CO2 Laser Amplifier Research Articles

Simplified UV-preionized long-pulse TEA CO2-laser amplifier

A simplified UV-preionized long-pulse TEA CO2-laser amplifier is proposed, which is powered by a common Marx generator for both the preionization arc discharges and the main laser discharge. The measurements of small-signal gain reveal that there exists an optimum capacitance for the capacitors which are provided in the preionization circuit for the control of preionization level. The physical interpretation for this is also given. A small-signal gain higher than 3%/cm and an extraction energy of nearly 14 J/l (at the input energy level of nearly 1 J) have been achieved. It has also been demonstrated that the amplifier can amplify a long laser pulse of the order of microseconds without serious distortion of pulse waveform.

Picosecond time response of a transverse electric atmospheric CO2 laser amplifier

The gain of an atmospheric pressure CO2 laser amplifier was measured as a function of the input pulse duration for the first time using optical free-induction decay pulses. It was found that the gain decreases rapidly for pulses shorter than 150 ps, in agreement with the gain bandwidth predictions.

Suppression of Internal Parasitic Oscillation in Large Volume CO2 Laser Amplifier

Suppression of Internal Parasitic Oscillation in Large Volume CO2 Laser Amplifier

Pulse propagation properties in a high-power CO2 laser system for laser fusion

The simulation results of nonlinear propagation properties in the CO2 laser system using a simulation model of the SF6 saturable absorbers and the CO2 laser amplifiers agree well with the experimental results. The technical problems of the simultaneous irradiation of multibeams to a target are also discussed.

Ionic and thermal instabilities in e-beam preionized CO2 EDCL devices in the presence of a laser beam

In the present paper a general model of plasma instabilities in high-power, e-beam-sustained CO2 laser amplifiers is presented together with a detailed discussion of some experimental cases. This model improves the Haas-Nighan one by allowing explicitly for the various degrees of vibrational freedom and taking into account the laser field. The above model has been used to test the degree of thermal and ionic instability of an EDCL device using four-component gas mixtures (CO2−N2−He−CO). The stabilizing effect of the laser beam intensity has been assessed, thus finding a dramatic reduction of the growth rate of thermal instabilities.

PLASMA INSTABILITY IN HIGH-POWER CO2 LASER AMPLIFIERS

In the present paper a general model of plasma instabilities m high-power e-beam sustained CO2 laser amplifiers is illustrated. This model improves the Nighan-Haas one by allowing explicitly for the various degrees of vibrational freedom and taking into account the stabilizing effect of the laser field intensity. Two numerical codes, which have been used to calculate the steady-state plasma properties needed to carry out the stability analysis, are also presented. The model has been used to test the degree of thermal and ionic instability of an EDCL device using four-component gas mixture (CO2 - N2 - He-CO).

Investigation of a CO2amplifier emitting microsecond pulses

A computer study was made of the energy and temporal characteristics of a CO2 laser amplifier, pumped by a nonself-sustained high-pressure gas discharge. The control of the shape and duration of the output pulse was investigated in a master-oscillator-amplifier system by varying the delay time of the input pulse relative to the initiation of pumping in the amplifier active medium. The dependence of the saturation energy on the pump energy density was determined and the influence of the excitation parameters and of the input radiation on the energy characteristics was considered.

Experimental Investigation of Heat and Mass Diffusion in a Porous Tube-Generated Flowfield

Flow of laser gas and a saturable absorber gas generated by two adjacent grids of porous tubes provides a fiav$ scheme for windowless gain isolation of large CO2 laser amplifiers. Turbulent mixing at the interface of the two parallel flows causes laser beam degradation, and a reduction in the available active laser volume due to mass diffusion. In the present study, hot wire anemometry, species concentration measurements, and interferometry were used to investigate and to visualize the turbulent diffusion of mass and heat at the interface between the laser and saturable absorber gas flows. The results show that the growth of the mixing zone is linear, a = 0.1, and the spreading of heat and mass are mutually equal. The interferometric data show that for CO2 wavelength radiation (10.6 /*) the turbulent mixing at the interface has little effect on the optical quality of the porous tube-generated flowfield for differences in /3 up to about

