On localized structures of radiation in a wide-aperture laser with a transverse inhomogeneity

The analysis of the stability of the generationless regime in a laser with a smooth transverse inhomogeneity of the resonator length is performed. Localized structures of radiation are considered that are possible in this laser when the nonlinearity of the gain and of the refractive index of the medium is taken into account.     

Optohydrodynamics of soft fluid interfaces: Optical and viscous nonlinear effects

Recent experimental developments showed that the use of the radiation pressure, induced by a continuous laser wave, to control fluid-fluid interface deformations at the microscale, represents a very promising alternative to electric or magnetic actuation. In this article, we solve numerically the dynamics and steady state of the fluid interface under the effects of buoyancy, capillarity, optical radiation pressure and viscous stress. A precise quantitative validation is shown by comparison with experimental data. New results due to the nonlinear dependence of the optical pressure on the angle of incidence are presented, showing different morphologies of the deformed interface going from needle-like to finger-like shapes, depending on the refractive index contrast. In the transient regime, we show that the viscosity ratio influences the time taken for the deformation to reach steady state.     

The development of turbulence in the active medium of a fast-flow gas-discharge laser

The effect of pump nonequilibrium on the heat generation and the parameters of turbulent flow in the active medium of a fast-flow gas-discharge laser is studied. It is shown that the amplitudes of the turbulent pulsations of the density and refractive index of the active medium depend on the pump method and the degree of saturation by laser radiation and grow with the pump level. Criteria for estimating the effect of turbulence on the quality of the active medium are presented. A model is proposed that relates the build-up of the turbulent pulsations of the gas density and refraction to the heterogeneous heat release in the nonequilibrium active medium. The effect is determined by both the growth of the isobar local temperature pulsations and the ionization-overheating instability. In the case of dc-discharge pumping, the optical inhomogeneity of the active medium may substantially exceed that for the high-frequency discharge pumping. Original Text ? Astro, Ltd., 2006.     

用于光生毫米波的双频微片激光器热致频差特性研究

研究了激光二极管端面抽运的双纵模双频微片激光器的热效应对其输出频谱的影响. 通过对端面抽运Nd:YVO₄微片的热传导方程求解得出晶体内部的温度场分布; 并进一步对微片晶体热致折射率变化进行分析, 由此计算出双频微片激光器的热致频谱变化. 实验结果表明: 当抽运功率较小时得到稳定的双纵模双频激光输出; 当抽运功率增大时, 晶体热效应使双频激光频谱双峰之间的频差减小, 各纵模波长频谱宽度加宽, 实验结果与理论分析相符合. 关键词： 光载无线通信 双频微片激光器 热效应 频率差     

High repetition rate femtosecond laser irradiation-induced elements redistribution in Ag-doped glass

A 250 kHz femtosecond laser was used to induce a one-step precipitation of Ag nanoparticles and the simultaneous element redistribution in Ag ion doped glass. After femtosecond laser exposure, a ring-shape region was formed. Besides, as a result of an optical spherical aberration effect, a nonuniform laser intensity distribution along the incident direction caused most of the formed Ag nanoparticles to precipitate in the lower part of cross section of this structure. By an EPMA analysis, the relative concentration of the Ag element was both high in the center and in the boundary of the ring-shape region. These Ag nanoparticles could potentially increase the refractive index in their precipitation regions. We proposed that the induced ion redistribution and the precipitation of Ag nanoparticles should be due to the heat accumulation effect by the high repetition rate femtosecond laser irradiation.     

