RF-excited CO2 Laser Research Articles

Highly sensitive Laser Photo-Acoustic SF6 Gas Analyzer with 10 decades dynamic range of concentration measurement

A highly sensitive Laser Photo-Acoustic (PA) SF6 Gas Analyzer with dynamic range of ∼10 decades (∼0.1 ppb–100% SF6) has been developed and tested. Gas analyzer is built on the base of RF-excited waveguide CO2 laser, resonant differential photo-acoustic detector (PAD), and sealed-off SF6-gas-filled Ref-cell. The resonant frequency of differential PAD is measured automatically in real time (∼1750–1780 Hz, error ± 0.1 Hz). While measuring SF6 concentration (up to 1000 ppm), normalization of PAD’s absorption signals according by Ref-cell absorption signals significantly reduce the error in PA gas analyzer readings caused by spontaneous tuning of waveguide CO2 laser wavelength near ∼10.6 μm. PA gas analyzer’s background readings was ∼0.1 ppb SF6, the standard dispersion of background signal was ∼10 ppt SF6. Minimum average power of waveguide CO2 laser radiation should be no less than ∼150 mW. Dependence of speed of sound vs gas mixture composition was used to measure SF6 concentration in 0.1–100% range. Resonant frequency of differential PAD varied from ∼1780 Hz (pure N2) to ∼690 Hz (pure SF6), which, after appropriate calibration, makes it possible to measure SF6 concentration up to 100%.

Comparative studies of photo-acoustic gas analyzers based on tunable CO2 lasers with external and intracavity detector arrangement

The results of comparative studies of laser photo-acoustic (PA) gas analyzers with external and intracavity arrangement of resonant differential photo-acoustic detector (PAD) are presented. An automated tunable waveguide RF-excited CO2 laser was used in the PA gas analyzers. During the experiments, the optimal design of the resonant differential PAD was determined, in which the various models of microphones were tested. Studies of the effect of gas flow rate through the PAD, measurements of gas absorption spectrum (CO2) at different PAD resonant frequencies have been carried out. Two versions of transportable PA gas analyzers with different detector arrangements are developed and tested: outside and inside the cavity of tunable CO2 laser. For an integration time of 1 s, the minimum detectable absorption for the external PA gas analyzer is αmin = 7.5 × 10−10 cm−1, for the intracavity PA gas analyzer αmin ~ (3.5 ± 0.4) × 10−10 cm−1. It is shown that the value of the minimum detectable absorption αmin is practically independent of the external or intracavity location of the differential PAD, i.e. from the power of CO2 laser radiation in the range of ~1.5–100 W.

Effect of excitation frequency on characteristics of mixture discharge in fast-axial-flow radio frequency-excited carbon dioxide laser

A one-dimensional fluid model has been used to describe the effect of radio frequency (RF) on the characteristics of carbon dioxide (CO2), nitrogen (N2) and helium (He) mixture discharge at 120 mbar in fast-axial-flow RF-excited CO2 laser. A finite difference method was applied to solve the one-dimensional fluid model. The simulation results show that the spatial distributions of electron density and current density rely strongly on the modulating driven frequency. When the excitation frequency changes from 5 to 45 MHz, the plasma discharge is always in α mode. Moreover, as the excitation frequency increasing, the higher densities of CO2V001 and N2*Vib can be obtained, which is important to get higher excitation efficiency for the upper laser level.

A new sealed RF-excited CO2 laser for enamel ablation operating at 9.4-μm with a pulse duration of 26-μs.

