Cw Laser Beam Research Articles

Comparative study between interferometric and Z-scan techniques for thermal lensing characterization

This work presents an original, comparative study between two optical techniques for the analysis of thermal lensing induced by a low-power, cw laser beam focused onto a sample cell containing a weak absorbing medium. It deals with an interferometric technique and a Z-scan technique in real time. The interferometric method permits the determination of the spatial profile of the thermal lens. The development of the work puts in evidence the high sensitivity of both techniques for the detection and measurement of low absorption coefficients and refractive index changes in dye solutions at very low concentrations. Improvements in the sensitivity of both methods can make possible the measurement of very small phase shift distortions of the wavefront. One shows also the mutual complementary character of two techniques for the characterization and measurement of linear and nonlinear properties of materials.

Crystallization of Glycine by Photon Pressure of a Focused CW Laser Beam

Abstract An intense CW-YVO4 laser beam of 1064 nm was focused into a supersaturated D2O solution of glycine, leading to its crystallization. At the focal point on an air–solution interface, a small single glycine crystal was trapped and grew quickly. The crystallization mechanism is considered in view of gathering and organization of large solute clusters of glycine by photon pressure.

Measurement of laser-induced refractive index change of inverted ferroelectric domain LiNbO_3

Optical nonlinearities of periodically poled LiNbO(3) crystals were investigated by the single beam Z-scan technique with a continuous wave (cw) laser beam at 532 nm. The nonlinear optical absorption coefficient and refractive index change are determined to be 8.1 x 10(-6) cm/W and 2.6 x 10(-4) at 0.5 MW/cm(2) light intensity, respectively. Both sign and magnitude of the measured refractive nonlinearity are considerably different from the Z-scan results in congruent LiNbO(3). The nonlinearities in the periodically poled LiNbO(3) induced by 532 nm continuous waves are believed to be mainly due to the photorefractive effect.

Hydrodynamical phenomena in the process of laser welding and cutting

The main objective of the paper is to outline the 'bridges' existing between the outcomes of fundamental researches and the results of investigations in the field of industrial laser materials processing (LMP). An analysis is presented on the models based on non-stationary hydrodynamic phenomena caused by deeply penetrating high power CW laser beam into materials. This is typical of laser welding (LW) and laser cutting (LC). A physical analysis pertaining to melt removal and melt layer instability mechanisms of gas jet assisted CW–CO2 laser fusion cutting is presented. The models deliberated here are melt squeezing out by gas pressure gradient, melt dragging by the friction force between melt surface and gas flow, formation of moving shelves at the cutting front. In case of high laser intensity, radiative flux interacts with material causing dynamical thermal transport onto the surface and phase transition at solid–liquid–gas interfaces. The solution is based on the non-stationary variables. Under these conditions the Mach number varies significantly due to laser intensity associated with laser flux energy instabilities. The connection among material surface temperature, laser intensity, laser flux and pressure in the plasma cloud is brought out. In addition, novel mechanisms based on hydrodynamics are proposed.

Combined laser spot imaging thermography and ultrasonic measurements for crack detection

A surface crack, close to a small, well-defined heated spot, impedes lateral heat flow and produces alterations in the shape of the thermal spot image that can be monitored by thermography. Spot heating has been achieved using both cw and pulsed laser beam illumination on stainless steel and titanium samples and the technique has been found to be successful for determining the location of fatigue cracks. When using a pulsed laser beam one can also simultaneously generate wideband ultrasonic signals in the sample; this can be used to detect the presence of surface and sub-surface defects in the sample. Results are presented that have been obtained using a fixed camera and cw laser beam position with the sample being moved through the field of view of the camera. Results are also presented using a fixed camera and sample, with a raster scanned cw or pulsed lasers beam moving across the sample. A demonstration of how a non-contact ultrasonic measurement can be performed simultaneously is presented. Thermal imaging results obtained using cw laser beam heating close to a surface crack are compared with predictions made using a preliminary 2D numerical model.

