A. N. Soldatov

A sealed-off strontium vapor laser

A sealed-off strontium-vapor laser for medical applications is examined. This is an integrated system that accommodates an excitation circuit, a laser cavity, and an active element. The active medium is excited by means of a modified Blumlein circuit. An unstable resonator of the telescopic type allows a near-diffraction-limited laser beam to be generated. Lasing is obtained in atomic strontium lines at λ=2.06, 2.2, 2.69, 2.92, 3.011, and 6.45 μm and in ionic strontium lines at λ=1.033 and 1.091 μm. We have studied experimentally the behavior of spectral distribution of the output power at varying power delivered to the discharge. It is found that 95% of laser radiation is concentrated in the line at λ=6.456 μm, which corresponds to a lasing power of ~ 2.5 W. Moreover, the time characteristics of lasing pulses are investigated. The radial inhomogeneity of the laser beam is examined. We have conducted lifetime testing of Sr-vapor active elements. The average output power exhibits a modest decrease (5%) within 300 h of a continuous operation. Notably, the pumping characteristics remain unchanged.

Ablation of soft tissue at 6.45 μm using a strontium vapor laser

A gas discharge strontium vapor laser has been shown to operate with up to 90% of its light emitted at 6.45 μm. We have investigated the use of this laser as a potential stand-alone, tabletop alternative to the FEL for ablation of soft tissue. This custom-made laser currently delivers up to 2.4 watts of average power at 13 kHz pulse repetition rate (range 5-20 kHz). Despite a poor spatial beam profile the laser has been shown to ablate both water and soft tissue. However, current pulse energies (< 185 μJ) are insufficient for single pulse ablation even when focused to the smallest possible spot size (130 μm). Instead, the high pulse repetition rate causes the ablation to occur in a quasi CW manner. The dynamics of ablation studied by pump-probe (Schlieren) imaging and macroscopic white light imaging showed micro-explosions but at a rate well below the pulse repetition frequency. Histological analysis of ablation craters in bovine muscle exhibited significant collateral thermal damage, consistent with the high pulse frequency, thermal superposition and heat diffusion. Efforts to increase the pulse energy in order to achieve the threshold for pulse-to-pulse ablation are ongoing and will be discussed.

The possibilities of a strontium vapor laser using for laser sensing of minor gaseous components of the atmosphere

The present work studies the possibility of application of the multiwavelength laser on SrI and SrII self-terminated transitions for laser remote sensing of minor gaseous components (MGC) and meteorological parameters of the atmosphere using the differential absorption lidar (DIAL) method.

PRR performance of Cu- and CuBr-vapor lasers

Results obtained from comparative analysis of the pulse repetition rate performance of Cu- and CuBr-vapor lasers operated at high pump pulse repetitions (approximately 100 kHz) are reported. For a CuBr laser with a 8 mm diameter discharge tube the laser pulse repetition rate as high as 270 kHz was realized.

Evaluation of metal vapor laser designs with radial separation of the active medium

Recent advances in self-terminating metal-vapor lasers have largely resulted from the feasibility of scaling laser characteristics in the cylindrical configuration of the active medium and longitudinal pulsed discharge, which makes it possible to provide the average power W > 100W from a large bore laser tube. Increasing the active volume, however, at the expense of a larger bore for this geometry of the gas discharge channel substantially reduces the specific energy Esp and the average specific power Wsp. Notably, the best laser characteristics have been realized with a low average specific input power Psp. The latter ranged between 1.5 and 0.5 W/cm3 for 6-12 cm bore tubes. As Psp was increased above a certain value, Wsp and W were found to decrease. As that took place, there appeared high radial inhomogeneities in the laser power distribution. Among the things which interfere with further increase of W, Wsp, and Esp as the input energy is increased, are radially nonuniform overheating of the active medium and very high degree of ionization. Given high input energies, these factors will give rise to a substantial deficit of ground state metal atoms N(O) at the center of the laser tube. As Psp is increased, the valley in the radial thermal distribution N(R) gets deeper due to ambipolar diffusion. The N(R) variation with excitation conditions has been studied experimentally for cylindrical laser tubes. The primary processes involved have been examined by means of the saturated power model. In this work we have studied laser action from Cu, I, and AuI in a tube whose configuration allows us to ameliorate the effect of a number of limiting factors on the output energy performance, on the one hand, and provides transversely separated excitation zones on the other, which, in turn, makes it possible to realize optimal thermophysical characteristics of the active medium, manipulate the spatial distribution of metal vapor, including the case of simultaneous excitation of different chemical elements.

