Pulse propagation in a nonlinear two-core fiber under the effects of intrapulse stimulated Raman scattering

The switching dynamics of short pulses under the effects of intrapulse stimulated Raman scattering (ISRS) and intermodal dispersion is studied. It is found that ISRS can generally reduce the pulse width and slow down the pulses simultaneously. With numerical examples, we demonstrate that the pulse narrowing effect of ISRS can be weakened by intermodal dispersion. The text was submitted by the author in English.     

Pulse propagation in a nonlinear two-core fiber under the effects of intrapulse stimulated Raman scattering

The switching dynamics of short pulses under the effects of intrapulse stimulated Raman scattering (ISRS) and intermodal dispersion is studied. It is found that ISRS can generally reduce the pulse width and slow down the pulses simultaneously. With numerical examples, we demonstrate that the pulse narrowing effect of ISRS can be weakened by intermodal dispersion.     

Effects of intrapulse stimulated Raman scattering on short pulse propagation in a nonlinear two-core fiber

The switching dynamics of short pulses propagating in a two-core optical fiber under the effects of intrapulse stimulated Raman scattering (ISRS) and intermodal dispersion is investigated theoretically. We find that, when the input pulses are short and intense enough, ISRS in general reduces the width of the pulses propagating in the launching core of the fiber and, at the same time, slows down the pulses. However, if the two-core fiber contains sufficient intermodal dispersion, the pulse-narrowing effect of ISRS can be weakened. Using typical fiber parameters, we demonstrate with numerical examples how the interaction of ISRS and intermodal dispersion affects the pulse shape. PACS 40.42.65.-k; 40.42.65.Dr; 40.42.65.Tg; 42.81.-i; 42.81.Dp     

光孤子脉冲在光纤放大器中的传播

本文建立了包括增益色散,受激喇曼散射,双光子吸收效应的理论模型,讨论了光孤子脉冲在光纤放大器中的传播.数值计算结果表明:在反常色散范围内,光孤子的放大是不稳定的.增益色散导致光脉冲对称分裂,而受激喇曼散射则导致不对称的分裂.在增益色散和受激喇曼散射的共同作用下可获得新的时域和频域特征.有限带宽的放大能抑制受激喇曼散射引起的自频移.本文对有啁啾的光孤子脉冲的放大也进行了分析.     

Effects of intermodal dispersion on short pulse propagation in multi-core fibers

The effects of intermodal dispersion on ultrashort optical pulse propagation through multi-core fibers are analyzed theoretically, which has been ignored in previous studies. A three-core fiber with collinear and triangular configuration and a four-core fiber are considered. We demonstrate with numerical examples that the intermodal dispersion can cause pulse breakup effect in multi-core fibers.     

Dark soliton switching in an NLDC in the presence of higher-order perturbative effects

Dark soliton switching in a nonlinear directional coupler in the presence of intermodal dispersion, cross-phase modulation (CPM), third-order dispersion, Raman effect, and self-steepening effect is reported for the first time. It is shown that, with the exception of CPM, all of the other perturbative effects have almost no effect on the switching characteristics of the dark soliton switch, which is an improvement over the corresponding bright soliton switch. Although the CPM increases the critical power of dark soliton switching, the soliton pulse remains stable inside the coupler during its propagation.     

All-optical limiting using nonlinear directional couplers composed of a self-focusing and a self-defocusing waveguide

We numerically demonstrate the all-optical limiting features in a mismatched nonlinear directional coupler (NLDC) composed of a self-focusing and a self-defocusing waveguide for both continuous wave and pulse cases. The working conditions required are analyzed. To obtain the limiting feature, the propagation constant of the self-focusing waveguide should not be larger than that of the self-defocusing waveguide. Cascaded asymmetric NLDCs are investigated to improve the limiting characteristics. The limiting threshold and the limiting output power can be adjusted by varying the coupler length or the ratio of the nonlinearity coefficients of the self-defocusing and self-focusing waveguides. Analytical solutions are presented in the case of a continuous wave. For the pulse case, numerical solutions show that the top part of the output pulse, if it exceeds the limiting threshold, will be tailored, while the rising and falling edges of the output pulse are almost the same as the input pulse. There is almost no pulse breakup. The influence of the second order dispersion and the intermodal dispersion on the limiting characteristics are analyzed. PACS 42.79.Gn; 42.79.Ta     

