Optical solitons switching in asymmetric dual-core nonlinear fiber couplers

With study on the switching characteristics in a asymmetric dual-core nonlinear fiber coupler, it is found the switch efficiencies when the dispersion values of two fibers have opposite signs are much higher than those when they have same signs. Meanwhile, when the dispersion values of two fibers have same signs, the switch efficiencies become much higher if the dispersion value of one core is decreased. In addition, the switch threshold power becomes lower than the one with the same sign if the nonlinearity coefficient of two fibers is opposite. The result shows that asymmetric coupler has much higher switch efficiencies and lower switch threshold power than symmetric coupler.     

Numerical simulation of quantum directional couplers

 A numerical analysis of a quantum directional coupler based on Π-shaped electron waveguides is presented with use of the scattering-matrix method. After the optimization of the device parameters, uniform output for the two output ports and high directivity are obtained within a wide range of the electron momenta. The electron transfer in the device is found more efficient than that in the previously proposed structures. The study of the shape-dependence of transmission for the device shows that the device structure with smooth boundaries exhibits a much better performance. Received: 4 July 1996/Accepted: 9 September 1996     

Zeno effect and switching of solitons in nonlinear couplers

The Zeno effect is investigated for soliton type pulses in a nonlinear directional coupler with dissipation. The effect consists in increase of the coupler transparency with increase of the dissipative losses in one of the arms. It is shown that localized dissipation can lead to switching of solitons between the arms. Power losses accompanying the switching can be fully compensated by using a combination of dissipative and active (in particular, parity-time-symmetric) segments.     

Numerical study of TE waves propagating in multibranch couplers

Abstract

The evolution of TE-polarized waves propagating in the linear and nonlinear multibranch coupler is studied. The propagation of the waves in both directions is calculated by the beam-propagation method (BPM). The numerical results of simulation show that the minimum insertion loss of the nonlinear multibranch coupler is less than 0.088 dB, whereas that of the linear multibranch coupler is more than 3 dB.     

Numerical study of TE waves propagating in multibranch couplers

The evolution of TE-polarized waves propagating in the linear and nonlinear multibranch coupler is studied. The propagation of the waves in both directions is calculated by the beam-propagation method (BPM). The numerical results of simulation show that the minimum insertion loss of the nonlinear multibranch coupler is less than 0.088 dB, whereas that of the linear multibranch coupler is more than 3 dB.     

All-optical soliton switching for the asymmetric fiber couplers

Coupled nonlinear Schrödinger (CNLS) equations for the fiber couplers with asymmetric self-phase modulation (SPM) and cross-phase modulation (XPM) are studied. With symbolic computation, one- and two-soliton solutions are obtained for the constant- and variable-coefficient CNLS equations. Switching dynamics of the solitons is discussed, and effects of the second-order group-velocity dispersion β ₂, SPM coefficient σ ₁, XPM coefficient σ ₂ and Kerr nonlinear intensity γ on the all-optical switching properties are studied, while other coefficients in those equations are seen not to affect the all-optical switching properties. For the constant-coefficient CNLS equations, we find that |β ₂| is proportional to the optical switching speed, and the optical extinction ratios increase with the decrease of σ ₁/σ ₂ and increase of |β ₂| and γ. A numerical simulation by the split-step Fourier and Runge-Kutta methods is presented on the constant-coefficient CNLS equations to analyse the stability of the one- and two-solitons with the random initial perturbations. For the variable-coefficient CNLS equations, effects of σ ₁/σ ₂, β ₂(z) = a ₂ e ^bz and γ(z) = a ₃ e ^bz on the optical switching are analyzed (where a ₂, a ₃ and b are all constants, and z gives the direction of propagation in the fiber couplers): optical switching speed increases with the increase of |a ₂| and decrease of |b|, and optical extinction ratios increase with the increase of |a ₂| and decrease of σ ₁/σ ₂ and |a ₃|.     

All-optical switching of solitons in an active nonlinear directional coupler

A numerical study of the all-optical switching properties of an active nonlinear directional coupler is presented. The switching power is reduced for both CW and soliton inputs because of the gain. The switching properties of the coupler are enhanced for solitons, which switch nearly completely. For ultrashort pulses, the effects of the bandwidth of the gain medium are considered by using a general form for the gain band. The gain spectrum of erbium is also considered as a specific example.     

Fixing multiple waveguides induced by photorefractive solitons: directional couplers and beam splitters

We show how to transform multiple real-time photorefractive solitons into permanent two-dimensional single-mode waveguides impressed into the crystalline lattice of the host material. We experimentally demonstrate two specific configurations of such fixed multiple waveguides: directional couplers and multiple beam splitters.     

Effects of third-order dispersion on soliton switching in fiber nonlinear directional couplers

The split-step Fourier method is used to study the energy switching characteristics of fiber nonlinear directional couplers with the third-order dispersion. The effects of the third-order dispersion increases with the third-order dispersion coefficient and input power and result in pulse shift and energy decreases. Adding high-order nonlinear can partly overcomes these effects.     

