Determination of attenuation coefficients of single mode optical fiber standards to be used in OTDR calibrations

The subject of this paper is the determination of attenuation coefficients of single mode optical fiber standards used in both loss and distance scales calibrations of OTDR instruments by applying “cut-back” method, and “loop transit time” measurements. In cut-back measurements a modified radiometer with InGaAs having 5 mm diameter active area, cooled to 77 K, was constructed and used. To derive attenuation coefficients after the completion of cut-back measurements, the loop transit time measurements were performed for standard fibers. Total expanded uncertainty was calculated as 3.30×10^-3 for determination of attenuation coefficients.     

Miniature microstructured fiber coil with high magneto-optical sensitivity

Magneto-optic Faraday Effect in a miniature coil wound from a microstructured spun-type optical fiber is investigated theoretically and experimentally for the first time. It is shown that such a microstructured fiber allows one to efficiently accumulate the Faraday phase shift in a magnetic field even when the fiber is wound into a coil of very small diameter. For example, a fiber coil of diameter 5 mm with 100 turns has a magneto-optic sensitivity of about 70% of its “ideal” value and agrees well with theoretical data.     

面向近红外脑功能成像的光学自导引漫射光断层成像方法

为降低近红外脑功能漫射光断层成像(DOT)固有的逆问题病态性,并避免多模态方法的图像配准等问题,提出了基于光学自导引提供先验功能信息的脑功能DOT方法(OT-DOT),并发展了图像重构方法.模拟验证表明:上皮厚度(TLT)已知时,OT-DOT获得的重构量化度(QR)约为传统DOT的4.2倍;当TLT的估计误差小于±10%时,OT-DOT重构的QR值可达92%以上,远远优于传统DOT;噪声鲁棒性测试表明,OT-DOT与传统DOT的噪声鲁棒性相近.利用连续光DOT测量系统的仿体实验重构结果表明,所发展的OT-DOT算法获得的重构结果优于传统DOT算法.     

Application of liquid crystal (LC) devices for optoelectronic reconstruction of digitally stored holograms

Recent advances in high-resolution CCD detectors allow the development of the concept of digital holography that relies on the capture of 2D microinterferograms and their further numerical reconstruction. This approach is valid for optical metrology applications as it gives direct access to the phase and intensity of an object at the selected distance “d”. However, it is not convienient for direct 3D-display purposes. This is the reason why the authors propose the optoelectronic reconstruction of a hologram that is digitally recorded and transferred to another optoelectronic medium. The application of LCD matrices and optically addressed liquid crystal cells as reconstruction media is discussed. Their applicability for computer-generated and optical holograms is tested by means of simple experiments. The problems connected with limited resolution of the recording (CCD) and reconstruction (LCD, DMD) devices are considered. The comparison of the results obtained by numerical and optoelectronic means is presented, together with a discussion of the limitations and further possibilities of these techniques.     

Inspection of nano-sized SNOM-tips by optical far-field evaluation

High resolution optical microscopy has many interesting applications in solid state physics, low temperature physics, biology and semiconductor technology. Unfortunately, the lateral resolution of conventional microscopes is limited by the Rayleigh-limit. “Scanning nearfield optical microscopy” (SNOM) seems to be a promising new approach to characterize the properties of materials optically with a high lateral resolution of 50–100 nm. The most important part of such a microscope is the scanning probe (a special glass fiber tip). However, the quality of the optical fiber tip is of decisive importance. Since the production process of pulled and coated glass fiber tips is still highly empirical and error-prone, a technique would be useful to determine the tips’ quality before they are shipped to the user or mounted in the microscope. The tips’ apertures are smaller than λ/2 and therefore they cannot be measured in a non-destructive way by conventional optical microscopy. This paper discusses an easy and fast method for the optical characterization of common glass fiber SNOM tips. The effective aperture of the tip is measured from the far-field distribution of the emitted intensity recorded by a CCD target. A numerical model is introduced to solve this inverse task and a simple optical setup is presented to detect light emitted by the tip at an angle of up to 90° from the optical axis. Experimental investigation, near/far-field calculations and scanning electron microscope investigations show the working principle of this measurement technique for the analysis and evaluation of a typical nanostructured object.     

Novel comprehension of “common path” principle

The idea of “common path” has been widely applied in optical instrument design for 30 years and even today. But the meaning of “common path” has not yet been explained clearly and sometimes confusion has been created. In this paper an “adaptive principle” is proposed and recommended on optical instrument system. It suggests that the designer not only arranges the measurement system to obtain measurement signal but also sets a channel to give prediction of noise or disturbance in real time or short term. Such a recommendation is based on the recent studies on nonlinear dynamics and atmospheric disturbance by means of experiments as well as theoretical analysis.     

