Laser Beam Deviation Research Articles

Accounting for speckle-scale beam bending in classical ray tracing schemes for propagating realistic pulses in indirect drive ignition conditions

We propose a semi-analytical modeling of smoothed laser beam deviation induced by plasma flows. Based on a Gaussian description of speckles, the model includes spatial, temporal, and polarization smoothing techniques, through fits coming from hydrodynamic simulations with a paraxial description of electromagnetic waves. This beam bending model is then incorporated into a ray tracing algorithm and carefully validated. When applied as a post-process to the propagation of the inner cone in a full-scale simulation of a National Ignition Facility (NIF) experiment, the beam bending along the path of the laser affects the refraction conditions inside the hohlraum and the energy deposition, and could explain some anomalous refraction measurements, namely, the so-called glint observed in some NIF experiments.

Cantilevered adaptive fiber-optics collimator based on piezoelectric bimorph actuators.

A novel, to the best of our knowledge, cantilever construction design of an adaptive fiber-optics collimator (AFOC) based on piezoelectric bimorph actuators for tip/tilt control is introduced. With this new cantilever structure, an AFOC with a diameter of only 6 mm was developed, and the output laser beam deviation angle and resonance frequency of the device were measured. The experimental results show that this new AFOC can provide more than 1 mrad deflection angle at a 20 V driving voltage, and the first resonance frequency is about 500 Hz. Further, in order to verify whether the cantilever structure can be used in a high-power fiber collimator, a high-power X-Y positioner with an 8 mm diameter fiber end cap was developed. The experimental results show that the high-power X-Y positioner can output more than 2 kW laser power and provide about 330 µm displacement of the fiber end cap in the X direction and about 770 µm in the Y direction at a 150 V driving voltage.

Pulsed laser welding and microstructure characterization of dissimilar brass alloy and stainless steel 308 joints

The laser welding of stainless steel and brass is difficult due to their different physical properties, including thermal conductivity and melting point. In this study, the laser welding of stainless steel 308 and brass was experimentally investigated. To systematically investigate the effects of different thermophysical properties, including the heat transfer rate, thermal conductivity, and laser beam interaction in relation to the beam absorption coefficient, the temperature of the melt pool zone was measured while changing welding conditions. Different temperature gradients were obtained by changing different parameters including the laser peak power, nozzle distance, and laser beam deviation. The results showed that an asymmetric melt pool formed, to which the melting of brass made a higher contribution. Due to the lower melting temperature and the higher thermal conductivity of brass, the measured temperature and the melt size of brass were higher. The microstructure of the melt pool consisted of intermetallic compounds. Variation of melt pool size (width and depth) and temperature could be affected by laser peak power and reductions in the nozzle distance than the other parameters. For instance, 1 mm increase in the nozzle distance reduced the melt pool zone temperature by approximately 20 °C for stainless steel. Furthermore, an increase in the laser peak power raised the maximum measured temperature to about 225 and 160 °C for brass and stainless steel sides, respectively. At the same time, the amount of temperature reduction at a period of 20 s for stainless steel is almost half that of brass.

Experimental investigation of temperature field and fusion zone microstructure in dissimilar pulsed laser welding of austenitic stainless steel and copper

Laser dissimilar welding of copper/ stainless steel was experimentally conducted to investigate the process parameters influence on the temperature field and formation mechanism of melt pool microstructures. The experimental approach is to estimate the influence of the Nd:YAG pulsed laser process parameters (e.g welding speed, laser power, focal length and laser beam deviation) on the temperature gradient and the resultant metallurgical aspects of the melt pool such as Solidification cracking, copper particle diffusion and microstructure of the fusion zone. Creating the appropriate temperature gradient during welding plays a crucial role to effectively control solidification cracking at the fusion zone in order to reduce heat conduction effect of the copper. Increasing welding speed from 2 to 6 mm/s not only reduces the temperature about 50 °C but also decreased the fusion zone cracking significantly. Evidently, increasing the laser power from 240 to 260 W transmitted the fusion zone cracks toward the stainless steel base due to creating higher temperature about 230 °C and 150 °C for steel and copper respectively. Therefore, higher discrepancy of the temperature field between copper and steel clearly changed the place and pattern of cracks propagation. The EDS analysis of the fusion zone confirms diffusion of copper particles inside the weld pool. The temperature measured near the molten pool region showed that clear discrepancy at the temperature gradient of the melt pool at the copper and stainless steel side has increased the possibility of cracking the stainless steel fusion zone. Also, high temperature gradient for compensating copper high rate of cooling could lead to excessive copper ablation or even diffusion of copper elements into the stainless steel fusion zone.

