Pulse-periodic Regime Research Articles

Stabilization of the parameters of closed-cycle electron-beam-controlled CO2lasers

A study was made of a closed-cycle electron-beam-controlled CO2 laser operating in the pulseperiodic regime and using a CO2–N2–He mixture with an oxygen content of ~0.2%. It was found that a rapid decrease in the deposited energy was due to the formation of traces of nitrogen dioxide (~ 10−3%) in the active mixture. Addition of carbon monoxide in amounts of [CO] – 2[O2] ≥ 0.1–0.3% and preliminary regeneration of the mixture improved the energy characteristics of the laser and stabilized them at a level 20% higher than the initial values. It was found that constancy of the deposited energy was achieved because of stabilization of the NO2 content in the mixture.

Ejection of material from a solid target subjected to the combined action of two laser pulses of differing durations

An experimental and theoretical investigation was made of the hydrodynamics of melting of a metal by the action of a laser pulse of complex shape. An increase in the efficiency of ejection of the melt was discovered experimentally. The motion of the melt under the action of combined (pulsed and cw) laser radiation obeyed hydrodynamic equations in the momentum approximation. The question of modeling the pulse-periodic metal-processing regime and optimizing the pulse shape for laser drilling of metals was considered.

Increase in the repetition frequency of XeCl laser pulses to 1 kHz

An investigation was made of the possibility of increasing the repetition frequency of XeCl laser pulses by altering the rate of flow, the degree and homogeneity of preionization, and composition of the gas mixture. The concentration of xenon had a strong influence on the homogeneity of a discharge in the pulse-periodic regime and, consequently, on the maximum repetition frequency of laser pulses. The experimental dependences were explained on the basis of a criterion allowing for the processes of multistage ionization and for the influence of inhomogeneities of the gas density, observed in the pulse-periodic regime of the operation of this excimer laser, on the maximum energy which could be deposited in the discharge.

Output characteristics of a gadolinium scandium gallium garnet laser operating in the pulse-periodic regime

A pulse-periodic neodymium laser was made from a new crystal, which was gadolinium scandium gallium garnet activated with chromium and neodymium ions (GSGG:Cr3+: Nd3+). When the pump energy was 7.3 J per pulse, the output energy in the free-running regime was 255 mJ, i.e., the absolute efficiency was 3.5%. At the maximum pulse repetition frequency (20 Hz), which was limited by the nature of the power supply unit, the average output power was 5 W when the pump power was 145 W. The differential efficiency was 4.5%, which was 2.4 times higher than for an YAG:Nd3+ crystal operated under similar conditions. The diameter of the GSGG active element was 5 mm and its length was 50 mm. The differential efficiency in the Q-switched regime was 3% when the pump energy was in the range 3–7 J.

Cascade CO2lasers with electric discharge excitation

The results are given of detailed calculations relating to continuous-flow CO2 lasers with electric discharge excitation, utilizing cascade transitions in the pulse-periodic regime. An analysis is made of systems with preliminary pumping of the active medium by a discharge and excitation of the gas by a continuous discharge occurring directly in the resonator. The likely energy parameters of such lasers are calculated and the optimal conditions for their operation are determined. Consideration is given to experiments designed to determine the probabilities of relaxation of the vibrational levels of paired CO2 modes.

Influence of thiourea on the emission characteristics of a laser based on an aqueous solution of rhodamine 6G

An investigation was made of the spectral (luminescence and lasing) characteristics of an aqueous solution of rhodamine 6G with an addition of thiourea. When the thiourea concentration in the solvent was ?30%, the absorption and fluorescence spectra changed greatly, the lasing threshold decreased approximately fourfold, and the output power increased by an order of magnitude. The good thermooptical properties of water as a solvent, in combination with the disaggregation properties of thiourea, made it possible to realize (without circulation of the solution) a pulse-periodic lasing regime at a repetition frequency of ?50 Hz and with output radiation parameters typical of a laser with continuous circulation of an ethanol solution of rhodamine 6G.

Characteristics of the pulse-periodic regime of excimer lasers

An investigation was made of the influence of inhomogeneity of the gas density in the pulse-periodic regime on the average power of an excimer laser. The experimental results agreed qualitatively with the proposed model. An average power of ~130 W was obtained from a pulse-periodic XeCl excimer laser at a repetition frequency of ~500 Hz.

Electron-beam-pumped multicomponent semiconductor laser emitting visible radiation

Stimulated emission of visible radiation was obtained from multicomponent semiconductor lasers pumped by an electron beam. The lasers operated at room temperature in a pulse-periodic regime. A cadmium sulfide laser emitted radiation pulses of 100 kW power.

Influence of acoustic waves on the output power of pulsed gas discharge lasers

It was established that acoustic oscillations are excited in the active media of pulsed gas discharge lasers having high specific pulse input energies. It was also found that in the pulse-periodic regime the excitation of acoustic oscillations leads to a redistribution of the density of the active medium in the discharge tube, and this results in a resonant dependence of the average laser output power on the repetition frequency of the excitation pulses.

Optimization of the average power of pulsed-periodic KrF and XeCl excimer lasers

The possibility is considered of increasing the average power of KrF and XeCl excimer lasers operating in the pulse-periodic regime without raising the circulation velocity of the gas mixture around the closed loop. By improving the homogeneity and stability of the volume discharge, an average power of ~40 W was achieved and laser operation over a period of many hours was obtained.

