新型超宽带双芯光子晶体光纤偏振分束器的研究

提出了一种新型超宽带双芯光子晶体光纤偏振分束器. 应用全矢量有限元法,系统地研究了光纤结构参数对偏振分束器带宽和偏振分束器长度的影响. 分析结果表明：增加掺氟区折射率,既能提高分束器带宽,也能减少分束器长度;增大光纤中空气孔的孔距及孔径与孔距比,可以增加分束器的带宽,但同时也会导致分束器的长度增大,使得器件尺寸增大. 因此,在设计中需兼顾分束器的带宽和长度来选取光纤的结构参数. 通过结构参数的优化,设计出一种短长度、高消光比和超宽带的偏振分束器,其长度为7.362 mm,消光比高于20 dB的带宽为600 nm. 关键词： 光子晶体光纤 偏振分束器 超宽带 全矢量有限元法     

光纤参变放大器光纤长度的优化设计

随着高输出功率掺铒光纤放大器和高非线性光纤的出现和使用,光纤参变放大器(OPAs)中出现了越来越多的增益饱和现象,这是光放大器中应该尽量避免的。对此,提出了依据光纤参变放大器的光纤非线性系数、抽运光功率、信号输入功率等参量对光纤长度进行优化设计的解决思路。明确提出最佳光纤长度的概念,即在其他参量一定的情况下使信号增益或信号输出功率达到最大所需的最小光纤长度,而且最佳光纤长度有利于提高光纤参变放大的增益带宽和波长转换带宽。通过数值积分求解描述光纤参变放大过程的非线性耦合方程,并运用控制变量法深入研究了最佳光纤长度与光纤非线性系数、抽运光功率、信号输入功率的关系。最后用最小二乘法进行数据拟合确定系数,得到简洁、实用的最佳光纤长度解析表达式。与已有实验结果比较表明,该解析式可很好地用来计算和优化光纤参变放大的光纤长度。     

Optimized design of two-pump fiber optical parametric amplifier and its noise characteristics

In the paper, based on the pump depleted model of two-pump fiber optical parametric amplifier (FOPA), the simulations are done to indicate the effect of amplifier parameters on gain characteristics, such as the input signal power, the wavelength separation between the two pumps, the fiber length, the nonlinear coefficient and the fourth-order dispersion of fiber. By optimizing the amplifier parameters, when FOPA attains low gain-saturation, the FOPA has a higher gain, a broader flat gain bandwidth and a lower noise figure. However, with the enhancement of gain-saturation the parametric gain decreases, the flat gain bandwidth becomes narrow and the noise figure also increases.     

啁啾长周期光纤光栅的超宽带滤波特性及其切趾优化

基于光纤光栅的模式耦合理论,采用传输矩阵法对啁啾长周期光纤光栅(LPFG)的超宽带滤波特性进行了分析。研究表明当啁啾LPFG的纤芯模同时与同向传输的多个阶次的包层模发生耦合,且与多个不同阶次包层模对应的谐振峰交叠时,其传输谱带宽可扩展到500nm以上,可用做超宽带带阻滤波器。传输谱带宽随模式序数、啁啾系数、光栅长度、周期和折射率调制量的增加单调递增,但随光纤包层半径的增大单调递减。采用高斯折射率切趾技术抑制了传输谱旁瓣,为设计超宽带滤波器提供了新的方法。     

基于少模长周期光纤叠栅的模式转换器

本文提出一种在同一段少模光纤上写入两个不同周期Λ1和Λ2的长周期光纤光栅构成叠栅的方法,实现了纤芯基模LP_(01)向高阶纤芯模LP_(11)模式转换的宽带宽的新型的全光纤模式转换器.利用有限元法和耦合模理论建立了模式转换器的理论分析模型.数值仿真分析了叠栅中两个子光栅周期间隔、光栅长度、耦合系数等光栅参数对模式转换器的影响.仿真分析和实验结果表明,通过改变两个子光栅的周期间隔来改变两个损耗峰的位置,形成一个损耗峰,从而可以实现宽带宽的模式转换器,其10dB带宽约是单栅的2倍.与传统的模式转换器相比,该转换器带宽宽、转换效率高,尺寸小、抗干扰能力强,可以在模分复用系统和光通信中得到广泛的应用.     

Unrepeatered transmission through ultra-long fiber laser cavities

We present a study on the potential use of ultra-long laser cavities for unrepeatered fiber communication, based on the theory of nonlinearity management. A comparison is offered between the performance of ultra-long lasers and standard bi-directional distributed amplification schemes in nonrepeated transmission. Links based on both traditional (SMF/DCF) and modern Ultrawave transmission fibers are considered.     

