Electromagnetic field distributions in waveguide-based axial-type microwave plasma source

We present results from simulations of 2D distributions of the electromagnetic field inside a waveguide-based axial-type microwave plasma source (MPS) used for hydrogen production via methane reforming. The studies are aimed at optimization of discharge processes and hydrogen production. We derive equations for determining electromagnetic field distributions and next determine the electromagnetic field distributions for two cases – without and with plasma inside the MPS. For the first case, we examine the influence of the length of the inner conductor of the coaxial line on electromagnetic field distributions. We have obtained standing wave patterns along the coaxial line and found resonances for certain positions of the coaxial line inner conductor. For the case with plasma inside the MPS, we perform calculations assuming that distributions of plasma parameters are known. Simulations are done for several values of maximum electron density. We have found that for values of electron density greater than strong skin effect in the plasma is observed. Consequently, plasma may be treated as an extension of the inner conductor of the coaxial line. We have used FlexPDE software for the calculations.     

Microdischarges in ceramic foams and honeycombs

Microdischarges in spatially confined geometries, such as microcavities and micropores of various materials, present a promising method for the generation and maintenance of stable discharges at atmospheric pressure. They have been successfully used in many biomedical, environmental and industrial applications. The paper presents two relatively new types of discharges in confined volumes – a capillary microdischarge in ceramic foams and a sliding discharge inside the capillaries of ceramic honeycombs – and describes their basic physical properties and mechanisms. Microdischarges inside the microporous ceramic foams develop from the surface barrier discharge if the amplitude of the applied voltage reaches given threshold, but only for a specific pore size. Sliding discharge inside the honeycomb capillaries is produced by a combination of AC barrier discharge inside catalytic pellet bed coupled in series with DC powered honeycomb monolith. Both discharges produce relatively cold microplasmas with high level of non-equilibrium. The basic characteristics of the microdischarges, addressing the effects of the applied voltage, discharge power, pore size, length and diameter of the capillaries are discussed.     

Simulation of the ignition transient in RF inductively-coupled plasma torches

The paper deals with the time-dependent numerical simulation of inductively-coupled plasma torches during the ignition transient, which is induced by a graphite rod and leads to the final, self-sustaining plasma condition. The study has been performed by using a 2D time-dependent fluid-magnetic code based on the SIMPLER algorithm within the assumptions of laminar flow, local thermodynamic equilibrium conditions and optically thin plasma. The graphite rod has been treated as a real obstacle for the gas and the electron emission due to the thermoionic effect has been suitably taken into account. The advantage of using a time-dependent code in order to select different plasma operating conditions that can lead to stable discharges is pointed out. Results for both argon and air discharges are presented for different torch geometries, RF frequencies and inlet gas configurations (also including the presence of a carrier gas injected along the axis of the torch). Moreover, the final self-sustaining plasma configurations obtained are compared, when available, with results coming from static models, which have been published by other authors. Received 29 December 2000     

Removal of paper microbial contamination by atmospheric pressure DBD discharge

In this paper the removal of the microbial contamination from paper material using the plasma treatment at atmospheric pressure is investigated. The Aspergillus niger has been chosen as a bio-indicator enabling to evaluate the effect of plasma assisted microbial inactivation. Dielectric barrier discharge (DBD) operated at atmospheric pressure was used for the paper sterilization. The working gas (nitrogen, argon and helium), plasma exposition time and the plasma power density were varied in order to see the effect of the plasma treatment on the fungi removal. After the treatment, the microbial abatement was evaluated by the standard plate count method. This proved a positive effect of the DBD plasma treatment on fungi removal. Morphological and colorimetric changes of paper substrate after plasma treatment were also investigated.     

AFM and contact angle investigation of growth and structure of pp-HMDSO thin films

HMDSO was plasma polymerized on silicon wafer and polyethylene (PE) substrates. The chemical structure of the pp-HMDSO was analyzed with Fourier-transform infrared (FT-IR) spectroscopy. The morphological structure of the thin films deposited on the different substrates was investigated by means of atomic force microscopy (AFM), indicating different coverage mechanisms. In order to investigate the growth process of the pp-HMDSO, films of different thickness were also deposited, varying the plasma deposition time from 10 s to 1800 s. Thickness and structure of such deposits was detected with AFM. Finally, hydrophobic characteristics of the different samples were evaluated by means of contact angle measurements and correlated with the morphological characteristics.     

Back discharge in multipoint-plane geometry in flue gases

The paper presents investigations of back discharge occurring in air and flue gases produced by the process of burning of liquefied petroleum gas or charcoal. The discharge was generated between a multineedle electrode and plate covered with fly ash layer. The aim of this work was to determine the effects of back discharge in multineedle-to-plate electrode configuration on the fly ash layer covering the plate electrode. Level of NO_x and CO emission was also measured. It was found that the chemical composition of flue gas can be changed in the domains where the back discharge occurs, for example, additional amounts of nitrogen oxides (NO and NO₂) are produced and also carbon oxide (CO) was produced at higher discharge current.     