Calculation of thermal processes upon the laser modification of HTSC ceramics aimed at increasing the critical current

A mathematical model for the heating of a surface layer of HTSC ceramics by a CW laser beam scanned at a constant rate is considered. The calculated depths of the molten region are presented for the heating of the Bi(2223) ceramics by a CW CO2 laser. The theoretical results are in reasonable agreement with the experimental data. The calculations can be used for the optimization of laser processing of superconducting ceramics aimed at increasing the critical current density.

RevoLix™ vaporesection of the prostate: initial results of 54 patients with a 1-year follow-up

Multiple laser systems for the treatment of benign prostatic hyperplasia (BPH) have been introduced. Current laser systems have limitations due to their laser physics. The RevoLix laser combines the advantages of the Holmium:YAG laser with the comfort of a continuous wave (cw) laser beam. This study reports the preliminary results of vaporesection (simultaneous vaporization and resection) of the prostate, using the 2 microm cw laser. A total of 54 consecutive patients were treated with the 70 W RevoLix laser for BPH. The mean age was 61 years. Mean prostate volume was 30.3 cc. A 550 microm RigiFib bare-ended fiber was used in combination with a 26 French laser resectoscope. Measured outcomes were resection time, decrease in hemoglobin and transfusion rate. Furthermore, the catheter time, improvement in the urinary flow rate (Q(max)), post-voiding residual urine (PVR), International Prostate Symptom Score (IPSS) and Quality of Life Index (QoL) were recorded. Average resection time was 52 min. After crossing the learning curve, a tissue ablation of 1.5 g/min was possible. Transfusions were not necessary in any patient. Catheter time was 1.7 days. Q(max) significantly improved from 4.2 to 20.1 ml on average. PVR decreased from 86 to 12 ml. IPSS and QoL-Score improved from 19.8 to 6.9 and 4 to 1, respectively. No patient required re-hospitalization. These preliminary results indicate that RevoLix vaporesection of the prostate is safe and efficient. One-year follow-up data showed a significant improvement in voiding symptoms and patients' quality of life. A longer follow-up is needed to prove the durability of these promising results.

Threshold Behavior of Photoinduced Plasmon-Resonant Self-Oscillation in a New Interdigitated Grating Gates Device

The new doubly interdigitated grating gate high-electron-mobility transistor (HEMT) has been subjected at room temperature to a 1.5 µm cw laser beam. The observed photoresponse shows a threshold behavior as a function of drain-to-source bias for different gate voltages. The result was interpreted as a clear signature of the self-oscillation of plasma waves.

Characterization of thermal diffusivity of micro/nanoscale wires by transient photo-electro-thermal technique

In this work, a transient photon-electro-thermal (TPET) technique based on step laser heating and electrical thermal sensing is developed to characterize the thermophysical properties of one-dimensional micro/nanoscale conductive and nonconductive wires. In this method, the to-be-measured thin wire/tube is suspended over two electrodes and is irradiated with a step cw laser beam. The laser beam induces a transient temperature rise in the wire/tube, which will lead to a transient change of its electrical resistance. A dc current is applied to the sample, and the resulting transient voltage variation over the wire is measured and used to extract the thermophysical properties of the sample. A 25.4-μm thick Pt wire is used as the reference sample to verify this technique. Sound agreement is obtained between the measured thermal diffusivity and the reference value. Applying the TPET technique, one can measure the thermal diffusivity of conductive single-wall carbon nanotube (SWCNT) bundles and nonconductive cloth fibers. For nonconductive wires, a thin (∼ nm) metallic film is coated on the outside of the wire for electrical thermal sensing. The measured thermal diffusivity for the SWCNT bundle is 2.53×10-5 m2/s, much less than the thermal diffusivity of graphite in the layer direction. For microscale cloth fibers, our experiment shows its thermal diffusivity is at the level of 10-7 m2/s.

Diffraction characteristics of a Fresnel zone plate illuminated by 10 fs laser pulses

Results of experimental and theoretical work performed to compare diffraction patterns and focal distributions of a Fresnel zone plate illuminated by ultrashort 10 fs pulsed and cw laser beams are presented. It is shown that the foci intensities of 10 fs pulses are considerably lower than those of cw beams while the focal widths in the axial and radial directions are broadened. Calculations also indicate the spectral modulation along the center of the diffraction patterns. These phenomena are explained by the coherent superposition of the composing frequency content.

