Light-induced Index Changes Research Articles

ALL-OPTICAL LOGIC GATES BASED ON SPATIAL-SOLITON INTERACTIONS IN OPTICAL COMMUNICATION SPECTRAL REGION

New designs of all-optical logic gates based on spatial-soliton interactions in optical communication spectral regions were proposed. The proposed structures are composed of local nonlinear Mach-Zehnder interferometer (MZI) waveguide structures with multi-input ports and two nonlinear output ports. They can be used to design various all-optical logic gates. The nonlinear MZI waveguide structure with local nonlinear waveguides functions like a phase shifter. It employs angular deflection of spatial solitons controlled by the phase modulation created in the local nonlinear MZI. The light-induced index changes in the local nonlinear MZI waveguide structures break the symmetry of structure and make the output signal beam propagate through different nonlinear output waveguides. By properly choosing the input control power, the spatial solitons will be switched to different output ports. The numerical results show that the proposed local nonlinear MZI waveguide structures could really function as all-optical logic gates in the optical communication spectral region.

Signal amplification based on the local nonlinear Mach–Zehnder interferometer

Using the phase modulation of spatial solitons, a new scheme for all-optical signal amplification has been proposed in this paper. The considered structure is composed of the nonlinear Mach–Zehnder interferometer (NMZI) with the straight control waveguide (CWG), the uniform nonlinear medium (NLM) and the linear output waveguide. The local NMZI functions like a phase shifter. The light-induced index changes in the local nonlinear MZI are responsible for the input beam routing in the uniform nonlinear medium. The coupling of the input beam to the output waveguide depends on its propagation direction in the NLM. It is shown that the signal launched at CWG can deflect the beam launched at the NMZI (input beam) and a modulated (amplified) output could be obtained at the output waveguide. Further, signal pulse may be reshaped by appropriately increasing the NLM length. In addition, amplification factor may be enhanced by increasing the NLM length and injecting an appropriate continuous wave beam along with the signal beam at CWG.

Correlation between photorefractive index changes and optical damage thresholds in z-cut proton-exchanged-LiNbO_3 waveguides

An interferometric Mach-Zehnder technique very recently developed has been applied to measure photorefractive index changes in different types of z-cut proton-exchanged planar waveguides in LiNbO(3). These measurements are complemented by determining the intensitythreshold for the onset of optical damage with a standard single-beam setup. In the intensity region just below the threshold-intensity obtained in the single-beam experiment the refractive index change is found to saturate at values around1x10(-4). Furthermore, we measure the dark conductivities of proton-exchanged waveguides by monitoring the decay of the light-induced index changes. Via the time constant of the decay we obtain dark conductivities of the order of about 5x10(-16) Omega (-1) cm (-1), that are negligible compared with the photoconductivity within the light intensity range used. The results of the measurements compare well with the predictions of a recent work, that uses a two-center model to explain the optical damage.

New all-optical logic gates based on the local nonlinear Mach-Zehnder interferometer

We propose new all-optical logic gates containing a local nonlinear Mach-Zehnder interferometer waveguide structure. The light-induced index changes in the Mach-Zehnder waveguide structure make the output signal beam propagate through different nonlinear output waveguides. Based on the output signal beam propagating property, various all-optical logic gates by using the local nonlinear Mach-Zehnder waveguide interferometer structure with two straight control waveguides have been proposed to perform XOR/NXOR, AND/NAND, and OR/NOR logic functions.

All-optical switch based on the local nonlinear Mach-Zehnder interferometer

We proposed a new all-optical switch by using the phase modulation of spatial solitons. The proposed structure is composed of the nonlinear Mach-Zehnder interferometer (MZI) with the straight control waveguide, the uniform nonlinear medium and the nonlinear output waveguides. The local nonlinear MZI functions like a phase shifter. The light-induced index changes in the local nonlinear MZI make the output signal beam routing in the uniform nonlinear medium. The all-optical switching scheme employs angular deflection of spatial solitons controlled by phase modulation created in the local nonlinear MZI. By properly launching the control power and increasing the length of the uniform nonlinear medium, this device can be generalized to a 1xN all-optical switch. It would be a potential key component in the applications of ultra-high-speed optical communications and optical data processing system.

Optimum exposure intervals for light-induced optical waveguides in lithium niobate crystals by laser micromachining

The optimal exposure intervals for optical waveguides induced by laser micromachining in LiNbO3 crystals are theoretically and experimentally investigated. When the focused laser beams scan along different directions, the optimal intervals for waveguide fabrications are numerically specified by solving the photorefractive dynamic equations. The simulations show that the index distributions of the fabricated waveguides are strongly dependent on the optimal exposure interval, and the optimal exposure interval is not dependent on the scanning directions of the writing beam. When the writing beam scans the crystal along the c axis for sandwich illumination, optical waveguides cannot be fabricated efficiently. However, in this case, symmetric refractive index changes can be obtained. Experimental demonstrations are performed by scanning LiNbO3:Fe crystal with a focused green laser beam. The light-induced index changes are measured by digital holography. The experimental results conform to the theoretical analyses. Additionally, a Y-branche waveguide is experimentally formed and the results of the guiding tests are showed in this paper.

