Influence of incoherent background on diffraction divergence of narrow light beams of He-Ne laser with wavelength of 633 nm due to refractive index changes of Fe-doped lithium niobate sample is experimentally studied. Incoherent background is produced by light-emitting diodes (LED´s) of different average wavelengths and laser diode emitting in blue region of visible. It has been experimentally demonstrated that nonlinear diffraction of coherent red beams of He-Ne laser with diameters on full width of half maximum (FWHM) near to 15 - 20 μm may be totally compensated by means of assistance of incoherent background with shorter average wavelengths 455 – 465 nm and even much lower intensity compared with that for coherent beam.
The influence of incoherent background illumination produced by light-emitting diodes (LED´s) of different average
wavelengths and laser diode emitting in blue region of visible on diffraction characteristics of narrow coherent light
beams of He-Ne laser due to refractive index changes of Fe-doped lithium niobate sample are studied. It has been
experimentally demonstrated that nonlinear diffraction of red beams with wavelength 633 nm and diameters on full
width of half maximum (FWHM) near to 15 μm may be totally compensated using background light with average
wavelengths 450 – 465 nm. To provide the necessary intensity of incoherent background, the combinations of spherical
and cylindrical concave lenses with blue LED and laser diode module without focusing its beam have been used.
Channel optical waveguides with spatial optical modulation of their parameters and one-dimensional phase diffraction
gratings are generated by laser radiation with wavelengths of 450 and 532 nm within lithium niobate surface layers
doped with iron, copper, and their combinations. The characteristics of induced elements are studied by diffraction of
light with wavelengths 532 and 633 nm. Formation of photorefractive channel waveguides along forbidden by
photorefractive properties of lithium niobate direction using point-by-point exposure of its photorefractive surface is
experimentally demonstrated. It is shown that phase diffraction gratings are optically induced only within the crystal
surface part doped with photorefractive impurities in studied crystal samples. The dependence of diffraction grating
thickness on wavelength of writing light is also demonstrated and explained by distinction of optical absorption within
doped layer for light waves of different wavelengths.
The features of linear and nonlinear propagation of light beams in one-dimensional photorefractive photonic superlattices
in bulk lithium niobate and in planar waveguides on this material are experimentally studied. The superlattices are
optically induced in bulk samples and in planar waveguides using two-beam holographic recording method and optical
projection scheme with coherent and incoherent light sources.
Formation of dark spatial optical solitons in planar waveguides produced by implantation of light ions into Fe- or Cudoped
X cut lithium niobate wafers is experimentally studied. The implantation both of protons and O3+-ions results in
the excellent waveguide layers with their thickness about 3 microns and optical losses less than 1 dB/cm. The soliton
states at light wavelengths of 532 nm and 633 nm are developed due to the self-defocusing photorefractive-photovoltaic
nonlinearity of lithium niobate. Extraordinarily polarized light beams are used in experiments to form dark solitons and
to probe the soliton-induced waveguide channels. Steady-state dark photovoltaic spatial solitons have been realized in
both, H+- implanted and O3+ - implanted planar waveguides at optical powers from 10 to 100 microwatts. The storage
time of soliton-induced channel waveguides makes up at least some hours without special illumination of a planar
waveguide and they may be erased within some seconds in a case of their permanent readout with stronger light beams.
The possibility to form more complicated channel waveguide structures in regimes of dark spatial solitons is also
demonstrated.
We experimentally investigate effects of linear and nonlinear propagation of light beams within one-dimensional
photonic superlattices fabricated in bulk photorefractive lithium niobate samples and in photorefractive planar
waveguides by optical induction technique. In other case similar superlattices are formed by optical modulation of
periodic waveguide arrays produced in lithium niobate by thermal diffusion of titanium and iron. The linear localization
of light power is experimentally observed in superlattices of all kinds and proved using numerical simulations of light
propagation within such structures. The features of nonlinear behavior of light at its propagation in superlattices is also
experimentally demonstrated in a configuration of their single-channel excitation.
Results of experimental investigations of temperature dependences of optical absorption coefficients and their light-induced
changes for Bi12SiO20, Bi12TiO20:Zn, and Bi12TiO2:Ca crystals are presented. Semiconductor light diodes
emitting quasimonochromatic radiation in red (625 nm), yellow (570 nm), green (505 and 515 nm), and blue (470 nm)
ranges of the spectrum were used as light sources. A theoretical model of thermal- and light-induced changes in the
optical absorption is considered that describes well the observed temperature dependences of the optical absorption for
the Bi12SiO20 and Bi12TiO20:Ca crystals and the dynamics of light-induced changes in light absorption for the
Bi12TiO2:Ca crystal at a wavelength of 470 nm and temperatures in the interval from 24 to 100°C.