Schlieren “PIV” for turbulent flows

The possibility of using commercial PIV equipment combined with schlieren optics to measure the velocity fields of turbulent flows is explored. Given a sufficiently high Reynolds number and adequate refractive flow differences, turbulent eddies can serve as the PIV “particles” in a schlieren image or shadowgram. The PIV software analyzes motion between consecutive schlieren or shadowgraph frames to obtain velocity fields. Velocimetry examples of an axisymmetric sonic helium jet in air and a 2D turbulent boundary layer at Mach 3 are shown. Due to optical path integration, axisymmetric flows require the inverse Abel transform to extract center-plane velocity data. Conditions for optimum schlieren sensitivity are examined. In its present embodiment, “schlieren PIV” is not useful for laminar flows nor for fully 3D flows. Otherwise it functions much like standard PIV under conditions where individual particles are not resolved and velocimetry is instead based on correlation of the motion of turbulent structures. “Schlieren PIV” shows significant promise for general refractive turbulent flow velocimetry if its integrative nature can be overcome through sharp-focusing optics.     

On the theory of temporal aberrations for cathode lenses

A new approach to the theory of temporal aberration for cathode lenses is given in the present paper. A definition of temporal aberration is given in which a certain initial energy of electron emission along the axial direction ε_z1 (0ε_z1ε_0max) is considered. A new method to calculate the temporal aberration coefficients of cathode lenses named “direct integral method” is also presented. The “direct integral method” gives new expressions of the temporal aberration coefficients which are expressed in integral forms. The difference between “direct integral method” and “τ-variation method” is that the “τ-variation method” needs to solve the differential equations for the three of temporal geometrical aberration coefficients of second order, while the “direct integral method” only needs to carry out the integral calculation for all of these temporal aberration coefficients of second order.All of the formulae of the temporal aberration coefficients deduced from “direct integral method” and “τ-variation method” have been verified by an electrostatic concentric spherical system model, and contrasted with the analytical solutions. Results show that these two methods have got identical solutions and the solutions of temporal aberration coefficients of the first and second order are the same as with the analytical solutions. Although some forms of the results seem different, but they can be transformed into the same form. Thus, it can be concluded these two methods given by us are equivalent and correct, but the “direct integral method” is related to solve integral equations, which is more convenient for computation and could be suggested for use in practical design.     

Feasibility of “intermittent” active control of combustion instabilities in liquid fueled combustors using a “smart” fuel injector

This paper describes an experimental investigation of the feasibility of an “intermittent” active control approach for suppressing combustion instabilities in liquid fueled combustors. The developed controller employs a “smart” fuel injector that can modify the spray properties in response to changes in combustor operating conditions. This action weakens or breaks up the coupling between the combustion process and combustor acoustic modes oscillations, thus preventing the excitation of large amplitude instabilities. This approach differs significantly from previously proposed active control methods, both in concept and implementation, as it requires only “intermittent” modification of the combustion process by a single control action as opposed to the continuous action required by most other active control methods. The “smart” fuel injector used in this study consisted of a double-staged, air-assisted atomizer in which counter swirling, primary (inner stage) and secondary (outer stage) air streams were supplied to the injector through separate sets of tangentially oriented orifices. Control of the ratio of air mass flow rates supplied to these two stages, by use of a diverter valve, resulted in significant changes in the spray shape and its axial, tangential, and radial velocity components. This variation in spray properties of the “smart” injector was characterized for different values of the inner to outer air flow rate ratio in cold flow tests with a PDPA system. These results were then correlated with the characteristics of the “intermittently” controlled combustor. Measured quantities included the instability amplitudes, axial dependence of the mean and oscillatory heat release amplitudes, and the characteristics of the recirculation zones, which were all shown to depend on the fuel spray properties. The results of this study demonstrate the feasibility of using “smart” fuel injectors with capabilities for varying the combustion process characteristics to reduce the amplitudes of detrimental combustion instabilities in real engines to acceptable levels.     

