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 characteristics of CO2laser amplifiers at 10.6 microns

Single-pass gain at 10.6 microns has been studied parametrically in nonflowing CO2or buffered CO2amplifying media. The gain profile across the amplifier diameter and integrated gain both were determined. Parameters varied included buffer gas type, mixture ratio, gas pressure, amplifier bore, discharge current, and wall temperature. Tube bores of 12, 22, and 34 mm and buffer gases of H2, He, Ne, A, and N2were studied. Optimum gain is relatively independent of current density, but decreases with increasing wall temperature. The pressure-diameter relationshipP_{CO_{2}} cdot D sim 4torr-cm was found to hold for CO2, CO2:He, and CO2:N2amplifying media at optimum gain. The gain depends strongly on the CO2partial pressure and is relatively insensitive to the buffer gas pressure except for the case of H2. The maximum gain decreased slowly with increasing amplifier diameter. The highest gain, 1.7 dB/meter, was achieved with a helium buffer gas in amplifiers with a diameter of 22 mm or less. No gain saturation was detected for a 30-dB range of input signal power, from a milliwatt to a few watts. Spectrograms showed that the principal spontaneous emission from CO2:He amplifiers in the 2000-7000-Å range consisted of CO bands; no CO2bands or He line spectra were observed.     

Continuously tunable optically pumped high-pressure DF → CO2transfer laser

The operational characteristics of a continuously tunable DF → CO2transfer laser optically pumped with radiation from a pulsed DF laser are experimentally and theoretically studied. The pump radiation is absorbed by DF in a high-pressure DF/CO2/He gas mixture, and subsequent V-V energy transfer to the CO2ν3mode provides the CO2laser population inversion. Continuous tuning of the CO2laser frequency between five CO2line centers from 29.14 to 29.30 THz has been demonstrated, using a 12 atm gas mixture. The maximum pulse energy was about 0.8 mJ. In experiments with a two-mirror CO2laser resonator, pulse energies up to 6 mJ and 35 percent slope quantum efficiency have been obtained at 10 atm gas pressure. The gas mixture typically contained 0.5 percent DF, 5 percent CO2, and 94.5 percent He, but this was not critical. Computer simulations based on a rate equation model of the laser have given results which are in reasonable agreement with those obtained experimentally.     

New techniques for determining vibrational temperatures, dissociation, and gain limitations in CW CO2lasers

We have developed an accurate method of determining vibrational temperatures and populations in CO2laser discharges. Our technique involves the use of both the regular 00 ° 1 and sequence 00 ° 2 laser transitions as probes of a CO2laser amplifier. We have been able to separately investigate the quantitative effects of gas heating, dissociation, and ν3mode excitation efficiency on the small-signal gain in typical CW CO2lasers. In general we find that the maximum gain attained in a typical flowing gas CW CO2laser is limited by dissociation of CO2at high discharge currents. To investigate the more fundamental limitations on the gain, we used a short discharge tube with fast flow rates. Contrary to many previous results, we find that thermal effects play a somewhat secondary role in the discharge dynamics, and that the lower laser level populations are small under all discharge conditions. Our results show that the chief limitation on gain in CW CO2lasers is the "saturation" of the ν3mode vibrational temperature T3at high discharge currents. This saturation effect is observed for a wide range of gas mixtures and pressures, and has been studied in detail. Gain coefficients as high as 3.3 percent/cm have been obtained in a conventional 1-cm bore CW discharge tube. We also report preliminary results of an experiment which uses a tunable diode laser to measure gain on a large variety of transitions in a CO2discharge. The diode laser measurements give a striking confirmation of the results described above, and provide the first direct experimental evidence that a Boltzmann distribution exists in the vibrational modes of discharge excited CO2.     

Amplification at 10.6 µ in the negative glow of a hollow-cathode discharge in a CO2-He mixture

An amplifier for 10.6-μ radiation of a CO2laser has been constructed using the negative glow of a hollow-cathode discharge. The single-pass gain of 10 percent per meter reported here from such a discharge in a CO2-He mixture is less than that realizable in the positive column of a glow discharge used for CO2lasers under comparable conditions. The addition of N2, CO, or O2was not found to increase the gain.     

