Generation of multicolor spatial solitons by stimulated Raman scattering

We report the experimental observation of Raman spatial solitons in a nonlinear planar waveguide. We show that, under certain conditions, stimulated Raman scattering (SRS) can stabilize the pump beam propagation and give rise to multifrequency spatial solitons. We present the different Kerr media with which we obtained self-focusing and SRS. The observations can be described in terms of Raman gain versus soliton power.     

Feasibility of nonlinear Raman lidar based on stimulated Raman gain spectroscopy without a tunable laser

A novel technique of lidar for atmospheric gas detection by use of stimulated Raman gain spectroscopy without any tunable laser is proposed. Detection sensitivity and detectable range are estimated on the basis of the lidar equation for CO2, CH4, and H2 in the atmosphere. The feasibility study clearly shows that the technique has a potential for application to lidar and that, in addition, the construction of the system is simpler than those of traditional differential absorption lidars.     

Multifractality of laser beam spatial intensity in a turbulent medium

We present the results of a laser beam passing through a turbulent medium. First we measure the geometric parameters and the spatial coherence of the beam as a function of wind velocities. A multifractal detrended fluctuation analysis algorithm is applied to determine the multifractal scaling behavior of the intensity patterns. The measurements are fitted with models used in the analysis of river runoff records. We show the surprising result that the multifractality decreases when the spatial coherence of the laser is decreased. Universal scaling properties could be applied to the spatial characterization of a laser propagating in a turbulent medium or random medium.     

Investigation of additive noise in power-stable Ar+ laser radiation

Translated from Izmeritel'naya Tekhnika, No. 5, pp. 30–32, May, 1990.     

Use of a laser beam with an oblique angle of incidence to measure the reduced scattering coefficient of a turbid medium

A simple and quick approach is used to measure the reduced scattering coefficient (μ(s)') of a semi-infinite turbid medium having a much smaller absorption coefficient than μ(s)'. A laser beam with an oblique angle of incidence to the medium causes the center of the diffuse reflectance that is several transport mean-free paths away from the incident point to shift away from the point of incidence by an amount Δx. This amount is used to compute μ(s)' by μ(s)' = sin(α(i))/(nΔx) where n is the refractive index of the turbid medium divided by that of the incident medium and α(i) is the angle of incidence measured from the surface normal. For a turbid medium having an absorption coefficient comparable with μ(s)', a revision to the above formula is made. This method is tested theoretically by Monte Carlo simulations and experimentally by a video reflectometer.     

Distortion of the spatial distribution of a laser beam reflected by a VO2 mirror

The switching of controllable VO₂ mirrors with dR/dT>0 or dR/dT<0 under the action of a Gaussian-profile laser beam is modeled. It is shown that when a high-intensity laser beam is reflected by a VO₂ mirror, it undergoes appreciable spatial distortion during switching. Pis’ma Zh. Tekh. Fiz. 24, 17–22 (March 12, 1998)     

Use of a sample translation technique to minimize adverse effects of laser irradiation in surface-enhanced Raman spectrometry

Surface-enhanced Raman spectroscopy (SERS) has proven to be a very powerful tool in the analysis of a wide range of compounds. However, continuous irradiation of the laser beam over the SERS substrate can promote the gross decomposition of the sample analytes and significantly broaden and diminish the intensities of observed spectral bands. In addition, the incident radiation can promote thermal or photolytic fragmentation of analytes, thereby altering the observable bands and possibly leading to a misinterpretation of analytical data. Finally, chemical or morphological changes in the SERS substrate are possible. This work presents the use of a sample translation technique (STT) as a means to minimize these adverse effects. By spinning the sample rapidly, the effective residence time of analytes and substrate within the irradiated zone is dramatically decreased without reduction of spectral acquisition time or the density of analyte in the zone. The technique is studied by acquiring SERS spectra of Naproxen USP, riboflavin, folic acid, Rhodamine 6G, and 4-aminothiophenol using silver islands on glass and silver-poly(dimethylsiloxane) composite substrates under various spinning and stationary conditions. In all cases, spectra show improvements upon spinning at laser powers as low as 4.2 (+/- 0.1) mW. Specific differences in the appearance of the spectra and the potential use of STT for improved SERS qualitative and quantitative determinations are presented.     

A new integral characteristic of the degree of difference of the spatial distribution of a laser beam from a Gaussian distribution

A new integral characteristic that makes it possible to rapidly estimate the degree of difference of a measured distribution of a laser beam in a cross-section from a Gaussian distribution is proposed. A high degree of sensitivity of the method to different types of distributions is demonstrated.     

