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Probe pulse preparation techniques play an important role for fiber-optic sensing. In this work, we investigate a setup for distributed sensors based on phase-sensitive optical time domain reflectometry ($\Phi$-OTDR) with the use of an erbium doped fibre amplifier (EDFA). In our $\Phi$-OTDR setup with pulse mode EDFA, we reveal a regime providing us both sufficiently large amplification of the probe pulse power together and adequately small probe pulse distortion. We then show that the use of pulse mode EDFA is a cost-effective solution, which allows one to avoid nonlinear effect and ensure sufficient sensitivity of the setup., Comment: 6 pages, 3 figures; improved version presented at the 38th Progress In Electromagnetics Research Symposium (St. Petersburg, May 22-25, 2017)

We propose a novel scheme for laser phase noise measurements with minimized sensitivity to external fluctuations including interferometer vibration, temperature instability, other low-frequency noise, and relative intensity noise. In order to minimize the effect of these external fluctuations, we employ simultaneous measurement of two spectrally separated channels in the scheme. We present an algorithm for selection of the desired signal to extract the phase noise. Experimental results demonstrate potential of the suggested scheme for a wide range of technological applications., Comment: 4 pages, 4 figures; published version

The influence of the laser frequency drift on the operation of phase-sensitive optical time-domain reflectometry ($\Phi$-OTDR) systems is considered. Theoretical results based on a new numerical $\Phi$-OTDR model demonstrating the influence of the laser frequency instability on a signal are reported. This model is verified based on experimental data. It has been used to calculate the signal-to-noise ratio (SNR) of the system for different parameters of the laser source stability. As a result, quantitative requirements for lasers used in $\Phi$-OTDR systems are formulated., Comment: 8 pages, 7 figure

An extension of hydrodynamic D'Angelo mode of inhomogeneous sheared plasma flow along the magnetic field into the short-wavelength limit, where the hydrodynamic treatment is not valid, has been considered. We find that D'Angelo mode in this wavelength range is excited by inverse ion Landau damping and becomes the shear flow driven ion-kinetic mode., Comment: 9 pages, 1 figure

The electrostatic shear-flow-driven ion cyclotron instability of magnetic field aligned sheared plasma flow is investigated analytically. It is shown that the shear-flow-driven electrostatic ion cyclotron instability can be considered as a combination of three different instabilities determined by theirs own mechanism of excitation: ion-kinetic, ion-hydrodynamic and electron-kinetic. Each of these instabilities are dominant in different ranges of the wavelength along the magnetic field. The linearized dispersion equation is solved within the limits of long and short waves along the magnetic field where effects of electrons and ions, respectively, are dominant in the development of these instabilities. The general criterion of instability excitation, which couples the flow velocity shear and wave number across the magnetic field, is obtained.

The linear and renormalized nonlinear kinetic theory of drift instability of plasma shear flow across the magnetic field, which has the Kelvin's method of shearing modes or so-called non-modal approach as its foundation, is developed. The developed theory proves that the time-dependent effect of the finite ion Larmor radius is the key effect, which is responsible for the suppression of drift turbulence in an inhomogeneous electric field. This effect leads to the non-modal decrease of the frequency and growth rate of the unstable drift perturbations with time. We find that turbulent scattering of the ion gyrophase is the dominant effect, which determines extremely rapid suppression of drift turbulence in shear flow.

The linear and renormalized nonlinear analysis of the temporal evolution of drift-type modes in plasma flows with strong time-varying velocity shear is developed. Analysis is performed in the time domain without spectral decomposition in time and admits time variation of the flow velocity with time scales which may be comparable with turbulent time scales. 52.35.Ra, Comment: 15 pages

The analytical treatment of nonlinear evolution of the shear-flow-modified current driven ion cyclotron instability and shear-flow-driven ion cyclotron kinetic instabilities of magnetic field--aligned plasma shear flow is presented. Analysis is performed on the base of the nonlinear dispersion equation, which accounts for a new combined effect of plasma turbulence and shear flow. It consists in turbulent scattering of ions across the shear flow with their convection by shear flow and results in enhanced nonlinear broadening of ion cyclotron resonances. This effect is found to lead to the saturation of ion cyclotron instabilities as well as to the development of nonlinear shear flow driven ion cyclotron instability. 52.35.Ra, Comment: 21 pages