光子晶体光纤中调制不稳定现象与超连续光谱的产生

研究了光子晶体光纤中调制不稳定性效应.从非线性薛定谔方程出发，计算和分析了光子晶体光纤中反常色散区以及正常色散区内的调制不稳定性现象，详细讨论了超短脉冲的脉宽、峰值功率、高阶色散和高阶非线性效应（如脉冲内喇曼散射、自陡峭效应）对调制不稳定性产生的影响.结果表明：二阶色散对调制不稳定性的影响要远大于三阶色散，同时也发现随着初始脉冲宽度的减小，调制不稳定性旁瓣增大但是强度有所降低.另外还发现高阶非线效应如自陡峭和喇曼效应会在不同程度上抑制调制不稳定性.     

零色散附近的交叉相位调制不稳定性分析

以三、四阶色散项的耦合非线性薛定谔方程为基础,考虑光纤损耗及高阶色散,研究了双光束在零色散附近的交叉相位调制不稳定性.理论上导出描述交叉相位调制不稳定性的色散方程,并进行数值模拟计算.结果表明：由于四阶色散的影响,在光纤的正常、反常色散区,交叉相位调制不稳定性均发生在两个频谱区.如光脉冲工作在最小群速度色散附近时,四阶色散对光纤的交叉相位调制不稳定性将起决定性作用,可使增益谱出现一个新的峰值.光纤损耗使增益的谱宽变窄.对给定的传输距离,随着光纤向零色散附近靠近,两个频谱区谱宽增加直到相互重叠.数值分析了两光波有差别时的交叉相位调制不稳定性.     

色散缓变光纤中宽带调制不稳定性谱的产生

本文报道色散缓变光纤中调制不稳定性效应研究.从非线性Schrodinger方程出发,获得了增益谱与纵向色散变化参量和光纤传输距离的一般关系.研究发现,色散缓变光纤中调制不稳定性的增益谱较常规光纤中的宽得多.调节光纤色散纵向变化参量,可以得到较宽的增益谱.该文的研究结果为光纤中利用调制不稳定性产生超短孤子脉冲提供了理论根据.     

高斯变迹布拉格光纤光栅中的调制不稳定性

利用激光脉冲在光纤光栅中传播时所遵守的相干耦合非线性薛定谔方程,研究了激光脉冲在高斯变迹布拉格光纤光栅中传输时，在反常色散区和正常色散区所产生的调制不稳定性.结果表明在反常色散区和正常色散区都能产生调制不稳定性；在反常色散区，当输入功率达到一定数值时，产生明显的有规律的增益谱；在正常色散区，在产生调制不稳定性功率区域，调制不稳定性存在并从给定值一直持续到无穷；并且，在反常色散区和在正常色散区，增益谱都受到高斯变迹函数的制约. 关键词： 高斯变迹 布拉格光栅 调制不稳定性 增益     

非线性高双折射阶跃光纤中单一频率传输区域调制不稳定性

利用光脉冲在高双折射非线性光纤中传播时所遵守的相干耦合非线性薛定谔方程,研究了单一频率的线偏振光,当偏振方向沿两个双折射轴的分量强度相等时,其在阶跃式光纤中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,调制不稳定性这一过程不仅产生在反常色散区,而且产生在正常色散区,对于阶跃式光纤,矢量调制不稳定性还随级数不同发生变化。并且,反常色散区产生的增益谱不同于正常色散区,较之多一个增益谱区域。     

色散缓变掺铒光纤放大器的调制不稳定性

由于掺铒光纤放大器(EDFA)存在增益,相比于传输光纤它有较小的调制不稳定性阈值,使其很容易受到调制不稳定性的影响.本文将用微扰法分析基本的非线性薛定谔方程,研究色散缓变掺铒光纤放大器的调制不稳定性,分析其调制不稳定性积分增益谱与输入信号功率、放大器增益、放大器的长度、光纤纵向色散变化参量的关系.结果显示增大光纤纵向色散变化参量值是减小调制不稳定性对放大器影响的有效途径.通过分析调制不稳定增益产生长度,表明合理的选择放大器的长度可以消除调制不稳定性增益的产生.     

