R branch tuning characteristics of the13CH3F Raman FIR laser

Summary We described a¹³CH₃F Raman laser pumped by a grating tuned 20 atmospheres CO₂ laser. The emission characteristics of the¹³CH₃F laser extends from 14 cm^–1–35 cm^–1 and from 49 cm^–1–72 cm^–1; about 65% of these frequency ranges can be covered with tunable radiation. The characteristics shows a strong dependence on the rotaional quantum numbers of the states involved in the Raman laser transitions and, within each tuning interval, on the frequency offset with respect to the frequencies of resonant transitions. We obtained, at 51 cm^–1, a maximum FIR laser pulse energy of about 800 J (at a pump energy of 200 mJ), corresponding to a photon conversion of about 8%. In some cases we have observed simultaneous emission at a Raman and a cascade frequency. In addition, FIR emission power dependence on¹³CH₃F gas pressure and pump pulse power were investigated for different J quantum numbers.     

Observation of new laser transitions and saturation effects in optically pumped NO

We report investigations of an NO laser employing specially profiled magnetic fields of up to 3.4T, and F₂ pump laser intensities as great as 20 MW cm^–2. We have observed laser oscillation at 226 nm on a rotational branch of the B'-X/it(3–11) band of NO for the first time, in addition to the previously reported oscillation at 218 nm on the B'-X/it(3–10) band. We have also observed visible laser emission on a rotational branch of the B ²-B ² II(3–1) band of NO. Saturation of the NO laser pulse energy with pump intensity has been observed, the total NO laser pulse energy having been increased to 490 J. The possibility of increasing the NO laser pulse energy towards 1 mJ per transition is discussed.     

Scaling and performance of CO2 lasers at supra-atmospheric pressure

The performance of a compact uv photo-preionized TE laser is studied in the pressure range 1–5 bar. As the pressure is increased, the laser pulse shape is little altered, but both the peak power and the total output pulse energy increase significantly with pressure, even for constant input electrical energy. For various gas mixtures and excitation source capacitors the measurements suggest approximate output energy scaling with the product of the source charge per unit electrode area [C.m^–2] and the molecular partial pressure [CO₂+N₂+CO]. This is explained in terms of the pressure-dependent discharge impedance. An input-energy-related discharge instability limits the optimum laser pressure to 1.5–2.5 bar, and we show that, at constant input energy, the instability boundary depends on the molecular partial pressure alone. The pre-ionization photo-electron yield varies negligibly with pressure, but the discharge tolerance to added oxygen decreases asp ^–3 top ^–4, dependent on gas mixture. Nevertheless sealed operation for >10⁵ shots has been obtained with a 5% CO₂5% CO3% N₂2% H₂85% He gas mixture at a total pressure of 5 bar.     

Tuning characteristics of a high pressure CO2 laser pumped CH3F Raman laser

Conclusion We described a CH₃F Raman laser pumped by a two stage 20 atmospheres CO₂ laser. The emission spectrum of the CH₃F laser at 11 Torr extends from 23 cm^–1 to 45 cm^–1 when the CO₂ laser is scanned over the 9R emission branch at a fixed pump power of 180 mJ. The emission spectrum shows a strong structure with large parts where the FIR energy decreases to zero. This fact makes the use of such a laser for spectroscopic scanning experiments in the FIR difficult. The laser is, however, very suited for working at fixed but adjustable FIR frequencies. The pulse energy in the maxima of the emission characteristics at a pump energy of 180 mJ exceeds 300 J, which corresponds a photon conversion coefficient of more than 6%.     

Femtosecond-pulse laser ablation of human corneas

A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 10^–5 cm², have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30–100 m range from the excision edge at all fluences above the ablation threshold of F _th60 J cm^–2 (I _th9 GW cm^–2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 m were observed at all fluences above ablation threshold of F _th1 J cm^–2 or I _th3 TW cm^–2 (3×10¹² W cm^–2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.     

Efficient production of13C2F4 in the infrared laser photolysis of CHClF2

We report the isotopically selective decomposition of chlorodifluoromethane. Chlorodifluoromethane is used industrially in high volume for the production of tetrafluoroethylene and its polymers; thereby it is an attractive working substrate for a medium scale isotope separation process, both in terms of its price and availability.We have studied the infrared multiphoton decomposition of carbon-13 substituted chlorodifluoromethane molecules present at their natural abundance (1.11%). A well defined CO₂ laser pulse (80 ns FWHM) was used and both the yield of carbon-13 enriched product and the net absorption of laser radiation were measured. These measurements were made as a function of substrate pressure (10-800 Torr), CO₂ laser line (9P 12–9P 32) and fluence (2–8 J cm^–2) and were used to determine the energy expenditure per carbon atom produced () at specified product carbon-13 content in the range 30%–96%. The results of these parametric studies were interpreted in terms of the kinetics of multiphoton absorption and dissociation, and allowed an initial optimization of the experimental conditions to minimize .Optimum results were obtained at 1046.9 cm^–1, 69 cm^–1 to the red of the¹²CHClF₂ v ₉ band center. Irradiation of 100 Torr of chlorodifluoromethane at 3.5 J cm^–2 gave tetrafluoroethylene containing 50% carbon-13 for an absorption of 140 photons (0.017 keV) per carbon atom produced. This efficiency compares favourably with existing carbon-13 enrichment technologies and would require an absorption pathlength of only 2 m to absorb half the incident photons.Issued as NRCC 20112     

