Numerical approach to laser beam propagation through turbulent atmosphere and evaluation of beam quality factor

In this paper, propagation of a Gaussian laser beam through turbulent atmosphere is evaluated numerically. The beam quality factor for the propagated beam has been estimated for different turbulent conditions that are characterized by parameter C_n. The calculations show that the beam quality can be affected dramatically by atmospheric turbulence and the laser beam size and wavelength have major role in the results. Furthermore, the propagation of laser beam in longer distance results in more spatial perturbation of the beam. The results of these calculations and evaluation of beam quality factor M² can be used for estimating the refractive index structure parameter (or atmosphere turbulent parameter) C_n.     

Steady state and transient self-interaction of a laser beam in a strongly ionized moving plasma

The steady state and transient self-interaction of a laser beam with a strongly ionized plasma flowing transverse to the direction of propagation have been investigated by a phenomenological approach using perturbation theory, WKB and paraxial ray approximations. The effect of the transverse motion of the plasma has been included by a convection term in the energy balance equation and is found to result in the non-symmetrical heating of electrons. As a result the beam is shifted towards the direction of transverse flow of the plasma by an amount that increases with the flow velocity. The extent of asymmetry in self-focusing along the transverse directions is, however very small. In a typical case of 7.6×10⁵ watt laser of ω=10⁴ GHz and initial beam widthr ₀=0.05 cm the transverse shiftx _p=0.1r ₀ is predicted in a distance of propagationz=0.34 cm in a strongly ionized plasma of electron densityN _e=10¹⁶ cm^?3 and transverse flow velocityW ₀=10⁷ cm/sec.     

Semiconductor laser amplitude and phase noises effects in coherent optical frequency domain reflectometry C-OFDR

We study the effects of the semiconductor laser noise on the generated beat signal, in Coherent Optical Domain Reflectometry device. By using a general laser noise analytical model, based on Volterra non-linear treatment of laser rate equations, we find a novel general expression of the beat photocurrent. We show that the coherence and the incoherence cases are observed for a different delay time intervals than the ones known in literature. We show that the coherence domain corresponds to a delay time lower than the half of the laser coherence time τ₀<1/2τ_c, and the incoherence domain corresponds for τ₀>2τ_c. An original shape of the beat signal autocorrelation function affected by the laser noises is obtained.     

2D laser collimation of a cold Cs beam induced by a transverse <Emphasis Type="Italic">B</Emphasis> field

We describe transverse collimation of a continuous cold cesium beam (longitudinal temperature 75 μK) induced by a two-dimensional, blue-detuned near-resonant optical lattice. The mechanism described for a lin-‖-lin configuration is made possible by the application of a transverse magnetic field B_⊥. The phenomenon described differs from gray molasses, for which any small magnetic field degrades cooling, as well as from magnetically induced laser cooling in red-detuned optical molasses, where there are no dark states. The lowest transverse temperature is experimentally found to vary as B _⊥ ² . The collimated flux density shows a dip as a function of B_⊥, the width of which is proportional to the cube root of the laser intensity, general features predicted by our semiclassical model. This technique provides a sensitive tool for canceling transverse magnetic fields in situ at the milligauss level.     

The energy of thermal electrons in electron beam created helium discharges

We have measured the electron energy of the thermal group of electrons in both longitudinal and transverse electron beam created helium glow discharges. The measurement technique employs the ratio of intensities of spectral lines in the 2s³S?np³P He I series. Values of kT_e between 0.07 and 0.11 eV were obtained. These energies are typical of the beam-generated electric field free plasmas. The competitive loss of helium ions by recombination and by charge transfer in a He?Hg electron beam created plasma is calculated. The results are applied to the Hg⁺ laser pumping scheme using a electron beam created He?Hg plasma.     

