Beam quality of a nonideal atom laser

We study the propagation of a noninteracting atom laser distorted by the strong lensing effect of the Bose-Einstein condensate (BEC) from which it is outcoupled. We observe a transverse structure containing caustics that vary with the density within the residing BEC. Using the WKB approximation, Fresnel-Kirchhoff integral formalism, and ABCD matrices, we are able to describe analytically the atom-laser propagation. This allows us to characterize the quality of the nonideal atom-laser beam by a generalized M2 factor defined in analogy to photon lasers. Finally we measure this quality factor for different lensing effects.     

Difference between a photon's momentum and an atom's recoil

When an atom absorbs a photon from a laser beam that is not an infinite plane wave, the atom's recoil is less than variant Planck's k in the propagation direction. We show that the recoils in the transverse directions produce a lensing of the atomic wave functions, which leads to a frequency shift that is not discrete but varies linearly with the field amplitude and strongly depends on the atomic state detection. The same lensing effect is also important for microwave atomic clocks. The frequency shifts are of the order of the naive recoil shift for the transverse wave vector of the photons.     

反射式热透镜时变信号测量固体热扩散率的研究

介绍了采用反射式热透镜时变信号测量固体热扩散率的一种简便方法,该方法结合表面热透镜技术原理,采用表面透镜的动态变化和反射的探测光发散的幅度来确定固体的热扩散率,相对于传统的热扩散率测量方法要快捷简单。对较大范围的一系列物质的热学特性进行了实验研究,证明了其实用性。     

About the correlation between the centre and the wings of a hard-apertured Gaussian beam

The paper considers the influence of near-field clipping upon the accuracy of angular divergence measurement of a laser beam. We have shown that power fraction that contain the centre and the wings of a diffracted beam are not necessarily correlated. This effect can lead to systematic important errors on laser beam divergence measurement even when the beam under consideration is weakly truncated. In particular, the monitoring of any variable lensing effect can be greatly affected by such effects since one could be mistaken about its sign (converging or diverging).     

A Novel Technique to Measure the Spatial–Temporal Intensity of an Ultrashort Pulse

Based on a modified cross-correlation technique, we experimentally demonstrate a technique for measuring the spatial–temporal intensity of laser pulses. Pulse widths at different spatial positions of the ultrashort pulse are measured by changing the scan position of the probe beam. Due to the existence of residual chirp in the transverse position, pulse widths at the center of the beam are less than that at the edge. By measuring the temporal evolution in the fastest growth area of spatial intensity during small-scale self-focusing, we find that its pulse width decreases as power increases because of the spatial–temporal coupling effect. The results show that this method not only can be used to accurately measure the pulse width at any one spatial position of the beam, but can also be useful for real-time monitoring of the spatial–temporal evolution.     

Experimental Research on the Influence of Laser-induced Plasma on the Beam Focusing During High-power CO_2 Laser Materials Processing

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Problems with small area surveys: lensing covariance of supernova distance measurements

While luminosity distances from type Ia supernovae (SNe) are a powerful probe of cosmology, the accuracy with which these distances can be measured is limited by cosmic magnification due to gravitational lensing by the intervening large-scale structure. Spatial clustering of foreground mass leads to correlated errors in SNe distances. By including the full covariance matrix of SNe, we show that future wide-field surveys will remain largely unaffected by lensing correlations. However, "pencil beam" surveys, and those with narrow (but possibly long) fields of view, can be strongly affected. For a survey with 30 arcmin mean separation between SNe, lensing covariance leads to a approximately 45% increase in the expected errors in dark energy parameters.     

Influences of pump beam distribution on thermal lensing spherical aberration in an LD end-pumped Nd:YAG laser

A numerical investigation is made on the thermal lensing and spherical aberration effect in an LD end-pumped Nd:YAG laser. Based on the finite element method (FEM), the laser rod temperature distribution is calculated and the focal length of the thermal lens is deduced, the influences of pump beam on the thermal lensing spherical aberration are mainly studied. The results show the thermal lens which focal length varied with the radial coordinate r is not an ideal lens. Given the heat dissipation boundary conditions, the radial dependent focal length will be shortened when the pump power raised or the waist radius of the pump beam reduced, meanwhile the radial differences of the focal length will decrease when the pump power increased. For a Super-Gaussian profile pump beam, the higher the exponent number, the more similar to flat top the pump beam is, and the less the radial differences of the focal length are.     

