd:YVO4 amplifier

The high phase-conjugate reflectivity of 150, by four-wave mixing in a diode-pumped Nd:YVO₄, has been demonstrated. The phase-conjugate mirror was capable of correcting the phase aberration of a probe beam. The experimental diffraction efficiency of the transmission gratings was 60. Received: 8 April 2002 / Revised version: 10 June 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +81/43-290-3490, E-mail: omatsu@image.tp.chiba-u.ac.jp     

Investigation of 13CH3OH methanol: new laser lines and assignments

We have reinvestigated ¹³CH₃OH as a source of far-infrared (FIR) laser emission using a CO₂ laser as a pumping source. Thirty new FIR laser lines in the range 36.5 μm to 202.6 μm were observed and characterized. Five of them have wavelengths between 36.5 and 75 μm and have sufficient intensity to be used in LMR spectroscopy. Using Fourier-transform spectroscopic data in the infrared (IR) and FIR regions we have determined the assignment for 10 FIR laser transitions and predict nine frequencies for laser lines which have yet to be observed. Received: 17 July 2000 / Published online: 6 December 2000     

Dynamic behavior of photoablation products of corneal tissue in the mid-IR: a study with FELIX

2 . Dynamic parameters such as the extension of the ablation cloud, the initial velocity and momentum of the ablated particles as well as the ablation threshold, the ablated mass, and the particle size were investigated. The ablation plume was made visible with a stroboscopic technique. For a fluence of 3.1 J/cm² the average initial velocity of the ejected particles was deduced from the extension of the plume to range from 120–400 m/s. Measurements of the recoil momentum using a sensitive pendulum led to values between 0.5 and 2.0 mm g/s. All measured properties were related to the spectroscopically determined absorption coefficient of cornea α_cornea. Where absorption due to proteins is high (at λ=6.2 and 6.5 μm), ablated mass, velocity and recoil momentum behave according to α_cornea. For the first time, variations of the ablation plume from pulse to pulse were observed. Those, as well as the particle size, not only depend on the absorption coefficient, but also on the predominant absorber. Received: 4 November 1997/Revised version: 7 September 1998     

Second-harmonic generation from a picosecond Ti:Sa laser in LBO: conversion efficiency and spatial properties

We report an experimental study of the second harmonic generated in type I interaction by a Ti:Sa laser operating in the picosecond regime at 786 nm in LBO crystals. A joint characterization of the dependence of conversion efficiency and spatial beam quality on crystal length and degree of pump focusing is given. A simple heuristic formula, reproducing over a broad range of parameters the predictions of classical Boyd–Kleinman theory, is derived and compared with the experimental results. The conditions for the optimization of the generation process using an elliptically focused pump beam are quantitatively evaluated. Received: 13 March 2002 / Revised version: 8 May 2002 / Published online: 12 July 2002     

4-W and 23-ps pulses from a lamp-pumped Nd:YAG laser passively mode-locked by polarization switching in a KTP crystal

Passive mode-locking of a cw lamp-pumped Nd:YAG laser using nonlinear polarization switching in a type-II SHG crystal is reported. Light pulses with more than 5 W of average power and pulse duration shorter than 25 ps have been obtained at 1064 nm. Received: 29 January 1999 / Revised version: 24 March 1999 / Published online: 1 July 1999     

Hard tissue ablation with a free running Er:YAG and a Q-switched CO2 laser: a comparative study

The Er:YAG and the CO₂ laser are competitors in the field of hard tissue ablation. The use of Er:YAG lasers (2.94 μm, pulse length _L of 100 to 200 μs) show smaller areas of thermal defects then ‘‘superpulsed’’ CO₂ lasers with pulse lengths of approximately 100 μs. Only the development of a Q-switched CO₂ laser (9.6 μm, τ_L=250 ns) allowed for similar results. In this paper new results for the Er:YAG and the Q-switched CO₂ laser under the influence of water spray will be presented. Several parameters are of special interest for these investigations: the specific ablation energy, which shows a minimum for the CO₂ laser at an energy density of 9 J/cm ² and a broad shallow minimum in the range of 10 to 70 J/cm² for the Er:YAG laser, and comparison of the cut-shape and depth. Surface effects and cutting velocity are discussed based on SEM pictures. Received: 19 July 2000 / Revised version: 1 November 2000 / Published online: 30 November 2000     

Efficient and compact intracavity-frequency-doubled Nd:GdVO4/KTP laser end-pumped by a fiber-coupled laser diode

A compact and efficient diode-pumped intracavity-frequency-doubled Nd:GdVO₄/KTP green laser is demonstrated with a flat–flat cavity design. With a 1.3 at. % Nd³⁺-doped GdVO₄ crystal and pumped at the weak-absorption peak of 806 nm, the second-harmonic output power at 532 nm was measured to be 1.95 W at an incident pump power of 8.4 W, corresponding to an optical conversion efficiency of 23.2%. The output characteristic at the fundamental wavelength of 1.063 μm was investigated with two different pump wavelengths. More than 4.5-W output power was generated when the laser was pumped at 806.2 nm. Received: 26 July 2000 / Revised version: 18 September 2000 / Published online: 7 February 2001     

In situ observation of dynamics of plasma self-channeling and bulk modification in silica glasses induced by a high-intensity femtosecond laser

The time-resolved dynamics of plasma self-channeling and refractive index bulk modification in silica glasses were first observed in situ using a high-intensity femtosecond (110 fs) Ti:sapphire laser (λ_p=790 nm). Plasma channeling is induced in silica glass at an irradiation higher than an input intensity of 1.5×10¹² W/cm² and photoinduces either the refractive-index modification or optical crack modification. In the domain of refractive-index modification, the lifetime of induced plasma self-channeling was 20 ps and the structural transition time for forming the refractive-index change was 10 ps. In the domain of optical cracks, however, the lifetime of induced plasma formation was 30 ps and the structural transition time for forming the optical cracks was 40 ps. According to electron spin resonance spectroscopic (ESP) measurement, it was found that the defect concentration of the SiE^′ center increased significantly in the refractive index modification region. A maximum value of the refractive-index change Δn was measured to be 1.6×10^-2. The intensity profile of the output beam transmitted through the refractive-index modification showed that the bulk modification produced a permanent optical waveguide. Received: 8 April 2002 / Accepted: 12 April 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: shcho@riken.go.jp