Two stable configurations of a continuous optical discharge (COD) were observed in experiments with plasma sustained continuously in xenon at high pressure by radiation of a medium power CW ytterbium fiber laser. One is the plasmoid of relatively small length with one temperature maximum and laser beam absorption of 10-30%. The other one is the plasma formation stretched along the laser beam with two or three local temperature maxima. The laser beam absorption in the second plasma configuration is increased dramatically up to 70-80% due to increased plasma length. Both plasma shapes were obtained under close conditions, so that oscillations between the two states were possible and also have being observed. The effect was studied and explained on the base of simplified consideration of the laser beam propagation through lens-like plasma medium surrounded by refractive near-spherical bounds between cold and hot gas. Other experimental results on the sustaining conditions of COD and plasma properties are also presented.
The results of laser beam refraction studies in a continuous optical discharge (COD) stabilized in a focused (f4.4) laser
beam and coaxial gas flow are reported. A plasma ball formed in COD acts as a defocusing plasma lens where lensing
effect occurs mainly due to distributed free electrons.
The properties of the plasma lens depending on the electron density distribution and controlled through a gas flow
velocity have being studied. Intensity profiles of sustaining laser beam (M2 = 6.6) transmitted through the plasma
together with intensity patterns of visible plasma images were simultaneously detected and analyzed.
It was found that the dependency of average refraction angle of the sustaining beam in COD plasma on a gas flow
velocity drops sharply as the velocity increased 1.5 m/s from initial value of 0.06 radians to milliradians, so that at higher
gas flow velocities the beam refraction does not affect plasma properties.
A shape of the curve of the refraction on gas flow velocity reveals the peculiarities of lensing properties of the plasma
ball. The possibility of controlling the lensing effect in plasma by means of the gas flow was also demonstrated.
To increase power density in gas-discharge lasers based on combined DC-CPD discharge technology (DC + Capacitively-coupled Periodic-pulsed Discharge) one have to increase pulse current density of CPD. Thus electrical and chemical processes in CPD were studied in pulse current density limits expanded up to 500 mA/cm2. It was shown that CO2 dissociation rate is increased with current density and pulse repetition rate and is different in near-electrode
sheath and in the discharge gap. Catalytic stabilization method of the gas mixture composition is proposed for CPD at higher pulse current densities.
Authors propose combined DC and capacitive periodic-pulsed discharge as an alternative to self-sustained DC and RF
discharges for excitation of fast-axial-flow CO2 lasers. Experiments show that output power 1.7 kW/m with discharge
efficiency more than 20% may be achieved in proposed discharge scheme. Measured discharge and active medium
characteristics are presented.
Characteristics of CO2 laser with 27.1 MHz transverse RF discharge in quartz tubes were significantly improved by distributed gold catalyst. Parameters of catalytic layer were optimized for RF discharge conditions to avoid RF-current shunting while maintaining good catalytic properties. Output power 0.77 W/cm and maximum efficiency 18.5% were demonstrated in prototype laser.
KEYWORDS: Resonators, Mirrors, Laser resonators, Transmission electron microscopy, Near field, Gas lasers, Near field optics, Output couplers, Carbon monoxide, Reflectivity
Different N-folded multiple-pass resonator schemes were realized and tested in high power fast-transverse-flow CO2 lasers. In all cases beam ellipticity occur due to gain saturation followed by beam self-action in active medium with fast transverse gas flow.
To avoid this effect new resonator structure was proposed where N-folded passes are arranged in different crossed planes declined to the gas flow direction. Fast-transverse-flow industrial CO2 laser "Lantan" with this resonator scheme demonstrates high beam quality up to 2.5 kW CW output power with differential efficiency of 16%.
Variable reflectivity output mirros (VRM) provide a good possibility for output coupling of diffraction limited beams both from gas and solid state laser media. A set of VRM resonators was designed and tested in fast-transverse-flow CO2 laser. It was shown that the phase profile of the output beam in VRM resonator may be improved by the appropriate design of the VRM output coupler. The results of experimental and theoretical studies of unstable VRM resonators operation in CW CO2 lasers with output power up to 3 kW are presented. It was shown that beam generated in VRM unstable resonator may be characterized correctly in terms of M2 and K factors.
Variable reflectivity output mirrors (VRM) provide good possibility for output coupling of diffraction limited beams both from gas and solid state laser media. A set of different VRM resonators was designed and tested in fast- transverse and fast-axial-flow CO2 lasers. It was shown that the phase profile of the output beam in VRM resonator may be improved by the appropriate design of the VRM output coupler. The results of experimental and theoretical studies of stable and unstable VRM resonators operation in CW CO2 lasers with output power up to 3 kW are presented.
Investigations aimed at the production of high power beams with diffractionally limited quality are carrying out in the Institute for Problems in Mechanics since 1992. Up to 5 kW CW output beam characterized by M2 less than 1.7 was produced with Lantan-5 industrial fast-transverse-flow CO2 laser by means of the unstable resonator with variable reflectivity output coupling mirror (VRM). Both theoretical and experimental investigation of the parameters of the beam produced by VRM unstable resonator are presented. The results of beam quality measurements are in good agreement with theoretical evaluations, and processing performance of the beam corresponds to the measured value of M2. The quality of the output beam in resonators with VRM may be further improved by appropriate choice of the parameters of variable thickness reflective layer.
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