Thermal Performance Testing of a Flat Plate Solar Air Heater Using Optical Measurement Technique

Experimental test set up at laboratory scale has been developed for thermal performance testing of flat plate solar air heater with simulated solar radiation intensity; 600W/m 2 . A test cell of size 1m x 0.5m x 0.1m was fabricated. Three designs namely (i) plane absorber (ii) transverse V- porous ribs and (iii) inclined V-porous ribs of absorber are tested. All the experiments are conducted with artificial solar radiation and in natural convection. Performances of these three designs have been compared on the basis of overall thermal efficiency and thermal gradient along normal to the base. Thermal gradient has been determined by laser beam deviation method. PT-100 temperature sensors have also been used to validate the optical results of thermal gradient. The overall thermal efficiencies of these designs have been found as 14.91%, 17.24% and 20.04% respectively. It has also been seen that thermal gradient tends to reduce with increase in efficiency.

Dispersion and anisotropy of thermo-optic coefficients in tetragonal GdVO4 and YVO4 laser host crystals

A detailed experimental study of dispersion and anisotropy of thermo-optic coefficients dn/dT and thermal coefficients of the optical path W=dn/dT+(n-1)α is performed for tetragonal YVO(4) and GdVO(4) laser host crystals by a laser beam deviation method. It is supported by theoretical description of thermo-optic effects, taking into account the volumetric thermal expansion effect and the temperature dependence of electronic bandgap E(g). Linear thermal expansion coefficients α were also determined by a dilatometric technique. Thermo-optic dispersion formulas describing temperature variation of the refractive index and the thermal lens effect are derived for a wide spectral range of 0.4-2 μm. It allows us to estimate the values of E(g) and dE(g)/dT for the studied crystals. The influence of materials parameters and pumping conditions on thermal lens properties is discussed, revealing the significant impact of anisotropic and temperature-dependent thermal conductivity.

Thermo-optical properties of uniaxial NaT(XO4)2 laser host crystals (where T = Y, La, Gd or Bi, and X = W or Mo)

Thermo-optic coefficients dn o/dT and dn e/dT were measured in tetragonal double tungstate and double molybdate crystals NaT(XO4)2 (where T = Y, La, Gd or Bi and X = W or Mo) by a laser beam deviation method in the spectral range 0.4–1.1 μm. Thermal expansion coefficients in the directions of a and c crystallographic axes were also measured. Analytical expressions for thermo-optic dispersion formulas were derived as series in 1/λ 2. All dn/dT values for NaT(XO4)2 crystals were found to be negative. Their absolute values satisfy the relation |dn e/dT| > |dn o/dT| for crystals without Bi and |dn o/dT| > |dn e/dT| for crystals with Bi. A clear tendency for dn/dT values to decrease with the increase of the volumetric thermal expansion coefficient α vol of the crystal was observed. This is related with dominant contribution of volumetric thermal expansion effect to the temperature dependence of the refractive index. Thermal coefficients of the optical path W = dn/dT + (n − 1)α T governing thermal lensing effect were calculated for different light propagation directions and polarizations as well as crystal athermal directions.

Experimental and numerical analysis of the boundary layer on a vertical plate at low temperature differences

The problem of heat convection from surface to air at very low temperature differences has been solved by experimental/numerical approach. The laser-based probing technique of laser beam deviation detection was developed for the measurement of air refractive index gradients. The data obtained were processed by interactive coupling with numerical modelling tools. As it was approved, the key properties of the boundary layer such as temperature and temperature gradient profiles can be analysed in detail and the impact of various factors can be studied. The numerical simulation and the experimental data comparison had shown good coincidence; nevertheless, at a temperature difference of only few degrees, the influence of ambient air movement has to be considered in simulation process. The presented boundary layer analysis can be successfully applied at specific conditions of building physics with the low temperature differences.

Beam displacement as a function of temperature and turbulence length scale at two different laser radiation wavelengths

Narrow laser beams directed from aircraft may at times pass through the exhaust plume of the engines and potentially degrade some of the laser beam characteristics. This paper reports on controlled studies of laser beam deviation arising from propagation through turbulent hot gases, in a well-characterized laboratory burner, with conditions of relevance to aircraft engine exhaust plumes. The impact of the temperature, laser wavelength, and turbulence length scale on the beam deviation has been investigated. It was found that the laser beam displacement increases with the turbulent integral length scale. The effect of temperature on the laser beam angular deviation, σ, using two different laser wavelengths, namely 4.67 μm and 632.8 nm, was recorded. It was found that the beam deviation for both wavelengths may be semiempirically modeled using a single function of the form, σ=a(b+(1/T)(2))(-1), with two parameters only, a and b, where σ is in microradians and T is the temperature in °C.