Light-induced scattering in SBN:Cr crystal under external electric fields and its suppression

In this paper, the enhancement of light-induced scattering in congruent SBN:Cr (Sr0.61Ba0.39Nb2O6:Cr) crystals in the presence of an externally applied electric field and its suppression are studied. If a coherent image is focalized in SBN:Cr crystal without applying external electric field, the output image will remain clear, because of the weak photorefractive effect in the crystal. When a field is applied properly along the crystal axis, markedly enhanced scattering from the signal beam and the output image dispersion can be observed due to the increase of the photorefractive two-beam coupling gain and the light-induced index change in SBN:Cr crystals. By introducing a coherent or incoherent beam with higher intensity the light-induced scattering can be suppressed through the erasure of scattering gratings. The difference between coherent and incoherent beam is that the former can also amplify the signal beam as the scattered light is removed, whereas the latter can only make the signal beam revert to its initial state. The results obtained under different experimental conditions are consistent with theoretical analysis.

Growth and characteristics of Fe-doped KTiOPO 4 crystals

Single crystals of Fe-doped KTiOPO 4 (Fe : KTP) of optical quality and dimensions up to 15×25×30 mm were grown by the flux method. During the growth process the formation of twin consisting of bicrystals was observed, even in the case of single-crystalline seeds. The Fe doping level in crystals is about 0.08–0.1wt%. The following physical properties were investigated: refractive indices, dielectric, piezoelectric, electro optical, elastic and thermoelastic constants. In order to determine whether Fe ions play a major role in photorefractive effect (PRE) of KTP, light-induced index change was measured and the results indicate that in KTP crystals Fe ions do not play a major role in PRE. It is also found that Fe : KTP shows different diffusion properties during the Rb : K ion exchange process in comparison with undoped KTP.

Light-induced index change with a fast response time and nonlinear absorption in the waveguide of a novel organic quinoid dye

A light-induced index change and nonlinear absorption were clearly observed in channel optical waveguides fabricated from a novel quinoid dye, 4,8-dihydroxy-2,6-bis(butylamino)-1,5-naphthoquinone. The measured response time of the index change was below 10 ps, and the magnitude of the light-induced phase shift was more than 2π at the wavelengths of 780 and 800 nm in a 10-mm-long waveguide. The values of the nonlinear refractive index and the nonlinear absorption of the dye estimated from the experimental results are of the same order as that measured by the third-harmonic-generation method. This waveguide should lead to the construction of all-optical functional devices that have fast operating times.

Photorefractive Properties of Paraelectric Copper Doped Potassium Tantalate Niobate

The photorefractive properties of Cu doped KTa1-xNbxO3 have been studied by using two-wave mixing method. The diffraction efficiency reaches as high as 52% under an electric field. The dependences of diffraction efficiency on interference angles and applied fields have been studied. The maximum light induced index change is found to be 2.33 × 10-5 and the photorefraction sensitivity calculated is 5 × 10-6 cm2/J. Experimental results demonstrate that electrons are the dominant charge carriers.

Ultraviolet light photosensitivity in Ge-doped silica fibers: wavelength dependence of the light-induced index change

A novel technique is reported for detecting permanent and transient light-induced refractive-index changes (photosensitivity) in optical fibers. The index change is detected by irradiating one arm of an unbalanced Mach-Zehnder fiber interferometer with UV light, thereby changing its optical path length. From a measurement of the change in the spectral response of the Mach-Zehnder interferometer, the change in the fiber core index as a function of wavelength can be determined. The equilibrium change in the core index is found to have an almost constant value of approximately 2.3 x 10(-5) over the measured wavelength range of 700 to 1400 nm.

Role of two-photon absorption in ultrafast semiconductor optical switching devices

We report measurements of the two-photon absorption coefficient of GaAs optical waveguide structures at 1.06 μm. We show that for pulse lengths longer than ∼1 ps, light-induced index changes sufficient to induce all-optical switching will be predominantly due to carriers generated by two-photon absorption. These results allow us to predict limitations for ultrafast all-optical GaAs devices.

Thermal index changes by optical absorption in group III-V semiconductor waveguides

Large fast-relaxing intensity-dependent refractive index changes would make certain all-optical signal processing devices in waveguides feasible. In III-V compound materials, light-induced index changes can arise from any of several physical mechanisms which have different strengths and relaxation times. Index changes from 1.06-μm pulsed and CW Nd: YAG laser excitation were measured in single-mode low-loss InP and GaAs waveguide Fabry-Perot structures. The dominant effect was found to be due to heating of the waveguide volume which accompanies an extremely small amount of free-carrier absorption.

Photorefractive effects and photocurrents in KNbO3: Fe

Holographic measurements of optically induced refractive index changes (light damage) of Fe doped KNbO3 are interpreted in terms of photoconductivity results. In “undoped” KNbO3 the light damage is one order of magnitude smaller than in LiNbO3-The light induced index changes can be drastically reduced or enhanced by applying a relatively small de-field along the polar ±c axis. The space-charge field due to the “photovoltaic effect” is then increased or decreased by photoconductivity.The spatial frequency dependence of the light-induced index changes indicates, that diffusion processes of charge carriers are important for the light damage in KNbO3 if the fringe spacing α of the recorded hologram is smaller than 2 μm. For α = 0.35 μm this term is two orders of magnitude larger than the “photovoltaic effect” and the photoconductivity term.The dispersion of the absorption-and photoinduced current-density measurements yield basic information on the energy levels of the photorefractive centers controlling the i...

Optically induced refractive index changes in KNbO3:Fe

Abstract The light induced index changes in KNbO3: Fe have been investigated by the holographic technique using optical power densities up to I = 300 W/cm2 at λ = 488 nm. For undoped crystals the light induced index change is smaller than 3 λ 106 for all light intensities used. The index change can be drastically reduced or enhanced by applying a small dc field along the ±c axis.