Analysis of Gain Competition Suppression in Multiwavelength Actively Mode-Locked Erbium-Doped Fiber Lasers Incorporating a Highly Nonlinear Fiber

A numerical model for multiwavelength actively mode-locked erbium-doped fiber lasers (AML-EDFL) is presented. Using the model, the influences of four-wave mixing (FWM) and phase modulation effects (i.e., self-phase modulation (SPM) and cross-phase modulation (XPM)) can be studied, respectively. We use it to investigate AML-EDFLs incorporating a highly nonlinear fiber (HNLF) and explore the mechanisms for gain competition suppression (GCS). The net dispersion of the cavity is found critical for GCS. In the small dispersion regime, FWM dominates GCS, but SPM and XPM deteriorate the output pulses. In the large anomalous dispersion regime, SPM and XPM, however, enhance GCS and produce high-quality pulses. Degree of GCS is also quantitatively evaluated. The simulation results are at last confirmed by experiments.     

Effects of SPM, XPM, and four-wave-mixing in L-band EDFAs onfiber-optic signal transmission

The significance of self-phase modulation (SPM) and cross-phase modulation (XPM) in L-band EDFAs are evaluated through numerical simulations of 20 Gb/s NZDSF transmission links. The results show that SPM and XPM in L-band EDFAs have negligible impairment, compared to that of SPM/XPM effects in NZDSF. An analytical theory describing FWM in an optically amplified multispan fiber line is developed with the inclusion of FWM in EDFAs. The theory predicts that the strength of FWM in L-band EDFAs with small dispersion and high output power can be comparable to that in NZDSF transmission links     

SPM/XPM/FWM对高速光传输的影响与对策探讨

文章探讨了SPM、XPM、FWM等与折射率相关的光纤非线性效应对高速长距离光传输的影响及减轻其影响程度的技术措施。     

10 GHz WDM/OTDM通信多波长光脉冲源

利用色散位移光纤中的复合非线性效应(自相位调制、四波混频、交叉相位调制)得到重复频率10 GHz、谱宽大于50 nm的超连续(SC)光谱,研究了泵浦光功率、波长与生成的SC光谱宽度和平坦度、光脉冲质量的关系.利用AWG从SC谱中滤出4个10 GHz不同波长光脉冲.     

Influence of nonlinear effects on 6.4 Tb/s dual polarization quadrature phase shift keying modulated dense wavelength division multiplexed system

In this paper, we have proposed a high‐performance transmission system of 32 channels based on dense wavelength division multiplexing (DWDM) through an optical fiber link consisting of 110 km per span. To check the influence of nonlinear effects, we have considered phase modulation (SPM), cross‐phase modulation (XPM), four‐wave mixing (FWM), stimulated Raman scattering (SRS), and self‐steepening (SS) which limits the performance of optical communication systems. In the proposed system, each channel is modulated at 200 Gbps of dual‐polarization quadrature phase shift keying (DP‐QPSK) format, with a 50‐GHz grid yield whole data rate of 6.4 Tbps. The results have proved that the nonlinear phenomena degrade the performance of proposed optical fiber communication systems, and XPM has the most dominant effect, followed by SS, SRS, and SPM. The simulation was carried out with Optisystem 14 which is powerful software for modeling and simulation of optical fiber transmission.     

超高速全光信息处理

P比特级光交换网络的发展,要求网络在传输、复用和交换方式上具有灵活性、多样性和高效性,因此基于超高速全光信息处理的网络功能存在较大价值。利用不同光子材料非线性效应(SPM\XPM\FWM等)已成功实现了组播、码型变换、逻辑门等不同的全光信息处理单元技术,其中高非线性光纤以其易与现有光纤网络相融合和成本相对较低等特点而具有较大潜力。在总结光信息处理的相关研究进展的基础上,文章重点介绍了偏振复用(PDM)系统中的高速全光信息处理技术,包括基于自相位调制效应(SPM)的全光再生和基于交叉相位调制效应(XPM)的波长转换实现。     

损耗作用下混频时调制不稳定性耦合模分析法

在考虑光纤损耗的情况下，以两载频以及混频过程中产生的边带频率成分为研究对象，给出了研究群速度色散（GVD）、自相位调制（SPM）、交叉相位调制（XPM）综合作用下混频过程中调制不稳定性（MI）的耦合模分析法。导出了当某一相位匹配较好、其他相位失配严重时，两载频自相位调制或交叉相位调制引起的调制不稳定性增益的解析表达式。分析发现，当调制不稳定性主要由自相位调制或交叉相位调制引起时，增益的最大值与色散无关，交叉相位调制在各边带引起的调制不稳定性最大增益相等，同时，考虑损耗后，幅度增益减小的大小受功率、非线性系数、色散情况、损耗系数和传输距离等因素共同影响，调制小稳定性产生的增益只有当频率偏移量足够大时才会出现，载频自相位调制引起的最大增益对应的频率偏移量减小。     

