取样光纤布拉格光栅特性的研究

用传输矩阵法从理论上计算了取样光纤布拉格光栅的反射谱特性。这种方法将光栅视为多层均匀薄膜的叠加,利用每一层的传输矩阵相乘获得了光栅的反射谱特性。研究表明,随着光栅长度的增加和采样率、折射率调制深度的减少,反射峰的均匀性得到了改善,旁瓣的反射率变小,带宽明显变窄,而反射峰间隔保持不变。反射峰的间隔由光栅周期决定,与采样率无关,而某些文献则要求采样率小于10%。这与频谱分析所得结论相吻合。     

光纤光栅应用于梳状滤波器和光分插复用器(OADM)的研究

研究了光纤光栅在梳状滤波器和光分插复用器(OADM)中的应用特性。基于传输矩阵法对Sinc取样光栅反射谱进行分析,分析了取样光栅长度、折射率调制深度和取样周期等光栅参数对Sinc取样光栅反射谱的影响规律。根据各参数对Sinc取样光栅反射谱的影响规律构造了4种反射率高、反射峰间隔均匀及谱宽稳定的Sinc取样光栅梳状滤波器,为梳状滤波器的设计与制作提供了新思路、新技术。用OptiSystem软件对光纤Bragg光栅型光分插复用器进行了仿真研究。研究表明,经过光纤光栅光分插复用器后,下载和除了下载的光谱峰值基本为-4dBm,下载的功率也基本是均衡的。     

保偏微结构光纤光栅 F-P腔折射率传感特性分析

针对提高光纤光栅折射率传感器抗干扰能力以及增加反射率的需求,本文提出了一种基于Fabry-Perot腔的保偏微结构光纤(PM-MOF)布拉格光栅折射率传感器.根据传榆矩阵法和有限元方法,分析了微结构光纤光栅F-P腔中被测物折射率与F-P腔反射谱中两个偏振模谐振波长差的关系,在此基础上讨论了中心孔直径、F-P腔长度等参数对传输特性的影响.研究结果表明,随着空气孔中填充物折射率的增加,保偏微结构光纤光栅F-P腔的两个偏振态的谐振波长差将逐渐减小;F-P腔的干涉作用使反射率较单个光栅有很大提高,便于长距离传输和实时解调;两个偏振模对外界干扰具有相似的响应,因此该传感器具有更强的抗干扰能力.本文研究结果为保偏微结构光纤光栅在折射率传感器及其生物传感器方面的应用提供了理论依据.     

棱镜耦合光子晶体表面波及其传感特性研究

构造一种棱镜耦合一维光子晶体结构,利用传输矩阵方法对该结构中光的传输特性进行研究.结果表明:光在这种棱镜耦合一维光子晶体结构中传播时,会在光子晶体表面产生非辐射传播模,即表面波,而且在反射谱中表面波所在的角度位置、反射率的大小以及反射峰的半峰全宽受到一维光子晶体的介质折射率和介质厚度的调制.此外,表面波对应的角度位置对...     

啁啾莫尔光纤光栅的多层薄膜分析新方法

将啁啾莫尔光纤光栅看成一多层薄膜体系,每层薄膜用界面传输矩阵和膜层传输矩阵表示,将每层的传输矩阵相乘后得到光栅的传输谱。数值计算结果表明,啁啾莫尔光纤光栅的传输谱的通道数取决于啁啾莫尔光纤光栅的长度、两线性啁啾光栅的中心波长的差值;通道间隔和带宽取决于光栅的啁啾系数和折射率调制幅度。该模型的精度满足设计要求。     

重叠体光栅相干组束的耦合波分析

目前基于重叠体光栅的相干组束系统还没有完整的理论模型和参数分析,针对这个问题,本文采用耦合波理论,建立了比较完整的基于重叠体光栅的双光束组束模型,给出了各光束在重叠体光栅内传播的解析解.模型表明,基于重叠体光栅的双光束组束系统其输出光强受两入射光的振幅和位相、体光栅间的相移和折射率调制振幅、光栅厚度,失谐量等多个光学参数的影响.通过数值计算,详细分析了各光学参数对组束结果的影响.最后总结了设计高效重叠体光栅相干组束系统时必须遵循的条件,如必须对光栅厚度进行优化、各光栅的折射率调制振幅必须相同,保证失谐量足够小等.     