We experimentally investigate some features of the self-defocusing of light beams as well as the relaxation of waveguide channels induced by dark spatial solitons at wavelength of 632.8 nm within slab optical waveguides formed in lithium niobate by iron diffusion or combined diffusion of titanium, iron and copper. We also study an influence of the externally applied electric field on both, spatial self-action of single beam within such waveguides and external focused beam which propagates in direction normal to the waveguide surface. Experiments show that for lowest TE modes the photovoltaic nonlinearity results in the fast but not very strong beam defocusing and for modes of higher order this effect is slower but much stronger. It is also demonstrated that at suitable light intensity the screening of the external electric field significantly contributes to the beam self-action at its initial stage even for mediate value of applied electric field.
We experimentally investigate the photorefractive nonlinearity within lithium niobate samples with surface doping by combinations of copper (Cu) and iron (Fe). In the configuration of closed Z-scan we use either helium-neon laser irradiation with wavelength of 633 nm or continuous wave semiconductor laser with wavelength of 650 nm. Light power in experiments ranges from 1 to 4 mW. We observe very strong Z-scan response in the closed scheme at the transient stage and the different temporal behavior of this response for different locations of the light beam waist with respect to the doped surface of samples studied. The approach of thin light-induced nonlinear lens is used to estimate the Z-scan response in the steady state in dependence on the nonlinear sample position. We also use this approach to compare the observed distortions of the light beam due to its spatial self-action with expected change of the beam profile.
A theoretical model which can describe the dynamics of spatial bright discrete solitons in photovoltaic photorefractive media is suggested. Four different types of staggered solitons, symmetric odd and even, twisted and antisymmetric odd one are observed numerically and their behavior is investigated with respect to small transverse phase offsets. A first experimental observation of steering of such symmetric odd staggered solitons in lithium niobate waveguide arrays is presented.
Photorefractive one-dimensional photonic lattices are optically induced in iron-doped lithium niobate. Discrete linear diffraction and formation of bright gap solitons due to the spatial self-action of light beams within such lattices are experimentally investigated at wavelength of 633 nm.
We experimentally investigate propagation of light beams of visible range within waveguide arrays of one-dimensional photorefractive waveguides formed in the Y cut lithium niobate wafers. We demonstrate formation of bright staggered discrete spatial solitons within such an array at simulataneous π out fo phase excitation of two waveguide channels by cw light beam with wavelength of 514,4 nm and total power of 200 μW.
We report the experimental investigation both, of the one - dimensional and two - dimensional bright screening spatial solitons in a promising photorefractive crystal of barium - calcium titanate doped with iron. The effects of the partial trapping of the spatially incoherent ordinarily polarized background light within the soliton - induced planar waveguide as well as the regular oscilations of the light beam intensity and width during the beam self - focusing are observed. For the crystal sample, doped with higher dose of iron (290 ppm), we also find the self - defocusing of light beams due to the light - induced negative photovoltaic lens in the absence of the externally applied electric field.
We study the processes of formation of dark photovoltaic spatial soliton in photorefractive LiNbO3:Ti:Fe waveguide. The 2-D distribution of the optical field is taken into account to define the nonlinear change of the refractive index.
We study the effects of self-defocusing and nonlinear interference for light beams propagating in photorefractive LiNbO3:Ti:Fe waveguide. The wave equation in paraxial approximation for this case of a crystal with the photovoltaic type of nonlinearity is used. A numerical approach allows to study the interference processes for waveguide modes both of the same and different orders. It is found that the behavior of interference minimum in the case of two parallel beams with a phase shift of π between each other looks like a channel waveguide induced by a dark soliton. The intensity dependence of defocusing angle and the nonlinear interference in waveguide formed by sequential diffusion of titanium and iron in Z-cut of LiNbO3 have been experimentally investigated.
The one-dimensional bright spatial screening solitons in a promising photorefractive crystal of barium-calcium titanate doped with iron are experimentally investigated. The effects of the regular oscillations of the light beam intensity and width during the beam self-focusing and in the soliton regime are observed. For the crystal sample, doped with higher dose of iron (290 ppm), we also find the self-defocusing of light beams due to the light-induced negative photovoltaic lens in the absence of the externally applied electric field. (Summary only available)
Effect of a light leakage may be observed as in planar waveguides formed in optically anisotropic materials as in primary isotropic multi-layer waveguide structures. This effect results in the exponential decay of the light intensity within the waveguide, but on the other hand it may be used to connect light beams within a substrate with guided light to excite waveguide modes. The characteristics of a light leakage in configurations corresponding to proton-exchanged waveguides in LiNbO3 are discussed.
