Spatial and spatio-temporal optical solitons

General Data

Acronym: SPASOL

Number/Data of the contract: 26/21.10.2005

Module: 1. Complex research & development projects

Project Director: Prof. Dr. Valentin I. Vlad

Project Coordinator: Institutul National de Cercetare Dezvoltare pentru Fizica Laserilor, Plasmei si a Radiatiei - INFLPR,

Partners: Institutul national de Fizica si Inginerie Nucleara - Horia Hulubei - IFIN HH,

               Institutul National pentru Optoelectronica - INOE2000

Proposal’s abstract 

The study of spatial and spatio-temporal optical solitons, that are localized physical objects, is at the core of a fascinating domain of Nonlinear Science, in which many nonlinear problems of physics, chemistry and biology can be modeled. The Nonlinear Optics, a very active research field in both Optics and Nonlinear Science, has a tremendeous potential for applications in all-optical communications based on photonic integrated devices.

The authors of  the present proposal have a remarkable scientific contribution to this specific domain. Their results have been published in the most prestigious Physics Journals, and have been cited more than 1500 times in ISI journals. They actively participate to important international colaborations, including a Network of Excellence of the European Union.

The main objectives of the project are:

-       obtaining of new advanced knowledge in the generation and characterization of spatial and spatio-temporal solitons in nonlinear optical media and of associated nonlinear processes, which will continue and will consolidate our international status in this domain;

-          study and realization of soliton waveguides with optimal profile for laser beam propagation;

-          study of soliton arrays and of their photonic functionality;

-          set-up of an experimental platform to study spatial solitons, nonlinear materials and photonic devices based on spatial and spatio-temporal solitons.

In the project we propose new ways to generate and to characterize spatial and spatio-temporal solitons for light guided by light and for new sources of optical information bits with maximum spatial and temporal confinement.

Of all possible means to obtain an optical waveguide, the soliton waveguide is the best way to induce single mode optical interconnections. The soliton waveguide has a refractive index profile optimized for the best possible propagation of laser beams, in comparison with the waveguides obtained by other technologies (chemical or photochemical ones).

This project will allow us to bring forward our studies of excellence to results that should enable new breakthroughs in the Information Society’s Technologies (optical information processing based on maximum bit confinement down to the fundamental principles of physics’ permitted frontiers).

Moreover, the procedures to realize soliton waveguides could become of low costs by using cheap light sources (laser diodes or even LEDs). This denote another project’s key point: results that can optimize the producing technologies of photonic devices at low costs, that being in accordance with a specific objective of PC6 and PC7 European Union programmes.

These photonic devices can solve many important practical problems: ultra-fast adaptive interconnections that could be induced or erased on demand, coupling between arrays of diodes and optical fibres, optical memories in fibres and waveguides, new types of photonic crystals etc.

We consider that this project can concentrate a main task force in Romania in order to reach high level scientific objectives in nonlinear and information optics that will raise our experience, will increase the number of young researchers with strong scientific education as well as the international recognition of Romanian Science, both in the European Research Area, in the Frame Programmes of the European Union and worldwide.

Objectives

We propose the next objectives, which are part of the advanced studies of exact sciences, promoted as priorities in CEEX programme as well as in Framework Programme 6 – EU (in which we are involved within a Network of Excellence ”PHOREMOST”-Nanophotonics and within COST P8 and P11 actions).

Objective 1 – To obtain new advanced knowledge in the field of generation and characterization of spatial and spatio-temporal optical solitons in nonlinear optical media and associated nonlinear processes, with results to be published in high impact factor international journals.

Objective 2 – To concentrate the equipment and the know-how for an experimental platform that will allow systematic investigation of solitons, nonlinear materials, processes and photonic devices based on spatial and spatio-temporal solitons.

These objectives are in agreement with general and specific objectives of CEEX Programme- Complex research & development projects:

-          to enhance the Romanian IRD system in order to accumulate first rank knowledge, results and experience in top scientific and technological domains, to spread and to transfer them to economic and social internal medium in order to increase its competitivity;

-          to concentrate and to use in an optimum way the existing, high level, scientific and technological potential of Romania;

-          to promote the Romanian R&D institutions participation to European and international research programmes and their connection to the European Research Area, including integration in technological platforms at European level;

-          to support the formation, development, integration and consolidation in endorsed domains of research networks of which activity reach the excellence level, recognized according to the international standards;

-    to increase the performance of Romanian research groups for an efficient integration in  international scientific programmes.

The project had as outcomes the obtaining of new and advanced knowledge bases regarding the generation and the characterization of spatial and spatio-temporal solitons in nonlinear optical media and of the nonlinear processes associated with, as well as the study of soliton type waveguides and their properties.

Following is a comprehensive presentation of the project results:

• The analytical description is achieved by determining and subsequently amending the propagation equations in photorefractive crystals with nonlinear birefringence, high absorption and high optical activity. The solutions show the existence of the spatial solitons in normal orientations and in the presence of the external electric field. In consequence, the Stokes parameters (the solitons’ polarization states) for different orientations of the crystal according to the distance of the propagation, to the ration between the soliton intensity and background intensity and to the external electric field can be calculated. The evolution of the photorefractive solitons’ polarization is complex and reaches relatively stable cycles at standard sizes of the crystal, high external electric fields and at very small or very high ratios between soliton intensity and background intensity. Our experimental and numerical results are in good accordance with the theoretical results.

