Wavefront reconstruction of high power laser diode array

A new method for the wavefront measurement of high power laser diode array with large dynamic ranges is presented. The wavefront sensor has a large dynamic range of −π/2 to π/2 and high precision on the measured wavefront slopes (±1′). According to the measurements of the slopes and intensity of the high power laser diode array, complex amplitudes can be obtained and the wavefront can be reconstructed. The reconstructed wavefront of high power laser diode array can be used in designing a laser beam shaping system in future work.     

Status of single-model optical fiber standardization in the electronic industries association (EIA)

This paper introduces and analyzes revolutionary laser system architecture capable of dramatically reducing the complexity of laser systems while simultaneously increasing capability. The architecture includes three major subsystems. The first is a phased array of laser sources. In this article, we discuss diode-pumped fiber lasers as the elements of the phased array, although other waveguide lasers can also be considered. The second provides wavefront control and electronics beam steering, as described in an IEEE Proceedings article on “Optical Phased Array Technology” [1]. The third is subaperture receiver technology. Combining these three technologies into a new laser systems architecture results in a system that has graceful degradation, can steer to as wide an angle as individual optical phased array subapertures, and can be scaled to high power and large apertures through phasing of a number of subapertures. Diode-pumped fiber lasers are appealing as laser sources because they are electrically pumped, efficient, relatively simple, and scalable to significant power levels (over 100 Watts has been demonstrated from a single diode-pumped fiber laser) [2]. The fiber laser design also lends itself to integration into a phased array. Fiber lasers have been phased. Initial phasing demonstrations have been at low power and were conducted by taking a single source, dividing it into multiple fibers, then phasing them together. To develop this technology further we need to use independent fiber lasers or fiber amplifiers, seeded by a common source, and to increase laser power. As we increase laser power, we will have to learn to cope with nonlinearities in the laser amplifiers. Optical Phased Array technology has demonstrated steering over a 90-degree field of regard [4], although this approach used additional optical components. If we use straightforward optical phased array beam steering without additional optics we can steer with high efficiency to about one-third λ/d, where d is the smallest individually addressable element. The one-third factor depends on the efficiency threshold. For example, if we use 1.5 μm light, and 5 μm center-to-center spacing, we can steer with high efficiency to about ±6 degrees, or a field of regard of 12 degrees. Last, we need to develop a subaperture receive technology. This can be a pupil plane receiver, an image plane receiver, or some combination of the approaches. When we have matured each individual technology and combined them into new laser systems architectures, we will have the ability to build simpler and more capable laser systems. The vision for an integrated, phased array laser concept is to enable a new class of laser systems with significant advantages, including high-efficiency, all-electric laser source; all waveguide beam transport; wavefront control at the sub-aperture level (enabling wavefront compensation, conformal apertures, and wide-angle electronic beam steering); random access beam pointing over wide angles; multiple simultaneous beam generation and control; and graceful degradation.     

新型离轴反射变焦距光学系统的多视场检测方法

基于对离轴反射变焦距光学系统进行计算机辅助装调研究, 需要检测离轴反射变焦距系统各个视场的波像差, 除零度视场外, 获得包括其他视场的波像差有助于提高计算机辅助装调的准确性, 但是目前已有的波像差检测方法往往只能获得系统零度视场的波像差. 本文针对这个难题提出了一种检测离轴反射变焦距光学系统各个视场波像差的方法, 并应用于离轴三反变焦距光学系统的各视场波像差仿真检测. 该方法在传统自准直干涉法的基础上进行改进, 关键在于采用变形镜代替扫描的平面镜, 并采用夏克-哈特曼波前传感器代替干涉仪, 配合精确标定的激光器光源阵列, 可以实现对离轴三反变焦距光学系统的多视场波像差同时检测. 由理论分析和仿真模拟得出, 该系统在视场(0°, 3°), (0°, 4.2°), (0°, 5.5°), (0°, 7°), (0°, 9.8°), (0°, 14°)处经过变形镜补偿后的剩余波像差的RMS值分别为0.00039λ, 0.00075λ, 0.0024λ, 0.00017λ, 0.00053λ, 0.0057λ, 分析仿真结果表明此检测方案是可行的, 且适用于离轴反射变焦距系统的计算机辅助装调技术的研究.     

Effect of screening on the emissivity of field electron emitters based on carbon nanotubes

The effect of screening on the emissivity of a field cathode built around a carbon nanotube array is analyzed. A numerical method of solving the Laplace equation for intricate-shape cathodes is developed that makes it possible to relate the amplification factor to the nanotube spacing in arrays containing as many as 225 emitters. Mutual screening of the tubes, which shows up in the dependence of the field amplification factor on the average emitter spacing, is studied numerically. The optimal spacing between the tubes that provides an emission current maximum density at a given applied voltage is determined. The role of edge effects in carbon nanotube screening is established.     

