By the end of 85 the Eureka "Eurolaser EU6" Project was started with a definition phase aimed at the generation of specific proposals in the field of High Power Lasers for materials processing. A number of industrial initiatives followed, leading to the definition of specific Eureka Projects in the areas of High Power CO2 Lasers (10 ÷ 100 kW), High Power Solid State Lasers (1 ÷ 5 kW) and High Power Excimers Lasers (up to 10 kW). A review will be given of the Eurolaser Projects originated in two years in the above areas and of possible additional projects considering other approaches to high power lasers. Other precompetitive European or national initiatives will be reviewed. The Eureka projects on laser processing at power levels lower than the Eurolaser ones will be also mentioned. Proposals for an additional coordination, possibly leading to Common European Standards on laser beams characterisation and quality assessment procedures, on mechanical and optical components for high power lasers and laser systems and on laser safety regulations, will also be outlined.

Within the power range 0.01~22kW, commercially-available CO2 industrial lasers now incorporate microwave, RF, AC and/or DC electrical excitation technologies, with a correspondingly wide variety of gas flow and optical extraction geometries. This paper summarizes from an applications viewpoint some of the advantages claimed for the various approaches, making brief comparisons with Nd:YAG performance. Particular emphasis is given to the importance of laser brightness, reproducibility, and fluctuation-stability for uses such as the heat-treatment, cutting, or welding of steels in the thickness range 10-25mm.

The results of our contribution to the first phase of the EUREKA project (EU-3) are discussed within this paper. Development of high power CO laser systems in the power range of 10 - 100 kw has been investigated theoretically concerning excitation techniques and resonator design. Rf-excitation of the active medium may have advantages due to high discharge stability and modulation bandwidth. Possible advantages and feasibility of an oscillator-amplifier system are discussed concerning beam quality, modulation capability, amplification length, and discharge efficiency. The oscillator-amplifier system appears to be an alternative to a oscillator system.

An original aerodynamic technique has been developped to achieve stable and uniform dc-discharges in wide rectangular channels. The discharge is spread out over the width of the laser channel by using parallel turbulent transverse flows (TTF). The technique insures stability and uniformity of the discharge without the need of a fine segmentation of the electrodes. Various experiments have been carried out in open and closed- gas cycles. At a pressure of 50 mbar, discharge power loading up to 6 W/cm3 has been obtained for a differential pressure limited to about 10 mbar. Experimental results are presented along with the design of a 3-kW laser.

Important features of an industrial CO2 laser to be considered during design are pointed out. An excellent overall efficiency of this laser is reported. Differences between DC and RF excitation are discussed.

This paper deals with some investigations of the gas discharge and the optical properties of a transverse flow cw CO2-laser in the output power range of 5 kW. The optical qualities of the generated laser beam depend very sensitively on the type and design of the optical resonator. Different unstable optical resonators and optical inhomogenities of the transversely excited active medium will be discussed. Experimental results (laser output power, beam cross sections, focal spot) will be presented.

Based on a continuous discharge slow gasflow CO. laser a high peak power, medium average power, high repetition rate laser has been developed. The system consists of a Q-switched oscillator plus a subsequent laser amplifier. Q-switching is done by a fast spinning chopper disk placed in the focal plane of an intracavity cylindrical telescope. Almost 500 W of average laser power have been achieved in the pulsed mode at 20 kHz repetition rate from laser tubes rated for 1 kW power in cw oscillator mode. Single pulse energies are 27 mJ to 50 mJ ( depending on duration of the tail) at repetition rates up to 10 kHz on line 9P(20). Up to to 50 kHz are easily achievable at reduced single pulse energy. Pulse duration is 200 ns (FWHM). The peak power of 150 kW is more than one order of magnitude higher than that with discharge current switching techniques in low pressure lasers. Therefore lasers of this type are suitable for applications in nonlinear processes like IR photochemistry or material processing without sacrificing much of the advantages of cw lasers as compared to TEA lasers. Investigations on stored energy, dependence on repetition rate, self oscillation problems, switching speed requirements and scaling properties are also reported.

This work deals with numerical model of a fast axial-flow dc - excited CO2 laser. A spiral motion of the gas mixture is considered. The preliminary results obtained without taking into account the influence of the turbulence are presented.

RF excitation has become the preferred method for excitation of CO2 lasers, from miniature waveguide lasers through to multi-kilowatt, industrial fast axial flow lasers. The underlying physics of transverse discharges in the 10 to 150 MHz frequency range is reviewed, and factors determining the choice of optimum frequency discussed. New discharge structures, giving industrially useful powers from very compact devices, are now becoming possible through RF excitation techniques. The new approaches are contrasted with the more conventional routes to high power density using fast axial flow.

The first RF-excited industrial carbon dioxide lasers began to appear on the market 3 - 4 years ago, and are rapidly establishing their superiority over traditional DC-excited designs in terms of useful performance factors and reliability in actual use. This paper presents the relative advantages and disadvantages of these two fundamental excitation modes for application to medium- to high-power carbon dioxide lasers as seen from the viewpoint of the industrial user, and reviews the fundamental physics and gas discharge phenomena involved.

The design and operation of a new 5/10 kW CO2 laser with unstable resonator and integrated RF power stag will be described. The system is characterized by a discharge power density. of 25 W/cm for cw operation and 100 W/cm3 for superpulse operation. The gas is circulated by three axial flow blowers. The volume of the laser system including the power stage is 1.8 x 1.7 x 1.6 m3. The output power can be controlled in the range between 10 and 100% using a 25 kHz pulse modulation and varying the duty cycle at constant power level. The beam quality and pointing stability was investigated. using a beam analyser (LBA) at the expanded far-field, generated by means of a concave mirror with large radius of curvature (R = 20 m). Good beam quality and pointing stability has been demonstrated for this transverse flow laser with RF excitation.

A new concept of a laser head with successfully integrated optical path, discharge tubes, gas delivery tubes and resonator rods has been developed. This laser contains eight discharge tubes each energized by a 1 kW solid state generator at a frequency of 13.56 MHz. With an input power of 4.68 kW a laser power of more than 860 W has been obtained in a non optimized state of operation. This output power entails a plasma efficiency of 19%.

A model of the homogenous CO2 laser plasma is presented which can be used to describe the efficiency of laser operation for both the DC and the RF- discharge. As laser gas a mixture of CO2, N2, and He with an admixture of Xe was used. To simplify the calculations the distribution of the electron energy was considered to be a Maxwellian one which makes it possible to avoid the solution of Boltzmann's equation. In addition only the most important collision cross sections were used, namely the cross sections for excitation of vibrational modes in CO2 and N2, for attach-ment and electronic excitation of CO2, for ionisation of CO2, N2, and Xe, for elastic collisions and for momentum transfer. Under these assumptions the energy and the particle density balances for electrons can be put up. As a result the energy transfer to the different levels in CO2 and N2 and the electron density can be calculated which can be used to estimate the efficiency of the conversion from electrical power to laser radiation. The evaluation of this model showes that the efficiency of the RF- discharge is a little bit higher than that of the DC- discharge which is mainly due to the more favourable conditions for ionisation in the RF- discharge. The admixture of xenon causes a sig-nificant increase of the electron density for a given value of E/N which raises the efficiency considerably.

