Design and simulation of a switch based on nonlinear directional coupler

In this paper, we have analyzed a nonlinear photonic crystal directional coupler by the finite difference time domain method. We have tried to increase the coupling efficiency and also reduce the coupling length in linear and nonlinear states in this device. In this coupler, refractive index of the rods of the central row is tuned by input signal power due to nonlinear Kerr effect; therefore, input signal beam can be controlled so as to be exchanged between two output ports. Physical length is chosen to be 24a so as to have the highest output power ratio and also the smallest amount of power required for nonlinear performance.     

掺铒光纤非线性耦合器的实验研究

提出一种耦合参量可用光调控的掺铒光纤非线性耦合器.用熔融拉锥法熔合两根掺铒光纤,拉制成工作波长为155nm的3dB掺铒光纤耦合器.通过调变输入耦合器一臂的980nm泵浦光功率,可以改变两臂的传播常量差,从而改变耦合器两臂信号光的相对输出功率.通过测量输入泵浦光功率和两臂信号光输出功率,得到直通臂耦合比依赖于泵浦光功率的实验曲线.实验研究表明,当泵浦光功率从0 mW变化至20 mW时,耦合比的变化可达到40％.与理论模拟的结果一致.     

非线性光定向耦合器解的简化表达式

本文将非线性定向耦合器在任意输入情况的解进行了简化.在本文解的基础上,非线性光定向耦合器的工作点,即其开关、调制和放大应用(几乎所有应用)的输入条件能很容易地由解析式确定,并在理论上提出了非线性光定向耦合器一个新的应用:一束强光的相位可以由一束弱光的功率加以控制.     

Demonstration of an all-optical switching operation using an optical fiber grating coupler

An optical fiber grating coupler (FGC) is a fused optical fiber coupler with a tapered region in which refractive index-modulated gratings are written. In the FGC, the light with specific wavelength satisfying the Bragg condition of the grating can be dropped to one output port and other lights are transmitted to another output port when lights with various wavelengths are launched into the input port. The FGC can operate as an all-optical switch by controlling the Bragg wavelength of the grating using a third order nonlinear optical effect caused by a control light that are launched with a signal light. In this paper, an all-optical switching operation due to a third order nonlinear optical effect in an FGC is first demonstrated for a signal light with 1.55 μm-wavelength to be changed from one port of the FGC to another one by the 720 W peak of a control light from a Nd:YAG laser with 1.06 μm-wavelength. The switching efficiency obtained was 7%. It was clarified that a longer pulse length of the control light compared to the grating length is required to obtain a large Bragg wavelength shift for the switching. It was also clarified that the Bragg wavelength shift is caused by a third order nonlinear effect and a photothermal effect. A contribution of the photothermal effect was estimated. We also estimated the switching efficiency for pump power in the FGC switch.     

Soliton switching study using new normalized nonlinear Schrödinger equations

Soliton switching in a nonlinear directional coupler (NLDC) is studied by using new normalized nonlinear Schrödinger equations (NLSE). The resulted equations have one parameter κL_D only, this feature greatly simplifies the numerical study of soliton coupling in a NLDC, and it enables us to derive the soliton switching conditions based on numerical analysis. Numerical results show that most of the input soliton energy remains in the launched waveguide when input soliton normalized peak power is bigger than 2.5. It is contrary to the existing theoretical expectations which were derived using variational approach. The fundamental soliton switching conditions is also expressed in terms of input soliton pulse width. In addition, it is found that frequency chirp degrades soliton switching performance in a NLDC.     

一阶模的滤除及在Y分支和多模干涉结构中的应用

Y分支和多模干涉结构广泛应用于平面光波导器件中，在这些器件中，Y分支和多模干涉结构输出对称性具有重要意义。引入Y分支和多模干涉结构输出的不对称主要原因是这两种结构中产生了一阶导模，设计一种新的耦合器结构，在这个结构中．两根单模波导分别放置在一根多模波导两边，并且单模波导的传播常量与多模波导的一阶模的传播常量相同，这种结构可以将一阶导模滤除。在Y分支和多模干涉结构中使用了这种结构，模拟结果表明，采用这种耦合器的Y分支和多模干涉结构具有非常好的输出对称性。     

A novel architecture of ultracompact pulse position modulator in photonic crystals

We propose and analyze a novel photonic crystal pulse position modulator utilizing a coupled cavity waveguide delay line. Also, a nonlinear photonic crystal directional coupler is used as an all-optical switch in the proposed structure. The input data acts as the control signal of the switch and activates the switching operation. We show that the device size can be reduced significantly by designing the delay line to achieve a reduced group velocity and a quasi-flat impurity band. The size of the designed modulator is 32a × 16a, where a represents the lattice constant of the photonic crystal. The characteristics of the device are investigated by finite-difference time-domain (FDTD) and plane wave expansion (PWE) methods.     

