Inverse four-wave mixing and self-parametric amplification in optical fibre

An important group of nonlinear processes in optical fibre involve the mixing of four waves due to the intensity dependence of the refractive index. It is customary to distinguish between nonlinear effects that require external/pumping waves (cross-phase modulation and parametric processes such as four-wave mixing) and those arising from self-action of the propagating optical field (self-phase modulation and modulation instability). Here, we present a new nonlinear self-action effect—self-parametric amplification—which manifests itself as optical spectrum narrowing in normal dispersion fibre, leading to very stable propagation with a distinctive spectral distribution. The narrowing results from inverse four-wave mixing, resembling an effective parametric amplification of the central part of the spectrum by energy transfer from the spectral tails. Self-parametric amplification and the observed stable nonlinear spectral propagation with a random temporal waveform can find applications in optical communications and high-power fibre lasers with nonlinear intracavity dynamics

A new type of macrophytic algae from Permian deposits of the North-East of the Samara region (Locality Russky Baitugan)

A new species of algae from the formal genus Algites Seward, 1894 — A. samarensis sp. nov. from the Lower Kazanian substage near the village of Russky Baitugan (northeast of the Samara Region) has been described. The cases offindingsof the fossilized Paleozoic macrophytic algae are not numerous. Each such finding is an important link for analyzing the evolution of algae as a whole. The literature review and geological characteristics of the area have been briefly presented. The related taxonomic composition of the fossilized fauna of this location, including brachiopods, bivalves, solitary corals, bryozoans, crinoids, and conularia, has been considered. A description of the morphological features of a new type of algae and a diagnosis in English have been given. The characteristic of phyllotaxy has been presented. On the basis of the results obtained in the course of the study, it has been concluded that the new described species belongs to macrophytic colonial sea algae. A comparison with the Paleozoic and Mesozoic macrophyte algae has been made, and the results have been analyzed. The main differences of the new form are the double dichotomous, even, thin and long thallome with the presence of the side needle-like «leaves», with the whorled location of the «leaves». It has been established that the burial of the described fossil algae occurred in a short period of time directly on the inhabitation or near their habitat. The well preservation of the algae without severe damage is an evidence of this.</jats:p