Dispersion error of a beam splitter cube in white-light spectral interferometry

We revealed that the phase function of a thin-film structure measured by a white-light spectral interferometric technique depends on the path length difference adjusted in a Michelson interferometer. This phenomenon is due to a dispersion error of a beam splitter cube, the effective thickness of which varies with the adjusted path length difference. A technique for eliminating the effect in measurement of the phase function is described. In a first step, the Michelson interferometer with same metallic mirrors is used to measure the effective thickness of the beam splitter cube as a function of the path length difference. In a second step, one of the mirrors of the interferometer is replaced by a thin-film structure and its phase function is measured for the same path length differences as those adjusted in the first step. In both steps, the phase is retrieved from the recorded spectral interferograms by using a windowed Fourier transform applied in the wavelength domain.     

Dispersive white-light spectral interferometry including the effect of thin-film for distance measurement

A spectral-domain white-light interferometric technique is used for measuring distances in a Michelson interferometer with a mirror represented by a thin-film structure (TFS) on a substrate. A fibre-optic spectrometer is employed for recording spectral interferograms that include wide wavelength range effects of dispersion in a cube beam splitter and multiple reflection within the TFS. Knowing the effective thickness of the beam splitter, its dispersion and parameters of the TFS and substrate, the positions of the second interferometer mirror are determined precisely by a least-squares fitting of the theoretical spectral interferograms to the recorded ones. We apply the technique to the beam splitter made of BK7 optical glass and to a uniform SiO₂ thin film on a silicon wafer. We compare the results of the processing that include and do not include the effect of the TFS.     

Thickness of SiO2 thin film on silicon wafer measured by dispersive white-light spectral interferometry

We present a white-light spectral interferometric technique for measuring the thickness of SiO₂ thin film on a silicon wafer. The technique utilizes a slightly dispersive Michelson interferometer with a cube beam splitter and a fibre-optic spectrometer to record channelled spectra in two configurations. In the first, a standard configuration with two identical metallic mirrors, the recorded channelled spectrum is fitted to the theoretical one to determine the effective thickness of the beam splitter made of BK7 optical glass. In the second configuration one of the mirrors is replaced by SiO₂ thin film on the silicon wafer and the recorded channelled spectrum is fitted to the theoretical one to determine the thin-film thickness. We consider multiple reflection within the thin-film structure, use the optical constants for all the materials involved in the set-up, and confirm very good agreement between theory and experiment. The technique is applied to four samples with various SiO₂ film thicknesses. PACS 07.60.Ly; 68.55.Jk; 78.20     

近红外消偏振分光膜设计近红外消偏振分光膜设计

大角度倾斜入射时,光学薄膜表现出强烈的偏振效应。入射光S分量和P分量的反射率和透射率曲线,以及反射或透射引起的两个分量的相位变化都不再相同,彼此有一定的偏振分离。通过对产生偏振效应内在原因的分析,采用一个在近红外波段对于45倾斜入射光的S分量和P分量等效导纳非常接近的膜系结构,结合单纯形法优化算法,设计得到一个1 500～1 600 nm范围内反射率都是50％的近红外消偏振分光膜。结果显示,在45入射时,S分量和P分量的反射率曲线偏振分离小,反射和透射引起的相位变化也控制在很小范围。     

超二代微光像增强器多碱光电阴极膜厚测量研究

介绍了多碱光电阴极的光学性能和光谱反射率特性,测量了多碱阴极的光谱反射率曲线.该曲线与普通光学膜层光谱反射率曲线相比,形状较不规则,原因是多碱阴极膜层存在光吸收.光谱反射率曲线上的干涉峰是入射光在玻璃与阴极膜层界面反射和在阴极膜层与真空的界面反射的两束光发生干涉的结果.根据干涉的原理,如果阴极膜层所反射的两束光的光程差为二分之一波长的偶倍数时,光谱反射将出现干涉加强峰;如果阴极膜层所反射的两束光的光程差为二分之一波长的奇倍数时,光谱反射将出现干涉减弱峰.根据超二代像增强器光谱反射干涉峰对应的波长,可以计算出其阴极膜层的厚度约为191 nm,比二代像增强器阴极膜层的厚度增加了38％.多碱阴极膜层厚度是影响多碱阴极灵敏度的一个关键参量,仅仅靠人眼观察阴极膜层颜色的方法不准确.实践证明,利用光谱反射的方法来计算阴极膜层厚度的方法简单有效.如果在多碱阴极的制作过程中进行光谱反射率的监控,那么将可以精确控制阴极膜层的厚度,对多碱阴极的研究将会更加深入,多碱阴极的灵敏度也将会得到进一步的提升.     

