Toward Defeating Diffraction and Randomness for Laser Beam Propagation in Turbulent Atmosphere

A large distance propagation in turbulent atmosphere results in disintegration of laser beam into speckles. We find that the most intense speckle approximately preserves both the Gaussian shape and the diameter of the initial collimated beam while loosing energy during propagation. One per 1000 of atmospheric realizations produces at 7 km distance an intense speckle above 28% of the initial power. Such optimal realizations create effective extended lenses focusing the intense speckle beyond the diffraction limit of vacuum propagation. Atmospheric realizations change every several milliseconds. We propose to use intense speckles to greatly increase the time-averaged power delivery to the target plane by triggering the pulsed laser operations only at times of optimal realizations. Resulting power delivery and laser irradiance at the intense speckles well exceeds both intensity of diffraction-limited beam and intensity averaged over typical realizations.     

Spatial fluctuations in intensity of dispersed laser beams with broad spectra

The statistical characteristics of spatial fluctuations of the intensity of dispersed laser beams are studied theoretically. The density of the probability distribution for the intensity is found and its transformation with a change in the longitudinal coordinate is studied. It is shown that a decrease in the intensity fluctuations and a drop in the contrast of the beam speckle structure occur with increasing distance from the initial plane in which the spatially inhomogeneous broadband field is assumed to be spatially coherent. A decrease in fluctuations is accompanied by an increase in the size of speckles in the direction of dispersion, as well as along the beam axis. An interpretation of the found regularities is given.     

A laser speckle method for measuring ultra-low thermal expansion

A method based on objective laser speckles for the measurement of ultra-low thermal expansion is described. The sensitivity of α = 10^-8 C^-1 can easily be achieved with a 25 C temperature change of a 2°5 cm long sample. The sensitivity is obtained by the rotation of a probe due to differential thermal expansion of the sample and reference rods. The method is discussed in detail along with the effects of some user-controlled parameters like speckle size, laser beam width, unwanted thermal expansions and the spectral width of the laser beam.     

Spatial structure of the degree of coherence for broadband dispersed laser beams

1D and 2D spatial distributions of a complex degree of coherence are obtained using numerical simulation for the broadband dispersed laser beams. The configuration of the spatial inhomogeneities (speck-les) of the degree of coherence is studied at various spectral widths and dispersions of the beam. It is shown that the speckle size increases with increasing distance from the initial plane where the light field is assumed to be spatially coherent. The small- and large-scale structures of the degree of coherence are shown for the transverse cross section of the beam. The results are compared with the analytically calculated correlation functions of the spatial coherence. The features of the speckle structure are interpreted using the concept of partial speckles.     

用零相关相位板匀滑散斑的理论研究

对零相关相位板匀滑散斑的特性进行了理论分析和散值计算,结果表明尽管零相关相位板是针对入射场为平面波面而设计的,但在入射场的振幅和相位有一定起伏的情况下,其远场光斑的散斑对比度明显低于采用随机相位板的情形.因而用零盯关相位板匀滑散斑是一种更为有效的方法.     

基于光克尔效应的径向光束匀滑新方案

针对惯性约束聚变装置中提高靶面辐照均匀性的要求, 提出了一种基于光克尔效应的径向光束匀滑方案, 其基本原理是利用光克尔介质和周期性高斯脉冲光束相互作用实现对激光束透射波前附加周期性的球面位相调制, 以周期性地改变激光束远场焦斑尺寸, 进而引起远场焦斑内部散斑的快速径向扫动, 从而在积分时间内抹平靶面焦斑的强度调制, 实现径向方向的光束匀滑. 通过建立基于光克尔效应的径向光束匀滑的理论模型, 分析了焦斑形态及其径向匀滑特性, 并讨论了光克尔介质的选取和径向扫动特性. 结果表明, 基于光克尔效应的径向光束匀滑方案可以有效地实现远场焦斑内部散斑的周期性径向扫动, 从而在积分时间内快速改善靶面辐照均匀性.     

Speckle pattern interferometer

We describe in this paper a speckle interferometer constituted by two partially diffusing plates. They are illuminated by a laser and the resulting intensity distribution is observed in the image plane of the source. Each speckle of this plane (Fourier plane) is modulated by a ring system whose interference order depends on its phase. Since the observation plane is a Fourier plane, a longitudinal or lateral shift of one of the diffusers results only into a change of the modulating fringes, the speckles themselves remaining unchanged. A photographic plate which records successively the intensity distribution lying in the Fourier plane before and after the considered shift, exhibits after processing a system of Moiré fringes, the shape of which depends on the nature of the translation given to the diffuser between the exposures.     

Numerical Simulations on the Formation of Speckles in Nanofluids Illuminated by a TEM00 Laser Beam

On the basis of a Rayleigh scattering model for a single nanoparticle illuminated by a TEMoo laser beam, we theoretically and numerically study the speckle formation when nanofluids are illuminated by a TEMoo laser beam. The results show that the laser speckles possess a Gaussian distribution, which are in agreement with the experimental results. The results may be useful for using a laser speckle velocimetry to determine the velocitiies of nanoparticles in nanofluids.     