Mechanism and rates of rotational relaxation of CO2(001) in He and Ar

Cross sections and rate constants for rotational relaxation of C02(001) in He and Ar are calculated using realistic (electron gas plus van der Waals) potential energy surfaces and 22 000 classical trajectories. Some propensity for small Δj transitions over large Δj transitions is noted, but there is certainly no Δj=±2 selection rule, and transitions with Δj as large as 30 are important in the relaxation kinetics. Calculated rates are compared with experiment and found to be within a factor of 2 for the total rate of relaxation into a depleted level. Relaxation of levels other than the depleted level is also modeled, and it is found that the present method, in contrast to earlier models, gives excellent agreement with experiment. It is suggested that the transitions used in multiline energy extraction from CO2 laser amplifiers be spread out rather than adjacent.

Improvement in energy extraction by a multiline/double-band CO2 laser

For short-pulse energy extraction from a CO2 laser amplifier it is desirable to use multiline/double-band amplification. Multiline/double-band oscillation including up to nine lines was obtained simultaneously from the 9.6- and 10.6-μm bands by placing an etalon perpendicular to the optical axis of a CO2 laser and adjusting its thickness in the oscillator cavity. Improvement in energy extraction was corroborated in the region of saturation amplification using a 1-atm electron-beam-controlled amplifier.

Energy storage and extraction in selected CO2 laser gases

The authors have calculated the optical energy storage and extraction capabilities of a variety of He/N2/CO2 gas mixtures suitable for use in short-pulse (10-9-5*10-7s) electron-beam-preionized CO2 laser amplifiers. For such pulse durations, which are of considerable practical interest in current laser-plasma research, a 3:1:2 gas mixture appears to be particularly useful. The saturating, non-linear, amplifier response is shown to be well characterized by three parameters, which are related to the rotational, intramode and intermolecular relaxation rates. At electrical field strength (E/N) and input energies of approximately 19*10-17 V cm2 and approximately 200 J litre-1 atm-1 respectively, the extractable energy is found to be typically 5-50 J litre-1 atm-1, depending upon the input pulse duration and band width.

Laser gain profiling with uniform-field electrodes

The small-signal gain was measured in a uv-sustained CO2 laser amplifier employing uniform-field electrodes of the type used by Chang. Four different profiles were utilized and the spatial distribution of the gain differed for each profile. For a given electrode shape there was an input energy which produced a region of constant gain. A dip or saturation of the gain occurred at the center of the discharge if the input energy exceeded the optimum value, while a monotonic decrease in gain away from the discharge center was produced below this value. The dip in the gain distribution appears to be caused mainly by thermal heating of the gas. The flatter electrode profiles produced wider uniform gain distributions and less saturation because less heating occurred.

Pulse propagation of CO2 laser amplifiers

Numerical predictions of pulse amplification in atmospheric CO2 laser amplifiers are discussed. A conventional four-temperature kinetic model is first used to establish the kinetic temperatures of relevant CO2 vibrational levels in a pumped amplifier or arbitrary gas composition and pressure. The power amplification of a (single-line) pulse or arbitrary duration propagating through this amplifier is then examined in the simplifying approximation that N2-CO2 relaxation can be neglected. The treatment includes multi-level (rotational and vibrational) energy exchange, and Fermi coupling between the (100) and (020) levels. Coherent effects, of importance for sub-nanosecond propagation, are handled with terms including the dipole-dephasing time tau 2. Asymptotic pulse shapes are observed.

Time-resolved gain of a volume-excited TEA CO2 laser amplifier

An investigation of the time-resolved gain of a volume-excited TEA CO2 laser amplifier is described. The amplitude and decay time of the small-signal gain are obtained as a function of molecular gas flow ratio and input energy density.

Cold-cathode electron-beam-controlled CO2 laser amplifier

A large-area—cold-cathode (10 × 100 cm) electron gun has been constructed and used to control of CO2 laser amplifier. Energy deposition of 226 J/liter in a time of 1 μsec has led to a gain of 4.9% cm in an atmospheric-pressure CO2 laser mixture.