载氢与光纤布喇格光栅

系统地研究了载氢条件对掺锗石英光纤的紫外光敏特性以及光纤光栅的影响．结果表明:光纤中氢气含量的大小决定了光纤光栅的折射率调制(即光纤光敏性的大小),也决定了光纤光栅的反射率;随着载氢压力的增大,光纤的光致折射率改变(即紫外光敏性)呈正比例增大,两者之间的关系为△n=1．34×10-5+4．66×10-5P;退火后的光纤光栅的反射率与载氢压力的关系为R3=2．76881+0．83537 P．这一结果有助于控制载氢压力获得发射率可调的光纤光栅,解决光纤光栅写入随机性大的问题;掺锗石英光纤的紫外光敏性的大小随着载氢时间的延长,呈指数增长,最后达到饱和;光纤光栅的反射率与载氢时间也呈现同样的规律,制作光纤光栅的最佳载氢时间为14 d(常温)．     

啁啾光子晶体光纤光栅反射谱和时延特性研究

用有限元法和传输矩阵法,对基于包层空气孔为柚子型结构的光子晶体光纤啁啾光栅的反射谱进行了理论分析。通过对均匀光栅仿真结果与实验结果的对比,可知此方法是可行的。研究表明光子晶体光纤的啁啾光栅有多个明显的反射峰,并且分析了啁啾系数、折变量对各谐振峰带宽和反射率等的影响。计算结果显示,随着啁啾系数和折变量的增大,光栅反射谱呈规律变化,当啁啾系数增大到一定程度时几个反射峰会连到一起,形成一个大的反射带宽。同时对光栅基模反射谱对应的时延特性进行了研究,可知其同样具有比较平滑的时延曲线,可以用于色散补偿。     

基于狭缝整形的低损耗飞秒光纤光栅制备

为了解决飞秒激光逐点法制备的光纤布拉格光栅（Fiber Bragg Gratings, FBG）损耗较高的问题,利用逐面法完成飞秒光纤光栅制备的实验研究。运用高斯光束传播的基本理论,通过狭缝整形技术对聚焦的飞秒激光能量分布进行等高宽整形,突破在光纤横截面内诱导产生圆形折射率调制的难点,最终制备得到低损耗的飞秒光纤光栅。开展不同狭缝宽度制备FBG的光谱特性对比实验,结果表明：利用光斑直径为5.0 mm的飞秒激光光束刻写FBG时,采用宽度为1.7 mm的狭缝制备得到的FBG插入损耗降低至0.15 dB,短波损耗降低至0.5 dB,验证了基于狭缝整形的低损耗飞秒光纤光栅制备方法的有效性。针对狭缝法制备的FBG反射率分散问题,提出控制折射率匹配液填充量以及调整飞秒激光能量的方法,并优化光束聚焦流程,成功降低FBG反射率的分散度。本研究对推动飞秒光纤光栅在大容量、高链路损耗等环境中的实际应用具有重要意义。     

光纤光棚温度调谐特性分析与实验

提出了均匀调制光纤光栅的热调谐数学模型,确定了储热与布拉格波长之间的数学关系,分析光纤光栅热敏度,计算出热调谐的响应时间.设计和建立了一套光纤光栅温度调谐实验装置,利用光谱分析仪测试出光纤光栅反射波长漂移量受温度变化的关系曲线.测试了0-100℃的温度变化过程中光纤光栅反射波中心波长调谐量,测得光纤光栅反射中心波长λB为1546.8～1548.8nm,每摄氏度光纤光栅反射光波的中心波长平均漂移量为0.0210nm/℃.讨论了波长调谐量与温度的变化方向的关系和调谐响应速度等问题.实验结果表明光纤光栅反射波长漂移量与温度具有比较好的线性关系,且与温度的变化方向无关.     

Proposal and analysis of digital concatenated gratings

A digital concatenated grating composed of five subgratings is proposed. Essential formulas on Bragg periods of subgratings and an analytical expression for the reflection-spectrum envelope are presented, which show how to use reflection-spectrum envelopes to construct a digital concatenated grating with uniform reflection peaks. For a perfect design, the spectral center separation of adjacent Bragg gratings will be chosen as the integral times of the reflectivity peaks' spacing. Gaussian apodization on the whole profile grating not only eliminates the sidelobes of each reflection peak but also provides a better flat reflection-spectrum envelope than that of the digital concatenated Gaussian-apodized grating.     

Secondary-phase-modulation method for open-loop fiber-optic gyroscopes

A proposed method of secondary phase modulation for open-loop fiber-optic gyroscopes is examined in general terms. To detect the rotation rate of a system through a beat-frequency channel, we employ linearly combined signals with different frequencies for the optical phase modulation. We find that the proper combinations of the modulation frequencies can optimize the sensitivity of gyroscopes. With this method we can employ a high-frequency band for optical phase modulations while keeping relative a lower-frequency band of the detection channel. The theoretically derived result is experimentally confirmed by using a lithium-niobate (LiNbO(3)) optical phase modulator. We also discuss the combination setup with an optical integrated-circuit device and digital signal processing.     

Errors in film refractive-index determinations from interference fringes

An analysis of interference fringes (channel spectra) in transmission or reflection spectroscopy of optical films can provide an accurate method for determining the refractive index or thickness of the film. If the index is changing and the common equation n = 1/(2tΔv) determined from the positions of two adjacent maxima in the interference pattern is employed, the resulting refractive index does not lie between the correct values at the frequencies of the two maxima as intuitively expected but well outside this range. This error can be large and is not restricted to the previously discussed case of an index with a linear dependence on wavelength.     

