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.     

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.     

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.     

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.     

Optical propagation and vibrational energy loss in gaseous waveguide lasers

It is observed that the condition for minimum optical attenuation in a dielectric waveguide laser simultaneously maximizes long range vibrational energy transfer from the upper laser level molecules to the waveguide wall. It is suggested that a specifically chosen thin coating on the waveguide wall could increase gain by both decreasing deactivation of the upper laser level and increasing deactivation of the lower laser level. These ideas are applied to the CO2laser.     

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.     

Saturation and excited-state absorption in neodymium laser glass

The existence of excited-state absorption of 1.06-μ radiation in transitions from the upper laser level upward (⁴F_{3/2}-⁴G_{9/2}) is suggested by the ground-state absorption spectrum of Nd³⁺in soda-lime glass. The strength of this absorption was measured as follows. In the unpumped material the upper laser level was populated at high temperatures (600 and 783°K) by saturating the laser transition with an intense 1.06-μ probe laser beam. The residual absorption was attributed to⁴F_{3/2}-⁴G_{9/2}transitions. The steady-state saturation behavior was calculated on the assumption of rapid spectral cross relaxation, using spectroscopic data appropriate to the elevated temperature. Comparison of theoretical and experimental saturation behavior yielded an effective excited-state absorption cross section equal to one-third the gain cross section of the laser transition. This sizable value may be expected to have a significant adverse effect on laser threshold and efficiency. Also, it provides a mechanism for internal fracture such that the fracture threshold decreases with increasing neodymium concentration and with increasing temperature.     

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.     

Collision effect on the saturation character of vibration-rotation transitions for 00°1-10°0 band of CO2

Experimental studies of collision effects on the saturation characteristics of vibration-rotation transitions for00deg1-10deg0 band of CO2is described. Saturation was studied in a passive absorption cell inside the laser resonator. The saturation value could be altered by varying the cell temperature and the pressure of CO2. Vibration-rotation transitions, at pressures greater than or equal to 1 mmHg, were found to be saturated homogeneously, in spite of the fact that the Lorenz width was much less than the Doppler width. This is explained by the high number of collisions during the lifetime in a vibrational state. In this case the spectrum of a single molecule corresponds to that of a Doppler profile. Cross sections for the destruction of levels of00deg1-10deg0by added gases have been obtained, which att = 800degK appeared to be σCO2-He =6 times 10^{-19};sigmaCO2-Ne =2.8 times 10^{-18}; sigmaCO2-CO2=6.6 times 10^{-18}; sigmaCO2-N2=1.2 times 10^{-17}cm²The introduction of sufficiently large absorption caused self-sustained radiation pulsation. When the field influenced the saturating system for only a short period of time, with the interaction time being commensurate with the period of time between collisions, the line was saturated nonhomogeneously. This was expressed by the fact that with the scanning of the laser frequency, a peak power output was observed, corresponding to Lamb's hole, in the center of a saturation line.     

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.     

The influence of CO and CO2laser performance

Impurities are necessary to achieve considerable CW laser action in CO2since in pure CO2the disintegration of the lower laser level is too slow to maintain a substantial population inversion. The dissociation product CO, provided by the discharge itself if initially pure CO2is used in a sealed system, enhances the gain mainly because of its effectiveness in accelerating the relaxation of the first level of the bending mode to which the lower laser level relaxes. The characteristic time for energy exchange between these two levels is20 pm 5 mus at 1 torr, 400°K. Experiments in He-CO2mixtures support the assumption that the main influence of CO is to accelerate the decomposition of the lower laser level.     

A time-resolved study of rotational and vibrational excitation and relaxation in the13CH3F optically pumped far-infrared laser

The rotational and vibrational energy transfer processes of the¹³CH3F optically pumped far-infrared (OPFIR) laser have been studied in a time-resolved experiment. The experiment uses a tunable millimeter and submillimeter spectroscopic technique as a diagnostic probe. Included are observations of the fastDelta Jprocesses that closely connect otherJstates withinK = 3to the directly pumpedJ = 5level, a vibrational swapping mechanism that transfers excitation fromK = 3to otherKstates, vibrational relaxation due to both wall collisions and molecule-molecule collisions, the nonunity probability of vibrational deactivation in a wall collision, and pump saturation and hole burning effects due to the CO2pump laser. All of these observations are accounted for in the context of a numerical simulation. This results in a complete map of all of the collision-induced rotation-vibration transitions of importance to this basic OPFIR system including quantitative cross sections for the relevant processes.     

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.     

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.     

Infrared pulse transmission in SF6-He mixtures

The transmission ofQ-switched CO2laser pulses through SF6-He mixtures is studied experimentally and compared with predictions generated by computer calculations based on a four-state model for SF6. The results show that for saturating fields the factor determining the rate of absorption is the rotational energy transfer process that feeds the levels interacting with the laser. This conclusion has implications for models of passiveQswitching in molecular lasers and infrared-infrared double resonance.     

Influence of collisions on radiative saturation and Lamb dip formation in CO2molecular lasers

Saturated gain profile measurements in CO2at pressures where a Lamb dip appears in the laser intensity reveal that individual rotation-vibration transitions saturate with a strong Doppler component in addition to the Lorentzian holes expected for a Doppler broadened line. This mixed inhomogeneous-homogeneous behavior represents cross-relaxation effects induced by collisions that change the molecule's velocity. A rate equation formulation of collision effects elucidates the role of elastic and rotational thermalizing collisions on saturation and Lamb dip formation in molecular lasers.     

Gain-optimized CO2waveguide laser

The gain of CO2waveguide lasers was measured for a wide range of He:CO2ratios and total gas pressures. It was found that maximum gain occurs at relatively low pressures. To minimize laser discharge length it is advantageous to operate in this high gain, low pressure regime even at the expense of a reduction in saturation intensity which falls off quadratically with gas pressure.     

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.     

The observation of diffusion as an effective vibrational relaxation rate in CO2

A differential absorption signal of 10.6-μ radiation by a CO2cell in the alternating presence and absence of a partially saturating beam on a different transition is observed as a function of increasing pressure of helium. The linear increase with pressure at low pressures is explained as a result of diffusion acting as a strong effective vibration relaxation rate. The behavior as a function over the entire observed pressure range is well described by combining simple collision and diffusion effects.     

Rotational level competition in CO2lasers

Competition effects between rotational levels of the rotation-vibration band of CO2at 10.6 μ have been investigated in both traveling-wave and standing-wave CO2lasers operated in a single mode and single frequency. In a ring laser, Doppler shift and gain proportionality as a function of gas flow can be used to generate a gain anisotropy as a function of frequency so that the ring laser operates as a unidirectional oscillator. Over a narrow frequency interval, two rotational levels can be made to oscillate with oppositely directed traveling waves with an intensity crossover between the two Doppler centers. In this way, a discriminant can be derived that allows frequency stabilization to 5 parts in 10¹²in frequency. In standing-wave lasers of high-frequency stability, the rotational level competition can be observed by synchronous detection of a low-frequency variation of the heterodyne beat frequency signal of two lasers. The competition effects are due to intensity-dependent anomalous dispersion arising from saturation.