Laser plasmatron for diamond coating deposition

An experimental installation with a laser plasmatron based on a continuous wave CO₂ laser with a radiation power of up to 3.5 kW has been created. The plasmatron design makes it possible to bring out the plasma jet into atmospheric air both along and across the laser beam direction. The spatial temperature distributions on the metal substrate surface heated by the plasma jet are measured. The threshold power for optical discharge maintenance as a function of the gas flow rate and the focal length of the focusing lens are obtained for an Ar and Ar/CH₄/H₂ gas mixture under atmospheric pressure; the radiation spectrum of the discharge plasma is measured. A one-dimensional model of the discharge for estimation of its geometrical parameters in a convergent laser beam with consideration of radiation refraction on the discharge is given.     

CO2 laser-induced plasma CVD synthesis of diamond

2 laser maintenance of a stationary optical discharge in a gas stream, exhausting over a substrate into the air (laser plasmatron). Nano- and polycrystalline-diamond films were deposited on tungsten substrates from atmospheric-pressure Xe(Ar):H₂:CH₄ gas mixtures at flow rates of 2 ?/min. A 2.5-kW CO₂ laser focused beam produced plasma. The deposition area was about 1 cm² and growth rates were up to 30–50 μm/h. Peculiarities and advantages of laser plasmatrons are discussed. Received: 15 January 1998/Accepted: 16 January 1998     

Nonlinear optical absorption in Bi3TiNbO9 thin films using Z-scan?technique

Bi₃TiNbO₉ (BTN) thin films with layered perovskite structure were fabricated on fused silica by pulsed laser deposition. The XRD pattern revealed that the films are single-phase perovskite and highly (00l) textured. Their fundamental optical constants, such as band gap, linear refractive index, and linear absorption coefficient, were obtained by optical transmittance measurements. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The nonlinear optical absorption of the films was investigated by single beam Z-scan method at a wavelength of 800 nm with laser duration of 80 fs. We obtained the nonlinear absorption coefficient β=1.44×10⁻⁷ m/W. The results show that the BTN thin films are promising for applications in absorbing-type optical devices.     

Performance of a zoom homogenizer for reshaping coherent laser beams

We have reshaped the TEM₀₀ beam emitted by a diode-pumped Nd-Yag laser by means of an optical homogenizer with zoom. The laser beam, first enlarged up to a size of (100 × 37.5) mm², is homogenized and resized to a final dimension continuously adjustable from (130 × 4.5) mm² up to (130 × 52) mm². We measured the plateau uniformity, the root mean square fluctuation and the edge steepness of the beam according to the ISO standard definitions, showing a poor reliability of the above parameter values due to the definitions themselves, and propose an amendment to overcome this problem. We obtained a very sharp edge steepness, but the spatial coherence of the laser beam put a lower limit to the high-frequency intensity fluctuations on the plateau region of the homogenized beam. Finally, we discuss the optimum homogenizer design for spatially coherent laser beams, including the depth of focus issue.     

Optical resonator for high-power transverse flow CO2 lasers

A new design of the U-type resonator is described. In this way, a laser beam with symmetrical intensity profile (regarding to a symmetry plane) can be extracted from an active medium that exhibits gain asymmetry along one of the transverse directions. The whole area of the active medium cross-section can be used, and consequently the laser efficiency will be increased. This resonator structure was applied for efficiency power extraction (as a low order TEM modes laser beam) from a DC excited transverse flow CO₂ laser with cylindrical geometry. Although the cross-section area of the discharge was entirely used (including the cathode fall region), a symmetrical intensity profile of the laser beam (regarding to the two orthogonal symmetry planes) was obtained in the near field as well as in the far field; the gain asymmetry along the flow direction was compensated by the gas circulation fluidodynamical circuit with two counterflowing discharge channels. A double-U optical resonator was introduced in order to provide a laser beam with axial symmetry.For the practical construction of these two types of optical resonators we have developed two new types of 90° deflection elements: the first one, which does not reverse the image (and which has the properties of the pentaprism), and the second one, which rotates the image with 90° angle. Both elements exhibit good focusability if they are equipped with two concave mirrors.     

A neat semi-Gaussian laser beam with no diffraction fringes: Theoretical analysis

We propose a simple optical device to convert a Gaussian laser beam into a neat semi-Gaussian laser beam without any diffraction fringe by using a spatial light modulator and a thin, sharp blade, and numerically calculate the diffracted, relative intensity distributions of both the semi-Gaussian laser field and the semi-Gaussian, pseudo-thermal light. We also study the dependence of the border width of the semi-Gaussian beam on the waist of the Gaussian beam. Our study shows that the proposed scheme can be used to cancel all diffraction fringes from both the straight edge of the blade and a finite lens aperture in all the planes vertical to the z axis and obtain a neat semi-Gaussian beam without any diffraction fringe, and find that the border width w_B of the generated semi-Gaussian beam is not dependent on the waist of the incident Gaussian beam.     

