Tea CO2 laser beam absorption in polyethylene

The absorption coefficient of polyethylene sheets has been measured for two wavelengths (0.6328 μm, He-Ne laser; 10.6 μm, CO₂ laser). For TEA CO₂ laser power densities up to 10⁴ W cm⁻², the absorption coefficient was found to be 35.36 cm⁻¹. The measured absorption coefficient shows that polyethylene sheets are good and simple attenuators for CO₂ laser radiation.     

High precision spectroscopy of SF6 with cw high pressure tunable CO2 laser

Investigations are reported on the frequency characteristics of the cw high pressure CO₂ laser and its use for high precision spectroscopy. The problem of using a cw tunable CO₂ laser with a mixture of a few isotopes (¹²C¹⁶O₂, ¹²C¹⁸O₂, ¹²C¹⁶O¹⁸O and others) in a cw high pressure tunable CO₂ laser is discussed. Broadening of the tuning range has given us the opportunity to find seven new absorption lines of SF₆ near the transition P(20) of the 00⁰1–10⁰0 band of the CO₂ laser.     

A new efficient far-infrared optically pumped laser gas: CH3 18OH

The¹²CH₃ ¹⁸OH molecule has been investigated for new far-infrared laser lines by optically pumping it with a cw waveguide CO₂ laser. The larger tunability (318 MHz) with respect to a conventional CO₂ laser permits the pumping of many¹²CH₃ ¹⁸OH lines. As a consequence 100 new laser lines have been discovered, ranging from 34.6 m to 653.2 m in wavelength. The infrared spectrum of¹²CH₃ ¹⁸OH has been observed and all the fundamental vibration energies measured.     

Isotope analysis by infrared laser absorption spectroscopy

The feasibility of IR laser spectroscopy as a technique for the measurement of small abundances of stable and radioactive isotopes has been examined. Theoretical considerations and first experimental results with two laser systems are presented: 1) Coincidences between emission lines of a CO₂-laser and absorption lines of¹³C-substituted ethylene can be used to determine the¹³C-concentration of C₂H₄. 2) A tunable PbS-diode laser emitting in the 4.3 μm-spectral region of the rotation-vibration bands of CO₂ can be used to determine abundances of¹²C,¹³C,¹⁶O,¹⁷O and¹⁸O in small samples of CO₂. With optimized performance, sensitivities up to 10⁻⁹–10⁻¹⁰ seem possible, and for higher abundances an accuracy of 10⁻³. This should allow geophysical isotope studies to be performed and it is hoped that the technique will eventually be applicable to measuring the activity of long-lived radioisotopes.     

Strengths of the SF6 transitions pumped by a CO2 laser

The absolute intensities of the SF₆ transitions from the ground state to ν₃=1 that fall within ±1.5 GHz of the CO₂ P(14), P(18) and P(20) laser lines have been calculated. Good agreement has been found between these theoretical results and the available experimental values, especially for the firm assignments of R(28) A⁰₂ and P(33) A¹₂ for SF₆ absorption nearest the CO₂ P(14) and P(18) laser emissions, respectively.     

Assignments of optically pumped CD3OH laser lines

Six FIR laser lines from CD₃OH pumped by the 10R(36) and the 10R(18) CO₂ laser lines are assigned to specific rotational energy levels in the excited C–0 stretch state. It is found that their upper laser levels are shifted by a Fermi resonance between the C–0 stretch vibration and the third and forth harmonics of the torsional mode. The Fermi resonance shifts are +0.332 cm^–1 and +2.251 cm^–1 for the upper laser levels pumped by the 10R(36) and the 10R(18) CO₂ laser lines, respectively. Calculated frequencies of the pump and the laser transitions agree with those of the pump CO₂ laser lines and the observed FIR laser lines within estimated accuracy.     

A compact,sealed, single mode TEA CO2 laser

A compact, sealed, TEA-CO₂ laser utilizing a double-discharge technique has been constructed and operated under single-mode conditions. Sealed operational lifetimes exceeding 10⁶ pulses have been obtained by the addition of small quantities of hydrogen and CO to the initial gas mixtures. It has also been found that operation without helium is possible giving single-mode output energies of about 20 mJ/pulse at peak powers of about 150 kW. The pulse repetition frequency was 0.5 s^-1. The use of an 18 cm long laser cavity allowed the oscillation of only one longitudinal mode when the frequency of the cavity was tuned closely to the CO₂ line centre frequency. An iris was used to restrict transverse modes to the TEM₀₀. Beat frequency measurements using a stable, CW CO₂ laser indicate a “chirp” of about 5–6 MHz/μs in the tail of the pulse.     

