Simple Gratings Research Articles

Microwave-photonic frequency doubling utilising phase modulator and fibre Bragg grating

A frequency doubling approach for optical generation of tunable microwave and millimetre-wave signals is proposed and verified experimentally. The experimental setup is based on an optical phase modulator and a simple uniform fibre Bragg grating. The generated signal is of high quality and can be tuned.

Periodic textures for enhanced current in thin film silicon solar cells

AbstractFor thin film silicon solar cells it is vital to increase the optical path of light in the absorber because this allows for thinner cells with better stability and higher production throughput. We discuss the effect of periodically textured interfaces for the case of thin film silicon solar cells in n-i-p configuration using embossed plastic substrate which allows us studying the effect of a wide range of random or periodic textures. Due to the moderate thickness of the individual layers the texture is carried into each interface with a high degree of conformity even for the front contact which is the last layer. Solar cells on periodic structures show excellent performance; in a microcrystalline cell on a simple sinusoidal grating we achieved a gain in current density of 30%. Furthermore, the periodicity serves as a useful tool for the study of light management because the underlying phenomena like diffraction or grating coupling to plasma excitations of the metallic back reflector are governed by a relatively low number of well defined parameters like the periodicity and the amplitude of the grating.

Polymeric nanofeatures of 100 nm using injection moulding for replication

The filling efficiency of replicated nanofeatures made with polymers using injection moulding with various moulding factors was investigated. Two simple calibration gratings with nanofeatures of 105 nm and 130 nm, respectively, were used as mould inserts to replicate nanofeatures by means of a custom-made injection machine. Nanofeatures made with polypropylene (PP), polystyrene (PS), polycarbonate (PC) and polymethylmethacrylate (PMMA) were tested because of differing viscosities and shrinkages in polymers. The results show that the surface configuration of the polymeric nanofeatures with low melt viscosity could be moulded well, while nanofeatures with high melt viscosity exhibited higher tolerance in depth. However, the moulding depth of nanofeatures could be effectively improved when mould temperature and forming pressure were higher. It was found that the reproducibility of the nanofeatures in large numbers could be replicated well after melt and mould temperatures became stable during moulding.

Full field phase imaging using a harmonically matched diffraction grating pair based homodyne quadrature interferometer

In this letter, the authors present a novel quadrature interferometry method based on the use of a harmonically matched shallow grating pair. Unlike a simple beam splitter or single shallow grating, the grating pair can confer a nontrivial interference phase shift (other than 0° or 180°) between the output ports of the interferometer. Using the grating pair as the beam splitter/combiner, the authors implement a homodyne quadrature full field phase interferometer and demonstrate the system’s capability to acquire phase and amplitude images.

Measurement of chloride-ion concentration with long-period grating technology

A simple and low-cost long-period fiber grating (LPG) sensor suited for chloride-ionconcentration measurement is presented. The LPG sensor is found to be sensitive to therefractive index of the medium around the cladding surface of the sensing grating, thusoffering the prospect of development of practical sensors such as an ambient index sensor ora chemical concentration indicator with high stability and reliability. We measured chlorideions in a typical concrete sample immersed in salt water solutions with different weightconcentrations ranging from 0% to 25%. Results show that the LPG sensor exhibited alinear decrease in the transmission loss and resonance wavelength shift when theconcentration increased. The measurement accuracy for the concentration of salt in watersolution is estimated to be 0.6% and the limit of detection for chloride ions is about0.04%. To further enhance its sensitivity for chloride concentrations, we coated amonolayer of colloidal gold nanoparticles as the active material on the gratingsurface of the LPG sensor. The operating principle of sensing is based on thesensitivity of localized surface plasmon resonance of self-assembled gold colloids onthe grating section of the LPG. With this method, a factor of two increase inthe sensitivity of detecting chemical solution concentrations was obtained. Theadvantages of this type of fiber-optic sensor are that it is compact, relatively simple toconstruct and easy to use. Moreover, the sensor has the potential capability foron-site, in vivo and remote sensing, and it has potential use as a disposable sensor.

