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.     

Dynamic characteristics of nonlinear Bragg gratings in photonic crystal fibres

We study numerically the temporal and spatial dynamics of light in Bragg gratings in highly nonlinear photonic crystal fibre for a CW input signal. Our numerical model is based on the plane wave mode solver and a set of nonlinear coupled-mode equations which we solve using a variation of implicit fourth order Runge-Kutta method. We observe not only bistability of the intensity versus transmitted and reflected light but also complex dynamics. We demonstrate that for values of input intensity above the bistable region the steady state may undergo a supercritical Hopf bifurcation. For some ranges of the input intensity we also observe a coexistence of two periodic attractors. The dynamics found, in particular the features in the bifurcation diagram, strongly depend on the parameters of the fibre. Consequently, we suggest that by proper design of the photonic crystal in the cladding we can adjust such nonlinear features of the Bragg gratings as the width of the bistable region, the intensity at which the bifurcation occurs and also the characteristics of the dynamics at high values of input intensity.     

分布反馈光纤激光器的研制

利用相位掩膜板,直接在掺饵光纤上紫外写入π／２相位光栅,研制出１０ｃｍ长相移分布反馈（ＤＦＢ）光纤激光器,并对其调制特性进行了研究。     

光纤布拉格光栅温度灵敏性的实验研究

在-150～550℃的温度范围内测量了光纤布拉格光栅中心反射波长的温度灵敏性。光栅波长的温度响应是非线性的。通过对试验数据的三次多项式进行拟合分析,结果表明:波长为1500nm左右的光栅具有相同的温度敏感性;波长对温度变化的响应(ΔλB/ΔT)主要是由折射率的变化(dn/dT)引起的,dn/dT随着温度的降低而减小;得到了温度灵敏系数关于温度的二次表达式,并给出了在-150～550℃温度范围内波长变化的快速估算。     

双体布拉格光栅光谱反馈特性的理论研究

基于Kogelnik理论模型,以平面波为入射光,分析了双体布拉格光栅（VBG）的反馈特性。理论分析表明,采用双VBG结构,通过控制两个VBG的中心波长差,可以使光栅反馈的效率提高,带宽加宽。对于由两块带宽100 pm、衍射效率20%的VBG组成的双VBG反馈系统,中心波长相同时,衍射效率最大（33.3%）;中心波长相差84 pm时,其带宽为单VBG带宽的1.8倍（180 pm）。由于VBG的中心波长存在温度漂移特性,因此通过改变双VBG结构中VBG的温度可以实现衍射效率和带宽的改变,为泵浦源的优化提供有效可行的方法。     

啁啾的引入对线性负切趾型布拉格光栅双稳特性的影响

基于耦合模理论，对线性负切趾型布拉格光栅中引入啁啾后其双稳特性的变化进行了研究.结果表明：引入正啁啾有利于改善线性负切趾型布拉格光栅的双稳态性能；通过合理选择正啁啾、线性负切趾型布拉格光栅的长度，可对其双稳态性能进行进一步优化. 关键词： 啁啾 线性负切趾布拉格光栅 双稳     

双体布拉格光栅光谱反馈特性的理论研究

基于Kogelnik理论模型,以平面波为入射光,分析了双体布拉格光栅（VBG）的反馈特性。理论分析表明,采用双VBG结构,通过控制两个VBG的中心波长差,可以使光栅反馈的效率提高,带宽加宽。对于由两块带宽100 pm、衍射效率20%的VBG组成的双VBG反馈系统,中心波长相同时,衍射效率最大（33.3%）;中心波长相差84 pm时,其带宽为单VBG带宽的1.8倍（180 pm）。由于VBG的中心波长存在温度漂移特性,因此通过改变双VBG结构中VBG的温度可以实现衍射效率和带宽的改变,为泵浦源的优化提供有效可行的方法。     