Design technique for all-dielectric non-polarizing beam splitter plate

There are many situations when, for the proper working, an opto-electronic device requiring optical components does not change the polarization state of light after a reflection, splitting or filtering. In this paper, a design for a non-polarizing beam splitter plate is proposed. Based on certain optical properties of homogeneous dielectric materials we will establish a reliable thin film package formula, excellent for the start of optimization to obtain a 20-nm bandwidth non-polarizing beam splitter.     

Quantitative analysis of surface plasmon interaction with silver nanoparticles

The present insight into plasmon effects on the nanoscale seems sufficiently advanced to allow the development of surface-plasmon-polariton- (SPP-) based optical devices. Therefore quantitative information describing SPP phenomena is required. We investigate a SPP beam splitter constituted by silver nanoparticles on a silver thin film, fabricated by electron-beam lithography. We acquire quantitative information on the beam splitter performance by monitoring SPP leakage radiation, yielding SPP reflection, transmission, and scattering efficiencies.     

A Fiber Optic Wavelength Modulation Sensor Based on Tantalum Pentoxide Coatings for Absolute Temperature Measurements

A fiber optic low-coherence sensor based on the spectral shift of tantalum pentoxide thin films for absolute temperature sensing up to 650°C is described. A tantalum pentoxide single layer was deposited directly onto the cleaved end-face of a single mode optical fibre and was illuminated with an super luminescence diode (SLD) source through a directional coupler. Interference fringes of the film on reflection were obtained within the optical bandwidth of the SLD using an optical spectrum analyser. The spectral shift versus temperature rise showed no turning points and the output was unambiguous, linear, monotonic and gave about 0.016 nm wavelength shift in the spectrum per°C. A semi-empirical calibration procedure based on the refractive index (n) and thickness (l) of the tantalum pentoxide film for absolute thermometric measurements is described.     

Spectral diffuse reflectance measurements of gadolinia, silica thin film systems using synchrotron radiation

Diffuse reflectance or optical scattering in thin films and multilayers can pose serious limiting factors to their desired or ultimate performances. Besides, such studies provide valuable information related to the buried microstructures and interfaces. Synchrotron radiation is the most appropriate source to record wavelength dependent polarized light scattering in thin films and multilayers. In the present experiment several gadolinia, silica thin films and multilayers were studied for their light scattering using the white light synchrotron beam. Various thin film layer geometries were selected to probe the results due to different types and combinations of interfaces. Due to phase coherent delay in certain optical non-wedged component used in the experimental setup very interesting spectral interference were noticed as the modulations in the diffuse reflectance signal. Appropriate modeling approach utilizing Gaussian function de-convolution technique is used to compute the pulse delay between the back reflected and forward propagating scattering signals that lead to such spectral interference. Alternatively inverse fast Fourier transform (IFFT) and analytical techniques were adopted to determine the group delays. The diffuse reflectance spectra were finally compared with their specular counter part and a shift both in the wavelength and phase were noticed. This may be explained on the basis of thin film roughness factors, different polarizations and incident geometries used in the measurements. However, all most all the spectral scattering signals exhibited the features resembling their specular reflection characteristic.     

Spectral interferometric technique to measure the relative phase change on reflection from a thin-film structure

A two-step white-light spectral interferometric technique to measure the relative phase change on reflection from a thin-film structure is presented. The technique is based on recording of the channeled spectra at the output of a Michelson interferometer and their processing by using a windowed Fourier transform to retrieve the phase functions. In the first step, the phase difference between the beams of the interferometer with a thin-film structure is retrieved. In the second step, the structure is replaced by a reference sample of known phase change on reflection and the corresponding phase difference is retrieved. From the two phase differences, the relative phase change on reflection from the thin-film structure is obtained. The feasibility of the simple method is confirmed in processing the experimental data for a SiO₂ thin film on a Si wafer of known optical constants. Four samples of the thin film are used and their thicknesses are determined. The thicknesses obtained are compared with those resulting from reflectometric measurements, and good agreement is confirmed.     