Design of tungsten complex gratings for thermophotovoltaic radiators

The concept of a kind of complex grating based on superposition of two simple binary gratings is described for potential application as thermophotovoltaic (TPV) radiators. The grating with one-dimensional microstructured surface is relatively easy to fabricate and the predicted directional-spectral emittance exhibits apparent enhancement over simple grating structures. Specifically, the emittance of the complex grating has a wider peak in the spectral region where the quantum efficiency of TPV cells is high. This enhancement can be explained by the excitation of surface plasmon polaritons coupled with the grating microstructures. At longer wavelengths, the emittance remains low to reduce the radiative heat transfer from the radiator to the TPV cells by low-energy photons that do not produce any photocurrent. Calculations using the rigorous coupled-wave analysis demonstrate that the emittance peak is insensitive to the direction, suggesting that the proposed structure may be well suitable for TPV applications.

Cellular cardiomyoplasty using embryonic cardiomyocytes and adult stem cells

We describe an extension of the method of Myers et al. (1982) to measure with high precision the uniformity of contractile motions that occur between sarcomeres in the isolated cardiac muscle cell (guinea pig and rat). The image of the striations, observed with modulation contrast microscopy, was detected by a linear array of photodiodes. Sarcomere length was measured greater than 500/s from the frequency of the array's video signal at two selectable regions of the cell. A precision test grating demonstrated that method resolves known differences in the spacing between two contiguous striations to +/- 0.01 micron and that the effects of image translation and microscopic resolution are minor. The distribution of striation spacing appears to be discrete in isolated segments of the cell, and patches of fairly uniform length can be identified that are laterally contiguous. When electrically triggered, contraction is synchronous and the sarcomeres shorten and relengthen smoothly. The contrast between the striations is transiently enhanced during relengthening, an indication that the contracting cell can not be treated as a simple grating. Pauses that occur during late in relengthening (and transient contractile alternans) are characterized by very synchronized activity. These forms of irregular contractile behavior are not explained by desynchronization of a mechanism of release of intracellular calcium. A companion article describes application of the technique to study the nonuniform motions that occur between sarcomeres.

Improving full-duplex radio-on-fiber transport system performance by employing an optical single sideband/vestigial sideband filter

A full-duplex radio-on-fiber (ROF) transport system based on wavelength division multiplexing (WDM) and optical add-drop multiplexing techniques, as well as employing a simple fiber Bragg grating (FBG) filter at the receiving site, is proposed and demonstrated. Over a long-haul fiber link, good transmission performance is obtained in our proposed systems. Since our proposed system does not use a sophisticated optical single sideband (SSB) transmitter, it reveals a prominent alternative with advantages in simplicity and cost. Such a proposed full-duplex ROF transport system is suitable for the long-haul microwave optical link.

Impact on Diffraction Efficiency of Binary Optical Elements for Spot Exposure Superposing Ratio

In fabrication of free form optical surfaces, spot exposure is a typical mode in laser photolithography. Proper selection of spot exposure superposing ratio is the key to optimize etching quality and writing speed. In this paper, a new composite diffractive grating structure is modeled for the influence on diffraction efficiency of binary optical elements caused by spot exposure superposing ratio. Composite grating can be transformed into one simple rectangular grating and two triangular gratings. Angular spectrum theory is used to describe the mathematical expression of the composite diffractive grating. Through simulation analysis, the relationships between diffraction efficiency and the other three items as grating depth, spot radius, and blaze wavelength, has been shown. Simulation results indicate that shape error caused by spot exposure ratio impact the diffraction efficiency greatly in deep etching, and the transmittance will reach maximal point approximately while spot exposure ratio is over 60%. This conclusion can be taken as the theoretical foundation for exposure frequency selection in laser photolithography figure generation.

A Nonlinear Optical Experiment: Stimulated Raman Scattering in Benzene and Deuterated Benzene

A nonlinear optical experiment is presented that utilizes a doubled Nd:YAG laser to produce stimulated Raman scattering (SRS) in benzene and deuterated benzene. A sampling arrangement is described that results in strong stimulated Raman emission involving symmetric CC and CH(CD) stretching modes in both C6H6 and C6D6. The wavelengths of the coherent beams are easily measured with a simple transmission grating and meter stick. Observations of both collinear and conical stimulated Raman beams are made and the reasons for these discussed. The measured frequencies permit calculation of CC and CH force constants in benzene.

