Twisted Waves near a Plasma Cutoff

This work considers twisted wave propagation in inhomogeneous and unmagnetised plasma, and discusses the wave properties in the cutoff region. The qualitative differences between twisted waves described by a single Laguerre–Gauss (LG) mode, and light springs resulting from the superposition of two or more LG modes with different frequency and helicity are studied. The peculiar properties displayed by these waves in the nonuniform plasma are discussed. The pulse envelope of a light-spring shows a contraction at reflection, which resembles that of a compressed mechanical spring. The case of normal incidence is examined, and nonlinear ponderomotive effects are discussed, using theory and simulations.

Estimation of Probability Distributions of Geoacoustic Signal Characteristics

The paper presents an estimation of probability distributions of geoacoustic signal characteristics. The studied signals have a pulsed nature. The ones have been recording at the geodynamic test site of the IKIR FEB RAS (Kamchatka Peninsula) for more than 20 years. To estimate the distribution of characteristics, such time intervals were determined in which histograms of the distribution did not change. The following characteristics were chosen for the estimation: maximum amplitude, the position of pulse envelope maximum, duration, filling frequency, and pulse-to-pulse interval. The obtained estimates made it possible to develop an empirical model of the geoacoustic emission signal. The model can help to test new and existing algorithms for the processing and analysis of geoacoustic signals. The paper also shows that the formalization of the selected characteristics makes it possible to search for anomalies, including those associated with seismic events, by the characteristic variations.

Band-Gap Solitons in Nonlinear Photonic Crystal Waveguides and Their Application for Functional All-Optical Logic Gating

This review paper summarizes our previous findings regarding propagation characteristics of band-gap temporal solitons in photonic crystal waveguides with Kerr-type nonlinearity and a realization of functional and easily scalable all-optical NOT, AND and NAND logic gates. The proposed structure consists of a planar air-hole type photonic crystal in crystalline silicon as the nonlinear background material. A main advantage of proposing the gap-soliton as a signal carrier is that, by operating in the true time-domain, the temporal soliton maintains a stable pulse envelope during each logical operation. Hence, multiple concatenated all-optical logic gates can be easily realized paving the way to multiple-input ultrafast full-optical digital signal processing. In the suggested setup, due to the gap-soliton features, there is no need to amplify the output signal after each operation which can be directly used as a new input signal for another logical operation. The efficiency of the proposed logic gates as well as their scalability is validated using our original rigorous theoretical formalism confirmed by full-wave computational electromagnetics.

High Power Linearly Polarized Fiber Lasers in a Linear Cavity

This thesis presents two designs for high power linearly polarized all-fiber linear cavity lasers, continuous wave (CW) and mode-locked. The cavity designs use Polarization Maintaining (PM) fibers for both gain medium and Fiber Bragg Gratings (FBGs). The FBG pairs select lasing wavelength and polarization. The fiber lasers incorporating specialty designed FBGs achieve an extinction ratio larger than 23 dB. Firstly, an all-fiber linear cavity design of a high power picoseconds mode-locked laser is introduced. The proposed configuration is based on Non-Linear Polarization (NPR) using PM Yb-doped active fiber and two matching FBGs to form the laser cavity. The combination of nonlinearity, gain and birefringence in cavity made the laser generate mode-locked pulses in picoseconds range and with high average output power. The output mode-locked pulses amplitude is modulated with an envelope whose mechanism is also investigated in this thesis project. Experimental data and numerical simulations of the self mode-locking fiber laser are presented. Main parameters affecting mode-locked pulses and its envelope are identified. In addition, a new theoretical model based on Nonlinear Schrödinger Equation (NLSE) is developed and implemented on the MATLAB platform. The model explains the self mode-locking mechanism and the source of the pulse envelope. In this model, it is proven that self phase modulation (SPM) plays an essential role in pulse formation and shaping. The theoretical model and experimental results are in a very good agreement at different pumping levels. A method of regulating the mode-locked pulses is presented. This is achieved by applying a pulsed current to pump diode. This method successfully stabilizes the mode-locked pulses underneath a Q-switched pulse envelope. Further scale-up of average power and pulse energy is realized by adding an amplifier stage. Secondly, a CW dual-wavelength all-fiber laser is presented. The laser consists of two pairs of FBGs and a PM Er/Yb co-doped fiber as a gain medium. The laser emits at both 1 μm and 1.5 μm wavelengths simultaneously with a stable output. This laser provides a compact fiber-based pumping source that is suitable for mid-IR generation.

