The paper describes an experimental method of determining probability density distribution for random threshold values
of laser fluence, damaging surface of optical components. The method is as follows. Numerous surface sites of an optical
component have been irradiated by a laser beam with a known non-uniform fluence distribution and magnitude
sufficient for surface damage. Subsequently, the minimal fluence value ε has beens determined at the boundary of the
damaged zone for each irradiation spot. It is shown that the sampling of minimal damage threshold values obtained from
the above-described data assembly makes it possible to construct the probability density distribution f(ε). Knowledge of
this distribution provides for the use of order statistics in damage threshold fluence data analysis and, thus, enhances
accuracy and reliability of determination of surface strength for sizable optic components tested by small-diameter
beams.
Results of statistical analysis for beam resistance tests obtained for a new KGSS-0180 neodymium glass are
presented. Methods of determining limitations for admissible levels of radiation energy density in the active elements of
laser systems based on calculation of statistics for the first term of an ordered data series on glass damage thresholds are
proposed. Use of this approach to minimize the impact of "size dependence" on accuracy of determination of a minimal
threshold value is considered.
Dependences of energy damage thresholds on radiation parameters and geometry for KGSS 0180 phosphate
neodymium glass have been studied. Laser-induced damage threshold dependences on the pulse duration for the range
of 1.5-20 ns for KGCC 0180 and K8 glasses are: εkgss = 16 τ1/2 εK8 = 18 τ1/2(J/cm2, when τ is in nanoseconds).
Damage thresholds for the entrance and exit surfaces of KGSS 0180 glass samples have been measured when the
samples were irradiated by a P-polarized beam at a Brewster's angle: εent ≈ 48 J/cm2; εex J/cm2. As has been
found out, the threshold energy density of the beam in the case of Brewster's incidence to the exit surface of the sample
is almost 1.5 times lower than in the case of a normal incidence.
Testing techniques for detection of platinum micro-inclusions in the laser glass have been developed. The energy
threshold for Pt-inclusions explosion has been determined at εtr ≈ 2.5 J/cm2 under laser pulse duration of 7 ns. Over 70
KGSS 0180 glass samples have been tested. It has been shown that platinum-induced damage was absent only in 25%
of glass melts out of the entire tested amount. An averaged concentration of inclusions was Npt ~ 5 liter-1.
Concentration of micro-inclusions in the last KGSS 0180 batch (2006 year) is ~ 1 liter-1.
In this paper we analyze some results of natural and numerical experiments on probability density of intensity
fluctuations on an axis for 1,06 microns and 0,53 microns laser beams in comparison with theoretical dependences
(lognormal, exponential and K-distribution). Beams were propagated in aviation engine exhaust at various angles
between the jet and beam axes. It has been shown that for a beam with a wavelength of 0,53 microns experimental data
can be approximated as exponential and K-distribution, while for radiation with a wavelength of 1,06 microns good
conformity to K-distribution has been observed.
Optimum conditions for image registration with CCD-cameras of laser beams distorted by turbulence have been
chosen. For this purpose transfer characteristics of several same type samples of CCD-cameras have been studied at
various irradiation modes and registration tunings. It has been shown that the dynamic range of the cameras is used to
maximum capacity for image recording when gamma-correction is applied.
Dependences of energy damage thresholds on radiation parameters and geometry for KGSS 0180 phosphate
neodymium glass have been studied. Laser induced damage threshold dependences on the pulse duration for the range
of 1.5-20 ns for KGCC 0180 and K8 glasses are: εkgss = 16 x τ1/2 И εK8 = 18 x τ1/2(J/cm2, when τ in nanoseconds). Damage
thresholds for the entrance and exit surfaces of KGSS 0180 glass samples have been measured when the samples were
irradiated by a P-polarized beam at Brewster's angle: εent ≈ 48J/cm2; εex ≈ 31 J/cm2. It has also been found out that
the threshold energy density of the beam in case of Brewster's incidence to the exit surface of the sample is almost 1.5
times lower than in the case of a normal incidence. At the same time the relation is reverse for the entrance surface of
the glass KGSS 0180 sample with the same radiation polarization.
Testing techniques for detection of platinum micro-inclusions in the laser glass have been developed. Over 70
KGSS 0180 glass samples totaling ~ 400 cm3 have been tested. It has been shown that platinum-induced damage was
absent only in 25% of glass melts out of the entire tested amount. An averaged concentration of inclusions was Npt ~ 5
liter-1. Concentration of micro-inclusions in the last KGSS 0180 batch is ~ 1÷2 liter-1. To automate and reduce testing
time for the entire blank volume of the active disc element (4-6 liters) an automated system for mapping Pt-inclusions
during irradiation has been developed. The efficiency of the proposed QC methods has been demonstrated
experimentally.