Stochastic resonance characteristic analysis of new potential function under Levy noise and bearing fault detection

Based on the output saturation of classcial bistable stochastic resonance (CBSR), a new type of piecewise nonlinear bistable stochastic resonance (PNBSR) system is constructed. The mean signal-to-noise ratio gain is regarded as an index to measure the stochastic resonance phenomenon. The laws for the resonant output of piecewise nonlinear bistable system governed by l, c, a, b and D of Levy noise are explored under different characteristic index α and symmetry parameter β of Levy noise. The results show that the output of PNBSR system has increased 4?dB by comparing with the output signal-to-noise ratio of CBSR system. And the stochastic resonance phenomenon can be induced by adjusting the piecewise nonlinear system's parameters under any α or β of Levy noise. The interval of the parameters of system which induces good stochastic resonance is roughly the same. And the output signal waveform of resonance is very similar to the input signal waveform, which has some reference value for the signal recovery. Moreover, we can find the good stochastic resonance interval of the system parameters do not change with D of Levy noise under the different noise intensity D of Levy noise. On the basis of this, adjusting the intensity amplification factor D of Levy noise, which induces good stochastic resonance, and the interval does not change with α or β. At last, the piecewise nonlinear bistable system is applied to detect bearing fault signals, which achieves better performance compared with the classical bistable system.     

Improved photonic crystal directional coupler with short length

We investigate a photonic crystal (PC) waveguide coupler which is formed by two closely spaced linear waveguides in a two-dimensional triangular lattice of air holes. Our study shows that shifting one row of the air holes between the waveguides affects the dispersion curves of the guided modes and if the triangular lattice of air holes between the waveguides is replaced by a rectangular lattice, this modification results in an ultra-short coupling structure with coupling length less than 3a, where a is the lattice constant. Also, we investigate the effect of changing the radii of air holes that are adjacent to or between the waveguides on the coupling length and show that increasing the radius of air holes between the waveguides decreases the coupling length. We analyze the output spectrum of an ultra-short channel drop filter designed based on this structure.     

Chirp controllable all-optical router in a nonlinear directional coupler

We analyzed the evolution of chirped optical pulses in a nonlinear directional coupler by coupled nonlinear Schrödinger equations. It is shown that an optical pulses launched into one channel can couple out from either of the two channels of the nonlinear directional coupler, with the routing determined by the chirp of the pulse. This is different from the previous routing mechanism of the nonlinear directional coupler where routing is determined by the peak power of the pulse. Based on the chirp-dependent feature, all-optical router and switch could be implemented by the pulse chirp modulation.     

Vector soliton switching in a fiber nonlinear directional coupler

This paper reports a detailed numerical study of soliton switching in a high as well as low birefringent nonlinear coupler. It is shown that by controlling the polarization angle one can have nearly 100% transmission with excellent switching characteristics. It is shown that soliton remains stable during its propagation inside the coupler. However it is observed that high birefringent coupler exhibits relatively better soliton stability. We show that the coupler could be used as a soliton switch even at an input peak power less than the critical power, the power at which 50-50 power sharing takes place between the two cores, just by a judicious choice of the polarization angle.     

Tunable optical bistability at microwave frequency based on 1D sandwich photonic structure consisting of a nonlinear dielectric slab and two magnetized cold plasma layers

In this paper, tunable optical bistability that denotes the relationship between input intensity and output intensity is numerically investigated in the microwave frequency region based on the one-dimensional (1D) sandwich photonic structure consisting of a Kerr-type nonlinear material slab and two magnetized cold plasma layers. Results show that, in the case of TM-polarized electromagnetic wave, width and switching thresholds of the bistability loops are dependent on the working frequency, initial incidence angle, layer thickness, plasma density, and external magnetic field, which should be judiciously selected to obtain a required bistability behavior. Compared to the case of switch-down threshold, the switch-up threshold in the bistability loop is more sensitive to the changes of parameters. Through this study, the suggested 1D sandwich photonic structure is beneficial to the all-optical signal processing.     

Theoretical analysis and heat dissipation of mid-infrared chalcogenide fiber Raman laser

Based on the structure of mid-infrared chalcogenide fiber Raman laser, the nonlinear coupled equations and heat dissipation equations are constructed. The effects of laser parameters including pump power, fiber length, reflectance of output coupler and fiber loss coefficient on laser performance are numerically analyzed. The results show that the Raman laser pumped at 2 μm can operate at high slope efficiency using the optimized structure parameters. In addition, the output laser power decreases dramatically with the increasing of fiber loss coefficient. Moreover, the temperature distributions along the fiber radial and axial directions and the maximum temperature versus launched pump power are calculated according to the heat dissipation equations. The results show that the maximum temperature in the fiber increases dramatically with the increasing launched pump power, which is above 300 °C for launched pump power of 21 W. The above obtained results can be used for theoretical guiding and optimizing design of practical chalcogenide fiber Raman laser.     