强双折射光纤中任意偏振方向矢量调制不稳定性

利用光脉冲在非线性双折射光纤中传播时所遵循的相干耦合非线性薛定谔方程,研究了偏振方向与双折射轴成任意角度时，在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，在反常色散区和正常色散区存在不稳定偏振区域和稳定偏振区域，对应不同的不稳定偏振区域，输入临界功率不同，脉冲有不同的增益谱. 关键词： 任意偏振方向 矢量调制不稳定性 非线性光纤 双折射     

五阶非线性下零色散附近的调制不稳定性

在同时考虑光纤损耗、高阶色散以及高阶非线性情况下,从广义非线性薛定谔方程出发,研究了零色散附近的调制不稳定性,分析了四阶色散和五阶非线性对增益谱的影响.结果表明：当光脉冲工作在零色散附近时,四阶色散对调制不稳定性起决定作用,它使反常色散区的增益谱变宽.在光纤正常色散区,正（负）五阶非线性使增益谱的谱宽和峰值增大（减小）;但在反常色散区,五阶非线性仅改变增益谱的峰值,几乎不影响谱宽.     

非线性色散渐减光纤中高峰值脉冲的激发和传输

本文基于变系数的非线性薛定谔方程,数值地讨论高峰值脉冲在色散渐减光纤中的激发和传输。首先,基于变系数非线性薛定谔方程的Peregrine孤子解,解析和数值地讨论精确的Peregrine孤子在色散渐减光纤中的传输特性。其次,通过输入不同的平面波背景上的局域脉冲,研究高峰值脉冲在非线性色散渐减光纤中的激发和传输。结果显示Peregrine孤子在色散渐减光纤中传输时,会产生一个空间和时间都局域化的高峰值单脉冲,并且当啁啾为负时,脉冲的幅值增加,脉宽被压缩。若光纤系统存在增益,脉冲的幅值也会增加。由于非线性光纤中的调制不稳定性过程,不同平面波背景上的小局部扰动都可激发出高峰值脉冲,除了峰值和宽度略有不同外,激发脉冲的形状几乎相同。     

非线性双折射色散光纤中极化调制不稳定性分析

利用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,研究了线偏振光在光纤中的传输特性.结果表明:由于线性双折射、非线性效应和色散的相互作用,导致光的偏振状态发生变化,产生交叉相位调制(XPM),从而导致调制不稳定性.这一调制不稳性不仅在反常色散区产生,在正常色散区也能产生,并且线性双折射发生变化时,增益谱也随之发生变化.     

Optical pulse evolution in the presence of a probe light in CW-pumped nonlinear fiber

We investigate theoretically and numerically the evolutions of optical pulses in the time domain due to modulation instability(MI), where CW pump accompanied with a probe is used as the input of nonlinear fiber. As the fiber length increases, we show that it exhibits beat frequency between the pump and the probe first when the probe lies outside the MI resonance region, and then gradually transforms into a pulse train resulting from spontaneous MI rather than induced MI. However, the regular pulse train is easier to generate in the whole fiber if the probe exists in MI resonance region,and the period of the pulse train is inversely proportional to the frequency spacing between the pump and the probe. It is emphasized that the pulse period can be adjusted only when the probe is in MI resonance region. The numerical simulations are in agreement with the theoretical results. The obtained results are guidable for generating and manipulating the optical pulse train in the fiber.     

