Sensitivity enhanced all-optical switching using prism-grating coupled surface plasmon modes

A sensitivity enhanced all-optical-switch design using prism-sinusoidal grating coupled surface plasmon modes is demonstrated numerically. Owing to the simultaneous excitation of surface plasmons for the pump and signal at the interface between a nonlinear waveguide layer and a metal layer, a sensitivity enhanced bistability effect in the structure can be obtained. The nonlinear refractive index of the waveguide layer experiences an abrupt change of the local pump field owing to this bistability effect. Such a characteristic greatly improves the performance of optical switch by reducing the threshold pump intensity. An all-optical-switch design with a pump threshold as low as 0.9 GW/cm² is presented.     

Propagation dynamic of a Gaussian in the inverted nonlinear photonic crystals

In this work, we study the evolution of a Gaussian beam inside a one-dimensional inverted nonlinear photonic crystals (INPC) with a Kerr nonlinearity. The INPC is a kind of virtual crystals which is generated by the optical induction via the electromagnetically induced transparency (EIT). The propagation dynamics of the Gaussian with different total power are identified. Four types of propagation behavior are found. They are collapse beam, breather beam, soliton and symmetry-breaking beam, respectively. The border between these four behavior types are given. For symmetry-breaking beam, an asymmetric profile of the beam is evolving from the symmetry Gaussian, which can be termed as a kind of dynamical symmetry breaking (DSB). The influences on the appearance of the symmetry breaking point are studied by varying input parameters of the Gaussian. The results of this work are both suitable in nonlinear optics and Bose-Einstein condensate (BEC).     

Optical modes in a nonlinear double-channel waveguide

We analytically address different types of optical modes in a coupler composed by two nonlinear optical waveguides. It is shown that the coupler not only supports symmetry-preserving modes but also symmetry-breaking modes. In addition, the properties on the existence and bifurcation of those modes are analyzed in detail.     

Enhanced third harmonic reflection and diffraction in Silicon on Insulator photonic waveguides

Enhanced third harmonic (TH) generation from Silicon-On-Insulator (SOI) planar waveguides as well as SOI photonic crystal (PhC) slabs is studied in different angular configurations, both in the visible and infrared energy ranges. In the SOI planar waveguide, the multilayer structure causes the optical properties such as TH reflection to be different from those of bulk silicon samples. This behavior is well reproduced by calculations of TH reflectance.Measurements of third-harmonic reflection and diffraction from one-dimensional PhC slabs etched in the SOI waveguide are also reported. The angular positions of TH peaks at various diffraction orders agree well with those calculated from a nonlinear grating equation. Both reflection and diffraction processes contribute to enhanced TH generation efficiency in the PhC slabs.TH reflectance measurements performed on PhC slabs in the near infrared show a resonant interaction between the incident beam and the photonic structure, dependent on the angle of incidence. This leads to a nonlinear conversion efficiency which is strongly enhanced with respect to that of the SOI waveguide, due to the excitation of strong local fields associated with the presence of photonic modes in the PhC slab.     

Gap solitons in defocusing lithium niobate binary waveguide arrays fabricated by proton implantation and selective light illumination

Photovoltaic photorefractive binary waveguide arrays are fabricated by proton implantation and selective light illumination on top of an iron-doped near stoichiometric lithium niobate crystal. Linear discrete diffraction and nonlinear formation of gap solitons were investigated by single-channel excitation using Gaussian light beams coupled into either wide or narrow waveguide channels. The results show that, at low power, linear light propagation leads to discrete diffraction, whilst for higher input power the focusing mechanism dominates, finally leading to the formation of gap solitons in the binary waveguide arrays. Our simulation of light propagation based on a nonlinear beam propagation method confirms the experimental findings.     

Detection of higher nonlinear harmonics of volume photorefractive gratings in reflection geometry

A comprehensive technique has been used to detect diffraction from the second and the third nonlinear spatial harmonic components, along with the fundamental harmonic of volume photorefractive gratings recorded in a channel waveguide in a LiNbO₃ substrate. For the first time to our knowledge, efficient diffraction from the higher spatial harmonic components of a photorefractive grating has been detected in reflection geometry. The dependence of a Bragg wavelength shift on the order of nonlinearity is also reported.     

Fabrication of integrated waveguide grating in azo polymers

In this Letter, we have introduced a technique, new to our knowledge, to fabricate gratings on a waveguide of azo-functionalized polymeric films using a slit mask and a fast, direct-writing method. To prevent the destruction of the waveguide by the grating formation on the waveguide, we placed a slit mask on the waveguide. By properly adjusting the resonance, this grating can be used as an integrated wavelength filter. We have produced an attenuation of 13.4?dB at 1562?nm with a FWHM of 3.45?nm. The grating has been fabricated as narrow as the width of the waveguide to couple filtered light into the waveguide by using a slit mask. Any light shifted from the resonance will pass through the waveguide undisturbed.     