一种用于细胞操作的单光纤光镊研究

采用一种抛物线形光纤针作为基本器件,设计制作了单光纤光镊系统.用时域有限差分（Finite Difference Time Domain,FDTD）的方法仿真了抛物线型光纤探针的出射场,并在稳态场下通过对麦克斯韦应力张量积分求出不同介质球半径和折射率情况下,介质球受到的横向和纵向光作用力.实验中此单光纤光镊系统实现了对水中酵母细胞的空间捕获,并且结合两个抛物线形光纤针实现了酵母细胞在两个光势阱中的转移交接.这种单光纤光镊系统结构简单、紧凑,操纵灵活,便于调整.可以适应更多的生物细胞和生物分子的光微操作需求.     

Beyond nanosizing: an approach to shape analysis of fluorescent nanostructures by SMI-microscopy

Using spatially modulated illumination (SMI) light microscopy it is possible to measure the sizes of fluorescent structures that have an extension far below the conventional optical resolution limit (“subresolution size”). Presently, the sizes are determined as the object extension along the optical axis of the SMI microscope. For this, however, “a priori” assumptions on the fluorochrome distribution (“shape”) within the examined fluorescent structure have to be made. Usually it is assumed that the fluorochrome follows a Gauss-distribution or a spherical distribution. In this report we overcome the necessity to make an assumption on the shape of the fluorochrome distribution. We introduce two new experimentally obtained parameters which allow the determination of a shape measure to describe the spatial distribution of the fluorescent dye. This becomes possible by independent measurements with different excitation wavelengths. As an example, we present shape parameter measurements on individual fluorescent microspheres with a nominal geometrical diameter (“size”) of 190 nm. In the case investigated, the experimental shape correlated well with a homogeneous fluorochrome distribution (“spherical shape”) but not with a variety of other “shapes”.     

Double-dielectric-mirror InGaN/GaN microcavities formed using selective removal of an AlInN layer

We describe the fabrication and optical properties of a 3λ/2 InGaN/GaN-based microcavity using “upper” and “lower” silica/zirconia mirrors. The fabrication of this structure involved selective removal of an AlInN layer following multistep thinning of a free-standing GaN substrate. Photoluminescence spectra show a narrowing of the excitonic emission from InGaN/GaN quantum wells in the microcavity, giving a cavity quality factor Q exceeding 400.     

Temperature Characteristics of Vibrating Type Sensor Using Micromachined Optical Fiber-Tip

An optical sensor using a quartz core microcantilever was fabricated by etching clad layer from optical fiber. The temperature dependence of the resonance frequency of this sensor was measured in atmosphere and water. The temperature coefficient of the resonance frequency in water was 1.3×10⁻³/°C, which was about one order larger than that (2.3×10⁻⁴/°C) in atmosphere. This was caused by increase of additional mass due to temperature dependence of the viscosity of water, while, the increase of the resonance frequency in atmosphere was caused by temperature dependence of Young’s modulus of the quartz core. These results were evaluated theoretically using a “string-of-beads” model.     

Worldwide activities in fiber optics: Part II—systems and field trials

This report is essentially in two parts. The first is a summary of systems and field trials in North America, Japan, and Europe, based on a paper entitled “A Survey of World Wide Optical Waveguide Systems” by R. L. Gallawa, T. E. Midwinter, and S. Shimada, at the 1979 IEEE/USA Topical Meeting on Optical Fiber Communications, March 64, 1979, in Washington, D.C.

The second part is a complete listing of the technical characteristics of over 100 operational and experimental fiber optic systems compiled by Information Gatekeepers, Inc., 167 Corey Road, Brookline, MA 02146.     

Temperature measurements using fiber optic polarization interferometer

A novel measurement method of temperature based on the phenomena that the phase difference between principle polarization states in the optical retarder is function of temperature is described. The polarization state of optical beam is changed as it passes through the optical retarder, which depends on the temperature. The temperature of optical retarder is determined by comparison of the power difference between principal polarization states. We demonstrate successfully the temperature measurement by using a polarization maintaining fiber as the optical retarder. With a 100 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.236 rad/°C and the measurement error was ±0.038°C over the temperature range of −2.6 – +3.4°C. With a 11.5 mm length of the fiber optic retarder, the change rate of phase difference on temperature was 0.021 rad/°C and the measurement error was ±0.79°C over the temperature range of −8.5 – +86.5°C.     

