Anal incontinence after lateral internal sphincterotomy is directly related to the length of the sphincterotomy. Whether secondary to preoperative sphincter abnormality or the result of lateral internal sphincterotomy, the external sphincter is thinner in incontinent patients than in continent controls.

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One of the unique features of mirrorless optical parametric oscillators based on counterpropagating three-wave interactions is the narrow spectral width of the wave generated in the backward direction. In this work, we investigate experimentally and numerically the influence that a strong phase modulation in the pump has on the spectral bandwidths of the parametric waves and on the efficiency of the nonlinear interaction. The effects of group-velocity mismatch and group-velocity dispersion are elucidated. In particular, it is shown that the substantial increase in temporal coherence of the backward-generated wave can be obtained even for pumping with a temporally incoherent pump. A configuration of a mirrorless optical parametric oscillator is proposed where this gain in spectral coherence is maximized without a penalty in conversion efficiency by employing group-velocity matching of the pump and the forward-generated parametric wave.

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We consider the basic problem of the parametric generation process from an incoherent pump wave. The analysis of the degenerate configuration of the two-wave interaction reveals that the mutual convection (i.e., group-velocity difference) between the incoherent pump and the signal (i.e., the daughter wave) may quench their parametric interaction, so that the gain experienced by the signal may become arbitrarily small. Conversely, in the absence of convection, the incoherent pump efficiently amplifies the signal wave, although this amplification process cannot lead to the generation of a coherent signal. However, in the case of nondegenerate three-wave interaction, we show the existence of a convection-induced phase-locking mechanism in which the incoherence of the pump is absorbed by the idler wave allowing the signal wave to grow efficiently with a high degree of coherence. We calculate explicitly the autocorrelation function of the generated signal in this regime of coherent-incoherent interaction. The analysis reveals that, owing to the convection-induced averaging process that accompanies the phase-locking mechanism, the degree of coherence of the signal increases as the degree of coherence of the pump decreases. We establish the experimental conditions that would allow for the observation of the transition between the incoherent and the coherent regimes of the three-wave parametric interaction.

An anisotropic plasma exhibits a transverse instability due to the mutual attraction of the microscopic current elements. A detailed study is made of this instability in an electron-ion plasma without any external magnetic field. The system is characterized by a temperature tensor and described by a quasi-Maxwellian distribution function. The dielectric tensor for such a system is calculated and then the dispersion relation is considered. It is found that there are two unstable modes: (i) a purely transverse mode and (ii) a coupled or transverse-longitudinal mode. The latter separates into two branches: (a) the first is an essentially unstable quasitransverse mode, (b) the second is the quasi-longitudinal ion acoustic mode driven into instability through its interaction with the quasitransverse mode. Growth rate, k and angular dependence, domain of instability, and polarization are analyzed in detail.

We present a solitary solution of the three-wave nonlinear partial differential equation ͑PDE͒ modelgoverning resonant space-time stimulated Brillouin or Raman backscattering-in the presence of a cw pump and dissipative material and Stokes waves. The study is motivated by pulse formation in optical fiber experiments. As a result of the instability any initial bounded Stokes signal is amplified and evolves to a subluminous backscattered Stokes pulse whose shape and velocity are uniquely determined by the damping coefficients and the cw-pump level. This asymptotically stable solitary three-wave structure is an attractor for any initial conditions in a compact support, in contrast to the known superluminous dissipative soliton solution which calls for an unbounded support. The linear asymptotic theory based on the Kolmogorov-Petrovskii-Piskunov assertion allows us to determine analytically the wave-front slope and the subluminous velocity, which are in remarkable agreement with the numerical computation of the nonlinear PDE model when the dynamics attains the asymptotic steady regime. ͓S1063-651X͑97͒13201-9͔