Absorption of CO2 laser pulses at different wavelengths by ground-state and vibrationally heated SF6

The absorption of CO₂ laser pulses by low pressure SF₆ gas has been investigated over a wide range of energy fluxes. For laser energy fluxes of 0.01–1 J cm^-2 the effective absorption cross section varies between 0.2 and 2 × 10^-18 cm². For each laser line an individual dependence on the energy is found and in some cases minor changes in the absorption behaviour seem to occur around 0.1 J cm^-2. SF₆ excited with an average vibrational energy content of up to 20 photons/molecule does not absorb measurable amounts of 9.4 μm laser light. The influence of various SF₆ and Ar pressures on the temporal shape of the transmitted pulses has been investigated.     

Absolute frequency measurement of CO2 laser lines with a femtosecond laser comb and new determination of the CO2 molecular constants and frequency grid

Absolute frequency measurements of a CO₂ laser stabilized on saturated absorption resonances of CO₂ laser lines are reported. They were performed using a femtosecond-laser frequency comb generator and two laser diodes at 852 and 782 nm as intermediate oscillators, with their frequency difference phase-locked to the CO₂ laser. Twenty ¹²C¹⁶O₂ laser lines in the P and R bands at 9 μm were measured with a relative uncertainty of a few 10⁻¹² limited by the CO₂ frequency reproducibility. A new determination of the CO₂ molecular constants was obtained from these data and previous measurements in the 10 μm band. The CO₂ frequency grid was also calculated, with an improvement of two orders of magnitude compared to the previous grid of Maki et al. [J. Mol. Spectrosc. 167 (1994) 211].     

Observation and assignment of submillimetre laser lines from CH3OH pumped by isotopic CO2 lasers

New laser lines have been generated using methanol when isotopically labelled CO₂ is used in the pump laser. 19 lines have been found when¹³C¹⁶O₂ is used and 29 when¹²C¹⁸O₂ is used. The wavelengths of the emission lines have been measured with a relative accuracy of 5×10^–3 using the submillimetre resonator as a scanning Fabry-Perot interferometer, and the polarizations of the lines have also been measured. Some of the laser lines have been assigned to pure rotational transitions in the excited state associated with the CO stretching mode, other lines are believed to originate from pumping in combination bands.     

Wavenumber measurement of weak CO2 laser lines around 10.6 μm

An experimental method is described in which a tunable semiconductor diode laser and the regular CO₂ laser lines are utilized to measure the wavenumber of CO₂ laser lines to an accuracy of about ±5 × 10^?4 cm^?1. Twenty new CO₂ laser lines have been measured over the 943 to 951 cm^?1 region.     

The infrared spectrum of the CO dimer

The infrared spectrum of (¹²C¹⁸O)₂ has been studied for the first time using a tunable diode laser spectrometer in the 2095 cm⁻¹ region to probe a pulsed supersonic jet expansion. Very dilute gas mixtures of CO in He were used, resulting in small consumption of ¹²C¹⁸O sample gas, as well as cold and simple spectra. The results were analyzed using a term value scheme to obtain model-independent energies for 7 rotational levels belonging to 2 stacks in the lower state, v_CO=0, and 22 levels belonging to 7 stacks in the upper state, v_CO=1. The two ground state isomers of the CO dimer were found to be separated by only 0.639 cm⁻¹ for (¹²C¹⁸O)₂. These results provide a foundation for future studies of the millimeter wave spectrum.     

Absolute frequencies of CO2 laser transitions by multiplication of CO2 laser difference frequencies

Frequencies of CO₂ laser transitions have been compared to the Cs standard by a four-step frequency chain using difference-frequencies from five CO₂ isotope lasers. A tungsten-nickel diode generates the differences and their harmonics. Measurement of the 10.71 μm R(6) transition of ¹³C¹⁸O₂ is described. The absolute frequency was found to be 27 979 469.512(65) MHz. Simultaneously, the four other CO₂ frequencies in the chain were also determined.     

Dissociation of NH3 to electronic ground-state fragments by high-intensity CO2 laser radiation

The dissociation of NH₃ into electronic ground-state fragments NH₂(²B₁) + H by high-intensity CO₂ laser radiation has been observed under essentially collision free conditions and as a function of NH₃ pressure, using laser fluorescence excitation for the NH₂ detection. Luminescence from excited NH₂ (²A₁) produced directly by the CO₂ laser has also been observed. The relative yield of NH₂ (²A₁) is estimated to be smaller than that of NH₂ (²B₁) by several orders of magnitude.     

