Generation of Subpicosecond Pulse Trains in Fiber Cascades Comprising a Cylindrical Waveguide with Propagating Refractive Index Wave

A cylindrical waveguide structure with the running refractive index wave has been recently demonstrated as a means for the generation of high-repetition-rate pulse trains. The operation mechanism involves a proper combination of the frequency modulation and modulation instability simultaneously experienced by the input continuous wave (CW) signal as it propagates through the cylinder waveguide. Here, we explore the same idea but employ the cylindrical waveguide only as a part of the cascaded optical fiber configuration now comprising both passive and active optical fiber segments. The new system design enables the improved control of the pulse train formation process in the cascaded system elements, relaxes strong requirements for the CW signal power, and provides an additional optical gain for the advanced pulse peak power scaling. In particular, using a low-amplitude, weakly modulated, continuous wave as an input signal we explore and optimize the nonlinear mechanisms underlying its cascaded transformation into the train of kilowatt peak power picosecond pulses.

Acoustic wave absorption in a waveguide with impedance boundary conditions

Object and purpose of research. The study of the acoustic pulse changes regularities during its propagation in con-fined media is one of the fundamental problems of acoustics, which allows to pose and solve the inverse problem of determining the dissipative and resonant properties of these media. The physical processes occurring during the propagation of a pulse in a cylindrical waveguide with rigid walls were investigated. Materials and methods. To analyze the mechanism of dissipation, experimental studies of pulse propagation in a hy-droacoustic tube were carried out, and the theoretical description of the obtained results was carried out using analytical methods. The simulation of the propagating pulse in the finite element waveguide model was used to confirm the theoretical assessments and the experiment. Main results. Experimental studies of physical processes during the propagation of an acoustic pulse in confined medium of cylindrical waveguide bounded by walls with characteristics close to absolutely rigid are carried out. The data showed that it is possible to control changes in the phase velocity, amplitude, and waveform, which made it possible to quantify the impedance of the internal walls of the waveguide and the dissipation of acoustic energy with a sufficient degree of accuracy. The numerical model calculation, taking into account the theoretically obtained quantitative assessments of the dissipation values and the impedance value of the waveguide inner surface, showed a good correspondence between the model and experimental characteristics of the change in the propagating pulse. Conclusion. In the studies devoted to the propagation description of acoustic waves in waveguides, the issues of energy dissipation are usually not considered, especially in cases where it has a weak effect on the measurement result. The theoretical value of the research is to quantify the wave energy dissipation by the parameters that can be determined with sufficient accuracy in the experiment: the phase velocity, the pulse form. Further accuracy improvement of the experimental data, especially in a wide frequency range, will improve the theoretical model of dissipation by taking into account the mechanism of inhomogeneous viscous and thermal waves near the inner surface of the waveguide. The practical significance of the research is to increase the reliability of experimental data and to develop physical and mathematical models of underwater sound absorption due to a forced variable flow with a highly transformed velocity of a viscous liquid in a thin surface layer near the elastic wall.

Numerical Design and Experimental Validation of a Plastic 3D-Printed Waveguide Antenna for Shallow Object Microwave Imaging

Microwave imaging of shallow buried objects has been demonstrated with holographic radar for landmine detection, civil engineering and cultural heritage. A key component of this system is the antenna based on a truncated cylindrical waveguide with two feeds. This paper investigates for the first time a manufacturing technology based on the 3D printing of a volumetric cylindrical plastic antenna. The investigation of this manufacturing technology was motivated by the reduction in the antenna size and customization of the electromagnetic characteristics to the radio frequency electronics mounted on the robotic scanning system. The antenna that was designed using a simulator and filled with polylactic acid plastic material (relative dielectric permittivity Ɛr = 2.5) is compared to the metal antenna, both operating at around 2 GHz. The goal was to replicate the characteristics of the void core antenna to be able to provide the same quality/information of the microwave images of shallow buried objects. Finally, we compared the scan results of dielectric and metal targets both in the air and in natural soil. From the observation of some of the characteristics of the images, such as dynamics, morphology of the target, signal-to-noise ratio, and operating distance, we demonstrate that 3D printing for volumetric cylindrical waveguide antenna could be used to obtain compact and easily adaptable antennas for different applications in remote sensing.