Effect of delay in the operation of helium-free TEA CO2 lasers with sequential and parallel spark preionisers

The operation of a helium-free TEA CO₂ laser is found to depend more sensitively on the delay between the pre and main discharges as against its conventional operation. Results pertaining to the optimum delay and the temporal shape of the emission are presented for the operation of the helium-free laser with both sequential and parallel spark-type preionisers.     

Spatial-dependent simulation of TEA CO2 lasers

We have developed a 1-D spatially-dependent model and simulation code to study the dynamics of TEA CO₂ lasers. In this paper we will discuss the motivation for developing the model and present the simulation details. We have compared and verified the simulation results with the previous 0-D model, and with the measurements of a TEA CO₂ laser. With spatial resolution, the 1-D simulation is in better agreement with experimental results compared with the existing 0-D model.     

A sensitive arc detection technique for reliable operation of high repetition rate TEA CO2 laser

A simple and sensitive technique, which utilizes the sensing of both the discharge current and the visible light emission from the discharge, has been developed to detect the arc formation during the operation of a high repetition rate TEA CO₂ laser. It is shown that the optical detection in the visible spectral region is more sensitive than that it is in the UV region and hence can be used more effectively to monitor the onset of arc formation, while the current signal is useful in the strong arc formation regime. We demonstrate that the trigger signals generated by both the methods can be employed for reliable protection of the laser system from catastrophic damage due to deleterious arc formation.     

A 3 kW average power tunable TEA CO2 laser

A high average power line-tunable TEA CO₂ laser is described. Average output power up to 3 kW is achieved at repetition rate of 180 Hz. The maximum output power is almost equal among the four spectrum bands of CO₂ laser (the P and R branches of 00°1–10°0 and 00°1–02°0 transition bands). Several special technologies including rotating spark gap switching, PCB (printed circuit board) preionization and zeroth-order grating coupling are employed.     

A compact TEA CO2 laser for field-based spectrochemical analysis of geological samples

A compact capacitor-transfer TEA CO₂ laser of 180 mJ output energy and 50 ns pulse duration in half width was designed and constructed for field use. This laser is suitable for laser microprobe spectrochemical analysis on geological and mining samples. Experiments show that the use of helium as a surrounding gas suppresses the continuous emission spectrum of the plasma, thus increasing the S/N ratio. Using glass as standard samples, the minimum detectable concentrations are estimated with Zn neutral line and F ionic line to be 60 ppm and 500 ppm, respectively.     

Injection locked single mode operation of a tea CO2 laser with high energy extraction

Generation of reproducible smooth pulses from longitudinal mode control of a TEA CO₂ laser by signal injection from a cw CO₂ laser is reported and the operating conditions investigated for several new geometries. Applied to unstable resonators, the efficient and spectrally pure, high power output obtains has important application in the area of optical pumping.     

A sequential discharge TEA CO2 laser with high repetition rate and high output power

In this paper, a novel excitation method named as sequential discharge is realized in a two-module TEA CO₂ laser by using a special rotating spark gap. It is demonstrated that the repetition rate and the output power of a laser can be multiplied through this method. For the two-module TEA CO₂ laser in the experiment, the repetition rate is 300 Hz and the average power is 356 W when each module discharges; the repetition rate is 600 Hz and the average power is 713 W when the two modules discharge sequentially.     

Performance of a self-switched TEA CO2 laser with multi/two/single spark channel preioniser

Satisfactory performance of a TEA CO₂ laser of active length 30 cm is reported when self-switched by its own preioniser comprising two mutually coupled parallel spark channels or even one spark channel. A comparison of the performance of the laser with respect to the maximum extractable energy and electro-optic efficiency as a function of number of preionising spark channels is presented. With reduced number of mutually coupled parallel spark channels, the operation approaches the near ballast-free regime, resulting thereby, in an improved performance of the laser.     

Mathematical modeling of tunable TEA CO2 lasers

A mathematical model, based on the Landau–Teller equations of six-temperature model for the CO₂–N₂–He–CO system, to describe the process of dynamic emission in tunable TEA CO₂ lasers is introduced. In this model, the Landau–Teller equations are rewritten with regard to fine longitudinal mode frequencies in the laser resonator. These revised equations can be utilized to estimate the laser output spectra as well as other laser output pulse parameters. Examples are given to show the modeling results of non-tunable, grating tuned or injection-locking TEA CO₂ lasers.     

