Radial gain profiles in CO2laser discharges

Measurements of the gain of the P(20), 10.6 μ, transition of CO2have been made in a flowing He, N2, CO2amplifier. Both small-signal and saturated gain conditions were investigated as a function of radial position. At low discharge currents, the radial small-signal gain profile followed a J0Bessel function distribution as predicted from the electron density distribution, while at higher currents, the gain was nearly constant across the tube diameter. Further increases in the current produced a lower gain on the tube axis than near the tube wall. This spatial behavior of the small-signal gain with discharge current can be understood following the theoretical models of Gordietz et al. or more recently of Wiegand et al. The theory indicates the small-signal gain behavior is largely explained by the increase of axial gas temperature with discharge current. Gain measurements at signal levels high enough to cause gain saturation indicate the gain is harder to saturate on the tube axis than near the tube wall. An analysis of the experimental data shows that the gain-saturation parameter increases with current density.     

Effects of gas flow on gain of 10.6 micron CO2laser amplifiers

Small-signal gain of flowing gas CO2laser amplifiers at 10.6 microns has been optimized for media including pure CO2CO2: N2, CO2: He, CO2: CO, CO2: O2, CO2: N2: He, CO2: CO : He, and CO2: CO : N2. Optimum gain of all flowing gas systems studied increases monotonically with increasing gas flow rate. In the low CO2flow rate region, 10 < RCO2: < 50 cm³/min, gas flow enhances the gain most for systems containing N2. Results provide strong evidence that the rapid increase in gain with flow rate in CO2: N2mixtures is due to removal by convection of the dissociated product CO. For 50 < RCO2< 200 cm³/min, a slow linear increase in gain of all gas mixtures with increasing flow rate occurs and is attributed to the cooling of gas temprature by convection. A stronger dependence of gainGon amplifier boreD, viz.,G propto I/D, was obtained for flowing gas media relative to that previously observed for nonflowing gas mixtures which is consistent with the proposed mechanism of gas cooling by convection. Highest gain values obtained were 7.8 and 6.2 dB/m with the flowing gas mixtures CO2: N2: He and CO2: CO : He, respectively, in a 12 mm bore water-cooled amplifier tube. Similarities between CO2: N2and CO2: CO systems suggest that pumping of the CO2laser by resonant transfer from CO* (upsilon = 1) can be significant.     

Gain measurement in the CO2laser discharge

In order to examine the CO2laser oscillation mechanism, a measurement was made of the unsaturated gain of CO2laser radiation in an active medium of gas discharge containing CO2, N2, and He. A two-beam optical balance method was used to measure the gain in an amplifier; the accuracy of the measurement was approximately 10 percent. The output of a CO2-N2-He laser was used as the radiation source. The absolute power of the probing beam, which has a diameter of approximately 5 mm, was maintained at approximately 15 mW. Saturation was not observed at probing signal levels up to 80 mW. Amplifier tubes with diameters of 55, 34, 12, and 5 mm were used. The dependence of the amplifier gain on the current density, pressure, composition of the gas mixture, and tube diameter was measured. Comparison was also made of the calculated and measured values for the laser population inversion.     

Whispering-gallery CO2laser

The design and operation of a CW CO2laser based on the whispering-gallery principle is reported. The optical cavity consists of a portion of the equatorial region of a torus terminated by two flat mirrors. In agreement with theoretical expectations, the output beam is found to be TE polarized, and to consist of low-order Airy-Hermite-Gaussian modes.     

Q-switched CO2laser

Experimental studies of a rotating-mirrorQ-switched CO2laser have been undertaken to obtain high peak powers at 10.6-μ wavelength. TheQ-switched pulse has a temporal structure consisting of two intensity peaks and is similar to the temporal behavior of a slowQ-switched CO2laser. Based on the gas pressure dependence it appears that the pulse structure and duration are influenced by collision-induced relaxations of the laser medium. Using an oscillator-amplifier system peak powers of 120 kW with pulsewidths of 200 ns have been obtained.     

