Comparison of computer simulation and experimental results for a 16 µm HBr/CO2laser

An improved version of a computer model simulates 16μm laser output from an optically pumped HBr/CO2/Ar laser cavity. A rate equation approach is used to examine the time history of vibrational and rotational excitation and subsequent lasing from the HBr/ CO2gas mixture. Rotational nonequilibdum phenomena in HBr and CO2are included. The effect of bleaching a particular vibrational-rotational transition with optical saturation is modeled in detail. The results of the computer simulation are compared to the laboratory observations from two separate experiments. The model predicts accurately the effect of changing partial pressures of the constituent gases on 16μm power and energy. The model reveals the important kinetic mechanisms yielding these trends. Finally, the model is modified to simulate optical pumping by an HF laser of an HF/CO2/Ar gas mixture. A case by case comparison with the results of the HBr model prediction show significantly lower 9.4 μm powers and energies for any given HF pressure and no evidence of 16μm laser output from the HF/CO2gas mixture.     

4B2--Pulsed and steady-state infrared emission studies of CO2laser systems

Transient and steady-state infrared emission at 2.5 to 15 μ from low-lying vibrational-rotational levels of CO2has been studied using a dc discharge. The time-dependent behaviors of several levels important for laser excitation and relaxation were examined following a pulsed discharge. The decays were, in general, different and were not simple exponentials. Relaxation of vibrational energy of CO2appears to occur by collisions involving vibration-vibration exchange between different vibrational modes and vibration-translation relaxation of the bending mode. The rate of transfer of vibrational excitation from N2to various vibrational modes of CO2was investigated as a function of CO2pressure. The addition of He reduced the emission from the lower laser levels with respect to that from the upper laser level and increased the nonradiative decay rate of the lower vibrational levels by CO2-He collisions. Under pulsed excitation, in addition to CO2laser action nearly coincident with the pulse, a weaker, delayed output was 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.     

Theoretical and experimental studies of a multiline TEA CO2laser with hot CO2as an intracavity absorber

The multiline behavior of a ballast resistance helical TEA CO2laser incorporating hot CO2as an intracavity absorber has been studied. Simultaneous laser oscillation onP(16), P(18), andP(20)lines of 10.6 μm has been obtained reliably. A rate equation model has been developed for the hot CO2absorber and is incorporated with the model for the multiline TEA CO2laser for calculating laser intensities onP(16), P(18), P(20), andP(22)lines of the 10.6 μm band in the presence of the absorber. The theoretical calculations agree quite well with the experimental observations. These studies show that a hot CO2column of Proper length and temperature inside a TEA CO2laser cavity can produce simultaneous laser oscillation on at least three rotational linesP(16), P(18), andP(20)of the 10.6 μm band with almost equal intensity.     

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.     

Radio frequency pumped mid-infrared waveguide lasers

RF discharge waveguide laser technology has been extended from the 10.6 μm CO2laser to include 2.7 μm HF, 3.8 μm DF, and several rare gas mid-infrared lasers. The maximum achieved electrical efficiencies of 5.3 and 4.0 percent were demonstrated in pulsed HF and DF systems, respectively. These, as well as several low efficiency rare gas lasers were demonstrated in a 20 cm gain length device. The output power and spectral distribution were determined as a function of the gas composition, pressure, velocity, and the RF power, pulse length, and repetition rate.     

Infared holographic optical elements with applications to laser material processing

Reflective holographic elements replayed at the 10.6 μm wavelength of the CO2laser can generate wavefronts that cannot be obtained using conventional optical elements. One proposed application is to use the projected holographic real image for laser materials processing, such as cutting or soldering. Three methods are discussed for producing the required holograms: computer generation, direct recording at 10.6 μm, and indirect recording in the visible. Results are presented on image intensity roll-off and aberrations introduced in image reconstruction.     

Initiation of a pulsed HF laser with a CO2laser-produced plasma

A CO2laser-produced plasma was used to initiate an HF chemical laser. It was found that fluorine atoms produced by the thermal dissociation of CF4in the hot plasma rapidly diffused from the focal volume and chemically reacted with the surrounding hydrogen gas to produce excited HF molecules. Output powers greater than 500 W at 2.79 μm and small-signal gains greater than 6 percent/cm at 2.73 μm were observed. This method of chemical laser initiation should be compatible with a wide variety of reaction systems.     

RF-pumped infrared laser using transverse gas flow

A transverse gas flow configuration has been developed utilizing RF discharge waveguide technology for several infrared lasers. Two potential applications have been identified: pulsed chemical laser and CW CO2laser. In the 3.8 μm DF laser, the flowing gas device provides rapid gas replenishment to maintain high electrical efficiency at high repetition rates. An average power of 0.6 W was achieved at 1 kHz. An order of magnitude power improvement can potentially be developed in a closed cycle system. In the CW CO2laser, the flowing gas provides efficient cooling so that high output power per unit gain length can be achieved. A 16 W output in a 20 cm gain length device, corresponding to a record 0.8 W/cm output has been demonstrated. This system can be developed into a 20-60 W laser with a 20-50 cm gain length.     