Comparing the state of development of high power gas lasers for civil applications, it can be seen that the CO2 laser is a well established tool; the CO laser, however, essentially remained a laboratory device. Hence, the question arises whether there will be an advantage to develop high power CO lasers for industrial applications, too. After a brief recapitulation of the typical CO-related properties, to help answering this question, the application potential of the CO laser, will be discussed. There are several wavelength-related advantages of the CO laser like increased absorption depth in glasses and crystals increased focal power density, and reduced plasma shielding. Furthermore, transmissive optical materials have considerably improved values for absorption and damage threshold, and finally power transmission through optical fibers is much more realistic in the near future for the 5 μm spectral range. In contrast to the variety of promising applications is the number of experimentally verified ones. This is due to the fact that only a few lasers are existing in the power range and in the developmental stage to be used for applications. In experiments CO lasers demonstrated advantages in the field of cutting and drilling metals and uranium isotope separation. Lasers in the high power range are developed in Japan, in the Soviet Union and in Germany. The types of lasers investigated in these countries differ from each other by the methods of gas cooling and excitation. Comparisons between Co- and CO2 lasers show that the system efficiencies of CO lasers are slightly higher by a factor of 1.3; the operation costs of CO lasers are reduced by the same factor. Investment and operation costs can be reduced considerably if for the planned application a high focal power density is used. Furthermore, the volumes of CO and CO2 lasers are comparable at present and in the future.

The mass spectrometric analysis was performed for a closed-cycle high-power CO laser to study the influence of gas composition on the discharge characteristics and the laser performance, and to measure the change of gas composition during a continuous operation. The addition of a small amount of 02 (0.2-0.3%) in the laser gas (CO/N2/He=4/16/80) enhanced the maximum discharge input power restricted by discharge instability, which enabled the maximum laser output power to increase. The addition of CO2 caused the drastic decrease of output power. During a continuous operation the CO concentration decreased, on the contrary, the CO2 concentration increased. At the same time the output power decreased monotonously. The prsence of 02 prevents the deposition of soot on the surface of electrodes, but accelerates the formation of CO2. The long time operation about 1 hour at the output power of 2 kW was firstly accomplished without a gas exchange.

A shock tunnel facility is fabricated and used for the study of a supersonic flow CO chemical laser. A high temperature mixture of CS2, CS, S2, S and an inert gas is produced in a shock tube, where the thermal dissociation of CS2 is accomplished by a reflected shock wave. The shock heated mixture is exhausted through supersonic nozzles mounted at the end of the tube and mixed with the supersonic streams of O2. Then, the vibrationally excited CO molecules are produced in the mixed streams by a chain reaction between CS and 02 initiated by S atoms. The shock tunnel facility is used to study the gain characteristics and to observe the flow field utilizing the chemiluminescence. A computer simulation of the supersonic flow CO chemical laser is also performed and the importance of the viscous effects on the gain characteristics is demonstrated.

Optimum conditions for the efficient operation of a cw CO laser excited by transverse dc glow discharge using a gas mixture of CO/N2/He/02 has been investigated by computer simulation. The simulation model combines direct excitation process by electron impact, vibration to vibration ( V-V ) and vibration to rotation/translation ( V-R/T ) energy transfer process, spontaneous and stimulated emission process, and semi 1-dimensional flow model. One of the important experimental problems that the laser power output decreases against the increase in gas flow velocity is analyzed by the simulation. According to the computed results, the problem is not serious if the gas temperature is low enough or sufficient discharge current is applied. The possibility of the high power and efficient laser operation even at around 200 K is predicted by the simulation.

Performance characteristics of a 1-kW transverse self-sustaind discharge-excited cw CO laser with a closed-loop subsonic flow are reported. Discharge and laser characteristics have been measured as a function of gas flow rate in a range of 10-37 m/sec. The measurements show that maximum discharge power loading drastically increases as the flow rate increases. Under the same cavity conditions, however, the output decreases rather than increases as the flow rate increases. From a discharge volume of about 480 cm3 the maximum laser output of 1250 W has been obtained with an electrical conversion efficiency of 26.6 % for a mixture of CO/N./He/02=6/16/78/0.05 (%) at a gas flow rate of 25 m/sec. The gas pressure and the entrance gas temperature are 40 Torr and 175 K, respectively. It has been also made clear that high-power,efficient operation can be achieved even at relatively high temperatures by increasing discharge input density. For a higher pressure of 50 Torr the output of 1180 W has been obtained with a conversion efficiency of 21.9 % at a temperature of 193 K.

The CO-laser is generated by exciting CO:N2:He:(02:C2H4) gas miksture cooled down to T= 110°K, N= 0.5 Amaga. The photoionization discharge is initiated by a short UV pulse of wavelength < 100 nm and duration t~ 1 μs at the reduced electric fill E/N < 3.3 x10-16 V.sm2. The energy consumed by direct photoionization of the (O2:C2H4) dope is 30% of the energy contribution to the volume discharge. The optimization of the photoionization pumping system parameters is examined. The dynamics of the recovery of optical homogeneity of active medium after the photoionization and discharge pulses is studied. The methods for improving the energy effectiveness of the photoionization CO-laser are proposed. The generation duration is shown to be adjustable within 2-600 μs, depending on the choice of the gas medium composition. The absence of He in the gas madium deteriorates but little the energy characteristics of the photoionization CO-laser. Effective excitation of the CO-CO2 laser media at temperatures of up to 145°K has been demonstrated to be in principle feasible.

The CO-laser is generated by the exitation of an CO:N2:(He:02:C3H4) gas mixture cooled to T=100 K, N=0.5 Amaga. The discharge is initiated by a short pulse (t < 1 μs) of a hard UV-radiation. The laser efficiency runs to 12%. The energy consumed by a direct photoionization is 30% of the energy contribution to the volume discharge. The optimization of the photoionization pumping system parameters is examined. The dynamics of the recovery of an optical homogeneity of an active medium after the photoionization and discharge pulses is studied. The methods for improving the energy effectiveness of the photoionization CO-laser are proposed. The generation duration is shown to be adjustable within 2-600 μs, depending on the choice of a gas medium composition. The absence of helium in a gas medium deteriorarates but little the energy characteristics of the photoionization CO-laser.

The performance of combustion-driven CO2 gasdynamic lasers (GDL's) was calculated and compared to experimental data for several gas mixtures produced by the combustion of different fuel/oxidizer combinations. The calculations were performed with a seven-temperature nozzle kinetics model which includes vibrational energy transfer processes involving ten combustion byproduct species. This model was used to analyze experimental results from a number of recent investigations which employed benzene and other liquid hydrocarbon fuels. The effects of large amounts of molecular O2 produced under oxidizer-rich combustion conditions were studied in detail. The calculated results agreed well with small signal gain data obtained for several fuels. The capability to make accurate predictions of the small signal gain for this wide range of gas mixtures and initial conditions makes this model useful in the design of optical resonators for GDL's.

Characteristics of a C6116-02-N2 type combustion-driven CO2 gasdynamic laser, which should be a radiation heating simulator for re-entry of a planetary probe and AOTV, are experimentally and theoretically investigated. Small-signal gain coefficients are obtained for supersonic nozzle expansion ratios of 5, 7.5, 10, 15 and 20. The result shows : 1) qualitative agreement is obtained between the present measurements and the numerical analysis, 2) the small-signal gain grows as the nozzle expansion ratio is increased, and 3) there exists an optimum plenum pressure for small-signal gain coefficient under a certain expansion ratio of supersonic nozzle.