Surface modification of the titanium implant using TEA CO2 laser pulses in controllable gas atmospheres - Comparative study

Interaction of a TEA CO₂ laser, operating at 10.6 μm wavelength and pulse duration of 100 ns (FWHM), with a titanium implant in various gas atmospheres was studied. The Ti implant surface modification was typically studied at the moderate laser beam energy density/fluence of 28 J/cm² in the surrounding of air, N₂, O₂ or He. The energy absorbed from the TEA CO₂ laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following titanium implant surface changes and phenomena were observed, depending on the gas used: (i) creation of cone-like surface structures in the atmospheres of air, N₂ and O₂, and dominant micro-holes/pores in He ambient; (ii) hydrodynamic features, most prominent in air; (iii) formation of titanium nitride and titanium oxide layers, and (iv) occurrence of plasma in front of the implant. It can be concluded from this study that the reported laser fluence and gas ambiences can effectively be applied for enhancing the titanium implant roughness and creation of titanium oxides and nitrides on the strictly localized surface area. The appearance of plasma in front of the implants indicates relatively high temperatures created above the surface. This offers a sterilizing effect, facilitating contaminant-free conditions.     

CO2 laser cutting of slate

Slate is a natural stone which has the characteristic that shows a well-developed defoliation plane, allowing to easily split it in plates parallel to that plane which are particularly used as tiles for roof building. At present, the manufacturing of slate is mostly manual, being noisy, powdery and unsafe for the worker. Thus, there is a need to introduce new processing methods in order to improve both the working conditions and the quality of the products made of slate.Following the previous work focused on the drilling and cutting of slate tiles using a Nd : YAG laser, we present in this paper the results of the work carried out to explore the possibilities to cut slate plates by using a CO₂ laser. A 1.5 kW CO₂ laser was used to perform different experiments in which, the influence of some processing parameters (average power, assist gas pressure) on the geometry and quality of the cut was studied. The results obtained show that the CO₂ laser is a feasible tool for a successful cutting of slate.     

High-power transverse flow CW CO2 laser for material processing applications

A transverse flow transversely excited (TFTE) CW CO₂ with a maximum output power about 15 kW has been developed. This is excited by pulser sustained DC discharge applied between a pair of multi-pins anodes and a common tubular cathode. Though the laser power in convective cooled CO₂ laser scales proportionally with the volumetric gas flow, it did not increase in this laser when the volumetric gas flow was increased by increasing the electrode separation keeping the flow velocity constant. The discharge voltage too remained almost unchanged with increase of the electrode separation. These observations are explained considering the electrical discharge being controlled by ionization instability. Laser materials processing applications often demand programming facilities for laser power modulation. A four-stage cascaded multilevel DC–DC converter-based high-frequency switch mode power supply has been developed to modulate the output power of the laser. Laser was operated up to 15 kW output power in four different modes viz. continuous wave mode, pulse periodic mode, single shot mode and processing velocity-dependent power mode with 1.2 kHz modulation bandwidth. We describe briefly the laser system, the SMPS, and the temporal behavior of laser beam.     

Optically pumped submillimeter laser lines from CD2Cl2 using a large tunability CW CO2 laser

Twenty-five new laser lines have been obtained in the wavelength region from 155 to 830 μm by optically pumping the CD₂Cl₂ (deuterated dichloromethane) molecule with a CW CO₂ laser having a tunability range of 300 MHz. The wavelength, polarization relative to that of CO₂ pumping radiation, and offset relative to the CO₂ center frequency were determined for all of the new lines and some other already known laser emissions. For all of them we give also the relative intensity and the optimum pressure of operation. Permanent address: Depto de Física e Química da FEIS — UNESP 15.378-000 Ilha Solteira-SP, Brazil     