The subject of this study is numeric simulation of laser beam propagation by methods of statistical testing, with calculation of the field in the space behind the randomly inhomogeneous phase screen based on Fresnel transformation and not on traditional spectral decomposition. This modification makes it possible to sufficiently increase efficiency of calculations for fields of disturbed beams in studies of radiation propagation through local zones of strong turbulence. Limitations of the method determined by conditions of the “sampling theorem”, by phase screen characteristics and edge effects have been considered. The possibility to simulate optical transformations of disturbed beams with increased accuracy of simulated natural experimental conditions has been noted. The results of numeric simulation have been analyzed and compared to the data obtained in natural experiments for radiation propagation through the jet of an aircraft engine. The model describes quite well experimentally identified peculiarities of spatial radiation characteristics going beyond the standard model of a turbulent layer, such as strong dependence of the angular divergence of distorted beam on the radiation wavelength and spatial anisotropy of its statistical characteristics. Finally, possible use of Fresnel transformation in modelling beam propagation in continuous randomly inhomogeneous medium represented as a sequence of phase screens has been examined.
This work presents a method for determination of specific amplification coefficient for stimulated Brilloin scattering (SBS) of radiation based on measuring amplification of the Stokes signal in a medium sample used as the SBS-amplifier. Registration of spatial distributions of amplified beam intensity allows minimization of a measurements error caused by saturation of Stokes radiation amplification in the sample under study. Values of specific amplification coefficient for SBS of radiation are obtained under identical experimental conditions for new phosphate glass KGSS 0180 (g = 1.9 cm/GW) and for glasses widely used in laser systems: GLS 22 (g = 1.4 cm/GW) and K 8 (g = 1.3 cm/GW). By means of optical heterodyning of radiation a SBS-frequency shift of laser radiation at a wavelength of 1.054 microns is measured for new glass KGSS 0180 (▵v = 14.57± 0.02 GHz). It makes it possible to estimate hypersound propagation velocity in the tested glass (V ≈ 5030 m/c) at this frequency.
Method for measuring nonlinear refractive index (n2) of optical materials is presented. It is based on the analysis of large scale beam distortions caused by the nonlinear aberrational lens induced by power radiation in the medium under study. Absolute n2-values for new Russian laser phosphate glass KGSS 0180/35 (n2 ≈ 1.2•10-13 CGSE units) and widely used glasses GLS 22 (n2 ≈ 1.9•10-13 CGSE units) and K8 (n2 ≈ 1.4•10-13 CGSE units) were obtained with this method for laser pulse duration of 4 ns. Also, evaluation of nonlinear phase delay induced in the laser beam that has passed through the glass samples under study for a longer pulse duration of -up to 25 ns has been performed.
A laser system designed for complex testing of optical elements, such a determination of a nonlinear refraction index, a specific coefficient of radiation amplification under stimulated Brillouin scattering (SBS), optical elements surface damage threshold and establishment of platinum inclusions in active elements is described. Descriptions of measurement techniques and results obtained for widely used in multi-stage Nd-glass laser systems glasses, including new phosphate glass KGSS-0180, are presented.
The experimental studies of spatial characteristics for laser beams disturbed by a turbulent aero-engine jet were carried out. The angular divergence, random wanderings of the beam's centroid and statistical characteristics of stochaistic intensity distribution in cross-section of laser beams with radiation wavelengths of 1.06 and 0.53 microns were determined for different angles of the beam-jet intersection.
On the basis of Monte-Carlo method a numerical mathematical model for lasre beam propagation through a turbulent jet of aero-engine has been designed. The model describes experimentally detected features beyond the framework of the standard model of a turbulent layer: strong dependence from radiation wabelength and spatial anisotropy of the distorted beam's statistical characteristics.
A numerical simulation experiment, verification and adjustment of the designed beam propagation model for a following comparison of results obtained from full-scale and simulation experiments were carried out.
With allowance for inevitable errors of measurements and calculations, there is a good agreement between almost all statistical characteristics of disturbed laser beams obtained in different configurations during full-scale and imitative experiments.
The results of studying laser beam propagation through a turbulent jet close to the jet lateral boundary, i.e. the zone, where intermittence of turbulent stae of air medium can be expcetd, are presented. In the experiments laser beams with diameters of 10 and 30 mm and wavelengths of λ = 1.06 and 0.53 microns crossed the jet in transverse direction not far from the engine nozzle. For the purpose of comparison, similar experiments for laser beams propagation through the central area of the jet were carried out. As a result of processing a large array of 'instantaneous' far-field images of the beams, the probability of light pulses with only weakly distorted spatial structure during propagation through the jet was determined. It has been shown that for a λ=1.06 μm and 30 mm diameter beams. For propagation of the ten-millimeter beam this probability is found to be ~0.02. In the case of the 'half-micron' beam propagation through the jet under similar conditions the probability for the occurrence of 'quasi-regular' pulses is very small in all the experimental situations. The analysis of the experimental data provides evidence for conclusion that successions of such 'quasi-regular' pulses obey Poisson's or binomial statistics.