Numerical characterization of nanopillar photonic crystal waveguides and directional couplers

We numerically characterize a novel type of a photonic crystal waveguide, which consists of several rows of periodically arranged dielectric cylinders. In such a nanopillar photonic crystal waveguide, light confinement is due to the total internal reflection. A nanopillar waveguide is a multimode waveguide, where the number of modes is equal to the number of rows building the waveguide. The strong coupling between individual waveguides leads to the proposal of an ultrashort directional coupler based on nanopillar waveguides. We present a systematic analysis of the dispersion and transmission efficiency of nanopillar photonic crystal waveguides and directional couplers. Plane wave expansion and finite difference time domain methods were used to characterize numerically nanopillar photonic crystal structures both in two- and three-dimensional spaces.     

Stability of solitons in parity-time-symmetric couplers

Families of analytical solutions are found for symmetric and antisymmetric solitons in a dual-core system with Kerr nonlinearity and parity-time (PT)-balanced gain and loss. The crucial issue is stability of the solitons. A stability region is obtained in an analytical form, and verified by simulations, for the PT-symmetric solitons. For the antisymmetric ones, the stability border is found in a numerical form. Moving solitons of both types collide elastically. The two soliton species merge into one in the "supersymmetric" case, with equal coefficients of gain, loss, and intercore coupling. These solitons feature a subexponential instability, which may be suppressed by periodic switching ("management").     

Phase sensitivity of directional couplers

Directional couplers of various design have been treated widely, both theoretically and experimentally. However, in most cases directional couplers are designed as 1 to 2 port circuits. If both input ports are used, the phase of the guided waves has to be taken into account. The influence of the phase term on the switching behaviour of Δβ-reversal couplers is discussed as an example. It is shown that the phase difference of two waves coupled to both coupler input ports will not affect crossover and straight-through states, but all states in between. Therefore electrical controlled 2 to 2 port switches will become very sensitive, if low crosstalk is required. This effect can be used to design an all optical working switch or optical logic.     

Theory of nonlinear directional couplers

A method is proposed for obtaining exact analytical solutions for the system of nonlinear equations for the propagation of electromagnetic waves in directional couplers with an arbitrary nonlinearity in the propagation constant. The resulting solutions can be used to determine the operating characteristics of nonlinear directional couplers. Zh. Tekh. Fiz. 69, 69–71 (August 1999)     

Soliton switching in nonlinear couplers

We present a theoretical overview of soliton switching phenomena in two-mode nonlinear couplers. By complementing numerical studies with perturbative or exact solitary wave solutions, one finds that nonlinear Schrödinger or sine-Gordon solitons tend to maintain their identity in the coupled systems. Moreover, the coupling itself may originate novel vector solitary waves, such as gap solitons in periodic media. The switching dynamics in the presence of dissipative perturbations such as linear gain or intrapulse Raman scattering is also discussed.     

All-optical switching via second harmonic generation in a nonlinearly asymmetric directional coupler

Taking advantage of the nonlinear phase-shift obtainable via up- and down-conversion of a beam to and from its second- harmonic, an efficient all-optical switch can be realized adopting the standard directional coupler configuration with strong nonlinear asymmetry. Using coupled-mode theory we numerically demonstrate power switching in a half-beat length coupler with remarkable functional characteristics in a wide range of parameters.     

Crosstalks of two-waveguide and three-waveguide directional couplers

Crosstalks of two-waveguide and three-waveguide directional couplers are analyzed mathematically based on the relationship between the coupled mode and the normal mode. Numerical techniques such as the finite-difference method and the beam propagation method are employed to confirm the validity of the derived expressions. The calculation results show that two-waveguide directional coupler is superior to any type of three-waveguide couplers from the practical viewpoint of the crosstalk and the coupling length.     

Slow-light switching in nonlinear Bragg-grating couplers

We study propagation and switching of slow-light pulses in nonlinear couplers with phase-shifted Bragg gratings. We demonstrate that power-controlled nonlinear self-action of light can be used to compensate for dispersion-induced broadening of pulses through the formation of gap solitons, to control pulse switching in the coupler, and to tune the propagation velocity.     

Nonlinear frequency conversion in waveguide directional couplers

Nonlinear optical effects for frequency conversion require a phase-matching condition to efficiently generate a coherent field at the new wavelength. We find that the phase-matching condition can be replaced by a resonance condition when the nonlinear effect takes place in a waveguide directional coupler. We apply this theory to second-harmonic generation and find a theoretical conversion efficiency of 100%, equivalent to the perfect phase-matching condition. An example of the design of such waveguide directional coupler is presented.     

Interactions between defects and propagating hydrodynamic solitons

<正>This paper studies the hydrodynamic solitons propagating along a long trough with a defective bed.The slight deviation from the plane in the bed serves as the depth defects.Based on the perturbation method,it finds that the free surface wave is governed by a Korteweg-de Vries(KdV) equation with a defect term(KdVD).The numerical calculations show that,for a single-convexity localized defect,the propagating soliton is decelerated as it comes into the defect region,and it is accelerated back to its initial velocity as it leaves,which has a dipole effect.As a result, its displacement is lagged in contrast to the uninfluenced one.And an up-step defect makes the propagating soliton decelerate simply.The opposite influence will occur for a single-concavity localized defect and a down-step one.The defect-induced influence on propagating hydrodynamic solitons depends on the polarity of defects,which agrees with that on non-propagating ones.However,the involved dipole effect of the single localized defect is not displayed in non-propagating cases.