Thermo-optical properties of pure and Yb-doped monoclinic KY(WO4)2 crystals

The thermo-optic coefficients, dn/dT, were determined for pure and Yb(20 at.%)-doped monoclinic KY(WO4)2 crystals for light polarized along the optical indicatrix axes (Np,Nm and Ng) in the wavelength range of 0.36–1.06 μm by a laser beam deviation method. The absolute values of thermo-optic coefficients satisfy the relation |dnp/dT|>|dng/dT|>|dnm/dT| and increase with the wavelength increasing. In the long-wavelength range, all the dn/dT values are negative: dnp/dT=−14.6, dnm/dT=−8.9, dng/dT=−12.4 [10−6 K−1] for pure KY(WO4)2 at 1.06 μm. The dependency of thermo-optic coefficients on the wavelength was modeled using an approach that takes into account contribution of volumetric thermal expansion and change of electronic bandgap with temperature. Large volumetric expansion of KY(WO4)2 plays a key role in the observed negative dn/dT values. Electronic bandgap and its temperature coefficient were determined for KY(WO4)2 crystals from thermo-optic dispersion curves as Eg=4.8–5.0 eV and −dEg/dT=0.7–1.1×10−4 eV/K. Athermal propagation directions were calculated for KY(WO4)2 crystals at the wavelength of 1.06 μm for light polarizations E∥Nm and Np.

Thermo-optic dispersion formulas for monoclinic double tungstates K Re(WO 4) 2 where Re = Gd, Y, Lu, Yb

Thermooptic coefficients dn/ dT for monoclinic potassium (rare-earth) double tungstates K Re(WO 4) 2 (where Re = Gd, Y, Lu, Yb) were determined at eight wavelengths in the range of 0.3–1.1 μm by a laser beam deviation method. In the long-wavelength range all the dn/ dT values were found to be negative and near constant, while dn/ dT( λ) dependence is especially pronounced in the short-wavelength range where thermooptic coefficients change sign to positive. Linear thermal expansion coefficients were measured in K Re(WO 4) 2 along the optical indicatrix axes by a dilatometric technique. Volumetric thermal expansion effect was found to dominate over the bandgap change influence on temperature dependence of the refractive index. Thermooptic dispersion formulas were derived for K Re(WO 4) 2 as series in 1/ λ 2. The influence of Yb and Nd doping on the dn/ dT values was investigated, resulting in prediction of thermooptic coefficients evolution in the isostructural KY x Yb (1− x )(WO 4) 2 and KYb x Lu (1− x )(WO 4) 2 series.

Charge-coupled devices combined with centroid algorithm for laser beam deviation measurements compared to a position-sensitive device

A position sensitive device (PSD) is frequently used in laser beam deviation measurement. However, it lacks the capability to retrieve the power distribution information of a laser beam. A charge-coupled device (CCD) gives much more information of a laser beam than a PSD. The requirement of a multifunctional sensor makes the replacement of a PSD with a CCD in measuring laser beam deviation to be a reasonable topic. In this paper a performance comparison between a PSD and a CCD combined with a centroid algorithm are discussed with special attention paid to the CCD-based system. According to the operating principle of the CCD-based system, several experiments were carried out to evaluate five factors of the CCD-based system: image window size, number of processed images, threshold, binning, and saturation. By applying the optimized parameters, several experiments were made to compare the CCD-based system with the state-of-the-art PSD-based system in terms of two performance indicators, namely resolution and speed. It is shown that, by applying the optimized parameters, the performance of a CCD-based system is comparable to that of a PSD-based system in measuring laser beam deviation.

Characterization study of silica aerogel for Cherenkov imaging

Different methods to measure the characteristics of silica aerogel tiles used as Cherenkov radiator in the CREAM and AMS experiments have been investigated to optimize the detector performances. The measurement accuracy dictated by the physics objectives on the velocity and charge resolutions set stringent requirements on the aerogel refractive index determination, namely Δ n ∼ 1.5 × 10 − 4 and Δ n ∼ 5 × 10 − 4 for the AMS and CREAM imagers, respectively. The matching of such accuracies for this material turned out to be a metrological challenge, and finally led to a full R&D program, to develop an appropriate characterization procedure. Preliminary studies performed with a standard refractive index measurement technique (laser beam deviation by a prism) have revealed a significant systematic index nonuniformity for the AMS tiles at a level ( 10 − 3 ) , not acceptable considering the aimed accuracy. These large variations were confirmed in a beam test. A second method, mapping the transverse index gradient by deflection of a laser beam entering normally to the tile has then been developed. It is shown that this procedure is suitable to reach the required accuracy, at the price of using both methods combined. The several hundreds of tiles of the radiator plane of the CREAM and AMS Cherenkov imagers were characterized using a simplified procedure, however, appropriate for each case, compromising between the amount of work and the time available. The experimental procedures and set-ups used are described in the text, and the obtained results are reported.