Compensating intermodal dispersion in photonic crystal directional couplers

Intermodal dispersion between the supermodes of a directional coupler may induce undesirable pulse breakup in a sufficiently large device. When this happens the device will no longer exchange power between its arms, and the extinction ratio is completely canceled. It is shown that, by carefully designing the coupling area of the directional coupler, one may compensate for intermodal dispersion. The compensating device should accomplish three basic requirements: inverse intermodal dispersion, balanced coupling of each supermode, and maximum power transfer while preserving the sign of the slope of the coupling coefficient with frequency for multiplexing-demultiplexing applications. This structure is designed and optimized with a genetic algorithm.     

Classical theory of sum-frequency generation by coherent nonlinear optical mixing of coherent and chaotic radiations I. Short-parameter solution

The nonlinear optical sum-frequency generation and the second harmonic generation with coherent and chaotic input radiations are treated in this paper. The short-time (-length) approximation method including the small parameter up to the eighth order is used. Effects of spectral widths of radiations as well as dispersion effects are neglected. Mean photon numbers, intermodal correlations and autocorrelations are calculated in the course of the nonlinear process. The obtained results are discussed from the point of view of the effect of intermodal correlations on nonlinear optical processes.The author would like to thank Dr. M. Krková from the Computer Centre of Palacký University for performing the numerical calculations.     

Acousto-optic mode coupling of partially coherent light in two-mode optical fibers

An acoustic pulse propagating on a two-mode fiber can act as a beam splitter in a scanning interferometer. When this device is employed in white-light interferometry, the effects of distributed coupling and dispersive interferometer arms need to be considered. A theory suitable for treating acousto-optic interaction of partially coherent light in a moving interaction region was developed. It was found that differential optical dispersion should be negligible and the acoustic pulse length short. Also the coherence time should be short but long compared to the intermodal group delay difference over a pulse length. Experiments with long acoustic pulses were performed, and fairly good agreement with theory was obtained.     

Analysis of femtosecond pulsed beam propagation in dispersive directional coupler

An analysis of ultrashort-pulse interaction inside directional coupler structures having dispersive material is conveyed using the well-known finite difference time domain (FDTD) technique and time domain beam propagation method (TD-BPM). The implicit TD-BPM uses a higher-order non-paraxial equation suitable for femtosecond pulse. The result obtained by the one-way technique is verified with that of the widely used FDTD. A rigorous study is conveyed in order to distinguish material dispersion from intermodal dispersion. The analysis reveals that material dispersion causes energy loss in the device, in addition to extra pulse broadening and splitting that occur to a low extent in the case of intermodal dispersion.     

超短激光脉冲在不同色散参量光子晶体光纤中传输的数值模拟

为了使得数值模拟更为精确,采用广义非线性薛定谔方程(GNSE)描述超短激光脉冲在光子晶体光纤中的传输演化过程,并利用二阶分步傅里叶方法通过求解方程,数值计算了相同脉宽和能量的超短脉冲在不同色散参量的光子晶体光纤中非线性传输和超连续谱的产生。比较了超短脉冲在光纤不同色散区传输时,高阶色散和非线性效应对超连续谱的产生以及对脉冲波形演化的影响。结果表明,相对于超短脉冲中心波长位于光子晶体光纤的正常和反常色散区,可以相应获得短波波段和长波波段的超连续谱输出,当超短脉冲中心波长位于零色散波长点时,通过色散和非线性效应的联合作用,更易于产生全波长段的平坦超连续谱。     

