Improving Efficiency of Supercontinuum Generation in a Low-Birefringent Dispersion-Flattened/Decreasing Fiber by Cross-Phase Modulation
Improving Efficiency of Supercontinuum Generation in a Low-Birefringent Dispersion-Flattened/Decreasing Fiber by Cross-Phase Modulation
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通过交叉相位调制提高低双折射色散平坦/递减光纤中超连续谱的产生效率
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发表时间：
2003
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通讯作者：
H. Sone
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作者：
H. Sone
通过激发和叠加两种正交偏振模式，提高了低双折射色散平坦/减小光纤中超连续谱（SC）的产生效率，并通过数值分析发现其光谱平坦且宽，为346nm。众所周知，高质量和宽的SC频谱主要是由于光纤中的自相位调制（SPM）具有色散平坦和色散减小的特性[1]。对于这种色散平坦/递减光纤（DFDF），已经从理论上和实验上讨论了几种SC光纤和泵浦激光器产生SC的一般准则[2,3]。此外，交叉相位调制（XPM）诱导脉冲压缩并导致双折射光纤中非DFDF[4]的群速度色散（GVD）异常区域的SC产生。与这些传统的SC光纤相比，光子晶体光纤（PCFs）[5]和锥形光纤[6]由于其不同寻常的色散特性和高效的非线性特性而成为高效SC产生的有前途的来源。然而，传统的DFDF仍然是必要的，因为通过在双折射DFDF光纤[7]的输入处激发两个正交偏振脉冲可以实现平坦和宽的SC。为了使孤子脉冲在无波矢量失配的情况下通过XPM过程相互作用，ddf的模态双折射必须小于10[8]。在这种条件下，我们在这里描述了通过叠加两个正交分量获得的输出脉冲显示出宽超过340 nm的平坦SC光谱。图1显示了在DFDF中利用XPM生成SC频谱的仿真模型。在插图中，也显示了主轴，即双折射光纤输入端的快轴和慢轴（x和y）。如图所示，θxy表示从快(x)轴测量的输入方位角偏振角。在DFDF的输入端需要λ/2波片来产生两个正交极化的基本孤子脉冲。偏振器用于检测与输入方位角相同角度的θxy光脉冲的叠加强度。在数值分析方面，采用分步傅立叶分析方法对耦合非线性薛定谔方程进行了数值求解。研究了自相位调制（SPM）、交叉相位调制（XPM）（同频率正交极化波之间的耦合）、自陡变和受激拉曼散射（SRS）的高阶色散和非线性的影响。广义拉曼散射磁化率可以在分子振动[8]的谐振子模型中近似得到，但在高度非线性光子晶体光纤中，SC的产生被认为是SRS和参数四波混频[9]的联合效应。超连续脉冲低双折射光纤
The efficiency of supercontinuum(SC) generation in a low-birefringent dispersion–flattened/ decreasing fiber is improved by exciting and superimposing two orthogonally polarized modes and its spectrum is found to be flat and broad as 346nm by numerical analysis. Introduction It is well known that a high-quality and broad SC spectrum is generated mainly due to self-phase modulation (SPM) in optical fibers with a dispersionflattened and dispersion-decreasing characteristic [1]. For such a dispersion-flattened / decreasing fiber (DFDF), general criteria of SC generation for several SC fibers and pump lasers have been discussed theoretically and experimentally [2,3]. Also, crossphase modulation (XPM) induces pulse compression and leads to SC generation in the anomalous region for group-velocity dispersion (GVD) in a birefringent fiber that is not a DFDF [4]. In contrast to these conventional SC fibers, photonic crystal fibers (PCFs) [5] and tapered fibers [6] have found its application as promising sources for efficient SC generation due to their unusual dispersion properties and high effective nonlinearities. However, the conventional DFDF is still necessary since a flat and broad SC may be achieved by exciting two orthogonal polarized pulses at the input of the birefringent DFDF fiber [7]. For soliton pulses to interact through the process of XPM without wavevector mismatch, the modal birefringence of DFDF is to be less than 10 [8]. Under such conditions, we describe here that the output pulses obtained by superimposing the two orthogonal components exhibit a flat SC spectrum as broad as over 340 nm. Analytical Model for SC Generation Figure 1 shows a simulation model of SC spectrum generation by utilizing XPM in a DFDF. In the inset, the principal axes, i.e., the fast and slow (x and y) ones at the input end of a birefringent fiber are also shown. As seen the figure, θxy denotes an polarization angle of input azimuth measured from the fast (x) axis. The λ/2 wave plate is required to generate two orthogonally polarized fundamental soliton pulses at the input end of DFDF. A polarizer is used to detect superimposed intensity of optical pulses at θxy that is the same angle as input azimuth. For numerical analysis, the coupled nonlinear Shrodinger equations are numerically solved by the help of a split-step Fourier analysis [8]. The effects of higher-order dispersion as well as nonlinearities of self-phase modulation (SPM), cross-phase modulation (XPM) (coupling between orthogonally polarized waves of the same frequency), self steepening and stimulated Raman scattering (SRS) are included. The generalized Raman scattering susceptibility can be approximated in the harmonic oscillator model for molecular vibrations [8], although the SC generation in a highly nonlinear photonic crystal fiber is identified as combined effect of the SRS and parametric fourwave mixing [9]. EDFRL +EDFA Supercontinuum Pulse DFDF(Low-Birefringent Fiber)