BBO晶体中飞秒脉冲的二次和三次谐波振荡产生

同时考察了群速色散失配、最低阶群速色散和二阶群速色散。研究了在ＢＢＯ晶体中飞秒激光脉冲的谐产生，计算了ＢＢＯ晶体的作为波长函数的晶体的最低阶和二谐群速色散，借助求解改进的耦合波方程的数值计算，得到了飞秒脉冲的二次及三次谐波辐射，分析了最低工阶及二阶ＧＶＤ对于基波脉冲和谐波脉的影响，最后考虑了三次谐波产生中群速失配的补偿。     

Allowance for angular divergence in the generation of the second harmonic in crystals with a regular domain structure

We obtained basic analytical expressions for the efficiency of the second harmonic in nonlinear crystals with a regular domain structure; in these expressions we took into account the angular divergence of the laser beam. The calculation was done in the geometric-optics approximation of the given field of laser radiation. We show that the expressions obtained are similar to the corresponding expressions for homogeneous nonlinear crystals. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 134–138, January–February, 2000.     

群速度色散对于纠缠光场二阶关联函数影响的研究

HBT干涉是量子测量中的一种重要手段, 其通过计算光场的二阶关联函数而得到测量结果. 在长距离测距中, 光场的二阶关联函数会受到光纤中群速度色散的影响而发生展宽和平移, 从而在一定程度上影响测量精度. 本文主要针对二阶关联函数半高宽受群速度色散的影响, 给出了半高宽与测量距离与群速度色散系数的关系.     

Ultrashort-pulse generation by lasers with saturable absorbers

A theoretical model for the ultrashort-pulse (USP) generation by lasers with saturable absorbers is presented. The gain medium is assumed to be a two-level system whereas the absorber is described by a four-level system which is characterized by a two-stage absorption process and the pertinent relaxation times. Laser dynamical equations are developed in the rate-equation approximation and boundary conditions appropriate for an unidirectional ring cavity are established. Evolution of USP is investigated for different combinations of parameter values appropriate for solid-state laser systems by computer simulation, employing a finite difference approximation for the dynamical equations. It is shown that USP output is attainable even if the cross-section of the excited-state absorption isgreater than that of the ground-state absorption and the laser is operating just above threshold. In fact, it is found that through the participation of a strong excited-state absorption the discrimination against satellite pulses is enhanced so that single-pulse output is more achievable. Furthermore, it is proposed that single picosecond pulses may be obtainable from relatively broad initial peaks by utilizing the high pulse-selection and pulse-shortening efficiency of the absorber due to the contribution of the excited-state absorption. The applicability of the present model to singlet-triplet crossing and photoisomer formation is also discussed. Work supported in part by the National Research Council of Canada under grant No. A6005. A preliminary version of this paper was presented at the Joint Congress of CAP-APS-SMF, Université Laval, Québec, Canada, 14–17 June 1976. This author is now with Welwyn Canada Limited, London, Ontario, Canada.     

Inversion of Rayleigh waves using a genetic algorithm in the presence of a low-velocity layer

The shear-wave velocity profile can be obtained by the velocity of Rayleigh waves through the back-calculation based on dispersion curves. However, the dispersion curves obtained in practical application are always discontinuous and correspond to different mode branches due to mode jumping, especially in the presence of low-velocity layer. Mode misidentification may be encountered in inversion based on these jumped dispersion curves. Mode analysis demonstrates that the mode jumping is caused by a different surface displacement distribution with frequency for each mode. This indicates that the surface displacement distribution of the modes should be taken into account for the case of a low-velocity layer. Shear-wave velocity profiles are inversed based on the (possibly discontinuous) dispersion curves of fundamental and/or higher modes using a genetic algorithm (GA). In addition to the dispersion characteristics, the surface displacement distribution is also taken into account for the case of a low-velocity layer; as a result, mode misidentification is avoided. Published in Russian in Akusticheskiĭ Zhurnal, 2006, Vol. 52, No. 6, pp 811–824. The text was submitted by the authors in English.     

Compensation of the active-medium dispersion in a semiconductor laser with an external resonator

Measurements of the dispersion parameter (the second derivative of the wave vector with respect to the frequency) of the active waveguide layer of semiconductor lasers operating in the red region of the spectrum (0.65–0.67 μm) were performed. It was revealed that for these lasers the dispersion parameter has a value of (0.13–1.40)·10⁻²⁵ sec²/cm. It is shown that the dispersion of the active medium of a semiconductor laser with an external resonator can be compensated by inserting a quartz-glass rod into the resonator. Reported at the Second International Scientific and Technical Conference on Quantum Electronics, Minsk, November 23–25, 1998. Translated from Zhurnal Prikladnoi Spektroskoii, Vol. 66, No. 5, pp. 707–710, September–October, 1999.     