Spatial-temporal instability and optical turbulence upon the formation of spontaneous gratings by S-polarized light in waveguide AgCl-Ag films

The spatial-temporal instability of the spontaneous gratings formed in waveguide AgCl-Ag films under irradiation with an S-polarized laser beam is investigated. The studies are based on the simultaneous observation of diffraction (D) patterns formed by the S _? gratings and small-angle scattering (SAS). The transverse instability of D and SAS patterns was found to be associated with excitation of TE₀ modes at different angles with respect to the plane of incidence by scattering centers of Rayleigh type with the following formation of microgratings by the scattered modes and with their competition. It is shown that the time evolution of the form and the dynamic shift of the D patterns is also connected with the time dependence of the effective refractive index for TE₀ modes. In the case when a focused laser beam was used, the optical turbulence was found in the SAS and D patterns, caused, mainly, by the competition of S _?-and S ₊-microgratings, which eventually leads to the destruction of the latter.     

激光准直Cr原子束的实验研究

利用多普勒原理对Cr原子束进行横向准直.应用激光感生荧光技术稳定激光器的频率，把激光器的中心频率稳定在偏离Cr原子共振中心频率-5±0.26MHz的位置.根据理论计算出准直激光束的最小尺寸为13.7mm.根据实验数据选择合适的参数，实现利用多普勒原理横向准直Cr原子束，使原子束的横向分布缩小到原来的1/3. 关键词： 激光准直 激光感生荧光稳频 多普勒冷却     

Allotropic conversion of carbon-related films by using energy beams

Energy beams, such as ion and laser beams, were employed to convert carbon allotropes into other ones at a specified position because these energy sources can be controlled precisely in time and space. The ion beam deposition technique employing mass-separated ions proved effective in studying the nucleation process by changing several growth parameters (ion species, incident energies, and substrate temperatures). Immersed nanosized diamonds were found in an sp ³-rich amorphous film prepared with 100-eV ¹²C⁺ ions at room temperature. Surrounding these nanodiamonds, regularly arrayed small bumps, “petals,” were formed around the periphery of bald circles upon cooling. Ar-ion laser illumination is effective in designing the array of high luminescent points on a C₆₀ film by careful control of the laser power, and the combination of a micro-Raman spectrometer with a piezoscanning system provides one with a tool for 2-dimensional processing of photosensitive materials. Simultaneous bombardment during C₆₀ evaporation results in an interesting pattern formation specific to the simultaneous treatment. The dependence of the surface nanoscale pattern on the ion energy and the substrate temperatures provides one with a new tool for designing nanoscale functional materials. As an extreme, the appearance of hexagonal diamonds was detected with disordered carbon and graphite under the condition of there being a high ratio between the Ne ion beam and the C₆₀ thermal beam.     

部分相干源实验研究

 在无阶梯诱导空间非相干光束平滑技术中，用作前端的部分相干源是重要的组成部分。介绍了利用准分子激光进行的部分相干源实验，发现聚四氟乙烯是一种较理想的散射材料。研究了聚四氟乙烯散射源对光束的散射性能，给出了光束均匀性、空间相干性和能量转换效率等参数。     

利用脉冲负反馈压缩准分子激光脉冲

 分析了准分子激光脉冲在放大过程中的展宽现象。将饱和开关技术应用于放电泵浦激光器放大腔的放大压缩,发展了脉冲负反馈饱和压缩技术。利用该技术在天光一号系统前端LPX-150激光器上成功获得能量100 mJ、脉宽10 ns的脉冲输出,其空间分布均匀性保持在3%以内。这证明该技术可与无阶梯诱导非相干技术联合使用,在保证光束均匀性基础上进行脉冲压缩。     

High brightness 735 nm tapered lasers – optimisation of the laser geometry

High brightness tapered laser diodes with different resonator geometries were fabricated and analysed. The devices consist of an index-guided straight section and a gain-guided tapered section. Lasers with a total length L = 2 and 4 mm and different length of the ridge waveguide L _RW (500 μm ≤ L _RW ≤ 1250 μm for L = 2 mm and 500 μm ≤ L _RW ≤ 2000 μm for L = 4 mm) were processed to study the influence of the straight section on the spatial mode filtering. The power–voltage–current-characteristics, the beam waist, the far field, and the beam propagation factor M ² were measured. From the experiments, it can be stated that the lasers with a small L _RW reach higher output powers compared to those with larger L _RW. Concerning the beam quality the length L _RW should exceed a minimal value to guarantee efficient spatial mode filtering. Devices optimised concerning maximum output power and excellent beam quality reach a beam propagation factor smaller than 2.1 at an output power P = 2 W.     

