Fast Axial Flow Research Articles

Novel considerations of functions of helium and nitrogen in a fast flow CO 2 laser

In a research of fast axial flow CO 2 laser sustained by 150 kHz silent discharge, we found the optimized gas mixing ratio was CO 2:N 2:He=1:22:5 or the content of helium was only about 18%. This result upset the situation of common CO 2 lasers in which the most important laser gas is helium. An explanation of our particular results and supporting experimental evidence are given.

Transient behaviors of output power in fast-axial flow-type CO laser

Transient behaviors of output power were experimentally investigated with the parameters of discharge current I/sub dis/, discharge length l, gas-flow velocity /spl nu/, and O/sub 2/ concentration for a compact dc discharged fast-axial flow (FAF) CO laser, and were theoretically explained, together with some empirical equations. Introducing the discharge time (t) dependence into the gas temperature and the concentration of O/sub 2/ and CO/sub 2/ molecules, based on the experimental data and the mass-spectrum analysis, respectively, the overall population density of CO molecule n/sub /spl nu/'/ toward an arbitrary vibrational level has been expressed as a function of t. Then, since the gain parameter can also be given as a function of t, together with the measured laser parameters such as the saturation intensity I/sub s/, a measure of partial homogeneity m, and the Voigt function /spl psi/, the transient behaviors of the output power as a function of t were well explained, being in good agreement with the experimental results and the calculations. From these analyses, it was found that control of the gas temperature rise and suppression of both the decrease of O/sub 2/ and the increase of CO/sub 2/ concentrations are required to relax this unnecessary power reduction due to the transient behaviors.

Optical inhomogeneities of the active media of high-power industrial CO2 lasers with fast axial flow

Theoretical and experimental investigations showed that a diverging thermal gas lens, which forms in high-power industrial lasers with fast axial flow of the active medium, can influence the diffraction losses in the laser oscillator and the beam parameters. The nonlinear optical properties of the lens, demonstrated by the dependence of its optical power on the intensity of the output radiation, are described.

Influence of a radial inhomogeneity of the active medium on the output radiation power from a cw CO2 laser with fast axial flow

A radial inhomogeneity of the gain in a cw CO2 laser with fast axial flow was studied experimentally and theoretically. It was found that this inhomogeneity resulted from a radial inhomogeneity of the temperature and contraction of a longitudinal self-sustained discharge towards the axis in a gas-discharge tube. This saturated the rise of the small-signal gain when the total discharge current was increased. It also reduced the rate of rise of the output radiation power with increase in the input energy when the fundamental Gaussian mode was generated.

Amplification and nonlinear losses in a cw CO2 laser with fast axial flow

A study is reported of the influence of a laser mixture relaxing outside the discharge zone on the parameters of the radiation emitted by a cw CO2 laser. It is shown that resonant nonlinear absorption of the radiation may appear, in a certain range of the laser parameters, behind the discharge zone. The dependence of such absorption on the radiation intensity is complex because of saturation and also because of a change in the relaxation length of excited molecules in the radiation field. The reflection coefficient of the exit mirror and its geometric parameters ensuring the maximum laser efficiency are determined, taking account of the additional amplification in the relaxing laser mixture.

Aerothermic characterization of a high-power fast axial flow CO 2 laser

Because the structure of the flow in glow discharge tubes for high-power CO 2 lasers is one of the main physical processes to control in order to avoid arcing, an experimental procedure for studying the velocity and temperature fields in such tubes was performed. Two kinds of injection system have been studied. For the first one, the gas mixture is injected by a single sonic nozzle located on the axis of the tube. Because the power of the laser may be increased by associating several tubes in line, the other injection system consists of a series of 12 holes radially arranged in order to move injectors away from the axis. For both cases, the axial mean time velocity profile was measured (without electric discharge) using a laser Doppler anemometry technique in several sections of the discharge tube. The temperature field was investigated during the discharge by using a special thermofluorescent probe. It is found that turbulence facilitates the homogenization of the discharge.

The Effects of Process Variables on the Case Depth of Laser Transformation Hardened AISI 01 Tool Steel Specimens

Abstract An investigation was carried out to study the effects of speed, nozzle gap and power input on the case depth of laser transformation hardened ASSAB DF2 (equivalent to AISI 01) tool steel specimens. The specimens, used were 5 and 10 mm thick and 10 and 25 mm in diameter. A defocussed beam from a fast axial flow C02 laser was used with continuous-wave power inputs of 0.4 to 0.6 kW. The traversing speeds used were 5, 10 and 15 mm/s. The nozzle gaps were 5, 10 and 15 mm which gave calculated beam diameters of 0.129, 1.174 and 2.276 mm respectively. It is found that, the case depth increases as either the specimen thickness or power input is increased, but decreases as the: traversing speed is increased. The beam diameter, however, does not seem to have a significant influence on the case depth.