Spatially Resolved Gain Measurements in a CO2 Amplifier

Spatially resolved gain measurements at 10.6 μm have been made transverse to the discharge in an axially flowing CO2 laser amplifier. The composition of the amplifier medium has been varied to include initially pure CO2, binary mixtures of CO2:CO, CO2:He, and CO2:N2, and triple mixtures of CO2:N2:He. Strong spatial variations of gain are observed which depend on the gas mixture, flow rate, temperature, pressure, and current. Corresponding spatial intensity profiles of the sidelight emission of CO2 at 4.3 μm are shown.

INFLUENCE OF DISCHARGE PROPERTIES ON CO2 LASER GAIN

The radial variation of small-signal gain α0 at 10.6 μm was measured in a cylindrical, electrically excited CO2 laser amplifier. The radial-gain profile changed markedly with increasing current. This was interpreted in terms of the dependence of gain on electron density and gas temperature. A modified Schottky analysis of the positive column was used to characterize the discharge properties. Values of electron density and gas temperature thus obtained were combined with the reported dependence of α0 on these parameters permitting calculation of the radial variation of α0. Comparison of calculated and measured values of α0 yielded good quantitative agreement, indicating that the analytical model accurately represents the features of the important physical phenomena which influence small-signal gain. These results emphasize the necessity of maintaining a low gas temperature to achieve maximum laser performance.

UNIFORM ELECTRICAL EXCITATION OF LARGE-VOLUME HIGH-PRESSURE NEAR-SONIC CO2–N2–He FLOWSTREAM

Aerodynamic techniques are described whereby continuous electric discharges may be produced uniformly over large volumes in near-sonic CO2–N2–He flowstreams. Experiment shows that discharge input power scales with pressure for the range 30–150 Torr thereby maintaining a fixed specific power load of ≃ 270 kW/lb(mass)/sec, while diffuse discharges at 1-atm pressure have been achieved at much lower specific power levels. Discharges occupying 4.5- and 43-liter volumes with the corresponding flowrates 2700 and 28 000 cfm are each uniform over their entire volumes, indicating that volume-flowrate scaling is not affected by wall diffusion. Using these discharge techniques, a closed cycle 28 000-cfm 5.6 × 76 × 100-cm3 CO2 laser amplifier channel has been designed to produce in excess of 20 kW laser power, continuous. In preliminary experiments small-signal gains reaching 1.9%/cm have been measured transversely across both cold and hot extremities of this device.

SPATIALLY RESOLVED GAIN MEASUREMENTS IN A FLOWING CO2 AMPLIFIER

Spatially resolved gain measurements at 10.6μ have been made transverse to the discharge in an axially flowing CO2 laser amplifier. These show a nonuniform gain throughout the discharge with maximum gain at the upstream end of the positive column. Increasing the flow velocity shifts the gain profile and leads to an over-all increase in gain. Spatial intensity profiles of the sidelight emission of CO2 at 4.3μ, CO at 0.48 and 2.3 μ, and N2 at 3371 Å are shown.

EFFECTS OF DIFFUSION ON THE SATURATION INTENSITY OF A CO2 LASER

The power and gain capabilities of a CO2 laser are dependent upon the saturation intensity of the laser media. Saturation intensities reported in the literature range from 22 to 100 W/cm2 for seemingly similar laser discharge tube bores, currents, gas flow rates, and gas mixtures. Measurements of saturation intensities between 7.5 and 57 W/cm2 in a CO2 laser amplifier indicate that this parameter is inversely related to the radius of the amplified beam. A significant increase in saturation intensity for small beam radii is attributed to diffusion of excited CO2 molecules into the beam. The experimental results are in qualitative agreement with a simplified derivation of a relation governing the saturation intensity which includes molecular diffusion. This effect can result in a serious overestimate of the capabilities of a large beam laser system designed with saturation intensities obtained from small diameter probe beams.