We have realized dark spatial photovoltaic solitons at wavelength of 632.8 nm in Fe and Cu - doped planar optical waveguides in lithium niobate. It has been shown that for lowest TE modes the photovoltaic nonlinearity only partly compensated the light diffraction effect at the experimental conditions used. The development time of corresponding light-induced photovoltaic lenses ranged from 0.1 s to 30 s depending on the waveguide sample. For the highest TE modes we observed the exact compensation of diffraction effects in the time scale of some minutes followed in some cases by the termination of dark solitons formed because of the light overdefocusing.
The steady-state bright spatial solitons have been realized in a planar photorefractive strontium-barium niobate waveguide for visible light, ranging from 514.5 to 780 nm. The build-up time of the steady-state condition was about 1 s. For the wavelength of 1047 nm, the strong self-focusing of a light beam was observed. In the time scale of several tens of milliseconds the transient self-trapping of red light was observed, which corresponds to the regime of quasi-steady-state solitons. The interaction of the steady- state spatial solitons has also been demonstrated at He-Ne laser wavelengths.
The features of intensity shape changes of single focused light beams caused by the photorefractive self-action effect in iron-doped lithium niobate optical waveguides are experimentally investigated and discussed.
The electrooptic modulation of the light intensity at excitation of leaky modes in annealed proton-exchanged optical waveguides in lithium niobate are experimentally investigated. The decreasing of a modulation efficiency due to the electrooptic coefficients reduction is observed and the possibility of such modulation method even for the axial light propagation is underlined.
Optical waveguides in melts of benzoic acid with the addition either of copper or iron oxides have been fabricated and two-beam recording of holographic gratings due to the photorefractive effect have been realized in waveguides produced.
Four-wave photorefractive parametric interactions in lithium niobate planar optical waveguides with advanced photorefractive sensitivity have been observed and experimentally studied. Photorefractive gratings of arbitrary periods at using the extraordinary radiation mode as a pump have been obtained in such interactions.
Excitation of leaky optical waves in optical waveguides on lithium niobate while total internal reflection of light beam with corresponding polarization from their surfaces has been investigated. The "mirror' configuration of waveguide operating elements on the ground of electrooptic and photorefractive interactions with leaky wave participation has been proposed and some results of their experimental investigations are presented.
Key words: Optical waveguide, leaky waves, electrooptical effect, photorefractive effect, light modulation.
Anisotropic optical waveguides in some cases can connect between each other external light beams and waveguide optical fields without any additional coupling elements, only due to existing of leaky optical waves in such waveguides. Because of the simplest design anisotropic waveguides with light leakage can be used in many integrated-optical elements including waveguide sensors. Some features of leaky wave excitation in planar waveguides LiNbO3:Ti and H+:LiNbO3 while total internal reflection of ordinary polarized light beams from a waveguide boundary have been experimentally investigated in this work. Besides, an influence of temperature changing on coupling angle variations at such excitation method has been considered as well with regard to possible applications of similar elements in integrated optics.
Dynamic holographic gratings can be used in optical waveguides on photorefractive electro- optical crystals for an information input in integrated-optical algebraic processors. An influence of the Fe and Cu additional diffusion on the refractive index profile and photorefractive properties of LiNbO3:Ti waveguides has been investigated and two possible schemes for dynamic holographic element formation at the single input light beam have been proposed. Holographic grating recording at photorefractive parametric interaction of radiation and waveguide modes is used in the first of them and formation of such grating while light diffraction at counter propagating and a little tilted surface acoustic wave beams -- in the second one.
Waveguide modes with their energy leakage into the substrate while light propagation even along the crystal main axes can exist in proton-exchanged optical waveguides on lithium niobate because of the considerable influence of that process on crystal anisotropic properties. This effect can be used to create planar coupling elements for integrated-optical processors by means of proton exchange in local areas of LiNbO3:Ti waveguides. Some results both of experimental and theoretical investigations of this effect and of the proton exchange coupling element are presented in this paper.
Holographic photorefractive gratings in optical waveguides with high photorefractive sensitivity on lithium niobate can be used for an information input in integrated-optical-vector- matrix multiplication devices. An influence of the Fe and Cu additional diffusion on the refractive index profile of LiNbO3:Ti waveguides and their photorefractive properties has been investigated and a matrix component input in the device with acousto-optical vector component input are demonstrated.
Several results of investigations of leaky optical wave (LOW) excitation in LiNbO3 planar waveguides at the nonaxial light propagation and of their acoustooptical interaction with surface acoustic waves (SAW) in such waveguides are presented. In experiments, leaky waves were excited by the total internal reflection of an ordinary polarized laser beam from the waveguide surface. The LOW excitation efficiency dependence and distortions of the reflected beam intensity distribution on the light propagation direction were investigated, the novel geometry of radiated ordinary mode conversion into the extraordinary one due to the diffraction of leaky waves on SAW was observed, and LiNbO3 acoustooptical modulators using LOW were realized. Effects of holographic grating photorefractive recording in LiNbO3 waveguides also were studied.
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