群速失配对CsLiB6O10晶体谐波脉冲展宽的影响

对超短强激光脉冲在CsLiB6O10晶体中的群速失配进行了理论分析,采用傅里叶变换法推导超短脉冲二次谐波的耦合波方程。在Ⅰ类相位匹配下数值计算了脉冲宽度50fs的基波532nm,在CsLiB6O10晶体长度为0．5mm时产生二次谐波(266nm)的群速延迟量约100fs,并且随着晶体长度的增长,二次谐波的脉冲宽度几乎线性展宽。并得到了群速失配量△β1^（2）226．243fs／mm,低阶和二阶群速色散系数分别为β2^（3）=210．120fs^2／mm,β2^(3)=65．587fs^3／mm。其结果表明由于群速延迟和群速色散导致了谐波脉宽展宽和形变。     

Numerical simulation and experimental investigation of parametric generation of light pulses in a KTP crystal

Numerical simulation and experimental investigation of the process of parametric generation of light inside a KTP crystal excited by laser pulses have been performed. The calculation method includes the solution of a system of coupled truncated equations, which describe the three-wave interaction in a laser resonator partially filled with a KTP crystal, with the use of reflection boundary conditions for amplitudes together with initial conditions corresponding to the level of noises. The time profile of the intensities of the interacting pulses and their energies were calculated. The results obtained make it possible to determine the optimal values of optical parameters of the resonator that ensure the maximum efficiency of conversion. The calculated values of the conversion efficiency are in good agreement with experimental data. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 56–60, January–February, 2000.     

Generation of soliton packets in a two-level laser

A variant of perturbation theory is constructed for a system of nearly integrable equations. Perturbations of a special type are considered, which makes it possible to represent the system in the form of compatibility condition for “deformed” linear systems. The corresponding deformation of the Whitham equations is found. The mathematical apparatus is used to theoretically examine the generation of a sequence of solitons in a two-level laser. The generation process is described by a system of Maxwell-Bloch equations with pumping of the upper level and with allowance for some relaxation effects. The dynamics of the transformation of the initial perturbation into a sequence of solitons under pumping is studied. Finally, the various generation regimes are analyzed and compared with the experimental data. Zh. éksp. Teor. Fiz. 112, 2237–2251 (December 1997)     

The effect of spatial dispersion on the interaction of short-wavelength oscillations with small-scale plasma irregularities

Using the statistical random-phase approximation, we derive the equations describing the interaction of short-wavelength oscillations with irregularities of a magnetized plasma taking into account thermal effects of the spatial dispersion. It is shown that an additional energy exchange between the wave and the plasma electrons, which is not related to resonance particles or collisions, occurs in a medium with random irregularities due to the effect of spatial dispersion. This energy exchange can be significant if the scales of the irregularities along the magnetic field are not very large, so that the dispersion effects in the interaction of a wave with irregularities are greater than the dissipative ones. We consider in detail the case, typical for the ionosphere, where the irregularities are field-aligned and the plasma oscillations are polarized almost perpendicular to the magnetic field. An equation describing the differential redistribution of plasma-oscillation energy in the wavenumber space due to multiple scattering by the irregularities was obtained taking into account the influence of spatial dispersion on the scattering process. We discuss application of the results obtained to the interpretation of some ionospheric experiments. Institute for Solar-Terrestrial Magnetism, lonosphere, and Radio Wave Propagation of the Russain Academy of Sciences, Troitsk, Moscow region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 4, pp. 310–324, April, 2000.     

“Instantaneous” spectrum of passively ionized electrons in a dielectric irradiated by a high-power electron beam

A technique is developed for theoretical analysis, and the “instantaneous energy spectrum,” that is, the energy spectrum of passively ionized electrons in a dielectric irradiated by an electron beam of moderate or high density which arises prior to electron-phonon relaxation is calculated. The source of the beam is usually a high-current electron accelerator. The computation algorithm is designed so as to make it possible to fully take into account and make use of the following data: first, the actual (measured) spectrum of the electron beam; second, the complete electron spectrum of the dielectric, including the spectrum of the density of occupied states, which is continuous within the valence bands and discrete in the region of the low-lying quasiatomic levels; and the spectrum of the density of unoccupied states, which begins in the conduction band and, in a continuous process, turns into the spectrum of quasi-free electrons in the crystal; third, the differential ionization cross-sections of the atoms due to electron impact, which the present report refines so as to take in into account the actual spectrum of the density of the final states; the latter spectrum is continuous within the valence bands and discrete in the region in which the law of dispersion of a band electron turns into the law of dispersion of a quasi-free electron. The results of the theoretical analysis are compared with the data of a numerical experiment carried out in the present report by the Monte-Carlo method using the same initial data. The agreement between the theoretical results and the data from the numerical experiment demonstrates the stability and reliability of the algorithm used to calculate the instantaneous spectrum. The precision of the calculation is determined solely by the initial data. The instantaneous spectrum which is obtained is the initial distribution function for all the kinetic equations describing subsequent relaxation of the electrons in the irradiated dielectric. Tomsk Polytechical Institute. Tomsk State University. Institute of High-Current Electronics, Russian Academy of Sciences (Siberian Division). Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 136–144, November, 1996.     