WDM系统中的非线①性研究及仿真实验

利用分离信道的分步傅立叶法对信道间距为0.8nm的8路10 Gbit/s波分复用色散补偿系统进行仿真实验，分析了在具有级联光放大器系统中光纤色散和各种非线性效应（自相位调制SPM、交叉相位调制XPM、四波混频FWM）对系统传输的影响。仿真结果表明，在色散补偿WDM通信系统中，SPW对系统传输造成的损伤最大。并且由于非线性效应、传输损耗及噪声的综合影响，应当选取合适的系统输入功率。     

Four-wave mixing of incoherent light in a dispersion-shifted fiberusing a spectrum-sliced fiber amplifier light source

We investigate the FWM efficiency of incoherent light in a dispersion-shifted fiber (DSF) using a spectrum-sliced fiber amplifier light source. A theoretical model is provided to describe the FWM mechanism of incoherent light. The FWM efficiencies of coherent and incoherent light are compared theoretically and experimentally. Unlike the FWM of coherent light, the FWM signals of incoherent light are mostly generated by nondegenerate FWM regardless of the number of input signals. Thus, when two input signals are mixed, incoherent light has about 6 dB higher mixing efficiency than coherent light due to the difference in their degeneracy factor     

A modified beam propagation method to model second harmonicgeneration in optical fibers

A finite-difference implementation of the beam propagation method (BPM) is used to solve the paraxial, scalar wave equation with a nonlinear source term. A transparent boundary condition capable of handling asymmetric modes is incorporated in the finite-difference algorithm. This nonlinear BPM is used to model the generation and propagation of second harmonic light in an optical fiber which has been prepared for second harmonic generation (SHG) by the formation of a _χ⁽²⁾ grating. This method can be used to predict the guided mode in which the generated second harmonic light propagates based on the modes of the writing (fundamental and second harmonic) and reading (fundamental only) light. The effects of self-phase modulation (SPM) and cross-phase modulation (XPM) are included in the model     

A 40-Gb/s/ch WDM transmission with SPM/XPM suppression throughprechirping and dispersion management

This paper proposes to combine prechirping with dispersion management scheme in such a way as to suppress the power penalty induced by self-phase modulation (SPM) and cross-phase modulation (XPM) in 40-Gb/s per channel wavelength-division multiplexed (WDM) transmission systems with long-amplifier spacing. First, we show that the optimum total dispersion to minimize SPM depends on prechirping and the local dispersion of the transmission fiber, unlike that for minimizing XPM. Next, it is shown that, by optimizing the combination of prechirping and local dispersion, these two optima can be made to match so as to improve the allowable maximum fiber input power. Finally, the operation of the proposed optimization scheme is confirmed experimentally, and 4×40-Gb/s WDM transmission over 400 km of nonzero dispersion-shifted fiber (NZDSF) is demonstrated successfully with the fiber input power of +10 dBm/ch and 250 GHz channel spacing     

Analysis of interchannel crosstalk in a dispersion-managed analog transmission link

A technique for computing the effect of cross-phase modulation (XPM) on two copropagating analog channels in an optical fiber link is presented. In this approach, the interaction between the two channels is linearized by keeping the self-phase modulation (SPM) and XPM interactions in the strong optical carrier components only at lowest order and then at the next order, deriving the effect on the modulation components of both channels when the optical carrier is strong relative to the other components of the channel. In contrast to some previously suggested approaches, it is not assumed that the pump is undistorted, and therefore, this method accurately describes distortions due to SPM, XPM, and dispersion management in both channels. This method is easily applied to systems with multiple spans employing dispersion management with loss and gain. The expressions for the received radio frequency power and crosstalk between the two channels when direct detection is used are then provided. Using this approach, new expressions for the amplitude modulation and phase modulation modes of the two channels are derived, and the way they exchange energy when SPM, XPM, and dispersion are all considered is explained. This method yields excellent agreement between theory and experimental data.     

The performance limit of coherent OTDR enhanced with optical fiberamplifiers due to optical nonlinear phenomena

This paper theoretically and experimentally clarifies the limit of incident optical pulse power in coherent optical time-domain reflectometry (C-OTDR) enhanced with optical fiber amplifiers. The critical pulse power, at which the performance of C-OTDR is degraded by the effect of optical nonlinear phenomena in a single-mode optical fiber, depends on the amplified optical pulse waveform and the pulse width. For a pulse width of 1 μs or longer, the incident pulse power is limited by the effect of self-phase modulation (SPM). When an optical pulse having a power gradient within the pulse width is incident to a single-mode optical fiber, the optical frequency of the backscattered signal is shifted by SPM, and the center frequency of the signal moves outside the receiver band, so the sensitivity of C-OTDR is degraded. For a pulse width of 100 ns, the incident optical pulse power is limited by four-wave mixing (FWM) which transfers the energy from the incident optical pulse to Stokes and anti-Stokes light as a result of the interaction between the incident optical pulse and amplified spontaneous emission. This paper also demonstrates the high performance of C-OTDR enhanced with EDF A's with 48, 44, 39, and 29 dB single-way dynamic ranges for pulse widths of 10 μs, 4 μs, 1 μs, and 100 ns, respectively, limited by the effect of SPM or FWM. These results are believed to be the best performance of C-OTDR with EDFA's     

