Modeling of flowing gas diode pumped alkali lasers: dependence of the operation on the gas velocity and on the nature of the buffer gas.

A simple, semi-analytical model of flowing gas diode pumped alkali lasers (DPALs) is presented. The model takes into account the rise of temperature in the lasing medium with increasing pump power, resulting in decreasing pump absorption and slope efficiency. The model predicts the dependence of power on the flow velocity in flowing gas DPALs and checks the effect of using a buffer gas with high molar heat capacity and large relaxation rate constant between the 2P3/2 and 2P1/2 fine-structure levels of the alkali atom. It is found that the power strongly increases with flow velocity and that by replacing, e.g., ethane by propane as a buffer gas the power may be further increased by up to 30%. Eight kilowatt is achievable for 20 kW pump at flow velocity of 20 m/s.

The I2 dissociation mechanisms in the chemical oxygen-iodine laser revisited.

The recently suggested mechanism of I(2) dissociation in the chemical oxygen-iodine laser (COIL) [K. Waichman, B. D. Barmashenko, and S. Rosenwaks, J. Appl. Phys. 106, 063108 (2009); and J. Chem. Phys. 133, 084301 (2010)] was largely based on the suggestion of V. N. Azyazov, S. Yu. Pichugin, and M. C. Heaven [J. Chem. Phys. 130, 104306 (2009)] that the vibrational population of O(2)(a) produced in the chemical generator is high enough to play an essential role in the dissociation. The results of model calculations based on this mechanism agreed very well with measurements of the small signal gain g, I(2) dissociation fraction F, and temperature T in the COIL. This mechanism is here revisited, following the recent experiments of M. V. Zagidullin [Quantum Electron. 40, 794 (2010)] where the observed low population of O(2)(b, v = 1) led to the conclusion that the vibrational population of O(2)(a) at the outlet of the generator is close to thermal equilibrium value. This value corresponds to a very small probability, ∼0.05, of O(2)(a) energy pooling to the states O(2)(X,a,b, v > 0). We show that the dissociation mechanism can reproduce the experimentally observed values of g, F, and T in the COIL only if most of the energy released in the processes of O(2)(a) energy pooling and O(2)(b) quenching by H(2)O ends up as vibrational energy of the products, O(2)(X,a,b), where the vibrational states v = 2 and 3 are significantly populated. We discuss possible reasons for the differences in the suggested vibrational population and explain how these differences can be reconciled.

Comparing modeling and measurements of the output power in chemical oxygen-iodine lasers: a stringent test of I2 dissociation mechanisms.

A parametric study of the output power of supersonic chemical oxygen-iodine lasers (COILs) is carried out, applying a kinetic-fluid dynamics model calculations as well as an analytical model and comparing the results to experimental studies. The I(2) dissociation mechanism recently suggested by Azyazov et al. [J. Chem. Phys. 130, 104306 (2009)], which was previously used for comparison of model calculations to measurements of the small signal gain [K. Waichman et al., J. Appl. Phys. 106, 063108 (2009)], is applied here for a similar, but more sensitive, comparison of the laser output power. The dependence of the power on iodine flow rate and on mirror transmission is studied for low and high pressure COILs, respectively. Good agreement between the calculated and measured power is obtained for both low and high pressure COILs only when the processes suggested by Azyazov et al. are included in the calculations. This is different from the situation for the gain where for high pressure COILs, the calculated values were insensitive to the assumed dissociation mechanism, although for low pressure the measurements were reproduced only by applying the Azyazov et al. mechanism. We believe that the results of the present work strongly support the application of this mechanism for modeling the COIL operation.

Analysis of the optical extraction efficiency in gas-flow lasers with different types of resonator.

The celebrated Rigrod model [J. Appl. Phys. 34, 2602 (1963)] has recently been shown to be inadequate for calculating the output power of gas-flow lasers when the quenching of excited species is slow and the optical extraction efficiency is high [Opt. Lett. 20, 1480 (1995)]. The previous analysis of two-level systems is presented here in detail and extended to include the chemical oxygen-iodine laser (COIL). For both two-level systems and COIL's, we obtained simple analytic formulas for the output power, which should be used instead of the Rigrod model. We present the formulas for Fabry-Perot, stable, and unstable resonators. Both the saturation parameter and the extraction efficiency differ from those appearing in the Rigrod model. The highest extraction efficiency is achievable for both stable and unstable resonators with uniform intensity distribution over the resonator cross section and is greater than that calculated by the Rigrod model. A rather surprising conclusion is that the extraction efficiency of unstable resonators can be increased substantially if one increases the length of the part of the mirrors lying downstream of the optical axis. The derived formulas are applied to describe published experimental results of supersonic COIL's. The dependence of the power on the threshold gain is evaluated and from this the plenum yield of singlet oxygen is estimated. The value of the yield is in better agreement with experimental measurements than that obtained by the Rigrod model.

Spectroscopy of D2O (2,0,1)

The (2,0,1) <-- (0,0,0) rovibrational transitions of D2O in the near infrared region were measured with a resolution of 0.07 cm-1 using photoacoustic laser absorption spectroscopy. We report on the assignment of newly observed transitions and determine new inertial and centrifugal constants of D2O (2,0,1) using the Watson-type A-reduced Hamiltonian for asymmetric tops.

Optical extraction efficiency in gas-flow lasers.