All optical Flip-Flop based on a nonlinear DFB semiconductor laser: Theoretical study

A new design of an all optical Flip-Flop is proposed. It consists of a nonlinear distributed feedback (DFB) laser. The wave guiding layer of the DFB laser consists of linear grating section followed by a nonlinear one, and both sections are separated by a phase shift section. In the OFF-state, the real part of the refractive index in the wave guiding layer forms a Bragg grating. In the ON-state, it forms a Bragg grating with a phase shift section. Optical gain is achieved by current injection in the semiconductor active layer. Nonlinearity in the nonlinear layers of the semiconductor grating was achieved by direct absorption at the edge of the absorption band (Urbach tail). Numerical simulation shows that the device switches in a nanosecond time scale.     

Analytical solution for the field in photonic structures containing cubic nonlinearity

In this work, we consider the exact solution of the stationary cubic nonlinear equation in a semi-infinite nonlinear medium in contact with a one-dimensional photonic crystal. Two kinds of analytical solutions are found for an arbitrary magnitude of the nonlinearity: a standing-wave-like one containing the inverse elliptic function Eli(?∣m), and a one-wave-type solution for transmitted TE-polarized waves. An approximate two-wave solution is proposed to describe the field propagation through the nonlinear film covering the photonic crystal. It is shown that the problem of a mixed linear-nonlinear structure may be reduced to a transcendental kernel equation determining the field inside the nonlinear part of the medium. The light reflection from a Si/SiO₂ layered structure in contact with an optically nonlinear medium is calculated. The angular-frequency photonic band diagram and power dependency are investigated. Local interface waveguide modes are considered.     

The influence of 3-photon absorption and pulse interactions on discrete X-waves in normally dispersive waveguide arrays

In this paper, we investigate the influences of 3-photon absorption on discrete X-waves in nonlinear normally dispersive waveguide arrays. It is found that 3-photon absorption can cause the decrease of the total power, which results in the appearances of the discrete diffraction for an intermediate input peak-power and the discrete X-wave for a higher input peak-power. Also, the interaction between pulses for different waveguide excitation are studied in detail. The results show that for the in-phase waveguide excitation of neighboring channels, the bound states can be formed by choosing properly the initial peak-power; for the in-phase waveguide excitation of distant channels, however, the bound states can not be formed. For the out-of-phase multiple waveguide excitation, due to interplay the repulsive force and nonlinearity, the interaction of two pulses can form the X-like wave or the double X-like wave as long as choosing the proper input peak-power.     

All-optical power limiting in waveguides with periodically distributed Kerr-like non-linearity

A slab waveguide with an embedded non-linear grating is considered as an all-optical power-limiting device. The limiting action is based on the power losses at boundaries between grating sections, where a mode-mismatch results from the self-focusing of the fundamental mode in each non-linear section. Time-harmonic and optical pulse propagation are both modelled numerically by finite-difference techniques to demonstrate the behaviour of the device. Received: 17 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Discrete nonlinear Schrödinger equation with arbitrary nonlocality: Linear stability analysis and localized solution

The modulational instability of a plane wave for a discrete nonlinear Schrödinger equation with arbitrary nonlocality is analyzed. This model describes light propagation in a thin film planar waveguide arrays of nematic liquid crystals subjected to a periodic transverse modulation by a low frequency electric field. It is shown that nonlocality can both suppress and promote the growth rate and bandwidth of instability, depending on the type of a response function of a discrete medium. A solitary wave (breather-like) solution is built by the variational approximation and its stability is demonstrated.     

Propagation of ultrashort pulses in a nonlinear long-period fiber grating

By solving a pair of normalized nonlinear coupled-mode equations, we analyze in detail the propagation of ultrashort optical pulses in a nonlinear long-period fiber grating and investigate the conditions for the grating to function as an optical limiter or a saturable absorber. We show that the function of the grating depends on whether the nonlinear effect counteracts or enhances the detuning effect, and the nonlinear effect can be weakened significantly by the group-delay difference between the core mode and the coupled cladding mode. We present simulation results to illustrate these effects and discuss the physical conditions required for an effective operation of a nonlinear long-period grating.     