High Power Linearly Polarized Fiber Lasers in a Linear Cavity

This thesis presents two designs for high power linearly polarized all-fiber linear cavity lasers, continuous wave (CW) and mode-locked. The cavity designs use Polarization Maintaining (PM) fibers for both gain medium and Fiber Bragg Gratings (FBGs). The FBG pairs select lasing wavelength and polarization. The fiber lasers incorporating specialty designed FBGs achieve an extinction ratio larger than 23 dB. Firstly, an all-fiber linear cavity design of a high power picoseconds mode-locked laser is introduced. The proposed configuration is based on Non-Linear Polarization (NPR) using PM Yb-doped active fiber and two matching FBGs to form the laser cavity. The combination of nonlinearity, gain and birefringence in cavity made the laser generate mode-locked pulses in picoseconds range and with high average output power. The output mode-locked pulses amplitude is modulated with an envelope whose mechanism is also investigated in this thesis project. Experimental data and numerical simulations of the self mode-locking fiber laser are presented. Main parameters affecting mode-locked pulses and its envelope are identified. In addition, a new theoretical model based on Nonlinear Schrödinger Equation (NLSE) is developed and implemented on the MATLAB platform. The model explains the self mode-locking mechanism and the source of the pulse envelope. In this model, it is proven that self phase modulation (SPM) plays an essential role in pulse formation and shaping. The theoretical model and experimental results are in a very good agreement at different pumping levels. A method of regulating the mode-locked pulses is presented. This is achieved by applying a pulsed current to pump diode. This method successfully stabilizes the mode-locked pulses underneath a Q-switched pulse envelope. Further scale-up of average power and pulse energy is realized by adding an amplifier stage. Secondly, a CW dual-wavelength all-fiber laser is presented. The laser consists of two pairs of FBGs and a PM Er/Yb co-doped fiber as a gain medium. The laser emits at both 1 μm and 1.5 μm wavelengths simultaneously with a stable output. This laser provides a compact fiber-based pumping source that is suitable for mid-IR generation.

Microwave Induced Thermoacoustic Imaging With Multi-Pulse in Geological Application

The accurate and real-time monitoring of fluid flow in porous media can boost the prediction of mass transport and chemical reactions, which profoundly impacts the subsurface exploration and hydrocarbon extractions. Our preliminary effort has shown the efficacy of employing a thermoacoustic (TA) technology for imaging an immobile rock sample. The results support the applicability of making this methodology to move forward for imaging a dynamical process. But the real-time monitoring of fluid flow requires the target under test to excite TA signals with a higher signal-to-noise ratio (SNR), which will promise a sufficient image resolution with fewer necessary measurements or less averaged times, and then lead to a faster scan. It is proved that the excitation pulse is directly proportional to the microwave absorption rate, and thus determines the observability of the corresponding TA signals. Unfortunately, due to the thermal and stress confinements, a microsecond-width pulse envelope is greatly limited and is not sufficient for achieving a high SNR. Although a recently proposed Frequency Modulation Continuous Wave (FMCW) showed an improvement on SNR, it signifies a deficiency of the long-time irradiation and additional electronic disturbance especially at a high peak power. To address this issue, we propose a new excitation envelope with multi-pulses, to favor the coherent frequency-domain signaling method for optimizing the image reconstruction while shortening the total envelope duration than that of the FMCW. In the present paper, the TA sensing of a dry sandstone sample is presented, which efficiently enhances the SNR of TA signals and the image resolution, thus validating the appropriateness of the proposed multi-pulse envelope. The current study also promises a future possibility towards its application for dynamically exploring the underground flow transport.