Modulation instabilities in a system of four coupled, nonlinear Schrödinger equations

The modulation instability of continuous waves for a system of four coupled nonlinear Schrödinger equations, two of which are in the unstable regime, is studied. In earlier studies, plane or continuous waves for a system of two coupled, nonlinear Schrödinger equations is shown to exhibit modulation instability (MI), even if both modes are in the normal dispersion regime, provided that the coefficient of cross phase modulation (XPM) is larger than that of self phase modulation (SPM). Requirements for MI in this system of four coupled, nonlinear Schrödinger equations can be relaxed. MI can occur even if the magnitude of XPM is less than that of SPM, and the magnitude of instability is generally larger than that of each mode alone. The implications for parametric process and wavelength exchange in optical physics with two pump waves are discussed.     

单模光纤中受激喇曼散射对调制不稳定性的影响

基于修正的非线性薛定谔方程，利用线性扰动理论和数值方法研究了单模光纤中的调制不稳定性.由于受激喇曼散射的作用，使得喇曼增益谱叠加到光纤中的调制不稳定性增益谱上.这样，原本调制稳定的光纤正常色散区也出现了调制不稳定性；而在反常色散区，随着初始功率的增加，常规的调制不稳定性增益谱的增益和频谱范围均增大，而喇曼增益谱的增益增大但其频谱范围基本不变，这样导致常规的不稳定区域逐渐侵入并最终掩盖喇曼增益区.数值模拟验证了解析结果的正确性，并证明了利用反常色散情形下的调制不稳定性可以产生超短脉冲序列，但这种脉冲序列的进一步传输将会出现喇曼孤子自频移现象.     

Modulational instability effects in PSK modulated coherent fiber systems and their reduction by optical loss

We show using the propagating beam technique that if optical loss is neglected, the efficiency of PSK modulation in a coherent fiber system is severely compromised by modulational instability effects at sufficiently high signal powers. We note, however, that the intrinsic optical loss of a single mode fiber will to a large degree counteract the modulational instability and preserve the initial PSK pulse shape.     

Investigations on order and width of RZ super Gaussian pulse in pre-, post- and symmetrical-dispersion compensated 10 Gb/s optical communication system using standard and dispersion compensating fibers

We show the effect of varied order and width of super Gaussian pulse at 10 Gb/s in dispersion compensated optical communication system. The optical communication system consists of standard single-mode fiber of 16 ps/nm/km of a certain length, whose dispersion is compensated using pre-, post- and symmetrical-dispersion compensation schemes with proportionate length dispersion compensating fiber of −80 ps/nm/km. Performance of these three compensation schemes is compared at 14 dBm values of Er-doped fiber amplifiers (EDFA) power at 1st, 2nd and 3rd order RZ super Gaussian optical pulse. The pulse width, full width at half maximum (FWHM) is also varied from 5 to 30 ps to highlight the optimum performance. The graphical results obtained show a relationship among the attributes pulse width, order of RZ super Gaussian optical pulse and dispersion compensation scheme implemented. It shows that to decrease BER and timing jitter in the system, smaller width and 3rd order super Gaussian pulse should be used. It is recommended that to decrease dependency of BER and timing jitter in the communication system on the pulse width i.e. FWHM, the symmetrical compensation scheme should be implemented.     

Nonlinear optical correction of the pulse shape from a directly modulated DFB laser

In this work, the effect of modulation instability (MI) in optical fiber is used to reshape nanosecond pulses form a directly modulated diode laser. Our configuration includes a fiber where MI causes the side lobes in the signal spectrum and a filter at the fiber output rejecting the side lobes. Simulations show abrupt drop of the transmission of the setup if pulse power is above some critical value. We investigated the transmission for fibers with lengths in the range between 62-m and 4.5-km. The critical power was found to be inversely proportional to the fiber length. An average scaled critical power is 2.16 W km. We demonstrated the application of the method for rejection of the transient peak in a directly modulated diode laser.     