Several studies over the past 20 years have shown that carbon dioxide lasers operating at wavelengths between 9.3 and 9.6-μm with pulse durations near 20-μs are ideal for hard tissue ablation. Those wavelengths are coincident with the peak absorption of the mineral phase. The pulse duration is close to the thermal relaxation time of the deposited energy of a few microseconds which is short enough to minimize peripheral thermal damage and long enough to minimize plasma shielding effects to allow efficient ablation at practical rates. The desired pulse duration near 20-μs has been difficult to achieve since it is too long for transverse excited atmospheric pressure (TEA) lasers and too short for radio-frequency (RF) excited lasers for efficient operation. Recently, Coherent Inc. (Santa Clara, CA) developed the Diamond J5-V laser for microvia drilling which can produce laser pulses greater than 100-mJ in energy at 9.4-μm with a pulse duration of 26-μs and it can achieve pulse repetition rates of 3 KHz. We report the first results using this laser to ablate dental enamel. Efficient ablation of dental enamel is possible at rates exceeding 50-μm per pulse. This laser is ideally suited for the selective ablation of carious lesions.

Evaluation of enamel surface modification using PS-OCT after laser treatment to increase resistance to demineralization.

At laser intensities below ablation, carbonated hydroxyapatite in enamel is converted into a purer phase hydroxyapatite with increased acid resistance. Previous studies suggested the possibility of achieving the conversion without surface modification. This study attempts to evaluate the thresholds for the modification without additional changes in physical and optical properties of the enamel. Bovine specimens were irradiated using an RF-excited CO2 laser operating at 9.4-µm with a pulse duration of 26-µs, pulse repetition rates of 100-1000 Hz, with a Gaussian spatial beam profile - 1.4 mm in diameter. After laser treatment, the samples were subjected to acid demineralization for 48 hours to simulate acidic intraoral conditions of a caries attack. The resulting demineralization and erosion were assessed using polarization sensitive OCT (PS-OCT) and 3D digital microscopy. The images from digital microscopy demonstrated a clear delineation between laser protected zones without visual changes and zones with higher levels of demineralization and erosion. Distinct changes in the surface morphology were found within the laser treated area in accordance with the Gaussian spatial beam profile. There was significant protection from the laser in areas that were not visually altered.

Analysis of Deformation of Reflector for RF-Excited CO<sub>2</sub> Laser

The paper aims to study the deformation and compensation of the reflector for RF-excited diffusion-cooled CO2 laser. Based on ANSYS, the thermal deformation and compensation of the reflector have been calculated, It has been proved that the effect of temperature compensation is obvious, and can correct the thermal deformation of the reflector to the tolerance of the parameter of optical cavity.

Laser Doppler velocimetry based on the optoacoustic effect in a RF-excited CO2 laser

We present a compact optoacoustic laser Doppler velocimetry method that utilizes the self-mixing effect in a RF-excited CO(2) laser. A portion of a Doppler-shifted laser beam, produced by irradiating a single wavelength laser beam on a moving object, is mixed with an originally existing laser beam inside a laser cavity. The fine change of pressure in the laser cavity modulated by the Doppler-shifted frequency is detected by a condenser microphone in the laser tube. In our studies, the frequency of the Doppler signal due to the optoacoustic effect was detected as high as 50 kHz. Our measurements also confirmed that the signal varied linearly with the velocity of the external scatterer (the moving object) and the cosine of the angle between the laser beam and the velocity vector of the object.

Output beam characteristics of the radio frequency–excited slab CO<sub>2</sub> laser with unstable-waveguide hybrid resonator

A positive branch unstable-waveguide hybrid resonator has been designed for a RF-excited slab CO2 laser of 100 W level. The output beam mode and the propagation characteristics in the two directions of unstable and waveguide were investigated by numerical and experimental methods. The results show that the output beam was collimated in the unstable direction, and was an approximate fundamental mode in the waveguide direction with a divergence angle of 12 mrad. The output power decreased by 50% with the misaligned angle of the tail mirror of 1.5 mrad, and was close to 0 with the misaligned angle more than 3 mrad.

