Wavelength modulation absorption spectrometry from optically pumped collision broadened atoms and molecules

A theoretical investigation of the influence of optical pumping on wavelength modulation absorption spectrometry (WMAS) signals from collision broadened atoms and molecules is presented. General expressions for the nf-WMAS signal from atomic and molecular systems, modeled as three-level systems that can accommodate both optical saturation and optical pumping, are derived by the use of a previously developed Fourier series-based formalism in combination with rate equations solved under steady-state conditions. The expressions are similar to those describing the nf-WMAS signal from two-level systems that can accommodate optical saturation [Schmidt FM, Foltynowicz A, Gustafsson J, Axner O, WMAS from optically saturated collision-broadened transitions. JQSRT 2005;94:225-54], the difference being the value of the saturation flux, wherefore the general parametric dependence of WMAS signals from optically pumped systems is the same as that from optically saturated systems. The additional effect of optical pumping on the WMAS signal is investigated for three typical cases: molecules or atoms in an ordinary atmosphere, atoms in an inert atmosphere, and atoms or molecules possessing metastable states. The possibility to describe any of these systems with a two-level model is investigated.     

Instabilities in optically pumped infrared lasers

The unstable emission of an optically pumped FIR gas laser is theoretically analysed through numerical resolution of the associated Maxwell-Bloch equations. The instability domain in the main control parameters space is determined and an example of chaotic behaviour is examined.     

Mid infrared and far infrared studies of the propyne optically pumped laser

An extensive study of the propyne optically pumped laser is presented. 32 new far infrared and 8 new mid infrared outputs are reported, together with pump frequency offset data. Complete assignments of some far infrared transitions and partial assignments of the mid infrared outputs are given. Very strong interactions occur between the mid and far infrared laser transitions. These interactions are of vital importance in obtaining high output energies.     

Diffusion and vibrational bottleneck in optically pumped submillimetre lasers

By observing the time dependence of the onset of the submm laser power in the millisecond time scale, just after the switching on of the pump laser, the vibrational bottleneck due to diffusion and vibrational relaxation has been clearly exhibited in CH₃F, D₂CO and HCOOH submm lasers. Experimental results are compared with those deduced from a simplified rate equation model. Both show a diffusion limited regime and a collisional regime.     

Biological research by optically pumped far infrared lasers

The FIR breeding for paddy rice, black bean and wheat, the chlorophyll mutation of paddy rice induced by optically pumped FIR laser, etc., are presented. The results of SDS electrophoresis analysis of soluble proteins of Drosophita melanrgaster irradiated by optically pumped FIR laser are described and discussed.     

High power optically pumped far infrared lasers1

A review of operating principles and achieved characteristics of high power methyl fluoride lasers at λ = 496 μm is presented. In addition, a novel linear pumping scheme is described which allows direct amplification of radiation from a narrow bandwidth (<60 MHz) methyl flouride oscillator. A gain of 7. output power ∼ 10 kW and conversion efficiency of 0.15% have been observed. The method offers an avenue toward a MW-level narrow bandwidth source for use in plasma diagnostics.     

Hybrid output mirror for optically pumped far infrared lasers

A hybrid metallic mesh multilayer dielectric coating mirror has been developed for use as an output mirror for optically pumped far infrared lasers. The metallic mesh provides a high reflectance in the far infrared while the multilayer dielectric coating is chosen to provide a maximum reflectivity at the pump wavelength (10 μm). This hybrid mirror has increased the output power of a CH₃F waveguide laser at 496 μm by a factor of 350 over that obtained with a hole coupling mirror. In addition, this mirror results in a far infrared output beam which has a minimum angular divergence limited only by the particular oscillating transverse waveguide mode (the EH₁₁ mode for this experiment).     

