A DC excited waveguide multibeam CO2 laser using high frequency pre-ionization technique

High power industrial multibeam CO₂ lasers consist of a large number of closely packed parallel glass discharge tubes sharing a common plane parallel resonator. Every discharge tube forms an independent resonator. When discharge tubes of smaller diameter are used and the Fresnel numberN ≪ 1 for all resonators, they operate in waveguide mode. Waveguide modes have excellent discrimination of higher order modes. A DC excited waveguide multibeam CO₂ laser is reported having six glass discharge tubes. Simultaneous excitation of DC discharge in all sections is achieved by producing pre-ionization using an auxiliary high frequency pulsed discharge along with its other advantages. Maximum 170 W output power is obtained with all beams operating in EH₁₁ waveguide mode. The specific power of 28 W/m is much higher as compared to similar AC excited waveguide multibeam CO₂ lasers. Theoretical analysis shows that all resonators of this laser will support only EH₁₁ mode. This laser is successfully used for woodcutting     

Spectroscopic Study of CO2 Decomposition in Sealed‐off CO2 Lasers

The CO₂ decomposition in a DC glow discharge in a sealed‐off CO₂ laser mixture (6% CO₂ : 20% N₂ : 74% He) in the pressure range 2 ‐ 22 Torr is investigated. Both the time evolution and the steady‐state CO₂ decomposition are studied by means of time resolved optical emission spectroscopy. A decrease of the time to reach steady‐state values of particles concentrations in the discharge system with decreasing pressure and increasing discharge current is observed. The degree of CO₂ decomposition rapidly decreases with increasing pressure and exhibits only a small increase with increasing discharge current. This behaviour can be explained by the variations of the reduced electrica field strength E/N and of the electron density. Also the influence of the balast volume is discussed in this work.     

A spark channel plasma electrode for a CO2 laser gas discharge

A plasma electrode potentially suitable for dc discharge pumped lasers has been developed based on the repetitive creation of a spark channel. The high-density plasma is a source of charge carriers for the dc discharge, thereby largely eliminating the cathode and anode falls. Potential reduction improves with increasing spark repetition frequency until about 9 kHz, at which point the falls are virtually eliminated. Fortuitously, the most beneficial regime of plasma electrode operation also appears to coincide with the optimum E/N range for CO₂ laser vibrational excitation.     

X-ray preionized CO2 laser

A short pulse (100 ns) high-energy x-ray source has been used to preionize a transversely excited carbon dioxide gas discharge laser of 600 cm³ active volume. The maximum output power of 60 MW in a 50 ns FWHM pulse was achieved from a CO₂–N₂–He–CO–Xe static gas mixture at 600 Torr pressure. The energy conversion efficiency was 6%.     

CO2-laser gas discharges in narrow gaps

We have studied RF discharges as excitation mechanisms for distributed feedback (DFB) CO₂ lasers. For CO₂ laser plasmas the reduced electric fieldE/N has to be in a well-defined range. The reduced electric fieldsE/N of gas discharges in the narrow gaps with widths of the order of 100 m required for DFB are considerably above this range. In order to study the feasibility of these RF-excited discharges for DFB CO₂ lasers we have measured the electron temperatureT _e in their plasmas. From helium-line-intensity ratios we have deduced a lower limit of the electron temperatureT _e of 4eV. The observed high intensities of bands of singly ionized nitrogen indicate an even higher electron temperature, but an efficient pumping of the upper laser level is not possible with an electron temperature above 2.5 eV.We have estimated the electron densityn _e and the current densityj _e from ratios of the intesities of forbidden and allowed helium lines. The high current densityj _e is in the range of abnormal glow discharges.In the gas discharges between narrow gaps the electron oscillation amplitudex _e is large than the electrode separationd. In order to replace the resulting high electron losses a high electron temperatureT _e is necessary to sustain the gas discharge. Because of this high electron temperatureT _e an efficient pumping of the upper laser level is not possible.     

