Pulsed high-power far-infrared gas lasers: Performance and spectral survey

We describe the practical performance of a pulsed mirrorless far-infrared gas laser pumped by a TEA CO2laser. The system is designed to serve as a high-power source for experiments in far-infrared nonlinear optics and saturation spectroscopy of solids. Out of numerous candidates, 11 gases were selected for a careful survey: CH3F,¹³CH3F, NH3, D2O, D2¹⁸O, D2S, HDS, CH3Cl, CH3Br, CH3I, and CH3CN. With an available pump energy of up to 10 J, a total of 203 far-infrared lines with pulse energies up to 12 mJ were found, covering the wavelength range from 42 μm to 1.2 mm. 78 of these lines had not been observed before. The position of further 39 lines is at variance with former literature reports.     

Pulsed high-power mid-infrared gas lasers

We present a survey in the spectral region of 200-700 cm^-1(14-50 mum) of pulsed mirrorless gas lasers pumped by a TEA CO2laser. 55 lines are found covering the wavelength range 15.8-57 cm^-1in the gases NH3,¹⁵NH3, D2O, CH3OH, CD3OH, and CD3OD. Most of these lines are reported for the first time. One of the strongest lines (26.4 μm in NH3) emits a considerable pulse energy of 100 mJ. No lines were found in C2H5OH and C3H6O.     

High power optically pumped far infrared laser systems

Single mode, 60-80 ns pulse width, 50-70 kW peak power laser oscillators operating on the 384.6 μm line of D2O have been developed. The characteristic linewidths of these oscillators are less than 25 MHz full width at half maximum which favorably compares with the intrinsic width of 6-8 MHz associated with the pulse length of about 60 ns. A 12.7 mJ, 195 kW, 384.6μm D2O laser oscillator-amplifier combination has been constructed and tested. Although single longitudinal mode operation is attained from this oscillator-amplifier system, amplified spontaneous emission (superradiance) from the amplifier adds low power level wide-bandwidth background radiation. Studies of far infrared lasing action in CH3F and CH3I are also described.     

Observation of new FIR superradiant lines from optically pumped D2O

Four new laser lines from optically pumped D2O vapor are reported at wavelengths of 235 μm, 280 μm, 97 μm, and 98 μm, the latter having a quantum conversion efficiency of 38 percent.     

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.     

Backward and foreward FIR emmission characteristics drom D2O in both Raman and laser regimes

Laser emission spectra for the 66 and 116 μ D2O lines pumped by the 9.66 μP32CO2line in both backward and forward directions have been obtained. Forward spectra are predominantly Raman dominated, except for a narrow pressure region near 2 torr where the 116 μ line is laser-like. Backward spectra appear to be entirely laser-like showing narrower bandwidths and constant frequency. The shape of the pulse outlines in different directions and pressures confirm the findings of the emission spectra. Total FIR energy emission is also asymetric, being significantly greater in the forward direction.     

New pulsed far infared laser lines in D2O

Eleven new far infrared (FIR) laser transitions were obtained by pumping a D2O oscillator with the 9P(12), 9P(18), 9P(44), and 10R(22) line radiation of a TEA-CO2-laser. The efficiency of at least two of the new lines is comparable to that of the strongest known lines in D2O.     

Investigation of D2O-laser emission at 50, 66, 83, 111, and 116 µm

A D2O laser oscillator axially pumped by a TEA CO2laser operating on the9P(32)line has been investigated. The laser emits five FIR lines whose emission characteristics are presented. For the three strongest lines (66, 111, and 116 μm) and an oscillator of length 45 cm, a single axial mode can be made to dominate. Special attention is drawn to the newly discovered and identified 111 μm line.     

Laser pumped molecular lasers--Part I: Theory

The density matrix formalism describing the interaction between a three level molecular system and two nearly resonant laser fields of arbitrary intensity is extended to include the molecularM-level degeneracy. Expressions for gain on the two possible relative polarizations of a laser pumped molecular laser are derived and its saturation behavior discussed. Numerical results for the 385 μm transition in D2O and 496 μm transition in CH3F are presented.     

SMMW laser emission and frequency measurements in doubly-deuterated methyl fluoride (CHD2F)

Submillimeter wave laser action is reported in CHD2F on 17 "parent" transitions plus three cascade lines. Accurate heterodyne frequency measurements are reported for nine lines. Useful output powers of greater than 1 mW were observed for several lines. The strongest line, which is at 384.3 μm, is particularly significant because of its potential as a local oscillator for the 385 μm D2O laser.     

A comparative study of D2O oscillators emitting at 385 µm

The generation of single-mode pulses of far-infrared (FIR) radiation of duration up to 700 ns at 385 μm has been achieved using a confocal unstable resonator with D2O as an active medium. Future volumetric scaling to the power levels necessary for measuring ion temperature in tokamak plasmas by Thomson scattering appears encouraging. A comparison between the performance of the unstable resonator and two hemispherical resonators is reported.     

Stimulated Raman emission in infrared excited gases

Using the pressure dependence of absorption, absorption coefficients and detunings were measured for CO2pump lines and the strong fat infrared emission in optically pumped D2O. The P (32) CO2line was found to be detuned ∼1.5 GHz from the ν2band transitions6_{6}, 6_{5} rightarrow 5_{5}, 5_{4}. The resulting emission lines at 50.3 μm and 66 μm were found to be detuned from their respective transitions by about the same amount. On the basis of these measurements and gain estimates for the far infrared, the resulting emission lines are identified as stimulated Raman emission.     