Interference Effects in Time-resolved Reflectivity Measurements in Thin Semiconductor Films

The study of non-equilibrium charge carriers in semiconductor films by time-resolved reflectivity measurements is greatly simplified if the change in reflectivity change varies linearly with the photoinduced change in refractive index. In the present work it is shown that interference effects in the transparent layer influence this dependence significantly. The sample reflectivity can either increase or decrease, depending on the film thickness. If the reciprocal absorption coefficient of the semiconductor is smaller than the film thickness, a refractive index profile is created inside the layer. The reflection at this profile diminishes the influence of the beams reflected at the semiconductor-substrate interface. The changes in reflectivity then vary linearly with the changes in refractive index, independently of the film thickness. Numerical calculations were performed to help interpret time-resolved reflectivity measurements on amorphous silicon films.     

Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite

The properties of highly oriented pyrolytic graphite (HOPG) as a substrate for external reflection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physisorbed hydrocarbon multilayers, and chemisorbed monolayers of p-substituted aryl radicals on HOPG were used as samples, and the experimental spectra were compared and complemented with the results of spectral simulations. From reflectivity measurements of clean HOPG surfaces with polarized light as a function of the light incidence angle and the frequency, the anisotropic optical constants n (refractive index) and k (absorption index) were determined for in-plane and out-of-plane directions with respect to the graphite basal plane. These constants express the semimetallic properties of HOPG, indicated by an intermediate reflectivity between a typical metal and a dielectric substrate and by asymmetric, distorted peak shapes in adsorbate film spectra, which represent a transition state between symmetrical, positive absorptions on metals and inverted, negative peaks on dielectric substrates. Regarding spectral sensitivity and surface selection rules, HOPG behaves much like a metal and is therefore an equally suitable substrate for external reflection infrared (IR) measurements.     

Sculpted-multilayer optical effects in two species of Papilio butterfly

The wing-scale microstructures associated with two species of Papilio butterfly are described and characterized. Despite close similarities in their structures, they do not exhibit analogous optical effects. With Papilio palinurus, deep modulations in its multilayering create bicolor reflectivity with strong polarization effects, and this leads to additive color mixing in certain visual systems. In contrast to this, Papilio ulysses features shallow multilayer modulation that produces monocolor reflectivity without significant polarization effects.     

Effects of fibre grating parameters on the spectral characteristics of cascaded long- and short-period gratings

Using coupled-mode theory combined with the transfer matrix method, this paper presents the impact of fibre grating parameters(grating period, grating length and average index modulation) and the separation between long-period fibre grating and fibre Bragg grating (FBG) on the reflection spectra of cascaded long-and short-period grating (CLBG). Two grating periods and two grating lengths have only impact on the two reflection peaks. The two resonance wavelengths shift towards the longer wavelength when the period of FBG decreases. Further, we study the sensing characteristics of coated CLBG. By analysing the relationship between the refractive index sensitivity and the thin film parameters (film refractive index and film thickness), the optimized parameters for the coated CLBG used as film sensor were obtained. Data simulation shows that the resolution of the refractive index of this coated CLBG film sensor is predicted to be 10^?6. This implies that the coated CLBG has an important application value in sensing and measurement field.     

Monitoring the deposition of an interference film by differential reflection of light

The change in the reflection (differential reflection) of light from an interference film as a result of the deposition of an ultrathin layer on it is investigated. Formulas describing the differential reflection around the reflectivity minima and maxima of the film are obtained by a perturbation method. It is shown that these formulas and the corresponding differential measurements can be used for an easy and unambiguous determination of the thickness and refractive index not only of ultrathin surface layers but also of the interference films themselves. The proposed method is especially convenient for monitoring the deposition of thin-film structures. Pis’ma Zh. Tekh. Fiz. 24, 78–86 (October 26, 1998)     

Sol–gel derived photonic bandgap coatings for solar control

Sol–gel derived photonic bandgap films have been investigated as possible multilayer coatings for solar control glass applications. Multilayer Bragg mirrors, in particular, have been modelled by the Transfer Matrix method, designed to have either near-UV or near-IR reflectivity, but visible transparency, based on alternating aluminosilicate glass/titania quarter-wave stacks. Such composite multilayer structures have been deposited by sol–gel processing on selected glasses and other types of substrates and their optical characteristics have been measured by optical absorption and reflection spectroscopies, as well as spectroscopic ellipsometry to determine the single layer refractive index and thickness. The UV–visible-IR absorption and reflection characteristics of these multilayer coatings revealed solar control properties, due to the presence of peaks near ∼350–400 nm and ∼900–1000 nm, with reflectivities of the order of 70%, which appear promising for solar control application.