Second-order theory for self-phase modulation and cross-phase modulation in optical fibers

The authors develop a second-order perturbation technique for the study of self-phase modulation (SPM) and cross-phase modulation (XPM) effects in optical fibers. When the dispersion distance is much shorter than the nonlinear length, it is found that the difference between the first- and second-order solution is negligible. However, as the dispersion distance increases, nonlinearity becomes a stronger perturbation, and the first-order theory is not adequate to describe the SPM effects. However, the results obtained using the second-order perturbation technique is in good agreement with numerical simulations even when the dispersion distance is longer than the nonlinear length. When pulses of different channels are copropagating in a fiber, they undergo amplitude distortion and timing shift due to XPM. The perturbation technique presented in this paper accounts for both amplitude distortion and timing shift of a pulse due to XPM.     

Intensity effects on the stimulated four photon spectra generated by picosecond pulses in optical fibers

The intensity dependence of stimulated four photon mixing (SFPM) spectra generated in 15 m of a 4-mode optical fiber by 25- ps pulses has been investigated. Despite the shortness of pulses, the SFPM conversion was highly efficient due to the intrinsic phase matching condition. In addition to usual features of SFPM spectra generated by nanosecond pump pulses, picosecond SFPM spectra were broadened by self phase modulation (SPM) and cross phase modulation (XPM). At the highest pump powers, intensity saturated frequency continua, arising from the combined effects of SFPM, stimulated Raman scattering (SRS), SPM, and XPM were generated all over the visible spectrum.     

Arrangement of orthogonal polarized signals for suppressing fiber four-wave mixing in optical multichannel transmission systems

The arrangement of signal polarization states for suppressing fiber four-wave mixing (FWM) in optical multichannel transmission is described. Experiments show that light power generated through fiber FWM becomes 0 or 1/4 of the maximum value when signals are orthogonal. Based on the experimental results, power reduction of FWM light is evaluated for various arrangements of signal polarizations, and the optimum arrangements is shown to efficiently suppress fiber FWM in a multichannel system.<>     

Simulation investigation on supercontinuum generation and noise characteristics in the normal dispersion photonic crystal fiber with a flattened dispersion profile

The propagation of picosecond pulses in the normal dispersion photonic crystal fiber(PCF) with a flattened dispersion profile is numerically investigated.The characteristics of the amplitude and phase noise in the supercontinuum generation(SCG) are also analyzed through the coherent sliced supercontinuum(SC).The effects of self-phase modulation(SPM) and four-wave mixing(FWM) on broadening of the pulse spectrum are presented,and the best amplitude and phase noise performance with a specific fiber length is o...     

Experimental study on channel crosstalk due to fiber four-wavemixing around the zero-dispersion wavelength

Channel crosstalk due to fiber four-wave mixing (FWM) in multichannel systems operated around the zero-dispersion wavelength is experimentally studied. After determining the wavelength at which FWM light is most efficiently generated, the FWM efficiency is measured for possible frequency combinations which generate FWM light at that wavelength. Using these data, FWM crosstalk in multiwavelength systems is evaluated. The results show that actual crosstalk, is less than the value estimated by the theoretical model assuming the uniformly distributed chromatic dispersion for 80-km-long fibers. It is concluded that the theoretical model can be applied to system design dealing with the worst condition     

Mid-Infrared Raman Fiber Lasers

As mid-infrared (MIR) lasers show numerous applications in the field of defense, medical, materials processing, and optical communications. Investigation on MIR Raman fiber lasers (RFLs) increasingly becomes a hot topic. Compared with the traditional silica fibers, fluoride and chalcogenide glass fibers possess higher nonlinear coefficients and excellent MIR transmittances. In this article, the latest developments of the MIR RFLs using fluoride and chalcogenide glass fibers as gain media are introduced, respectively. This review article mainly focuses on the developments of MIR RFLs in aspects of output wavelength, output power, and optical efficiency. Besides, the prospect of MIR RFLs is also discussed.     

Comparison of FWM- and XPM-induced crosstalk using the Volterra series transfer function method

New analytical tools to calculate the variance due to cross-phase modulation (XPM) and four-wave mixing (FWM) induced intensity distortion are derived based on the Volterra series transfer function method. The analysis for both the XPM and FWM effects is based on the same system configuration with a continuous-wave (CW) probe channel plus modulated pump channels, which makes possible a fair comparison between the two nonlinear effects. Effective ways to reduce the XPM- and FWM-induced intensity distortion are given. The new results on the variance of the nonlinearity-induced intensity fluctuation also make it possible to study both synchronous wavelength-division multiplexing (WDM) systems with fixed channel delays and asynchronous WDM systems with random channel delays. The new analytical results provide accurate and efficient ways for system parameter optimization to reduce these two nonlinear effects.