Nonlinear level crossing in the infrared transitions of optically pumped far infrared laser media

In conjunction with the Stark field induced power enhancement in optically pumped far infrared lasers, absorption transitions in a degenerate two level system under the influence of an weak static Stark field have been analyed. A third order theory to the density matrix formalism leads to zero field level crossing signals for absorption in rate equation approximation and in Doppler limit. For fast relaxation among Zeeman sublevels, the zero field level crossing signal arises from population effect. Optoacoustic measurements have been carried out on the ₅,^qQ(16,8) absorption line of CH₃OH which produces the familiar 119-m laser with the CO₂ 9P(36) pump line. The optoacoustic absorption signals observed are well reproduced by the theoretical expression.     

New submillimeter laser lines in optically pumped gas molecules

A small, pulsed CO₂ laser has been used to optically pump gaseous formic acid, ethyl fluoride and ozone as well as other molecules previously know to lase. A large number of new far infrared lines with high intensity have been observed and accurately calibrated.     

Thermal pump saturation in CW optically pumped far infrared lasers

Vibrational-translationally relaxing far infrared (FIR) laser gases are heated by relaxation from the first vibrational state containing the lasing levels to the ground state. This reduces the molecule density, thus leading to a lower effective pump absorption compared with a cold gas. This phenomenon is investigated theoretically by solving the appropriate heat diffusion equation, considering both the traditional circular symmetric waveguide structure and the sandwich (or parallel slab) structure. The most important results are discussed and compared with previously reported experiments. A method is given for an estimation of the effective pump absorption and the excess temperature.     

Pulsed optically pumped far infrared laser and maser action in OCS

Twenty new optically pumped far-infrared laser transitions have been observed in OCS, including four “maser” transitions of 3.6–8.2 mm wavelength. The operating conditions and assignments of these lines are reported. Rabi splitting of the FIR gain profile caused by the high intensity of the CO₂ TEA laser pump is found to be very important for some lines.     

A variable reflectivity output coupler for optically pumped far infrared lasers

In this paper we describe the design and performance of a variable reflectivity output coupler for optically pumped far infrared (FIR) lasers. The output coupler is a compact, tunable Michelson interferometer. The output coupling ratio (i.e. the Michelson transmission) is adjustable between 0 and 60% for laser line wavelengths between 110 and 500 m. This output coupler provides increased output power and flexibility, since it can be tuned to optimum coupling ratio for different laser lines. Beam profile measurements show that the Michelson output coupler produces a well collimated Gaussian laser beam.Design features are a) the use of 10 m reflection coated quartz vacuum window which acts as a dichroic mirror for the pump radiation and b) the high mechanical stability obtained by a leaf spring flexure mount of the movable Michelson mirror and by restricted alignment devices.     

Nuclear spin diffusion effects in optically pumped quantum wells

We studied the influence of the nuclear spin diffusion on the dynamical nuclear polarization of low dimensional nanostructures subject to optical pumping. Our analysis shows that the induced nuclear spin polarization in semiconductor nanostructures will develop both a time and position dependence due to a nonuniform hyperfine interaction as a result of the geometrical confinement provided by the system. In particular, for the case of semiconductor quantum wells, nuclear spin diffusion is responsible for a nonzero nuclear spin polarization in the quantum well barriers. As an example we considered a 57 Å GaAs square quantum well and a 1000 Å Al _x Ga_1?x As parabolic quantum well both within 500 Å Al_0.4Ga_0.6As barriers. We found that the average nuclear spin polarization in the quantum well barriers depends on the strength of the geometrical confinement provided by the structure and is characterized by a saturation time of the order of few hundred seconds. Depending on the value of the nuclear spin diffusion constant, the average nuclear spin polarization in the quantum well barriers can get as high as 70% for the square quantum well and 40% for the parabolic quantum well. These results should be relevant for both time resolved Faraday rotation and optical nuclear magnetic resonance experimental techniques.     

