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.     

10.6-µm amplification in metallic waveguide by external discharge pumping

Amplification of 10.6-μm radiation in a metallic waveguide and waveguide CO2-laser action was achieved by excitation of the CO2(00⁰1) mode by vibrational energy transfer from metastableNmin{2}max{ast}(upsilon = 1)molecules. Excitation of N2was accomplished in a separate dc discharge tube. The N2-He mixture, after flowing through the discharge region, was pumped into the waveguide and there CO2was added. Maximum small-signal gain values of 25.6 and 15.3 dB/m were obtained at amplifying waveguide sections of 2 and 6-cm length, respectively. A theoretical analysis, based on rate equations for the (00⁰1) and the (10⁰0) states of CO2and the concentration ofNmin{2}max{ast}(upsilon = 1)molecules, is presented, which leads to predictions for the small-signal gain and the saturation intensity. In the pressure range covered by experiments the calculated gain values were found to be consistent with measurements.     

Gain characteristics of a MAGPIE coaxial CO2laser system

Gain coefficient measurements of a MAGPIE (magnetically stabilized, photoinitiated, impulse-enhanced, electrically excited) coaxial CO2laser discharge are presented. The effects of gas composition, input power, pulser ionization, and magnetic field on gain are examined. Measurements of the radial gain profile and saturation intensity are also discussed. A maximum small-signal gain of 0.30 m^-1is observed, along with a saturation intensity of 190 W/cm².     

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.     

A high-voltage hollow-cathode Au-II 282-nm laser

Laser operation on the Au-II 282.3-nm line is obtained from a high-voltage, segmented hollow-cathode discharge tube with external mirrors. Measurements of the laser output power and the small-signal gain demonstrate that, for a given total discharge current, the optimum performance of the laser is obtained for a discharge length for which the linear current density is approximately 65 mA cm^-1. The gain is approximately 2546 m^-1 at this current density and increases with current density up to the highest value used (250 mA cm ^-1), at which the gain exceeds 50% m^-1. The threshold current decreases monotonically, with decreasing length, the lowest observed value being 6.28 A for a length of 5 cm. A quasi-CW output power of 20 mW is obtained for a gain length of 20 cm and a discharge current of 3.2 A in a partially optimized tube with unoptimized output coupling     

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.     

Effect of inductance on the small-signal gain of a transverse-excitation atmospheric-pressure discharge in carbon dioxide

An external inductance was placed in the circuit of a resistive transverse-excitation atmospheric discharge to observe its effect on the small-signal gain of a CO2-He-N2gas mix. The results of the measurements indicate that by lowering the inductance of the circuit, less helium is required in the gas in order to obtain a uniform discharge; consequently, more CO2and N2may be used with higher gains resulting.     

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.     

Gain characteristics of an atmospheric sealed CW CO2laser

Experimental and analytical investigations have been made on unsaturated gain g0of a CO2electric-discharge convection laser, in which discharge current flow, gas flow, and the optical axis are mutually perpendicular. Stable glow discharges in sealed gas mixtures of CO2, CO, N2, and He were maintained at pressures up to 780 torr with an input power density of about 90 W/cm³. The ratio of electric field to neutral particle densityE/Nwas1.7 times 10^{-16}V . cm²and was independent of the total gas pressureP. The electron density in a positive column of the glow discharge was about4 times 10^{10}cm^-3. Detailed spatial distributions of g0at a wavelength near 10.6 μm were measured in the pressure range from 100 to 780 torr. Measurements were also made on the current dependence of g0and on the change in gowith discharge time. The g0distributions along the gas flow direction were found to agree with those calculated from the electron density distribution and the relaxation rate constant of the upper laser level on the basis of continuity equations for a two-level model. The integrated value of g0along the flow direction was proportional to P^-0.8whenE/N, electron density, and gas temperature were held constant. A maximum value of the g0distribution, which was proportional to P^-0.3, was 0.14 percent/cm at 780 torr.     

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.     

Gain limitations in TE CO2laser amplifiers

The factors which limit the small-signal gain of TE CO2laser amplifiers are investigated with a novel technique based on gain measurements of the sequence, hot, and regular CO2laser bands. This new technique enables us, for the first time, to determine accurately and independently the rotational and vibrational temperatures characterizing the CO2laser system. The gain ratio of the sequence band (00° 2) to the regular band (00° 1) provides a simple and accurate determination of the ν3mode vibrational temperature. It is found experimentally that the ν3mode vibrational temperature saturates at a high input discharge energy. This saturation sets an upper limitation to the gain attainable in TE CO2laser amplifiers. As we can measure all the characteristic temperatures relevant to the gain medium, a detailed comparison between the calculated and experimental gain can be carried out with no variable parameters. The result of such a direct comparison confirms both the validity of the conventional "mode temperature" model for CO2laser dynamics and the validity of our measurement technique for vibrational temperatures.     

