Application of in situ ellipsometry in the fabrication of thin-film optical coatings on semiconductors

Thin-film interference filters, suitable for use on GaAs- and InP-based lasers, have been fabricated by use of the electron-cyclotron resonance plasma-enhanced chemical vapor deposition technique. Multilayer film structures composed of silicon oxynitride material have been deposited at low temperatures with an in situ rotating compensator ellipsometer for monitoring the index of refraction and thickness of the deposited layers. Individual layers with an index of refraction from 3.3 to 1.46 at 633 nm have been produced with a run-to-run reproducibility of 0.005 and a thickness control of 10 A. Several filter designs have been implemented, including high-reflection filters, one- and two-layer anitreflection filters, and narrow-band high-reflection filters. It is shown that an accurate measurement of the filter optical properties during deposition is possible and that controlled reflectance spectra can be obtained.     

Algebraic wavelet filters

A method to compute the discrete wavelet transform for certain wavelet filters is proposed that takes advantage of conjugacy properties in number fields. It is shown that wavelet filters derived from compactly supported orthonormal wavelets can be approximated with arbitrary precision by the proposed wavelet filters. © 1996 John Wiley & Sons, Inc.     

Improved optimization of soft-partition-weighted-sum filters and their application to image restoration

Soft-partition-weighted-sum (Soft-PWS) filters are a class of spatially adaptive moving-window filters for signal and image restoration. Their performance is shown to be promising. However, optimization of the Soft-PWS filters has received only limited attention. Earlier work focused on a stochastic-gradient method that is computationally prohibitive in many applications. We describe a novel radial basis function interpretation of the Soft-PWS filters and present an efficient optimization procedure. We apply the filters to the problem of noise reduction. The experimental results show that the Soft-PWS filter outperforms the standard partition-weighted-sum filter and the Wiener filter.     

Rugate filters fabricated by a radio frequency magnetron sputtering system by use of an optical in situ monitoring technique

We propose, for the first time to our knowledge, a new feedback fabrication technique for rugate filters with sinusoidal refractive index distribution. The technique uses an in situ optical monitoring system, in contrast to conventional techniques for rugate filters that are based on time control, which is generally unsuitable for accurate fabrication of a continuous index distribution. We employed a-SiOx:H thin film as the material for the rugate filters because its refractive index can be successively controlled. Using the proposed technique and material, we fabricated near-infrared rugate minus filters having multiple and continuous refractive index distributions. The experimental and calculated spectra were well correlated as a result of applying the proposed feedback fabrication technique.     

Unconstrained correlation filters

A mathematical analysis of the distortion tolerance in correlation filters is presented. A good measure for distortion performance is shown to be a generalization of the minimum average correlation energy criterion. To optimize the filter's performance, we remove the usual hard constraints on the outputs in the synthetic discriminant function formulation. The resulting filters exhibit superior distortion tolerance while retaining the attractive features of their predecessors such as the minimum average correlation energy filter and the minimum variance synthetic discriminant function filter. The proposed theory also unifies several existing approaches and examines the relationship between different formulations. The proposed filter design algorithm requires only simple statistical parameters and the inversion of diagonal matrices, which makes it attractive from a computational standpoint. Several properties of these filters are discussed with illustrative examples.     

Multiple delay line shaping: a new class of weighting functions suitable for digital signal processing

We introduce a whole class of weighting functions, that can be implemented with an analog prefilter, an analog-to-digital converter and a FIR or IIR digital filter. These weighting functions feature a finite duration and can have a flat top within less than 1%. General rules are given about the design of such general class of filters. The shape of the obtained weighting function is strictly related to the chosen prefilter, and its duration may be effectively shrinked by the digital processing. However the number of samples required in order to keep a finite duration of the weighting function is shown to increase with the complexity of the prefilter. The developed class of weighting functions can be alternatively implemented with analog delay lines or switched-capacitor filters.     

Shadow-invariant classification for scenes illuminated by daylight

A physics-based method for shadow compensation in scenes illuminated by daylight is proposed. If the daylight is represented by a simplified form of the blackbody law and the camera filters are of infinitely narrow bandwidth, the relationship between red/blue (rm) and green/blue (gm) ratios as the blackbody's temperature changes is a simple power law where the exponent is independent of the surface reflectivity. When the CIE daylight model is used instead of the blackbody and finite bandwidths for the camera are assumed, it is shown that the power law still holds with a slight change to the exponent. This means that images can be transformed into a map of rm/gmA and then thresholded to yield a shadow-independent classification. Exponent A can be precalculated from the CIE daylight model and the camera filter characteristics. Results are shown for four outdoor images that contain sunny and shadowed parts with vegetation and background. It is shown that the gray-level distributions of the pixels in the transformed images are quite similar for a given component whether or not it is in shadow. The transformation leads to bimodal histograms from which thresholds can easily be selected to give good classifications.     