高非线性光纤正常色散区产生超连续谱的数值研究

基于广义非线性薛定谔方程,通过数值计算对高非线性光纤正常色散区产生超连续谱进行了研究.结果表明,抽运脉冲的峰值功率、脉冲宽度以及脉冲初始啁啾对该光纤正常色散区超连续谱的形成有极其重要的影响;在高非线性光纤正常色散区产生超连续谱的过程中,三、四阶群速度色散甚至更高阶群速度色散对超连续谱的影响完全可以忽略,但与其他高非线性效应相比,自陡峭效应对超连续谱产生的影响更为明显.高功率超短光脉冲在高非线性光纤正常色散区,得到了没有泵浦成份残余、－20 dB谱宽达400 nm以上而频谱强度起伏小于10 dB的超宽而平坦超连续谱.     

Spectral-domain intermodal interference under general measurement conditions

Spectral-domain intermodal interference is analysed theoretically at the output of a few-mode optical fibre alone and at the output of the optical fibre in a tandem configuration with a Michelson interferometer. The theoretical analysis is performed under general measurement conditions when a light source of a given spectral bandwidth and a spectrometer of a given spectral bandpass are considered and when first- and second-order intermodal dispersion effects in the optical fibre are taken into account. Thus, the effect of a variable spectral bandpass of a low-resolution spectrometer on the spectral fringe visibility functions for a two-mode optical fibre of known intermodal dispersion is specified.     

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.     

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.     

Analysis of nonlinear gain-induced effects on short-pulse amplification in doped fibers by use of an extended power equation

Optical pulse amplification in doped fibers is studied using an extended power transport equation for the coupled pulse spectral components. This equation includes the effects of gain saturation, gain dispersion, fiber dispersion, fiber nonlinearity, and amplified spontaneous emission. The new model is employed to study nonlinear gain-induced effects on the spectrotemporal characteristics of amplified subpicosecond pulses, in both the anomalous and the normal dispersion regimes.     

负折射介质中三阶非线性色散项对超短脉冲传输的影响

采用分步傅里叶法研究了三阶非线性色散项对超短脉冲传输的影响.通过模拟数值计算得出,在德鲁德模型频率选取的一定范围内,三阶非线性色散项对超短脉冲传输所造成的影响是不可忽略的.结果表明:三阶非线性色散项系数取负值时,在正常色散区超短脉冲不对称展宽,脉冲中心向前沿偏移;在零色散点超短脉冲不对称展宽程度加剧并在脉冲前沿形成振荡,脉冲中心向前沿偏移;在反常色散区超短脉冲不对称变窄,脉冲中心仍向前沿偏移.     

Chirped and chirpfree soliton solutions of generalized nonlinear Schrödinger equation with distributed coefficients

Using the F-expansion method, we systematically present exact solutions of the generalized nonlinear nonlinear Schrödinger equation with varying intermodal dispersion and nonlinear gain or loss. This approach allows us to obtain large variety of solutions in terms of Jacobi-elliptical and Weierstrass-elliptical functions. The chirped and unchirped spatiotemporal soliton solutions and trigonometric-function solutions have been also obtained as limiting cases. The dynamics of these spatiotemporal soliton is discussed in context of optical fiber communication. To visualize the propagation characteristics of chirp and unchirped dark-bright soliton solutions, few numerical simulations are given. It is found that wave profile of solitons depend on the group velocity dispersion and the gain or loss functions.     

Reverse propagation of femtosecond pulses in optical fibers

We present a numerical technique for reversing femtosecond pulse propagation in an optical fiber, such that given any output pulse it is possible to obtain the input pulse shape by numerically undoing all dispersion and nonlinear effects. The technique is tested against experimental results, and it is shown that it can be used for fiber output pulse optimization in both the anomalous and normal dispersion regimes.