The two-dimensional vibrational non-equilibrium flow field and the nozzle shape effects on CO2 GDL (Gasdynamic Laser) performance are numerically analyzed by a point implicit scheme. It is shown that the Shock Free Minimum Length nozzle (SFML-nozzle) is superior to other nozzles in the viewpoint of GDL characteristics such as the small-signal gain and the maximum power available. Furthermore, the effects of combustion by-products such as O2,CO on the small-signal gain are investigated. It is concluded that such by-products have a negative effect on GDL gain characteristics. The effects of H20 concentration on the small-signal gain are also investigated considering H2O vibrational relaxation. These numerical analyses are performed for the predictions of CO2 GDL.

The report results indicate that the minimum time needed for cw gasdynarnic laser active cavity to adjust itself to a new steady-state defined by sudden change of the boundary gain value and/or of the resonator quality is: (i) in zero order approximation equal to the average time of the excited states residence within the cavity region, (ii) in first order approximation exceedes the mentioned value by several orders depending, moreover, of the sign and type of the perturbation. Within the frame of the summing-up comments a single strongly non-linear partial equation of the third order is formulated. It covers all cases of the non-stationary operation of transverse flow lasres. The problem is treated in the frame of Riigrod-Marlow rate equations description of the active lasers cavities.

A hot gas generator and a multiple nozzle module were employed to simulate the flow of a gas dynamic laser. Interferometric measurements performed with a Fizeau interferometer allow the mapping of density profiles in the cavity region. Density nonuniformities caused by sidewalls and artificial obstructions in the subsonic section of the flow field were observed. Typically, density variations of five to eight percent originated from the shock waves off the sidewall boundary layers. Additional density nonuniformities of about five percent were caused by artificial obstructions arranged directly upstream of the nozzle structure. An explanation is based on heat transfer from the hot gas into the obstruction. To minimize the effect of these medium nonuniformities on optical performance, the obstructions were mounted in a skewed orientation relatively to the optical axis. Interferometric measurements showed excellent cancellation of phase differences across the observation area. Results of numerical resonator calculations including the measured medium inhomogenities show the effect on laser beam quality.

The aim of this paper consists in estimating the influence of the intensity of radiation field enterin back the active cavity of a cw gasdynamic source laser after being reflected by the target surface. It is shown that this back reflected beam increases the source laser efficiency (net output power) and, ny the same token, the thermal load of its cavity mirrors. It changes significantly the range of the source laser transmittivity admissible values. The lower bound of the reflectivity of a cw gasdynamic laser resonator decreases. The upper bound of the same, parameter coincides with its value at which the emitted light energy goes theoretically ad infinitum causing destruction of the source laser. It is suggested that mentioned effects are the result of the hybryde, generator - master oscillator power amplifier, conditions at which the source laser active cavity is operating.

The present paper treats the analytical predictions for characteristics of solar-pumped CO2 mixing gasdynamic laser that is going to be investigated experimentally by solar collector in Tohoku University. As the efficiency of solar collector is high in space, CO2 gasdynamic laser can be the hopeful candidate for space laser because of its high power and possibility of combination with heat engines. Experimental approach is planned by the solar collector of 10m aperture in Research Institute for Scientific Measurements. With this collector in mind, numerical estimation on CO2 mixing gasdynamic laser is conducted. According to data of the collector, the stagnation conditions of solar-thermally heated N9 are estimated. Mass flux of N2 from plenum can be calculated with including the assumea thermal efficiency, and cross sectional area of N2 throat is obtained. Quasi-onedimensional analysis for vibrationally nonequilibrium supersonic flow is conducted. The extracted power is also estimated numerically. Parametric studies are performed on this mixing laser concept, especially for CO2 stagnation conditions, and optimum operation conditions are clarified.

Properties of a folded multipass unstable circular mirrors cavity designed for a gasdynamic laser have been studied by using a code developed for numerical modelling of a GDL performance. The results concerning the steady-state field patterns, gain and inversion population distributions along the GDL channel as well as values of the output power for different optical condigurations are discussed in the paper. The characteristics of the cavities considered are compared with those obtained for a two-oblong-cylindrical mirrors unstable resonator. Due to the high diffraction losses, incre-asing with the number of folding mirrors the output power of the beam obtained for the multipass cavity is considerably lower than that for the two-oblong mirrors resonator. Taking into account technological difficulties connected with manufacturing of oblong cylindrical mirrors appropriate for a large Fresnel number cavity, the multipass configuration seems to be more practicable.

A simple linear mathematical model of gas flow and gas temperature distribution in the excitation channel of cow, gas flow lasers with transverse electrical glow discharge is proposed. We aim to describe, explain and control the temperature distribution in order to minimize the effect of the so called temperature-overheating instability of the discharge. The most essential assumption of the model is: gas flow is one-dimensional and relatively slow. The model is determined by a single partial differential equation for temperature, and relatively simple algebraic equations for the velocity and the density'. Inspection of the model and preliminary numerical calculations show, that, in the channel, there exist two near-electrode maxima of temperature /which is in accordance with experimental observations/. Location and height of these maxima are mainly dependent on a competition between on-electrode cooling and near-electrode heating of the gas. This fact seems to enable us to control the temperature distribution to avoid the temperature-overheating instability of glow discharge in the channel.

The most common method of "pumping" the active medium, in the field of high-power CO2 lasers, is still the self-sustained electric discharge[1-4]. This paper presents the experimental results for an electrode configuration citied in an ealier work [5,6]. The experimental investigations were carried out in the experimental chamber of a high-power fast-flow carbon dioxide laser facility [7] in which the circulation of the gas occurs in a closed loop as described in [8]. The measured V-I characteristics are used to analyse the characteristic properties of the excitation system. These investigations help to illuminate the general quality of the glow discharge and define the region of application of the electrode configuration for exciting the gaseous medium of a CO2 laser. The electric discharge characteristics are tested with a gas mixture of CO2 : N2 : He in the proportions 1 : 2 : 7 at an operating pressure in the range 20 - 80 hPa and a gas flow velocity which is variable from 30 to 100 m/s and the length of the discharge zone is between 10 and 70 mm along the gas flow.

Until now there is no clearness no the nature of instability of a non-self-sustained discharge in gas mixtures, though such a discharge is widely used for lasers pumping. To advance in understanding of reasons in restrictions of limiting energy contributions in a discharge a comparision of numerical simulation and experiment should be performed.

Investigations of the laboratory set-up of a high-power CO2 laser have been carried out to determine the best conditions of performance. The measurement results and sample characteristics are presented and discussed in this paper. Also a brief description of the facility is given. The laser, developed for investigation of beam generation processes and of laser beam interaction with matter is characterized by relative good efficiency of generation of cw radiation power in the range up to 8 kW as well as by its experimental potential that has been confirmed by a number of tests.

A computer code has been assembled to analyze the CS2-O2 flame as an infrared light source. The particular application is to experiments performed at Sanders, Inc., Nashua, New Hampshire, USA in 1987. The code contains calculations of fourteen chemical populations in the flame and eighteen excited states of carbon monoxide. CO* is the vibrationally excited CO produced in the reaction, CS+O=S+CO*. We find that the radiation from the experiments at 30 torr can be approximated by the model when the deactivation, third body term, is minimized. The match between experiment and calculation was somewhat obscured by apparent recirculation of spent gases in the experiment observation chamber. Our conclusion is that the CS2-O2 flame can be used as an infrared light source up to at least a pressure of one atmosphere.

We describe the performance of two lasers with homogeneously saturated gain, coupled through central hole in a common mirror of symmetric unstable resonators. New iterative procedure is proposed based on combining the Fox-Li and Prony methods. The suggested procedure allowed us to calculate several resonator modes and find a stable phase-locked operating region for various sizes of coupling hole and small signal gain values. Far field patterns from combined lasers are computed for various operating regimes.