Use of high-concentration Li–Nd–La phosphate glass inQ-switched lasers

Results are presented of an experimental study of the use of high-concentration Li--Nd--La phosphate glass in Q-switched lasers for the giant-pulse and pulse-periodic regimes. When an electro-optic switch and a rotating prism were used for Q switching, a 1% efficiency was achieved. When a passive switch utilizing color centers in LiF was used, the laser efficiency was 0.5%. A method of optimizing a Q-switched laser for a given output energy is described.

Investigation of the temperature characteristics of an ammonia laser

An investigation was made of the temperature characteristics (200–400 °K) of an ammonia laser pumped optically with CO2 laser radiation and utilizing both pure ammonia and an NH3–N2 mixture as active media. It was established that cooling of the active mixture resulted in a considerable increase in the laser efficiency in the case of pure ammonia, while the addition of nitrogen increased the operating temperature range in the case of the NH3–N2 laser. It was found that when the mixture was cooled additionally, the average output power increased in the pulse-periodic regime.

Numerical investigation of a pulse-periodic copper vapor laser

A method is proposed for numerically investigating a copper vapor laser. The results obtained for transversedischarge lasers with a high buffer gas pressure are in agreement with experiment. The optimal transverse dimensions of the active zone are determined under conditions of efficient gas heating. A method is given (and confirmed by numerical calculation) for estimating the electron density and temperature and the population of the metastable levels in the pulse-periodic regime.

Plasma parameters in the afterglow of a copper vapor laser

In the afterglow discharge of a copper vapor laser, operating in the pulse-periodic regime at a frequency ƒ ~ 7 kHz, measurements were made of the electron density and temperature and of their distributions over the tube radius. An estimate was made of the degree of copper atom ionization on the axis of the discharge.

Changes in the energy characteristics caused by pulseperiodic operation of electron-beam-controlled discharges in CO2–N2–He mixtures and in technical-grade nitrogen

A study was made of the changes in the characteristics of electron-beam-controlled discharges in CO2–N2–He mixtures and in technical-grade nitrogen which resulted from the operation of the laser in a pulse-periodic regime. It was established that in the range E/p≤5 kV cm−1 atm−1 and for specific input energies not exceeding 500 J liter−1 atm−1 the changes in the discharge characteristics of the investigated CO2–N2–He mixtures and technical-grade nitrogen were entirely due to the action of the electron beam. The reduction in the specific input energy was due to the formation, as a result of plasmachemical reactions, of nitrogen oxides whose molecules had very large (~10−10 cm3/sec) electron-attachment rate constants.

Electric-discharge excimer lasers

It is demonstrated that excitation systems of the type AIL–1 nitrogen can be applied also to obtain stimulated emission from XeF*, KrF* and XeCl* molecules. The dependences of the output radiation energy on the composition and pressure of the mixture and oscillograms of the radiation pulses are given. For discharges in chambers of 100×1.8×0.3 cm dimensions and the He:Xr:CCl4=750:12:1 mixture the output radiation energy was 21 mj (λ≈308 nm) and for the He:Xe:NF3=100:1:1 mixture it was 15mJ (λ≈350 nm). It was found that the halogen compound CHCl3 was a desirable additive to lasers operated in the pulse-periodic regime.

Preionization of a gas by electrodeless capacitative pulses in lasers operating in the pulse-periodic regime

A method was developed for the preionization of a gas in high-pressure pulsed lasers. An electrodeless breakdown was induced in the gas by high-voltage pulses applied to the bulk of the gas by a capacitative method. This method was applied to a pulse-periodic CO2 laser. The experiments carried out on this laser confirmed the effectiveness of this relatively simple preionization method, which ensured a high homogeneity of the discharge at atmospheric pressures.

Characteristics of stimulated emission from LiNdLa phosphate glass

An investigation was made of the characteristics of LiNd0.3La0.7 phosphate glass with 8?1020cm-3 of Nd3+ These characteristics were obtained under free-oscillation, Q-switching, and pulse-periodic conditions. At moderate pumping rates the emitted radiation consisted of the TEM00 mode with near-diffraction divergence. The investigated glass could emit in the pulse-periodic regime at a repetition frequency up to 100 Hz. The maximum average output power was 6.5 W when the pump power was 1 kW.

Resistance of dielectric-coated interference mirrors to laser-induced damage

Laser-induced damage in multilayer dielectric mirrors made from TiO2 and SiO2 was investigated for both single-pulse and pulse-periodic operation. The damage thresholds were measured and found to be lower in the pulse-periodic regime for mirrors having a low reflection coefficient at the laser wavelength. The thresholds of single-layer TiO2 and multilayer TiO2?SiO2 coatings were governed by the damage resistance of the TiO2 surface layers. The effect on the damage threshold of varying the radiation intensity at the boundaries of the TiO2 layers was examined by shifting the spectral characteristics of a mirror relative to the laser wavelength.

Electron-beam-controlled gas-flow CO2laser

Solutions are obtained describing cw and pulse-periodic lasing regimes in a CO2 laser with high mean output power. In the pulse-periodic regime the mean power and the repetition frequency are at present limited to 450 W and 33 Hz in a 40?10?10 cm discharge. The laser properties are all described accurately by theoretical expressions obtained on the basis of a four-temperature model. The use of this model, together with the gasdynamic equations, provides a reliable method of predicting the operation of a supersonic-flow laser in the cw regime.