Simple data acquisition method for multi-dimensional EPR spectral-spatial imaging using a combination of constant-time and projection-reconstruction modalities

A combination of the constant-time spectral-spatial imaging (CTSSI) modality and projection-reconstruction modality was tested to simplify data acquisition for multi-dimensional CW EPR spectral-spatial imaging. In this method, 3D spectral-spatial image data were obtained by simple repetition of conventional 2D CW imaging process, except that the field gradient amplitude was incremented in constant steps in each repetition. The data collection scheme was no different from the conventional CW imaging system for spectral-spatial data acquisition. No special equipment and/or rewriting of existing software were required. The data acquisition process for multi-dimensional spectral-spatial imaging is consequently simplified. There is also no “missing-angle” issue because the CTSSI modality was employed to reconstruct 2D spectral-spatial images. Extra reconstruction processes to obtain higher spatial dimensions were performed using a conventional projection-reconstruction modality. This data acquisition technique can be applied to any conventional CW EPR (spatial) imaging system for multi-dimensional spectral-spatial imaging.     

A structured light-based system for human heads

The 3-D modeling of heads by using optical triangulation techniques is of great interest in the context of virtual reality, telecommunication and computer animation. This paper presents a structured light-based system mainly for human heads. It is named “3-D Laser Color Scanner” (3DLCS). A 3-D model is obtained with a cylindrical scan. The laser beam is switched on and off using a “light valve” and two successive CCD frames are captured, one with the laser line showing and one without. We can simplify the laser line extracting by subtracting these two images.In this system, two CCD cameras are used to avoid occlusion problems. Color information is read from the CCD when the laser light is absent. Since traditional laser scanner will miss the range data in the low-reflectance areas such as the hair area of human head, a shape from silhouette algorithm is presented to overcome this problem. Finally, we give some results using our system. The resulting model is suitable for many applications.     

Trellis ternary line coding scheme for asynchronous optical CDMA systems

In this paper, we investigate the employment of a ternary line coding technique based on Ungerboeck's trellis-coded method in asynchronous optical CDMA systems. The ternary coding we use is predicated upon the equal-weight orthogonal (EWO) scheme. Each user transmits two mutually orthogonal signature sequences to represent “+1” and “−1”, respectively, and nothing is transmitted for “0”. The receiver employs a maximum-likelihood soft-decoder to select the path with minimum Euclidean distance as the preferred path. This trellis ternary coding scheme applies set partitioning with partially overlapping subsets to increase the free Euclidean distance, which considerably improves system performance. Furthermore, due to line coding technique, such scheme comprises sufficient clock information, and thus benefits for baseband timing extraction (i.e. clock recovery). Numerical results reveal that the proposed trellis ternary coding scheme can significantly reduce the error floor and allow more active users to be accommodated in the network.     

Analysis of Off-Axial Optical Systems (2)

The extended 4×4 Gaussian bracket matrix Gij represents the lowest order quantity of “aberration coefficient tensor quantities” which are defined to as the peculiarity of off-axial optical systems and are independent of azimuths. We newly confirmed this extended 4×4 Gaussian bracket matrix of deflection (refraction or reflection), transmission and “twisting.” The result determined by use of a new representative method of asymmetrical surfaces and a method of paraxial expansion along the folded reference axis shows that the 4×4 Gaussian bracket matrix is the extended form of the 2x2 Gaussian bracket matrix which is used in co-axial rotational symmetric optical systems. Furthermore, we analyze and formalize the crossterm effects, which are the most serious problem in optical systems having multiple off-axial surfaces, using the concept of a chain of “optical systems divided into former and latter” and the vector-tensor analysis method. The result of this analysis reveals the structure of the cross-term effects and proves the usefulness of the vector-tensor analysis method in general image analysis.     

Polarized He spin-filters using MEOP for wide-angle polarization analysis

Polarized ³He spin-filters are currently employed on a wide range of neutron instruments at the ILL, primarily for diffraction, reflectometry and fundamental physics. A wide range of recent and ongoing improvements are enabling the implementation of this technique for wide-angle polarization analysis for inelastic measurements. These include

• Progress in metastability-exchange optical pumping (MEOP), resulting in on-beam polarization levels of up to 80%.

• 1st generation “Pastis-1” coils for rotating the neutron polarization at the sample position, allowing for “XYZ” polarization analysis.

• 2nd generation “Pastis-2” coils with no blind angles in the equatorial plane.

• Spin-filter cells with glued silicon windows, allowing for wide-angle “banana” cells with very low background scattering.

• Polarization-preserving capillaries for transferring polarized ³He gas into the cell without manual access.

The development of capillary transfer also allows for a completely new way of working with ³He spin-filters: connecting the cells on the instruments directly to the MEOP filling station several tens of meters away and allowing for quasi-continuous operation.