Analysis of all-optical light modulation in proteorhodopsin protein molecules

We present a detailed steady-state and time-dependent theoretical analysis of all-optical light modulation in the recently discovered, wild-type proteorhodopsin (WTpR) protein molecules based on excited-state absorption. Amplitude modulation of cw probe laser beam transmissions at 520, 405, 555 and 560 nm, corresponding to the peak absorption of pR, pR M, pR K and pR N intermediate states of pR photocycle, respectively, by cw and pulsed modulating pump laser beam at 520 nm have been analyzed. The effect of various spectral and kinetic parameters on modulation characteristics has been studied. There is an optimum value of concentration for a given pump intensity value for which maximum modulation of the probe beam can be achieved. The switching characteristics of probe beam at 405 and 520 nm exhibit dip and peak, respectively, which can be removed by decreasing the absorption of pR M state at 520 nm. The modulation in WTpR is at lower pump powers with smaller contrast in comparison to WT bacteriorhodopsin (bR) and WT pharaonis phoborhodopsin ( ppR). The modulation characteristics exhibit unique features compared to bR and ppR.

تأثير سرعة المسح للحزمة الليزرية على خصائص منطقة التقسية في الفولاذ الكربوني

Surface heat treatment of steel by using laser technology is the most developed method of hardening for machine elements and tools, where it formes on the surface structure with optimal properties. The structure depends on phase's transformation which occures in steel after laser treatment. The mechanism of mutual reaction between laser beam and steel is bind by kind of laser generation, wave length, power and scanning speed. The scanning speed of CW laser beam became the most important parameters of laser surface treatment. This paper concentrate on the study of the effect of laser scanning speed for steel surface on the depth of heating zone in three kind of carbon steel and the microhardness inside heat effected zone. It has been shown that both of microhardness and depth of hardening layer are decreased when scanning speed is increased. Also, it has been studied the relation between microhardness and the containment of carbon in steel.

Characterization of glass and high-power near-infrared cw laser beams using nonlinear optical techniques

Absorption coefficients of highly transparent optical materials (glass) have been measured using a z-scan technique. Absorption coefficients as low as 10−5 cm−1 and lower can be measured by scanning a substrate of a transparent optical material in the focal region of a lens. Glass substrates with relatively high absorption coefficients are used for calibration and for verifying the results of measurements. The sensitivity of this technique proves to be high enough to reveal differences between absorption coefficients of different substrates made of the same material (BK7). We have suggested and used a novel procedure, scanning nonlinear lens profiling, for characterization of homogeneity of glass substrates and other transparent optical materials. Optical nonlinearity of glass substrates allows fast and high-precision measurement of the power and power density of high-power near-IR laser beams without large temperature increases and related problems.

Laser Beam Filamentation in Fractal Aggregates

We investigate filamentation of a cw laser beam in soft matter such as colloidal suspensions and fractal gels. The process, driven by electrostriction, is strongly affected by material properties, which are taken into account via the static structure factor, and have impact on the statistics of the light filaments.

Experimental demonstration of novel effects on the far-field diffraction patterns of a Gaussian beam in a Kerr medium

We report on the experimental observation of the effects on the far-field patterns obtained when a divergent or convergent beam interacts with a thin self-defocusing material. The measurements were performed using a CW laser beam propagated through a cell containing a colloidal system that presents a very large nonlinear refractive index. The high nonlinearity of the colloid allowed for the observation of patterns never investigated before, revealing a quite different behaviour on the far field from a convergent or a divergent beam due to the opposite interaction with the negative Kerr nonlinearity of the medium. An unpredicted relation between the number of generated rings and the nonlinear phase-shift is obtained in the case of a convergent beam. Our results are in good agreement with a model for thin nonlinear media.