Normally incident resonant grating reflection filters for efficient narrow-band spectral filtering of finite beams

Resonant grating reflection filters operating at normal incidence are investigated. At normal incidence these structures are shown to have characteristics different from those of structures operated at oblique incidence. We show how higher-order diffraction within the structure laterally confines the incident energy about the point of incidence and results in a broadened angular selectivity. Multimode structures are shown to exhibit broader angular selectivities and narrower spectral linewidths than those of single-mode structures, achieving angular selectivities hundreds of times broader than what could be obtained at oblique incidence. When compared with that for oblique incidence, the increase in angular selectivity is shown to greatly improve the performance of these filters for spectrally filtering finite incident beams.     

Control of resonance separation over a wide spectral range in multiwavelength resonant grating filters

The spectral separation of resonances within multiwavelength resonant grating filters is studied through the use of a grating layer with constant structural parameters in combination with single-layer, multimode waveguides and multilayer, multimode waveguides. The use of multilayer, multimode waveguides are shown to provide the ability to control resonance separations for multiwavelength filters from hundreds of nanometers to only a few nanometers.     

Adiabatic state preparation in a cavity

Abstract

The paper discusses the single-mode Jaynes-Cummings model with time-dependent parameters. Solvable models for two-level systems are utilized to consider the changes in the photon distribution effected by the passage of atoms through the cavity. It is suggested that such systems may be used as filters to modify the photon distribution. The effect can be enhanced by repeatedly sending new atoms through the cavity. We show that such filters can cut out either small or large photon numbers. It is also shown that the method can be used to narrow down photon distributions and in this way achieve highly non-classical sub-Poissonian states. Some limitations and applications of the method are presented.     

Filter multiplexing by use of spatial Code Division Multiple Access approach

The increasing popularity of optical communication has also brought a demand for a broader bandwidth. The trend, naturally, was to implement methods from traditional electronic communication. One of the most effective traditional methods is Code Division Multiple Access. In this research, we suggest the use of this approach for spatial coding applied to images. The approach is to multiplex several filters into one plane while keeping their mutual orthogonality. It is shown that if the filters are limited by their bandwidth, the output of all the filters can be sampled in the original image resolution and fully recovered through an all-optical setup. The theoretical analysis of such a setup is verified in an experimental demonstration.     

Noise robustness of nonlinear filters for image recognition

We analyze the performance of the Fourier plane nonlinear filters in terms of signal-to-noise ratio (SNR). We obtain a range of nonlinearities for which SNR is robust to the variations in input-noise bandwidth. This is shown both by analytical estimates of the SNR for nonlinear filters and by experimental simulations. Specifically, we analyze the SNR when Fourier plane nonlinearity is applied to the input signal. Using the Karhunen-Loève series expansion of the noise process, we obtain precise analytic expressions of the SNR for Fourier plane nonlinear filters in the presence of various types of additive-noise processes. We find a range of nonlinearities that need to be applied that keep the output SNR of the filter stable relative to changes in the noise bandwidth.     

A Review of HTS Thick-Film Microwave Filter Technology

For 10 years, thick-film YBCO has been used to make radio frequency filters out of three-dimensional superconductor coated structures. These filters are presented in a detail never before revealed. The chronology of solving technical problems such as coating and construction will be recounted from a historical perspective. The most recent design, which results in a 16-pole, 1.9 GHz filter in a housing measuring 145 mm×60 mm×35 mm, will be shown in detail. We will discuss the challenges in resonator design, substrate fabrication, coating technology, filter design, and filter realization associated with this filter. This overview demonstrates the progression of size over the past decade, resulting in an order of magnitude reduction in filter size and lessening size as a concern for system application of thick-film HTS filters. The impact of this size reduction on performance metrics such as Q and realizable filter order, as well as device characteristics such as intermodulation distortion, will be shown.     