Tunnel regenerated coupled multi-active-region large optical cavity laser with high quality beam

A novel coupled multi-active-region large optical cavity structure cascaded by tunnel junction is proposed to solve the problems of catastrophic optical damage (COD) of facet and large vertical divergence caused by the thin emitting area in the conventional laser diode. For the laser with three active regions, a slope efficiency as high as 1.49 W/A, a vertical divergence angle of 17.4°, and a threshold current density of 271 A/cm² are achieved. By optimizing the structure parameters, the beam quality is greatly improved, and the level of COD power increases more than two times compared with that of the conventional laser.     

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.     

A simple kinetic model of a Ne-H2 Penning-plasma laser

A simple kinetic model of the Ne-H₂ Penning-Plasma Laser (PPL) (NeI 585.3 nm) is proposed. The negative glow of a hollow cathode discharge at intermediate pressures is considered as the active medium. The balance equations for the upper and lower laser levels, electrons, ions and electron energy are solved. The dependences of the laser gain on the discharge conditions (Ne and H₂ partial pressures, discharge current) are calculated and measured. The calculated values are in a good agreement with the experimental data.     

Theoretical consideration of pits recording and etching processes in chalcogenide vitreous semiconductors

We propose theoretical consideration, computer modeling and comparison with our recent experimental results for information pits recording and etching processes in chalcogenide vitreous semiconductors using Gaussian laser beam and selective etching. Our calculations demonstrate that photo-transformed region cross-section could be almost trapezoidal or parabolic depending on the photoresist material optical absorption, exposure, etchant selectivity and etching time. Thus our approach open possibilities how to select the necessary recording procedure and etching conditions in order to obtain pits with the optimum shape and sizes in As₄₀S₆₀ chalcogenide semiconductor. Obtained results quantitatively describe the characteristics of pits recorded by the Gaussian laser beam in thin film of As₄₀S₆₀.     

Optical actuation of a macroscopic mechanical oscillator

An intensity-modulated HeNe-laser beam was utilized to optically actuate the mechanical resonance of a macroscopic torsional silicon oscillator (f₀ = 67 700 Hz, Q = 42 100 at p = 1 mbar and T = 300 K). Both radiation pressure and photothermal effects may cause optical actuation of a mechanical device. Both excitation effects were studied. In actuation through radiation pressure, the actuating laser beam was focused on the high-reflectivity-coated oscillator surface. In the case where the intensity-modulated laser beam was incident on the uncoated silicon surface the photothermal effect was shown to be the dominating excitation factor. Oscillation amplitudes due to the actuation through radiation pressure and photothermal effects were Δ x_rad = 1.4 pm and Δ x_ph = 4.3 pm with the same optical power of 1.5 mW. The measured resonance frequency and quality value were not changed when purely mechanical and radiation pressure actuation mechanisms were compared. With photothermal actuation the absorbed optical power heats the oscillator, introducing a slight decrease in the resonance frequency. Our experiments demonstrate that optical actuation combined with sensitive optical interferometric measurements can be utilized to perform dynamic vibration analysis of micromechanical components. Prospects of using micromechanical devices for observing extremely weak external forces are discussed.     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Creation of atomic W state in a cavity by adiabatic passage

We propose two relatively robust schemes to generate controllable (deterministic) atomic W states of three Λ-like atoms interacting with an optical cavity and a laser beam. Losses due to atomic spontaneous emissions and to cavity decay are efficiently suppressed by employing adiabatic passage technique and appropriately designed atom-field couplings. In these schemes the three atoms traverse the cavity-mode and the laser beam and become entangled in the free space outside the cavity.     

A tunnel regenerated coupled multi-active-region large optical cavity laser with a high quality beam

A novel coupled multi-active-region large optical cavity structure cascaded by a tunnel junction is proposed to solve the problems of facet catastrophic optical damage (COD) and the large vertical divergence caused by the thin emitting area in conventional laser diodes. For a laser with three active regions, a slope efficiency as high as 1.49 W/A, a vertical divergence angle of 17.4 , and a threshold current density of 271 A/cm 2 are achieved. By optimizing the structural parameters, the beam quality is greatly improved, and the level of the COD power increases by more than two times compared with that of the conventional laser.     

Time-dependent thermal lensing and Kerr nonlinearity in CS2 with CO2-laser radiation

We report on optical limiting and z-scan in CS₂ as a function of the pulse duration of 10 µm TEA CO₂-laser radiation. We present a comprehensive and simple model for the shape of a laser pulse transmitted through this limiting device. For the first time, we propose the use of an optical limiting device for beam shaping.     