High power spectrometer for the characterization of photovoltaic cells in a controlled atmosphere or vacuum

The complete description of a high power spectrometer, allowing for large area illumination of up to 10 cm2 in the wavelength range 200–800 nm with average power densities of 2 mW cm2, is provided. The setup employs a stationary 150 W xenon lamp used in conjunction with a simple stationary single monochromator grating. The photovoltaic sample under test is placed in a stainless-steel vacuum chamber allowing for operating pressures down to 10−6 mbar equipped with a quartz window and electrical connections. The entire vacuum chamber was placed on a moving arm allowing for positioning in a selected part of the diffracted beam of the monochromator. The typical spectral resolution was 12 nm cm−1 which gave bandwidths of 25 nm with a 2 cm sample width. The electrical characteristics of the photovoltaic device under test was measured using a source meter giving an experimental current sensitivity of 10 pA. We finally demonstrate the application of the instrumental setup for the characterization of a polymer based photovoltaic.

Two-color photorefractive effect in Mg-doped lithium niobate.

The two-color photorefractive response of near stoichiometric lithium niobate (SLN) doped with Mg above a critical threshold has been investigated. Striking differences as compared with non Mg-doped material were observed: The intermediate level in the two-color writing process has approximately a two orders of magnitude longer lifetime in SLN:Mg than in nominally undoped SLN, the grating is written in a shallower level but can be fixed via a simple thermal process and complementary electron-hole gratings are formed. It is proposed that the Fe impurity level moves from below the small-polaron level in nonMgO-doped material to above it, resulting in the increased lifetime of the small polaron. These changes are associated with a shift of the Fe from a Li site to a Nb site. The two-color sensitivity is higher than in the absence of MgO but the dynamic range is much lower.

Learning to discriminate simple sinusoidal gratings is task specific.

Learning transfer effects of a modified spatial frequency discrimination task with simple sinusoidal gratings on various untrained test tasks, testing performance in relative position, local width and global size discrimination, have been investigated. Six subjects were exposed to grating stimuli of varying spatial frequency and size but constant relative position while they had to respond only to spatial frequency. In the course of 7 consecutive days all subjects showed significant reduction of spatial frequency discrimination thresholds. Comparison of discrimination thresholds for the untrained test tasks, taken before and after the learning epoch, reveals complete learning transfer to spatial frequency discrimination with gratings of constant size and variable relative position, but complete lack of transfer when grating spatial frequency is shifted about one octave lower. Further, there is improvement of relative position discrimination and width discrimination of single luminance bars, which is not very specific for the learnt spatial frequency. Although size variation was part of the learning procedure, discrimination of global stimulus size did not improve. Generally, the observed scheme of learning transfer reveals that there is learning only for stimulus attributes that are behaviorally relevant in the learning task. The differential scheme of improvement and code usage in the test tasks strongly indicates involvement of higher stages capable of independent access to different coding domains, as well as attentionally guided attribute selection and suppression. Supported by other recent findings, it is suggested that discrimination learning can be understood as a higher level process of gradual refinement of code selection out of a rich code base provided by lower level stages.

Novel technique for long period gratings fabrication using broad spectrum ultraviolet source

AbstractA novel technique using broad spectrum UV light source and amplitude mask to write LPGs is presented. Based on this technique, writing LPGs becomes simple, flexible, and multiple fiber gratings could be written at same time. It can be used by industry to manufacture LPGs in large volume with very low cost. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 368–370, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10324

Hemispheric asymmetry in visual discrimination and memory: ERP evidence for the spatial frequency hypothesis.