用于单频光纤激光器的光纤光栅双腔Fabry-Perot结构传输谱特性理论研究

分析了基于光纤光栅的全光纤型双腔Fabry-Perot(F-P)结构传输谱特性. 理论推出了两腔F-P结构传输率具体计算公式,给出在光栅中心波长处产生单谐振传输峰时,腔长与组成光栅反射率各需满足的条件. 基于理论分析结果,分两部分数值模拟了对称及非对称两腔F-P结构传输谱,讨论了计算结果并给出定性的解释,总结了腔长及光栅长度、折射率调制深度的设计原则. 结论表明,当单腔F-P结构腔长增大到阻带内出现多个谐振峰时,通过合理选取两腔结构的腔长及光栅参数,双腔F-P结构能够在整体长度不变条件下,抑制中心波长两侧的次谐振峰,而中心波长处的主谐振峰不受影响. 关键词： 光纤光栅 Fabry-Perot结构 光纤激光器     

Modeling of resistance characteristics of a continuously-graded distributed Bragg reflector in a 980-nm vertical-cavity surface-emitting laser

The resistance characteristics of a continuously-graded distributed Bragg reflector(DBR) in a 980-nm verticalcavity surface-emitting laser(VCSEL) are modeled in detail.The junction resistances between the layers of both the p-and n-DBR mirrors are analysed by combining the thermionic emission model and the finite difference method.In the meantime,the intrinsic resistance of the DBR material system is calculated to make a comparison with the junction resistance.The minimal values of series resistances of the graded p-and n-type DBR mirrors and the lateral temperature-dependent resistance variation are calculated and discussed.The result indicates the potential to optimize the design of the DBR reflectors of the 980-nm VCSELs.     

Numerical analysis of apodized fiber Bragg gratings formation using phase mask with variable diffraction efficiency

Scalar diffraction theory is applied to analyze the intensity distribution in the fiber core during fiber Bragg grating (FBG) fabrication using an apodized phase mask. The averaged diffraction field distribution in fiber core was calculated as a function of optical fiber position. The results show that in a case of apodized FBGs fabrication, averaged field intensity profile, and thus refractive index changes in fiber core have complex form. Moreover, it was shown that the influence of optical fiber position behind the phase mask on average intensity distribution in fiber core decreases with increasing of its diameter.     

Bidirectional oscillations in Er-doped fiber ring cavity with polarization splitting for rotation sensing

The structure of an all-fiber Erbium-doped fiber ring cavity with bidirectional single-mode oscillations and a rotation-sensitive beat signal for gyro applications is reported in this letter. The structure is achieved by introducing a polarization splitting method to filter the bidirectional oscillations and to control the mode coupling of the counter-propagating waves. Two distinct behaviors of the beat signal are identified, namely, the linear mode coupling situation caused by the back-reflection of the optical elements and the nonlinear mode coupling situation attributed to spatially non-uniform saturation in gain medium. Experiments are conducted to show the cavity rotation rate dependence of the beat frequency shift. The results are in good agreement with the theoretical predictions.     

1.3μmDFB半导体激光超短脉冲的产生及其压缩

本文首次报道１．３μｍ单模超短脉冲产生及压缩实验。通过梳状波电脉冲直接调制ＤＣＰＢＨ－ＤＦＢ半导体激光器，获得２２ｐｓ单模光滑脉冲；然后利用Ｇ－Ｔ干涉仪压缩得到近变换极限超短脉冲。脉宽为９．３ｐｓ，峰值功率超过２００ｍＷ，重复频率１ＧＨｚ，时间带宽乘积为０．３９，脉冲压缩比为２．４。最后对实验结果进行了讨论。     

取样布拉格光纤光栅的传输矩阵

根据在一般坐标系下均匀布拉格光纤光栅的传输矩阵,得到了取样布拉格光纤光栅的传输矩阵。利用傅里叶变换得到了取样布拉格光纤光栅的谐振方程。结果表明,在不考虑平均折射率变化的情况下,谐振峰的位置是由光栅的周期和取样周期共同确定的,与取样时的占空比、光栅长度和耦合系数没有关系。类似于物理光栅,取样布拉格光纤光栅也存在缺级现象,给出了出现缺级的条件。     