This paper deals with experimental and analytical investigation on supersonic CO2 mix-ing laser with N2 axial glow discharge. Measurements of small signal gain coefficient and laser power are conducted by the setup of cluster of glass channels for supersonic N2 glow discharge and CO2 conical screen nozzles. The discharge characteristics are also monitored. Supersonic flow of population inversion is maintained for about 10 seconds in the experiments. To estimate the characteristics of our CO2 mixing laser, vibrationally non-equilibrium analysis for steady and quasi-onedimensional supersonic flow is performed, including N2 molecular activation effects by electron impact in glow discharge. As for the results, steady glow discharge in supersonic N2 flow is obtained, and measured small signal gain and power dependencies on flow parameters, and on discharge parameters are discussed in comparison with the results of numerical estimation.

The time behavior of the small-signal gain g0 of TE CO2-laser amplifiers is investigated for different CO2:N2 laser gas mixtures. It is found that two different decay times T1 and T2 for g0 occur. whereas T2 is in agree with known relaxation rates T1 describes a much more faster-decay process immediately after g0 maximum. T1 decreases abrupt with increasing input energy density and represents a new gain limitation. The g0 rise is caused by the vibrational energy transfer from nitrogen. The influence of He on the g0 rise by V-T relaxation of the V2-mode can be neglected.

A flame sheet model is presented which describes continuous wave (cw) HF/DF chemical laser performance and includes the effects of translational and rotational nonequilibrium. The model is then simplified by the realistic assumption that translational and rotational relaxation rates are fast compared with convection, chemical pumping, and collisional deactivation rates. It is also assumed that the homogeneous width Δvh is small compared with the Doppler width Δvd. The resultant system of equations is independent of rotational relaxation. Amplifier and oscillator solutions are discussed. It is concluded that a reasonable first estimate for cw HF/DF chemical laser performance can be obtained by assuming rotational equilibrium and translational nonequilibrium.

A semiconductor preionised pulsed chemical DF laser is operated using SF6 and D2 as fuels. The technique results in uniform, stable and high energy density discharges and provides output energies and peak powers of 40 mJ and 1.2 MW from 20 cc active volume. Discharge measurements are correlated with the output characteristics of the laser to show that the important excitation parameter is the total charge passed by the discharge. A fluorine production rate coefficient is estimated to be 3.67 x 10-9 cm /s and this is shown to be consistent with a dominant role for fluorine production by ionisation of SF6 in the discharge.

Initial performance of an electric-discharge initiated HF/DF chain reaction laser is presented. The discharge is triggered by a short X-ray pulse while a voltage just under the breakdown voltage is applied accross the electrodes. Information is presented on the effects of several parameters on laser performance. Using a 6 F2/2 H2/1 02/91 Ne gas mixture, laser energies up to 13 J/1 were obtained at a pressure of 0.7 bar. This corresponds to an electrical efficiency of 15 % and a chemical efficiency of 4.5 % based on the H2 content.

The high efficiency, high energy operation of an intense electron-beam (e-beam) initiated hydrogen fluoride (HF) chain chemical lasers was theoretically analyzed using a new computer code including both neutral and ionic reactions. The used gas mixture is F2/H2/SF6/02. Both theoretical HF laser output energy and pulsewidth were in good agreement with those of the experiment, respectively. Using this code, we calculated the performance characteristics as a function of e-beam current pulsewidth, H2 partial pressure, initial total gas pressure, and gas temperature. In order to optimize he total gas pressure in terms of laser engineering, we introduced a new parameter which shows the effect of the pressure rise by the exothermic chemical reactions. The optimum total gas pressure of 850 Torr was predicted in our system. Moreover, lowering the initial gas temperature from 300 to 220K both the output energy and the chemical efficiency were found to be improved. In consequence of the experiment based on the theoretical simulation results, a maximum output energy of 5.0kJ(268J/1) was obtained from the low total pressure mixture(282Torr) of F2/H2/SF6/02 = 139/58/45/42 (Torr) with a high electrical efficiency of 284% and a chemical efficiency of 20.3%. Aiming at the achievement of the high chemical efficiency, a maximum chemical efficiency of 29.6% was attained with a 317 Torr mixture of F2/H2/SF6/02 = 167/30/70/50 (Torr). The maximum output energy was 4.9kJ(259J/1), corresponding to an electrical efficiency of 246%.

An efficient, kW-class chemical oxygen iodine laser has been constructed. The highest laser power of 1020 W has been achieved at overall efficiency of 15.6%. The system shows very stable output indicating the possibility of industrial applications. Numerical analysis has been carried out and compared to experimental results.

The effect of water vapor on the operation of a chemical oxygen-iodine laser without a water vapor trap is described. The maximum cw laser power of 87 W was obtained without the water vapor trap at C12 flow rate of 740 mmol/min. An alkaline H202 solution (90 wt% H202, 50 wt% KOH) was cooled down to about -30°C in order to control the saturated H202-H20 vapor pressure less than 100 mTorr. Two porous pipes made of carbon were utilized as a singlet oxygen generator.

The dissociation of molecular iodine by metastable oxygen (O2( 1Δg)) is poorly understood. Knowledge of this process is important to assess the ultimate efficiencies of high-power Chemical Oxygen Iodine Lasers.

The laser technology needs a pulse repetition laser with the controlled pulse length in the range from 1 mcsec to 1 msec and high efficiency in the whole this interval. We demonstrate in this paper that the chemical oxygen - iodine laser allows to attain pulses length in this interval. The calculations show also that under high pressures of active medium ( up to 100 torr ) one can change the time duration of radiation in a wide range at high efficiency of photolysis.

A photolyzed 02(1Δ)-CH3I-N2 mixture was made to lase with an output energy of over 160 mJ per pulse. The utilization efficiency of the stored 02(1Δ) energy attains 12%. The evidence provided by our experiments confirms that the major contribution to the laser energy comes from the energy transferred from 02(1Δ) to the iodine atom. A comparison between 02(1Δ)-CH3I-N2 and 02(1Δ)-CF3I-N2 mixtures suggests that the laser performance is strongly affected by the molecular species of the iodides and the former exceeds the latter in many respects. It is also shown that N2 is as good a buffer gas as Ar in the pulsed chemical oxygen-iodine laser with respect to laser output. The paper also demonstrates for the first time the feasibility of an electrically initiated pulsed oxygen-iodine laser. It is confirmed that the reactions of the oxygen-iodine laser can be efficiently initiated by using low energy electrons. The electrical efficiency is 350 times higher than that obtained with iodide photolysis.

The pressure broadening coefficients of the atomic iodine 52P112 - 52P312 transition are important parameters for the modelling of photolytically pumped iodine lasers. Various measurements have been reported in the literature corresponding to a relatively high pressure range (1 atm.) of interest for photolytically pumped iodine lasers (see Ref. I and references therein). Little has been done at low pressure in particular for 02 and I2 broadening. indeed the pressure broadening coefficients (α of 02 and 12 have been measured only in two works2,3 at high temperature. Following the scaling factor used for extrapolation at room temperature, the results lead to (X values which differ by a factor of two. We report here about the Ar, O2 and I2 measurements of optical broadening coefficients α at low pressure that are of interest for the Chemical Oxygen Iodine Laser. As a matter of fact it has been shown that the hyperfine relaxation rates K and K* between the hyperfine levels of I (2P3/2) and I (2P1/2 ) (see Fig.1) are of importance in determining the active laser cavity width and the total laser power extraction.45 The K and K* rates are directly related to the optical broadening coefficient a. As only the order of magnitude of K and K* have been estimated6'7 the determination of the coefficient (X will serve as a check for these estimates of K and K*.