芯泵浦高功率宽带谱平坦长波段光纤光源

为了获得高功率、宽带宽及谱平坦的长波段掺铒光纤光源,基于2级双程芯泵浦,应用偏振复用技术实现泵浦瓦级供给,在泵浦总功率和光纤总长度都不变的情况下,数值分析了4种光源结构的输出特性受泵浦和光纤分配比例的影响。结果表明,4种结构基本都能工作于L波段（1 565 nm~1 610 nm）,带宽受结构影响较小,但只有双程后向+双程后向结构可同时拥有高输出功率和高平坦度。其在总泵浦功率750 mW,第一级泵浦功率为300 mW,第二级泵浦功率为450 mW时,和光纤总长度21 m,第一级光纤长度为18 m,第二级光纤长度为3 m时,可实现输出功率314 mW,带宽32.41 nm,中心波长1 584.84 nm,平坦度2.23 dB的L波段超荧光光源。     

TE01-TE11模式转换器的带宽特性

针对模式转换器设计中在相同的中心频点转换效率下模式转换器越短带宽越宽的传统观点,研究发现了最佳长度的存在,即当结构长度大于最佳长度时,带宽随着结构长度增加而减小,当结构长度小于最佳长度时,带宽随着结构长度增加而增加。通过对中心频率35 GHz、半径13.6 mm的TE01-TE11模式转换器进行的验证发现,最佳长度为550 mm,此时带宽达到最大值6.28%。     

长周期光纤光栅(LPFGs)的谱结构研究

利用相位匹配条件,从理论和实验上研究了均匀LPFGs的谱结构特点.结果表明,导模和不同包层模耦合产生的光谱特性有很大差别.在模序较小时,谱宽较小;模序增加,谱宽也增加;在某一模序附近,谱宽达到最大;以后随模序增加,谱宽迅速减小.给出了LPFGs中透射谱的最大损耗率及3 dB带宽与包层模序、光栅周期、耦合系数和栅长的关系.这些关系对设计满足特定最大损耗率和带宽的带阻滤波器或增益均衡器提供一种理论参考.选用不同的包层模、耦合系数和栅长,可以设计出谱宽很大或很小的LPFGs.     

Design of triangular FBG filter for sensor applications using covariance matrix adapted evolution algorithm

The design of triangular fiber Bragg grating (TFBG) filters using covariance matrix adapted evolution strategy (CMAES) algorithm is presented. The TFBG filter can be used as a readout device in FBG-based sensor applications. Two different filter design problems are investigated. The first problem is the design of TFBG filter for a specified bandwidth. The other is maximizing the bandwidth of the TFBG filter for a given grating length and maximum index modulation of the FBG. A more practical asymmetrical triangular FBG filter design is also considered. The results show that for the same grating length and maximum index modulation, a larger bandwidth can be achieved with a chirped grating. The maximized bandwidth design can be used to achieve the maximum possible bandwidth for the specified grating length, maximum index modulation and chirp rate. The results obtained using CMAES algorithm (codes available at http://www.ntu.edu.sg/home/epnsugan) is compared with the results of PSO and other algorithms. Comparisons show that the CMAES algorithm is more consistent in obtaining the best solution with reduced computation time. Hence, the CMAES algorithm is effective for the design of TFBG filters for sensor applications.     

双带通啁啾光纤光栅光谱特性仿真及分析

针对啁啾光纤光栅的带通滤波特性,设计了纤芯/包层复合结构的啁啾光纤光栅滤波器.基于光纤耦合模和传输矩阵理论及坐标变换方法,对其光谱特性进行仿真分析,验证了该光栅的双带通滤波特性;对周期为536、537、538nm,长度为8、10、12mm,折射率调试深度为0.0002、0.0003、0.0004,啁啾系数为0.05、0...     

掺铒光纤环形激光器混沌带宽特性数值研究

利用掺铒光纤环形激光器非线性克尔效应模型,研究抽运功率和非线性系数对激光器输出混沌信号带宽的影响.研究表明：在非线性系数一定的情况下,混沌带宽随着抽运功率的增加先增大后减小;固定抽运功率为01 W,当非线性系数增加到45 W^-1km^-1时,掺铒光纤激光器输出的混沌带宽达到最大值（153 GHz）,非线性系数继续增加,输出混沌的带宽将逐渐减小. 关键词： 克尔效应 掺铒光纤激光器 混沌 带宽     

Dispersion measurement of Yb-doped fiber by a spectral interferometric technique

<正>The dispersion of Yb-doped fiber is measured by a spectral interferometric technique.The experimental verification is achieved by comparing the measured data with published data of the Nufern 1060xp fiber and the measurement relative error is 1.36%.The parameters of the experimental system,such as minimum required source bandwidth and minimum fiber length,are introduced and analyzed in the measurement. The minimum required source bandwidth predicted through theoretical calculation at the center wavelength of 1070 nm is 19.3 nm,which perfectly agrees with the experimental value.     