Relationships Among Control Charts Used with Feedback Control

Feedback control is common in modern manufacturing processes and there is a need to combine statistical process control in such systems. Typical types of assignable causes are described and fault signatures are calculated. A fault signature can be attenuated by the controller and an implicit confounding among faults of different types is discussed. Furthermore, the relationships between various control statistics are developed. Control charts have been proposed previously for deviations from target and for control adjustments. We describe why one or the other can be effective in some cases, but that neither directly incorporates the magnitude (or signature) of an assignable cause. Various disturbance models and control schemes, both optimal and non‐optimal, are included in a mathematically simple model that obtains results through properties of linear filters. We provide analytical results for a widely‐used model of feedback control. Copyright © 2006 John Wiley & Sons, Ltd.     

Harmonic GHz surface-acoustic-wave filters with unidirectional transducers

The fabrication of fundamental-mode surface-acoustic-wave (SAW) filters for gigahertz frequencies requires an elaborate photolithographic process. In order to reduce process demands or to increase the possible frequency range, interdigital transducers can be designed to work on spatial harmonics. However, harmonic filters are known to be unattractive because of their high insertion loss. It is shown that harmonic gigahertz filters made with unidirectional transducers can have moderate insertion loss. Two third-harmonic filters at 1.4 GHz and 2.5 GHz, using two-phase unidirectional transducers, are described. A simple model for calculating the input impedance of harmonic surface-transverse-wave (STW) transducers is presented.     

Feasibility study of particle filters for mobile station receivers

The application of an equalisation method for mobile station terminals using particle filters is presented. To use particle filters for equalisation, a mathematical model is shown which allows the transmitted symbols to be represented as the state of a stochastic system, which can be estimated by particle filters. The authors propose an equaliser structure with particle filters for application in mobile station receivers, especially for GSM/EDGE (global system for mobile communications/enhanced data rate for GSM evolution). Several improvement strategies, which help to obtain better estimation results with a lower number of particles, are discussed. In addition, performance evaluations of the particle filter equaliser for GSM/EDGE are presented.     

Narrowband multichannel filters and integrated optical filter arrays

We propose and demonstrate three approaches to achieve narrowband multichannel filters. These are multiple heterostructures with defects, guided-mode resonance (GMR) Brewster filters with multiple channels, and integrated narrow bandpass filter arrays. Transmission studies for multiple heterostructures with defects are presented. We show that the enlargement of the forbidden band and multiple-channel filtering can be reached simultaneously with these configurations. GMR Brewster filters with multiple channels can be obtained with a single-layer grating. The same properties can be obtained by use of double-layer structures that consist of a homogeneous layer and a grating with equal refractive index. We developed a combinatorial etching technique that has 32 elements on a single substrate with which to fabricate integrated narrow bandpass filters. Single- and double-chamber integrated optical filter arrays were fabricated by use of this etching technique. These narrowband multichannel filters and narrow bandpass filter arrays show good filtering features and can be utilized in many optical applications.     

Extreme-ultraviolet radiation filtering by freestanding transmission gratings

Measurement of energetic neutral atoms fluxes in space requires efficient suppression of exceptionally strong background extreme-ultraviolet (EUV) and UV radiation. Diffraction filters make it possible to separate (transmit) charged and neutral particles from the background radiation (which would be suppressed). Recently developed freestanding transmission gratings look especially promising for implementation in a new family of diffraction EUV/UV filters. The first results of our experimental study of filtering properties of freestanding transmission gratings with a period of 200 nm are presented. The grating transmission was measured in the 52-131-nm wavelength range, and grating polarization properties were determined at 58.4 nm. It is shown that transmission gratings can be used efficiently as filters and polarizers in the EUV/UV spectral range.     

Computer-generated optical multiwavelet filters for hybrid image-classification systems

Optical coherent Fourier correlators are applicable in real-time image analysis such as image classification. The functionality of Fourier correlators can be increased by use of multifunctional filters, which have many spatially multiplexed impulse responses. The concept of multiresolution analysis on the basis of wavelet theory offers profitable methods to design multifunctional filters for image analysis. The applicability of such filters is demonstrated by an example in which different characteristic textures of medical images are extracted. The physical implementation of multiwavelet filters is restricted by modulation-domain constraints imposed by the use of spatial-light-modulator or of diffractive-element fabrication technology. Coding methods of diffractive optics are shown to be helpful to transform the original complex-valued distributions of multiwavelet filters into light-efficient quantized phase-only distributions by preservation of the original filter functionality. The quality of the designed diffractive phase filters is documented by computer experiments.