Photorefractive LiNbO3 Crystal Research Articles

Bending optical soliton-induced waveguide channels in a photorefractive LiNbO3 crystal

Bending optical soliton-induced waveguide channels in a photorefractive LiNbO3 crystal

Optical limiting and speckle of low power continuous wave laser beams using nonlinear scattering in photorefractive Zr: LiNbO3 crystals

The optical limiting caused by nonlinear scattering in the photorefractive Zr-doped LiNbO3 crystals was studied for lower-power laser beams with wavelengths of 514.5 and 532 nm. The measurements of speckle evolution indicate that the samples with 0.625 ≤ [Zr] ≤ 1.25 mol% exhibit the very large gain factor for holographic amplification of noise gratings. Based on the direct measurements of the output power depending on the input one, we conclude that these samples behaved as hard optical limiters for the laser beams with a low incident power. Thus, these crystals find their potential application in the optical limiting devices.

Parameters of nonlinear scattering evaluated by open-aperture Z-scan technique in photorefractive LiNbO3 crystals

An open-aperture Z-scan experimental technique was used to evaluate parameters of the nonlinear scattering (NLS) in the photorefractive (PR) LiNbO3 crystals. NLS is related to the photoinduced light scattering caused by the holographic amplification of the seed scattering due to PR effect. The values of the NLS coefficient, amplification gain factor and seed scattering ratio were evaluated and found to be increasing with the light intensity.

Non-linear light scattering in photorefractive LiNbO3 crystals studied by Z-scan technique

Experimental study of non-linear scattering (NLS) in nominally pure, Mg- and Zr-doped LiNbO3 crystals with a varying dopant concentration are reported in this paper. The study is undertaken by Z-scan technique with a cw-excitation at 514.5 nm. A modified open-aperture Z-scan experimental setup is used to evaluate the contributions of NLS in the observed transmission attenuation. Study of these open-aperture Z-scan traces shows that NLS has marked magnitude at moderate and high light intensities in undoped, strongly Zr-doped and Mg-doped LiNbO3, and a very significant magnitude in the moderately Zr-doped LiNbO3 crystals even at low intensities. NLS is related to the photoinduced light scattering (PILS), which is explained by holographic amplification of the seed scattering due to photorefractive effect.

Structural homogeneity of photorefractive LiNbO3 crystals doped with 0.03–4.5 mol % of ZnO

Using the electronic spectroscopy method, the laser-conoscopy method, and the Raman light-scattering method, we have studied the structural homogeneity of LiNbO3 crystals doped with 0.03–4.5 mol % of ZnO. We have found that, as the laser radiation power is increased to 90 mW, the conoscopic patterns of crystals show additional distortions, which are attributed to the manifestation of the photorefractive effect. For the LiNbO3 crystal doped with 4.5 mol % of ZnO, in which the photorefractive effect is low, we have revealed a considerable shift (compared to the remaining crystals) of the optical absorption edge toward the shortwavelength range, which indicates a high structural homogeneity of this crystal. We have shown that, in the LiNbO3 crystal doped by 0.05 mol % ZnO, due to the displacement of NbLi and Li□ structural defects by Zn2+ cations, the crystal structure is ordered and, simultaneously, the number of defects with localized electrons decreases.

Writing and probing light-induced waveguides thanks to an endlessly single-mode photonic crystal fiber

We demonstrate writing and probing of light-induced waveguides in photorefractive bulk LiNbO3 crystal using an endlessly single-mode photonic crystal fiber. The optical waveguides are written at visible wavelengths by slightly raising the ferroelectric crystal temperature to benefit from the pyroelectric-driven photorefractive effect and the guiding properties are investigated at telecom wavelengths using the same photonic crystal fiber. End butt coupling with this photonic crystal fiber enables writing and probing of optical waveguides due to the self-alignment properties of spatial solitons.

Multiple-hologram recording with one-beam encoding

This research proposes a method that uses a single object beam to record multiple images in a photorefractive crystal medium without having to use any reference wave. The object beam in this study is modulated using a lenticular lens array sheet to produce a set of sub-object beams. These beams are then angularly separated on the recording plane and their scattered waves overlapped in an iron-doped photorefractive LiNbO₃ crystal. This single-exposure, multiple-holographic-recording method is simple and proven successful via the experiments that recorded four holograms in a 30 x 1 mm³ LiNbO₃:Fe crystal with single exposure.