Investigation of a Q-switched pulsed discharge CO2laser

AQ-switched pulsed discharge CO2laser is investigated and the dependence of peak power on delay between pulse excitation andQswitching is determined for different gas mixtures. Peak power of 5 kW with 13 percent efficiency is obtained.     

Velocity dependence of the gain of a CO2laser

Measurements of the small signal gain and saturation intensity of a CO2-N2-He laser are made as a function of gas flow velocity over the range fromsim 0-10m/s. The small signal gain increases and the saturation intensity decreases with increased gas velocity. For intermediate flow velocities with a gas transit time in the laser of a second, the gain also depends on the direction of propagation of the amplified beam with respect to the gas flow. The directional dependence is due to an axial gradient in the saturation intensity. The transit time of the gas in the 2.5-m amplifier spans the time required for appreciable generation of CO by dissociation of the CO2and the variation of the laser gain with velocity is attributed to the effects of CO on the inversion of the laser medium.     

Electrical and gain characteristics of a multiatmosphere UV-preionized CO2laser

The scaling parameters of a UV-preionized TE CO2laser which permit the direct comparison of small-signal gains as a function of laser pressure have been investigated in the pressure range of 4-19 atm. Careful measurements of the gain as a function of laser pressure in both the 9.4 and 10.4 μm vibrational bands were made under the appropriate scaling conditions. A theoretical model for the gain incorporating regular, hot band, and sequence band rotational lines, with proper account taken of non-Lorentzian line overlap effects, predicts the observed pressure dependence of the small-signal gain.     

Simultaneous CO and CO2laser

CW laser action at both the 5- and 10.6-μm wavelength regions has been achieved simultaneously from CO and CO2molecules in a slowly flowing mixture of He-air-CO. Two dc discharges through the premixed gases within a segmented laser cavity were used to provide molecular excitation and generate CO2molecules by discharge-initiated CO oxidation. Both liquid nitrogen and room-temperature water cooling were used concurrently. A total power of about 2 W has been achieved with approximately half the power in each wavelength region.     

Folded-path atmospheric-pressure CO2laser

A folded-path transversely excited atmospheric-pressure CO2laser utilizing shower or brush discharges is described. The output pulse has an initial peak 0.4 μs wide followed by a tail2-3 mus long. A peak power of 0.2 MW with 4.4 percent efficiency is obtained. By rotating one of the mirrors of the resonator the tail is eliminated, yielding a pulse 0.2 μs wide of the same peak power.     

Investigations of a UV preionized electrical discharge and CO2laser

Investigations of a moderate-volume high-pressure electrical discharge that employs volumetric photoionization of the gas by ultraviolet radiation are reported. Operation of the discharge in a self-sustained mode at high values of energy input for several microseconds duration has been achieved. Use of the method in a pulsed electrical CO2-laser configuration has yielded an optical energy extraction of 40 J/1 atm at a conversion efficiency of 24 percent. Experiments related to the physics of the photoionization process and parametric behavior of the discharge and laser are described.     

CO2laser excited by a long pulse capacitively coupled discharge

A CO2TEA laser is excited by a preionized discharge through dielectric electrodes with single pulse durations of 100-300 ns (FWHM). For constant input energy, output energy and efficiency are found to increase with pulse duration giving rise to a maximum enhancement of approximately 20 percent over the range tested. An output comparable to metallic electrode devices was obtained. Optimum performance was achieved for a total circuit inductance on the order of 2μH.     

Thin film-coated waveguide CO2laser

A new type of radio frequency (RF) excited waveguide laser with a rectangular cross section is proposed which is composed of two dielectric or lossy materials and two metallic electrodes which are coated by a thin film with small absorptions. Theoretical and experimental analyses show that the maximum output powers of the new waveguide laser are much larger than those of conventional ones.     