Diode end pumped Nd:YAG laser at 946 nm with high pulse energy limited by thermal lensing

Diode lasers with peak powers in the kW range and pulse durations of micro- to milli-seconds have been available since several years. Pumping solid state lasers with such sources yield high output pulse energies in spiking or Q-switched operation. The output energy is limited by the thermal lens effects, which are measured and calculated. The time dependent heat conduction equation in the laser crystal is solved numerically to predict the overall temperature rise and thermal lensing. The thermally induced optical path difference is approximated by a quadratic distribution to obtain the focal length f of the thermal lens. The thermal lens coefficient K=1/(f⋅P _av), which depends only weakly on the heat transfer coefficient H of the laser crystal to the heat sink, decreases exponentially with increasing pump frequency until the steady state is reached. Experiments were done with a Nd:YAG crystal at different pump frequencies up to 100 Hz. The thermal lens coefficients obtained by the power maxima of asymmetric flat-flat resonators agree with our calculations.     

Temporal extension of stable glow discharges in fluorine-based excimer laser gas mixtures by the addition of xenon

The effect of addition of xenon on the long term homogeneity of discharges in F₂and ArF excimer laser gas mixtures was investigated in a small-volume discharge chamber. The gas mixture in the discharge chamber was preionized by X-rays. A special electrical excitation circuit containing a pulse forming line provided a long, square-shaped current pumping pulse of a predetermined duration to the discharge electrodes. The initiation and the development of the discharge was monitored via its fluorescence signal with an intensified CCD camera. We found that adding Xe up to partial pressures of 0.53 mbar extended the homogeneous phase of the discharge from 80 ns to approximately 200 ns in He/F₂as well as in He/Ar/F₂and Ne/Ar/F₂excimer laser gas mixtures. Monitoring of the ArF and XeF spontaneous emission signals showed that the formation of ArF excimers remained unaffected by the addition of xenon (up to 1.3 mbar) to the laser gas mixture.     

Investigation of radial profiles of vibrational and rotational temperatures in nitrogen DC glow discharge

Spatially resolved measurements of vibrational and rotational temperature determined from the N₂(C) nitrogen bands intensities have been performed by means of optical scanner of original construction. It has been found that radial variations of studied bands are independent of pressure and discharge current under our experimental conditions, i.e. in the pressure range (100–300) Pa and for discharge current up to 40 mA. Moreover, it has been found that vibrational as well as rotational temperatures stay almost constant in the radial direction. No radial changes of both temperatures can be explained by good thermal conductivity of the positive column of DC glow discharge. This research was supported by grants: Charles University No. GAUK 194/01, Ministry of Education of Czech Republic MSM 11320002, and Grant Agency of Czech Republic GAČR 202/03/0827. The theme of presented article was included in the EU project No. G1RT-CT-2002-05083 “Plasmatech”.     

KrF pulsed laser ablation of polyimide

In the present paper, polyimide surfaces were processed with pulsed KrF laser radiation at fluences near the ablation threshold. The morphology of the processed surfaces was studied by scanning electron microscopy and chemical analyses performed by electron dispersive spectroscopy. The formation of conical structures was observed for radiation fluences lower than 0.5 J/cm². The areal density of cones increases with the number of pulses and decreases with the radiation fluence. At low fluences (<150 J/cm²), cones are formed due to shadowing by calcium phosphate impurities while for higher fluences the main mechanism of cones formation is believed to be radiation hardening.     

Modulation enhancement of a laser diode in an external cavity

The influence of Zn-atom additive on ‘pure’ copper vapor laser output characteristics is studied. The laser pulse width, energy and power are found to increase under the diffusion of zinc atoms from a zinc-containing reservoir into the discharge of the copper-vapor laser at temperatures above 500°C. Additional absorption experiments and calculations are consistent with the conclusion that not only optical resonant pumping by the 213.9-nm Zn I line, but also other processes, should be taken into account to explain the effects of additive influence.     

Frequency stabilization of CO laser using RF optogalvanic Lamb-dip

The Lamb dip of CO rovibrational transition is detected by a room temperature extracavity RF optogalvanic cell and employed to stabilize the frequency of a CO laser. The S/N ratio of optogalvanic signal is about 2000  at optical power < 1 W. The relative depth of Lamb dip is 2.3%. The S/N ratios of first and third harmonic demodulated saturation signals are about 40  and 10  , respectively. The CO laser is stabilized using the first harmonic demodulated signal, and the frequency stability is better than 300 kHz. Concurrently, the influences of operational parameters, which include the coil current, partial pressures of gas mixture, are investigated. A simple model for the influence of coil current is presented, and further improvements are addressed as well.     