New far infrared laser lines in optically pumped CD3F

Summary Using a high pressure CO₂-laser we have studied laser action in¹²CD₃F stimulated by R-branch pumping. We used an arrangement with low-feedback mirrors as suitable for Raman laser action. However, no Raman laser action has been observed in¹²CD₃F though similar experimental conditions were met as for¹²CH₃F and¹³CH₃F. The reason for the different behavior of the gases is not yet clear.Instead of Raman laser action we found 15 FIR resonant laser lines with 13 of them unknown up to now in the frequency range from 50 cm^–1 to 68 cm^–1. We reached FIR pulse energies up to 900 J, corresponding to a photon conversion efficiency of 12%. For our arrangement the optimum operation pressure varied from 40 torr at low J values (J=36) to 70 torr at high J values (J=49).     

Far infrared absorption by pairs of nonpolar molecules

Recent progress in the field of binary collision induced spectra of nonpolar gases and mixtures in the far infrared (FIR) region of the spectrum includes accurate measurements of a variety of molecular systems and temperatures, and rigorous quantum calculations. The latter are based on the isotropic potential approximation and either on ab initio induced dipole data obtained with highly correlated wavefunctions, or on the classical multipole induction model. The contributions of both free pairs of molecules in collisional interaction, and bound pairs (van der Waals molecules), are accounted for in equilibrium proportions. The effects of the anisotropy of the intermolecular interaction potential on the spectra are also being understood in quantitative terms. On an absolute intensity scale, the agreement of theory with the laboratory measurements is typically well within the uncertainties of the measurements if all theoretical dimer features are flattened by convolution with with an instrumental profile of 10 or 20 cm^–1 width; certain dimer features have been seen in the FIR spectra of the atmospheres of the outer planets and their big moons. For astrophysical and other applications, the results of the quantum computations have been cast into simple analytical expressions which reproduce collision induced spectra accurately as function of frequency and temperature on computers of small capacity in seconds for a selection of molecular systems.     

Effect of helium addition on output power of a CW optically pumped FAR infrared methanol laser

The results of an experimental study of the effect of He addition as a buffer gas on the output power of a CW optically pumped methanol FIR laser are presented. Results of the measurements are compared with results of calculations executed on the basis of the theoretical model proposed earlier [J.S. Bakos, Preprint KFKI-1990-171D, Hungarian Academy of Science (1990)], and that of the results of gain and line broadening measurements [J.S. Bakos, T. Kárpati and Z. Sorlei, Int. J. Infrared Millimeter Waves 6, 867 (1985)], and good agreement is found.     

Atomic coherence effects on optically pumped cesium atoms in static magnetic field

The population difference between ground states F = 4 and F = 3 is calculated for Cs atoms pumped on the D₂ line by a resonant laser beam. The pumping efficiency for Cs atoms in a static magnetic field on two hyperfine transitions (6S_1/2 F = 4 → 6P_3/2 F′ = 3 and F′ = 4) is calculated for various pump laser intensities. The population difference as a function of the static magnetic field exhibits a dip centered at the zero magnetic field, which corresponds well with the Zeeman coherence between sublevels of the F = 4 state. The full-width at half-maximum (FWHM) of the dip as a function of the pump laser intensity shows abnormal power broadening behavior that differs for different hyperfine transitions. We present experimental results that agree with the theoretical calculations.     

Observation of new laser transitions and saturation effects in optically pumped NO

We report investigations of an NO laser employing specially profiled magnetic fields of up to 3.4T, and F₂ pump laser intensities as great as 20 MW cm^–2. We have observed laser oscillation at 226 nm on a rotational branch of the B'-X/it(3–11) band of NO for the first time, in addition to the previously reported oscillation at 218 nm on the B'-X/it(3–10) band. We have also observed visible laser emission on a rotational branch of the B ²-B ² II(3–1) band of NO. Saturation of the NO laser pulse energy with pump intensity has been observed, the total NO laser pulse energy having been increased to 490 J. The possibility of increasing the NO laser pulse energy towards 1 mJ per transition is discussed.