空间调制傅里叶变换光谱仪分束器色散特性研究

在基于多级微反射镜的傅里叶变换光谱仪中,由于分束器的色散特性, 不同波长的干涉图具有不同的横向偏移量,其在横向空间会发生混叠. 同时,不同波长的干涉图还具有不同的光程差偏移量,使其在纵向空间也会发生混叠. 根据几何光学原理,干涉图的横向混叠会减小干涉图元的有效面积. 计算表明,本研究中干涉图元的横向混叠面积为总面积的3.4%, 通过数据处理扣除混叠部分可以消除横向混叠的影响. 干涉图的纵向混叠会产生一个附加的相位延迟,削弱谱线的强度. 经分析,附加相位延迟与分束器和补偿板之间的厚度差成正比. 本文给出了发生对比度反转时分束器与补偿板厚度差的极限值, 并利用解线性方程组的方法对纵向混叠的干涉图进行了校正, 反演出的光谱与理想光谱的标准偏差仅为1.76×10^-14.     

Slightly dispersive white-light spectral interferometry to measure distances and displacements

A new spectral-domain interferometric technique of measuring distances and displacements is realized when the effect of low dispersion in a Michelson interferometer, which comprises two coated plates of a beam splitter and a compensator, is known and the spectral interference fringes are resolved over a wide wavelength range. First, processing the recorded spectral interferograms by an adequate method, the unmodulated spectrum, the spectral fringe visibility function and the unwrapped phase function are obtained. Then, knowing the dispersion relation for the fused-silica plates, the ambiguity of the unwrapped phase function is removed and the thickness of fused silica and the nonlinear phase function due to the effect of the coatings are determined by using a new procedure. It is based on the linear dependence of the overall optical path difference between interferometer beams on the refractive index of fused silica. Once the thickness and the nonlinear phase function are known, the positions of the interferometer mirror are determined precisely by a least-squares fitting of the theoretical spectral interferograms to the recorded ones.     

White-light spectral interferometric technique to measure a nonlinear phase function of a thin-film structure

We present a new two-step white-light spectral interferometric technique to measure a nonlinear phase function of a thin-film structure. The technique is based on recording of channeled spectra at the output of a Michelson interferometer and their processing by using a windowed Fourier transform to retrieve the phase functions. First, the phase function including the effect of a thin-film structure is retrieved. Second, the structure is replaced by a reference sample of known phase change on reflection and the corresponding phase function is retrieved. From the two functions, the nonlinear phase function of the thin-film structure is obtained. The feasibility of this simple method is confirmed in processing the experimental data for a SiO₂ thin film on a Si wafer of known optical constants. Four samples of the thin film are used and their thicknesses are determined. The thicknesses obtained are compared with those resulting from reflectometric measurements, and a good agreement is confirmed.     

Optical bistability based on surface plasmon coupled between a noble metal film and a metal grating involving Kerr materials

A phenomenon about optical bistability is successfully investigated in a layered structure consisting of a silver film with Kerr medium and a silver grating sandwiched between semi-infinite linear dielectrics.This type of structure can lead to the optical bistability phenomena occurring in reflection and transmission.There exists an optimal thickness of the metal grating that can cut off the effect of the near-field enhancement and may have the lowest effect on conversion from surface plasmon to light.This structure can realize the functions of the beam splitter and the polarization splitter and will be essential for future classical and quantum information processing.     

Dependence of wavDependence of wavefront errors on nonuniformity of thin films

In contrast to uncoated substrate, a nonlinear relationship of phase shift with the thicknesses of the thin film makes the calculation of wavefront aberration complicated. A program is compiled to calculate the wavefront aberration of multilayer thin film produced by thickness nonuniformity. The physical thickness and the optical phase change on reflection are considered. As an example, the wavefront aberration of the all-dielectric mirror is presented in ArF excimer lithography system with a typical thickness distribution. In addition, the wavefront errors of the thin film at wavelengths of 193 and 633 nm are compared in the one-piece and two-piece arrangements. Results show that the phase shift upon reflection of the thin film produced by thickness nonuniformity is very sensitive to the incident angle, wavelength, and polarization.     

Spectral interferometry and reflectometry used to measure thin films

A new method for a precise measurement of the oscillatory part of phase change on reflection (interferometric phase) from a thin-film structure is presented. The method, which is based on phase retrieval from the spectral interferograms recorded at the output of a slightly dispersive Michelson interferometer, is combined with reflectometry. The interferometric phase of the thin-film structure is measured precisely using a reference sample of known phase change on reflection. The spectral reflectance of the thin-film structure is also measured in the interferometer. The feasibility of the method is confirmed in processing the experimental data for SiO₂ thin film on a silicon wafer of known optical constants. Four samples of the thin film are used and their thicknesses are determined. We confirm very good agreement between the thicknesses obtained from the interferometric phase and reflectance measurements. PACS  07.60.Ly; 68.55.Jk; 78.20.Bh     

