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.     

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.     

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.     

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.     

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.     

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.     

Experiment on cross relaxation in CO2

The details of the hole-burning process are studied experimentally and theoretically in a Doppler-broadened gain or absorption line in pure CO2and in CO2-N2mixtures contained in a test cell. The change of absorption or gain of the test cell when irradiated by a saturating laser in aPtransition is probed with another laser operating in one of manyRtransitions. In this way one may determine the rate of velocity cross relaxation within a single rotational-vibrational level and the rate of cross relaxation between different rotational levels of the upper and lower vibrational states. When the probing laser shares a common upper or lower level with the saturating laser, a pip is observed in the differential absorption or gain profile at total pressures of less than 0.5 torr. At higher pressures the relaxation across the velocity profile eliminates the pip. An analysis is presented of the population distribution in the multilevel system of CO2. Three of the relaxation rates (describing the relaxation across the velocity profile of a single level, the relaxation among rotational levels of the vibrational state, and the phenomenological relaxation rate of entry into and departure from the upper and lower vibrational states) are determined. The last of the three rates is found to be dominated by diffusion at the pressures used in the experiment.     

Stimulated rotational Raman scattering in CO2-pumped para-H2

Stimulated rotational Raman scattering in a 300 K multipass cell filled with para-H2with a single-mode CO2pump laser was studied using a single-mode OPO as a probe laser at the Stokes frequency for the So(0) transition. Amplification and pump depletion are examined as a function of incident pump energy. For an incident CO2pump laser energy of 1.5 J, a photon conversion efficiency of 47 percent is observed.     

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 and saturation intensity measurements in a waveguide CO2laser

The small-signal gain and saturation intensity in a waveguide CO2laser are determined by means of a least squares curve-fitting technique. The method allows the simultaneous estimation of the two parameters from an arbitrary number of data points relating power output to the incidence angle of an internal coupling plate.     

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 high-pressure RF-excited CO2waveguide laser

The design and frequency tuning characteristics of a pulsed RF-excited CO2waveguide laser operating at 10 atm are reported. Continuous tunability has been obtained over a 300 GHz wide spectral range from theR(12)to theR(26)line in the 10.4 μm band. The laser output has good temporal and spectral characteristics.     

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.     

External cavity CO2-pumped InSb spin-flip laser

A pulsed CO2-pumped external cavity spin-flip laser has been operated with the output frequency governed to a large extent by the Fabry-Perot (FP) modes of resonant output couplers. An output spectral width of 0.027 ± 0.003 cm^-1has been attained.     

Review of CW high-power CO2lasers

Various forms of CO2lasers have achieved CW powers in the 60-kW range, operating efficiencies approaching 30 percent, pulse energies of approximately 2000 J, pulsewidths less than 1 ns, peak pulse powers in excess of 10⁹W, a frequency stability of a few parts in 10¹², and sealed-off tube lifetimes of many thousands of hours. In addition, the laser can be easily Q-switched as well as gain-switched and has been electrically, optically, gas-dynamically, and chemically pumped. In addition to all these attributes, the CO2laser output wavelength lies within one of the best atmospheric windows. It should be no surprise then that during the last eight years, the CO2laser has firmly established itself as a candidate for recognition as the most important among the numerous laser devices presently known. Depending on the gas pressure, gas flow rate, pumping mechanisms, gas mixture, etc., CO2lasers can exhibit a wide range of noise, bandwidth, gain, and power saturation characteristics. This flexibility enables a designer to optimize the performance of CO2laser stable-frequency master oscillators; power oscillators; low-noise high-gain preamplifiers; intermediate-power or high-power amplifiers. As a result, CO2laser oscillator-amplifier chains can be designed utilizing guidelines similar to those which have been extensively applied in the design of transmitters in the RF and microwave region of the electromagnetic spectrum.     

Narrow-band CO2-TEA laser for efficient FIR laser pumping

To reduce the linewidth mismatch between the emission lines of a CO2-TEA laser and the absorption lines of a low-pressure FIR gain medium a tiltable Ge etalon has been introduced into the TEA resonator. The resulting spectral compression and tunability increase the energy conversion efficiency in a TEA laser pumped CH3OH laser by as much as a factor of 37 and has led to the observation of 14 new FIR laser lines.     

Gain of high-pressure CO2lasers

Gain formulas are shown for the 10.4- and 9.4-μm CO2bands for the high-pressure broadening case to point out a new regime of operation for this laser. Computer-simulated results of this gain are presented as a function of wavenumber, with density equivalent pressure and population inversion ratio as parameters. The results show gain curves that suggest continuously tunable lasers over the entire 9.4- and 10.4-μm bands with no rotational structure.     

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.     

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.     

Gas dynamic CO2-He-(N2) laser investigations

Experiments on IR laser action and gain measurements are reported for a CO2-containing gas mixture flow that cools as a result of expansion. The mixture is preheated by a reflected shock wave, and the gas is expanded into a vacuum through a slit. Laser action is obtained in a CO2-He mixture, while the addition of nitrogen increases the gain. The gain was found to depend on the distance from the slit and the gas pressure before the slit.