Optimizing Weak Measurements to Detect Angular Deviations

We analyze and compare the angular deviations for an optical beam reflected by and transmitted through a dielectric triangular prism. The analytic expressions derived for the angular deviations hold for arbitrary incidence angles. For incidence approaching the internal and external Brewster angles, the angular deviations transverse magnetic waves present the same behavior leading to the well‐known giant Goos‐Hänchen angular shift. For incidence near the critical angle a new region of large shift is seen both for transverse magnetic and transverse electric waves. While a direct measuring procedure is better in the vicinity of the Brewster region, a weak measurement breaks off the giant Goos‐Hänchen effect, preserving the amplification in the critical region. We discuss under which conditions it is possible to optimize the amplification and we also determine when a weak measurement is preferred to a direct measuring procedure.     

Detection of a semirough target in turbulent atmosphere by a partially coherent beam

The inverse problem of the interaction of an isotropic Gaussian Schell-model beam with a semirough target in turbulent atmosphere is investigated. It is found that we can determine the target size and the transverse correlation width of the target by measuring the transverse beam widths and the transverse coherence widths of the beams at the source plane and the receiver plane. Our results are useful for remote sensing and bistatic LIDAR system.     

Pump power dependence of the spatial gating properties of femtosecond optical Kerr effect measurements

The pump power dependence of the spatial gating properties of femtosecond optical Kerr gate (OKG) was investigated using coaxial two-color optical Kerr measurements in CS₂. As the pump power increased, the spatial pattern of the optical Kerr signals changed from a Gaussian spot to a ring form, and then a spot surrounded by a concentric ring, successively. By comparing the experimental data with the calculation results and measuring the pump power dependence of the OKG signal intensity, we demonstrated that the spatial variation of OKG transmittance could be attributed to the non-uniform spatially distributed phase change of the probe beam, due to the transient birefringence effect induced by pump beam with transverse mode of a Gaussian distribution.     

Nanoparticle sizing by a laser-induced breakdown detection using an optical probe beam deflection

A method for determining the size of a colloidal nanoparticle by measuring the magnitude of the probe beam signal which can be remotely measured in a non-contact manner has been developed. The method using a probe beam signal is intended to employ a principle by which the path of a probe beam is changed by a laser-induced shock wave, accompanying the occurrence of a laser-induced breakdown of colloidal nanoparticles. It was observed that the peak and full width at half-maximum of a frequency distribution curve of the measured magnitude of a probe beam signal appear in direct correlation with the size of a nanoparticle at the fixed pulse energy of a laser beam for the breakdown. A calibration curve for a particle sizing is presented for particle diameters ranging from 20 to 60 nm, with reference polystyrene particles. An application is demonstrated for measuring hexavalent uranium colloidal particles generated by the hydrolysis of free uranyl ions.     

热透镜控制激光光束质量技术研究

在端面抽运固体激光器中,就如何改善在高抽运功率时输出激光的光束质量,提出一种激光器的设计方法。在激光器谐振腔中放入多根掺杂浓度不同的激光介质,利用激光介质内部产生的热透镜控制抽运光和基模振荡光的空间分布,并且最大限度地使抽运光的分布区和基模振荡光分布区重叠,实现抽运光与基模振荡光在空间上高度匹配,进而提高抽运光能量的利用效率和振荡光的光束质量。实验表明,在不同抽运功率下,抽运光和基模振荡光在晶体内部的光斑的空间分布可通过热透镜加以控制。在端面抽运功率200 W附近时,实现了抽运光与基模振荡光较高程度匹配,光束质量因子M2由14.7改善为4.1。     