Noise reduction by diaphragming laser beam in the optical detection system of an atomic force microscope

We propose to use a diaphragm in order to decrease the contribution of the transverse wandering of a laser beam to the total noise level in a system of the cantilever bending measurement using laser beam deviation in an atomic force microscope (AFM). The efficacy of this method has been experimentally verified, and it was found that, using a diaphragmed laser beam, the noise level can be reduced to 0.01 nm.

Laser-based optical facility for determination of refractive index of liquids

Refractive index knowledge is an important parameter when physical properties of liquids are investigated. For the easy and fast determination of refractive index of liquids, we established a computer-controlled optical facility in National Metrology Institute of Turkey, based on laser beam deviation technique. Basic components of the established facility are stabilized laser sources, temperature-controlled rectangular cells, servomotor-controlled knife edges and trap detectors. The facility was used to measure the refractive index of four liquids; pure water, acetone, methanol and n-propyl alcohol. Temperature and dispersion characterizations of each liquid were also investigated and are presented in this paper.

Refractive index of supercritical CO 2 -ethanol solvents

In-line analysis of refractive index is required for efficient design and monitoring of supercritical fluid extraction and precipitation processes. In the present work, a robust method has been developed based on measurements of laser beam deviation using an interferometer and image processing system. Data on refractive index of CO 2 -ethanol mixtures were obtained at pressures between 70 and 200 bar and temperatures between 308 and 363 K, for continuous flow of premixed solvents and, in addition, for equilibrium gas phase below the mixture critical pressure. The refractive index of a mixture is a linear function of ethanol mole fraction and can adequately describe mixing and phase behavior in the vessel. For pure CO 2 , refractive index was determined as a function of pressure and density and its Lorentz-Lorenz functions determined.

Threshold of inelastic strain formation in Si and GaAs surface layers under multiple pulsed laser irradiation

For the first time, a contactless local photoacoustic technique based on the spectroscopy of laser beam deviation was used to estimate maximal values of elastic shear strains ϕ0 in micrometer-sized surface regions of Si and GaAs; the values obtained fall in the range of 10−5 ϕ0 is demonstrated. Studying the diffuse and Raman scattering of light near the thresholds ϕ0 suggests that the early stage of the inelastic cyclic strain photoinduced in the surface layers is accompanied by the generation and spatial redistribution of point rather than extended defects (e.g., dislocations). A number of threshold values, such as the photoinduced increase in the temperature and the mean shear stresses that appear in the surface layers of Si and GaAs samples exposed to local submicrosecond radiation, are estimated. The physical nature of the low-threshold effects is discussed.

Analysis of laser beam deviation fluctuations in a turbulent nonisothermal flow and relevance to εθ

The fluctuating displacements of a laser beam crossing a strongly nonisothermal channel air flow have been measured in the range [10 4-3.5 × 10 4] of the Reynolds number based on the hydraulic diameter. The measured instantaneous displacements are useful data for testing direct numerical simulations of nonisothermal turbulent flows. It is shown that the variances of these displacements are related to two-point correlation functions of temperature and temperature spatial derivatives. A first approach developed here to predict the beam displacement variances is based on a four supplementary equation, near-wall, turbulence model. These variances are shown to be related to components of the dissipation rate ε θ of temperature fluctuations. A satisfactory agreement between measured and predicted displacement variances is obtained.

Transurethral Laser Surgery with a Conventional Modified Resectoscope

A new 1,800-microns side fiber made of pure silica with laser beam deviation of 82 degrees and divergence of 18 degrees was inserted into a modified transurethral resectoscope and used to treat in vitro some prostates removed during radical prostatectomy. Sterilization of prostatic tissue with a depth of penetration of 12 mm was observed after treatment with 20 W for 2 min. Irradiation with 60 W for 1 min produced vaporization and explosion of the tissue and a depth of penetration of 15 mm. The laser resectoscope was then successfully employed for the treatment of 6 cases of benign prostatic hyperplasia (BPH) as well as in high risk patients with bulky bleeding bladder tumors (4 cases) and with transitional carcinoma of the prostate invading the parenchyma (2 cases). 3 patients with small size BPH underwent only laser irradiation. All others were submitted to traditional transurethral resection followed by laser treatment with Nd: YAG using the same instrument. Our laser resectoscope lowers costs since the side fiber is reusable for 4-5 treatments and brings laser treatment nearer to traditional endoscopic treatment, thereby making its application more comfortable for the urologist.

Laser beam deviation as a local density probe

In axisymmetric flows, we show that the laser beam deviation technique can be associated with an Abel inversion calculation in order to give the local density and its gradients. We report density profile measurements in the different regions (zone of silence and barrel shock) of nitrogen and argon supersonic free jets near the nozzle exit. Three different nozzle geometries have been tested at different stagnation pressures. Results are compared to calculations obtained by the method of characteristics.