Badr Mohamed Ibrahim Shalaby

Spatial beam self-cleaning in multimode fibres

The Kerr effect in graded-index multimode fibres drives a spatial beam self-cleaning phenomenon that withstands fibre bending and does not necessitate dissipative processes such as stimulated scattering. Multimode optical fibres are enjoying renewed attention, boosted by the urgent need to overcome the current capacity crunch of single-mode fibre (SMF) systems and by recent advances in multimode complex nonlinear optics1,2,3,4,5,6,7,8,9,10,11,12,13. In this work, we demonstrate that standard multimode fibres (MMFs) can be used as ultrafast all-optical tools for the transverse beam manipulation of high-power laser pulses. Our experimental data show that the Kerr effect in a graded-index (GRIN) MMF is the driving mechanism that overcomes speckle distortions, and leads to a counterintuitive effect that results in a spatially clean output beam robust against fibre bending. Our observations demonstrate that nonlinear beam reshaping into the fundamental mode of a MMF can be achieved even in the absence of a dissipative process such as stimulated scattering (Raman or Brillouin)14,15.

Spatiotemporal characterization of supercontinuum extending from the visible to the mid-infrared in a multimode graded-index optical fiber

We experimentally demonstrate that pumping a graded-index multimode fiber with sub-ns pulses from a microchip Nd:YAG laser leads to spectrally flat supercontinuum generation with a uniform bell-shaped spatial beam profile extending from the visible to the mid-infrared at 2500 nm. We study the development of the supercontinuum along the multimode fiber by the cut-back method, which permits us to analyze the competition between the Kerr-induced geometric parametric instability and stimulated Raman scattering. We also performed a spectrally resolved temporal analysis of the supercontinuum emission.

Polychromatic filament in quadratic media: spatial and spectral shaping of light in crystals

We demonstrate that monochromatic infrared laser pulses can generate polychromatic light in noncentrosymmetric crystals simultaneously covering the ultraviolet, visible, and infrared domains. The spatial shape of the beam and its energy can influence this multicolor frequency conversion, unveiling complex and interesting dynamics. We performed our experiments in a bulk crystal of periodically poled lithium niobate, working close to the optimal condition for second-harmonic generation. We used an input laser beam wide enough that, at very low intensities, the diffraction leaves its diameter unchanged along the propagation in the crystal. At high intensities instead, as we show in this work, such a spatially wide laser beam can be reshaped into a beam of much smaller diameter and guiding multispectral components. We also show how this outcome may permit exploitation of other parameters, like the crystal temperature, for tuning the spectrum of the generated multicolor light.

Equivalent Model of Photoswitch: Application to the UWB Antenna Design Integrating Impulse Feeding

Optoelectronic devices triggered by a laser ∞ash and operating in linear switching regime allow the generation of short pulses with small time jitters (2ps typically). An Ultra Wide Band antenna array combining as many of this photoswitches as antennas has the advantage to increase the radiation power on one hand and to ofier the agility of the radiation beam on the other hand obtained by time delay of laser illumination. During the step of antenna design, it becomes important to take into account the photoswitch integration in order to increase the peak power and the frequency band of the generated output signal. This paper presents an equivalent model of photoswitch obtained with the transient solver of CST Microwave Studio coupled within CST Design Studio. The second part of this article is dedicated to the integration of a photoswitch even within the antenna.

Dispersive Wave Emission in Dual Concentric Core Fiber: The Role of Soliton–Soliton Collisions

Soliton-soliton collisions have a crucial role in enhancing the spectrum of dispersive waves in optical fibers and collisions among in-phase solitons lead to a dramatic enhancement of the dispersive wave power, as well as to its significant spectral reshaping. We obtained a simple analytical model to estimate the spectral position, width, and amplitude of the dispersive waves induced by a collision of two in-phase solitons. We tested our theory in the case of a dual concentric core microstructured fiber.