Dispersive solid-state dye laser oscillators

Narrow-linewidth multiple-prism grating solid-state dye laser oscillators are analysed using ray transfer matrices. Attention is given to beam divergence measurements and the effect on them of thermal lensing, at the solid-state gain medium. It is found that although thermal lensing leads to an increment of beam divergence, it induces oscillation in the unstable resonator regime, thus contributing to the attainment of TEM00 beam profiles. In addition, narrow-linewidth emission in dispersive oscillators, incorporating rhodamine 6 G doped HEMA:MMA gain media, is reported for the first time. Laser efficiency is 4–5% at a laser linewidth ≤1.8 GHz for double-longitudinal-mode emission. Beam divergence is measured in the 2–2.3 mrad range, in good agreement with theory. This revised version was published online in November 2006 with corrections to the Cover Date.     

A novel beam–spin asymmetry in double-hadron inclusive lepto-production

We show that a new beam–spin asymmetry appears in deep inelastic inclusive lepto-production at low transverse momenta when a hadron in the target fragmentation region is observed in association with another hadron in the current fragmentation region. The beam leptons are longitudinally polarized while the target nucleons are unpolarized. This asymmetry is a leading-twist effect generated by the correlation between the transverse momentum of quarks and the transverse momentum of the hadron emitted by the target. Experimental signatures of this effect are discussed.     

Steady-state vortex-screening solitons formed in biased photorefractive media

We report the observation of steady-state photorefractive vortex-screening solitons. As a singly charged circular vortex nested on a broad beam propagates through a biased strontium barium niobate crystal, it self-traps in both transverse dimensions despite the inherent anisotropy of the photorefractive nonlinearity. When the vortex beam is a doughnut-shaped narrow beam, it breaks up into two elongated slices (with a self-defocusing nonlinearity) or into two focused filaments (with a self-focusing nonlinearity). We demonstrate the optical guidance of a probe beam in a circular waveguide induced by the self-trapped vortex.     

Theory of thermally-induced interference and lensing in transparent materials

The vector Kirchhoff approximation is employed to calculate thermally-induced interference and lensing effects on the transmitted intensity, when a laser beam is incident on a thin solid sample with parallel faces.     

Optical bistability in gold nano-colloid due to thermal lensing effect

In this study, experimental results are concerning “the optical bistability due to thermal lensing effect”, in colloidal solution of gold nanoparticles (Au Nps) which has been studied under CW laser illuminations. Two different CW laser beams (He–Ne and Nd-Yag) were used to interact with a vertical Fabry–Perot interferometer, containing colloid of the Au Nps. Hysteretic loops have been observed with regard to optical bistability and the results were fitted with theoretical curves. The radiated power to the resonator was elevated up to 50 mW for both the laser beams with the same spot size. Also, new method was defined to precise evaluation of the theoretical hysteretic loop which is describing the relation between nonlinear refractive index and the beam intensity inside the resonator. This dependency is due to thermal lensing effect and numerical simulations of this effect were also performed, in order to better understanding of it.     

Pseudo-nonlinear and athermal lensing effects on transverse properties of Cr based solid-state lasers

Several experiments (time-resolved Z-scan experiments based on pulsed and CW pump lasers, time-resolved divergence diagnostics) have been performed to examine and clarify the question of the converging or diverging population lensing effect occurring in a Cr^3 +:Al₂O₃ ruby laser. The dynamics of the laser far-field divergence of such a laser indeed indicated initially a diverging effect while Z-scan measurements conclude to a converging one. The origin of this discrepancy is thus analysed and elucidated here by introducing the general concept of correlation collapse between the centre and the wings of a laser beam having some clipping.     

Estimation of thermal lensing effect in the high-power end-pumped direct-cut crystal lasers

At this working, analytical (with Gaussian pump beam profile) and numerical FEM (with super-Gaussian pump beam profile) methods were compared for estimation of thermal lensing effect in the diode-end-pumped solid-state laser rods. These results were applied to the design and realization of a direct-cut (i.e., cut in right-angle ) crystal Nd:YVO₄ laser. The results showed that for such crystal (direct-cut), two methods obtained almost the same results, but slightly different from experimental results. However, estimation of thermal lensing effect led to realization of a diode-end-pumped Nd:YVO₄ cw high-power laser with good optical quality.