Nitrogen related excitation processes in a mercury bromide laser discharge

The role of nitrogen as a buffer gas constituent in an avalance discharge mercury bromide laser has been examined, and it is shown that a change over in excitation from electron impact dissociation of HgBr₂ to energy transfer dissociation by excited nitrogen occurs at 1.5%N ₂. This is explanable in terms of excitation cross-sections if considerable pumping occurs from excited states of nitrogen which are higher in energy than the metastableA state. When excitation via nitrogen is dominant, a high dischargeE/N value is needed to avoid coupling of energy to unwanted vibrational excitation.     

Design considerations and scaling laws for high power convective cooled CW CO2 lasers

Various criteria for designing high power convective cooled CO₂ lasers have been discussed. Considering the saturation intensity, optical damage threshold of the optical resonator components and the small-signal gain, the scaling laws for designing high power CW CO₂ lasers have been established. In transverse flow CO₂ lasers having discharge of square cross-section, the discharge lengthL and its widthW for a specific laser powerP (Watt) and gas flow velocityV (cm/s) can be given byL = 1.4 x 10⁴ p ^1/2 V ^-1 cms andW = 0.04P ^1/2 cms. The optimum transmitivity of the output coupler is found to be almost constant (about 60%), independent of the small signal gain and laser power. In fast axial flow CO₂ lasers the gas flow should be divided into several discharge tubes to maintain the flow velocity within sonic limit. The discharge length in this type of laser does not depend explicitly on the laser power, instead it depends on the input power density in the discharge and the gas flow velocity. Various considerations for ensuring better laser beam quality are also discussed.     

First lasers created at the Institute of High Current Electronics of the Russian Academy of Sciences (Siberian Division)

This paper presents the designs and radiation characteristics for lasers operating by self-limited transitions of nitrogen (λ=337.1 nm) and neon (λ=614.3 nm) and pumped by a pulsed longitudinal discharge, and for atmosphericCO ₂ lasers (λ=10.6 μm) pumped by a transverse electron-beam-initiated discharge or by a transverse discharge with uv preionization. These lasers were put into operation at IHCE in 1969 (the nitrogen and neon lasers), in 1971 (theCO ₂ laser pumped by an electron-beam-initiated discharge), and in 1972 (theCO ₂ laser pumped by a transverse discharge with uv preionization). Institute of High Current Electronics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 14–17, August, 1999.     

Optimization of the CO2:N2:He Ratio in the Active Media of Continuous-Wave Electric-Discharge CO2 Lasers

It is shown that the optimum ratio between the main components CO₂:N₂:He of the active medium of continuous-wave electric-discharge CO₂ lasers depends on the temperature of the active medium. Continuous-wave CO₂ lasers can operate effectively at high temperatures of their active medium (T 1000 K) if in their composition the fraction of N₂ molecules is increased as compared to the fraction of CO₂ molecules (CO₂:N₂ < 1:15) and the fraction of He atoms is decreased (He n< CO₂ + N₂).     

Present status of research into the electron-beali-controlled discharge excitation method

The present status of research into electron-beam-controlled discharge (EB) lasers is considered briefly, and the main trends in the development of the investigations in this field are indicated. A concise review is given of the latest results on the development of pulsed and cw high-power EB CO₂ lasers, on the improvement of the design systems and optimization of the pumping regimes of such lasers, on the development of the theory of EB lasers, on the construction and investigation of EB lasers using, new active media, studies of the stability of nonautonomous discharge excited by the EB method, and the use of EB discharge to stimulate chemical reactions.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Akademii Nauk SSSR, Vol. 116, pp. 3–6, 1980.     