新型小尺寸窄带偏振分光器在光纤通信中的应用

在光纤通信中,为了在不改变调制波长范围的基础上仍能实现更多数据通道的波分复用,设计了一种新型的小尺寸窄带偏振分光器,用于对现有的数据通信网络进行扩容以及提高光信号的信噪比。在该分光器上蒸镀了两种新设计的膜系,一层是窄带滤光膜,另一层是偏振分光膜。采用TFCalc软件仿真,设计结果中窄带滤光膜的带宽约为0.4 nm,偏振分光膜在1 530～1 560 nm范围内对p光的通透性能优于99.8%。基于以上膜系设计在BK7光学玻璃上实际制备两组膜系,实验采用Agilent 8164-A型光波测量系统对经过膜后的光进行光谱分析。结果显示,窄带滤光膜的实际带宽优于0.4 nm,增益平坦度小于-0.05 dB,相比现有常见的0.8 nm滤光膜具有更窄的带宽,可以实现在调制波长范围不变的条件下增大波分复用的数据通道总量。偏振滤光膜的实际p光透光率为99.6%,相比仿真值略低,但仍优于设计要求,相比传统分光器的光信号强度保留效果更好,具有更高的信噪比。综上所述,该分光器具有更好的应用价值和实用意义。     

基于金属狭缝阵列的各向异性偏振分束器

在金属-电介质结构的基础上提出了一种基于金属狭缝阵列的各向异性偏振分束器,并采用有限元法研究了横磁(TM)和横电(TE)偏振光入射后结构所表现出的负反射和镜面反射等特性.计算结果表明,当偏振光的入射角设定在20?—70?时,入射的TM光发生强烈的负反射,而TE光的负反射很弱,并随着波长的增加而急剧下降.分析可得偏振分束光栅的理想负反射点和反射面的完美对称响应效果.通过仿真得到了理想负反射点的取值范围.结合严格耦合波法软件,计算不同偏振光入射时负反射和镜面反射条件下的反射率,其消光比高达10~6.     

傅里叶变换光谱仪多次反射杂光对调制度的影响分析

为满足遥感探测领域对高光谱干涉探测仪的高信噪比和高光谱分辨率的要求,针对我国目前在研高光谱分辨率傅里叶变换光谱仪,详细分析了采用分束器及补偿器分光方案的迈克尔逊干涉仪中,增透面上多次反射杂光对光谱仪调制度的影响。分析表明增透面透过率越低,光谱仪调制度越低。理论分析发现,引入分束器和补偿器的楔角和倾斜角可以实现正常光斑与多次反射光斑分离,从而减弱甚至消除多次反射杂光的影响。通过理论计算给出了二次反射杂光与正常光路夹角与楔角和倾斜角的关系式,并依据光斑分离要求确定了楔角和倾斜角优化方法。由于干涉仪结构参数的相互关联特性,在优化楔角和倾斜角后,需要对入射角进行调整,使得反射光路与透射光路关于分束面对称,以消除其对光谱分辨率的影响;需要对补偿器厚度进行调整匹配,消除由于角度调整引入的正常光路光程差改变导致的调制度下降。按此步骤进行设计优化,在保持原有高精度设计指标的前提下,可消除干涉仪多次反射杂光的影响,该干涉仪优化设计步骤及结构参数调整方法适用于采用分束器补偿器分光方案的傅里叶变换光谱仪。     

傅里叶红外光谱仪分束器自动装调系统设计

为了提高傅里叶红外光谱仪干涉系统的精度,设计了一种基于一体化分束器的自动装调系统。使用光学胶水将棱镜与分束器固定在一起组成一体化分束器,在一体化分束器上设置三点调整机构,O点位置进行固定作为参考点,在X点、Y点位置装入压电陶瓷,利用压电陶瓷驱动分束器进行微小位移的调整。在动镜匀速运动的情况下,利用四象限检测器检测三路携带相位差信息的信号,对信号进行放大滤波和数字化处理后,采用DSP的ECAP模块捕捉三路相位差信号,并利用增量式PID算法根据相位差来对三个点位的调整幅度进行实时控制,使用DSP的AD模块采集三路信号,并利用USB2.0与上位机进行通信,在上位机上实时显示三路信号图形及相位差信息。利用光学胶水的紫外固化特性,在支架上安装紫外照射灯,调整过程中不断增加照射强度使光学胶水进行深度固化,从而制作出符合傅里叶变换红外光谱仪应用需求的一体化分束器。实验结果表明,所设计的基于一体化分束器的自动装调系统具有结构简单,调整精度高等优点,其能够满足常规红外分束器装调需求。