Bistability of a CO2 laser with SF6 intracavity as an absorber: Transient effects

We investigated time-dependent effects related to the bistable operation of a CO₂ laser with an intracavity nonlinear absorber (SF₆). The response of the system to small perturbations was sensitively dependent on the operating laser line [P(12)→P(30)] of the 10.6 μm band. The narrow bistable region forP(12), due to a very narrow hysteresis loop, can be effectively utilized for optical switching, because of its pronounced sensitivity to variations in operational parameters. The maximum admissable beam blocking time (intracavity) for spontaneous return to lasing was measured forP(16), for a range of absorber pressures. This τm is related to the relaxation time of the upper level in the absorber.     

Bistability and passive Q-switching of a CO2 laser with saturable absorber

The coexistence of bistability and repetitive passive Q-switching of a CO₂ laser with a gaseous saturable absorber has been investigated experimentally. With CH₃F as the saturable absorber bistability has been observed simultaneously with passive Q-switching, whereas with CH₃OH either one of these phenomena can only exist, as demonstrated for the first time. The measured features of both systems can be explained qualitatively by rate-equation theory.     

Self-focussing of CO2 laser radiation in SF6

Self-focussing of high-power TEA CO₂ laser pulses for a number of 10 m P-band lines is reported in SF₆ molecular gas. Application of this effect to estimating the intensity-dependent refractive index of the gas is demonstrated.     

Sensitive intracavity photoacoustic measurements with a CO2 waveguide laser

A photoacoustic intracavity configuration is presented; a resonant photoacoustic cell excited in its first longitudinal mode is placed inside the cavity of a CO₂ waveguide laser. Due to the high laser power and the sharp intracavity focus, saturation effects occur in the excitation and relaxation process of absorbing C₂H₄ molecules. A more optimal configuration is applied to measure the C₂H₄ emission of several Rumex species. A detection sensitivity of 6 ppt (parts per trillion) C₂H₄ is reached, equivalent to a minimal detectable absorption of 1.8×10^–10 cm^–1.     

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.     

Identification of the SF6 transitions pumped by a CO2 laser

The quantum numbers (J values and octahedral symmetry types) of the SF₆ transitions from the ground state to v₃ = 1 that fall within ± 1.5 G Hz of the CO₂ P(14), P(18), and P(20) laser lines have been assigned. The SF₆ absorptions nearest these three laser frequencies are R(28) A⁰₂, P(33) A¹₂, and an F₂ component of P(59) or P(60), respectively.     

Bistability and spatial hysteresis in an Nd:GdVO4 laser with an intracavity twisted-nematic liquid crystal

In this study, optical bistability accompanied by spatial hysteresis was demonstrated in an Nd:GdVO₄ laser with an intracavity twisted-nematic liquid crystal. The low branch of power hysteresis mainly has the spatial distribution of the fundamental mode as the pump power increases, whereas a four-mode and varying transverse pattern exist in the high branch of power hysteresis as the pump power decreases. The result revealed that the power and spatial hysteresis is controlled by the director axis reorientation and the order parameter modification in twisted-nematic liquid crystals, which is determined by the non-uniform transverse distribution of the intracavity laser intensity.     

Geometrical effects on the isotopically selective photodissociation of SF6 under CO2 laser pulses

A linear relationship is found between the³⁴S isotopic enrichment factor per pulse and the focal distance of the lens used to concentrate the laser beam. From this, one derives a threshold power of 26 MW/cm² for photodissociation, a mean absorption of 100 photons of 10.59 μm wavelength per³²SF₆ dissociation, and a dependence of the enrichment factor on the 3/2 power of the laser pulse energy.     

Analysis of Q-switch in a CO2 laser with saturable absorber

Intracavity experiments with an SF₆ saturable absorber have been performed with a CO₂ infrared laser. Accurate measurements of the passive Q-switch pulses versus the laser parameters have been made. A comparison with theoretical models is presented, discussing the involved approximations.     

Multiphoton absorption of broad-band CO2 laser radiation by SF6

Increase of the emission bandwidth of a high-pressure CO₂ laser up to 1.5 cm^–1 increases the multiphoton absorption cross-section of SF₆. Comparison with the previously found [9] increased absorption for shorter pulses suggests that this is also a bandwidth effect. Spectral structures as narrow as 1 cm^–1 above the 10^th absorption step are invoked to explain the observations. The temperature effect, which disappears in the broad-band case, confirms this view.     