On the effect of molecular chlorine on the CO2laser

Vibrationally excited Cl2was found to strongly decrease the gain in the CO2laser amplifier whereas an increase in gain was produced by vibrationally unexcited Cl2. The results are interpreted in terms of the near resonant V-V energy transfer between Cl2and the ν2mode of CO2.     

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.     

Relaxation of CO2laser levels by collisions with foreign gases

Relaxation time of the 00⁰1 upper and 10⁰0 lower CO2laser levels as a function of H2O, CO, and Xe gas pressure has been measured using the afterglow pulse-gain technique. Lifetime data for these gas mixtures and also for mixtures of CO2, CO2-H2, CO2-He, and CO2-N2, obtained previously, are analyzed and compared with available ultrasonic and fluorescence data. Results indicate that the 10⁰0 and 01¹0 levels of CO2are strongly coupled and depletion of the lower laser level population is essentially limited by the relaxation rate of the 01¹0 level. Other processes involving energy exchange between CO2and foreign gases are detailed.     

Gain and fluorescence characteristics of flowing CO2laser systems

Single-pass gain has been measured for flowing CO2, CO2-N2, CO2-He, CO2-N2-He, and CO2-N2-H2mixes. The gain for CO2-N2mixes varies as d^-0.9, wheredis the tube diameter. The diameter dependence of the gain is less pronounced for CO2- N2-He mixes; a peak gain of 4.7 dB/m was obtained in a 1/2 in diam tube. Fluorescence data indicate that the upper laser level population is saturated at 100 mA in all cases. The addition of He, H2, or O2depopulates the lower laser level; helium further increases the population of the upper laser level. The addition of CO increases the population of the upper laser level, probably by resonant transfer from the excited vibrational states of CO.     

A radiatively pumped CW CO2laser

A proof of principle experiment to demonstrate the physics of a radiatively pumped laser has been carried out. For the first time, a blackbody cavity has optically pumped a CW CO2laser. Results are presented from a series of experiments using mixtures of CO2, He, and Ar in which maximum output power was obtained with a 20 percent CO2- 15 percent He-65 percent AR mixture. The dependence of the output power on the blackbody temperature and the cooling gas flow rate is also discussed. By appropriately varying these parameters, continuous output powers of 8-10 mW have been achieved.     

Waveguide CO2laser gain: Dependence on gas kinetic and discharge properties

Using a simple rate equation approach we examine the gas kinetic and discharge properties of waveguide CO2lasers. We calculate the dependence of the population inversion and laser small-signal gain on gas pressure, gas mixture, pumping rate (discharge current), tube bore diameter, and wall temperature. The results indicate, for example, that at a pressure of 50 torr and a tube-bore diameter of 0.125 cm, the gain is optimized with a gas mixture in the ratio CO2:N2:He of 1:0.75: 1.5. At higher pressures the gain is optimized by using more helium-rich mixtures. We also calculate the dependence of laser tunability on the gas kinetic properties and cavity losses. We find that for low-loss cavities the laser tunability may substantially exceed the molecular full width at half-maximum. Furthermore, the more helium-rich gas mixtures give greater tunability when cavity losses are small, and less tunability when cavity losses are large. The roles of the various gases in the waveguide CO2laser are the same as those in conventional devices. By contrast with conventional lasers, however, the waveguide laser transition is homogeneously broadened. Thus the dependence of gain on gas pressure and other kinetic properties differs substantially from that predicted by scaling results from conventional low-pressure lasers.     

Self-focusing of 10.6-µm radiation in liquid CS2

Self-focusing of 9-kW CO2laser pulses was observed in liquid CS2. The calculated coefficient of nonlinear index of refraction (n2) from experimental data is approximately 10^-17MKS, which is nearly three orders of magnitude larger than the nonlinear index coefficient observed for visible radiation.     

New SMMW laser transitions optically pumped by a tunable CO2waveguide laser

Optical pumping of a submillimeter wave (SMMW) laser with a relatively compact RF-excited CW CO2laser is described. The increased frequency tunability of the waveguide pump laser has resulted in new low threshold SMMW emissions in C2H2F2, CDF3, and CD2F2by pumping into absorption lines which are beyond the tuning range of a conventional CO2laser. Frequency offsets and some assignments obtained with the aid of a tunable diode laser heterodyne spectrometer are reported.     

