Table of contents

Theoretical models and methods for calculating the characteristics of distributed-feedback lasers for the optical range are examined. An asymptotically exact method for introducing an effective Q is described. This method reduces the description of the generation of a distribution-feedback laser to simple balance equations for a system with lumped parameters. The generation regimes of lasers with a light-induced stochastic distributed feedback are examined. The possibility of an optical bistability, the generation of ultrashort pulses, and a fluctuation mechanism for a lowering of the threshold are studied. Explicit expressions convenient for practical calculations of parameters in the generation of ultrashort pulses are derived. Experimental results are reviewed. The distinctive features and advantages of distributed-feedback lasers as sources of ultrashort light pulses are analyzed.

Two problems involving the propagation of laser radiation in a distributed-feedback system are discussed. One involves the transformation of noise and a determination of the conditions under which quantum nondemolition measurements are possible. The second problem concerns an experiment on the induction by light of hydrodynamic instabilities (of the Rayleigh–Bénard convection type) in an anisotropic liquid (the treatment is classical). The discussion stresses the possibility of observing these states, which are of fundamental importance to physics, in the field of stable cw lasers with high coherence properties. This group of problems is related to one of the scientific interests of professor S. A. Akhmanov in the later years of his life. The present study, carried out under the strong influence of his ideas, is devoted to his memory.

A new method is proposed for carrying out calculations on the propagation of coherent pulses through optical systems which contain dispersive elements, modulators, and regions of a dispersive medium. In this method, a matrix analogous to a ray matrix in the optics of Gaussian beams is associated with each element of the optical system. A multiplication of the matrices of the individual elements of the system yields the matrix of the overall system. The latter matrix can be used to construct an integral relation which describes the propagation of a coherent pulse through an optical system of a general type. This relation can be used to construct an algorithm for analyzing the propagation of coherent pulses through optical systems. This algorithm is analogous to the method of Gaussian beams. It substantially simplifies the analysis and makes it more graphic. The method is illustrated by several examples.

A method is proposed for substantially reducing the length of superradiance pulses and increasing their intensity. The idea is to introduce an additional coherent resonant component, with a higher Rabi frequency, in a superradiance medium. The superradiance intensity of a two-component medium of this sort can be substantially higher than that of either component individually. The pump pulse can be several orders of magnitude longer than the generation pulse. The parameters of the generation pulse are controlled by varying the length and composition of the active medium.

A quantum theory is derived for the propagation of two electromagnetic waves in an anisotropic medium with a cubic nonlinearity. In general, the dynamics are described by a Hamiltonian that corresponds to an anisotropic two-dimensional anharmonic oscillator. The degree of suppression of quantum fluctuations in the quadrature component of one wave decreases when another wave is present. When the nonlinear phase corrections associated with self-interaction of the waves are different, it is possible to create a polarization state of the optical field that is nonclassical, with quantum fluctuations in one of the Stokes parameters smaller than in the coherent state.

We investigate the possibility of scaling "Brillouin" combiners up to pump intensity levels ∼ 0.2 GW/cm² at an energy of 15 J and a wavelength λ_p= 1.06 μm. We obtain a simple numerical criterion for operational stability of a system consisting of a master oscillator, an amplifier, and a combiner. We analyze the parasitic noise in the combiner. We show experimentally that the combiner possesses an energy efficiency near 0.8 while preserving the diffraction-limited directivity of the amplified signal. We have simulated the experimental situation in the periodic-pulse regime.

Exact, previously unknown solutions are derived for the system of equations for the slowly varying amplitudes of the circularly polarized components of an optical field. These solutions describe solitary waves with nontrivial degrees of ellipticity. The ranges of parameter values of an isotropic nonlinear gyrotropic medium in which various types of solutions are possible are determined. A numerical analysis of their stability is carried out.

We investigate the changes in the frequency spectrum of a partially coherent pulse brought on by a Kerr nonlinearity and by multiphoton ionization in a gas. We find that fluctuations in the parameters of the laser light cause the average spectrum of a pulse in a medium with a Kerr nonlinearity to be symmetric, with a single maximum at the center frequency. In the case of free-carrier generation, a long wing forms in the average spectrum that extends into the anti-Stokes region. We show that the ionization-induced nonlinearity gives rise to two time scales in the correlation function of the light.

A multistage amplifier utilizing counterpropagating stimulated scattering is proposed. This idea has been implemented experimentally. This design makes it possible to achieve characteristics in a counterpropagating amplifier which approach those of a copropagating amplifier. In a two-stage amplifier with an overall gain of 20 and at an input signal level equal to 10⁻⁵ of the pump level, it is possible to achieve a gain coefficient ∼ 10⁵ and a conversion efficiency ∼ 0.95.

Energy exchange in a parametric amplifier in which the frequencies involved are multiples of one another is analyzed. There can be a complete conversion of the energy of a pump wave at the frequency 3ω into a signal wave at 2ω. The conversion efficiency is calculated as a function of the wave detunings.

The self-focusing of wave beams in a nondispersive medium with a cubic nonlinearity is analyzed. The analysis is based on a field equation analogous to a nonlinear Schrödinger equation. Waves whose temporal profile is a periodic train of shock waves are studied. Calculations are carried out in the aberrationless approximation. The cubic nonlinearity causes not only a self-focusing but also, because of the shock fronts, an absorption of a wave. The self-focusing, however, is only slight. Although the beam does contract, the wave amplitudes undergo essentially no increase.

The Hartmann probe-beam method has been used to measure the first ten aberration coefficients of Nd:YAG and Nd:YAlO₃ crystals. The pump power was varied over the range 0.7–3.5 kW.

A new instrument consisting of an autocorrelator and a recording device is capable of rapid analysis of pulse lengths over the range 0.2–50 ps. The operation of this instrument is described. The autocorrelator determines the autocorrelation function of ultrashort laser pulses from the noncollinear generation of the second harmonic of the light under study. The recording device records the spatial distribution of the light with the help of a linear charge-coupled-device array.

Resonant absorption at the fundamental frequency makes second-harmonic generation irreversible. The absorption losses decrease as the conditions approach the optimum conditions for generation.

Ultrashort pulses, 85 fs long, have been generated in a Cr:Mg₂SiO₄ (forsterite) laser pumped by a cw Nd:YAG laser. Self-mode locking was observed in experiments with no intracavity elements except two prisms used to compensate for group-velocity dispersion.