Spectral-domain tandem interferometry to measure the group dispersion of optical samples

A spectral-domain technique based on tandem interferometry is used for measuring the group dispersion of optical samples over a wide wavelength range. The technique utilizes a tandem configuration of a Michelson interferometer and an unbalanced Mach–Zehnder interferometer with a sample inserted into its test arm. First, the theoretical background of the technique is presented and then experiments with individual interferometers and their tandem configuration are specified. In all the experiments the spectral signals are recorded to measure the equalization wavelength as a function of the path length difference, or equivalently the group dispersion. We measure the group refractive index as a function of wavelength for a glass sample of known thickness and for a quartz crystal as well.     

Measurement of refractive index variation by differential surface plasmon resonance technique

A differential detection technique combined with a surface plasmon resonance technique is proposed for refractive index variation measurement. The experimental results demonstrate that our system can attain a resolution of 1.2 × 10^?5 RIU (refractive index unit). In addition, a method for two-dimensional detection of the refractive index variation is demonstrated. The system is free from the influence of optical source fluctuation, and can improve the measurement sensitivity.     

Design of novel DS-OCDMA encoders/decoders using phase sampled long period grating

A novel long period grating structure with phase sampled technique is proposed to act as encoders/decoders in the direct sequence optical code division multiple access (DS-OCDMA) system. The structure with address code of 31 chips is designed and the relation between the sample period, the input pulse width and the refractive index difference is given. The encoded and decoded results are demonstrated to verify the feasibility of utilizing the coupling ratio to the cladding mode. The whole structure is expected to move on the photonic crystal fiber, for it can support the large refractive index difference between the core mode and cladding mode, as well as minimize the wavelength shift caused by the temperature change.     

Numerical correction of coherence gate in full-field swept-source interference microscopy

A big problem in low-coherence interference microscopy is the degradation of the coherence signal caused by shift of the angular and temporal spectrum gates. It limits the depth of field in confocal optical coherence microscopy and degrades images of sample inner structure in most interference microscopy techniques. To overcome this problem we propose numerical correction of the coherence gate in application to full-field swept-source interference microscopy. The proposed technique allows three-dimensional sample imaging without mechanical movement of the microscope components and is also capable of determining separately the geometrical thickness and the refractive index of the sample layers, when the sample contains a transversal pattern. The applicability of the proposed technique is verified with numerical simulation.     

Group refractive index measurement by fringes of equal chromatic order

Fringes of equal chromatic order in transmission across a thin liquid or a thin solid sample inside a wedge interferometer, followed with a grating spectrograph, are produced. A single-shot interferogram of the air and sample regions is recorded. Locations of fringes maxima in the air region are fitted in a numerical procedure based on Cauchy's dispersion function. Then it is used for measuring the interferometric gap thickness. The order of interference in the sample region is represented by a third-order polynomial in the wavenumber for deducing the sample group refractive index. An error analysis of the measured group refractive index is given. The method is applied for measuring the group refractive index of water and mica samples across the visible spectrum. The method measures both the sample thickness and its group refractive index. It is static with no moving parts and suitable for thin liquid or solid samples without immersion liquids.     

基于全相位滤波技术的光纤表面等离子体共振传感解调算法

基于生物样品检测对折射率传感的迫切需求,构建一种全光纤表面等离子体共振(surface plasmon resonance,SPR)系统,并针对其设计了基于全相位滤波技术的SPR特征波长传感解调算法.基于系统仿真,理论计算了光纤SPR传感器的折射率传感灵敏度.采用全相位滤波技术提取光纤SPR传感器透射光谱的特征波长,理论推导了全相位滤波器的解析表达式.实验结果表明,使用本算法的光纤SPR传感器折射率传感灵敏度为1640.4 nm/RIU,折射率检测的分辨率是7.36×10~(-4)RIU,与传统方法相比,有效提高了系统的检测精度和抗光源扰动性能,降低了实验成本.     

Measurement of Refractive Index for High Reflectance Materials with Terahertz Time Domain Reflection Spectroscopy

A method to measure the refractive index for high reflectance materials in the terahertz range with terahertz time domain reflection spectroscopy is proposed. In this method, the THz waveforms reflected by a silicon wafer and high reflectance sample are measured respectively. The refractive index of the silicon wafer, measured with the THz time domain transmission spectroscopy, is used as a reference in the THz time domain reflective spectroscopy. Therefore, the complex refractive index of the sample can be obtained by resorting to the known reflective index of the silicon and the Fresnel law. To improve the accuracy of the phase shift, the Kramers-Kronig transform is adopted. This method is also verified by the index of the silicon in THz reflection spectroscopy. The bulk metal plates have been taken as the sample, and the experimentally obtained metallic refractive indexes are compared with the simple Drude model.     

