Ultrafast soliton-trapping AND gate

An ultrafast, all-optical, soliton-trapping AND gate that consists of a birefringent optical fiber followed by a frequency filter is demonstrated. The gate is sensitive to the timing of the input pulses and provides an output with a large energy contrast. The performance of the gate is characterized by varying the total input energy, the ratio between the energies of the two input pulses, and the arrival-time difference between the input pulses. It is shown that the gate efficiency (characterized by its ON-OFF contrast ratio) increases with increasing pulse energy up to the limit where Raman effects become dominant in the fiber, and the optimal performance of the gate is obtained with two input solitons having equal energies. The gate efficiency degrades with increasing difference of arrival time of the two input pulses, but a contrast ratio of 5:1 can still be obtained for a full pulse width of timing mismatch. The experimental results are in agreement with numerical simulations using the coupled nonlinear Schrodinger equations     

具有高阶耦合色散系数双芯非线性光纤耦合器开关的数值分析

基于耦合非线性薛定谔方程(NLSE),利用分裂步长傅里叶方法(SSFM),研究了具有高阶耦合色散系数时,双芯非线性光纤耦合器中的传输和开关特性.模型中,一阶耦合色散系数的存在类似于群速度失配,二阶耦合色散系数类似于群速度色散,并且会对另一个纤芯中的信号产生线性互调制.研究表明当一阶耦合色散系数较小时,光脉冲仍能在二纤芯之间周期性耦合传输,并且表现出良好的开关特性,当一阶耦合色散系数增大时,脉冲耦合传输的周期性和陡峭的开关特性遭到破坏,光脉冲在传输中发生分裂;二阶耦合色散系数使耦合器的耦合长度减短,开关阈值功率增加,开关特性也变得更加陡峭.     

基于沃尔泰拉理论的集总光纤非线性噪声补偿

通过沃尔泰拉(Volterra)级数理论求得了非线性薛定谔方程(NLSE)的半解析解,在考虑光纤损耗、色散及非线性效应的情况下,推出了长距离在线级联掺铒光纤放大器(EDFA)光纤通信系统中信号和自发辐射噪声(ASE)之间耦合串扰的半解析表达式,得到了多跨距传输接收端输出信号和发射端输入信号之间的关系式.根据维纳(Wiener)滤波理论的时域滤波原理,在多跨距传输系统接收端设计了对非线性乘性噪声有滤波作用的维纳滤波器,并对预集总色散补偿、后集总色散补偿、分布链路色散补偿系统及一对三"跨距成比例平移对称的色散非线性同步补偿系统进行了仿真模拟研究.结果表明了提出的设计思路及方法的可行性,为进一步提高传输距离增大入纤功率提供了新的思路.     

Switching dynamics of short optical pulses in a nonlineardirectional coupler

It has been shown recently that the propagation of short optical pulses in a nonlinear directional coupler consisting of two parallel waveguides or fibers should be described more generally by a pair of coupled nonlinear equations that take into account the dispersion properties of the coupling coefficient. In this paper, we use the Fourier series analysis method to solve these equations, and study the effects of a dispersive coupling coefficient on the switching dynamics of short optical pulses in a nonlinear directional coupler. We demonstrate, with numerical examples, that a dispersive coupling coefficient could cause significant pulse distortion and affect the threshold switching intensity. The effects of introducing frequency chirps in the input pulses are also discussed     

All-optical pulse switching in twin-core fiber couplers withintermodal dispersion

Due to the dispersion mismatch between the two supermodes of a two-channel coupler, pulse switching cannot be analyzed without taking into account the effect of intermodal dispersion. Using full numerical simulations, we show the influence of intermodal dispersion on all-optical switching with ultrashort pulses in twin-core fiber couplers. For the nonlinear regime, however, only subpicosecond and femtosecond pulses are affected in the cross state since the parallel state remains almost unaffected     

Effects of Second-Order Coupling Coefficient Dispersion on Short-Pulse Propagation and Switching in an Active Two-Core Nonlinear Fiber Coupler

The propagation and switching of short pulses in an active two-core fiber nonlinear directional coupler have been investigated theoretically by using the split-step Fourier method. The analysis highlights the effects of the second-order coupling coefficient dispersion, the linear gain coefficient, and the finite-gain bandwidth on the switching and propagation of short pulses. The research indicates that the linear gain can sharpen the switching characteristic and reduce considerably the switching threshold power, as well as significantly increase the switching efficiency with the influences of the second-order coupling coefficient dispersion. However, both the second-order coupling coefficient dispersion and the finite-gain bandwidth degrade the switching characteristics. In addition, the finite-gain bandwidth of linear gain not only suppresses significantly the pulse compression and amplification caused by the linear gain coefficient, but also suppresses effectively the frequent pulse fluctuation on pulse propagation induced by the second-order coupling coefficient dispersion; consequently, as in the case of the passive fiber coupler, optical pulses tend to restore periodical coupling propagation in active two-core fiber coupler.      