A mathematical model of the bubbled singlet oxygen generator is presented taking into account a variation in the volumetric fraction of gas in gas-liquid mixture with the distance from the reactor bottom, as well as the effect of water vapour pressure in gas bubbles on singlet oxygen loss. With this more exact model, in comparison with our previous one, a good agreement of experimental and calculated dependences of singlet oxygen yield vs. liquid height in reactor was achieved. This confirmed that the process responsible for major loss of singlet oxygen in the bubbled generator is its dimolar reaction occuring in bubbles within gas-liquid mixture.

We describe the modeling of a pulsed iodine laser that operates by first producing iodine atoms from photodissociation of RI, followed by energy transfer with electronically excited oxygen O2('Δ). 35 rate processes are considered. By using the Runge-Kutta-Gill integral method in computing, the performance of such oxygen-iodine pulsed lasers is obtained under various operating conditions. The results show that this type of pulsed oxygen-iodine chemical lasers possesses high chemical efficiency. Increasing the partial pressure of O2('Δ) is shown to be the key step to improving the performance.

The mechanism of Iodine dissociation by the singlet molecular oxygen O2(1Δg) is poorly understood. For energetic reasons (see Fig.1) it cannot occur following a single collision. Probably various processes are in competition involving two, three or even four steps.

Chemical kinetics and fluid dynamics including diffusion are combined in a model which predicts the behaviour of an iodine jet in a flow of singlet oxygen mixed with water vapor.

Recent advances in the research aimed at achieving short-wavelength chemical laser via detonation of primary explosives are presented. Lead azide, Pb(N3)2, is initiated by a laser pulse and the ensuing chemiluminescence and laser induced fluorescence are monitored. As a result of the detonation, electronically excited N2 is formed and a number of Pb states are preferentially populated by energy transfer from the N2. Most of the chemiluminescence originates in Pb and appears in the time interval 5 - 50 μs following the initiation, depending on the geometry of the detonation vessel. The temporal behavior is explained by a "moving cloud" hydrodynamic model and by a coupled hydrodynamic-kinetic model. Calculations based on the coupled model point out that population inversion between potential laser levels of Pb is feasible. Both experiments and calculations indicate that high concentrations of excited, effectively long-lived Pb states are maintained as a result of radiation-trapping and that these states can be exploited to obtain laser transitions.

A Pulsed Short Wavelength Chemical Laser (SWCL) is essentially a bomb that produces coherent light. Current leading candidates have exothermicities ranging from 68 to 248 kcal/mol. The challenge to the fluid dynamicist is to tame the explosion and produce a uniform optical media. Fluid dynamic analyses and experiments are essential to achieving scalable pulsed SWCL devices. We have originated designs of the three leading SWCL concepts with emphasis on the attendant fluid dynamics. With some additional analysis, we converted the device designs for use as flow simulators operating on the well known detonation of H2/02 which liberates 77 kcal/mol in the gas phase. In this paper we briefly present the analyses used to predict the pulsed SWCL flow behavior and discuss the conceptual designs of the flow simulators of the laboratory scale pulsed laser devices.

The development of lasers operating in the wavelength range 30-200A using laser produced plasmas as lasant has been widely demonstrated. Recombination into hydrogen and lithium-like ions leads to population inversion and gain in expanding and cooling plasmas. The general principles of such devices will be discussed, and experimental demonstration of their operation considered. At present the most flexible device uses small plasmas generated from a restricted target-fibre or stripe. In principle such systems allow scaleability to below the water window at 44A from the present demonstrations at 50-200A. The detailed design of such devices involving target geometry and composition, variation and pump pulse characteristics, will be discussed.

Laser energies of several joules per pulse are obtained in XeCL lasers with X-ray preionized discharge. High average power are accessible with this technique at high repetition rate, but reliability problems occur, especially for the electron gun of the X-ray source. In this work, a secondary emission electron gun is used for the X-ray source. This gun has a potential of more than 106 pulses at a pulse repetition frequency of 1 kHz. The maximum energy is 1.2 j/pulse from a 40 x 4.2 x 2.3 cm3 discharge. The minimum X-ray dose is shown to be nearly 40 mrad/pulse, i.e. more than one order of magnitude larger than in a similar smaller system.

An imaging system with a two dimensional photo diode array was developed for the excimer laser beam diagnostics. It is featured with a fast frame rate, high spacial resolution, and responsibity at the uv wave length. With a simple optical set up for the imager, we measured a near and a far field pattern simultaneously and the spectral band width of the consecutive KrF excimer laser pulses at 250Hz. These measurements shows our imaging system possesses enough spacial resolution for the beam pattern and spectral measurements.

Theoretical investigations on the generation and propagation of density perturbations have been performed using a two-dimensional unsteady finite-difference method. In comparing typical results for CO2 and excimer lasers it becomes evident that both the formation and the physical impacts of the perturbations differ substantially. At equal values of deposited energy density the relative density variation is ore than an order of magnitude higher in the CO2 laser, whereas, due to their different wave lengths the resulting phase shift is about the same in both systems. In the excimer laser the beam quality will be detracted by only the perturbations of the preceeding pulses whereas in the CO2 laser the density change due to the pulse under consideration itself additionally leads to phase distortions.

Experimental study of waves behaviour in a high repetition rate excimer laser has been undertaken. Excitation of active medium in a subsonic loop is achieved by means of a classical discharge, through transfer capacitors. The discharge stability is controlled by a wire ion plasma (w.i.p.) X-rays gun. The strong acoustic waves induced by the active medium excitation are analysed by means of a Michelson interferometer and fast pressure transducers.

X-rays have been used to preionise all three mercury halide lasers (HgX: X=C1, Br, I) for the first time. This paper describes the design of the laser head, the discharge circuitry and the X-ray preionisation source used. The minimum X-ray source strength required to saturate the laser energy dependence on the preionisation electron density has been determined, this corresponding to a dose of ~4mR in a 7Ons FWHM pulse for the optimum mercury bromide laser gas mixture. Typical maximum energies and efficiencies obtained with the three mercury halides in N2/Ne/HgX2 gas mixtures were: HgC12 93mJ (0.27%); HgBr2 100mJ (0.34%); HgI2 - 2mJ (0.08%). The possibility of scaling the mercury chloride and mercury iodide laser energies and efficiencies to levels comparable to the known best results for mercury bromide is discussed.

Chemiluminescences emitted during recombination of ground state Cl atoms were studied in the aim of obtaining a laser emission on the B-X electronic transition of the chlorine molecule. Cl atoms were produced by dissociation of pure C12, in a turbulent electric discharge, at pressures between 10 and 50 Torr. Chemiluminescences were observed for the same pressures downstream from the discharge. The experimental spectra were compared to synthetic ones obtained from a modelling of the chlorine afterglow. A population density of Nv'=0 ≈ 1013 molecules/cm3 was estimated for the v' = 0 level of the B state, for an equilibrium vibrational temperature of 700 K. Assuming a long radiative lifetime of the B state, the calculated optical gain is weak, α ≤ 10-5/cm at λ = 1.1 gm.

The aim of this paper is first the numerical study of the flowfield structure in a DF chemical laser resulting from a high average pressure in the exit plane of the nozzles and second, the analysis of the influence of the diffusion coefficient multiplier model on the evolution of the small signal gain coefficient accross and along the laser cavity. The numerical code is based on the solving of the complete unsteady two-dimensional Navier-Stokes equations in the nozzles and in the laser cavity with a finite difference explicit predictor-corrector scheme and a time splitting technique.