Molecular dissociation of SF6 by ultra-short CO2 laser pulses

We have measured the absolute dissociation probability of SF₆ gas irradiated by CO₂ laser pulses varying in duration from 0.5 nsec to 100 nsec. We find a threshold for dissociation of ? 1.4 J/cm², independent of pulse duration. For fixed energy density, the dissociation probability increases as the pulse duration decreases, but not nearly to the degree expected from theory. We have also determined the dissociation probability in the ν₂ + ν₆ combination band and find it to be 10³ times less than in the ν₃ band, in contradiction to some conclusions recently reported by Ambartzumian et al. The dissociation mechanism takes place without collisions, and if collisions occur, they tend to be detrimental to the dissociation rate.     

Regular and random output-power pulsations of a TEA CO2 laser with an intracavity nonlinear medium

The lasing dynamics of a pulsed atmospheric-pressure (TEA) CO₂ laser with a nonlinear medium (semiconductor InAs or InSb) inside the cavity is experimentally investigated. It is observed that introduction into the CO₂ cavity of nonlinear media having high nonlinear-absorption and a nonlinear refractive index leads, on the one hand, to a limitation of the emission intensity in the cavity and to an increase of the pulse duration (FWHM) from 0.1 sec to 2 sec, and on the other to appearance, on the output-power pulse envelope, of spikes having irregular amplitudes and durations, and separated by irregular intervals. It is established that the transition from the intermode-beat-induced regular pulsations to chaos has an emission-intensity threshold.Quantum Radiophysics Division, Lebedev Physics Institute. Translated from Preprint No. 136 of the Lebedev Physics Institute, Academy of Sciences of the USSR, 1989.     

Dependence of characteristics of a gas laser on the parameters of an intracavity absorber

The characteristics of a gas laser with an intracavity saturable absorber are calculated in the third order approximation of Lamb's selfconsistent theory. Attention is paid especially to the dependence of the amplitude of the inverted Lamb dip on the characteristics of the absorber (Lorentz width, shift of central frequency etc.). The results are compared with those of a hole-burning theory and discrepancies are explained.Behová 7, Praha 1, Czechoslovakia.The author wishes to express his thanks to Prof. W. R. Bennett Jr. for pointing out this problem and continuous attention during author's stay at Dunham Laboratory of Yale University and to Ing. A. Novotný for his help with developing the program for the computer.     

Radially polarized 3 kW beam from a CO2 laser with an intracavity resonant grating mirror

A 3 kW radially polarized beam is generated from a CO(2) laser by means of a resonant grating mirror. The design results and spectral characterization of a broadband polarizing mirror are presented as well as the mirror's operation in a CO(2) laser resonator. In a long-term experiment a radially polarized beam with a power of 2.7 kW was observed for several hours, proving the high stability of this polarizing scheme.     

Optical bistability of a CO2 laser with intracavity saturable absorber: Experiment and model

The optical bistability in the output of an ir CO₂ laser containing an intracavity SF₆ absorber was observed as a function of the laser pumping inversion. The laser parameters were determined through a set of independent measurements. A theoretical model valid within the mean field approximation and taking into account the exponential amplification and attenuation of the radiation inside the Fabry-Perot cavity was developed. The state equation of the laser with the saturable absorber was solved graphically and the solution was compared to the experimental results.     

Efficiency of an electron-beam-pumped chemical laser with an SF6-H2 working mixture

Results are presented from a study of HF lasers pumped by non-chain chemical reactions initiated by a radially convergent and by a planar electron beam. The main channels of formation of vibrationally excited HF molecules are analyzed. The distribution of the energy density of the radiation in the output beam of a wide-aperture laser is measured. In 30 liters of a mixture of SF₆:H₂=8:1 at a pressure of 1.1 atm an output energy of ∼200 J is obtained at an ∼11% efficiency with respect to the energy deposition. It is shown that the admixture of a buffer gas of neon or argon improves the uniformity of the radiation energy distribution in the output beam of an HF laser pumped by a non-chain chemical reaction and initiated by an electron beam, and it also increases the output energy. Zh. Tekh. Fiz. 69, 76–81 (January 1999)     

Collisionless dissociation of SF6 using two resonant frequency CO2 laser fields

The collisionless dissociation of SF₆ has been studied using simultaneous irradiation by two frequencies from a CO₂ laser which are both nearly resonant with the SF_6v3 absorption band. It was found that the dissociation was enhanced, and occurred over a wider frequency range, than for single frequency dissociation. No threshold effect was observed for a weak resonant and a much higher energy field pumping slightly off-resonance. For such two frequency irradiation, the peak in the dissociation curve was found to be shifted to lower frequencies with respect to that for single frequency dissociation.     

On the shape of power resonances in a CO2/SF6 laser with a linear three-mirror cavity

Results of experimental and theoretical studies of the shape of power resonances of a single-mode CO₂/SF₆ laser with a linear three-mirror cavity for cases when the amplifying and the absorbing media are located in various sections are reported.