• Bright optical solitons open the way for an andvanced, all optical, tehnology for writing solitonic waveguides (SWGs) in the nonlinear optical materials usually used in photonics, i.e. lithium niobate (NL). The SWGs in NL are characterized by an optimal adjustment of their refractive index profile with the fundamental laser profile which opens the opportunity to obtain long lifetime SWGs as well as the possibilities of SWGs removal, fixing and multiplexing in the crystal volume (3D). There have been created SWGs in non-doped NL crystals using continuous-wave, small power, lasers emitting in the green spectral domain and using lasers with fs pulses, high repetition rate, in the near-infrared spectral domain (800 nm) assisted by a green background and an external electric field. The formation of the SWGs was observed after accumulating a large number of pulses because the characteristic photorefractive growth time is much longer than the period of the pulses. The background intensity determines a decrease of free carriers’ average free path value, making the screening effect to dominate the photovoltaic effect. We tested the propagation of light in SWGs on a distance of approx. 15 diffraction lengths. We registered gratings (arrays) of SWGs in LN using the screening effect and the photovoltaic effect with a controllable transversal profile depending on the writing time and with a long lifetime. We tested the reciprocal influence of parallel SWGs, registered at distances of 100 μm between them, over their capacity of guiding light. At this distance between solitonic waveguides the registration of new guides in crystal does not practically affect the light propagation in previously registered waveguides. SWGs present a registration time controlled transverse profile as well as a long lifetime and a smaller dispersion for ultra short pulses. SWGs and matrix of SWGs can be used in efficient, reconfigurable optical interconnections.

• Analytical modeling of optical field propagation, the determination of the conditions for spatial solitons existence in photorefractive crystals with photovoltaic effect and the obtaining of photovoltaic-screening spatial solitons, which emphasizes the possibility to change the type (profile) of these solitons (bright/dark) depending on the choice of the applied external electric field versus the internal photovoltaic field.

• The study of the stimulated photorefractive effect by small intensity fs pulses in the near infrared (IR) spectral domain, in non-doped LN. An externally applied voltage leads to a self-focusing effect that confines the beam. Demonstration of the confinement mechanism produced by the second harmonic of small intensity IR fs pulses generated in mismatch phase conditions in LN. These results provide a method for extending the photorefractive effect produced by the fs pulses in the photorefractive crystals with quadratic nonlinear effect, such as LN, towards IR, without a phase match in the second harmonic generation or high peak intensities of the pulses. The analysis of the material and guide dispersion effect over the fs laser pulse propagation in solitonic waveguides induced in NL. Due to the spatial solitons’ refractive index, the guide dispersion induced by such SWG is negligible in comparison with the material dispersion for visible spectrum and IR communications band wavelengths. The near IR spectral domain has the advantage of a much smaller material dispersion than the visible domain.

• The modeling of laser beam propagation in spatial soliton periodical lattices induced in photorefractive crystals in order to find allowable and forbidden bands of the photonic crystal formed by the spatial soliton lattice.

• Study of the conditions for spatial optical solitons generation in photorefractive media with periodical refractive index.

• The modeling and the study of multidimensional (spatio-temporal) soliton stability confined by radial symmetric optical lattices of Bessel type. The dynamic system’s Hamiltonian and its dependence of soliton’s total energy value were determined. The stability domain in the parameter space of multidimensional solitons confined by Bessel optical lattices was identified. The collapse phenomenon of multidimensional solitons for sufficiently high energies as well as the possibility of stable multidimensional solitons excitation in the case of propagation from spatiotemporal input of Gaussian type was shown. A fork-type “swallow-tail”, a physical-mathematical concept that rarely appears in concrete physical problems was identified.

• The mathematical modeling and the study of “hidden” vortex spatial solitons stability, consisting of two incoherent coupled components with different topological charges, which are formed in saturable nonlinear media. It was determined the soliton’s spatial shape dependence of the control parameter in the experiment, namely the soliton’s nonlinear wavenumber which is biunique connected to the soliton power. The stability domain of spatial solitons, which constitute a one-parametrical family determined by soliton’s nonlinear wavenumber was identified. It was established that solitons are linear stable if the parameter of one-parametrical family (i.e. the nonlinear wavenumber of soliton) is bigger than a certain threshold. It was marked out the fact that these spatial solitons with net topological charge equal zero are extremely robust to propagation in the presence of some perturbations of “white noise” type as input if total power exceeds a certain threshold. This threshold was estimated being of the order of few hundreds of mW for typical photorefractive crystals.     

• Study of the existence and stability conditions of fundamental spatio-temporal solitons (with null vorticity) and with non-null vorticity (S=1) in nonlocal nonlinear media using the Crank-Nicholson propagation numerical methods, which are specific for the parabolic dynamic problem, and the calculus of the increasing rate of instabilities using Lyapunov linearization method around stationary soliton solutions. It was developed an efficient calculus code using the numerical scheme with Crank-Nicholson finite differences for solving the optical spatial solitons propagation problem in nonlocal nonlinear media. The propagation under the action of some input perturbations of stationary solutions and some Gaussian incident beams was monitored, and it was also studied the azimuthally instability of 3D non-zero vorticity solitons. These new results can be also applied to the study of 3D soliton generation in Bose-Einstein atomic condenser of attractive type.  

• Identification of existence and stability domains in the parameter space of fundamental spatio-temporal optical solitons (without vortex) in nonlinear media with arbitrary nonlinearities (of quadratic-cubic type, a saturable type, with nonlocal nonlinearities, etc.).

• Calculus of the nonlinear wavenumber dependence (propagation constant) of the solitons energy, which is, in fact, the nonlinear relation of dispersion, and calculus of the dependence of dynamic system’s Hamiltonian of the soliton norm (energy).

• Demonstration of existence of a generic fork-type “swallow-tail” in the study of spatio-temporal soliton stability in nonlinear optical media, as well as in the study of 3D solitons in Bose-Einstein condenser of attractive type confined by optical lattices. This generic fork leads to the appearance of two cuspidal points that separate the stability domain of solitons from the domains of instability (usually there have been identified two domains of instability).