Most electric discharge pumped excimer laser with x-ray preionization use water transmiss-ion line as power supply, in this paper, a lumped low inductance capacitor bank was used for deriving the x-ray preionized XeCl laser to increase the laser output energy density. In case of the water transmission line, the laser pulse was a sharp spike with FWHM of 65 ns and the specific laser energy of 6 Joule / liter, with low inductance capacitor bank, the laser pulse duration was increased to 85 ns with specific laser energy as high as 9 Joule / liter. The superior characteristics of the low inductance capacitor bank to the water transmission line are very effective for repetitive pulse operation with pulse energy of 1.5 Joule at 25 pps. The effects of gas flow velocity and input energy density on the laser performance were studied in detail.

The aim of this work is to describe the unsteady two-dimensional flow in an excimer laser cavity. In order to do this, a numerical model based on a finite difference scheme associated with a corrected transport algorithm (SHASTA model) has been developped to calculate the flowfield in "L.U.X." laser of Institut de Mecanique des Fluides de Marseille. The evolution of longitudinal and transverse waves along the cavity and the damping of density perturbations in the discharge volume have been drawn. The numerical results are compared to the experimental evolution of density at an upstream discharge volume position.

Theory of population inversion, light gain and laser action is presented for the case of combustion of finely dispersed metal particles sprayed in the gaseous oxidiser. A variety of pairs metal-oxidizer is shown to be applicable as active media for IR and visible lasers.

A theoretical analysis is presented of the conditions necessitating the occurence of the partial population inversion on electronic states of diatoraics as a result of the electronic energy transfer from N(A3∑u+). It is shown that the light gain can exceed 10-4 cm-1 in highly diluted gas mixtures under atmospheric pressure at the N2(A) concentration close to 1014 molecules cm-3.

An all solid-state pulser for the excitation of a high repetition-rate TEA CO2 laser is described. The pulser uses a fast switching thyristor, a fast pulse transformer and a two-stage magnetic pulse compression network. The pulser interfaced with the laser has been succesfully operated at 200 Hz with a pulse energy of 7 J, peak voltage of 20 kV, pulse duration 0.6 μs, and an overall efficiency of 70%.

A corona preionization system for a high repetition gas laser is described. The system has been applied to a 200Hz repetition rate TEA CO2 laser with a 1.5x1.5x40cm3 discharge volume. Because the self inductance of the laser head can be kept at a very low value, short excitation pulses and fast rising discharge voltages are possible. Since the preionizer is incorporated in the main circuit and consumes only a small fraction of the total excitation energy, high overall efficiencies can be obtained.

Gas laser preionization techniques using formed ferrite plasma sources (FFPS) are proposed and experimentally demonstrated in two different ways of both a long-distance uniform UV light source and a new type of plasma cathode intended for x-ray generation. The UV source worked well as a preionizer by creating a linear plasma over a 36-cm span along CO2-laser electrodes in an 1:1:8 mixture of 1 atm, and enabled the laser to operate under high energy depositions up to 300 J/l-atm. An x-ray gun diode employing a 6-cm FFPS as a plasma cathode showed high perveance for the small cathode dimension allowing a high anode current of 150 A at 70-kV applied voltage for generating x-rays. The possibilty of the FFPS as new preionization sources for both single-shot and high-repetition-rate operation of excimer and CO2 lasers is discussed.

The auxiliary discharge concept offers potential as a simple means for preionization of large-aperture, repetitively-pulsed gas lasers. The results of small-scale experiments and comprehensive numerical modeling are presented. By dynamically profiling the space-charge density distribution and, thus, the electric field, ignition occurs some distance from the electrodes ensuring discharge stability.

Turbulence of a gas flow in a high-repetition-rated TEA CO2 laser was investigated with an interferogram method. The average laser power increased in proportion to the repetition rate up to 1 kpps. The clearing ratio was smaller than 2 even in the repetitive operation as high as 1 kpps. However, once arc discharge occured, the recovery time became larger by a factor of two than one after a glow discharge. Our observation showed that allowance of the medium density disturbance for stable outputs was less than 1.8%.

Gas optic elements can be thought of as weak lenses which can be tailored to the needs of the FEL and are in fact well suited for them. This paper describes experimental and analytical work on defocusing gas elements which use fluid flows in their operation.

We have previously investigated effects of misalignment in empty unstable strip resonators for various ranges of equivalent Fresnel number, Feq, for a fixed value of magnification, M, of 2.0. When Feq is fixed near N + 3/8 and the misalignment parameter ε is allowed to vary, the power retention ratio PR is quasi-oscillatory and behaves poorly for certain values of ε, including fairly small misalignments such as could readily occur inadvertently. The present work extends misalignment investigations to a variety of values of magnification, including 1.6, 1.8, 2.0, 2.2, and 2.5. The same qualitative conclusions apply for all values of magnification, but the magnitude of the variations in PR as a function of 6 becomes less pronounced as magnification is increased.

The excimer laser - once heralded as the greatest laser invention since the CO2 laser - has had a continuing patnership with the stimulated Raman process since excimers were first made to lase in the mid 70's. These excimers, which operate in the near ultraviolet region, were found to be efficient (12% reported for KrF), volumetrically scalable (output energies of over 5 kJ have been reported) and most amenable to pulsed format (radiative lifetime of a few to 100's of nanoseconds). This means that intense pulses of UV light are reasonably easily generated from even small discharge devices. This makes them ideal pumps for most stimulated Raman processes, which scale on-linearly with pump intensity (W/cm2).

The structure in an initial mixing region of a two-dimensional curved jet along a circular arc is investigated by measurering the mean velocity, the turbulent intensities, the turbulent shear stress, the terms in the turbulent energy equation and terms in the Reynolds stress transport equation. It is found that the profiles of the rms intensity for the axial fluctuating velocity component and the Reynolds stress exhibit an obvious asymmetry due to the influence of the centrifugal force. By using the measured distribution of the mean velocities and the Reynolds stresses, the curvature factor and the eddy viscosity, which is included in the expression given by Sawyer, are estimated along the jet centerline and it is shown that the Sawyer's expression is somewhat inadequate in the initial mixing region of a two-dimensional curved jet. The profiles of the terms in the turbulent energy equation and the Reynolds stress transport equation also exhibit an obvious asymmetry. However, the generation term and the pressure-strain redistribution term in the Reynolds transport equation exhibit an obvious asymmetry only within the region of about a few slot thicknesses downstream of the nozzle exit and the asymmetry of these terms

The development of CO2-lasers tends to high power devices with an average power of ten kilowatt and more. In the same way as laser power is raised, the thermal load on the output coupling window increases. So it is necessary to investigate either materials for high power solid state windows or gasdynamic windows as we did. Preliminary investigations with a supersonic free-vortex aerodynamic window proved both, a good optical quality and fluid-mechanical sealing of a simulated laser cavity. Basing upon these results an experimental prototype was designed. To minimize the flow rate needed for the maintaining of the sealing gas jet, the aperture area is chosen very small, 5mm x 5mm. The outcoupled laser beam is focused through this opening by means of two asphere mirrors. The sealing properties and optical quality were demonstrated by operating the prototype mounted at a 5 kW CO2-laser. One particularity is the active controlling of the gas flow in the aerodynamic widow by adjusting part of the diffuser. So it is possible to vary the achievable cavity pressure and to adapt it to the actual pressure level in the laser.

