渐变折射率平板波导耦合器研究

介绍了渐变折射率平板波导耦合器的结构特点,并用波导理论详细分析了其工作机理及制作方法,给出了相应的设计方程。尤其是针对半导体光放大器与单模光纤耦合的问题,介绍了渐变折射率平板波导耦合器在光通信系统中的实际应用,由于渐变折射率平板波导耦合器可以有选择性地对半导体光放大器的输出模斑进行有效变换并压缩光束的束散角,因此实现了半导体光放大器到单模光纤的高效率光耦合,其光耦合损耗约为3dB,可基本满足工程使用要求。从光学特性上,渐变折射率平板波导耦合器可以在许多领域替代柱面镜。     

双锥单模光纤渐变器中的模式耦合及功率传输特性

本文对匹配阶跃型单模光纤,利用芯子——包层——外介质双包层波导结构中的场作为本地简正模,应用本地简正模耦合波理论分析了双锥单模光纤渐变器中的模式耦合及基模功率传输特性。我们提出的一种新的耦合模型——耦合相位匹配的分区耦合,可以对各种实验现象给出较好的理论解释。对通信用单模光纤,我们的研究表明,只要在渐变区内芯径的变化半满足:|da/dz|≤1/3·a/(2π)(β_1-β_2),就可忽略由于芯径变化而引起的基模功率损耗。     

太赫兹波导传输中的模式稳定性

太赫兹波在基模波导中传输的欧姆损耗很大,因此在传输太赫兹波时经常采用过模传输的方式来降低传输损耗。但采用过模传输,会引起波导中传输模式的变化,因此如何保证传输中的模式稳定是过模传输中的重要问题。本文采用仿真与实验的方法,对220 GHz圆传输波导中的圆波导半径渐变情况下的模式稳定性进行了研究。结果表明,适当延长渐变波导长度,可以抑制模式耦合,保持单模传输。     

退火质子交换LiNbO3光波导低损耗耦合分析

利用光束传播法分析了退火质子交换铌酸锂光波导模场参数随波导宽度、深度和表面折射率改变的变化情况,指出波导模场受这些参数的综合影响,并讨论了波导结构参数对光纤．波导耦合效率的影响。结果表明,要实现最佳耦合,不同模场直径的光纤对波导结构参数的要求是不同的,这可供相应波导器件的设计和制备参考。     

弱耦合多芯少模光纤的声波导布里渊散射特性

光纤中的声波导布里渊散射(GAWBS)可在较低光功率水平激发，该散射噪声对大容量空分复用光纤通信、数字相干传输及光量子传输影响较大。针对空分复用光纤通信系统，文章对自制弱耦合7芯4模光纤的GAWBS特性进行了理论和实验研究。结果表明，由于不同纤芯与光纤中心的距离差异，弱耦合7芯4模光纤中不同纤芯具有明显的GAWBS分布差异。未来需要在不同的纤芯中采取不同的GAWBS噪声补偿方法以实现高信噪比和低误码率的空分复用传输。     

渐变太赫兹椭圆-双曲线形连接器的理论分析

为了实现太赫兹高斯光束与双曲线形金属波导的高效耦合,设计了一个渐变的椭圆-双曲线形金属波导连接器。此波导连接器的输入端是矩形的,输出端是椭圆-双曲线形的。在渐变的过程中,矩形金属波导的TE01模逐渐转变成双曲线形金属波导的椭圆偏振模。根据WKB近似,这个缓慢渐变的波导消除了反射和散射,TE01模和椭圆偏振模的耦合效率可以高达94%。最终,经过这个渐变连接器,高斯光束和双曲线形金属波导的耦合效率可以提高到69.1%。     

应用等效折射率模型研究光子晶体光纤

应用等效折射率模型研究了光子晶体光纤(PCF)的传播特性.介绍了光子晶体光纤的等效折射率模型.通过求解标量波动方程得到了光子晶体光纤包层基空间填充模的模式折射率,利用阶跃光纤的理论来研究光子晶体光纤的导模特性.应用此模型对不同结构光子晶体光纤包层区的等效折射率与波长的关系进行了讨论.包层区等效折射率与芯子的折射率差随波长的增加而增大,并由此阐述了光子晶体光纤的单模特性.数值分析得到光子晶体光纤的基模的模式折射率,并由此研究了光子晶体光纤的波导色散与结构参量的关系.分析表明,光子晶体光纤的波导色散随空气孔孔距的变化符合Maxwell方程的比例性质.空气孔的相对孔径对波导色散有重要的影响.这些分析表明光子晶体光纤具有可以灵活设计其色散特性的潜在应用前景.     