Indigenous development of a 2 kW RF-excited fast axial flow CO2 laser

RF-excited fast axial flow CO2 lasers in kilowatt regime are presently being used for various new scientific applications in addition to laser material processing because of its versatility and superior beam quality. We have indigenously developed a compact 2 kW RF-excited fast axial flow CO2 laser with moderate beam quality. In this paper the key design features of the laser and the associated high power capacitively coupled RF excitation technique are discussed in detail. Operational characteristics of this system are described along with the experimental findings.

Dissociation and regeneration kinetics of carbon dioxide in the active medium of sealed-off transverse RF-excited CO2 lasers

An improved mathematical model describing the dissociation and regenerations kinetics of carbon dioxide in the active medium of sealed-off transverse RF-excited CO2 lasers is presented. It is shown that the calculation of the active medium composition of such lasers requires the equations of the gas-mixture kinetics to take into account the diffuse flow of oxygen atoms on metal electrodes and on the surface of heterogeneous catalysts used to reduce the degree of the carbon dioxide dissociation. The rate constants of the heterogeneous recombination reaction CO + O → CO2 on the surface of alumina ceramics and Al2O3 are determined.

Frequency stabilization of a radio frequency excited CO2 laser using the photoacoustic effect

A frequency and a power of rf excited CO(2) laser are stabilized using the photoacoustic effect generated from the laser itself. As the radiation intensity in a laser resonator changed, the pressure of laser medium gas changed due to its photoacoustic effect. A condenser microphone built in the resonator is used to convert delicate variations in the pressure into voltage signals to be used as input signals for lock-in stabilizer. As a result of stabilizing the frequency and power of the laser oscillated from P(20) line, at the apex of gain curve, the relative stability of the optical frequency and the rate of change in laser power are 5.57 x 10(-8) and 9.3%, respectively.

Laser-Produced Plasma EUV Light Source for Microlithography

The current status of the laser produced plasma light source development for EUV lithography is presented. Our key technology is based on an RF-excited CO2 driver laser combined with a Sn droplet target, because this approach enables cost-effective high-conversion efficiency. The RF-excited CO2 laser has a MOPA (master oscillator power amplifier) configuration. It currently generates an average output power of 2.6kW at 130kHz repetition rate. Due to negligible wavefront distortion of the laser beam during amplification, a laser focus of 100μm (FWHM) has been obtained. The Sn target and magnetic ion mitigation development as well as an evaluation of the collector mirror lifetime are presented.

RF-excited unstable-resonator planar CO2 laser on all-metal electrode—waveguide structure

The radiation characteristics of a planar CO2 laser excited by a diffusion-cooled rf discharge at a frequency of 40 MHz are studied. A single-mode cw lasing power of ≈50 W is achieved with an efficiency of ≈10% for a nearly diffraction-limited radiation divergence of 4—7 mrad. The spatial structure, output power, stability and laser radiation quality are studied as functions of longitudinal and angular alignments of the resonator mirror for two types of hybrid unstable-waveguide resonators of the laser. It is shown that for the resonator corresponding to the negative branch of the stability diagram, a misalignment of 0.02 rad of the mirrors leads to a 50% decrease in the output laser power, while its value for the positive branch resonator is about 100 times smaller. It is found that for the resonator corresponding to the negative branch, the sensitivity to the violation of confocal arrangement of the mirrors upon an increase in the resonator length is an order of magnitude higher. The dependence of the density of input rf power on the working gas pressure is studied experimentally in the interval 50—110 Torr. Power density values of 1—4 W cm-2 are obtained for normal discharge current density. These values are important for optimisation and scaling of the lasing characteristics of high-power planar CO2 lasers.

Active and passive frequency stabilization for a Q-switched Z-fold radio-frequency-excited waveguide CO_2 laser with two channels

We report on passive offset frequency stability in free-running applications and active offset frequency stability achieved by use of a frequency-locking technique for a Q-switched Z-fold rf-excited waveguide CO2 laser with two channels. The laser structure of common electrodes and two channels has the advantage of compensating for the frequency variation caused by variations in temperature, cavity length, gas refractive index, and mechanical vibrations, so its offset frequency stability is higher than that of two separate lasers. In the experiments, the offset frequency shift was less than 6 MHz for 3 min in free-running mode. The technique of active offset frequency locking by counting was also introduced. The beat frequency shifting value was smaller than +/- 0.5 MHz in the long-term.