High Power CO2 Laser Excited by TM010-Mode Microwave Discharge.

Microwave discharges for excitation of CO2 laser have been investigated. A resonant microwavecavity (2.45 GHz) has been employed to get high power laser operation. Laser experimentsusing a system composed of fast axial gas flow and axial electric field parrallel to the laserbeam have been performed. Both electric field distribution and resonant frequency of thecavity calculated by using the SUPERFISH were compared with the results of the measurements.Homogeneous discharges caused by the resonance of TM010 mode was obtained usingcylindrical cavity with reentrant type tuners on the both end plates. The laser output of 259 Wand the efficiency of 14% were obtained from a newly developed two-cavity CO2 laser module.

Correlation between CO2 Laser Beam and Material Processing.

This paper briefly introduces experimental results on beam focusing properties and material processing phenomena using a fast axial flow CW CO2 laser.

1-kW, capacitively coupled radio frequency discharge excited CO laser with a closed-cycle fast axial flow

A capacitively coupled transverse radio frequency discharge has been successfully applied to excite a CO laser with a closed-cycle fast axial flow. The discharge and laser characteristics are described and the role of the O2 added to CO/N2/He mixtures is discussed. For a mixture of CO/N2/He/O2=7.5/12.5/80/0.8 an output of about 1 kW has been obtained at an entrance gas temperature of 160 K. Stable output with good beam quality has been demonstrated.

Transverse mode selection in a convectively cooled cw-CO2 laser with a cyclonic flow configuration

Strong cyclonic flow induced discharge confinement has been used for the first time to efficiently select the lower order transverse modes in a fast axial flow high power cw-CO2 laser. The laser performance with and without the cyclonic flow has been compared. Comparison has also been made with the conventional aperture method of selection of lower-order transverse modes.

An inexpensive method of efficiency improvement in fast axial flow, high-power cw CO 2 lasers by the addition of argon

It is shown that a substantial increase in the efficiency and output power of a fast axial flow cw CO 2 laser is possible by the addition of small quantities of argon to the normal three component gas mixture. We have studied the increase in output power as a function of argon concentration and determined that a sharp power maxima occurs at 2.6% concentration.

Correction to the dissociation rate in an open-cycle FAF CO2 laser amplifier.

On the basis of the transient mass-spectrum analysis of CO2 gas dissociation for various input probe powers Pin, plasm alengths l, gas-flow velocities v and discharge currents Idis, the dissociation rate (1-δ) and dissociation factor ξ of a CO2 molecule are found to be modified in an open-cycle fast-axial flow (FAF) CO2 laser amplifier. The effective dissociation constant Deff related to (1-δ) is found to be corrected as a function of v and input probe irradiance Iin, and the dissociation-rate constant ad concerned with the dissociation factor ξ is found to be modified to be v-dependent, though these have been regarded as a constant in a sealed-off system. Furthermore, suction rate δf due to the difference in the evacuation rate between CO2 and CO or O2 is found to be taken into consideration in (1-δ). Corrections for Deff and αd are also qualitatively discussed. By using these corrected parameters, the small-signal gain γ0 is calibrated to explain experimental results satisfactorily.

Fast axial flow CO/sub 2/ laser excited by silent discharge

A fast axial flow CO/sub 2/ laser excited by silent discharge has been developed. With a new electrode system applied to the laser, homogeneous discharge of power density of up to 80 W/cm/sup 3/ is obtained without applying any stabilization techniques on the discharge. An output laser power of 920 W in stable TEM/sub 00/ mode operation is attained with an efficiency of over 14%. Beam fluctuation common to fast axial flow lasers is suppressed by the combination of smooth gas flow and low CO/sub 2/ molar fraction. >

Transient gain phenomena and their improvement in a fast-axial flow CO2 laser amplifier.