Large aperture CO2laser discharges

Large aperture high-pressure gas laser discharges are a prerequisite for the development of high-energy gas lasers of sufficient power for the production of plasmas of thermonuclear interest. Of the several approaches being followed toward the attainment of such discharges, one utilizing weak volumetric preionization of the active gas region produced by UV radiation is described. The use of this technique has resulted in the successful generation of atmospheric-pressure CO2laser discharges between electrodes separated by 30 cm, having total cross sections of ∼600 cm². With input energies of ∼200 J/1 small signal gain values of 4-5 percent cm^-1were measured in 1 : 1 : 3 gas mixtures of CO2, N2, and He, respectively. It is thus concluded that this excitation technique could be incorporated into the fabrication of large volume gas laser amplifiers having beam cross sections in excess of 10³cm²and total output-energy capabilities ofsim 10^{4}J.     

Q-switching of the CO2laser

The laser pulses obtainable from aQ-switched CO2laser are calculated and compared with the results of a number of different techniques of performing theQ-switching. The continuously operating laser is considered first. The transition rates between the molecular vibrational states and their occupations are derived from the measured CW power. The laser tube was 1.9 meters long, had a diameter of 2.4 cm, and used flowing CO2-N2-He gas. For rapidQ-switching, maximum pulses of 4.5 mJ energy and 85 ns half width are predicted. Such pulses were observed with a rotating mirrorQ-switch. However, that technique has a limited pulse repetition rate and experiments on closely spaced pulses are difficult to interpret. A more flexible technique, which allows a much greater variation in the experimental parameters, is the use of a fast shutter to interrupt the laser beam in the cavity. While this switch is somewhat slower than the rotating mirror it does produce pulses of the same energy at repetition rates up to 5000 per second, and smaller pulses at any desired higher rate. From these measurements the upper and lower laser level lifetimes are deduced. They are found to agree well with the values obtained from the CW measurements.     

Mixed isotope multiatmosphere CO2laser

A sealed transversely excited multiatmosphere pulsed laser is reported in which the active media is a combination of three isotopes:¹²C¹⁶O2,¹²C¹⁸O2, and¹²C¹⁶O¹⁸O. Lasing action is obtained on all 12 vibrational-rotational bands with continuous tuning observed between line centers at pressuresfrac{1}{2}tofrac{1}{4}of those required with conventional¹²C¹⁶O2lasers.     

A pin-electrode atmospheric-pressure CO2laser

A 1.2-m-long transversely excited atmospheric-pressure (TEA) CO2laser utilizing shower discharges is described and its operating characteristics as a function of voltage examined up to 60 kV. Pulse energies in excess of 2 J, peak powers in excess of 20 MW, and efficiencies up to 15 percent have been achieved.     

Prediction of CO2laser signatures

Based on previous work, a method is presented to predict the signature of a CO2laser, i.e., the output power as a function of the cavity length. For that purpose the normalized laser intensity curve versus cavity length is determined for a single line. The calculation is based on a homogeneously broadened line (Lorentzian shape) and takes into account the line dispersion. The curve is a parabola and its shape is assumed as being the same for all lines considered. From calculated positions of line centers and small-signal-gain data the signature can be drawn with the help of a parabolic French curve. The transition from one line to the next is assumed to occur at crossings of the normalized intensity curves. The theory agrees well with the experiment.     

A long-pulse high-energy CO2laser pumped by an ultraviolet-sustained electric discharge

A UV-sustained electric-discharge CO2laser has demonstrated optical energy-density extraction and long-pulse performance competitive with currente-beam sustained devices. Atmospheric-pressure CO2-N2-He mixtures seeded with tri-n-propylamine are volumetrically photoionized using the 1200-1700-Å radiation from a spark-array source. Laser pulse lengths up to 37 μs have been achieved with a specific energy of 47 J/1.atm. The best specific energy demonstrated was 60 J/1.atm in a 23-μs pulse.