Mourad Zghal

Full vector modal analysis of microstructured optical fiber propagation characteristics

Microstructured optical fibers (MOFs) are optical fibers having a periodic air-silica cross-section. The air holes extend along the axis of the fiber for its entire length. The core of the fiber is formed by a missing hole in the periodic structure. Remarkable properties of MOFs have recently been reported. This paper presents new work in the modeling of the propagation characteristics of MOFs using the Finite Element Method (FEM) and the Galerkin Method (GM). This efficient electromagnetic simulation package provides a vectorial description of the electromagnetic fields and of the associated effective index. This information includes accurate determination of the spectral extent of the modes, cutoff properties and mode-field distributions. We show that FEM is well adapted for describing the fields at abrupt transitions of the refractive index while GM has the advantage to accurately analyze MOFs of significant complexity using only modest computational resources. This presentation will focus on the specific techniques required to determine single mode operation, dispersion properties and effective area through careful choice of the geometrical parameters of the fibers. We demonstrate that with suitable geometrical parameters, the zero dispersion wavelength can be shifted. This tool can also provide design criteria for fabricating MOFs and a corresponding map of effective area. This approach is validated by comparison with experimental results and measurements on actual MOFs fabricated at IRCOM and at Alcatel Research and Innovation Center.

Accurate measurement of the cutoff wavelength in a microstructured optical fiber by means of an azimuthal filtering technique

A simple self-referenced nondestructive method is proposed for measuring the cutoff wavelength of microstructured optical fibers (MOFs). It is based on the analysis of the time-dependent optical power transmitted through a bow-tie slit rotating in the far-field pattern of the fiber under test. As a first demonstration, the cutoff wavelength of a 2 m MOF sample is found to be close to that provided by numerical predictions (approximately 25 nm higher). Because of the high dynamics of the measurement, the uncertainty is limited to Dlambda= +/-10 nm.

Spontaneous emission based on 2D photonic crystal and quantum dot

Spontaneous emission of light in semiconductor LEDs produces light that lack fixed-phase dependence; it is a question of incoherent light. However, it was proved that spontaneous emission depends not only on intrinsic property of material but also on the surrounding environment. Association between Quantum Dots (QD) and Photonic Crystals (CP) can have this role and modify characteristics of spontaneous emission and obtain wished results. In this paper we present another way to produce spontaneous emission with new interesting performances to arrive in produce a single photon source which produce indiscernible unique photons. Such sources are useful in quantum cryptography and quantum computing.

New design of As2Se3-based chalcogenide photonic crystal fiber for ultra-broadband, coherent, mid-IR supercontinuum generation

In this paper, we propose a new design of all-normal and ultra-flat dispersion As2Se3-based chalcogenide photonic crystal fibers (PCF). The generation of supercontinuum (SC) in the designed fibers is investigated, which has flat and smooth profile, covers a broad range extending from 2 to 8 μm. The significance of this work is that it provides a new type of mid-infrared SC source with flat shape, broadband and high coherence properties by pumping the As2Se3-based PCF. Thus many applications can be performed such as fiber lasers, pulse compression and multi-wavelength optical sources in the mid-infrared region.

Supercontinuum generation in a large mode area photonic crystal fiber

In this paper, we report about a numerical and experimental study of a high energy, micro-joule range, supercontinuum (SC) spectrum which is generated in a large mode area (LMA) photonic crystal fiber (PCF). Our experiment consists of launching a train of 100 femtosecond pulses into a 20 cm-long span of a PCF delivered from a Ti:Sapphire pump parametric amplifier. The optical properties of the PCF were accurately calculated using a finite element method mode solver and the real cross section of the fiber. The PCF exhibits a zero dispersion wavelength at 1250 nm and has an effective area of 660 μm2 at λ=1250 nm. We observed an octave-spanning SC with spectral components propagating in the fundamental mode. The physical processes leading to the construction of the continuum spectrum were studied by monitoring the growth of the SC while increasing the input optical power. The main mechanisms behind the spectral broadening are mainly ruled by the effects of self-phase modulation, the stimulated Raman scattering, and the soliton propagation. Our experimental results are compared with the numerical solution of the nonlinear Schrodinger equation and good agreement between experimental and numerical results is found.