新一代多模光纤

新一代渐变折射率多模光纤提高了当今高性能网络的传输带宽和距离。文章阐述多模光纤的新特性——差分模延迟(DMD)的规范和测量方法,以及精密控制光纤折射率剖面以改善DMD性能的方法,并介绍OFS公司几种渐变折射率多模光纤的新产品。     

光纤色散效应对脉冲展宽的影响

为了研究光通信系统中光纤色散特性对通信系统传输性能的影响,基于单模光纤和多模光纤的色散特性,采用数值模拟计算的方法,对脉冲展宽、光纤内部的偏振模色散、色度色散、波导色散和模间色散的物理机制进行了分析,分别得到了折射率n=1.516和n=1.458的标准单模光纤经过10km传输距离后色散导致脉冲展宽的结果,比较了传输波长在850nm和1310nm时多模光纤的色散效应,通过对不同光源LD(Δλ=1nm)和LED(Δλ=70nm)的比较,分析了光谱宽度对脉冲展宽的影响。结果表明,纯石英光纤在系统传输波长为1.27μm处群速度色散等于0;折射率渐变多模光纤工作在常见的850nm以及1310nm通信窗口时,其模内色散表现为负色散;色度色散和模间色散引起的脉冲展宽随光纤的数值孔径、材料折射率和光源光谱线宽的增大而增大。     

大芯径光纤基模功率传输特性与折射率分布形式关系的研究

研究和分析了光纤芯区径向折射率分布对大芯径光纤基模的功率传输特性(主要包括最大功率密度和等效模面积这两个参数)的影响。采用一种可适用于多种光纤实际折射率分布的独特数学表达式,研究了折射率分布形状变化时大芯径光纤基模在横截面内功率密度分布与等效模面积的变化,并将结果与阶跃型折射率光纤进行对比。计算结果表明,在传输功率相同、光纤基模与高阶模等效折射率差大于10-4的前提下,折射率在芯区中心有一定凹陷的分布可以有效降低横截面内基模功率密度的最大值,增大基模的等效模面积。这一研究为设计和制作可以传输更大功率的大芯径的无源和有源单模光纤提供了理论基础。     

Optical Fiber Index Taper–Theoretical Analysis and Experiment Demonstration

We discuss theoretical analysis of optical fiber taper with its refractive index gradually changing alone its propagation axis caused by photosensitivity. The numeric results for two different taper shapes in step-index fiber show that the couplings of fundamental mode to the reflected mode and to high-order modes is relatively small provided the change is gradual. The fundamental mode has the capability of excellent transmission that can adjust itself adiabatically to the refractive index change. The experiment demonstrates its effectiveness for optical interconnection with different mode-fields     

Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing

Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing is studied in this paper. The modes propagation properties and coupling properties in cladding index modulated FBG are studied based on the classical coupling mode theory while considering interactions among multiple modes and developed on a three-layer step-index fiber geometry. Evolution of cladding mode resonance spectra of cladding index modulated FBG with the decreasing of fiber diameter is simulated and compared with the core grating. Sensing characteristics of cladding modes resonance of cladding index modulated FBG to the variation of ambient refractive index is detailed and emphasized for cladding modes resonances has a much narrower bandwidth. Results show that cladding index modulated fiber grating has better spectrum for ambient refractive index sensing applications under the same sensitivity with core index modulated fiber grating and may be applied to many field including biochemical sensing and environmental monitoring.     

Fiber Bragg grating coherence spectrum modeling, simulation, andcharacteristics

The fiber Bragg grating (FBG) coherence spectrum, which comes from mode coupling with both core-bounded and cladding-bounded modes, was modeled and simulated using a dynamic mode selecting rule together with the coupling equations. Grating saturation effects were included in the index distribution modeling for its significant influence on the coupling process. The grating spectrum under normal experimental testing conditions was modeled and simulated for the purpose of experimental analysis and device design     

Ultra-broadband optical filter based on chirped long-period fiber grating using leaky mode coupling

An ultra-broadband optical filter was proposed and demonstrated based on leaky mode coupling in a coated chirped long-period fiber grating (CLPFG). The CLPFG was coated with a material whose refractive index (RI) was higher than that of the fiber cladding, enabling the coupling of the core mode to leaky modes, to achieve a desired coupling efficiency. Complex coupled-mode theory was used to investigate the power evolution of the core mode that resulted from the coupling. From this, the conditions in which the core mode power attenuates the most rapidly were identified. In addition, phase matching turning point (PMTP) was used in the design, to overcome the conflict between the range of grating period change and the grating length in the CLPFG. Finally, an optimized CLPFG-based filter with a length of 3.5 cm was obtained, which has a symmetrical attenuation band with an operating bandwidth over 300 nm. Within the operating bandwidth, the flatness is less than 2.5 dB and the transmittance is lower than 0.1%.     