The present paper reviews our recent work on transient gain phenomena caused by both thermo & gas dynamic cooling mechanism and dissociation of CO2 molecules in a fast-axial flow (FAF) CO2 laser amplifier. Taking into account a time-dependent gas temperature T and an inverted population n2/n1, we theoretically formulate a transient gain as functions of the gas-flow velocity ν and the discharge current Idis including their spatial distributions along the radial direction from the plasma-tube axis to explain the transient power-decrease ΔP and the transient time tst required to be a stable value with respect to a plasma length l and an input probe power Pin Theoretical calculations are in good agreements with, corresponding experimental results. Possibilities for improving the above transient behavior are also demonstrated by cooling a plasma-tube wall and by use of an Au catalyst.

Spatial variation of gain and saturation in a fast axial flow CO_2 laser amplifier

The spatial variation of gain and saturation is analyzed in weakly and heavily saturated cases for a fast axial flow (FAF) CO(2) laser amplifier with laser gas in laminar and turbulent flow regions. Expressiong the laser parameters by fourth-order even functions of the radial distance, the development of the amplified radiation is predicted and compared successfully to the experimental results for the 10P(18) line. The distinctive feature of a FAF system is enhancement of the gain and saturation irradiance as well as the mode volume. The condition of spatial soliton is also discussed in a heavily saturated case.

Enhancement of Small-Signal Gain and Mode-Volume in a FAF CO2 Laser Amplifier due to Cooling of Wall Temperature

Cooling the wall temperature of a fast-axial flow (FAF) CO2 laser amplifier improved both the small-signal gain and its spatial distribution and increased the effective mode-volume significantly under the turbulent flow condition. Considering the dependences of the gain parameter G and the line-shape function denoted by the Voigt function on the wall temperature Tw and the radial distance r, the experimental results of the Tw-dependent small-signal gain γ on the axis and its radial distribution are theoretically explained. On cooling Tw from room temperature to -30°C, about 17% increase of the gain was obtained and its mode-volume was much improved. The spatial distribution of the small-signal gain becomes almost uniform over the cross section of the plasma tube particularly under the turbulent flow region greater than the gas-flow velocity of 170 m/s.

Dependence of Spatial Gain Distribution on Gas-Flow Velocity and Discharge Current in a Fast-Axial Flow CO2 Laser Amplifier

Taking into account the radial distributions of the gas-flow velocity v, the discharge current Idis, the gas temperature T and the effective pumping rate ηe, we theoretically derive the gain coefficient γ as a function of the radial distance from the plasma-tube axis for a fast-axial flow (FAF) CO2 laser amplifier. For various combinations of v and Idis, the calculated γ's are compared with the experimental results corresponding to both laminar and turbulent flows, resulting in good agreement in both cases. Increase of v leads to a linear enhancement of γ: for example, γ becomes about two times larger with increasing v from 80 to 180 m/s, while there is an optimum current near 45 mA to provide the highest gain in our FAF system. It is especially noteworthy that the high-gain region tends to spread towards an area far off-center of the discharge tube axis with increasing v, and its spreading area becomes significantly broader at a turbulent flow greater than v = 170 m/s.

Dependence of transient gain on gas-flow velocity and discharge current in the FAF CO/sub 2/ laser amplifier

Transient behaviors in which the output power amplified through the fast-axial-flow (FAF) CO/sub 2/ laser amplifier decreases temporally after initiating the discharge are discussed. The output power becomes stable within a few minutes, i.e. the transient time. These phenomena should strongly depend on input irradiance, plasma length, gas-flow velocity v, and discharge current I/sub dis/. The small signal gain gamma /sub 0/ is formulated as a function of v and I/sub dis/ and the discharge time for a partially-homogeneously-broadened, slightly saturated gain medium at individual vibrational-rotational transitions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theoretical analysis of transient gain phenomena in a fast-axial flow type CO/sub 2/ laser amplifier

A decrease of output power observed after discharging plasma tubes in a fast-axial flow CO/sub 2/ laser amplifier with a short dwell time of about 1 ms is discussed. For the 10P(18) line at 50 torr pressure, the small-signal gain goes from 1.308/m to 1.068/m within 135 s after discharging the plasma tubes, for a gas-flow velocity of 180 m/s and discharge current of 60 mA. The fraction of power decrease is about 15% for the plasma length of 80 cm and the input power of 5 W. Dependences of the observed fraction of power-down and the transient time to reach a stable power on input power and plasma length are theoretically analyzed by introducing the CO/sub 2/ dissociation factor as a function of gas-flow velocity, discharge current and plasma length, and gain saturation, resulting in good agreement between the experimental results and computation. >