Speckle size and decorrelation time; space-time correlation analysis of coherent light dynamically scattered from turbid media

We report on the dependence of the decorrelation time on the spatial intensity correlation of speckles generated in the far-field by back-scattered photons from turbid media. The effects contribute to an explanation of an earlier observation that the average Doppler width of the power spectrum of detector current fluctuations depends on the size of the illuminating laser beam. The space-time correlation of the speckles generated by a particle suspension illuminated by a collimated laser beam is analyzed from serial images taken by a high speed camera. It was found that larger spatial correlation distances, associated with large speckles, exhibit a slower temporal decorrelation.     

Sampled-speckle photography for measurement of deformation

Speckles usually are of two kinds: laser speckle and white-light speckle. An additional kind, termed a sampled speckle, is proposed. Whereas laser speckles arise from multiple interference of light scattered from an object illuminated by a coherent laser beam and white-light speckles are a physically generated speckle pattern on the surface of the object, sampled speckles are generated as a result of the sampling of a digital image. The generation of these speckles and their application to displacement measurement are demonstrated.     

Dynamic laser speckles and their application to velocity measurements of the diffuse object

In connection with the parameters of the optical configuration used to produce speckles and under illumination of a Gaussian beam over a moving diffuse object, this paper discusses the statistical properties of dynamic speckles produced in the diffraction and image fields by the diffuse object moving in a plane with constant velocity. Especially, the space-time correlation function of the dynamic speckle intensity is examined in detail. Two typical speckle motions of translation and boiling are interpreted for various optical configurations in the diffraction and image fields. As one of the interesting applications of dynamic speckles to metrology, various methods for velocity measurements of the diffuse object are introduced.     

Theorie der nichtstationären Anregung von Exziton-Polaritonen durch Zweiphotonenabsorption

Theory of nonstationary Exciton-Polariton Generation by Two-photon Absorption A theory of nonstationary two-photon absorption for nonlinear non-centrosymmetric crystals is presented. The time-dependent radiation flux density of the generated polariton beam is calculated under the condition of an ultrashort laser pulse duration T_L ? ΓT^?1 (ΓT is the exciton line width) and a stationary incoherent test field. The “mechanical” response of the excitons and different group velocities of the polariton and the laser and test fields are taken into account. the spatial dispersion of the exciton motion being neglected. With a rectangular laser pulse shape the analytically nonstationary solutions are discussed for identical and for different group velocities of laser and test fields. Using the conservation laws for energy and quasimomentum we illustrate, by considering the polariton dispersion branches, the various possible cases of two-photon absorption in uniaxial piezocrystals.     

Chaotization and decay instability of a diffraction-catastrophe field in the presence of focusing of high-power laser radiation in a nonlinear medium

Focusing of a high-power laser beam, whose initial wave front is deformed by spherical aberration and astigmatism, in a medium with refractive-index nonlinearity is investigated by the computer experiment method. It was found that the extended formations which arise near the focus are no longer diffraction catastrophes, since they are structurally unstable. It is shown that weak disturbances of the shape of the initial wave front produce extremely strong distortions of the optical field in the focusing region. As the wave propagates in the z direction, spatial chaos in the distribution of the optical field develops in the region of focusing (from 137 to 142 mm). Optical turbulence arises in the focusing region as a result of self-diffraction of light by self-induced nonuniformities of the refractive index of the medium. After the region with the smallest cross section of the formation near the focus, the three-dimensional optical field has the form of chaotically dispersing “splashes” and extended “filamentary ejecta.” Zh. Tekh. Fiz. 68, 74–83 (September 1998)     

Measurements of axial displacements undergone by a diffusing object in speckle photography

The speckles of the image plane of an object are both radially shifted and decorrelated when the object is axially translated through an amount ?. We demonstrate that this radial shift, which is related to the position of the pupil of the optical system, disappears when the pupil lies in the back focal plane of the imaging lens. However, if the irradiance of the image plane is twice recorded on a photographic plate which is laterally shifted through ζ_H between the exposures, the minimum value of the contrast of the Fourier fringes exhibited by the plate after processing, gives the value of ? if ? is less than a particular value, ?_M which will be defined. Also we propose a new speckle measurement method in which the fringes are automatically removed when ? is greater than ?_M. We record the image of the object illuminated in convergent light through an amplitude diffuser placed in the Fourier plane of the object. The mean speckle speckle size of the diffuser is equal to the mean size of the speckles generated by the object in its Fourier plane.     

