Far infrared laser emission from15NH3optically pumped by a CW sequence band CO2laser

Two new coincidences between theP(31) andP(35) CO210.6 μm sequence band emission lines and the¹⁵NH3absorption lines have been found to produce far infrared (FIR) emission lines atlambda = 111.9and 218 μm. In addition, the known coincidence between the regularR(42) 10.6 μm band CO2emission line and theasR     

Emission frequencies of the CF4laser

104 (nu_{2} + nu_{4}) leftarrow nu_{2}hot-band transitions, some of which correspond to known(nu_{2} + nu_{4}) leftarrow nu_{2}CF4laser lines, have been identified and measured in high-resolution Doppler-limited spectra of¹²CF4near 16 μm, obtained with a tunable diode laser. From an analysis of these frequencies, the rotational and tensor splitting constants for the ν2vibrational level have been determined. These constants, together with thenu_{2} + nu_{4}pump-band spectrum previously obtained, allow the laser emission frequencies for any given CO2pump line to be calculated with an accuracy of 0.01-0.003 cm^-1.     

CO2laser preamplifier capabilities for low-level 10.6-µm direct-detection receivers

We examine the potential of CO2laser preamplifiers for sensitivity enhancement in low-level, direct-detection 10.6-μm receivers. For the condition in which a gain-dependent competition exists between the background noise and amplifier spontaneous emission noise (assuming negligible thermal noise), the analysis predicts an optimum useful SNR enhancement of only 6 dB for a blackbody background field of 300 K and 4.1 dB for a background of 260 K, when the amplifier gain bandwidth perp-line is 100 MHz and the infrared (IR) filter bandwidth is 0.10 μm. Based on preselected choice of gain and bandwidth, a two-stage, water-cooled, flowing-gas amplifier of optimized design was constructed. A maximum gain of 3.12 dB was attained forP(20)with a He : CO2: N2mixture of 5.0 : 1.0 : 0.6 at a coolant temperature of 285 K and a slow gas refresh rate of 0.2 volumes/s. Using a fast-flow system with 12-volume/s refresh rate, we measured an amplifier gain of 3.9 dB, close to the design estimate of 4.1 dB. With a calibrated HgCdTe detector,f/4cold shield, and narrow-band (0.25 μm) cold filter, a spontaneous emission flux density ofsim 1.0 times 10^{14}photons/ cm². s was measured at the 3.12-dB gain level, in close agreement with the theoretical estimate. Excess noise resulting from amplifier discharge was undetectable above the basic detector noise.     

Line competition in the neutral argon laser

Uncertainties in transition assignments for the neutral argon laser lines at 1.792 and 2.208 μm have been resolved by operation of a CW neutral argon laser with several simultaneous output wavelengths. The observed competition effects have established the correct transition for the 1.792-μm line to be3d(1/2)_{1}deg -4p(3/2)_{2}and for the 2.208-μm line to be3d(1/2)_{1}deg -4p'(3/2)_{2}.     

4B7--Precision spectroscopy of new infrared emission system of molecular nitrogen

Stimulated emission and laser oscillation resulting from transient population inversions on a number of electronic transitions of molecular nitrogen have made it possible to more accurately specify energy differences between several excited singlet states of the molecule and in one case, that of thew ^{1}Delta_{u}state, to more directly determine its absolute position with respect to the electronic ground state. Measurement accuracy has been improved on a system which we previously reported. A new emission system has been identified as resulting from transitions between thew ^{1}Delta_{u}state and thea^{1}Pi_{g}state. We have observed for the first time in the unperturbed region at lowJvalues Λ doubling inupsilon = 0ofa^{1}Pi_{g}. Very complex emission systems of more than eighty lines falling into four groups between 5.35 μm and 8.06 μm have been observed. No identification has yet been made.     