Scaling of strontium-vapor laser active volume

Variations in the energy performance of a self-terminating Sr-vapor laser (SrVL) are examined. The active laser volume is varied between 20 and 650 cm3. A linear relation is revealed between the average power delivered by the SrVL and its active volume. The SrVL efficiency is found to increase with active volume and to be comparable with that of a copper-vapor laser for an active volume V = 650 cm3 (0.45 %). As the volume is increased, the total lasing pulse duration increases from 30 to 120 ns. The beam divergence problems associated with the use of a Fabry-Perot cavity or an unstable resonator of the telescopic type are discussed. A total average power of 13.5 W is obtained from V = 650 cm3 at a lasing PRR F = 19 kHz. The output power generated at different laser wavelengths is as follows: 10.4 W at λ = 6.456 μm, 2.6 W at λ = 3 μm, and 0.5 W at λ = 1 μm. The wavelength dependence of the lasing pulse duration is considered.

Radial characteristics of the stimulated emission of copper-vapor lasers

Copper-vapor lasers are currently under investigation with the definite aim of increasing the efficiency and mean power of such lasers. One way of doing this is to increase the repetition frequency of the pulse discharge and increase the volume of the active medium. However, if the interval between the pulses is reduced, cumulative effects begin to play a part in determining the state of the active medium before a new pulse. The main such effects are evidently the accumulation of heat, electrons, and metastable atoms. Ultimately, they reduce the pulse power and mean power of the laser. The reduction is characterized by the fact that the power of the laser emission decreases primarily in the center of the tube, and the transverse structure of the beam acquires an annular shape. Moreover, with increasing diameter of the working channel, these features are manifested at lower repetition frequencies. [1]. The present paper gives experimental data on this phenomenon and an attempt to explain it on the basis of the radial characteristics of the active medium.

Thermal Cleavage of Optical Glass by Strontium-Vapor Laser Radiation

Controlled thermal cleavage of optical glass by strontium-vapor laser radiation was studied. It was shown that high-quality complete separation of display glass is determined by the multiwave spectrum of the absorbed radiation. Sr-Laser radiation (λ = 6.45 μm) absorbed in a about 200 μm thick ‘top’ layer forms a surface microcrack, just as in the case of CO2 laser radiation (10.6 μm). Radiation in the about 1 and 3 μm range forms a ‘volume’ microcrack. Together these cracks create an additional effect making it possible to increase in the periodic-pulse regime the rate and quality of the laser separation of optical glass at comparatively low average lasing power.

On the Mechanism of Limitation of the Frequency-Energy Characteristics of Metal Vapor Lasers

Electrophysical processes in the discharge circuit of a pulsed metal vapor laser are analyzed. The greatest attention is paid to the initial period of the discharge development and conditions for the inversion formation. It is shown that the limitation of the frequency-energy characteristics (FEC) of lasing is due to the process of populating the metastable levels of metal atoms on the excitation pulse front and redistribution of the rates of population of the laser levels in favor of metastable ones with an increase in the prepulse electron density. Which of the processes plays a decisive role in limitation of the lasing FEC depends on the electrophysical process in the discharge circuit of the laser, the development of which is significantly influenced by the arrangement of electrodes in the gas-discharge tube (GDT). The arrangement of electrodes in the GDT also determines the conditions for the inversion formation and the choice of the optimal pumping parameters. Technical solutions are discussed under which the pumping efficiency of a copper-vapor laser attains ~10%.

Development of multiple-wavelength oscillation in plasma of a pulse-periodic He + Ne + Sr laser

Mechanisms of multiple-wavelength oscillation at the lines of He I, Ne I, Sr I, and Sr II ions in the active medium of a high-voltage pulse-periodic laser based on a He + Ne + Sr mixture at a total pressure of ∼200 Torr and duration of the current pulse of about 150–300 ns are analyzed. An important role played by collisional thermal mixing of the laser levels of Ne I and Sr I in multiplets, as well as by collisions of the second kind between metastable and unexcited atoms, in populating and depopulating atomic levels is demonstrated. Absolute populations of the upper and lower levels of the laser transitions in Ne I are found. It is discovered that the degree of ionization nonequilibrium of plasma, both during and after the pulse of current, determines the mechanism of population inversion in laser transitions.