Modeling of plasma-controlled surface evaporation and condensation of Al target under pulsed laser irradiation in the nanosecond regime

Phase transition on the surface of an aluminium target and vapour plasma induced by laser irradiation in the nanosecond regime at the wavelengths of 1.06 and 0.248 μm with an intensity of 10⁸-10⁹ W/cm² in vacuum are analysed. Particular attention is paid to the wavelength dependence of the observed phenomena and the non-one-dimensional effect caused by the Gaussian laser intensity distribution. A transient two-dimensional model is used which includes conductive heat transfer in the condensed phase, radiative gas dynamics and laser radiation transfer in the plasma as well as surface evaporation and back condensation at the phase interface. It is shown that distinctions in phase transition dynamics for the 1.06 and 0.248 μm radiation result from essentially different characteristics of the laser-induced plasmas. For the 1.06 μm radiation, evaporation stops after the formation of hot optically thick plasma, can occasionally resume at a later stage of the pulse, proceeds non-uniformly in the spot area, and the major contribution to the mass removal occurs in the outer part of the irradiated region. Plasma induced by the 0.248 μm laser is much more transparent therefore evaporation does not stop but continues in the subsonic regime with the Mach number of about 0.1.     

Interferometric technique for the determination of thermal nonlinearities in semiconductor glasses

An experimental method for the determination of thermal nonlinearities of semiconductor doped glasses induced by cw laser radiation is presented. An estimation of the intensity dependent refractive index is given, at steady state conditions, which is in good agreement with experimental data reported in the literature for similar materials.     

High Photosensitivity in SnO 2 :SiO 2 Optical Fibers

Enhanced photosensitivity has been achieved in tin-doped silica optical fibers. Refractive index modulations (~3 . 10 -4 ) have been achieved by use of 248 nm UV radiation. Studies performed on preform slides proved that the refractive index change can be ascribed to structural rearrangements induced by photochemical reactions. The modified structure has shown extreme stability, and gratings written in this fiber start to be erased above 600C. Exposure of the fiber to 255 nm laser radiation showed that, at high fluence, the refractive index modulation saturates and does not exhibit any sign of decrease.     

High Photosensitivity in SnO 2 :SiO 2 Optical Fibers

Enhanced photosensitivity has been achieved in tin-doped silica optical fibers. Refractive index modulations (~3 . 10 -4 ) have been achieved by use of 248 nm UV radiation. Studies performed on preform slides proved that the refractive index change can be ascribed to structural rearrangements induced by photochemical reactions. The modified structure has shown extreme stability, and gratings written in this fiber start to be erased above 600°C. Exposure of the fiber to 255 nm laser radiation showed that, at high fluence, the refractive index modulation saturates and does not exhibit any sign of decrease.     

Effects of transient local heating of spatially and spectrally heterogeneous biotissue by short laser pulses

Summary A new laser method for the spectroscopy of locally absorbing microvolumes much smaller than the radiation wavelength in size is discussed. Such an absorption is characteristic of biotissues and other biological media. The method allows data to be obtained on the size of local microvolumes absorbing at the radiation wavelength. It is based on the possibility of transient overheating of the microvolumes by means of an ultrashort pulse with a duration much shorter than the time it takes to heat to diffuse from the microvolumes. The pulse-heated microvolumes must have an altered refractive index leading to an additional scatter of another probe laser pulse, that is made to irradiate the medium under study after some delay. Besides, the locally heated microvolumes will have temperature-altered fluorescence induced either by the heating laser pulse or by an additional probe pulse at another wavelength more suitable for fluorescence excitation. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Radiation reflection from an optical inhomogeneity moving in a right- and left-handed medium

The parametric Doppler effect in a medium whose dielectric permittivity and magnetic permeability become negative in some spectral range is analyzed with respect to uniform plane waves. The regime in which the medium is right-handed (the refractive index is positive) for incident radiation and left-handed (the refractive index is negative) for reflected wave is demonstrated. The possibility of the occurrence of additional branches in the dispersion dependences is demonstrated.     

Interferometric investigation of the effects, on the refractive index of PVA, of doping with lead acetate and gamma irradiation

Non-localized interference fringes, resulting from illuminating a film of PVA with a laser point source, have been used for measuring the refractive index of such a film. The refractive index of pure polyvinyl alcohol (PVA) film and five PVA films, doped with lead acetate of concentrations of 5, 10, 15, 20 and 25% by weight, have been investigated before and after irradiation by doses of 2, 4, 6, 8 and 10 Mrad gamma rays. It is found that, before irradiation, the refractive index increases with increasing the concentration of doped lead acetate. After irradiation the refractive index increases with increasing the dose up to a certain limit then decreases again. This indicates that the radiation deformation of gamma radiation is a chain scission effect.     

