75 - 13(2)75.PDF
file, full text
Wide beam stabilities and instabilities in one dimensional arrays of Kerr-nonlinear channel waveguides
J. Meier, D.N. Christodoulides, G.I. Stegeman, H. Yang, G. Salamo, R.
Morandotti, J.S. Aitchison, and Y. Silberberg
The propagation of wide (plane-wave like) and narrow high intensity beams at 1550 nm was investigated in 1D arrays of AlGaAs channel waveguides which are nearest-neighbour coupled via evanescent fields. Spatial diffraction (beam spreading) by evanescent field coupling leads to dispersion relations that are periodic with propagation direction and hence exhibit regions of both normal and anomalous diffraction. This results in propagation directions in which filamentation of high intensity beams with superimposed noise occurs, and other regions in which these beams are stable against noise. Both cases were observed experimentally, and the modulation instability gain was measured versus the spatial frequency of the noise. Good agreement with theory was found.
Keywords: discrete nonlinear optics, modulation instability, solitons.
85 - 13(2)85.PDF file, full text
Spatial solitons in photorefractive lattices
B. Hanna, W. Krolikowski, D. Neshev, A.A. Sukhorukov, E.A. Ostrovskaya, and
Yu.S. Kivshar
We present a summary of the recent results of our studies of nonlinear localization effects in optically-induced periodic index structures in photorefractive crystals. We demonstrate the generation of spatial bright solitons, controllable excitation of gap solitons and their anomalous steering properties.
Keywords: periodic structures, gap solitons, nonlinear optics.
93 - 13(2)93.PDF file, full text
Discrete diffraction in an optically induced lattice in photorefractive media with a quadratic electro-optic
effect
G. Staron, E. Weinert-Raczka, and P. Urban
Propagation of light in an optically induced waveguide array in biased photorefractive media with a quadratic electro-optic effect is investigated numerically with the beam propagation method. The refractive index distribution of the array is induced by two coherent plane waves interfering in a guiding layer in a photorefractive multiple quantum well (MQW) planar waveguide. The influence of modulation depth and the space period of an interference pattern as well as the external electric field intensity on diffraction properties of the array is analysed. The potential possibility of all-optical switching due to the dependence of the guided wave output distribution on the external waves parameters is shown.
Keywords: discrete diffraction, optically induced lattice, photorefractive effect.
103 - 13(2)103.PDF file, full text
Optics in non homogeneous waveguide arrays
R. Morandotti, H.S. Eisenberg, D. Mandelik, Y. Silberberg, D. Modotto, M. Sorel, C.R. Stanley, and J.S. Aitchison
Optical discrete solitons possess extremely interesting dynamical properties. Such properties lead to the realization of several ultrafast, all-optical switching mechanisms, based on the use of uniform and nonuniform waveguides array.
Keywords: discrete solitons, waveguide arrays, integrated optics, all-optical switching, semiconductor devices.
107 - 13(2)107.PDF file, full text
Nematic liquid crystal waveguide arrays
K.A. Brzdakiewicz, M.A. Karpierz, A. Fratalocchi, G. Assanto, and E. Nowinowski-Kruszelnicki
We investigate linear and nonlinear light propagation in a voltage-tunable array of waveguide channels in undoped nematic liquid crystals. This novel geometry, based on a photonic structure with a periodic modulation of refractive index controlled by an electric field, offers a wealth of possibilities for the study of discrete optical phenomena. The structure, in conjunction with a giant, non-resonant and voltage-dependent reorientational nonlinearity, allows us to drive the system from bulk diffraction to discrete propagation. Theoretical and experimental investigations, carried out with near infrared light wavelength and powers of a few milliWatts, show the possibility of transverse light localization, resulting in discrete spatial solitons. Such array, with its voltage- and light-adjustable guided-wave confinement and coupling, exhibits potentials for the realization of multifunctional routers and all-optical signal processors with nematic liquid crystals.
Keywords: nonlinear optics, solitons, self-action effects, optical nonlinearity in liquid crystals, discrete diffraction.