The results of experimental studies of angular divergence for laser beams intersecting an aero-engine jet at different angles to its axis - 90 degree(s), 45 degree(s) and 10 degree(s), are presented. The experiments were carried out on the ground with radiation wavelengths of 1.06 and 0.53 microns. Depending on conditions of the experiment, the angular divergence of radiation impacted by the jet increased 6-35 times as compared to the initial values for undisturbed beams. It has been found that the angular width of the half- micron beam is significantly (two or three times) higher than that of the one-micron beam, which does not fit in with the model of radiation scattered by typical a turbulent medium. Besides, radiation intensity angular distribution demonstrated azimuthal asymmetry correlating with physically selected spatial directions - along and across the jet stream. Based on relevant selection of spectral density for refractive index fluctuations (composition of a turbulent spectrum and additional high frequency spectral components, anisotropy in the outer scales of turbulence) analytical relationships for evaluation of the angular divergence of disturbed beams matching experimental data have been obtained.
The results of experimental measurements of dispersion of a laser beam centroid's random wanderings for beams disturbed by a high-speed turbulent aero-engine jet stream are presented. The duration of light pulses radiated synchronously at wavelengths of (lambda) equals 1.06 and 0.53 microns did not exceed 50 ns, which made it possible to record instantaneous far-field distributions of radiation intensity. In each experimental cycle that differed from one another by geometry of experiment (beam diameter of 10 or 30 mm, different angles of beam-jet intersection) not less than 1500 frames were recorded in the computer. The range of measured values for dispersion of the laser beam wanderings depends on conditions of experiment and varies within 60 divided by 180 (mu) rad for (lambda) equals1.06 microns and within 110 divided by 300 (mu) rad for (lambda) equals0.53 microns. A certain difference in dispersion of beam wanderings in horizontal and in vertical (orthogonally to jet's axis) directions has been detected. The analytical model to calculate dispersion of laser beam's wanderings agreeable with experimental data is presented. The results of the experiment have allowed it to estimate the type of a high- frequency component of refraction index fluctuations spectrum in the jet, which supplements von Karman's spectrum for a turbulent medium.
The study describes a method that, with a series of experimental data obtained from a single sample, allows not only to estimate the threshold fluences but to take into account statistical nature of surface-damage. In a number of experimental situations this method makes it possible to estimate damaging fluences even from the result of a singular exposure of the studied surface. Estimated threshold. Fluences for various optical elements are presented: K8-glass, experimental phosphate laser glass KGSS-0180, high-reflecting and anti-reflecting thin-film coating of elements. Half-height duration of 1.06 micrometers - radiation pulse of 4 ns and irradiation spot of approximately 4 mm in diameter were used in the experiment.
One-, half- and ten-micron wavelength radiation was used to study laser beam propagation through turbojet aircraft engine exhaust. A feature of the methods was that instantaneous distributions of the beam intensity were recorded during the experiment. Analysis of experimental data has shown that turbulent stream has a dramatic impact on spatial characteristics of a laser beam. For example, the averaged angle divergence for 30-mm one-micron beam becomes about ten times higher than its diffraction divergence. Results of different experiments showed that the average angle divergence of the narrow one-micron beam disturbed by the jet plume is several times less than that of the ten-micron beam which is characterized by a large diffraction divergence, and that of the half-micron beam stronger subjected to disturbances. Experiments in which the beam crossed the plume close to a nozzle at (phi) = 90 degree(s), 45 degree(s) and 10 degree(s) have shown that angular divergence increases with decreasing cross-angle, practically doubling the value when coming from the maximal angle of (phi) = 90 degree(s) to the minimal (phi) = 10 degree(s). Mathematical models have been derived, based on the experimental studies. The value of the structural characteristic in a turbulent stream is in the range of Cn2~10-9m-2/3.
One and ten `micron' wavelength radiation was used to study laser beam propagation through a turbojet aircraft engine's exhaust. Pulse lengths were 0.1 and 2 microsecond(s) respectively, i.e. instantaneous realizations of intensity distribution (up to several thousand frames) were registered.
An optical scheme for a kJ channel of a neodymium glass laser system with SBS-compressor for a 70 ns pulse has been described. The use of a pump pulse of long duration - 70 ns - makes it possible to operate a near-steady SBS mode in compressed gasses. An experimental study of the SBS oscillator-compressor stage with pump focusing and a two- pass pattern has been carried out and a 70 ns, 8 J pump pulse has been compressed down to 1 ns.
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