Beam deflection and isolation in laser amplifiers

A new beam deflection and collimation technique is described which utilizes transverse gain variations in laser amplifiers. This method may be used to achieve beam mixing or optical isolation between the stages of an amplifier chain. Results with a CO₂ amplifier are decribed.     

Propagation parameters of the beam extracted from a diode pumped Nd:YAG ceramic slab laser with a hybrid stable–unstable resonator

We analyze the propagation properties of the beam extracted from a diode pumped ceramic Nd:YAG slab laser adopting a hybrid stable–unstable resonator. Such a resonator produces a beam characterized by an Hermite–Gauss mixture-like distribution in one transverse direction and a hard-edge diffracted distribution on the other transverse direction. The beam propagation parameters M_x² and M_y² are measured for different values of the diodes driving current. We obtain a beam parameter product smaller than 3 mm mrad in both transverse directions and in the whole range of powers, up to an extraction of 220 W in a QCW regime.     

A neat semi-Gaussian laser beam with no diffraction fringes: Theoretical analysis

We propose a simple optical device to convert a Gaussian laser beam into a neat semi-Gaussian laser beam without any diffraction fringe by using a spatial light modulator and a thin, sharp blade, and numerically calculate the diffracted, relative intensity distributions of both the semi-Gaussian laser field and the semi-Gaussian, pseudo-thermal light. We also study the dependence of the border width of the semi-Gaussian beam on the waist of the Gaussian beam. Our study shows that the proposed scheme can be used to cancel all diffraction fringes from both the straight edge of the blade and a finite lens aperture in all the planes vertical to the z axis and obtain a neat semi-Gaussian beam without any diffraction fringe, and find that the border width w_B of the generated semi-Gaussian beam is not dependent on the waist of the incident Gaussian beam.     

Amplitude and phase clipping: Strehl ratio versus divergence

We consider the transverse characteristics of a Gaussian laser beam subject to a phase or amplitude clipping due to a pupil which is a π-plate or an opaque disc (stop). In particular, we consider the correlation between two features, the Strehl ratio and divergence angle, usually used for characterising the focusability of a diffracted beam. It is demonstrated that the Strehl ratio does not give systematically a global view, from a divergence point of view, on the transverse properties of a Gaussian beam suffering amplitude or phase diffraction. In addition, we consider the case of self-diffraction of a Gaussian beam upon a Gaussian phase aberration of same width, and it is found that the on-axis intensity describes correctly the whole diffracted beam cross-section, from a divergence point of view, only if the central phase shift is smaller than π. Another example showing that the focusability of a pure high-order Laguerre–Gauss TEM_p0 beam, free from any clipping, cannot be correctly described by Strehl ratio is also considered.     

Electron magnetic chiral dichroism in CrO2 thin films using monochromatic probe illumination in a transmission electron microscope

Electron magnetic chiral dichroism (EMCD) has been studied in CrO₂ thin films (with (100) and (110) growth orientations on TiO₂ substrates) using a gun monochromator in an aberration corrected transmission electron microscope operating at 300 kV. Excellent signal-to-noise ratio is obtained at spatial resolution ∼10 nm using a monochromatic probe as compared to conventional parallel illumination, large area convergent beam electron diffraction and scanning transmission electron microscopy techniques of EMCD. Relatively rapid exposure using mono probe illumination enables collection of EMCD spectra in total of 8–9 min in energy filtered imaging mode for a given Cr L_2,3 energy scan (energy range ∼35 eV). We compared the EMCD signal obtained by extracting the Cr L_2,3 spectra under three beam diffraction geometry of two different reciprocal vectors (namely g=110 and 200) and found that the g=200 vector enables acquisition of excellent EMCD signal from relatively thicker specimen area due to the associated larger extinction distance. Orbital to spin moment ratio has been calculated using EMCD sum rules for 3d elements and dichroic spectral features associated with CrO₂ are compared and discussed with XMCD theoretical spectra.     