Efficiency of a high pressure CO2laser pumped CH3F Raman laser

An experimental investigation of the conversion efficiency of a high pressure CO2laser pumped CH3F Raman laser is reported. We show that resonance absorption of the CO2laser radiation in the CH3F gas can lead to a severe limitation of the efficiency. At CO2laser frequencies where the stimulated Raman action is strongest, a quantum efficiency for conversion of CO2laser radiation into far infrared radiation of the order of 0.1 is observed.     

Excited-state absorption of SF6at 936.8 cm-1

An effect that is opposite to saturable absorption in the infrared was observed when a CO2laser beam was passed through SF6. An increase of absorption occurred when the radiant flux of the laser was increased to 10 W/cm². The effect was observed for the P(28) line of the00deg1-10deg0band of the CO2laser, and was attributed to absorption from excited vibrational states of SF6.     

Continuous wave 16 µm CF4laser

A 16 μm CF4laser oscillator has operated continuous wave in a cooled static cell. Pump powers required from the low-pressure continuous wave CO2laser were approximately 3 W. The laser cavity was a multiple-pass off-axis-path two-mirror ring resonator. Unidirectional CF4laser power at 615 cm^-1exceeded 2 mW. Rate equations were used to estimate scaling of this laser source. For modest pump powers (40 W) approximately 1 W of emission power is predicted from this small and simple system.     

Submillimeter laser action of CW optically pumped CD2Cl2, CH2DOH, and CHD2OH

More than 30 laser lines, several of them with an intensity in the order of mW have been obtained in the submillimeter wavelength range from 100 to 900 μm by optically pumping the deuterated molecules of dichloromethane (CD2Cl2) and methanol (CD2HOH, CH2DOH) with a step tunable CO2laser.     

Population inversion and gain measurements for soft X-ray laser development in a magnetically confined plasma column

We present population inversion and gain measurements from an experimental investigation of possibilities to obtain high gain and lasing action in the soft X-ray region. Our approach to soft X-ray laser development is based on rapid plasma cooling after the laser pulse by radiation losses, leading to fast recombination and collisional cascade into upper excited levels of CVI, for example, while the lower excited levels depopulate rapidly by radiative transitions, thus creating population inversions and gain. A ≈ 0.5 kJ CO2laser was focused onto a target of solid carbon or teflon; or CO2, O2, Ne gas, and the resulting plasma confined in a 50-90 kG magnetic field. Spectroscopic diagnostics with absolute intensity calibration were used to measure level populations. Population inversions were observed between the4dand3dlevels in the lithium sequence ions: CIV, OVI, FVII, and NeVIII, and a gain of 0.1 (10 percent) was estimated for the OVI4f-3dtransition at 520 Å. In experiments with a solid carbon target, we observe relatively high CVI 182 Å emission in the axial direction compared to the transverse direction, which, if due to stimulated emission, would correspond to a gain of 4. Extension of these results to potential lasing transitions below 100 Å will be discussed.     

The influence of diffusion upon the amlification of a Gaussian laser beam in a CO2laser amplifier

Gain saturation for filamentary parts of a Gaussian laser beam were measured in a CO2laser amplifier using a pinhole technique. The results show that the detailed growth of a Gaussian beam in the amplifier as well as the overall growth can be described by the well-known homogeneous saturation function. The small-signal gain constant was found to have the same value of 2 dB/m for all radial positions on the beam. The saturation parameter, however, decreased from a maximum Value of 40 W/cm²on the beam axis to 18 W/cm²on the beam wings. The profile of the saturation parameter had an approximately Gaussian shape with a width of 3.9 mm, the same as the width of the laser beam. Diffusion is suggested as the reason for this radial variation of saturation characteristics.     

Photon-induced current changes in a CO2laser amplifier

Experiments have been conducted with a CO2laser amplifier and various mixtures of the active gas media. It has been found that there is a decrease in the current through the amplifier concurrent with transmission of the incident laser beam. A correlation exists with the gain of the amplifier and decrease in current. A possible mechanism is suggested.