Diffraction properties study of reflection volume holographic grating in dispersive photorefractive material under ultra-short pulse readout

Based on the modified Kogelnik diffraction efficiency equation, we study the diffraction intensity spectrum and the total diffraction efficiency of reflection volume holographic gratings in photorefractive media. Taking photorefractive LiNbO3 crystal as an example, the effect of the grating parameters and the pulse width on the diffraction properties is presented under the influence of crystal material dispersion. Under the combined effects, the diffraction pulse profiles and the total diffraction efficiency are compared with and without crystal material dispersion. The results show that the dispersion will decrease the diffraction intensity. Moreover, when pulse width is smaller or the grating spacing and the grating thickness are larger, the influence of dispersion on diffraction is large. The results of our paper can be used in pulse shaping applications.

Instantaneous characteristics study of the diffracted and transmitted light of a staticphotorefractive reflection volume holographic grating read by an ultrashort pulselaser

The time-domain diffraction and transmission of reflectionvolume holographic gratings (RVHGs) recorded in photorefractiveLiNbO3 crystals are theoretically studied and compared with transmission volume holographicgratings (TVHGs) when they are illuminated by ultrashort pulse laser light. The modifiedKogelnik’s coupled-wave theory, combining the dispersion effect in the crystal, is used.The results show that larger diffraction intensity can be obtained by choosing alarger grating period, grating thickness and modulation index for a given pulseduration. The RVHG has higher diffraction intensity than the TVHG with thesame grating parameters. Also, it is found that the diffracted pulse of the RVHGand the TVHG shift in opposite directions along the time-axis. The values ofthe shifts are functions of the readout pulse duration and grating parameters.The overmodulation phenomenon appears in the diffracted pulse of the TVHGat high values of the grating thickness and refractive index modulation, but itdoes not occur in the RVHG. The results of our study can be used in choosingproper grating parameters for pulse shaping and signal processing applications.

Wavelength multiplexing and demultiplexing with one single volume hologram in photorefractive crystal

Based on anisotropic diffraction of a volume phase grating in a photorefractive crystal, we theoretically discuss an optical multi- and demultiplexing scheme implemented by one single grating in photorefractive LiNbO3 crystal. It shows that our scheme can simultaneously demultiplex 93 channels in the telecommunication wavelength around 1550 nm. Using only one grating to realize WDM can avoid the multiple exposure problems encountered by multiple hologram scheme. Moreover, in our scheme, the transmitted and diffracted beams are orthogonal to each other, thus we don't need to use a normal recording and readout structure. Our theoretical results can be used in the design of a WDM device.

Improved Holographic Recording Techniques for Data-Page Rewriting

Novel holographic recording configurations capable of selective erasure of multiplexed holograms for data-page rewriting in holographic storage are presented. By using a half wave plate and a polarizing beam splitter as the phase control device of the interference pattern in holographic recording/erasure procedure, comparatively large-area and finespun data-pages may be recorded, selectively erased, and rewritten in simple operations. We experimentally demonstrate the selective erasure of multiplexed data-pages in photorefractive iron-doped LiNbO3 crystals, and show the effectiveness of the configuration. We also propose a method to improve the in-plane uniformity of the erasure making use of the diffuser in the optical path of the object beam which is employed for dispersing the object image and the phase conjugate readout technique.

Laser induced memory bits in photorefractive LiNbO3 and LiTaO3

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO3 and LiTaO3 crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10−4 was recorded in undoped LiNbO3 at an average light intensity of ∼TW/cm2 that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed.

光折变LiNbO3晶体中单光栅实现波长解复用方案

光折变LiNbO₃晶体中单光栅实现波长解复用方案

Three-dimensional write–read–erase memory bits by femtosecond laser pulses in photorefractive LiNbO3 crystals

We report on a three-dimensional single-shot optical recording by 150fs pulses at 800nm wavelength in Fe-doped and stoichiometric undoped LiNbO3. The highest refractive index modulation of ∼10−3 per single pulse has been achieved at close to the dielectric breakdown irradiance of ∼TW/cm2, and was found to be independent of polarization of the writing beam. The quasi-permanent and rewritable bits have been demonstrated in Fe-doped LiNbO3. Recording in undoped stoichiometric LiNbO3 was of transient nature with full optical contrast recovery on a 0.1–1s scale. The observed refractive index modulation is explainable by the photovoltaic effect in LiNbO3.