Ultrafast optical switching of an ionized medium by interfering ultrashort laser pulses

Multibeam ionization of gas and condensed-phase media with interfering ultrashort laser pulses can help to substantially increase the maximum field intensity and the density of free electrons attainable in the focus of the laser field relative to the regime of single-beam ionization. Multibeam ionization schemes offer new solutions for laser micro- and nanomachining, micro- and nanosurgery, spectral and temporal transformation of ultrashort light pulses, as well as remote sensing of the atmosphere. Subfemtosecond changes in the local refractive index induced in the regime of multibeam tunneling ionization enable the high-speed switching of optical signals.     

Far-infrared frequency shifting by optically induced gratings in semiconductors

A first approach to frequency shifting of far-infrared radiation by optically induced “moving” refractive index gratings in semiconductors is reported. The factors determining the performance of this frequency shifting technique are inferred using a one-dimensional analytical model. In a proof-of-principle experiment frequency shifting of 119 μm radiation was observed up to 80 MHz using a 300 mW, 488 nm Ar⁺ laser for optical excitation.     

An electrodynamic model of the optical characteristics of blood and capillary blood flow rate

A model for the prediction of the optical characteristics of blood (refractive index and absorption coefficient) and for the determination of the rate of blood flow in the capillary bed under irradiation of a laser beam is proposed. The dependences of the intensity of the laser radiation on the refractive index and absorption coefficient of the system of blood vessels in the upper layer of the dermis and on the rate of blood flow in the capillary bed are obtained.     

非线性介质中强光对弱光聚焦的控制研究

从理论和实验上分析了强光控制弱光脉冲聚焦的方法, 该方法可以灵活地控制弱光的聚焦过程和特性. 通过研究两光束共同传输, 利用强抽运光在相互作用时为弱探测光提供诱导非线性效应, 从而使弱探测光发生聚焦. 得到了在抽运光保持不变情况下探测光的解析解, 并在实验上得到验证. 进一步研究了强抽运光对弱探测光焦点位置的影响. 结果显示: 通过改变抽运光功率、束宽能够有效地控制弱探测光的聚焦过程和焦点位置. 关键词： 诱导聚焦 自聚焦 非线性折射率     

Frequency Characterization of the Nonlinear Refractive Index in Optical Fiber

The Kerr and the electrostrictive contributions to the intensity-dependent nonlinear refractive index in optical fibers are discussed on both theoretical and experimental bases. Particular attention is devoted to the electrostrictive contribution that under particular conditions is enhanced by resonances. The resonant behavior of the electrostrictive effect is calculated, measured, and discussed for standard single-mode fibers and dispersion shifted fibers. Experimental results are also reported. Measure ments have been carried out with a simple and effective technique based on the cross-phase modulation effect that also allows one to evaluate the nonlinear refractive index at any bit rate. Electrostriction contributes 20 % to the total nonlinear refractive index but in the resonant regime becomes comparable to the Kerr effect.     

Coherent effects in the incoherent channel of resonant radiation scattering from excited atoms

Scattering of a resonance electromagnetic field from excited atoms cannot be described by the semiclassical theory of radiation operating with nonquantized electromagnetic fields. Field quantization effects are manifested in this case on the macroscopic level and lead to evolution of statistical properties of radiation in the course of scattering. It is found that a combined process coupling elastic scattering from an atom and induced emission from the same atom, which cannot be studied by the methods of the standard perturbation theory, plays a significant role in this effect. The process of combined scattering in extended media exhibits coherent properties that cannot be described by the standard refractive index.     

掺杂偶氮苯聚合物光致异构Z扫描测量及其理论解释

用He_Ne激光对掺有偶氮苯聚合物DR 13的PMMA薄膜样品进行Z扫描研究 ,发现其折射率改变量与光强的关系存在着饱和效应 ,且线偏振光引起的折射率改变明显大于圆偏振光引起的折射率改变 .对于这些现象 ,用光致异构的角烧孔机理进行解释