Wavefront Sets in Algebraic Quantum Field Theory

The investigation of wavefront sets of n-point distributions in quantum field theory has recently acquired some attention stimulated by results obtained with the help of concepts from microlocal analysis in quantum field theory in curved spacetime. In the present paper, the notion of wavefront set of a distribution is generalized so as to be applicable to states and linear functionals on nets of operator algebras carrying a covariant action of the translation group in arbitrary dimension. In the case where one is given a quantum field theory in the operator algebraic framework, this generalized notion of wavefront set, called “asymptotic correlation spectrum”, is further investigated and several of its properties for physical states are derived. We also investigate the connection between the asymptotic correlation spectrum of a physical state and the wavefront sets of the corresponding Wightman distributions if there is a Wightman field affiliated to the local operator algebras. Finally we present a new result (generalizing known facts) which shows that certain spacetime points must be contained in the singular supports of the 2n-point distributions of a non-trivial Wightman field. Received: 27 July 1998 / Accepted: 3 March 1999     

Electric field enhancement in field-emission cathodes based on carbon nanotubes

The problem of determining the field enhancement factor in field-emission cathodes based on carbon nanotubes (CNTs) is considered. The electrostatic problem of finding the field enhancement factor for nanotubes with different shapes of the tip as a function of the angle the nanotube makes with the cathode surface and of the interelectrode spacing is solved. The dependence of the electric field enhancement factor on the spacing between vertically oriented nanotubes constituting an array is derived. Making allowance for this dependence gives an optimal value of the surface density of nanotubes in the array at which the emission current density is maximal. The I—V characteristic of CNT-based cathodes is studied with regard to the statistical straggling of their orientation angles. This I—V characteristic is compared with the characteristic obtained with regard to the statistical straggling of the CNT geometrical parameters.     

基于超指向性多极子矢量阵的水下低频声源方位估计方法研究

针对水下低频声源的方位估计问题,基于基元紧密排列的二维矢量阵,建立了一种超指向性波束形成方法.根据矢量基元差分运算构建各阶多极子模型,获得了几乎与频率无关的模态函数,并经加权计算可在低频条件下实现超指向性波束,以解决阵列孔径对波束性能的限制.同时,结合输出信噪比最大准则所得波束,分析了超指向性波束形成算法的稳定性与波导的影响程度,探索模态阶数与阵列参数的选取原则.通过阵列性能的仿真计算与实际阵列的测量数据表明,该算法可在小尺寸阵列孔径下获得良好的阵列波束,兼具了水下线型超指向性阵和环形超指向性阵的优点,可有效实现水下低频声源的水平方位估计;且波束性能可通过调节模态阶数与基元间距以达到最佳,并受水下声波导多途与频散效应影响有限.     

Configuration optimization of laser guide stars and wavefront correctors for multi-conjugation adaptive optics

Multi-conjugation adaptive optics(MCAOs) have been investigated and used in the large aperture optical telescopes for high-resolution imaging with large field of view(FOV).The atmospheric tomographic phase reconstruction and projection of three-dimensional turbulence volume onto wavefront correctors,such as deformable mirrors(DMs) or liquid crystal wavefront correctors(LCWCs),is a very important step in the data processing of an MCAO's controller.In this paper,a method according to the wavefront reconstruction performance of MCAO is presented to evaluate the optimized configuration of multi laser guide stars(LGSs) and the reasonable conjugation heights of LCWCs.Analytical formulations are derived for the different configurations and are used to generate optimized parameters for MCAO.Several examples are given to demonstrate our LGSs configuration optimization method.Compared with traditional methods,our method has minimum wavefront tomographic error,which will be helpful to get higher imaging resolution at large FOV in MCAO.     

Field characterization of therapeutic ultrasound phased arrays through forward and backward planar projection

Spatial planar projection techniques propagate field measurements from a single plane in front of a transmitter to arbitrary new planes closer to or further away from the source. A linear wave vector frequency-domain projection algorithm is applied to the acoustic fields measured from several focused transducer arrays designed for ultrasound therapy. A polyvinylidene difluoride hydrophone is first scanned in a water tank over a plane using a three-dimensional positioning system to measure the complex pressure field as a function of position. The field is then projected to a series of new planes using the algorithm. Results of the projected fields are compared with direct measurements taken at corresponding distances. Excellent correlation is found between the projected and measured data. The method is shown to be accurate for use with phase-controlled field patterns, providing a rapid and accurate method for obtaining field information over a large spatial volume. This method can significantly simplify the characterization procedure required for phased-array application used for therapy. Most significantly, the wavefront propagated back to a phased array can be used to predict the field produced by different phase and amplitude settings of the array elements. A field back-projected to the source could be used as an improved source function in acoustic modeling.     