Mathematical modeling of hybrid CO2 laser

A Teller–Landau six-temperature model describing the dynamic emission of single-mode TEA CO₂ laser has been adapted. This model has been also used to describe the mechanism of obtaining relatively high-power output pulses from hybrid TE-TEA or CW-TEA CO₂ laser consisting of high- and low-pressure sections. The suggested mathematical model allows to investigate the mechanism which limits the TEA oscillation to single longitudinal mode (SLM) due to the narrow gain bandwidth of low-pressure section, and also to study the effect of the laser input parameters on the smooth output laser pulse parameters. In addition, numerical solutions of non-linear rate equation system of the suggested model are quantitatively discussed. The solutions describe the radiation field intensity, the population inversion, and the energy transfer processes. The calculated values of maximum peak power, total energy in pulse, pulse width, etc. are in a very good agreement with the observed experimental values.     

Phase-locking of a multi-channel radial array CO2 laser

A theoretical analysis of phase-locking in a multi-channel radial array CO₂ laser is presented. The concepts are based on the theories of injection phase-locking and matrix optics. The mutual optical coupling occurring within the central region is demonstrated by theoretical deduction. Interrelated graphs of the coupling coefficient and the distance between the coupling position and the cavity mirror are calculated by numerical simulation. The results of the analysis are in accord with experimental results.     

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).     

AC-chopper application for CW CO2 laser

A microprocessor-based algorithm was developed to control the power supply to a CO₂ laser using an AC-chopper method. This system was connected directly to a CO₂ laser tube without the need for a dc-dc converter or the storage capacitance of a multilevel circuit. The typical CO₂ laser power supply system had a full-bridge series resonant inverter or voltage multiplier. AC-to-AC converter schemes using zero voltage switching (ZVS) can be used efficiently and economically for low and medium power applications.This paper reports the performance characteristics of a symmetrical AC-chopper technology that can maintain the quality of the ac output of a CO₂ laser tube, regardless of the amount of switching loss. The laser was operated to an output power, maximum system efficiency total gas mixture of 37.2 W, 92%, and CO₂:N₂:He=1:9:15, respectively. The laser system, AC-chopper power supply and its operation were examined.     

CO2 laser assisted removal of UO2 and ThO2 particulates from metal surface

Pulsed laser assisted removal of uranium dioxide and thorium dioxide particulates from stainless steel surface have been studied using a TEA CO₂ laser. Decontamination efficiency is measured as a function of laser fluence and number of pulses. Threshold fluence for the removal of UO₂ particulates has been found to be lower than that required for the removal ThO₂ particulates. Usage of a ZnSe substrate, that is transparent to the laser wavelength used here, enabled us to decouple the cleaning effect arising out of absorption in the particulates from that in the substrate and has contributed towards understanding the mechanism responsible for cleaning. The experimental observations are also corroborated by simple theoretical calculations.     

High-power multibeam CO2 laser for industrial applications

Multibeam CO₂ lasers consist of a large number of closely packed parallel glass discharge tubes, all sharing a common plane parallel resonator. This paper describes construction and operation of a CW multibeam CO₂ laser consisting 20 discharge tubes and cooled length of 1500 mm, delivering 1 kW power. A high-frequency pulser is used for producing preionization in all discharge sections for initiating the main DC discharge simultaneously in all discharge tubes. Plane parallel resonator consists of a plane ZnSe mirror of 90 mm diameter having 60% reflectivity and a gold-coated copper mirror of same diameter. This laser operates in waveguide regime and laser power is not critically sensitive to mirror misalignment.     

Numerical investigation of a fast-axial-flow CW CO2 laser

This paper presents the results of the calculation of the parameters of the active medium of a fast-axial-flow CO₂ laser using numerical methods in the framework of a one-dimensional approximation of the set of continuity equations, Bernoulli equation, equation of gas state, energy equation and multi-temperature rate equations with regard to diffusion for the gas flow in the cylindrical discharge tube. The spatial distribution of the small-signal gain and gas temperature along the gas flow direction have been calculated for a given set of initial conditions, namely, gas flow velocity, gas pressure and the tube diameter. In addition, the dependence of small-signal gain, the asymmetric stretch vibrational temperature of CO₂ (T₃) and the gas temperature on the discharge current were studied.