UWB antenna array with autonomous scanning capability using integrated opto-electronic feeding device

The originality of the radiation source presented here is localized in its ultrafast speed and its autonomy of scanning a large spatial area. That behavior is obtained by means of a trigger mode of radiation perfectly controlled by a single optical source. The transmitter consists of a powerful picoseconds laser, an optical power divider and n optoelectronic ultra-wideband radiation sources which produce several electric pulse trains with a repetition rate slightly different one with each other. Each train is then addressed toward a single antenna. When transmitted the electromagnetic waves interact to produce an ultrafast orientation of the array emission lobe allowing scanning of large spatial field in less than 1μs.

Spectro-temporal shaping of supercontinuum for subnanosecond time-coded M-CARS spectroscopy

A supercontinuum laser source was designed for multiplex-coherent anti-Stokes Raman scattering spectroscopy. This source was based on the use of a germanium-doped standard optical fiber with a zero dispersion wavelength at 1600 nm and pumped at 1064 nm. We analyzed the nonlinear spectro-temporal interrelations of a subnanosecond pulse propagating in a normal dispersion regime in the presence of a multiple Raman cascading process and strong conversion. The multiple Raman orders permitted the generation of a high-power flat spectrum with a specific nonlinear dynamics that can open the way to subnanosecond time-coded multiplex CARS systems.

A Pulsed Modulator Combined With Very High PRF Photoconductive Switches to Build a Self-Scanning UWB Radiation Source

The development and construction of self-scanning capability short-range ultrawideband (UWB) radar is forthcoming for the French Military for Defense. This paper is devoted to the presentation of the self-scanning radar principle and of the development, construction, and tests of the different blocks needed to implement the elementary part of the forthcoming radar. The elementary radiation source is composed of a microstrip line with photoconductive switches, an optical system to trigger the switches, a pulsed modulator used as bias voltage to load the line, and a UWB antenna to radiate high electric fields in a broad frequency band. The main proposed innovations are devoted to the integration of the photoconductive switches within the antenna and to the use of the pulsed modulator as bias source instead of a dc voltage source. This modulator allows considering the increase in the yield of the whole source as well as the increase in the radiation bandwidth. Experimental results are presented in order to compare the switching efficiency with a dc bias source against this pulsed modulator bias source. Finally, photoswitches triggered by an 80-ps/20-μJ laser source are able to generate monopolar or bipolar pulses in the kilovolt range with subnanosecond duration and a 33-MHz pulse repetition frequency. First radiated electric field results of the whole device are also reported.

Supercontinuum Generation in an Ytterbium-Doped Photonic Crystal Fiber for CARS Spectroscopy

An optimized broadband source emitting from 1064 to 1600 nm was specially designed for coherent anti-Stokes Raman scattering spectroscopy. This source is based on the use of a ytterbium-doped photonic crystal fiber with a large core in which a supercontinuum is generated from a signal wave at 1064 nm regenerated by ytterbium ions pumping. A particularly flat spectrum with high spectral power density and perfectly synchronized spectral components is obtained.

A pulsed power source to drive an innovative autonomous scanning UWB radiation system

Ultra Wide Band (UWB) antenna arrays offer the possibility to generate a wide band signal in a sharp direction, which can drastically improve the detection sensitivity. With a configuration including as many antennas as generators, an UWB array presents the advantage of increasing the radiation power on one hand and offering the agility to the array on the other hand. Indeed, the application of time delays between the feeding pulses of the antennas permits to steer the radiated fields in each wanted direction and to realize a coherent sum of each initial power. However, a major difficulty with such a configuration is the minimization of the radiation source jitters, to obtain the best synchronization possible. A solution consists of using pulsed optoelectronic devices, operating in linear switching regime, which permits to bypass this difficulty and to obtain ultra-short electrical waveforms with small temporal jitters (2ps typically). Many applications such as transient radar cross section (RCS) measurements, UWB synthetic aperture radar (SAR) and high power UWB radiation source can be developed by using such systems. In this paper, the design of an innovative autonomous scanning radiation system will be approached. The first experimental results will be also presented. These results are essentially focused on the high voltage pulsed power source which is previously described. This source aims at driving the pulsed optoelectronic devices. It consists of a series-connected IGBTs component.