A sealed high-repetition-rate TEA CO2laser

A compact atmospheric pressure CO2laser utilizing a double-discharge technique has been constructed and operated at repetition rates to 100 pulses/s. With the addition of small amounts of hydrogen and carbon monoxide to give a gas mixture of He:N2: CO2:CO:H2= 69.3:11:15:4:0.7, sealed operational lifetimes exceeding2 times 10^{6}pulses have been obtained. Operating in this mode, the output energy density is about 8-9 J/l at repetition frequencies of 30-40 pulses/s for input energy densities of 60-70 J/l. The operation of the sealed laser has been studied by means of mass spectroscopic measurements of the gas mixture. It has been determined that sealed operation is possible as long as the oxygen concentration is kept below 1-2 percent. It has also been found that the addition of small amounts of H2and CO will keep the oxygen concentration below 2 percent by reducing the CO2decomposition, allowing sealed operation. The experimental results are compared to the predictions of a theoretical model in which neutral and negative-ion processes have been included. The calculations indicate that when small amounts of oxygen or water are present in the discharge the negative-ion population is significantly increased and the ratio of negative-ions to electronsN_{n}/N_{e}can approach values near unity. These are the conditions under which discharge arcing was found to occur. The model also predicts that the dissociation equilibrium of the CO2can be controlled by the addition of the above concentrations of hydrogen and 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.     

ReactiveQswitching in a CO2multimode-multipass laser oscillator

Experiments with a CO2-N2-He multipass laser having a movable-end mirror are described. At constant mirror velocities ranging from 0.5 cm/s to 40.0 cm/s, pulsing was observed with repetition rates between 1-60 kHz and with apparent peak-pulse amplitudes of 10 times the CW output. Measurements were made for various cavity optical path lengths.     

Gain and relaxation studies in transversely excited HF lasers

The results of measurements of the spatial and temporal dependence of the gain of a pulsed HF laser are reported and are compared with those for CO2lasers. While the propagation in space of a locally excited gain medium is observed to be comparable for the two lasers, the time developments are vastly different. The extremely fast gain decay rates that were observed in the HF laser are shown to be a serious detriment to efficient high-power CW or long-pulse operation. Short pulse HF lasers, however, can be operated relatively efficiently at high energies. This was demonstrated by obtaining 350-mJ pulses from a 75-cm "pin" laser excited by 50-ns pulses from a Marx bank.     

Pulsed CO2laser with double modulation

For the pulse pumped CO2laser we used Q-switching to investigate inversion kinetics. The pump was synchronized with the Q-switching mirror. It was possible to obtain the Q-switch pulses at any phase of the pump pulse. The repetition rate was 50 Hz and the pump pulse duration was 4 or 10 ms. It was found that there is an optimum delay in switching the cavity Q-factor after the discharge started. The giant pulse intensity increased several times after the discharge was switched from CW to pulses. The inversion rise time was 1-2 ms and its lifetime was strongly dependent on the discharge current, due to plasma heating during the current pulse. Increasing the helium percentage in the discharge prolonged the inversion lifetime because of high thermal conductivity of helium gas. Gaseous BCl3was chosen to obtain the CO2laser giant pulses with bleachable filter. The vibronic frequency V3of the B¹¹Cl3molecules coincides with thePlines of the CO2laser. Rarer B¹⁰Cl3corresponds to less activeRlines. Pure BCl3did not give the giant pulses. Adding helium to the BCl3cell gave the pulses. The best results came from adding very small amounts of ammonia NH3because of the effective vibrational energy transfer between colliding BCl3and NH3molecules.     

Far infrared frequency synthesis with stabilized CO2lasers: Accurate measurements of the water vapor and methyl alcohol laser frequencies

A far infrared (FIR) frequency synthesis technique using saturated-absorption stabilized CO2lasers and a point-contact diode has been used to measure frequencies of a number of strong CW H2O, D2O, and CH3OH laser lines. The first frequency measurements of the 79-μm H2O, the 73- and 108-μm D2O, and 11 CO2-pumped CW¹²CH2¹⁶OH laser lines are reported. This measurement is the first demonstration of the general usefulness of CO2lasers for accurate synthesis of FIR frequencies.     

Supersonic transverse electrical discharge laser

An electrically excited CO2-N2laser has been developed using high repetition rate discharge pulses from a pin electrode array transverse to a supersonic flow. To date volumetric laser power density of 34 W/cm³from a Mach 3 flow at 17-kHz repetition rate and 160-torr static pressure has been achieved. Power densities of at least 150 W/cm³at atmospheric pressure appear possible. The supersonic blowdown facility and electrical pulse generator are described and experimental results are discussed.     

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.     

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.     

New CW FIR laser action by Stark tuning from optically pumped CH3OH and CH3OD

Fifteen new CW FIR laser lines are reported from Stark tuned CO2optically pumped CH3OH and CH3OD. Four new CW laser lines have also been observed from zero field optically pumped CH3OD, three with a CO2laser, and one with an N2O laser.     

Gas storage technique for the repetitively pulsed operation of a continuously excited laser

This paper reports a technique by which repetitively pulsed outputs may be obtained from some continuously excited gas lasers by supplying the resonator with pulses of excited gas. The technique is applied to a CO2laser in which 14 kW, 1 ms (FWHM) pulses are produced at a repetition frequency of 200 Hz.     

A frequency stabilized compact high repetition rate TEA-CO2laser

A compact atmospheric-pressure hybrid CO2laser utilizing a transverse double-discharge technique has been constructed and operated at moderate repetition rates. A pulse output energy of 80 mJ has been obtained under single-mode conditions at repetition frequencies of 100 Hz. Using the hybrid technique to obtain single-longitudinal-mode operation, we have been able to reach a 7 kHz long-term relative frequency instability between the TEA laser and a local oscillator. Measurements of the frequency sweeping during the pulse tail (chirp) are also presented along with a direct measurement of the resonant frequency pulling effect associated with the real part of the electric susceptibility of the gas. Limitations on the pulse repetition frequency for this laser configuration are also discussed.     

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.     

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.     

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.     

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.     

Frequency and amplitude characteristics of a high repetition rate hybrid TEA-CO2laser

The envelope and frequency characteristics of the output pulse of a high repetition rate hybrid TEA-CO2laser are presented. Both the intrapulse and interpulse laser frequency stabilities are experimentally determined at repetition rates up to 300 Hz. The recovery of the CW laser signal following the generation of the TEA laser pulse is analyzed theoretically and experimentally. Short term reproducibilities of ±2 MHz are observed at a pulse repetition rate of 300 Hz with initial chirp rates of about 1.5 MHz/μs. Improvements and limits on power and repetition rate are discussed.     

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.     

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.