G. N. Tsikrikas

The effect of voltage pulse polarity on the performance of a sliding discharge pumped HF laser

The influence of the applied voltage pulse polarity on the performance of a HF laser pumped by a sliding surface discharge is investigated. Details are presented for the polarity dependence of the discharge properties as well as for their relation to the physical processes that affect the sliding discharge evolution. The output energy and efficiency were higher when the initiating or stressed electrode was in relative positive potential, irrespective of which electrode was grounded. The experimental results indicate that for positive polarity the discharge was more uniform, self terminating in type, with reduced electron density and temperature. The increase in output energy and efficiency is attributed to the improvement of the spatial uniformity of laser pumping, to a more efficient utilization of discharge input energy due to an increase in the steady state discharge time phase, as well as to the reduced electron temperature and to the increased steady state voltage which may also result in more efficient laser pumping.

A semiconductively preionised TEA nitrogen oscillator-amplifier laser system

Abstract The gain characteristics of a semiconductively preionised TEA nitrogen oscillator-amplifier laser system were investigated and a small signal gain value of 53 dB and a gain value of 15 dB at maximum output energy were obtained. This novel design system provided laser pulses of low beam divergence of 1.0→1.5 mrad, with output energy of 0.5 mJ, pulses duration of 7 ns, repetition rate up to 80 Hz and excellent shot to shot reproducibility of 2%.

Performance studies of a pulsed HF laser with a sliding discharge plasma cathode

The plasma electrode design concept is applied for the first time to an HF laser. The discharge along the surface of a dielectric (sliding discharge) is used as a plasma cathode for the main laser discharge. The laser operates at atmospheric pressure with a gas mixture of He/SF6/C3H8. Details are presented on the efficiency of energy transfer, the dependence of laser performance on circuit parameters, gas mixture, relative energy loading and time delay between the plasma electrode and main discharges. The F atom production rate is estimated from the linear dependence of the output energy on the electric charge passed through the discharge. Output energies of 600 mJ were obtained at 1.6% efficiency from a small active discharge of 108 cm3 volume and 38 cm length, while the maximum specific input and output energies were 370 J/1 and 5.7 J/1, respectively. These values compare favourably with those reported in the literature for non-chain-reaction-type gas mixtures at 1 atm pressure and demonstrate that the plasma electrode design is a powerful scheme for developing gas-discharge lasers.

Development of a sliding discharge pumped HF laser

Abstract Direct excitation by a surface sliding discharge is applied for the first time to an HF laser. The laser operates at atmospheric pressure with a gas mixture of He/SF 6 /C 3 H 8 . Details are presented on the efficiency of energy transfer, the dependence of laser performance on input energy deposition, circuit parameters and gas mixture. The F atom production rate is estimated from the linear dependence of the specific output energy on the electric charge passed through the discharge. Output energies of 135 mJ were obtained at 1.2% efficiency from a small active discharge of 10cm 3 volume and 38 cm length. These values compare favourably with the values reported for similar laser systems, while the maximum values of specific input and output energy extraction obtained, of 1700 and 15 J/l respectively, are among the highest reported for non-chain-reaction type gas mixtures. These results demonstrate that the sliding discharge, although simple in design, is an efficient scheme for developing gas discharge lasers.

Infrared laser ablation of hard tissue

The aim of this work is the experimental and theoretical investigation of the influence of variable laser parameters (wavelength, fluence, pulse repetition rate) and the optical and thermophysical properties of bone tissue (absorption coefficient, tissue inhomogeneity) on the ablation thresholds and the extent of thermally influenced zones. Ablation and perforation experiments were realized by using (1) a semiconductively preionized TEA CO2 laser (10.6 micrometers ) and (2) a sliding discharge TEA HF laser (2.9 micrometers ).

Novel design: sliding discharge plasma cathode hf laser in multicomponent tissue ablation

The performance characteristics of a novel design, sliding discharge plasma cathode pulsed HF lasers are given and their advantages, as far as laser and tissue interaction is concerned, are analyzed. With these lasers, intense multiline operation in the 2.5 to 3.5 micrometers range is observed from the sliding discharge plasma cathode three electrode system, with a wide cross section beam, good beam profile quality and high efficiency at moderate voltages. These characteristics are very crucial for obtaining reproducible results leading to a well-defined dosimetry in biomedical applications. To the best of our knowledge this is the first report on the HF chemical laser operating in the sliding discharge plasma cathode three electrodes excitation mode and the first time this novel design lasers are being engaged in some laser and tissue interaction applications. With these recently developed, novel design gas lasers, operating in the mid-infrared part of the spectrum, it has become very interesting to examine their efficacy in ablating multicomponent tissues, such as occluded carotide arteries. Some preliminary experimental results are presented and the role of the HF laser in optimization of soft and hard biological tissue ablational treatment is discussed.

Dentin ablation-rate measurements in endodontics witj HF and CO 2 laser radiation

Recent studies focused on the ability of the laser light to enlarge the root canal during the endodontic therapy. The aim of this research is the experimental and theoretical study of the ablation rate of two infrared laser wavelengths on dentin. Thirty freshly extracted human teeth were longitudinally sectioned at thicknesses ranged from 0.5 to 2 mm, and irradiated on the root canal dentin. The measured ablation rates in dentinal wall of the root canal showed that the HF laser at 2.9 micrometer can more effectively penetrate into the tissue, whereas the carbon dioxide laser at 10.6 micrometer leads to high thermal damage of the ablation crater surroundings.

Design and development of a low pumping capacity, compact dc-discharge-excited cw HF chemical laser

The design and development of a compact, low cost, subsonic cw HF chemical laser with expected output power of the order of approximately 100 mWatts that requires less than 5 lt/s pumping capacity is presented. A theoretical estimation of the minimum pumping capacity required in order to obtain an output power of 100 mWatts is given. The laser operates with a He/SF6/H2/O2 gas mixture at an overall pressure of 4 - 8 mbar. A dc electric discharge is used for the SF6 dissociation. In order to operate at such low gas flow rates the mixing channel dimensions were reduced down to a cross section of 0.2 cm height by 13 cm width. Hydrogen is transversely injected into the flow through approximately 285 holes of 0.03 cm diameter. This low cost compact laser system is suitable for a wide range of experimental requiring mid-infrared cw laser radiation such as laser-tissue interactions and environmental studies.

Stimulated Raman scattering in H2 and D2 using a pulsed Nd:YAG laser at 355 nm

The stimulated Raman scattering (SRS) technique has been sued for upward and downward shifting the 3rd harmonic of a pulsed Nd:YAG laser into the 274-503 nm wavelength region. The gases investigated were hydrogen (H2) and deuterium (D2), using helium (He) as buffer gas. Experimental results on the output Stokes and anti-Stokes conversion efficiencies of the forward SRS technique in H2, D2 and He, versus Raman gas pressure and input pump energy, are presented. Conversion efficiencies more than 40 percent were achieved and the output energies obtained at the various ultraviolet and visible wavelengths were of the order of several mJ.