Moving inhomogeneity in a magnetohydrodynamic medium

The integral equations of ideal magnetohydrodynamics are derived by the Green’s function method, taking into account the velocity of the medium inside the inhomogeneity before and after its disturbance by the incident field. The extinction principle of of magnetohydrodynamics is demonstrated for a moving half space. A comparative analysis is made with the results of an analogous problem in which only the velocity of the interface between the two media is taken into account. Zh. Tekh. Fiz. 67, 6–11 (May 1997)     

Angular structure of laser radiation in a Markov binary mixture

Laser-radiation transfer in a Markov binary mixture is compared to transfer in an equivalent homogeneous medium. Within the framework of a small-angle approximation a system of equations that describe the angular moments of the second order (angular dispersion) for a laser beam and the equation for laser-beam intensity is obtained. The results of the calculations show a significant effect of the stochastic nature of the medium on the intensity and the angular structure of the laser beam. Deceased Institute of Applied Optics, National Academy of Sciences of Belarus, 11, Belynitskii-Birulya St., Mogilev, 272793. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 36–42, January–February, 1999.     

Influence of local dispersion on transient processes accompanying the generation of rf radiation by an electromagnetic shock wave

Transient processes accompanying the conversion of a video pulse into a radio pulse in a nonlinear transmission line having hysteretic properties are studied. It is established that the transition process leading to the establishment of “steady-state” (close in amplitude) oscillations has a minimum when the electromagnetic shock wave front is phase-matched with the wave excited by it at a frequency near the minimum local dispersion of the group velocity. Zh. Tekh. Fiz. 68, 89–95 (January 1998)     

Probing vacuum birefringence by phase-contrast Fourier imaging under fields of high-intensity lasers

In vacuum high-intensity lasers can cause photon–photon interaction via the process of virtual vacuum polarization which may be measured by the phase velocity shift of photons across intense fields. In the optical frequency domain, the photon–photon interaction is polarization-mediated described by the Euler–Heisenberg effective action. This theory predicts the vacuum birefringence or polarization dependence of the phase velocity shift arising from nonlinear properties in quantum electrodynamics (QED). We suggest a method to measure the vacuum birefringence under intense optical laser fields based on the absolute phase velocity shift by phase-contrast Fourier imaging. The method may serve for observing effects even beyond the QED vacuum polarization.     

光强对短脉冲在准周期光学超晶格中传输的影响

 在考虑自相位调制和高阶群速色散的情况下，分析了超短脉冲在准周期Fibonacci超晶格中传输时，入射光强对脉冲传输特性的影响。结果表明：入射光强度对脉冲传输特性影响很大，不但使频谱展宽，而且限制超晶格的有效长度，是超短脉冲进行频率转换的主要考虑因素之一。     

Magnetic properties of an amorphous microconductor in the microwave range

The frequency dispersion of the magnetic permeability is taken into account in a calculation of the impedance of an amorphous microconductor. Possible methods of measuring the magnetic parameters are analyzed, and the main characteristics of an amorphous microconductor in the microwave range are described. Zh. Tekh. Fiz. 68, 136–137 (January 1998)     

Two-period model for calculation of level populations in subbands of multi-period quantum-cascade superlattice structures

For two periods of quantum-cascade laser structures, we propose a system of closed balance equations that make it possible to calculate the occupancy of the energy levels, the quasi-Fermi levels, and also the injection current density taking into account different charge carrier scattering mechanisms. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 801–804, November–December, 2007.     

Modeling of the surface nucleation and growth on active centers from the vapor phase

The formation of nuclei of a new phase and their growth on the inhomogeneous substrate from a vapor phase are studied. Basic kinetic equations describing such a process are solved numerically. The effect of the depletion of active sites during the growth of nuclei is taken into account. Basic characteristics of the nucleation process, such as size distribution function, nucleation rate and number of nuclei formed on the unit surface are determined. It is shown that the size distribution of nuclei evolves by a nontrivial way as a function of time. This process is fully nonstationary from the viewpoint of nucleation rate. The total number of nuclei reaches the number of active centers for a sufficiently long time. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. This work was supported by Grant No. 202/99/0403 of the Grant Agency of the Czech Republic.