Modelocked quantum dot vertical external cavity surface emitting laser

We report the first successful modelocking of a vertical external cavity surface emitting laser (VECSEL) with a quantum dot (QD) gain region. The VECSEL has a total of 35 QD-layers with an emission wavelength of about 1060 nm. In SESAM modelocked operation, we obtain an average output power of 27.4 mW with 18-ps pulses at a repetition rate of 2.57 GHz. This QD-VECSEL is used as-grown on a 450 μm thick substrate, which limits the average output power.     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

High-power diode-pumped Tm:YLF slab laser

A 2% Tm³⁺-doped LiYF₄(Tm:YLF) slab is double-end-pumped by two laser diode stacks. The pumped volume has a rectangular cross section. The Tm:YLF laser produced 148 W of continuos-wave output at 1912 nm in a beam with M _x ²≈199 and M _y ²≈1.7 for 554 W of incident pump power. The slope efficiency with respect to the incident pump power was 32.6%, and the optical-to-optical efficiency was 26.7%.     

Near- and mid-infrared laser radiation interaction with eye tissue

Comparison of the eye tissue transmission for laser radiations with the wavelengths 0.75, 1.08, 1.34, 1.44, 1.54, 1.66, 2.01, 2.69, and 2.94 μm was made with the aim to understand the possible tissue injury by the particular light. From the step by step transmission measurements of the various human eye tissues (in vitro) it was recognized that the value of the transmitted energy in particular segments is different for various wavelengths yielding substantial fluence differences on the retina and anterior segments, mainly.     

High-efficiency,high-speed VCSELs for optical interconnects

High-efficiency, high-speed, tapered-oxide-apertured vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm have been demonstrated. By carefully engineering the tapered oxide aperture, the mode volume can be greatly reduced without adding much optical scattering loss for the device sizes of interest. Consequently, these devices can achieve higher bandwidth at lower current and power dissipation. In addition, the parasitics are reduced by implementing deep oxidation layers and an improved p-doping scheme in the top mirror. Our devices show modulation bandwidth exceeding 20 GHz, a record for 980 nm VCSELs. Moreover, 35 Gb/s operation has been achieved at only 10 mW power dissipation. This corresponds to a data-rate/power-dissipation ratio of 3.5 Gbps/mW. Most importantly, our device structure is compatible with existing manufacturing processes and can be easily manufactured in large volume making them attractive for optical interconnects.     

Three-dimensional modelling of inductively coupled plasma torches

A three-dimensional model has been developed for simulating the behaviour of inductively coupled plasma torches (ICPTs), using customized CFD commercial code FLUENT ^?. The helicoidal coil is taken into account in its actual 3-D shape, showing the effects of its non-axisymmetry on the plasma discharge. Steady state, continuity, momentum and energy equations are solved for argon optically thin plasmas under the assumptions of LTE and laminar flow. The electromagnetic field is obtained by solving the 3-D vector potential equation on a grid extending outside the torch region. In order to evaluate the importance of various 3-D effects on calculated plasma temperature and flow fields, comparisons of our new results with the ones obtainable from conventional 2-D models and from an improved 2-D model that includes 3-D coil effects are presented. The presence of wall temperature hot spots due to plasma discharge displacement from the torch axis is evidenced, while the use of the new 3-D code for optimization of induction coil geometry and plasma gas inlet features is foreseen. Received 5 September 2002 Published online 13 December 2002 RID="a" ID="a"e-mail: colombo@ciram.ing.unibo.it     

Optical gain on the 476.5 nm Argon-ion laser line in a gas-target excited by a heavy ion beam

Optical gain on the 476.5 nm Ar II 4p–4s ion laser transition has been observed in argon-gas excited by 2.5 ns pulses of 90 MeV³²S ions with a repetition rate of 4883 Hz. The energy per pulse was 23 J. The projectiles were stopped in the target at pressures between 5 and 20 kPa. Gain was determined from a measured transient increase of the intensity of a 476.5 nm probe laser beam sent along the ion beam axis and back reflected by an aluminum foil. The maximum gain observed was (0.4±0.1)×10^–3 at a target-gas pressure of 5 kPa. Control experiments using krypton as target-gas were performed and yielded a null result. The optical gain observed in argon is consistent with the result from an analysis of spectroscopic studies of rare-gas targets excited by heavy ion beams.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday. This work has been funded by the German Federal Ministry for Research and Technology (BMFT) under contract No. 06 TM 310 I, Gesellschaft für Schwerionenforschung (GSI), Darmstadt, and the Tandem accelerator laboratory, Munich     

Long period gratings written in large-mode area photonic crystal fiber

We report for the first time, to the best of our knowledge, on the fabrication and characterization of CO₂-laser written long-period gratings in a large-mode area photonic crystal fiber with a core diameter of 25 μm. The gratings have low insertion losses (<1 dB) and high attenuation (>10 dB) at the resonant wavelengths, making them particularly interesting for high power applications.