Velocity sensitivity of laser-speckle velocimeters

The method of increasing sensitivity in laser speckle velocimetry is studied theoretically and experimentally. The sensitivity of the velocimeters is shown to have a fairly strong dependence on the optical configurations, such as the diameter of the incident laser beam-waist, or its position relative to the moving diffuse object to be measured. It is pointed out that the minimal sensitivity appears in the configuration where the converging incident light beam produces its beam waist at the position of a detecting pinhole. In some cases when using image speckles to obtain finite spatial resolution in the object plane, an optimum condition exists which gives the maximal sensitivity. The sensitivity of a laser speckle velocimeter using a spatial filter and two point cross-correlation technique is also discussed.     

Improvement of ultrasound speckle image velocimetry using image enhancement techniques

Ultrasound-based techniques have been developed and widely used in noninvasive measurement of blood velocity. Speckle image velocimetry (SIV), which applies a cross-correlation algorithm to consecutive B-mode images of blood flow has often been employed owing to its better spatial resolution compared with conventional Doppler-based measurement techniques. The SIV technique utilizes speckles backscattered from red blood cell (RBC) aggregates as flow tracers. Hence, the intensity and size of such speckles are highly dependent on hemodynamic conditions. The grayscale intensity of speckle images varies along the radial direction of blood vessels because of the shear rate dependence of RBC aggregation. This inhomogeneous distribution of echo speckles decreases the signal-to-noise ratio (SNR) of a cross-correlation analysis and produces spurious results. In the present study, image-enhancement techniques such as contrast-limited adaptive histogram equalization (CLAHE), min/max technique, and subtraction of background image (SB) method were applied to speckle images to achieve a more accurate SIV measurement. A mechanical sector ultrasound scanner was used to obtain ultrasound speckle images from rat blood under steady and pulsatile flows. The effects of the image-enhancement techniques on SIV analysis were evaluated by comparing image intensities, velocities, and cross-correlation maps. The velocity profiles and wall shear rate (WSR) obtained from RBC suspension images were compared with the analytical solution for validation. In addition, the image-enhancement techniques were applied to in vivo measurement of blood flow in human vein. The experimental results of both in vitro and in vivo SIV measurements show that the intensity gradient in heterogeneous speckles has substantial influence on the cross-correlation analysis. The image-enhancement techniques used in this study can minimize errors encountered in ultrasound SIV measurement in which RBCs are used as flow tracers instead of exogenous contrast agents.     

Cylindrical vector axisymmetric Bessel-modulated Gaussian beam

Focusing properties of the cylindrical vector axisymmetric Bessel-modulated Gaussian beam with quadratic radial dependence (QBG beam) in high numerical aperture system is investigated theoretically by vector diffraction theory. Results show that intensity distribution in focal region can be altered considerably by beam parameter μ and polarization angle. Polarization angle may adjust transverse intensity distribution, for instance from one focal spot to one ring shape. While μ alters axial intensity distribution remarkably, focal splitting may occur with tunable focal shift, and real value μ also may induce local intensity minimum. For certain case, with increasing imaginary value μ, transverse focal spot shrinks accompanied with higher full width half maximum of axial intensity distribution.     

粗糙表面对散斑统计特性的影响

为研究高速摄影中激光相干噪声的影响因素,降低激光照明中散斑噪声对高速摄影成像质量的影响,对激光照射表面的粗糙程度与形成的散斑场统计特性之间的关系进行了研究。基于菲涅尔-基尔霍夫衍射原理,分析了高斯光束入射在随机粗糙表面上经反射形成的散斑图样,通过对大量散斑图样的数据统计,得出了激光散斑的信噪比、自相关函数、概率密度函数与入射表面粗糙度之间的变化关系。计算结果表明：粗糙度的增大使散斑场信噪比降低、相干程度减小、概率密度降低,即表面越粗糙散斑噪声越严重。     

Peculiarities of filamentation of sharply focused ultrashort laser pulses in air

Peculiarities of the self-focusing and filamentation of high-power femtosecond laser pulses in air have been experimentally and theoretically studied under conditions of broad variation of the beam focusing parameter. The influence of the numerical aperture (NA) of the initial radiation focusing on the main characteristics of laser-induced plasma columns (characteristic transverse size, length, and concentration of free electrons) is considered. It is established that, for a rigid (NA > 0.05) initial laser beam focusing, the transverse size of the plasma channel ceases to decrease at a level of R _pl ≈ 2–4 μm as a result of strong refraction of radiation on the plasma formed at the focal waist, which prevents further contraction of the laser beam due to its focusing and self-focusing.     

Single-frequency operation of a laser diode side-pumped Q-switched Tm:YAG laser using seeding injection technique

A laser diode (LD) side-pumped 2 μm single-frequency Q-switched Tm:YAG laser was demonstrated. The laser was injection seeded by a CW single frequency Tm:YAG laser with a twisted-mode cavity. The maximum single-frequency pulse energy was 16.3 mJ, with a pulse width of 570 ns and a pulse repetition rate of 10 Hz. The linewidth of the 2 μm single-frequency Q-switched laser was 0.68 MHz, measured by using the optical heterodyne technique. The M ² of the laser beam was measured to be 1.09 and 1.03 for x direction and y direction, respectively.