Small-signal gain and time-resolved spectroscopy of the D2-F2/CO2pulsed chemical transfer laser system

Measurements made of the small-signal gain and time-resolved spectral output of a flash-initiated D2-F2/CO2chemical transfer laser system are reported. Small-signal gain measurements indicate a possible lack of rotational equilibration among the rotational levels of the CO2during the DF-CO2V-V energy transfer process. Time-resolved spectral output of this system, operated as a laser oscillator, is presented as verification of the small-signal gain results.     

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.     

A compact cylindrical CO2TEA laser

A cylindrical discharge scheme which utilizes a fine wire as the central electrode is investigated for compact CO2TEA laser applications. It is found that the strong radial dependence of electrical energy deposition into the plasma, a consequence of the large ratio of outer to inner electrode radii, produces a radial gain profile which is well localized in the center of the discharge tube. This allows for efficient matching of low-order stable optical modes to the active gain region while providing a high degree of mode selectivity in favor of these modes. Oscillation in the TEM01Laguerre-Gaussian mode has been obtained at electrical efficiencies approaching 4 percent.     

Small-signal gain of a CO2laser amplifier utilizing a white optical reflector design

A small-signal gain of 39 dB in a nonresonant multi-path CO2laser amplifier with a discharge length of 65 cm is reported. The multipath system employed is a modified White optical reflector design. An unsaturated gain of 39 dB was observed for signals smaller than 10^-5watt. No noise component, due to the amplifier, was detected for signals as small as5 times 10^{-8}watt.     

An electro-aerodynamic CO2scan laser

An electro-aerodynamic CO2scan laser with a vanadium dioxide control reflector has been demonstrated. Gas flow, electrical discharge, and optical path are coaxial. A gain coefficient of 0.015 cm^-1has been achieved in a tapered discharge tube with 3.175 cm limiting aperture. Scanning speed near 10⁵directions per second can be utilized over3.7 times 10_{4}resolvable directions in space. Scanning output power up to 9 W was shown.     

Effects of time-delayed amplification in TEA CO2lasers

A novel technique is reported for studying the behavior of the time-dependent gain in a TEA CO2amplifier. The method involves the incorporation of an additional amplifier tube into a laser cavity already containing a laser gain tube. The two tubes are independently operated, but so arranged that they can be fired with a controllable time delay between the discharge current pulses. This system permits effects of the additional gain tube on the lasing properties to be investigated as the time delay is varied. In particular, there is a time delay between the discharge current pulse of the laser tube and the onset of lasing. The variations in this delay produced by the firing of the additional amplifier tube have been investigated. The observed time-delay changes can be related to a simple theory for the time-dependent gain. The analysis of the measurements can be used to determine parameters describing the time-dependent gain. This method has been used to measure decay times of the gain for various gas mixtures. The techniques reported here can also be used to study other time-dependent effects within laser amplifiers.     

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.     

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.     

Accurate measurements of pressure-broadened linewidths in a transversely excited CO2discharge

A novel technique is used to measure He-dominated pressure-broadening coefficients in the 10μm band of CO2. Gain measurements in a pulsed transversely excited CO2discharge are made at line center using a CW CO2laser as a probe, and at a known offset frequency using a CW N2O laser. By measuring this gain ratio as a function of discharge pressure, we determine the linewidth with an accuracy of ∼ 2 percent. Linewidths are measured for nine different transitions in the 10 μmP-branch, and theJ-dependence of the CO2-He pressure-broadening coefficient is determined and compared to theory. In addition, we examine the temperature dependence of the linewidth under conditions of constant number density and find that the linewidth increases asT^{0.42 pm 0.06}. This agrees well with a recent theoretical prediction of T^0.38(R. T. Pack, "Pressure broadening of the dipole and Raman lines of CO2by He and Ar. Temperature dependence," J. Chem. Phys., vol. 70, pp. 3424-3433, 1979). To our knowledge, these experiments represent the first direct linewidth measurements in a transversely excited CO2discharge.