Three-Mirror Multiple-Pass Cavity of a Transversely RF-Excited Planar Waveguide CO2 Laser

A theoretical model for determining the optical losses in the three-mirror, multiple-pass cavity of a transversely rf-excited planar waveguide CO2 laser with an exit mirror of arbitrary radius of curvature has been developed.

The influence of an external magnetic field on a radio frequency excited CO2 laser

The influence of a dc external magnetic field B0 on the output power of a rf excited CO2 laser is considered. The voltage-current density characteristic curve, electron temperature, deposited power, and gas mixture temperature of the plasma are numerically investigated in the presence of the external magnetic field. The electric discharge of the magnetized rf excited CO2 laser is seen to be stable. It is shown that the deposited power into the plasma increases with increasing magnetic field intensity. Mean temperature of the gas mixture of the magnetized rf excited CO2 laser is computed as a function of B0, which is substantially increased at a relatively strong B0. An experimental setup is conducted to measure the output power of a rf excited CO2 laser in the presence of B0. It is found by applying B0 upon the laser the output power increases.

Theoretical investigation of discharge parameters in magnetized radio frequency excited CO2 lasers

In the present paper the magnetic field effects on discharge parameters in rf excited CO2 lasers are calculated. A rf generated plasma imbedded in an external, constant, and homogeneous magnetic field is considered. The continuity equation is used to derive the electron density. Quasineutrality condition and ambipolar diffusion are used. Electron attachment coefficient is neglected. Local electric field, local electron density, and thickness of charge layers are derived as a function of distance from the electrodes and magnetic field. The thickness of charge layers in the presence of magnetic field is always smaller than one without the magnetic field. When the magnetic field increases, the electron density increases in all regions of discharge, and the electric field reduces in the charge layers but increases in the middle part of discharge.

The laser pulse from a rf waveguide CO 2 laser

The behaviour of a rf-excited waveguide CO2 laser in the pulse regime is studied experimentally. The specific time sections of the pulse have been defined and described in detail. The output pulse evolution versus input power, pressure of the laser mixture, output coupling, repetition frequency and pulse width has been investigated. The spectral content of the pulse has been analysed. A model explaining the effect of laser tuning during the pulse duration is given. The ultrahigh pulse (UHP), defined as the pulse for which the bulge effect occurs, is predicted.

Compact High-Efficiency CO2 Slab Waveguide Lasers with Segmented RF Excitation

RF-excited slab waveguide CO2 lasers based on either internal or external electrodes were studied. A compact multisegment RF excitation configuration was used for pumping the gain medium. The use of a separate driving circuit to excite each discharge segment led to a very uniform discharge along the slab waveguide. A resultant maximum output power of 82 W with efficiency up to 20.5% was recorded from a 3 mm×10 mm×450 mm slab waveguide. The gain coefficient, saturation power and the effect of Xe in the gas mixture were also studied. These lasers can be easily scaled to obtain low-cost high-power operation.

Intracavity beam behavior in hybrid resonator planar-waveguide CO_2 lasers

We describe a combined computer simulation and experimental investigation of the intracavity spatial beam profile characteristics of a planar-waveguide rf-excited CO(2) laser that incorporates a hybrid waveguide confocal unstable negative-branch resonator. The study includes results for the intracavity lateral beam intensity profile and output power of the laser as a function of resonator mirror misalignment. In addition, the behavior of the unstable resonator, observed experimentally and predicted by the simulation, in generating localized high intensity hot-spots when it is subjected to relatively large misalignment angles is reported.