Direct optical pumping of high-pressure CO2and N2O lasers with a pulsed HF pump laser

Pulsed HF laser radiation has been used for direct optical excitation of CO2and N2O lasers at 10 and 5 atm gas pressures, respectively, which are the pressures required for continuous laser frequency tuning between the line centers in these gases. The maximum demonstrated quantum efficiency is about 14 percent, which seems to be limited by the formation of pressure waves in the gas, in addition to losses in the resonator optics. We find that quantum efficiencies close to unity are theoretically feasible with a low-loss resonator, and by using a sufficiently short pump pulse to avoid a serious influence from pressure waves. The excitation scheme can be used with any of the CO2and N2O laser isotopes.     

A study of the CW 28-µm water-vapor laser

The low signal gain of a CW water-vapor laser at 28 μm was measured as a function of the discharge current and pressure. Together with the measurement of other quantities such as the axial electric field and the concentration of OH, a partial interpretation of the mechanisms involved in pumping the 28-μm transition was possible. Thermal equilibrium between the ν0,2nu_{2}, and ν3vibrational levels will result in a large absorption at the elevated gas temperatures observed (800-1000 K). The strong dependence of gain on the electron temperature strongly suggests that the vibrational excitation proceeds through electron-impact excitation. Only the electron-impact excitation of H2O is quantitatively capable of overcoming the large thermally induced absorption. Although vibrational-excitation transfer from H2to H2O seems insufficient, by itself, to overcome this absorption, it may provide appreciable additional gain. Pumping of the 28-μm line through electron-ion recombination and by reactions involving OH can be ruled out.     

Nanosecond pulse transmission of buffered SF6at 10.6 µm

The 10.6 μm transmission properties of SF6were measured using nanosecond-duration pulses for various buffer gases and buffer gas pressures up to 3 atm. The addition of a buffer gas strongly increased the SF6absorption at moderate energy fluences without significantly altering the high fluence pulse transmission. For pressures above 1 atm, the transmission behavior appeared independent of the pulse duration over the range of parameters investigated in this experiment. The implication of these results for CO2laser applications will be discussed.     

New HF laser pumped molecular lasers in the middle infrared

Stimulated emission has been observed in five molecules optically pumped by a pulsed HF laser. Emission was observed in¹⁵N2O,¹⁴N¹⁵NO, and¹⁵N¹⁴NO at 4.6 μm, HCOOH at 5.7 μm, and in¹³CS2at 6.9 μm. Lasing pulse shapes and delays after the pump pulse were measured. Lasing due to rotational relaxation induced by collisions with He was observed in¹⁵N¹⁴NO.     

Theoretical analysis of combustion-driven 16µm CO2gas dynamic lasers

A numerical analysis is presented with a combustion-driven 16 μm CO2gas dynamic laser operating on liquid fuel and liquid oxidizer constituted of benzene (C6H6) and nitrous oxide (N2O), respectively. Adopting a sharp-edged nozzle with an area ratio of 100, optimization of equivalence ratio φ, reservoir pressure P0, and nozzle throat heighth*was performed by considering conditions of no steam condensation and exhausting the combustion gas to the atmosphere. The analysis showed that the maximum value ofG_{16} = 0.48m^-1for small-signal gain andE_{16}^{max} = 9mJ/(l . atm . pulse) for available specific energy would be attainable with theP(15)line of the (02⁰0)-(01¹0) transition.     

Dispersion in a laser-pumped molecular laser

The dispersion equations are derived for two laser fields of arbitrary intensity interacting with a homogeneously broadened, three-level molecular system. The results obtained apply to laser-pumped molecular lasers and may be used to analyze frequency pulling of the emitting laser and self-focusing or defocusing of the pump laser. The laser fields are allowed to be of arbitrary intensity and to be on or off resonance. The dispersion function for the emitting laserchi'(omega_{s})is evaluated in various limits. The present theory is applied to analyze previous experimental data for cavity frequency pulling in CO2laser-pumped molecular lasers, including a 385-μm D2O laser and a 496-μm CH3F laser. Good agreement is found between theory and experiment.     

The identification of candidate transitions for optically pumped far infrared lasers: Methyl halides and D2O

Two hundred and forty-four candidate transitions for optically pumped far infrared lasers have been identified in C¹²H3F, C¹³H3F, C¹²D3F, C¹²H3Cl³⁵, C¹²H3Cl³⁷, C¹²H3Br⁷⁹, C¹²H3Br⁸¹, and D2O using as candidate pump lines, 781 in number, the isotopic forms of CO2, the sequence and hot bands of C¹²O2¹⁶, and the N2O bands. The dominant transition parameters responsible for strong emission from TEA, CW, and long pulse modes of operation were identified by a correlation between known experimental behavior and the pertinent transition parameters, the results being that strong TEA laser emission correlates with ground state population while strong CW and long pulse emission correlate with absorption coefficient. The candidate transitions were subsequently selected on the basis of these parameters using calculated transitions and assignments based on band data from high resolution laser-Stark spectroscopy and measured candidate pump frequencies.     

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.