变耦合系数的简并拉曼耦合J-C模型中场熵的演化

研究了考虑耦合系数随时间线形变化的简并拉曼耦合J-C模型中场熵随时间演化特性,分析了耦合系数变化的快慢和不同的初始光场对场熵的影响。结果表明:(1)无论光场初始态如何,场熵均呈周期性演化,平均光子数n-的增加并不改变场熵演化周期的大小;(2)初始光场统计性质的不同对最大场熵值和最大场熵值变化的幅度产生较大的影响;(3)线形调制使场熵呈现的完美周期性振荡遭到破坏。当原子-场耦合系数变化缓慢时,原子进入统计混合态的速度被减缓;而当原子-场耦合系数变化较快时,则不仅使场熵演化曲线的振荡周期逐渐减小、振荡频率越来越快,而且使原子与场退纠缠的时刻增多。     

Modeling of waveguide UV-induced Bragg gratings with arbitrary apodization function: Tools for designing high-performance related devices

We present a semi-analytical modeling suitable for the investigation of the spectral characteristics of UV-waveguide Bragg gratings (UV-WBGs) having arbitrary apodization function and extending both in the film layer and in the overlayer. The modeling is based on a proper combination of coupled mode theory and transfer matrix formalism and provides a powerful tool for designing and simulation of complex UV-grating based structures. The results of a detailed investigation of UV-WBGs in doped silica based channel optical waveguides are presented and critical issues for upgrading the performances of related device are discussed. Validation of the modeling is provided by means of an extensive comparison between theoretical and experimental results relevant to UV-WBGs realized in phosphorous (P)- and boron (B)-doped silica based waveguides.     

长周期光纤光栅傅里叶模式耦合理论

建立了长周期光纤光栅傅里叶模式耦合理论.在分析同向模式耦合时,发现了同向耦合模式的振幅系数间存在傅里叶变换关系.推导了长周期光纤光栅的同向耦合谱和透射谱的通用表达式.该理论是用傅里叶变换分析得出长周期光纤光栅折射率微扰的空域谱,再对该空域谱进行模式同向耦合分析,从而得到长周期光纤光栅光谱特性的通用表达式.根据该理论模拟分析了长周期光纤光栅在不同长度和微扰幅值时的光谱特性,与传统耦合模理论进行了对比分析.结果表明,该长周期光纤光栅傅里叶模式耦合理论具有简单、精确和高效的特点,与实际长周期光纤光栅的透射谱特性一致.应用该理论可分析无过耦合的任意轴向折射率微扰分布的长周期光纤光栅光谱特性.     

Bragg光纤光栅傅里叶模式耦合理论

建立了Bragg光纤光栅傅里叶模式耦合理论.在分析光纤光栅的耦合模时,发现了耦合模式的振幅系数间存在傅里叶变换关系.推导了将傅里叶变换和模式耦合融合在一起的Bragg光纤光栅反射谱和透射谱的通用表达式.该理论是用傅里叶变换得到Bragg光纤光栅折射率微扰的空域谱,再对该空域谱进行模式耦合分析计算,从而得到Bragg光纤光栅的光谱特性.根据该理论,仿真分析了Bragg光纤光栅的谱特性,与耦合模理论、直接傅里叶变换法进行了对比分析.结果表明,傅里叶模式耦合理论与传统的耦合模理论及实际Bragg光纤光栅的光谱特性一致,具有简单、清晰、直接、精确和分析效率高的特点,可分析任意轴向折射率微扰分布的Bragg光纤光栅结构.     

Characterization of thermal (in)stability and temperature-dependence of type-I and type-IIA Bragg gratings written in B-Ge co-doped fiber

The temperature-dependence and the thermal stability of UV-exposed type-I and type-IIA fiber Bragg gratings written in boron-germanium co-doped silica fiber were examined, in terms of their refractive-index modulations and the change in the Bragg wavelengths. The decay in the reflectivity of the gratings with time at different temperatures was studied and modelled according to a power-law and a master aging curve approach. The distribution of the activation energy, for these types of gratings, was evaluated to describe their thermal stability and then the comparison of their stabilities has been made. Temperature-induced reversible and irreversible changes in the grating characteristics were observed. The temperature-induced reversible spectral characteristics of type-I and type-IIA gratings were found to be different in nature. All these results were discussed within the framework of the current theoretical approaches for predicting the stability and operational lifetime of these types of gratings.