Comparative measurements of plasma position using coils,hall probes,and bolometers on CASTOR tokamak

Plasma position on CASTOR tokamak is measured by several systems based on different experimental methods. Vertical and horizontal plasma displacements are deduced from data of 4 Mirnov coils spaced poloidaly by 45°. Output of the Mirnov coils is routinely used as an input for automatic feed-back control of plasma position on CASTOR. Independently, we used an array of 16 Hall sensors, which measure the required magnetic field directly. Additional information on plasma position is obtained from two arrays of bolometers that measure horizontal and vertical profile of plasma radiation. Principle design of the all used diagnostics is given. Comparison of horizontal and vertical displacements deduced using various experimental methods is presented.     

Magnetic dipolar coupling and collective effects for binary information codification in cost-effective logic devices

Physical limitations foreshadow the eventual end to traditional Complementary Metal Oxide Semiconductor (CMOS) scaling. Therefore, interest has turned to various materials and technologies aimed to succeed to traditional CMOS. Magnetic Quantum dot Cellular Automata (MQCA) are one of these technologies. Working MQCA arrays require very complex techniques and an excellent control on the geometry of the nanomagnets and on the quality of the magnetic thin film, thus limiting the possibility for MQCA of representing a definite solution to cost-effective, high density and low power consumption device demand. Counter-intuitively, moving towards bigger sizes and lighter technologies it is still possible to develop multi-state logic devices, as we demonstrated, whose main advantage is cost-effectiveness. Applications may be seen in low cost logic devices where integration and computational power are not the main issue, eventually using flexible substrates and taking advantage of the intrinsic mechanical toughness of systems where long range interactions do not need wirings. We realized cobalt micrometric MQCA arrays by means of Electron Beam Lithography, exploiting cost-effective processes such as lift-off and RF sputtering that usually are avoided due to their low control on array geometry and film roughness. Information relative to the magnetic configuration of MQCA elements including their eventual magnetic interactions was obtained from Magnetic Force Microscope (MFM) images, enhanced by means of a numerical procedure and presented in differential maps. We report the existence of bi-stable magnetic patterns, as detected by MFM while sampling the z-component of magnetic induction field, arising from dipolar inter-element magnetostatic coupling, able to store and propagate binary information. This is achieved despite the array quality and element magnetic state, which are low and multi-domain, respectively. We discuss in detail shape, inter-element spacing and dot profile effects on the magnetic coupling. Numerical Finite Element Method (FEM) simulations show a possible microspin arrangement producing such magnetostatic coupling.     

A novel wavelength dispersive device with a dispersive element based on staircase-like straight and parallel arrayed waveguides

We propose a new type of arrayed waveguide grating (AWG) multiplexer/demultiplexer based on modified group refractive index. This device is composed by an array of straight and parallel waveguides of equal length and each waveguide consist of two sections with different width. The length of the two sections are changed from a waveguide to the adjacent one following a linear dependence resulting in a wavelength dispersive waveguide array. An example of the device design for silicon-on-insulator (SOI) platform is provided and numerical simulations have been carried out for various arrayed waveguide parameters. We demonstrate that the group index modification can be used for tailoring device dispersion properties, and that it can also result in new dispersion characteristics predicted numerically not observed in conventional AWGs. Additional advantages are that the demultiplexer does not necessarily require bending waveguide sections as in a conventional AWG (de)multiplexers, and thus yields highly compact devices with potentially very low insertion loss. Channel spacing of 1 nm have been predicted for sub-micron waveguides sizes. In this paper it is also proposed a novel wavefront converter based on waveguide array lens-like element with waveguides broadened sections. Numerical results for different input/output geometries are analized.     

Limitations of the Shack–Hartmann sensor for testing optical aspherics

Aspheric lenses and surfaces are increasingly used in modern high-quality optics. Therefore, new measuring methods for an accurate quantification of these aspheres are also necessary. The current approach to quantify aspheres is to apply null systems such as computer-generated holograms as a part of a null lens in a interferometer. An alternative to this method is the Shack–Hartmann wavefront sensor. The dynamic range of this sensor can be adjusted by the optical parameters of the applied microlens array. Hence, large wavefront aberrations can be measured directly without a null lens. However, there are basic limitations in the dynamic range of a Shack–Hartmann sensor (SHS) depending on the curvature of the incident wavefront. In this paper, an analytical expression to determine the strongest wavefront curvature which can be measured with a defined microlens array of an SHS is derived. It allows to calculate the microlens parameters required to measure the wavefront of a test lens. Particularly, the influence of rotational symmetric aspherical wavefront shapes to the dynamic range of an SHS has been studied. A comparison between interferometry and the SHS has been accomplished. Numerical solutions using scalar diffraction theory illustrate the analytical predictions.     