Intensity noise measurement of strongly attenuated laser diode pulses in the time domain

Developing the ability to characterize photon statistics of light sources has been one of the important driving forces of Quantum Optics. Photon statistics is also a crucial parameter to evaluate of quantum key distribution security. As practical quantum cryptographic systems encode information on faint laser pulses, we present a simple method to measure and calibrate their intensity noise with respect to shotnoise reference. The technique is based on the record of photodetection timetags in the photon counting regime. Two different methods are considered to produce light pulse: first, direct pulsing of a laser diode driving current, second, chopping the CW laser beam emitted by a laser diode with an acousto-optical modulator. As predicted by basic Quantum Optics theory, levels of attenuation used in practical quantum key distribution systems lead in both cases to Poissonian photon number distribution in the generated light pulses.

3D-soliton waveguides in lithium niobate for femtosecond light pulses

We show that efficient waveguides can be written by bright spatial solitons in the volume of lithium niobate photorefractive crystals by cw and pulsed laser beams. Using high-repetition-rate femtosecond laser pulses, an efficient formation of soliton waveguides (SWGs) is possible, after accumulating a large number of pulses, because the characteristic photorefractive build-up time is much longer than the pulse period and the efficient two-photon absorption may contribute to the solitonic confinement. These results open the possibility of writing reconfigurable single SWGs and SWG arrays (with any spatial orientation and large range of periods) and optimally guiding the femtosecond pulsed laser beams through them, creating a graded refractive-index profile matched to the spatial beam profile. Our experiments also show a small increase in pulse duration (small dispersion) in these waveguides.

Light-induced deformation and instability of a liquid interface. I. Statics

We study in detail the deformations of a liquid-liquid interface induced by the electromagnetic radiation pressure of a focused cw laser beam. Using a simple linear model of static equilibrium of the interface under the effect of radiation pressure, buoyancy, and Laplace pressure, we explain the observed hump height variations for any value of the optical Bond number Bo=(omega0/lc)2 (lc is the capillary length and omega0 is the waist of the beam) in the regime of weak deformations and show that the deformations are independent of the direction of propagation of the laser. By increasing the beam power, we observe an instability of the interface leading to the formation of a long jet when the laser propagates from the more refringent phase to the less refringent one. We propose that the total internal reflection of the incident light on the highly deformed interface could be at the origin of this instability. Using a nonlinear model of static equilibrium of the interface taking account of the angular dependance of radiation pressure, we explain the measured beam power threshold of the instability P, as well as the shape of the interface deformations observed at large waists just below the instability onset. According to this model, the instability should occur when the interface slope reaches the angle of total reflection, theta(TR). We find experimentally that, just below the instability threshold, the maximum incidence angle along the interface, theta(imax), is significantly smaller than theta(TR) and that our nonlinear model does not present any instability up to theta(imax)=theta(TR). Thus, although the proposed instability model correctly predicts the instability threshold P, it fails to describe the actual instability mechanism. We finally discuss possible additional effects that could explain the instability.

Preparation of thick enriched isotopic Si targets by the HIVIPP method

A thick enriched isotopic 28Si target is required for a hyper-nuclear spectroscopy experiment. We have successfully prepared Si targets of 50–125 mg/cm 2 in thickness, 15–25 mm in diameter and without backing support by using the high intensity vibrational powder plating (HIVIPP) method. The targets properties were tested with 3.2 MeV Ne + heavy ions and a high-power CW laser beam.

Laser Induced Modification in Glasses by CW Laser Backside Irradiation (CW-LBI)

We describe filamentary crack-free modification in silica glass, pyrex glass and soda-lime glass by a CW Laser Backside Irradiation (CW-LBI) method. In this method, an absorbent, which is attached on one side of a glass plate, is irradiated from the opposite side through the glass with CW laser beam. Ar ion laser is used and metal foil is chosen as absorbent. An intense flash moving from the absorbent toward the opposite direction of the incident laser beam at the speed of approximately 20 cm/s is observed and then modification appears in the flashed zone. With increasing irradiated laser power, the length of modification increases but the diameter is almost constant. Our experiments indicate the start of this modification may be due to thermal absorption from the heated absorbent by laser irradiation.