Simultaneous operation of distributed feedback and grating tuned dye lasers

Multiphoton absorption (MPA) of the CO₂ laser 9 μmR(30) line (pulse width 60 ns, FWHM) by NH₃ and NH₃/N₂ mixtures was studied as a function of gas composition, pressure and laser fluence (Φ). MPA occurs for Φ smaller than those that have been used for optically pumped NH₃/N₂ lasers; consequently a simple two-level absorption model will not adequately describe laser action in these systems. The photon energy deposited in NH₃/N₂ mixtures can be calculated from the MPA cross-section and the fluence dependence of the illumination geometry. An examination of the efficiency of conversion of this absorbed energy to the reported radiant energy of optically pumped NH₃/N₂ lasers shows an optimum value which depends on pressure, and gas composition.     

Negative ion effects in CO2 convection laser discharges

Negative ions are computed to be formed on a time scale and in quantities such that they may be a cause of plasma instability observed in low pressure electrical discharge convection CO₂ lasers. In a typical CO₂−N₂−He−H₂O laser mixture the principal ions are CO ₃ ⁻ , CO ₄ ⁻ and H⁻ with the total negative ion densityn ₋ given by 0.1n _e <n ₋<n _e , wheren _e is the electron density: but if the gases are re-cycled or if there is an air leak NO ₂ ⁻ and NO ₃ ⁻ are formed in significant amounts andn ₋ can become greater thann _e in a time considerably less than the gas dwell time in the electrical excitation discharge. CO is effective in reducingn ₋ in a system without re-cycling, but is ineffective in a re-cycled system with the oxides of nitrogen present.     

Study of a non-equilibrium pulsed nanosecond discharge at atmospheric pressure using coherent anti-Stokes Raman scattering

Time-resolved CARS measurements of rotational and vibrational temperatures of nitrogen in nanosecond pulsed discharge at atmospheric pressure are reported. Experiment is first performed with a discharge in pure air where spatial and temporal evolution of temperature distribution is recorded by delaying the probe lasers relative to the discharge pulse in the range 10 ns to 1 ms. The experiments demonstrate that a strong vibrational non-equilibrium can be sustained in N₂ at 1 bar. The effect of different colliding partners on the vibrational relaxation of N₂ is studied for discharges in CH₄/air mixtures with different equivalence ratio. The observed temperature distributions suggest that thermal equilibrium is not fully achieved in this mixture. Effect of the discharge on the ignition of a premixed CH₄/air flame is also investigated for various equivalence ratio.     

Output characteristics and parameters of the plasma from a gas-discharge low-pressure ultraviolet source using helium-iodine and xenon-iodine mixtures

The output characteristics and parameters of the plasma of a powerful gas-discharge source of UV radiation are studied. The UV source uses He-I₂ and Xe-I₂ mixtures and is excited by a longitudinal glow discharge. The pressure of the gas mixtures is varied from 100 to 1500 Pa, and the discharge power falls into the range 15–250 W. The source (lamp) emits in the spectral interval 200–390 nm, which covers the spectral line of the iodine atom at 206.2 nm, the spectral band of XeI(B-X) with a maximum at 253 nm, and the spectral band of with a maximum at 342 nm. For He(Xe)-I₂ mixtures at a pressure of 800–1000 Pa (this pressure range is near-optimal according to our experimental data), the electron energy distribution functions and the electron kinetic coefficients as functions of parameter E/N (E is the electric field strength, and N is the particle concentration in the discharge) are calculated. The calculated plasma parameters are used in the qualitative analysis of key electronic processes in the plasma of an exciplex halogen UV source and will be subsequently employed in numerical simulation of the process kinetics and output characteristics of a UV source based on helium-iodine or xenon-iodine mixtures.     