Phase conjugated waves via degenerate four wave mixing with picosecond laser pulses having a full width at half maximum (FWHM) of 80 ps have been observed in alcali vapours. The dephasing time of the 2P3/2 state in sodium was found to be 7.4 ns. This is the lifetime of the transient volume grating of the atomic states which are involved in the phase conjugated experiment. The average time of the perturbation of the atomic wave functions due to a "hot" collision (from a frequency modulation of a Markov type) was found to be 5x10-14 sec.

Apodized (soft) apertures(AA) of 2-50 mm diameter with the near-Gaussian and the flat top spatial profiles in ultraviolet, visible and infrared regions are presented. They have the high thresholds for intensity damage.

Shaping of CO2 laser beam by Kaleidoscope, in which uniform intensity is obtained by multiple reflections, is analyzed by geometrical and physical optics. Gaussian beam is transformed into square beam spot with nonuniformity less than 2% when the beam reflects 2-3 times. Effects of angular and positional misalignments of the optics with respect to laser beam axis are also discussed. Through the analysis of the beam shaping, a novel optics has been developed, which produces rectangular-Gaussian beam spot with variable aspect ratio, unlike the case of Kaleidoscope where the aspect ratio of the beam spot is unchangeable. This optics provides higher efficiency of beam shaping and higher absorption in metal working by utilizing plane-polarized laser beam. Laser-surface hardening is demonstrated without absorption coating by using this optics.

Light interactions with materials in laser material processing operations occur by a variety of mechanisms depending on the material being processed, the wavelength of the laser light, the gaseous environment, and the physical state of the material surface. The high reflectivity of metals limits the fraction of the beam power absorbed by the solid metal surface. For metals in the solid state, reflectivity increases as the wavelength of the laser light and the electrical conductivity of the metal increase. The reflectivity of metals is reduced upon heating to the melting point, and further reduced upon melting. At high power densities the liquid metal surface is heated so quickly that very rapid vaporization occurs. The recoil force produced by the evaporation causes a depression in the liquid/vapor interface. The "keyhole" resulting from this depression allows for multiple reflections and thus increases beam absorption into the liquid. The vaporized metal above the liquid surface can cause attenuation of the beam. Both plasma absorption and fine particle scattering theory have been proposed to account for this beam attenuation. The attenuation is strongly affected by shielding gas type and pressure. In the laser cutting the beam interacts with the material through a high angle of incidence. Under these conditions the degree of beam polarization becomes important.

Since the early seventies, research scientists and engineers in the Institute of Mechanics, Chinese Academy of Sciences have been working in the field of theoretical and experimental studies on the gas flow lasers and laser-gas flow medium interactions. The primary results in the field of laser-gas flow medium interactions (mostly, theoretical results) obtained in the Institute are surveyed in this overview paper. Only when necessary, some research work of foreign colleagues is mentioned.

This paper is concerned with a basic study of the cavitation induced by a high intensity laser beam. The present work is dealt in a first approach with the model of the initially only one spherical bubble, then followed by exploring the collective effects between several bubbles. The experimental setups producing one or several cavitation bubbles in the liquid (water) are at once described. The experimental apparatus (shadowgraph or Schlieren type) used to visualize bubbles and shock waves at high speed are also presented. Bubbles dynamics in infinite medium and near a solid wall are showed on sequential photographs (framing and streak). Oscillations, shock waves radiated upon collapse and jets developped towards the wall are also displayed. The comparison between experimental dynamics and numerical calculations prove that the evolution of optical cavitation bubbles is in good agreement with classical laws. This study of cavitation is extended to interacting effects when four bubbles are induced independently and simultaneously. The achieved visualizations show such interesting results as strong asymmetries, deformations, attraction or repulsion and premature collapses.

A breakdown generated by laser radiation in a gas contaminated by aerosol particles is known to occur at much lower radiation intensities than in case of pure gases, Laser radiation is heavily attenuated by sparks of plasma formed at breakdowns. Energy loss estimation is important at radiation propagation in the atmosphere and in laser resonators. The breakdown phenomenon may be used in diagnostics of the atmospheric aerosol contamination events. The report presents experimental data on the influence of aerosol size distribution and concentration on optical breakdown generation and other results.

The results of isotopic CO2 - laser radiation absorption measurements carried out under laboratory and field conditions are presented. Water vapour continuum and water vapour, ammonia and sulphur dioxide selective absorption at the 12C1602 , 13C1602, 12C1802 laser wavelengths is studied. An improved model of the water vapour continuum is proposed. Spectroscopic parameters of some H2O , NH3 and SO2 absorption lines are refined. The results of laboratory experiments are compared with laboratory data of other authors and with our field measurement results.

Propagation through the atmosphere of a high power laser beam is limited by gaseous and particulate extinction and above all by optical breakdown induced by aerosols with relatively low intensity thresholds. The plasma created strongly absorbs the incident laser radiation and thus limits the maximum energies transmitted behind the breakdown zone. The object of this paper is to study breakdown thresholds in terms of energy density and incident intensity as well as transmission ratios of a CO2 laser beam as a function of natural atmospheric conditions. The setting up of an automatic station for meteorological and fine granulometric measurements enabled us to know the state of the atmosphere over the first ten meters of the surface boundary layer at each moment during laser shots. Five kinds of characteristic situation from the meteorological point of view with visibility greater than 5 km was studied : A-cold anticyclone conditions with the approach of an atmospheric disturbance 6-humid low pressure conditions C-disturbed conditions : rain and drizzle D-maritime and continental wind conditions E-breaking wave sea-state simulation with criteria of similarity. The analysis was carried out in two steps. The first consisted in correlating the variations in meterological parameters and those of the granulometry of the atmospheric aerosol. The second step linked the granulometric fluctuations to the variations in break-down threshold and transmission ratios. Intensity and fluence thresholds from 70-90 MW/cm2 ; 2-4 J/cm2 up to 150-160 MW/cm2 ; 6,5-7,5 J/cm2 have been measured for these different conditions. Comparing the best transmission ratios obtained in a cleaned atmosphere with the worst in strongly breaking sea conditions, we have obtained for 62 J/cm2, energy transmission of 90 % and 40 % respectively and 25 % to 15 % for 250 J/cm2.

The fundamental process which determines the efficiency of laser materials processing is the absorption of the high power laser beam at the materials surface. Coupling photons into the surface of a metal mainly involves the interaction of photons with the conduction electrons in the surface layer. Excited electrons are then scattered by phonons and lattice defects with the conversion of electronic energy derived from the light flux into heat. Due to the low efficiency of this process metals have,in general, a high reflectivity to optical radiation . At present high power CO2 lasers are used to process various materials although higher than average reflectivity metals such as Cu and Al have proved difficult to treat with CO2 lasers, in addtion anti-reflection coatings are usually required for transformation hardening. Enhanced beam coupling of a Ferranti 400w cw CO2 laser beam has been observed on Cu and Al surfaces via interaction with a plasma generated by a Lumonics Excimer laser. This paper presents the initial results of the experimental investigation into mixed wavelength processing

For mild steel, the light emitted from the oxidation process at the cut front was observed in order to detect the melt flow behaviour. Through a beam splitter within the laser beam guidance the portions of radiation, which passed the nozzle orifice, were measured by a Si-photodiode within the visible and the near infrared range. A comparison to the geometrical structure of the surface shows agreement between the frequency of the sensor signal and that fo the formation of groovings. Therefore, the quality and the structure of roughness of the cut surface could be registrated on-line by frequency analysis of the sensor signal. This behaviour was studied with varying cut parameters.