Bottle beam from a bare laser for single-beam trapping

We demonstrate that a laser beam converging from a specific transverse mode is a bottle beam, as described in J. Opt. Soc. Am. B 20, 1220 (2003). To our knowledge, this is the first time that a bottle beam has been generated directly from a laser. By calculating the radiation forces on a dielectric Rayleigh sphere in the bottle beam, we show that the single beam can trap high-refractive-index particles at the multiple axial sites of intensity maxima, and it can confine low-index particles on a transverse plane within the bottle regions. Such a novel laser beam may have other applications.     

Thermal noise of a laser gravitational antenna with an elastic clip

The behavior of the spectral density of the thermal noise of laser gravity antennas fastened by an elastic clip with dissipative losses in the mounting is investigated, using a smooth perturbations method based on the fluctuation-dissipation theorem. Translated from Izmeritel'naya Tekhnika, No. 3, pp. 3–5, March, 1998.     

Development of the 2.8 microm emission doubly shifted Raman laser using stimulated Brillouin scattering in a cascaded cavity

A doubly shifted Raman laser using CH(4) gas has been developed for 2.8 microm generation, pumped by a Nd:YAG laser with 65.5 mJ at 17 ns. A dichroically coated meniscus-type lens is modified to utilize the backward stimulated Brillouin scattering and backward Stokes beams from a previous laser design [Appl. Opt.46, 5516-5521 (2007)APOPAI0003-693510.1364/AO.46.005516]. A maximum output energy of 4.76 mJ at 2.80 microm wavelength has been achieved in the cascaded resonator. A maximum conversion efficiency of 8.9% has been achieved at a CH(4) gas pressure of 600 psi. The obtained spatial beam profile is quite smooth, and its output pulse width is 10 ns.     

相关偏置信号调制噪声和加性噪声驱动线性系统随机共振

研究了偏置信号调制噪声和加性噪声驱动线性系统的随机共振现象。根据线性系统理论,得到了系统输出信噪比的精确表达式。研究结果表明：系统输出信噪比是系统衰减常数、调制信号偏置参数、信号调制噪声强度和加性噪声强度的非单调函数。同时,系统输出信噪比随两种噪声的相关强度成非单调关系。     

Development of an algorithm for corneal reshaping with a scanning laser beam

The corneal-ablation rate, the beam-intensity distribution, and the initial and the desired corneal topographies are used to calculate a spatial distribution map of laser pulses. The optimal values of the parameters are determined with a computer model, for a system that produces 213-nm radiation with a Gaussian beam-intensity distribution and a peak radiant exposure of 400 mJ/cm(2). The model shows that with a beam diameter of 0.5 mm, an overlap of 80%, and a 5-mm treatment zone, the roughness is less than 6% of the central ablation depth, the refractive error after correction is less than 0.1 D for corrections of myopia of 1, 3, and 6 D and less than 0.4 D for a correction of myopia of 10 D, and the number of pulses per diopter of correction is 2500 when the beam-intensity distribution is Gaussian and 580 when it is flat.     

Betatron contributions to the angular gain spectrum of a free-electron laser with an imperfect beam trajectory

The effect of betatron oscillations on the angular gain spectrum of a free-electron laser is examined in the case of an imperfect beam trajectory. The use of an axial magnetic field is suggested to improve the beam–mode coupling and gain of the laser.     

Numerical simulation of a laser resonator with an in-phase doughnut-like output beam

A numerical model of the toric concave mirror laser resonator is founded by using the eigenvector method. Numerical calculation shows that an in-phase doughnut-like beam mode with high beam quality can be obtained in this resonator, whose diffraction loss is the lowest, and whose intensity distribution covers the whole resonator mirror. Systematical simulations indicate that, different from the spherical stable resonator, the effects of Fresnel number of the resonator and curvature radius of the toric concave total reflector on output beam quality is not very obvious. Under the condition of curvature radius of 15 m, the M ² factor of the output beam of this resonator with a large range of Fresnel number from 6.2 to 12.6 is from 1.3 to 1.9. Furthermore, the diffraction loss is close to 1.0% or less than 1.0%. Under the condition of Fresnel number of 7.07, the M ² factor of the output beam of this resonator with curvature radius from 6 to 30 m is from 1.60 to 1.24, and the diffraction loss is close to 1.0% or less than 1.0%.     

Analytical characteristics of stimulated Raman scattering in a multimode fiber obtained with an optical time-domain reflectometer

The nonlinear behavior of stimulated Raman scattering (SRS) in a low-loss optical fiber has been analytically and experimentally investigated by the assumption of certain restraint conditions. As a result, it has been found that the SRS light propagating along the optical fiber is expressed in a simple formula by the introduction of nonlinear parameters, which are experimentally identified and whose maximum value is restricted by the restraint conditions. The parameters, moreover, are available for compensating the SRS intensity in the presence of noninteracting light. Therefore the formula for SRS intensity presented is useful for investigating the SRS effect in a multimode fiber.