一种提高HWS采样率的透镜式微扫描方法

为改善传统哈特曼-夏克波前传感器对待测波前采样不足的缺点,对哈特曼-夏克波前传感器和微扫描进行了分析,提出一种提高哈特曼-夏克波前传感器采样率的透镜式微扫描方法。通过在微透镜阵列之前加入由PZT驱动的透镜扫描装置,对CCD采集的光斑分布情况进行高分辨率微扫描图像重建,通过对重建后的光斑分布进行波前重构,提高了哈特曼波前传感器对待测波前的采样率。通过对比实验验证,波前复原精度提高了41%,可以有效提高哈特曼传感器对波前探测的精度。     

Fracture spacing in layered materials.

We perform an elastostatic analysis of a periodic array of cracks under constant loading. We give an analytical solution and show that there is a limitation to the fracture spacing, due to a transition from an opening to a compressive loading. For this configuration, the threshold of the fracture spacing depends on neither the applied strain nor the elastic parameters of the material. This result shows that, in the general case of layered materials, the physical mechanism that is responsible for the limitation in the density of fractures is related mainly to the geometry of the problem. This is in agreement with observations and with recent numerical results.     

二维光子晶体耦合腔阵列的慢波效应研究

设计了一种六角晶格二维光子晶体耦合腔阵列,平面波展开法计算能带表明,处于禁带中的耦合缺陷腔模的色散曲线在光子晶体平面内所有k矢量方向更加平坦.模拟了横电波沿ΓK方向的透射谱.与光子晶体单缺陷腔相比,耦合腔阵列结构的缺陷腔模透射率提高三个量级以上,而群速度降低一个量级,得到0.007c的结果.该慢波效应在构造微型可调谐光延迟器和低阈值光子晶体激光器等方面具有潜在的应用前景. 关键词： 光子晶体 耦合腔阵列 慢波 透射率     

Field-of-view shifted Shack-Hartmann wavefront sensor for daytime adaptive optics system

For the daytime adaptive optics system, a field-of-view shifted Shack-Hartmann wavefront sensor (FSWFS), which is used to measure the aberrant wavefront under daytime conditions, is proposed. Because the field angle of the object signal in adaptive optics systems is much less than that of the sky background, the effective object signal is separated from the strong sky background. Experimental results indicate that FSWFS with a single focal-plane array can precisely and stably measure the aberrant wavefront information with a strong sky background under daytime conditions.     

High TcYBa2Cu3O7−δ superconducting transition-edge microbolometers

High T_c superconducting thin film YBa₂Cu₃O_7−δ (YBCO) bolometers have been fabricated on various substrates such as MgO, LaAlO₃, YSZ and Si using laser ablation technique. Performance of these IR bolometers operating with a Joule-Thomson refrigerator has been investigated. Measurements of the responsivity and low frequency noise near T_c in the current biased YBCO bolometers show that reliable devices can be fabricated. Measured noise equivalent power (NEP), for YBCO/YSZ bolometer, reaches 6 × 10⁻¹⁰W/Hz^1/2 at 165 Hz and has a responsivity of 60 V/W with a blackbody source. This performance is comparable to that of the optimized pyroelectric detectors. The characteristics of YBCO films deposited on Si substrates reveal that superconducting thin film multi-elements or focal plane array with silicon integrated readout circuit are feasible. Such bolometers exhibit NEP of 7 × 10⁻⁹W/Hz^1/2, and significant improvement appears possible. Electrical measurements show no noticeable film degradation after the bolometer is exposed to atmosphere for three months.     

Coherent electronic properties of coupled two-dimensional quantum dot arrays

In this paper we review our recent study of coherent electronic properties of coupled two-dimensional quantum dot arrays using numerical exact-diagonalization methods on a Mott–Hubbard type correlated tight-binding model. We predict the existence of a novel kind of persistent current in a two-dimensionalisolatedarray of quantum dots in a transverse magnetic field. We calculate the conductance spectrum for resonant tunneling transport through a coherent two-dimensional array of quantum dots in the Coulomb Blockade regime. We also calculate the effective two-terminal capacitance of an array coupled to bias leads.