X-ray preionized CO2 laser

A short pulse (100 ns) high-energy x-ray source has been used to preionize a transversely excited carbon dioxide gas discharge laser of 600 cm³ active volume. The maximum output power of 60 MW in a 50 ns FWHM pulse was achieved from a CO₂–N₂–He–CO–Xe static gas mixture at 600 Torr pressure. The energy conversion efficiency was 6%.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.     

Laser-induced free-carrier absorption in Si single crystal

The transmitted energy density in thin single Si crystal, wafers is measured at=1.06 m as a function of the incident energy density for a Nd laser pulse of 30 ns duration. Non-linear effects begin to become important at about 0.3 J/cm². The contribution due to free-carriers is separated from the interband one by using measurements made at low energy density and at different sample temperatures in the 20°–150 °C range. The time dependence of the free-carrier concentration and of the lattice temperature is computed for different values of the Auger constant. The experimental data in the 0.2–2.5 J/cm² energy density range are fitted with an Auger constant of 10^–30 cm⁶s^–1.Work supported in part by M.P.I. and G.N.S.M.-C.N.R.     

Defects in pulsed laser and thermal processed ion implanted silicon

The evolution of the nature and concentration of the defects produced by 100 or 300 keV As ions at fluences 1 to 4×10^–12 cm^–2 inn-type, Fz Silicon doped with 10¹⁵ to 10¹⁶ cm^–3 has been studied as function of thermal treatments (in the range 500°–900 °C) and of the energy density (in the range 0.3–0.6 J cm^–2) of a light pulse from a ruby laser (15 ns, 0.69 m). Deep-level transient spectroscopy (DLTS) combined with capacitance — voltage (C-V) measurements were used to get the characteristics (energy level, crosssection for the capture of majority carriers) of the defects and theirs profiles. The difficulties encountered in the analysis of the results, due to the large compensation of free carriers in the implanted region and to the abrupt defect and free carrier profiles, are discussed in detail and the corrections to apply on the C-V characteristics and the DLTS spectra are described. The defects resulting from the two types of treatments are found to be essentially the same. Only, for laser energies higher than 0.5 J cm^–2, the laser treatment appears to introduced new defects (atE0.32 eV) which should result from a quenching process. The fact that a laser energy smaller than the threshold energy for melting and recrystallization is able to anneal, at least partially, the defects produced by the implantation, demonstrates that the annealing process induced by the laser pulse is not a purely thermal process but is enhanced by a mechanism involving ionization.     

Infrared multiphoton dissociation of13CF3Cl induced by CO2 laser pulses

Selective infrared multiphoton dissociation of¹³CF₃Cl induced by CO₂ laser pulses adjusted on = 1071.9 cm^–1 has been studied in the energy rangeE between 0.5 and 2 J per laser pulse or fluence range between 5 and 25 J per cm², and in the pressure range between 0.10 and 60 Torr. The effect of these parameters on the isotopic selectivity of the dissociation gives information on the rotational relaxation constants. As for the dissociation probabilities, they vary exponentially withE ^–1. The applicability of such an Arrhenius-type relation is discussed and semi-quantitatively justified.     

Study of Direct Amplification of Ultrashort Light Pulses in a Laser Amplifier with a View to Obtaining High Radiation Contrast

An amplification scheme for ultrashort laser pulses of high radiation contrast was used to perform experiments on ablation pressure symmetrization using a prepulse upon acceleration of thin foils. It is shown that the spontaneous radiation of the regenerative amplifier restricts the energy contrast in the amplification of chirped pulses at a level of 10^-4–10^-3. The possibility of direct amplification of a short pulse with a view to increasing the energy contrast ratio was considered. Experiments were performed on the PICO laser facility to demonstrate that a 10-ps pulse amplification achieved an intensity 100 GW/cm², a gain factor of 1.2, and an inversion dumping factor >30%.     

Transient absorption and laser output of YAG : Nd

YAG : Nd grown under 98% Ar 2% H₂ protective atmosphere free of nitrogen or hydrocarbons showed after UV irradiation broad absorption peaked at 1·9×10⁴ cm^–1 which disappeared relatively slowly at room temperature. It was more intensive in oxygen treated samples than in those annealed in hydrogsn. Transient absorption suppresses laser output by the increase of absorption at 0·94×10⁴ cm^–1 (1064 nm) and, particularly in CW mode, by the anomalous rod deformation. YAG : Nd containing Fe ions (2·10^–4 wt%) showed no transient absorption.     