Efficient multikilowatt mid infrared difference frequency generation in CdGeAs2

Multikilowatt mid infrared emission with high pump photon conversion efficiency (∼ 15 percent) has been obtained by difference frequency generation in CdGeAs2utilizing the double wavelength output from a grating tuned twin-cavity TEA CO2system, one signal of which is frequency doubled to provide efficient synchronized sources for both pump and idler. Tuning over a spectral range8-14 mum can be achieved and further extended to 17 μm using 12.8 μm emission from an efficient TEA CO2pumped NH3laser.     

Atomic laser action in rare gas-SF6mixtures

Pulsed-laser action is described on a number of atomic transitions of ArI, one of which has not been reported previously. The addition of SF6to Ar shifts the laser action in the Ar from the3d'(3/2)min{1}max{0} - 4p'(1/2)_{1}transition to six ArI transitions originating from the three lowest states of the3p^{5}3delectron configuration. Significant enhancement of the intensity is observed over that resulting from a discharge in pure argon. In addition, when a small amount of SF6is added to a He discharge, laser oscillation results from FI at 0.7129 μm. Increased SF6pressure produces an unidentified oscillation at 2.639 μm. These last two transitions have been observed also in He-NF3, He-CF4, and He-PF3mixtures.     

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.     

Electronic- and phonon-terminated laser emission from Ho3+in BaY2F8

Infrared spontaneous and stimulated emission from Ho³⁺in BaY2F8is reported. In addition to the familiar⁵I_{7} rightarrow⁵I8transition at 2 μ,⁵F_{5} rightarrow⁵I5emission at 2.4μ and⁵I_{6} rightarrow⁵I7emission at 2.9μ are discussed. There are several unusual features of the 2-μ laser emission. At room temperature, phonon-terminated laser emission is observed at 2.171 μ. At 77 K a complex CW laser output is observed in a wavelength interval lying on the shoulder of a fluorescence line. The complex output is attributed to oscillation in transverse modes of the resonator. Oscillation is not observed in the strongest emission line, despite a large terminal state splitting of 310 cm^-1. These results are explained on the basis of a theory developed earlier for transition metal ion lasers. The validity of the model is supported by demonstrating the tunability through loss modulation predicted by theory. The observation of these effects is made possible by the very low internal scattering loss in the crystals. The⁵F_{5} rightarrow⁵I5laser lines near 2.4 μ represent relatively low gain transitions with pulse durations limited by accumulation in a longer lived terminal state. The dynamics of laser emission indicate the possible absence of thermal equilibrium in the excited state. For the 2.9-μ transition the bottleneck posed by a longer lived terminal state may be eliminated by the addition of Eu³⁺or Pr³⁺, but laser emission could not be obtained.     

Application of laserQswitching for infrared spectroscopy

PassiveQswitching of IR lasers (N2O as well as CO2) has been applied for the detection of coincidences within a few 100 MHz between laser lines (νL) and molecular transitions (νM). Polar (¹²CH3F,¹³CH3F, C2H5OH, and C6H5CH3) as well as nonpolar molecules (SiH4, GeH4, CCl4, C2H4, C3H4, and C6H6) have been studied. The phenomenon depends strongly on the frequency mismatchDeltanu = nu_{M} - nu_{L}. 81combinations between laser lines and molecular absorption lines were found to produce theQ-switch effect.     

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.     

Five color CO2laser stabilization and switching

A CO2laser is described which may be actively stabilized in any of four differentR/Pline pairs in the 10.4 μ band. The laser may be caused to rapidly switch between line pairs. The degree of frequency stability achieved issim1:10^{10}, 5 Hz and above, andsim1:10^{8}, 0-5 Hz. The laser may be switched through the four line pairs in ∼100 ms.     

Nonlinear properties of cuprous halides

A study of the nonlinear properties of cuprous halides in the near infrared (YAG Nd³⁺laser) and in the medium infrared (CO2laser) is presented. The second-order susceptibilities have been compared to those of quartz and GaAs. Coherence lengths were measured at 1.06 μ and for the four principal transition lines of CO2laser. In addition some linear parameters such as refractive indices in the visible and absorption coefficients from 0.3 to 25 μ were determined. The large values of Miller's δ14of those compounds are consistent with Levine's bond-charge model.     