Anomalous behavior of optogalvanic signal in a miniature neon discharge lamp

A strong optogalvanic effect has been observed in a negative glow of a miniature neon discharge lamp using tunable pulse dye laser pumped by a copper vapor laser. A comparative study on temporal evolution of optogalvanic signal in a positive and negative dynamic resistance region of the discharge is described. Dye laser beam was tuned to various neon transitions 1s_i → 2p_j (Paschen notations) within 570-617 nm wavelength range. Anomalous behavior of optogalvanic signal was observed at 588.2 nm for (1s₅ → 2p₂) neon transition at low discharge current (<220 μA). This anomalous behavior is the attributes of damped oscillations of optogalvanic signal that correlate with negative dynamic resistance (dV/di < 0) of the discharge. Penning ionization at low discharge current and small energy mismatch is assumed to be the main cause of the negative dynamic resistance. Penning ionization process has been explained by resonantly ionizing energy transfer via collisions between neon buffer gas atoms in the lowest metastable state (1s₅) and electrode sputtered atoms in ground state using their partial energy level diagram.     

Optimization of a laser mitigation process in damaged fused silica

One of the major concerns encountered in high power laser is the laser-induced damage of optical components. This is a main issue of the development of the Europe's biggest laser, known as Laser Méga Joule (LMJ) especially in the section where the beam wavelength is 351 nm. This study deals with the development of a laser treatment process to improve the laser damage resistance of silica optical components. First, by irradiating the component at 355 nm in the nanosecond regime, defects of the silica optic are revealed and evolve as damage. Next, the damaged sites are irradiated with a CO₂ laser at a 10.6 μm wavelength in order to melt and evaporate the silica in the damage neighborhood. In this study, we performed a variation of the CO₂ laser parameters to obtain the most efficient stabilization. To check this stabilization, damage resistance tests were performed with an UV laser representative of the LMJ (at 355 nm/2.5 ns). The results show that we can stabilize weak points and thereby make the component resistant to subsequent UV laser irradiation.     

Photorefractive spatial solitons as waveguiding elements for optical telecommunication

Spatial solitons permit optical waveguiding. This holds true for the soliton write beam (i.e. the driving laser beam), as well as for additional probe beams, which may carry optically encoded information. This feature of spatial solitons is of significant interest for applications in optical telecommunication. We present systematic experimental investigations on single and multiple spatial solitons in the infrared spectral regime (i.e. around optical telecommunication wavelengths), applied as controllable all-optical devices. In particular, we present the implementations of a Y-coupler as an optical signal divider, a switchable Y-coupler as an optical add multiplexer, and a novel design for a 1 × 3 optical beam switch, i.e. applied as a router for infrared signal beams. We report large waveguiding efficiencies up to 40% and transmission rates of 90 Tbit/s in our setups. The presented experimental data are confirmed by numerical simulations.     

Angle-of-arrival variance’s dependence on the aperture size for indoor convective turbulence

We analyze the angle-of-arrival variance of an expanded and collimated laser beam once it has traveled through an indoor convective turbulence. A continuous position detector is set at the focus of a lens collecting the laser beam. The effect of the different turbulent scales, between the inner and the outer scales, is studied by changing the diameter of a circular pupil before the collector lens. The experimental optical setup follows the design introduced by Masciadri and Vernin [Appl. Opt., 36(6) (1997) 1320]. Tilt data measurements are studied using the fractional Brownian motion model for the turbulent wave-front phase introduced in a previous paper [Pérez et al., J. Opt. Soc. Am. A 21(10) (2004) 1962]. The Hurst exponents associated to different strengths of turbulence are obtained from the here proposed D^2H−2 dependence.     

Theoretical and experimental investigations of nanosecond 177.3 nm deep-ultraviolet light by second harmonic generation in KBBF

We have presented theoretical and experimental investigations of nanosecond (ns) deep-ultraviolet (DUV) 177.3 nm radiation by means of second harmonic generation (SHG) from a frequency-tripled Nd:YAG laser (355 nm, 49 ns and 10 kHz) in KBe₂BO₃F₂ (KBBF) nonlinear crystal for the first time. A DUV KBBF-SHG numerical model, accounting for linear absorption, pump depletion, beam spatial birefringent walk-off and diffraction, is performed in the Gaussian approximation of spatial and temporal profiles. In the experiment, a maximum average output power of 14.1 mW at 177.3 nm was obtained. The dependence of 177.3 nm output power on the 355 nm pump power was simulated. The calculated results are in good agreement with the measured data. We used the model further to investigate the optical conversion efficiency, pulse width, beam spatial intensity profile and beam quality factor of the generated 177.3 nm light, in particular the effect of beam birefringent walk-off.