表面膜层参数对长周期光纤光栅特性的影响

采用严格的耦合模理论建立了表面涂覆弱吸收薄膜长周期光纤光栅（LPFG）的理论模型，用微扰法对复数超越特征方程进行了求解。研究发现，对应某一膜厚，存在一折射率范围，有效折射率的变化和谐振波长偏移特别明显；但消光系数对有效折射率和谐振波长的影响则很小。从功率密度分布曲线可看出低阶奇次包层模式与纤芯模式的耦合要强于低阶偶次包层模式与纤芯模式的耦合，消光系数引起的功率损耗较小。薄膜折射率对透射谱的变化（幅度和谐振峰偏移）影响很大；消光系数对谐振峰偏移影响很小，对谐振峰幅度的影响与具体的薄膜折射率有关。结果表明，敏感薄膜折射率和消光系数与传感器的灵敏度高低有直接关系，传感器的结构优化非常必要。     

具有混合形状空气孔的OAM传输光纤设计研究

本文提出了一种新型轨道角动量模(OAM)传输的光子晶体光纤(PCF)结构,该光纤由最中心空气孔、高折射率环形层和外包层构成,其中外包层由一圈椭圆形空气孔和多圈呈周期排列的扇形空气孔共同组成,无需通过额外的掺杂,就可以使中心空气孔和外包层空气孔之间形成一个用来传输OAM光束的等效高折射率环形区。通过对该PCF的传输特性进行仿真分析,发现在1.55μm处,各模式的限制损耗维持在10^-6 dB/m-10^-10 dB/m,在C波段的色散值均维持在310 ps/(nm×km)以下,HE2,1模在1.55μm-1.6μm波段内的色散变化为3.1 ps/(nm×km)。在1.55μm处最大的Δnneff能够达到4.83×10^-3,大的有效折射率差可有效地抑制了模式间的简并,避免HE模和EH模耦合成LP模。验证了该光纤具有大带宽、小而平坦色散、大有效折射率差、低限制损耗和低非线性系数等优良性能。在1.25μm-2.0μm波段内共可以支持34个OAM模式的有效传输,每个模态都可作为独立的信道传递信息,适合用于大量数据的传输,大幅提高了光通信的系统容量和频谱效率。     

长周期光纤光栅谐振波长及耦合系数的研究

运用Matlab7.0软件进行建模,研究发现,当光栅周期确定后,波长改变时高次包层模与导模耦合的谐振波长处于长波长处,而低次包层模式与导模耦合的谐振波长处于短波长处;光纤参数的变化对耦合系数的影响程度不同,耦合系数对纤芯半径的变化更敏感,特别是纤芯半径变小时,奇次耦合系数的最大值所对应的模式次数向相反方向漂移.由此可见,光纤参数的变化对谐振波长和耦合系数会产生一定的影响,而这些影响会进一步引起透射谱损耗峰的漂移和分裂.     

Theoretical analysis of cladding-mode waveguide dispersion and its effects on the spectra of long-period fiber grating

A new method to control the free spectral range (FSR) of a long-period fiber grating (LPFG) is proposed and theoretically analyzed. As the refractive index decreases radially outward in the silica cladding by graded doping of fluorine, waveguide dispersion in the cladding modes was modified to result in the effective indexes change and subsequently the phase-matching conditions for coupling with the core mode in a LPFG. Enlargement of the FSR in a LPFG was theoretically confirmed.     

Proposal for Second-Harmonic Generation Based on Mode Coupling in Coaxial Optical Fiber

We propose the quasi-phase-matching (quasi-PM) conditions for second-harmonic generation, which can be realized through the mode coupling in coaxial multilayer optical fiber. The coupling behavior between core and tube modes can be explained by the propagation of multiple supermodes with the modified propagation constants, which can be used to compensate for the index mismatch between fundamental and second-harmonic modes. Compared to periodically spatial modulation of quadratic nonlinearity, the advantage of these PM schemes is the simple multilayer structure and the available long-distance interaction in the coaxial fiber.