Table of contents

A review of studies on lidar sensing of the environment by the method of IR differential absorption is presented. The differential-absorption method is described and its various applications are considered. A comparison of this method with other methods of lidar sensing showed that a differential-absorption lidar successfully supplements a Raman lidar. The basic parameters are presented for IR lidars fabricated recently by various research groups. The outlook for the IR lidar sensing of the atmosphere is discussed.

The supercontinuum conical emission of a 50-fs laser pulse focused into a K 108 glass is studied experimentally and numerically. It is found that, as the pulse energy was increased from 2 to 30 μJ, the continuous picture of conical emission decomposed into speckles upon focusing with a lens and split into narrow rings upon focusing with an axicon.

The features of propagation of femtosecond laser pulses in the atmosphere are studied by numerical methods. The type of propagation is determined by the relation between processes of multiphoton ionisation and multiple small-scale self-focusing (filamentation). It is found that the process develops step by step and filamentation occurs without the pulse energy loss until the intensity threshold is achieved. After the achievement of the threshold, ionisation losses appear which are accompanied by the defocusing action of the electron plasma resulting in the intensity decrease. When the intensity becomes lower than the ionisation threshold, self-focusing can again appear. The characteristic features of the final stage of filamentation can be explained by the Bespalov—Talanov theory. By using the time-averaged equations and parallel methods, a program complex is developed for qualitative simulations of the propagation of high-power laser pulses over long distances.

The propagation of an optical ultrashort pulse in a resonance Bragg grating is considered taking into account the polarisation of electromagnetic radiation. It is assumed that the grating is formed by thin films containing two-level atoms with the triply degenerate upper energy level. The system of equations is derived for the envelopes of electromagnetic pulses counterpropagating in such a grating. In the long-wavelength (continual) approximation, the system of equations generalising the known system for scalar waves is obtained. The solutions corresponding to elliptically (in particular, linearly and circularly) polarised stationary pulses are found. An arbitrary degree of ellipticity is possible only in a medium with a preliminary prepared stage of resonance atoms.

The scenario of the experiment on the amplification of gamma rays upon stimulated transitions from metastable states of isomeric nuclei in a Bose—Einstein condensate (BEC) is presented. The experiment involves the successive transport of atoms along the so-called quantum conveyer. It is shown that if the excess broadening of the gamma-ray emission line is eliminated, the total gain G > 1 can be achieved despite the presence of negative factors such as the 'laser lethargy', spontaneous decay of metastable states, etc.

A device for optimal coupling between a laser resonator and the external medium, having the form of one-dimensional wire grating conjugated with a plane mirror with an aperture, is proposed, developed and tested experimentally. The dependences of the output laser power on the separation between the grating and the mirror, diameter of the aperture in the plane mirror, and the grating period, are studied. The obtained results not only confirm the possibility of using such a coupling device, but also point towards the ways and principles of its application.

It is shown with the help of numerical simulation that various mechanisms of self-oscillation instability exist and self-oscillation perturbations of various types may appear in an unstable resonator with a transverse flow of the active medium and a nonuniform internal pumping. The interaction of different self-oscillation perturbations, manifested in pulling and locking of oscillation frequencies and a variation of their increments, is studied. Analytic relations describing the spatial structure of perturbation modes and used for evaluating their frequencies and increments from steady-state generation parameters are presented.

The distribution of polarisation of a light field in the cross section of a beam at the sum frequency is investigated upon the collinear interaction of two elliptically polarised Gaussian beams in a nonlinear isotropic gyrotropic medium. It is shown that the ellipticity, the angle of rotation of the principal axis of the polarisation ellipse, and the rotation direction of the electric field vector of radiation at the sum frequency in the beam cross section strongly depend on the angle in the polar coordinate system. The ranges of parameters of elliptically polarised fundamental Gaussian beams are found where the cross section of the sum-frequency beam is divided into sectors with different rotation directions of the electric field vector. The equations of the straight lines determining the boundaries of these sectors contain parameters specifying the shape and orientation of polarisation ellipses of the fundamental waves and the ratio of their wave vectors. In the case of opposite circular polarisations of these waves, the ellipticity of the sum-frequency beam does not change in the beam cross section and the principal axes of polarisation ellipses of the light field are oriented perpendicular to the radius in polar coordinates.

Configurations of unstable resonators are considered depending on the basic parameters of a high-power chemical oxygen—iodine laser and the design of an unstable resonator is proposed which provides the compensation of the inhomogeneity of the small-signal gain downstream of the active medium, a high energy efficiency, and stability to intracavity aberrations. The optical scheme of this resonator is presented and its properties are analysed by simulating numerically the kinetics of the active medium and resonator itself in the diffraction approximation.

Actual aspects of using a new method of laser interference microscopy (LIM) for studying nerve cells are discussed. The peculiarities of the LIM display of neurons are demonstrated by the example of isolated neurons of a pond snail Lymnaea stagnalis. A comparative analysis of the images of the cell and subcellular structures of a neuron obtained by the methods of interference microscopy, optical transmission microscopy, and confocal microscopy is performed. Various aspects of the application of LIM for studying the lateral dimensions and internal structure of the cytoplasm and organelles of a neuron in cytology and cell physiology are discussed.

The problem of data communication over a wavelength division multiplexing (WDM) high-data-rate communication link where wave division multiplexing is performed by using quasi-rectangular Bragg filters is studied numerically. It is shown that the optimisation of the mean dispersion of the link, the width and shape of the transfer function of filters provides the increase in the data communication range up to 1550 km.

Two related problems are studied by numerical simulations using the KARAT code: the reflection of the TM₀₁ mode of an electromagnetic pulse from the subcritical taper of the section of a circular waveguide and the reflection of the same pulse from a 'cold' collisionless plasma with a density increasing up to a supercritical value along the waveguide axis. It is shown that in the former case the pulse is totally reflected with an insignificant distortion of its shape, in accordance with the linear theory. In the latter case, the character of reflection depends substantially on the plasma density increase length, the pulse duration, and the wave field amplitude, a significant field deceleration and amplitude growth occurring near the critical point; the pulse absorption in the plasma far exceeds the absorption due to the linear transformation of the incident transverse wave to the longitudinal plasma oscillations.

The passage of CF₃I molecules excited by high-intensity IR laser radiation to high vibrational states (with energy E_v ≥ 0.3–1.5 eV) and unexcited molecules in a pulsed beam through a converging truncated hollow metal cone cooled to T_s ≈ 80–85 K and mounted at an angle to the beam axis is studied. It is found that the excited molecules pass much more readily through the cone than the unexcited (vibrationally cold) molecules. This opens the possibility for studying the processes of energy transfer and redistribution over a cold surface covered by molecular (cluster) layers, and for separating excited and unexcited molecules in a beam.