113 - 13(2)113.PDF file, full text
Arrays of weakly coupled, periodically poled lithium niobate waveguides: beam
propagation and discrete spatial quadratic solitons
R. Iwanow, R. Schiek and G. Stegeman, T. Pertsch, F. Lederer, Y. Min, and W. Sohler
Discrete optical systems can be realized as arrays of parallel, weakly coupled, channel waveguides where light normally undergoes "discrete diffraction" via the weak coupling between adjacent channels. Here we describe how light can be forced to maintain a constant field profile on propagation in waveguide arrays, i.e., to localize into a discrete spatial soliton, by using the second order nonlinearity of periodically poled lithium niobate near phase-matching for second harmonic generation. Detailed sample characterization and experimental verification of the excitation of discrete quadratic solitons is reported.
Keywords: solitons, nonlinear optics, integrated optics.
123 - 13(2)123.PDF file, full text
Stable spinning optical solitons in two and three dimensions
D. Mihalache
A brief overview of recent results in the field of multidimensional spinning (vortex) optical solitons is given. We address the stability problem of two- and three-dimensional spinning solitons in media with competing nonlinearities. We conclude that these solitons could be stable, provided that their external size and power (energy) are large enough. The stability of vorticity-carrying solitons is a generic feature of media with competing self-focusing and self-defocusing nonlinearities.
Keywords: optical vortices, spinning optical solitons.
129 - 13(2)129.PDF file, full text
Evolution of light bullets propagating in saturable Kerr-like media with higher order effect
J. Jasinski
In the paper, propagation of spatio-temporal pulses (light bullets) in inhomogeneous, Kerr-like nonlinear medium is considered. For ultra-short bullets the higher-order nonlinear effects n self frequency shift and nonlinear dispersion together with saturation of nonlinear susceptibility are taken into account. The equations describing evolution of temporal and spatial width of the bullet parameters are derived. The stationary solutions of these equations are found analytically. Small oscillations around the stationary bulletís widths are considered and frequency oscillations are obtained. The regime of stable oscillations is discussed.
Keywords: light bullets, spatio-temporal pulses, temporal solitons, higher-order nonlinear effects, saturable Kerr media.
135 - 13(2)135.PDF file, full text
Spatial solitons in biased photorefractive media with quadratic electro-optic effect
A. Ziolkowski and E. Weinert-Raczka
Propagation of optical beams and properties of one-dimensional (1D) spatial solitons in biased photorefractive media with quadratic electro-optic effect are analysed. An exact analytic solution of the corresponding propagation problem is presented as well as a numerical investigation of the evolution of optical beams.
Keywords: spatial solitons, beam evolution, photorefractive effect.
141 - 13(2)141.PDF file, full text
Second harmonic generation in Ru-derivative chromophores incorporated within PMMA polymers
stimulated by acoustic and optic fields
A. Migalska-Zalas, I.V. Kityk, and B. Sahraoui
We have demonstrated the co-existences of second harmonic generation (SHG) and
c(2) grating using acoustical and optical
fields in Ru derivatives incorporated within the PMMA polymer matrices. The investigated compounds possess metastable
trapping levels originated both from the d-states of Ru as well as the effective charge transfers due to the presence of
p-conjugated
states. The investigated compounds possess long-lived c(2)
grating which decreases less than 76% under 600 min of laser treatment. Electrostatic field causes additional reanimation of the SHG. Values of the acoustically induced optical second
harmonic generation (AIOSHG) for the best Ru chromophres were substantially higher than for known pure inorganic
materials. The effective value of the second-order optical susceptibility for the wavelength 1.89 µm is about 1.94 pm/V. With
increasing acoustical power, the AIOSHG for fundamental YAB-Gd laser light (l = 1.76 µm) increases and achieves its maximum
value at acoustical power density of about 1.45 W/cm2.
Keywords: c(2)-gratings, acoustically induced second harmonic generation, nonlinear optics.
147 - 13(2)147.PDF file, full text
Nonlinear cut-off integrated optical isolator
K. Gniadek
The paper presents a concept of an integrated isolator which operation combines the magnetooptic nonreciprocity and nonlinear effects. The isolator is based on nonreciprocal cut-off of TM modes in magnetooptical planar structure where one layer (e.g. top one) is nonlinear. The power dependence of the propagation constants in such configuration allows for easy control of the nonreciprocal behaviour of the counterpropagating TM modes and nonreciprocal cut-off thickness.
Keywords: magnetooptics, nonlinear waveguides, integrated isolators.