熔锥平行排列3×3单模光纤耦合器的研究

平行排列 3× 3单模光纤耦合器在光开关、光缓存器、光谐振腔、光滤波器等方面有很重要的应用 ,偏振态的演化和波长特性是耦合器应用中的关键问题。利用模耦合理论对平行排列 3× 3单模光纤耦合器的输出性能进行了详细分析 ,得出了光纤引线对耦合器的输出偏振态演化起主要作用 ,而耦合区的影响则比较小 ;通过理论推导模耦合系数 ,分析了耦合器的分光比随波长的变化关系 ,得出了输出分光比比较稳定的耦合器所需要的合适芯间距(对于我们研制的为 12 .5 μm)。并采用可移动火焰实际研制出了具有长耦合区的平行排列 3× 3单模光纤耦合器 ,测试表明输入光波长在 1 5 0 0～ 1 5 75 μm范围内变化时 ,耦合器的输出分光比变化不大 ,理论与实验结果基本相符     

阶跃光纤中相近频率传输区域的调制不稳定性

利用激光脉冲在光纤中传播时所遵守的相干非线性薛定谔耦合方程,研究了非线性双折射色散阶跃光纤(SWDF)中两相近频率的激光脉冲,其偏振方向相互正交且平行于光纤的双折射轴,且偏振方向沿2个双折射轴的分量强度相等时,在同为反常色散区和正常色散区所产生的调制不稳定性.结果表明,在反常非线性区和正常色散区都能产生调制不稳定性;在正常色散区存在不同的调制不稳定性功率区域,对应不同的功率区域导致增益谱表现出明显的不同,并且,当输入功率和波长差(或频率差)一定时,增益谱随传输距离变化保持形态不变.     

Polarization independent narrow channel wavelength divisionmultiplexing fiber couplers for 1.55 μm

A polarization-independent narrow-channel (PINC) wavelength division multiplexing (WDM) coupler for operation at 1.55 μm has been designed based on the birefringent properties of fused single-mode couplers. These couplers are made from both conventional single-mode fiber (SMF) and dispersion shifted fiber (DSF). The couplers are designed so that the coupling-coefficient-length products for the two orthogonal polarizations are π out of phase. As a result of the long coupling lengths required, these couplers have very small cross-sectional dimensions, typically on the order of 10 μm     

Raman-assisted femtosecond soliton switching and wavelength demultiplexing with optical fiber rocking filters

We analyze the femtosecond polarization and wavelength-dependent nonlinear switching response of birefringent fiber rocking filter. We show by perturbation theory and numerical simulations that, in the presence of the Raman self-frequency shift, the soliton switching efficiency may be optimized by injecting pulses in the fast fiber axis and with a negative wavelength offset from the center of the filter bandpass.     

Nonlinear propagation of pulses in multimode fiber with strong linear coupling

Multimode fiber has a richer spatial dimension than single-mode fiber, and is an ideal platform for studying many novel nonlinear effects. We established a strong linear coupling and short-range fiber model to understand the interactive effects of linear coupling and nonlinear effects. We find that strong linear coupling can compensate for the group delay between eigenmodes and cause energy fluctuation between modes which weakens the nonlinear effects. In high energy pulses, the interaction of linear coupling and nonlinear effects can help producing weak dispersion waves when the spectrum is broadened. Since linear coupling in a mode group is common and unavoidable, these results may provide a certain theoretical explanation for multi-mode nonlinear phenomena.     

