Large signal-to-noise-ratio enhancement of ultrashort pulsed optical signals using a power-symmetric Nonlinear Optical Loop Mirror with output polarisation selection

A numerical study of a fibre-based scheme for the regeneration of ultrashort-pulsed optical signals is presented. The setup is made of a power-symmetric Nonlinear Optical Loop Mirror (NOLM) followed by a polariser. The NOLM operates through nonlinear polarisation rotation, and includes twisted, anomalous-dispersion fibre and a quarter-wave retarder. When the orientations of the linear input polarisation and of the output polariser are properly adjusted, the output energy characteristic flattens at high power, a property that can be used to eliminate large amplitude fluctuations on the logical ones of an optical signal. When the input pulse parameters closely match those of fundamental solitons or of stable elliptically polarised solitary waves, a wide and flat plateau is obtained, allowing the reduction of ～30% amplitude fluctuations to less than 1%. Very large amplitude fluctuations beyond 50% can also be reduced down to a few %. Although the output pulses are slightly chirped, they are free of pedestal, thanks to the zero low-power transmission of the NOLM, which also allows the simultaneous regeneration of logical zeros. We believe that this setup will be useful for the regeneration of highly degraded signals in future ultrafast transmission networks.     

Ultrafast polarisation-independent all-optical switching using a polarisation diversity scheme in the nonlinear optical loop mirror

Polarisation-independent 32 to 8 Gbit/s demultiplexing in the nonlinear optical loop mirror is successfully demonstrated using a novel polarisation diversity scheme in which two independent switching operations are performed along the two principal axes of a polarisation-maintaining fibre loop. The variation of the switched signal powers is measured to be less than 1 dB as the input polarisation direction is varied over 180 degrees .<>     

高阶孤子在色散渐减光纤环中的传输特性研究

研究了高阶孤子在对称结构和不对称结构的色散渐减光纤环形镜的脉冲传输特性,比较了不同耦合系数下的色散渐减光纤环形镜的开关性能及脉冲压缩性能.得出了使色散渐减光纤环具有良好脉冲输出的耦合器系数范围,对实验中参数的选取有一定的指导意义.发现不对称结构光纤环形镜亦能产生无基座超短脉冲.     

基于非线性光学环镜中SOA的XPM效应生成UWB信号的研究

利用非线性光学环镜(NOLM),提出了一种基于半导体光放大器(SOA)的交叉相位调制(XPM)效应生成超宽带(UWB)信号的方法,设计了系统结构,并进行了仿真实验,得到了中心频率约为6.7GHz、相对带宽约为161%的UWB信号,该信号符合美国联邦通信委员会(FCC)标准,从而验证了该方法的可行性。另外,所生成的UWB信号的中心频率及带宽还具有可调性。     

Effects of Higher-order Dispersion on Time Delay in Femtosecond Pulse Generated from a Nonlinear Optical Loop Mirror Constructed from Dispersion Decreasing Fiber

The time delay（TD） of femtoseeond pulses is studied for the first time, which generated from the nonlinear optical loop mirror composed of dispersion decreasing fiber（DDF-NOLM）. The results show that the higher-order dispersion and high order nonlinearities such as Raman frequency shift play a key role in producing TD, and that the time delay ean be suppressed by the third-order dispersion（TOD） in DDF-NOLM. The mechanism of the time delay suppression is also discussed in detail.     

Simultaneous 4 × 10 Gb/s NRZ-to-RZ Modulation Format Conversion in Nonlinear Optical Loop Mirror With a Photonic Crystal Fiber

Orthogonal cross-phase-modulation (XPM) scheme in a nonlinear optical loop mirror (NOLM) is proposed to simultaneously convert four synchronized 10-Gb/s nonreturn-to-zero signals into return-to-zero (RZ) format. Modulation format conversion is achieved by using a synchronized optical control pulse train as the pulse carver in a NOLM. The control pulse train and the targeted signals are orthogonal in their states of polarization. This orthogonal nonlinear interaction substantially suppresses the undesirable four-wave mixing (FWM) induced crosstalk in the multichannel operation. Experimental demonstration of the proposed scheme shows that the suppression of the generated FWM idler is >26 dB. Error-free operation is achieved for the four converted 10-Gb/s RZ signals in a single NOLM by sharing the nonlinear XPM effect induced by the control pulse train.     

基于非线性偏振旋转锁模孤子光脉冲的产生及模拟

非线性偏振旋转(NPR)技术是被动锁模光纤激光器中实现超短脉冲的一种有效方式,因其结构紧凑,可靠性高而备受关注。本文利用基于NPR锁模的掺铒光纤激光器,在1557.7 nm波段,获得了脉冲宽度为1.35 ps,基频重复率为9.49 MHz的脉冲序列输出。利用耦合的金兹堡-朗道方程,数值模拟了激光器中锁模孤子光脉冲的产生,并对锁模建立过程中孤子时域和频域演化进行了分析,模拟分析和实验观察相吻合。该结果有助于加深人们对NPR锁模光纤激光器中孤子锁模动力学特性的理解。     

Optical and structural properties of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films obtained by pulsed laser deposition in a H<Subscript>2</Subscript>S atmosphere with subsequent annealing in a N<Subscript>2</Subscript> atmosphere

The structural and optical properties of Cu₂ZnSnS₄ thin-film layers formed by reactive pulsed laser deposition in a H₂S atmosphere at room temperature with the use of a Cu metal target and a Zn–Sn alloy target are studied in relation to the parameters of annealing in a N₂ atmosphere.