Vibrational energy transfer in CO2under laser conditions with and without water vapor

It is shown that in the case of a dry 1.5-torr CO2gas fill the upper laser level is indirectly excited by vibrationally excited CO produced during the discharge, whereas in the case of a 1.5-torr CO2and 0.2-torr H2O mixture the upper laser level is directly excited by the electrons in the discharge. The collision relaxation times measured under laser conditions for the symmetric valence vibration of CO2in a CO2-H2O mixture and in a CO2-CO mixture as produced during a discharge of an initially pure CO2fill were 19 and 73 μs, respectively. If the reasonable assumption was taken that half of the CO2was dissociated into CO then this result shows that H2O was 14 times more effective in depopulating the lower laser level than CO. The growth in laser intensity for the dry fill was shown to be due to the CO (nu = 1) transfer of energy to the asymmetric vibration of CO2(00°1) with a characteristic increase that was exponential strictly only for a time short compared with the relaxation time of the symmetric vibration. The characteristic transfer time for excitation of the asymmetric vibration was dependent upon the fraction of CO present. If we make the assumption of 50 percent dissociation, the intermolecular energy transfer time between CO and CO2was found to be 40 torr-μs. Results obtained with N2and He added to the laser mixture indicated that He was not more effective in relaxing the lower laser level than N2or CO and was less effective than H2O.     

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.     

Two photon induced optical bistability in CdHgTe at room temperature

We report the first observation of quasi-steady state optical bistability in a 360 μm thick, uncoated Cd0.23Hg0.77Te etalon at room temperature as a result of the generation of free carriers through two photon absorption of pulsed 10.6 μm radiation from a hybrid TEA CO2laser (FWHM = 1.75 μs). The experimental results are compared with the predictions of a computer model.     

Compact, high-power FIR NH3laser pumped in a CO2laser cavity

A compact CO2-NH3FIR laser system, with an NH3laser cavity inserted in the pump CO2laser cavity, was designed. Temporally smooth and reproducible single mode NH3laser pulses with an energy of 5 mJ (20 kW peak power) were obtained at the 152 μm line when optically pumped by the injection-locked single-mode CO2laser with a pulse energy of 0.3 J.     

J-dependence of rotational relaxation in the CO200°1 vibrational level

Observations are presented on theJ-dependence of rotational relaxation in the CO200°1 vibrational level for mixtures of CO2, He, and N2. The experiment consists of injecting a saturating ∼2-ns pulse at theP(20)line in the 10.4-μ CO2band into a low-pressure CO2-laser amplifier while simultaneously monitoring the transient gain response of an overlapping weak probe beam in the 9.4-μ band restricted to operate on any of the transitionsP(10)-P(34). The data show that the decay times of the variousJstates in the CO200°1 level get progressively longer for increasing or decreasingJvalue centered about the perturbedJ = 19state. Such behavior may be expected to have a significant effect on the efficiency of energy extraction and pulse shapes in CO2amplifiers for nanosecond and, especially, subnanosecond laser pulses. An analysis using a coupled set of rate equations to describe the rotational level populations is presented in which consideration is given only tomid DeltaJ mid = 2changes in collision. The analysis, when compared with the data, indicates that collisions in whichmid DeltaJ midchanges by more than two units must also be considered.     

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.     

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.     

Laser surgery: CO2or HF

Laser surgery may be improved by modifications based on similar processes in industrial applications. A major problem in materials processing is minimizing heat diffusion from the site of laser exposure. The same problem exists in the surgery of tissue with a CO2laser. A model based on vitreous surgery is described which indicates that radiation at 2.9 μm (HF laser) in short duration pulses, shorter than the thermal relaxation time (1.7 μs) of its 1μm thick absorption depth in water, will minimize thermal diffusion, and also take advantage of the large amount of heat removed by the phase change of water into steam. This model suggests that, for deep cuts, many pulses are preferable to a single long duration exposure, and that more delicate surgery may be possible with such short pulse, shallow absorption depth types of energy delivery. For coagulation (hemostasis control) as well as ablation, two simultaneous wavelengths are required: 2.9μm for cutting, and another with less absorption (such as Nd: YAG or argon lasers) for more penetration and heating of deeper layers and blood vessels. Among the secondary benefits of the use of the HF laser at 2.9μm is the ready availability of flexible optical fibers for a delivery system.     

Stark cell noise reduction of a CO2laser beam

A technique is demonstrated for reducing the rapid intensity fluctuations in the beam of a single frequency, polarized CO2laser tuned to theP(20)transition at 10.6 μm. The method uses a Stark-tunable absorption line in NH2D to compensate for the fluctuations of the laser output. Substantial reductions in laser beam intensity noise have been obtained. The method may be applied to other lines of the CO2laser and indeed to other types of lasers.     

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.