Stochastic instability in nonlinear anisotropic fiber couplers

Transmission characteristics of a nonlinear coherent coupler composed of anisotropic fibers are investigated. The system mode approach and the channel normal mode approach are compared and shown to produce identical results so long as weakly guiding and weakly coupling structures are involved, as in the linear case. It is found that the critical state at which the power is equally divided into two channels at any long coupler lengths, as predicted in the early literature, does not exist in a conventional nonlinear coherent coupler unless the initial excitation beam is oriented along the coupler geometric axes. The existence of stochastic instability in a nonlinear anisotropic fiber coupler is shown even when the modes of the individual cores are excited. This result is different from the conclusion reported in the early literature with the assumption of negligible intensity-induced power exchange between the axes. The reason for such instability and the way to avoid it are also discussed     

Time-resolved switching characteristic of the nonlinear directionalcoupler under consideration of susceptibility dispersion

Two new forms of nonlinear coupled mode theory are presented which allow a quantitatively correct calculation of the temporal and spatial field evolution in a nonlinear coupler. The resultant set of coupled equations for the Fourier transforms of the pulse envelopes of the basis functions are numerically solved for representative coupling and excitation conditions. The approach permits the treatment of the power and time dependent switching characteristics of ultrashort pulses by accounting for the frequency dependence of the susceptibilities and coupling integrals. The results show that switching efficiency is low in the positive dispersion region due to pulse breakup. The pulse breakup can also be observed in the negative dispersion region, with pulses representing higher order solitons having a peak power on the order of the continuous-wave switching power     

All-optical regeneration based on highly nonlinear photonic crystal fiber

An all-optical regeneration based on selfphase modulation in a highly nonlinear photonic crystal fiber is proposed. The dispersion and nonlinearity properties of a series of photonic crystal fibers are analyzed, and the results show that the nonlinearity coefficient is closely related to the structure of the fiber.In this paper, the nonlinearity coefficient is increased by reducing the effective mode area, and a highly nonlinear photonic crystal fiber with a large air-filling fraction is used as nonlinearity medium in optical regeneration. The numerical results show that good optical regeneration results can be obtained by using a relatively short fiber length due to the high nonlinearity of the fiber. The input peak power launched into the photonic crystal fiber and the parameters of the filter have much influence on optical regeneration. To achieve good optical regeneration, those parameters need to meet certain requirements.Furthermore, the transfer characteristic of the regenerator is also discussed. By adjusting the input peak power and filter parameters, the regenerator can deal with input pulses of different pulse widths.     

Polarization-dependent parametric and Raman processes in abirefringent optical fiber

The generation of new frequencies by the parametric four-photon mixing process, stimulated Raman processes, and subsequent stimulated mixing processes, all of which arise from the third-order nonlinearity in a single-mode birefringent fiber, is reported. It is observed that different processes are dominant for different input pump polarization angles with respect to the birefringence axes of the fiber. By examining the evolution of the nonlinear output spectra at various input pump power levels for different polarization angles, the pump-polarization-dependent third-order nonlinear processes are studied in detail. The effect of the birefringence-compensated group-velocity matching between the pump and parametric frequencies of different polarizations results in effective nonlinear interactions over long distances for the picosecond pulses     

Self-switching with a nonlinear birefringent loop mirror

We investigate switching characteristics of the nonlinear birefringent loop mirror which consists of a symmetric directional coupler, a birefringent fiber, and a symmetry-breaking element that converts linearly polarized light to circularly polarized light, and vice versa. The condition for total reflection in the linear regime is obtained using the Jones matrix formalism, The switching curves have been obtained through numerical computations both in the nondispersive pulse regime and in the soliton-like pulse regime. Parameter values suitable for practical applications of our device as an intensity filter are obtained     

微结构光纤正常色散区飞秒激光脉冲传输光谱展宽的功率饱和效应

利用自制的微结构光纤进行了飞秒激光脉冲传输实验,输入激光脉冲的中心波长为800 nm,位于微结构光纤的正常色散区。随着输入激光脉冲的平均功率从220 mW逐渐增大到300 mW,输出光谱同时向长波和短波方向展宽,光谱中的反斯托克斯线对应的能量逐渐占据主导地位。平均功率达到280 mW时,光谱的展宽范围不再增大,显示出了一种功率饱和效应。在微结构光纤的正常色散区实现了波长从560~960 nm的20 dB展宽谱。     

Nonlinear optical pulse coupling dynamics

The pulse coupling dynamics in a nonlinear directional coupler (NLDC) is analyzed by using the new normalized coupled nonlinear Schro/spl uml/dinger equations. The operation of an NLDC can be divided into three regions according to L/sub D//spl kappa/: Region 1 (L/sub D//spl kappa//spl les/1), Region 2 (L/sub D//spl kappa/>50), and Region 3 (1相似文献   

双折射光纤中三阶色散对孤子稳定性的影响

本文研究了单模光纤中双折射效应和高阶色菜和效应对弧 子传输稳定性影响。研究结果表明：线性双折射所导致的两偏振孤子分离的振幅阈值三阶色散的影响而增大,如果增大脉冲 振幅的阈值以上,那么两个垂直方向的偏振孤子将锁定在一起传输。