Wide band (2.5 GHz) infrared heterodyne spectrometer

Conclusion A double beam heterodyne spectrometer with 2.5 GHz bandwidth was built in Reims. The receiver's bandwidth was measured by heterodyning radiation from a CO₂ laser with the radiation from a tunable diode laser. The spectrometer was used for laboratory studies. Ammonia absorption spectra were recorded. Line positions in agreement with other works were measured.This apparatus will soon be used for atmospheric ozone studies in association with an isotopic¹²C¹⁸O₂ laser to avoid atmospheric CO₂ absorption.     

Absolute diode-laser frequency calibration using varactor mixing with a stabilized CO2 laser. Ozone spectrum near the 9R6 CO2 frequency

Efficient infrared varactor mixing near 9 μm has been performed between diode-laser and CO₂ laser. Difference frequency up to 17 GHz has been detected using non linear mixing between the two radiations and the first four harmonics of a microwave frequency (f = 4.2 GHz). As a result, the absorption spectrum of ozone near the 9R₆ CO₂ line is frequency scaled over a 1.7 cm^-1 range with microwave accuracy.     

Characterization of a Br(2P1/2)–CO2(1001-1000) transfer laser driven by photolysis of iodine monobromide

2 laser operating on the 10⁰1-10⁰0 transition at λ=4.3 μm and pumped by E –V energy transfer from Br(²P_1/2) has been demonstrated. The dynamics and performance of this device were characterized by observing the time-resolved stimulated emission and the steady-state spontaneous side fluorescence after photolysis of IBr or Br₂ by a frequency-doubled Nd:YAG laser or Ar⁺ laser, respectively. Although the E –V excitation kinetics are favorable, rapid vibrational relaxation limits laser action to CO₂ pressures of less than 1 Torr. Numerical modeling of laser pulse shapes and the dependence on IBr and CO₂ pressure and photolysis energy establish a relatively high gain of 0.33%/cm, a CO₂-pressure-dependent optical loss of 0.04–0.06%/cm, and an efficiency of 2×10^-5 4.3-μm-laser photons per incident photolysis photon. The CO₂ fluorescence after photolysis of a fixed Br₂/CO₂ gas mixture decreases as a function of photolysis time by about 30%/h, indicating the photolytic production of an important quencher. Received: 23 June 1997/Revised version: 23 September 1997     

Evaluation of downstream-mixing scheme for 9.4-µm CO2 gasdynamic laser

A theoretical analysis of a downstream-mixing 16-μm CO₂ gasdynamic laser revealed the possibility of utilizing the downstream-mixing scheme for the generation of 9.4-μm radiation using a CO₂ gasdynamic laser. The flow-field has been analyzed using complete two-dimensional, unsteady laminar form of Navier-Stokes equations coupled with the finite rate vibrational kinetic equations. The analysis showed that integrated small-signal gain of 11.5m⁻¹ for Lorentzian broadening and 4.8m⁻¹ considering Voigt function can be obtained for N₂ reservoir temperature of 2000°K and velocity ratio 1:1 between the CO₂ and N₂ mixing streams. These results (presented in graphs) clearly highlight the large potential of downstream-mixing CO₂ gasdynamic laser for 9.4-μm laser generation.     

Discovery and characterization of optically pumped far-infrared laser emissions using laser magnetic resonance spectroscopy

A laser magnetic resonance spectrometer has been used to discover and subsequently measure a far-infrared laser emission: the 166.6-micron line of CH₂F₂, optically pumped by the 9P24 CO₂ laser. By recording spectra for the NH radical, the frequency of this laser emission has been determined to be 1799950±13 MHz. Spectra for the NH radical were also recorded with two other far-infrared laser emissions: the 160.4-micron line of N₂H₄ (9P46 CO₂ pump) and the 328.6-micron line of ¹³CH₃OH (9P12 CO₂ pump). From the NH spectra, a discrepancy of 2.1 GHz with the previously measured laser frequency was identified for the 160.4-micron line. A three-laser heterodyne system was then used to remeasure the frequency to be 1868475.5±0.5 MHz. The NH spectra were also used to determine the frequency for the 328.6-micron line to be 912366±7 MHz, in agreement with the value previously calculated from the Rydberg–Ritz combination principle. PACS 07.57.Hm; 32.60.+i; 42.62.Eh     

Mixing of 30 THz laser radiation with nanometer thin-film Ni-NiO-Ni diodes and integrated bow-tie antennas

Mixing experiments with 30 THz CO₂-laser radiation as well as the detection of 35 ps 30 THz pulses of an optical-free-induction-decay CO₂-laser system have been performed with the first nanometer thin-film Ni-NiO-Ni diodes with a minimum contact area of 0.012 µm². Difference frequencies up to 85 MHz were detected by mixing two different CO₂-laser beams coupled to the diode with an integrated bow-tie antenna. The dependence of the beat signal on bias voltage, laser power and polarization of the infrared laser radiation was determined.