Frequency-doubling in self-induced waveguides in lithium niobate

Efficient frequency-doubling is experimentally demonstrated in presence of beam self-trapping in congruent lithium niobate crystal. The self-trapping is induced by the generated second harmonic beam via photorefractive effect under an external applied field. The local space charge field distribution, formed by the second harmonic beam, is shown to efficiently trap both wavelengths. The dynamics of self-focusing is studied along with the power evolution of the second harmonic beam. Fast tuning of phase matching conditions in the written waveguide is realized by an externally applied voltage also used for the photorefractive confinement.     

Volume grating preferential-order focusing waveguide coupler

A volume grating focusing waveguide coupler has been designed, fabricated, and tested. The volume grating is designed to couple a 633-nm wave guided in a single-mode polyimide waveguide preferentially into the air cover. The beam is outcoupled normally to the surface of the waveguide and focused to a line 25 mm above the waveguide. The slanted-fringe volume grating is holographically recorded by the interference of two coherent 364-nm ultraviolet waves focused with a cylindrical lens to produce the chirped grating. The 1-mm-long volume grating coupler exhibits a preferential-coupling ratio of 0.98, a coupling efficiency of 95%, and an almost diffraction-limited focal line with a full width at half-maximum of 10.49mum.     

Stratified waveguide grating coupler for normal fiber incidence

We propose a new stratified waveguide grating coupler (SWGC) to couple light from a fiber at normal incidence into a planar waveguide. SWGCs are designed to operate in the strong coupling regime without intermediate optics between the fiber and the waveguide. Two-dimensional finite-difference time-domain simulation in conjunction with microgenetic algorithm optimization shows that approximately 72% coupling efficiency is possible for fiber (core size of 8.3 microm and delta=0.36%) to slab waveguide (1.2-microm core and delta=3.1%) coupling. We show that the phase-matching and Bragg conditions are simultaneously satisfied through the fundamental leaky mode.     

On the distribution of 1550-nm photon pairs efficiently generated using a periodically poled lithium niobate waveguide

We report on efficient generation of 1550-nm photon pairs in a periodically poled lithium niobate waveguide using the spontaneous parametric down-conversion process. Such photon pairs are expected to find applications in fiber-based long-distance quantum communication. Pumping the waveguide with a pulsed semiconductor laser with a pulse rate of 800 kHz and a maximum average pump power of 50 μW, we obtain a coincidence rate of 600 s⁻¹. Despite only two single-photon detectors are used, we gain some information about the photon-number distribution. Our measurements are found to be in agreement with a Poissonian photon-pair distribution, but clearly differ from the expected outcomes for both conventional and two-mode squeezed states, the latter corresponding to a thermal photon-pair distribution. The Poissonian photon-pair distribution is also explained by comparing the coherence time of the pump light and of the detected photons. An average of 0.9 generated photon pairs per pulse can thus be inferred.     

Femtosecond third-harmonic generation:. self-phase matching through a transient. Kerr grating and the way to ultrafast computing

The third harmonic of 810-nm 100-fs pulses at 130 μJ is generated very efficiently when ultrashort pulses from two noncollinear beams interfere in an optical medium to create an instantaneous transient grating via the optical Kerr effect. The grating couples two pathways for third-harmonic generation, each taking two photons from one beam and one photon from the other beam, respectively. The coupling enables self-phase matching in the complete process, resulting in a conversion efficiency of ≈3%. Scattering an independent beam at the transient grating confirms a lifetime limited by the pulse duration, with a reaction on the order of one optical cycle. Using the second harmonic of a Ti-sapphire laser at 405 nm, it is shown that the generation of the transient Kerr grating is a general feature, requiring less than 20 μJ/pulse. By introducing a third femtosecond beam we are able to emulate various digital logic units with femtosecond response. Received: 16 October 2001 / Published online: 6 June 2002     

Determination of nonlinear coefficient and group-velocity-dispersion of bismuth-based high nonlinear optical fiber by four-wave-mixing

Nonlinear coefficient and group-velocity-dispersion of bismuth-based nonlinear fibers were determined by four-wave-mixing measurements. The wavelength dependence of refractive index of bulk bismuth-based glasses was also measured to estimate the material dispersion and waveguide dispersion. A newly developed bi-directional four-wave-mixing configuration enabled us to determine all fiber parameters simultaneously. The obtained fiber nonlinearity γ ∼ 1000 W⁻¹ km⁻¹ of bismuth-based nonlinear fiber is the highest one in the step-index fiber made of oxide glasses as expected from a high refractive index at 1550 nm. Dispersion analysis reveals that the both material dispersion and waveguide dispersion affect to the large group-velocity-dispersion of bismuth-based nonlinear fiber.     

Theory of chirped gratings in broad band filters

Closed form expressions for the reflection of light in a dielectric waveguide incident on a grating with a nonlinear chirp are presented. The method is based on a direct integration of the coupled mode equations using the method of stationary phase.