Polarization dynamics of intracavity frequency-doubled Nd:YAG laser

Experimental study of low-frequency dynamics of an intracavity frequency-doubled Nd:YAG laser demonstrates the influence of the interaction of orthogonally polarized modes, participating in frequency doubling (type II phase matching), on the stability of the laser output. At a sufficiently low pump rate and low conversion efficiency, the laser shows stable operation with a low noise level at the frequencies of relaxation oscillations. At a high pump power and/or a high conversion efficiency, the laser emission becomes unstable as a result of Hopf bifurcation at the frequencies of relaxation oscillations that are responsible for the anti-phase polarization dynamics of the laser.     

Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

Identification of a dissipation coefficient by a variational method

A generalized inverse problem for the identification of the absorption coefficient for a hyperbolic system is considered. The well-posedness of the problem is examined. It is proved that the regular part of the solution is an L ₂ function, which reduces the inverse problem to minimizing the error functional. The gradient of the functional is determined in explicit form from the adjoint problem, and approximate formulas for its calculation are derived. A regularization algorithm for the solution of the inverse problem is considered. Numerical results obtained for various excitation sources are displayed.     

Sub-Doppler absorption resonances induced by strong radiation

New possibilities are demonstrated for eliminating uncompensated Doppler broadening in different types of nonlinear optical processes by means of atomic coherence effects in strong electromagnetic fields are demonstrated. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 12, 964–968 (25 June 1998)     

Nonequilibrium gas absorption in rotating permeable media

The absorption of ammonia, sulfur dioxide, and carbon dioxide by water and aqueous solutions in rotating permeable media, a cellular porous disk, and a set of spaced-apart thin disks has been considered. The efficiency of cleaning air to remove these impurities is determined, and their anomalously high solubility (higher than equilibrium value) has been discovered. The results demonstrate the feasibility of designing cheap efficient rotor-type absorbers to clean gases of harmful impurities.     

On the numerical modeling of the dynamics of homogeneous isotropic turbulence

Using a series of mathematical models based on the closed Karman-Howarth equation, the numerical modeling of the dynamics of homogeneous isotropic turbulence was carried out. Computational results agree with known experimental data. Mathematical models are compared.     

Solving the Problem of Gravitational Compression of a Layered Sphere (by the Example of the Earth)

The problem of spherically symmetric, gravitational compression of an isotropic hyperelastic layered sphere which modeling the region of the Earth below the Mohorovii boundary is solved. The known mechanical characteristics of the Earth in the compressed state are used to find its characteristics in the unstrained state obtained by adiabatic or isothermal stress relief. The stress state differs significantly from the state of purely hydrostatic compression. The minimum bulk compression and maximum radial tension occur not on the boundary of the sphere but in depth at certain distances from the boundary.     

Intracavity laser spectroscopy

The main features of broadband intracavity laser spectroscopy (ILS), the most sensitive technique of absorption-spectrum measurements, is investigated. The factors that limit the sensitivity of the ILS method are studied, and the conditions for reaching its maximum sensitivity are formulated. The main practical applications of the ILS method are described, such as detection of weak absorption spectra and measurements of low gas concentrations, registration of amplification spectra and of the spectral distribution of external radiation, measurement of the cross sections for nonlinear processes, investigation of transient processes in an electric discharge, and investigation of the kinetics of multistage chemical reactions.Translated from Preprint No. 187 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1988.