通过以太无源光网的射频电视传送技术

论述了1550 nm射频电视(CATV)通道与1490 nm数据通道的相互干扰问题。针对1490 nm光波通过光纤中受激拉曼效应对1550 nm光波的非线性串扰和1550 nm光波通过粗波分复用(CWDM)分波器对1490 nm光波的线性串扰提出了理论计算公式,进行了定量分析和计算。结果表明,数据非线性串扰可使射频电视系统低频端的载噪比下跌9 dB;而在常规融锥型粗波分复用分波器情况下,射频电视线性串扰可使数据光接收机的光功率代价达到2.5dB。最后指出了克服串扰的方法,如以太空闲字符扰码和高隔离度粗波分复用等。     

On the defective NR-PC flat lens for lightwave target detection and imaging

Influence of the defective cylinder air holes of negative-refraction photonic crystal (NR-PC) lens on the performance of lightwave target detection and imaging are studied with finite-difference time-domain method in the paper. Numerical simulations indicate that significant enhancement of the scattering signal can be obtained using the NR-PC flat lens; consequently, great improvement of the refocusing gain as well as the imaging resolution will be provided. We further study the effect on target detection and imaging using the defective NR-PC flat lens. Using dynamic scanning scheme, it is found that its focusing resolution is better than the non-defective lens. We can also get better resolution by appropriately decreasing the radius of the defective cylinder (R = 0.3a). In conclusion, appropriate radius of the defective cylinder (R = 0.3a) will provide better image resolution.     

All optical method of developing OR and NAND logic system based on nonlinear optical fiber couplers

Logic gates are the fundamental building blocks in any digital data processing system. Several all optical logic operations are already proposed by scientists and technologists. Here the authors proposed a new method of all optical logic operation based on nonlinear directional coupler. Effective optical switching is achieved by modifying the coupling length between the coupler waveguides by means of an optical signal.     

Vehicular motion through a sequence of traffic lights controlled by logistic map

We study the dynamical behavior of a single vehicle through the sequence of traffic lights controlled by the logistic map. The phase shift of traffic lights is determined by the logistic map and varies from signal to signal. The nonlinear dynamic model of the vehicular motion is presented by the nonlinear map including the logistic map. The vehicle exhibits the very complex behavior with varying both cycle time and logistic-map parameter a. For a>3, the dependence of arrival time on the cycle time becomes smoother and smoother with increasing a. The dependence of vehicular motion on parameter a is clarified.     

Vehicular traffic flow through a series of signals with cycle time generated by a logistic map

We study the dynamical behavior of vehicular traffic through a series of traffic signals. The vehicular traffic is controlled with the use of the cycle time generated by a logistic map. Each signal changes periodically with a cycle time, and the cycle time varies from signal to signal. The nonlinear dynamic model of the vehicular motion is presented by a nonlinear map including the logistic map. The vehicular traffic exhibits very complex behavior on varying both the cycle time and the logistic-map parameter a$a$. For a>3$a>3$, the arrival time shows a linear dependence on the cycle time. Also, the dependence of vehicular motion on parameter a$a$ is clarified.     

Experimental circuit realization of zero-dispersion nonlinear resonance via the generic input-output analysis

When a nonlinear oscillator is driven by an external force which is calculated on the basis of the zero-dispersion nonlinear resonance (ZDNR) theory, it can be transparent, i.e., the output of the response system is the same as the magnitude of driving force without any dispersion or distortion. In fact, this is a unique phenomenon, called zero-dispersion nonlinear resonance (ZDNR). Herein, we used the generic input-output analysis to realize the ZDNR circuit experiment of two unidirectional coupling self-excited oscillators. The experimental observations also show good agreement with numerical simulations.     

Analysis and optimization of optical bistability in one-dimensional nonlinear photonic crystal with (HL)(D)(LH) and (LH)(D)(HL) structures

Although one-dimensional nonlinear photonic crystal (1D-NPC) has been widely studied, there is no comprehensive analysis on decreasing the bistability threshold power.In this paper, conditions required to create bistability have been specified for two types of structures of alternative high and low refractive index layers and defect layers with Kerr nonlinearity effect, in the order of (HL)^p(D)^q(LH)^p and (LH)^p(D)^q(HL)^p, where L and H denote low and high refractive index layers, respectively, D stands for the defect layers, p is the number of LH or HL layers and q is the number of defect layers.One of the essentials for the bistability is appropriate shifts of frequency of the defect mode, so the effect of the order of layer's arrangement in (1D-NPC) structures have been studied. Different structures have been introduced and the best structure for the lowest bistability threshold power has been proposed. Nonlinear finite-difference time-domain (NFDTD) in C++ has been employed for simulation.