IR operated novel Ag0.98Cu0.02GaGe3Se8 single crystals

In this work, we report on the structural and optical properties of novel Ag_0.98Cu_0.02GaGe₃Se₈ single crystals that were synthesized by the Bridgman–Stockbarger technique. We have performed illumination by 10.6 μm CO₂ pulsed laser working in the microsecond time duration regime. Such illumination allows causing substantial changes for both pure electronic nonlinear optical effects like optical second harmonic generation as well as piezooptical effects described by the fourth rank tensors. The measurements of the piezo-optical effects were carried out at different temperatures. The effects are observed only during the IR CO₂ laser illumination and are disappeared after switching off the illumination. Simultaneously the IR induced optical second harmonic generation at Er:glass laser fundamental wavelength 1540 nm was performed during illumination by nanosecond Nd:YAG and Er³⁺:glass laser. The observed effects allow to use the studied materials as promising for IR-optoelectronic devices.     

Thermal fields in laser–multi-layer structures interaction

In the present paper, we are dealing with the thermal fields for laser–periodic multilayer structures interaction. Our point of view is originally that we consider any order transverse laser beams, like heating sources. We consider that the laser beam acts in IR (Nd:YAG or CO₂ laser beam) and it is in one transverse mode or in only a few decoupled modes. In order to solve this problem, we will use the Green function method. Specific results are presented for a laser beam (Nd:YAG laser) operating in the mode TEM₀₁ and a two-layer structure.     

Recording of a hologram by using the second-order nonlinearity

The observation of “spatial-modulation” resonances of saturated absorption in the interaction of iodine-127 vapors with a superposition of frequency-nondegenerate TEM₀₁ and TEM₁₀ transverse modes of a linear laser is reported. The modulation of the total radiation power of the probing wave, recorded at twice the beating frequency of transverse modes of the laser, is interpreted to be the result of the transformation of modulation of the spatial distribution of the field into its amplitude modulation under the action of nonlinearly absorbing iodine vapors. Resonances of saturated absorption of the weak line R(127)11-5 of iodine-127 (633 nm) in an extracavity cell are observed by the method proposed.     

Experimental evidence of photoinduced valence change of Fe3+ in BaTiO3 and mechanism for growth of new grating in depleted pump condition: An EPR investigation

With a view to understanding the role of photo-induced valence changes of impurities in BaTiO₃ in the phenomena of photorefraction, EPR experiments were conducted under in situ He-Ne laser illumination. These experiments gave evidence for photoinduced valence change of Fe in BaTiO₃ at room temperature. The EPR signal due to trivalent iron was found to reduce in intensity with laser illumination The kinetics of the valence change has been investigated. Under large fringe width condition, the time constant of the decay is identified as the dielectric relaxation time τ_d. The changes in line shape on laser illumination to Dysonian form, appeared most predominantly in mechanically poled crystal compared to electrically poled single domain crystals. This demonstrated the possible role of domain walls and the defects there, as source or sinks of charge carriers on photo excitation. It is observed, that there is transient growth of Fe³⁺, when the laser illumination was put on, before its decay. This was attributed to charge transfer between electrons in oxygen vacancies and Fe⁴⁺. This predicted the growth of a transient grating under depleted pump condition in a two beam coupling experiment. This was experimentally proved by following the diffracted signal of the reading beam under the depleted pump condition.