Improving the performance of fiber gratings with cladding being etched as hyperbolic function

A new grating structure of cladding being etched as hyperbolic function along the grating length is introduced to improve their performance. The fiber grating exhibits a flattop spectrum with steep edges and high reflectivity when tension is applied to it. The bandwidth utilization defined as the ratio of −1:−30 could be achieved a larger value (>0.9689). The −3 dB bandwidth and the bandwidth utilization could be tunable with the increment of tension applied to the grating and the etch factor.     

Multimode fiber bandwidth prediction based on preform profile

A new technique is suggested for multimode fiber bandwidth prediction based on a preform profile. It allows one to calculate the output pulse distortion and allows the bandwidth of a fiber to be drawn dependent on the excitation conditions under which it will operate. It is shown that the accuracy of the bandwidth prediction for 1.5- to 2.5-km fibers is about 10%, the bandwidth being 1–1.5 GHz/km. To achieve this accuracy it is necessary that the excitation conditions and differential mode attenuation be exactly reproduced in the calculation. The profile variations along the length should not be considerable. The prediction technique was adapted to the P101 Preform Analyser (York Technology) and can replace alpha-approximation curve-fitting during routine manufacture of multimode preforms.     

Multimode fiber bandwidth prediction based on preform profile

A new technique is suggested for multimode fiber bandwidth prediction based on a preform profile. It allows one to calculate the output pulse distortion and allows the bandwidth of a fiber to be drawn dependent on the excitation conditions under which it will operate. It is shown that the accuracy of the bandwidth prediction for 1.5- to 2.5-km fibers is about 10%, the bandwidth being 1-1.5 GHz/km. To achieve this accuracy it is necessary that the excitation conditions and differential mode attenuation be exactly reproduced in the calculation. The profile variations along the length should not be considerable. The prediction technique was adapted to the P101 Preform Analyser (York Technology) and can replace alpha-approximation curve-fitting during routine manufacture of multimode preforms.     

Character analysis of slow light with communication waveband in fiber optical parametric amplifier

The comprehensive characters of delay time and delay bandwidth with distortion, signal wavelength dependence and delay bandwidth product in fiber optical parametric amplifier are investigated theoretically when the pump locates at anomalous dispersion region. The delay character of single pulse, data stream with certain bit pattern and pseudo random data stream are analyzed comprehensively. In this category of fiber optical parametric amplifier, the delay time and delay bandwidth product vary from signal wavelengths and bit rates. For the same nonlinear coefficient, pump power and fiber length, the diverse dependences of bit pattern in different signal wavelengths are found by numerical simulation. The impacts of pump power and fiber length on the delay character and distortion are also discussed. These relations are conductive to the design of slow light based on fiber optical parametric amplifier in telecommunication waveband.     

变迹啁啾Bragg光纤光栅的矩阵分析

本文用传输矩阵法分析了各种变迹与线性啁啾Bragg光纤光栅的响应特性,构造了各种变迹函数,进行了比较.分析了变迹函数、啁啾系数、折射率变化量以及光栅长度对反射带宽、最大反射率的影响.找到了最佳变迹函数和量佳啁啾系数.特例中的数值分析表明,补偿100km光纤色散带宽为1nm的最佳光栅长度是140mm,最佳啁啾系数为0.048nm/cm.     

Improving the performance of chirped fiber grating with cladding being etched as sinusoidal function

A new grating structure of cladding being etched as sinusoidal function along the grating length is introduced to improve their performance. When a tension is applied to it, the fiber grating not only exhibits a flattop spectrum with steep edges and high reflectivity, but also has a smoothly time delay curve. The bandwidth utilization defined as the ratio of −1:−30 could be achieved a larger value (>0.90). The −3 dB bandwidth and the bandwidth utilization could be tunable with the increment of tension applied to the grating and the linearly chirped modulus.     

Analysis of the bandwidth of resonant spectra for tilted fiber grating

Based on the coupled-mode equations, the formula of null-null bandwidth for tilted fiber grating is deduced in this paper. Numerical simulations and theoretical analysis of the effects of the tilt angle on bandwidth and maximum reflectivity are performed. Furthermore, under the maximum reflection of 95%, the relation of bandwidth with the tilt angle, modulation amplitude, and the needed length of grating are demonstrated in two three-dimensional diagrams. It reveals that the grating tilt can obviously narrow the bandwidth, especially in the case of large modulation amplitude and grating length, which is meaningful to its application in optical communications and fiber sensing.