It is shown that the celebrated Rigrod model [J. Appl. Phys. 34, 2602 (1963)] is inadequate for calculating the output power of gas-f low lasers in an important range of parameters in which the quenching of the excited species is slow and the optical extraction eff iciency is high. For two-level systems very simple analytical formulas for the output power are obtained that should be used instead of the Rigrod model. The formulas are given for Fabry-Perot, stable, and unstable resonators. Both the saturation parameter and the extraction efficiency differ from those appearing in the Rigrod model. Applications of the formulas to systems with more complex kinetic schemes are also indicated.

Spectroscopy and laser characteristics of copper-vapor-laser pumped pyrromethene-556 and pyrromethene-567 dye solutions.

We measured the basic optical properties of Pyrromethene-567 (P567) and Pyrromethene-556 (P556) dye solutions that are relevant to their application as dye lasers. The fluorescence spectra of methanol solutions show mirror images in relation to the absorption spectra, with Stokes shifts of 29.5 and 37.5 nm, respectively, for the two dyes. The central fluorescence peaks were at 546 and 535 nm, with widths of ~40 and ~50 nm (FWHM). The quantum yields were 97% ? 5% and 78% ? 5% for P567 and P556, respectively. Fluorescence lifetimes of 6.0 ? 0.2 ns were obtained for both dyes in methanol. Laser action, obtained by pumping with the green emission line (510.6 nm) from a copper-vapor laser, was measured in a Hänsch-type cavity. Tunability ranged from 531 to 590 nm for P567 and from 522 to 590 nm for P556. Lasing thresholds were ~0.27 and ~0.16 mJ/pulse, with 25% and 27% slope efficiencies for P567 and P556, respectively. Spectroscopy and lasing were studied in other solvents as well.

Analysis of lasing in gas-flow lasers with stable resonators.

A model is developed that describes the power extraction in chemical oxygen-iodine lasers (COIL's) and CO(2) gasdynamic lasers with stable resonators when a large number of transverse Hermite-Gaussian eigenmodes oscillate. The extraction efficiency, mode intensities, and intensity distribution along the flow depend only on two parameters. The first is the ratio gamma(0) of the residence time of the gas in the resonator to the O(2)((1)D) or N(2)(v) energy extraction time and the second is the ratio of the threshold to the small-signal gain. The efficiency is maximum for gamma(0) ? infinity and decreases rapidly as gamma(0) decreases. It is found that for a range of parameters corresponding to the highest efficiencies the intensity distribution along the flow is nonuniform and has two peaks near the upstream and downstream sections of the resonator. In this case only the highest-order modes that totally fill the resonator cross section oscillate (the so-called, experimentally observed sugar scooping bimodal intensity distribution). For the range of parameters corresponding to smaller efficiencies the intensity is uniform. In this case all the modes participate in lasing; however, the intensities of the high-order modes are larger than those of the low order. The current model is compared with the plane-mirror Fabry-Perot resonator model and with the constant intraresonator intensity and rooftop models of COIL's with stable resonators. The extraction efficiency calculated with the last two models is close to that estimated from our model. However, the intensity distribution cannot be calculated correctly using the Fabry-Perot, the constant intraresonator intensity, or the rooftop model.

Dinitrobenzene detection by use of one-color laser photolysis and laser-induced fluorescence of vibrationally excited NO.

Trace concentrations of 1, 4-dinitrobenzene (DNB) are detected by a combination of laser photolysis and laser-induced fluorescence. A one-color laser is applied to induce DNB photodissociation and for subsequent detection of NO photofragments by excitation and emission through A(v' =0) <-- X(v" = 0 - 2) and A(v' =0) --> X(v" = 0,1) transitions, respectively. The resulting NO rovibrational excitation spectra serve as markers for the presence of DNB. The NO is produced in vibrational ground and excited states with peak height ratios of (v" = 0):(v" = 1):(v" = 2) = 1:0.5:0.13. The limits of detection of DNB mixed with 100 or 500 Torr of air with v" = 2 excitation at 248 nm are 13 and 11 parts in 10(9) by weight, respectively, for a 30-s integration time. The application of this scheme for DNB detection has the advantage that no ambient ground state NO interferes and that the fluorescence is collected at shorter wavelengths than the exciting radiation, precluding background fluorescence.

Laser photobleaching leads to a fluorescence grade adenosine deaminase.

The enzyme adenosine deaminase (adenosine aminohydrolase EC 3.5.4.4) from calf intestinal mucosa is commercially available at high purity grade yet, at the sensitivity at which fluorescence studies may be undertaken, a nonpeptidic fluorescence is detectable at lambda exmax = 350 nm and lambda emmax = 420 nm. A sevenfold decrease of this nonpeptidic fluorescence was obtained upon irradiation by the third harmonic (355 nm) of a Nd:YAG laser for 16 min, at 5 mJ/pulse, with a pulse width of 6 ns at a repetition rate of 10 Hz. The decline of fluorescence was accompanied by a negligible loss of enzymatic activity. Moreover, the integrity of the protein was ascertained by (i) its fluorescence (lambda exmax = 305 nm, lambda emmax = 335 nm) and lifetime distribution and (ii) its kinetics in the presence of the substrate adenosine and two inhibitors, all of which remained essentially unaltered. Laser photobleaching is a simple way to achieve a fluorescence grade adenosine deaminase.