Low-threshold self-induced modulational instability ring laser in highly nonlinear fiber yielding a continuous-wave 262-GHz soliton train

Modulational instability (MI) is employed in a self-induced ring cavity configuration based on highly nonlinear dispersion-shifted fiber (HNL DSF) and an erbium-doped fiber amplifier to generate a continuous-wave 262-GHz train of 365-fs optical solitons. The laser operates around 1540 nm, with an average output power of 15 mW. MI is achieved at a low threshold as a result of low average cavity dispersion and high fiber nonlinearity. It is shown that, because of the normal dispersion of the HNL DSF, the solitons exist in the average soliton regime.     

Optimizing 10 Gbps optical communication system with duty cycle selection of return to zero pulse

We have investigated the return-to-zero (RZ) pulse duty cycle for single-channel Standard Single mode fiber (SSMF), Non Zero Dispersion shifted fibers (normal NZDSF and anomalous NZDSF fiber) for 10 Gbps optical fiber communication system. We give a comprehensive look on the behavior of variable duty cycle optical RZ pulse indicating that lowest bit error rate for duty cycle 0.8 among the duty cycle values 0.2, 0.4, 0.6 and 0.8 investigated for the case of SSMF. The single repeaterless mode fiber length is increased from existing 55 km at duty cycle 0.2 to fiber length 85 km by keeping duty cycle at 0.8. The result is also emphasized through the 10 dB Q value improvement and corresponding improvement in average eye opening diagram. The normal NZDSF show similar improvement but at greater fiber length, it offers BER 10⁻⁹ at length 110 km with duty cycle 0.2. NZDSF operating length can further be increased to length 160 km by keeping duty cycle 0.8. The corresponding 8 dB Q value improvement and Average eye opening improvement also supports the result through its graphical variation. Thirdly Anomalous NZDSF for same optical communication system showed that 0.2 duty cycle value give operational length of 130 km which could be extended to 160 km if 0.8 duty cycle is kept. The corresponding 8 dB Q value improvement, average eye-opening improvement endorsed the fact in the graphs.     

All-optical switchable UWB pulses generation,modulation and transmission

We proposed and demonstrated all-optical polarity- and shape-switchable ultrawideband (UWB) Gaussian pulses generation schemes using a single polarization interferometer (PI) or a semiconductor optical amplifier (SOA) cascaded by a PI. When an optical signal with dark return-to-zero (RZ) format propagates directly through a single PI, a pair of polarity-reversed UWB monocycle pulses and a positive UWB doublet pulse is acquired respectively only by adjusting the polarization controllers, without changing the bias of the intensity modulator. If a SOA is connected at the input of the PI to firstly implement the first-order differential function to the input dark RZ pulse utilizing gain saturation effect of the SOA, both polarity- and shape-switchable UWB Gaussian monocycle and doublet pulses are all acquired from the cascaded system. Moreover, if a nonreturn-to-zero control signal is introduced into the SOA, utilizing the cross-gain modulation (XGM) effect of the SOA and the differential characteristic of PI, all-optical shape modulation to UWB signals can be realized, and the polarity of the modulated UWB pulses can also be switched. Also, UWB signal transmission characteristics in fiber link were investigated.     

Modulational instability in the cubic-quintic nonlinear Schrödinger equation through the variational approach

The dynamics of nonlinear pulse propagation in an average dispersion-managed soliton system is governed by a constant coefficient nonlinear Schrödinger (NLS) equation. For a special set of parameters the constant coefficient NLS equation is completely integrable. The same constant coefficient NLS equation is also applicable to optical fiber systems with phase modulation or pulse compression. We also investigate MI arising in the cubic-quintic nonlinear Schrödinger equation for ultrashort pulse propagation. Within this framework, we derive ordinary differential equations (ODE’s) for the time evolution of the amplitude and phase of modulation perturbations. Analyzing the ensuing ODE’s, we derive the classical modulational instability criterion and identify it numerically. We show that the quintic nonlinearity can be essential for the stability of solutions. The evolutions of modulational instability are numerically investigated and the effects of the quintic nonlinearity on the evolutions are examined. Numerical simulations demonstrate the validity of the analytical predictions.