Theoretical and experimental study of V–I characteristics of UV pre-ionized TEA CO2 laser for variety of laser gas mixtures

Experimental and theoretical study of V–I characteristics of UV pre-ionized TEA CO₂ laser has been carried out for a variety of gas mixtures emitting different optical pulse shapes suitable for various applications. Coupled differential equations have been solved to model the pulse excitation circuit using the numerically calculated values of ionization coefficient (α), attachment coefficient (β) and drift velocity (U_d) as functions of E/N (i.e. electric field to neutral particle density ratio) for chosen gas mixture. Calculated and experimental V–I characteristics for gas mixtures (CO₂:N₂:He::1:2:3, 1:1:4, 1:1:5 and 1:0:4.7) show a good agreement. It has been shown that gas mixture has a dominant effect on the delay between pre-ionization and main discharge; thus, determining the long-term stability of discharge. The excitation pulse duration increases with increase in molecular content of gas mixture (i.e. amount of CO₂ and N₂ in gas mixture).     

Theoretical prediction on the wavelength-temperature shift in pulsed TE CO2 lasers

The wavelength-temperature shift observed in pulsed TE CO₂ lasers is discussed theoretically by means of Six-temperature model rate equations for tunable TE CO₂ lasers. Numerical calculations of the temperature-wavelength shift in a pulsed TE CO₂ laser with a simple plano-concave stable resonator, whether excited by conventional low-inductance fast-discharge scheme or by a long-pulse Pulser/sustainer discharge scheme, show that the laser output wavelengths are within the 10P branch as the ambient temperature varies from 228 to 338 K, but will change as the ambient temperature varies. The laser output wavelengths will move to the transition lines with longer wavelengths in the 10P branch as the ambient temperature increases and vice versa. The calculated results also illustrate that near the ambient temperature of 310 K, the laser is more likely to operate on multi-transition lines. Considering this wavelength-temperature shift, the chilling device adopted in high-power high repetition rate TE CO₂ lasers is important in maintaining a stable laser output spectra as well as a stable laser output power. The numerical results also suggest that a frequency agile resonator is highly recommended if stable laser output spectra are required in TE CO₂ lasers.     

Der zeitliche Verlauf einer Elektronenstrahl-gesteuerten Hochdruckentladung in CO2-Lasergasgemischen

The energy which may be introduced into the plasma of an e-beam sustained laser discharge depends on the electron loss processes as recombination and attachment. Measuring the time behaviour of the discharge current both the recombination and the attachment rate coefficients can be determined. The results of such investigations reported here suggest that some widely used assumptions have to be proofed carefully once more. It could be shown that in the case of pure nitrogen the recombination coefficient decreases very rapidly immediately after the discharge ignition even if the reduced electric field strength remains unchanged. In CO₂-discharges such a behaviour could not be observed. Moreover, it has been found that CO₂-discharges are stable only as long as they are recombination determined. These results are supported by additional investigations in N₂—CO₂-mixtures. Some conclusions according to the operation of tunable high pressure lasers are given.     

Operational characteristics and power scaling of a transverse flow transversely excited CW CO<Subscript>2</Subscript> laser

Transverse flow transversely excited (TFTE) CO₂ lasers are easily scalable to multikilowatt level. The laser power can be scaled up by increasing the volumetric gas flow and discharge volume. It was observed in a TFTE CW CO₂ laser having single row of pins as an anode and tubular cathode that the laser power was not increasing when the discharge volume and the gas volumetric flow were increased by increasing the electrode separation keeping the gas flow velocity constant. The discharge voltage too remained almost constant with the change of electrode separation at the same gas flow velocity. This necessitated revision of the scaling laws for designing this type of high power CO₂ laser. Experimental results of laser performance for different electrode separations are discussed and the modifications in the scaling laws are presented.     

Investigations to the increase of the specific energy of a gasdynamic CO-laser

The input power density and hence the output power of electrically excited gasdynamic lasers is limited by instabilities of the glow discharge. The application of theoretical results, which have been obtained with respect of convection or flow lasers, to the discharge region of an electrically excited gasdynamic CO-laser shows, that especially thermal instabilities cause this glow collapse. An increased convection of local heat concentrations in front of the anode surface results in an improved stability behaviour. Input power densities of up to 100 W/cm³ are now accessible to operate the glow discharge and hence specific laser output powers of 57 kJ/kg are obtained.