A numerical method for determining refractive index of a glass sample from its implicit transcendental function

The refractive index of a glass sample was determined from an implicit function of its optical path within the sample arm of a Michelson interferometer.On rotation of the sample from normal incidence, the light beam suffers increasing refraction, causing the optical path for air to decrease whilst that for the glass sample increases. This is observed as a shift in the number of fringes, which were captured and counted in real time on a computer, as rotation proceeded. The angle of rotation and the fringe shift were entered into an implicit function of optical path versus refractive index written to an Excel worksheet. A refractive index matching the wavelength of the He-Ne laser light source was read-off to three decimal places. A new source of uncertainty has been identified, misalignment on a micrometer scale of the laser normal to the sample surface. Whilst driving the sample to find normal incidence a finite angular region of insensitivity (dead space) occurs due to misalignment of the laser normal to the sample surface by a few micrometers. A linearization technique was employed to compensate for this offset and hence reduce its uncertainty contribution.     

Phase-Doppler Anemometry with the Dual Burst Technique for measurement of refractive index and absorption coefficient simultaneously with size and velocity

The principle of the dual burst technique (DBT) based on phase-Doppler anemometry (PDA) is proposed for simultaneous particle refractive index, size and velocity measurements. This technique used the trajectory effects in PDA systems to separate the two contributions of the different scattering processes. In the case of forward scattering and refracting particles, it is shown that from the phase of the reflected contribution, the particle diameter can be deduced, whereas from the refracted contribution the particle refractive index and velocity can be obtained. Furthermore, the intensity ratio of these two scattering processes can be used for absorption measurements. Simulations based on generalized Lorenz-Mie theory and experimental tests using monodispersed droplets of different refractive indices and absorption coefficients have validated this technique.     

A proposed new experimental setup for determination of refractive indices of liquids by mode line measurements

A new experimental method is proposed for measuring the refractive indices of liquids based on mode line measurements. The experimental setup consists of a dielectric substrate in which a rectangular hollow is formed containing the concerned liquid. This is covered by a trapezium-shaped prism. The liquid, confined between both basin walls and the prism, forms a wavegude. Thus, the refractive index of the liquid can be determined by measuring the propagating modes in the structure. Using this technique, the refractive indices of liquids can be measured to an accuracy of the order of 1×10^-4.     

间插取样光纤光栅多通道色散补偿器

对间插取样光纤光栅（ISFBG）进行了理论分析和仿真,给出了ISFBG的折射率表达式,提出了一种在较低的折射率调制深度条件下实现通道间平坦度较好的色散补偿器的制作方法.实验制作了由两个取样光纤光栅间插而成的10通道色散补偿器,其时延抖动大都小于20 ps,通道间平坦度小于1 dB.     

Automated interferometric technique for express analysis of the refractive indices in isotropic and anisotropic optical materials

This paper deals with the technique for the refractive index measurements based on the interferometry of a rotated parallel plate (IRPP). The device consists of the Michelson interferometer, the sample rotation system and the optoelectronic registration system. A refractive index of parallel plates is determined by their rotation through measuring simultaneously a shift of interference fringes. Although the IRPP technique is known from long ago [Shumate MS. Appl Opt 1966;5:327] several considerable improvements have been done in order to improve the accuracy of the method. The measuring process is completely automated. The method has been tested on the model crystals of the lithium niobate giving the magnitudes for ordinary and extraordinary refractive indices as n₀=2.2865±0.0007 and n_e=2.2034±0.0007. A considerable increase of accuracy is reached in our case by an automation of the measuring procedure, development of a new software as well as implementing the interferometric method for a precise determination of a sample zero position. The automated refractometer is offered for use in research laboratories and industry.     

Measurement of refractive index variation of liquids by surface plasmon resonance and wavelength-modulated heterodyne interferometry

In this study an alternative method based on surface plasmon resonance is proposed for in-situ monitoring of variation in the refractive index of a test sample. A wavelength-modulated light source and an unequal-path-length optical configuration heterodyne interferometer are used to detect the phase difference change, which can then be used to estimate the change in the refractive index of a test sample. The experimental results demonstrate a phase stability of 0.02°. The resolution power of the refractive index is 1.5 × 10^− 6 RIU. This method has several advantages over previously used methods such as simple optical setup, easier operation in real time, and low cost.     