Optical pumping by CO2and N2O lasers: New CW FIR emissions in 1,1-difluoroethane

53 new CW FIR laser lines are reported in 1,1-difluoroethane optically pumped near 10 μm by CO2and N2O lasers. The emission spectrum initially reported in the literature consisted of four lines between 770 and 458 μm and has now been extended to the 2.39 mm- 319 μm region. The reason for this extension, especially to the long wavelengths, is analyzed.     

A laser augmented reaction: SF6+ SiH4→S*2+ SiF4+ HF + H2· retention of isotopic selectivity during detonation

A single unfocused pulse of a free running CO2laser, area ∼ 8 cm², initiates an explosive reaction between SF6and SiH4. This occurs at a minimum energy of 4 J [full width at half maximum (FWHM)sim 1.5 /mus] of which about one half is absorbed in an 8 cm long cell; total pressure 12 torr; 0.65 <p(SiH4)/p(SF6) < 1.8. The spectral and temporal distributions of the emitted chemiluminescence depend sensitively on the fuel to oxidizer ratio, and on the pulse energy; we investigated the range 4 → 20 J. The principal emission is due to S2(B^{3}Sigma-_{u} rightarrow X^{3}Sigma-_{g}). Transitionsupsilon' (0-4) rightarrow upsilon" (2-15)were recorded. In the³Sigma-_{u}state, vibrational temperatures range from 3000-13000 K. The luminosity peaks sharply at (SiH4)/(SF6) = 1.0 ± 0.05. On each side of the maximum of the emission versus composition curve [at (SiH4)/(SF6) ≈ 0.95 and 1.22, for a 12 J pulse] the residual SF6(0.2-0.5 percent of initial amount) is enriched in³⁴SF6; the observed fractionation factors at these two compositions are 8 ± 2. The separation between the two sharply peaked optimum compositions appears to increase with increasing pulse energy. Preliminary results with other fuels suggest that the concurrent absorption of CO2laser radiation by the fuel, as well as a highly exothermic reaction, are pre-requisite for fine tuning of composition, injected power, and total pressure for optimum isotope fractionation.     

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.     

Analyses of mode-hopping phenomena in an AlGaAs laser

Intensity fluctuations of the longitudinal modes of a 0.8 μm AlGaAs laser were precisely measured during the occurrence of hopping between two modes. It was found from this result that mode hopping follows the stochastics of a Poisson process. The frequency of mode hopping was measured asf_{c} = [exp [-95(I/I_{th} - 1)]] times 10^{7}(Hz). whereI/I_{th}is the injection current normalized to its threshold value. Results of analog computer simulations showed that spontaneous emission worked as a triggering force for mode hopping. Results of the analysis based on the Fokker-Planck equation were compared to the experimental results, from which the root-mean-square value of the fluctuating electric field of spontaneous emission was estimated as2.3 times 10^{2)(V/m)leqlanglesim{E}_{N} leq 3.2 times 10^{2}(V/m). It is concluded that an effective reduction of mode hopping is achieved if the laser is operated at a higher bias or if the coupling constant between the two modes is increased.     

Near single-line operation of a free-burning CS2/O2/N2O flame laser with a nondispersive optical cavity

The CS2/O2/N2O flame laser has been operated for the first time under conditions in which the spectral output is nearly single line. This transition is theP_{10-9}(17) of CO at 5.4265 μm, the same transition which was observed to oscillate in single-line fashion by Hirose et al. in an electrically initiated CO chemical laser. It is suggested that the unique behavior of this line may be due to its close proximity to aPbranch transition in an adjacent band, namely theP_{9-8}(23) line, such that the gain profiles of the two lines overlap. Calculations suggest that at the conditions of these experiments, the separation of the line centers for this pair is about 0.3 Å or less. TheP_{10-9}(17) transition was also found to be totally absent under certain conditions of high multiline power, particulary at low O2and N2O flows. This may be due to absorption by a high-bandRbranch transition at 5.4266 μm, namely theR_{15-16}(32) line.     

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.