Dynamics of photodeformations and space charge field in photorefractive Fe:LiNbO3 studied with synchrotron area diffractometry

The method of synchrotron area diffractometry has been used to map with a high spatial resolution in real time the distribution over the crystal surface of the lattice deformation tensor components induced by a macroscopic visible light beam on a z-cut iron-doped photorefractive LiNbO3 crystal. The xy planes exhibit a tensile strain, on the order of 10−4, in the center and long range, extending up to 780μm, shear deformations at the borders of the illuminated region, respectively. Photodeformations evolve with illumination time with relaxation type, time dependence, and time constants of the order of minutes. The observed lateral distribution of deformation tensor components, as well as their temporal evolution, has been examined, considering the coupling of the converse piezoelectric effect with the strong space charge field generated by the damage inducing beam along the z axis due to the bulk photovoltaic effect. The observed strain in the center can be attributed to the bulk photovoltaic field of the order of 107V∕m, while the long range shear deformations are mainly associated with the lateral components of the electric field which are present at the borders of the space charge regions. Both photodeformations and space charge field evolve at the same time scale, the dependence of time constants on the incident light intensity following the predictions of the one-center model charge redistribution due to the bulk photovoltaic effect. This work demonstrates that the method of synchrotron area diffractometry is a very powerful tool to study in situ the dynamics and spatial variation of microstuctural changes (deformations) induced by an external field (electric, magnetic, or temperature).

Angle multiplexed storage with object beam alone

Making use of the interference of incident light waves and the accompanying fanning waves in a photorefractive LiNbO3 crystal, the authors have recorded binary images as well as gray-tone images in a crystal with only an object beam. Angle multiplexing has also been demonstrated with this one beam recording method. Basing on a model that the recording is accomplished through the interference of the object light and its fanning light, the authors have estimated the minimal angular separation for achieving a cross-talk-free multiplexing.

Diffraction of an ultrashort pulsed beam with arbitrary polarization state from a volume holographic grating in LiNbO 3 crystals

Based on a modified coupled wave theory of Kogelnik, we have studied the diffraction of an ultrashort pulsed beam with an arbitrary polarization state from a volume holographic grating in photorefractive LiNbO3 crystals. The results indicate that the diffracted intensity distributions in the spectral and temporal domains and the diffraction efficiency of the grating are both changed by the polarization state and spectral bandwidth of the input pulsed beam. A method is given of choosing the grating parameters and input conditions to obtain a large variation range of the spectral bandwidth of the diffracted pulsed beam with an appropriate diffraction efficiency. Our study presents a possibility of using a volume holographic grating recorded in anisotropic materials to shape a broadband ultrashort pulsed beam by modulating its polarization state.

Pulse shaping properties of volume holographic gratings in anisotropic media

Based on a modified coupled wave theory, the pulse shaping properties of volume holographic gratings (VHGs) in anisotropic media VHGs are studied systematically. Taking photorefractive LiNbO(3) crystals as an example, the combined effect that the grating parameters, the dispersion and optical anisotropy of the crystal, the pulse width, and the polarization state of the input ultrashort pulsed beam (UPB) have on the pulse shaping properties are considered when the input UPB with arbitrary polarization state propagates through the VHG. Under the combined effect, the diffraction bandwidth, pulse profiles of the diffracted and transmitted pulsed beams, and the total diffraction efficiency are shown. The studies indicate that the properties of the shaping of the o and e components of the input UPB in the crystal are greatly different; this difference can be used for pulse shaping applications.

Formation of photovoltaic bright spatial soliton in photorefractive LiNbO3 crystal by a defocused laser beam induced by a background laser beam

We have shown, for the first time to our knowledge, both theoretically and experimentally that a bright photovoltaic (PV) spatial soliton (SS) can be formed in a photorefractive crystal with large Glass constant and negative refractive index perturbation, provided that the Glass constant of the background beam is larger than that of the self-trapped (signal) beam. The ratio of the effective Glass constants of the self-trapped beam and the background beam is the key parameter that determines whether the PV SS is bright or dark. We have demonstrated experimentally that bright SSs can be formed in LiNbO3 crystals using all four combinations of polarizations of the signal beam and the background beam.

Holographic angle multiplexing combined with peristrophic multiplexing in a photorefractive LiNbO3 crystal

We have investigated angle multiplexing combined with peristrophic (rotational) multiplexing in a photorefractive LiNbO3 crystal of cylindrical shape. The peristrophic multiplexing was achieved by rotating the recording medium while the angle multiplexing, by varying the incident angle of the reference beam. Angle multiplexing in synchronization with rotation of the sample made it possible to continuously record and retrieve holograms. In the combined multiplexing, holograms were stored at different peristrophic positions and thus their angular selectivity could be much improved in comparison to that of only angle-multiplexed holograms. The theoretical angular selectivity was derived, along with its dependence on the sample rotation. The experimentally measured selectivity was in agreement with the theoretical one. The angular selectivity of angle-multiplexed holograms was measured to be 0.03° in the absence of sample rotation and it became less than 0.0075° with a sample rotation by 5.4°.