Z-scan technique as a tool for the measurement of blood glucose

The measurement of the blood glucose level utilizing the optical nonlinearity observed by a single-optical beam Z-scan technique is proposed. The glucose level is calculated by measuring the nonlinear refractive index (n ₂) of the sample. The results are compared with the measurement made by the conventional colorimetric method and are found to be in good agreement.     

Critical Angle Refractometry for Simultaneous Measurement of Particles in Flow: Size and Relative Refractive Index

The principle of the optical technique critical angle refractometry, used to determine the size and refractive index of spherical particles (with relative refractive index below unity) in liquid flows, was investigated. This technique is based on the observation of the particle scattering pattern around the critical angle. Similarly to the recent technique developed for rainbow scattering pattern analysis for droplet temperature and size measurements, it is shown that the relative particle refractive index (m_r<1) and size can be determined from the position of the primary diffraction fringe and from the angular spacing between two fringes. Explicit equations for refractive index and particle size measurement were derived from the first-order term of the physical optics approximation. An experimental validation test and numerical computations based on the Lorenz-Mie theory were used to validate the principle of the proposed technique and to estimate its sensitivity, which was shown to be of the same order as that of the rainbow technique. This technique is considered to be useful for various applications in liquid multiphase flows where the particles size and material are to be characterized.     

Method for retrieving the refractive index of ordered particles from data on the photonic band gap

A method for retrieving the refractive index of spherical particles arranged into ordered structures is proposed. It is based on the solution of the inverse problem using data on the photonic band gap. The solution has been obtained within the quasi-crystalline approximation of the multiple wave scattering theory and the transfer-matrix method. Quantitative results are presented for systems of silicon oxide particles. The effective refractive indices of synthetic opal particles have been found from the available experimental data on the spectral position of the photonic band gap. The described technique is applicable for retrieving not only the refractive index of particles but also other characteristics of ordered particulate structures from the coherent transmittance spectra.     

Efficient Technique to Control the Zero-Dispersion Wavelength of a Microstructured Optical Fiber

Abstract

An efficient technique to control the zero-dispersion wavelength of a microstructured optical fiber is proposed and numerically demonstrated in this article. This technique is based on the variation of the linear refractive index and the thickness of a thin film covering the microstructured optical fiber holes' inner surfaces. A powerful and accurate code based on a full-vector finite-element method formulation in conjunction with perfectly matched layers was used. A maximum of 570-nm zero-dispersion wavelength displacement is demonstrated. This thin film can be included after the microstructured optical fiber has been fabricated, as that means the zero dispersion of such fiber can be tailored as needed.     

工字形太赫兹超材料吸波体的传感特性研究

利用超材料吸波体对材料参数的电磁响应, 可将其应用于传感. 本文设计了一种工字形单元结构的超材料吸波体, 基于频域算法对其在太赫兹频段的传感特性进行数值模拟, 研究了待测样品折射率、厚度及电介质隔层厚度对超材料吸波体传感器的频率灵敏度、振幅灵敏度及品质因数的影响. 研究结果表明:当待测样品厚度为40 μm时, 折射率频率灵敏度可达到153.17 GHz/RIU, 折射率振幅灵敏度可达到41.37%/RIU; 待测样品折射率一定时, 厚度频率灵敏度随其厚度的增大而线性减小; 随着待测样品厚度的增加, RFOM呈增大趋势, 但增大幅度在逐渐减小; TFOM随待测样品厚度的增加而减小.     

Theoretical study of saturable Kerr nonlinearity using top-hat beam Z-scan technique

Saturable Kerr nonlinearity is theoretically investigated by use of the top-hat beam Z-scan technique. The saturation intensity changes the nonlinear refractive profile and decreases the sensitivity of the Z-scan measurements, which were quantitatively analyzed. An empirical formula for the saturable Kerr nonlinearity, which gave the relationship between the light intensity and the peak-valley transmittance difference, was accomplished. A high-accuracy method to determine the nonlinear refractive index and the characteristic saturation intensity was proposed.     

Direct measurement of dispersion of the group refractive indices of quartz crystal by white-light spectral interferometry

We present a white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the dispersion of the ordinary and extraordinary group refractive indices of a quartz crystal over the wavelength range approximately from 480 to 860 nm. The technique utilizes a dispersive Michelson interferometer with the quartz crystal of known thickness to record a series of spectral interferograms and to measure the equalization wavelength as a function of the displacement of the interferometer mirror from the reference position, which corresponds to a balanced non-dispersive Michelson interferometer. We confirm that the measured group dispersion agrees well with that described by the dispersion equation proposed by Ghosh. We also show that the measured mirror displacement depends, in accordance with the theory, linearly on the theoretical group refractive index and that the slope of the corresponding straight line gives precisely the thickness of the quartz crystal.