Submillimeter laser action of CW optically pumped CF2Cl2 (Fluorocarbon 12)

Eleven new CW far infrared (FIR) laser lines have been observed in the 600 m–1200 m range from the CF₂Cl₂ (Fluorocarbon 12) molecule optically pumped by a CO₂ laser. A 510^–4–10^–3 accuracy is achieved in the measurement of the FIR wavelengths.The frequency offset between the CO₂ pump center and the absorption line centers are measured using the transferred Lamb dip technique. Owing to a recent spectroscopic study of the CF₂ ³⁵Cl₂ molecule three lines may be assigned with great confidence as rotational transitions in thev ₆ vibrational band 923 cm^–1 of this main isotope.     

High-resolution degenerate four-wave-mixing spectroscopy of OH in a flame with a novel single-mode tunable laser

The first application of a novel single-mode tunable laser system to nonlinear spectroscopy is reported. The device uses a modeless dye laser, pumped by a single longitudinal mode (SLM) Q-switched Nd:YAG laser, as a narrow-bandwidth amplifier of the output of a SLM diode laser. The system provides pulses of 5-ns duration, 30-mJ energy and 165-MHz spectral line width tunable in the range 632–639 nm at 10-Hz repetition rate. The frequency-doubled output of the laser is used to record spectral line shapes of degenerate four wave mixing (DFWM) signals from the P₁(15) line of the A²–X² (0,0) band of OH in a methane/oxygen flame. Pressure broadening of the DFWM line shape is studied for the first time in a low-pressure flame and a pressure-broadening rate of 1.31±0.09×10^-4 cm^-1/Torr is derived from the data. Power-broadening effects are measured and compared with predictions of the standard perturbative model and of an analytical solution derived from a non-perturbative treatment of DFWM with arbitrary pump and probe intensities. PACS 42.55.Px; 42.62.Fi; 42.65.-k     

Co:LaMgAl11O19 saturable absorber Q-switch for a 1.319 μm Nd:YAG laser

Passively Q-switched output of a flashlamp-pumped 1.319 μm Nd:YAG laser is realized by using Co²⁺:LaMgAl₁₁O₁₉ (Co:LMA) as saturable absorber. When initial transmission of the saturable absorber T₀ is 78%, a Q-switched output pulse with pulse width (FWHM) 44.8 ns and pulse energy 17.4 mJ is obtained, corresponding to 19.3% of the free-running energy under the equal pumping energy of 45.4 J. The experimental results show that the higher T₀ will result in a lower pumping threshold of the laser, but lower T₀ can make the laser generate pulses with higher single-pulse energy, narrower pulse width, and accordingly higher peak power.     

A Compact Pulsed HF Laser Based on an Auto-Wave Photon-Branched Chain Reaction

Huge energy gain is detected theoretically in a pulsed chemical laser-amplifier based on a photon-branched chain reaction initiating in a gaseous disperse medium composed of H₂–F₂–O₂–He and Al particles by focused external infrared radiation. It is shown that this effect is observed due to the other optical effect of diffractive multifocal focusing of the input radiation on a certain type of bicomponent optical system coupled structurally with the input mirror of an unstable telescopic laser cavity. Such a relatively simple bicomponent diffraction system, consisting of two plane screens with circular apertures on a given optical axis, enables one to focus the input beam without using classical refraction elements such as lenses and prisms. The focusing of the input signal ensures the minimization of the initially excited volume of the laser active medium and the appropriate sharp lowering of the necessary energy of the input pulse up to 10^-6 J. This enables the laser system to reach a high value of the energy gain of 10⁹. The huge laser energy gain allows us to use a master oscillator in the form of a small microjoule laser powered by an accumulator, and consequently makes it possible to construct a completely self-contained compact pulsed chemical HF-laser.     

Dynamics of the CO2 lower laser levels as measured with a tunable diode laser

A tunable diode laser operating in the 4.3 m region is used to probe a conventional cw CO₂ laser discharge. Vibrational populations in the 10⁰0, 02⁰0, 02²0, and 01¹0 levels of CO₂ are measured under lasing conditions, i.e., in the presence of intense 10.4 and 9.4 m fields. The tunable diode laser is also used to monitor the energy transfer processes between the four levels after the passage of an intense 10.4 m pulse. The detailed information provided by the tunable probe laser enables us to determineseparately all the vibration-vibration (V-V) and vibration-translation (V-T) rate constants of importance in the relaxation of the lower laser levels in CO₂. The V-V rate constants are found to vary from a low value of 4.5×10⁴ s^–1 Torr^–1 for the coupling of 01¹0 to 10⁰0 to a high value of 8.0×10⁵ s^–1 Torr^–1 for the coupling of 01¹0 to 02²0.This work was supported by the National Science and Engineering Research Council of Canada and the Provincial Government of Ontario     

He–SrCl2 vapor laser excited by Blumlein discharge circuit

An investigation of a He–SrCl₂ vapor laser excited by Blumlein discharge circuit is performed. Dependences of laser output power on the working parameters are obtained by experiment. The optimal operating parameters for lasing on several strontium atom and ion lines are found. The multi-line average output power of 1.32 W and the power specific of 12.46 mW/cm³ are achieved, of which more than 78.1% is concentrated at the 6.45 μm laser line.