The topography of a laser cut edge shows a mixture of periodical and stochastical deviations from the ideal surface geometry /1/. Different surface profiles can lead to different functional properties of laser cut workpieces. Therefore they are of interest, especially if further production operations, e.g. forming processes are limited by surface structure.

A DC-discharge CO2 laser oscillator can be alternatively operated under cw conditions or Q-switched by a mechanical chopper wheel. The laser output is fed to a DC-discharged, axially convectively cooled amplifier. After amplification laser pulses of 70 mJ energy, 0.2 μs half width and 250 kW peak power with a repetition rate of 20 kHz and an average power of 1.4 kW with a Gaussian intensity profile are available. Under pure cw operation the cw laser power of the oscillator of 80 W is amplified to 4.2 kW. This laser system is used for material processing like cutting of aluminum and copper. The results of the cutting velocity and quality of the surface of section using either cw or pulsed oscillator-amplifier system are compared. The pulsed laser system allows the treatment of aluminum plates with a higher velocity and a better cutting quality than it is possible with a cw laser at comparable average power. Aluminum plates up to 10 mm thickness can be cut in the pulsed operation mode of the laser with a high quality of the surface of section.

Although welding experiments on light alloys (aluminium or titanium based) have been performed since the beginning of the 1980's, the relative techniques have rarely been applied to production. The major difficulties in laser welding these alloys is only partially due to the differing thermal properties of the materials: another feature of light metals is the high reflectivity to laser radiation of aluminum and the extreme intensity of the plasma generated by laser on titanium. Welding tests were carried out on light alloys under abnormal conditions (controlled atmosphere, special nozzles, non-traditional covering), with various working parameters, have been analysed by means of fractography, Auger spectroscopy, X-rays and electronic and optical microscopes, to check that the quality control specifications of the aerospace industry have been fulfilled.

Laser welding processes require nozzles with a " large " outlet in order both to avoid turbulent gas flow which can cause removal of the melt and to ensure an adequate protection of the melt against atmospheric oxidation. The gas can be supplied in a variety of ways but in this work the coaxial gas-laser beam geometry was used. CO2, He and N2 have been tested as covering gases with flows between 30+100 Nl/min. It has been observed that, once the gas and the steel have been set, the penetration depth and the width of the melted zone are, in the majority of cases, virtually constant when the flow rate increases from 30 to 100 Nl/min.

Some industrial laser applications demand high quality and repeatability levels, and therefore precise control of induced effects. In the case of CW CO2 surface hardening, the microstructural transformation depends on the initial structure and on thermal cycles, and has been widely studied. In order to control these thermal cycles accurately, optimal intensity distributions have been computed with the help of a two dimensional finite element code. The choice of the cost function to be minimized appeared to be quite important, in agreement with theoretical considerations. The cost function is minimized under a set of constraints chosen to describe quenching of a 0.35% weight Carbon low alloyed steel. Numerical results exhibit distributions constituted of several "strips". Such intensity distributions have then been realized and characterized with ETCA beam analysers on the Laser Lab workstation devoted to laser-material interaction studies. A kaleidoscope has been used either for top hat intensity shaping or for interference patterns creation. Both Spectra-Physics 5kW beam and CI1000 after amplification by the 5kW cavity have been used in experiments. Temperature measurements have been performed during treatment both in the samples bulk and on their surface. The microstructure after transformation has been characterized by hardness measurements.

Laser sources can create plasmas at metallic surfaces when radiation fluxes are above 106 W/cm2. These plasmas grow rapidly, absorbing the incident laser radiation and strongly influencing the laser coupling to the surface.

Photolytic processing is especially attractive for semiconductor device fabrications at low temperature. Specially excimer laser chemical vapor deposition are expected as.new method for selective deposition of the semiconductor films. We report here the formation of single crystalline silicon films were grown on the polycrystalline silicon substrate at 450°C substrate temperature using an ArF excimer laser. Laser beams were irradiated simultaneously parallel and perpendicular to the substrate. The focused parallel beam induced two photon decomposition of SiH4 near the surface of a substrate to produce radicals. Amorphous silicon films were deposited by parallel irradiation only, and poly or single crystalline silicon films were grown with the additional perpendicular irradiation. Crystallinity was confirmed by measurements of refraction index, optical band gap, infrared absorption, electron diffraction and X-ray diffraction.

The difficulties encountered in welding two high carbon content materials are well known. If used properly, laser technology may provide new approaches for their solution. Due to persistent difficulty in welding C-43 shafts and C-33 wings, in particular, research was extended to determine the feasibility of these parts being laser welded. The troublespots were soon found to originate in a number of basic phenomena, as illustrated in this paper. Findings so far have been encouraging and, we feel, throw new light on both welding procedure and associated laser welding phenomena.

The use of c.w. laser cutting of mild steel and aluminum alloys was already been investigated. As others authors already pointed out, a converse proportion relation between thickness of material subjected to the cutting and speed of travel of the material itself takes place. In this work, refering to the previously obtained experimental data, we made some remarks on the above mentioned relation, in order to find a parameter which would be able to give useful instructions on the machinability of a fixed material by laser beam. In particular, we examined the effects on the above mentioned relation of three gases (Helium, Nitrogen, Oxygen) for different source power levels and on three kinds of materials (Aluminum alloy, Stainless steel, Mild steel). The law of correlation between thickness and speed was reviewed by some illustrative hypoteses. The obtained results seem to provide useful directions on the possibility to point out a technological parameter which could provide to laser systems consumers some information on optimization of processing conditions, by varying the maximum power of the source, the speed, the thickness, and the kind of material.

Laser welding of stainless steel AISI 304 and AISI 405 is investigated. Laser source with "flat-top" energy distribution is employed which produces welded joints with similar geometry to other conventional welding techniques. Mechanical properties of the welds are however very similar to those typical of deep penetration processes. Microhardness and bend tests as well as micrograph examinations have been carried out on the welds. The test pieces exhibited good mechanical strength and in particular acceptable ductility. Theoretical considerations based on widely accepted thermal models have also confirmed that the process proceeds at high efficiency energy transfer.

In a series of experiments we have investigated the NF2 UV photolysis product distributions, the kinetics of those products, and the reaction of the parent NF2 with atomic hydrogen. Our experiment utilizes a teflon coated photolysis chamber which can be heated (420 K) and in which we flow the reagent tetrafluorohydrazine (N2F4: 10% mixture in Ar). At the photolysis cell temperature, the N2F4 pyrolyzes (98%) to form 2NF2. An excimer laser (248 nm, 193 nm) or a tunable UV laser (frequency up converted Nd-Yag pumped dye laser) is used as the photolysis source. The Nd-Yag pumped dye laser in the visible also serves as a pump in the laser-induced fluorescence (LIF) studies of NF(X3∑). Two PMT's, are simultaneously used to detect the LIF from NF(X3∑) and fluorescence emission from NF(a1Δ). This capability allows us to simultaneously monitor both ground and excited state products following laser photolysis. Two computers are necessary for the experiment. The first monitors the LIF signal as a function of delay time between photolysis and probe laser, both laser energies, and the probe laser wavelength. The second computer is used to acquire data of the fluorescence emission as digitized by a fast transient recorder. We also monitor all gas mass flow meters, the cell temperature, and pressure.

We present in this paper a quantitative analysis of the process of multiphoton ionization and dissociation of benzene molecules in the visible. In particular, we have investigated on the presence of ionized atomic hydrogen in the laser mass spectra of benzene, put forward a possible explanation for the main routes leading to the formation of H+, and estimated the cross section for a number of multiphoton absorption processes and fragmentation rates for different ionic species.