A simple wavelength division multiplexing system for active learning teaching

The active learning project consists in a series of workshops for educators, researchers and students and promotes an innovative method of teaching physics using simple, inexpensive materials that can be fabricated locally. The objective of the project is to train trainers and inspire students to learn physics. The workshops are based on the use of laboratory work and hands-on activities in the classroom. The interpretation of these experiments is challenging for some students, and the experiments can lead to a significant amount of discussion. The workshops are organized within the framework of the project ‘‘Active Learning in Optics and Photonics” (ALOP) mainly funded by UNESCO, with the support of ICTP (Abdus Salam International Centre for Theoretical Physics) and SPIE. ALOP workshops offer high school, college or university physics teachers the opportunity to improve their conceptual understanding of optics. These workshops usually run for five days and cover several of the topics usually found in any introductory university physics program. Optics and photonics are used as subject matter because it is relevant as well as adaptable to research and educational conditions in many developing countries [1]. In this paper, we will mainly focus on a specific topic of the ALOP workshops, namely optical communications and Wavelength Division Multiplexing technology (WDM). This activity was originally developed by Mazzolini et al [2]. WDM is a technology used in fibre-optic communications for transmitting two or more separate signals over a single fibre optic cable by using a separate wavelength for each signal. Multiple signals are carried together as separate wavelengths of light in a multiplexed signal. Simple and inexpensive WDM system was implemented in our laboratory using light emitting diodes or diode lasers, plastic optical fibres, a set of optical filters and lenses, prism or grating, and photodiodes. Transmission of audio signals using home-made, simple, inexpensive electronic circuits was also demonstrated. The experimental set-up was used during national ALOP workshops. Results are presented and discussed in this paper. Current explorations to further develop these and other closely-related experiments will also be described.

Full vector beam propagation method modelling of dual core photonic crystal fiber couplers

We analyse the coupling characteristics of dual-core photonic crystal fibre couplers by a 3D finite difference vector beam propagation method. Beam propagation analysis of photonic crystal fibre couplers is performed in terms of coupling length and coupling efficiency. The determination of the guiding properties such as the propagation constants is evaluated using a mode solver based on plane wave method. We study the influence of the photonic crystal fibre coupler geometrical parameters on the coupling length at different wavelengths. Variable size of the central hole is considered to improve the coupling between the two cores. It is shown that it is possible to design shorter photonic crystal fibre couplers with coupling lengths of hundred micrometers compared to conventional optical fibre couplers. We demonstrate that the designed coupler can operate as a polarization preserving directional coupler. This study confirms that this device can act as an efficient ultra small wavelength selective coupler.

Analysis of coupling in two dimensional photonic crystal waveguides

The behaviour of a straight two-dimensional photonic crystal waveguides is analysed. Considering the two guides as a single system, we implement a photonic crystal directional coupler with micrometer dimensions. By fitting the interaction region separating the waveguide cores, composed of dielectric rods, we demonstrate that a suitable designing of the geometry of the interaction region separating the waveguide cores, composed of dielectric rods allows a more efficient coupling. We show that with only one row between the two parallel waveguides either than two rows, we can reduce the coupling length and increase the coupling coefficient of coupler.

Popularisation of optical phenomena: establishing the first Ibn Al-Haytham workshop on photography

Within the framework of its scientific activities, the Optical Society of Tunisia organized the first photographic workshop called Ibn Al-Haytham session. This activity enabled, through conferences, the evocation of the research done by one of the most distinguished and prolific mathematicians in the medieval tradition of Arabic Islamic science. The camera obscura that he thoroughly studied was the theme of a training where more than twenty participants build and used this basic camera. The adopted training approach based on active teaching and learning made possible the achievements of interesting results in spite of the heterogeneity of the group of trainees.

Numerical solution to modal field equation with a finite difference beam propagation method: application to Bragg fiber

The Beam Propagation Method (BPM) is the most widely used tool for the investigation of complex photonic structures. Since the original BPM was introduced, many improvements and extensions have been proposed. We have developed a computer program based on the Finite Difference BPM for modeling propagation in optical waveguides. This method has been successfully applied for several 3D problems such as propagation on Bragg Fiber. The main drawback of this method is its complexity and long computation time using a personal computer. In this paper, a simple efficient numerical solution method, we called double 2D-BPM, is proposed. This technique is based on the decomposition of the 3D field propagation equation onto two 2D equations related to transverse plans. Propagation along the x and y axes is computed separately in two steps. Using a similar technique, a finite difference approximation for each propagation step involves the solution of two equations and the complete problem splits into two independent 2D problems. We performed propagation tests in elementary 3D problems but also on Bragg fiber. The numerical results of 3D-BPM and double 2D-BPM have been compared. The propagation step along the propagation axis has been experimentally determined. Parameters that affect the accuracy and the stability of this method were discussed. Losses induced by propagation on Bragg fiber were also considered. We have established that the global effect of the double 2D-BPM is equivalent to 3D-BPM technique. Comparison with exact results obtained from analytical expressions also shows excellent agreement.