151 - 13(2)151.PDF file, full text
Dye-enhanced nonlinearity threshold measurements in liquid crystals
M. Sierakowski and L. Waga
Since early eighties it is well known, that optical field can interact with liquid crystalline structures inducing molecular motion in similar way to quasi-electrostatic fields. This nonlinear optical effect (NOE) causes changes of refractive index distribution in the structure and in this way leads to several optical phenomena, like self-diffraction of the propagating beam, harmonic generation, four-wave mixing, nonlinear wave-guiding, etc. At first, NOE was observed in nematics for relatively high optical power densities. In the middle of nineties, Istvan Janossy discovered, that small addition of specific dopants to LC-material can significantly reduce the light power required for NOE. At present, NOE can be induced in certain LC-materials by sub-miliwatt laser beams. This achievement opened a way to many applications of the accompanied optical phenomena. Mechanism of the Janossy effect is still not quite clear. NOE is generally a threshold effect, which means that it starts after a certain threshold value of the optical power. In this work we examine the threshold of the self-diffraction of light in doped nematics in selected conditions, trying to understand its nature. Final goal of the work is examination of NOE in chiral liquidcrystal structures (cholesterics).
Keywords: optical nonlinearity, liquid crystals, Janossy-effect.
157 - 13(2)157.PDF file, full text
Steady state photorefractive gratings in multiple quantum wells at high modulation depth
M. Wichtowski, E. Weinert-Raczka, and A. Gajda
The steady-state analytical expressions for the space-charge field harmonics generated by the high contrast interference pattern in biased photorefractive material with ambipolar transport are presented. As an example semi-insulating multiple quantum wells film operating in the Franz-Keldysh geometry was considered. Parameters of photorefractive grating created by a quadratic electro-optic effect were analysed. Calculations based on the band transport model were limited to the low external field case what corresponds to the linear transport regime. In particular, it was found that the photogenerated carriers density has nearly harmonic distribution. Moreover, the fundamental component can be obtained from the linear equations what enables us to improve the solution for the space-charge field in bulk, nonresonant materials described by the classical Kukhtarevís model.
Keywords: photorefractive multiple quantum wells, band transport model, harmonic gratings.
171 - 13(2)171.PDF file, full text
Polarization degree fading during propagation of partially coherent light through retarders
A.W. Domanski
In the paper, degree of polarization fading during transmission of partially coherent light through bulk birefringent optical materials is analysed. It is also shown that classical Mueller matrix formalism for such cases should be changed and a depolarisation matrix should be added. As an example influence of degree of polarization fading on optical retarders parameters as well as on accuracy of degree of polarization measurements are discussed and a result of polarization degree fading measurements of laser diode light passing through a lithium niobiate crystal are presented.
Keywords: polarization, coherence, birefringence, Mueller matrix.
177 - 13(2)177.PDF file, full text
Propagation properties of photonic crystal fibers filled with nematic liquid crystals
T.R. Wolinski, K. Szaniawska, K. Bondarczuk, P. Lesiak, A.W. Domanski, R.
Dabrowski, E. Nowinowski-Kruszelnicki, and J. Wojcik
The paper presents the latest experimental results on propagation properties of a photonic crystal fiber infilled with a nematic liquid crystal characterized by either extremely low (of the order of ~0.05) or relatively high (of the order of ~0.3) material birefringence. The nematic liquid crystal was introduced into the micro holes of the photonic crystal fiber by the capillary effect. Due to anisotropic properties of the obtained photonic liquid-crystal fiber, guiding mechanism that has to be attributed to the photonic band gap effect has been demonstrated.
Keywords: liquid crystals, photonic band gap, optical fibers.
183 - 13(2)183.PDF file, full text
Simultaneous twist and longitudinal strain effects on polarization mode dispersion in highly birefringent
fibers
P. Lesiak and T.R. Wolinski
Simultaneous twist and longitudinal strain effects on polarization mode dispersion (PMD) in highly birefringent (HB) bow-tie and photonic crystal fibers have been investigated. External twist has been found to modify both modal birefringence and differential group delay (DGD) in HB fibers, and in particular to influence the range of DGD changes under longitudinal strain.
Keywords: polarization mode dispersion, twist, strain, highly birefringence fibers.