Event related potentials (ERPs) were recorded during delayed discrimination of simple spatial frequency gratings in the high and low frequency bands. Analyses of the waveforms N170, P220, N310, P400, and slow wave (SW) indicated significant and regionally specific interaction of spatial frequency and hemisphere for N170 amplitude. This interaction was independent of memory conditions, and appeared to be in the opposite direction of what is predicted by the spatial frequency model of hemispheric asymmetry. Additional interactions between spatial frequency and hemisphere were observed for N310 in the encoding process (reference stimulus) and for SW in the retrieval process (test stimulus). The general hypothesis of an interaction of spatial frequency and hemisphere in visual cognition is supported, but the findings indicate caution in interpreting an increase in physiological measures as an indication of more efficient brain processing. Moreover, several stages of information processing may contribute to the asymmetry observed in behavioral studies, and hemispheric balance may change dynamically during the time course of processing.

Analysis and design of electromagnetic crystals in microstrip technology using a fibre grating model

Electromagnetic crystals (EC) in microstrip technology have been recently proposed as efficient Bragg reflectors. They are implemented by etching the ground plane or by drilling the substrate following a periodic pattern. In this paper we propose a simple and fast fibre Bragg grating (FBG)-based model for the synthesis and analysis of these microstrip ECs with sinusoidal patterns. The method rests on the analogy found between the frequency responses of a microstrip EC and a FBG. Based on this analogy, an equivalence relationship between the physical parameters of a microstrip EC and those of an equivalent FBG has been established. The equivalence assures that the frequency response of the microstrip EC is a down-shifted replica of the one corresponding to its equivalent FBG. This model avoids the use of the time-consuming electromagnetic calculations involved in the analysis of the microstrip ECs as well as the trial method employed until now in the synthesis, since a FBG response arises immediately from coupled-mode theory. At the same time the theoretical difficulties encountered in the formal derivation of coupled-mode theory for microstrip ECs are also avoided by the equivalence relationship found.

Liquid-crystal phase grating based on in-plane switching

A simple phase grating is constructed by insertion of a liquid-crystal layer between two glass plates, upon one of which a pair of transparent interdigitated electrodes is formed. With a bias application, liquid-crystal molecules align themselves along the electric field lines, which are substantially parallel to the glass plates. By controlling the degree of this in-plane switching for the liquid-crystal molecules, one can generate various phase-shift distributions for the light passing through the device. The grating characteristics are altered accordingly. Versatile design and ease of fabrication are potential advantages of this device for some future applications.

Cortical responses to visual motion: complex human and tool motion compared with simple radial gratings

Cortical responses to visual motion: complex human and tool motion compared with simple radial gratings

Simple grating synthesis algorithm

Many authors (including this one) have suggested that the problem of synthesizing a grating from its reflection spectrum is significantly more difficult than the direct problem of obtaining the spectrum from the grating. Fortunately, this view is mistaken. One can synthesize a grating from any reflection spectrum by direct solution of the standard coupled-mode equations while simultaneously evaluating a simple integral to obtain the grating strength. This approach has the advantage of being based on a well-understood set of equations, making it amenable to physical interpretation, mathematical approximation, and generalization.

Perception of three-dimensional shape from texture is based on patterns of oriented energy

This paper presents empirical support for a new observer model of inferring three-dimensional shape from monocular texture cues. By measuring observers’ abilities to estimate the relative three-dimensional curvature along a textured surface from two-dimensional projected images, and concurrently examining the local spectral changes occurring in the projected image for various texture patterns, we have found that correlated changes in oriented energy along lines corresponding to the lines of maximum and minimum curvature of the surface are crucial for conveying the three-dimensional shape of the surface. Energy along these lines of maximum and minimum curvature can be used to compute the orientation of local surface patches. Texture patterns consisting of simple and complex sinusoidal gratings and plaids, and filtered noise were drawn onto a surface that was corrugated sinusoidally in depth about the horizontal axis and projected in perspective onto an image plane. The perceived relative surface curvature was reconstructed from measurements of local ordinal depth around a central fixation point at 12 different phases of the corrugation. Our results show that: (1) it is neither necessary nor sufficient to identify individual texture elements or texture gradients in order to extract the shape of the surface; (2) one-dimensional frequency modulation is insufficient for conveying complex three-dimensional shape. (3) Veridical ordinal depth is seen only when the projected pattern contains changes in oriented energy along lines corresponding to projected lines of maximum curvature of the surface. (4) For a surface corrugated in depth about the horizontal axis, this pattern of oriented energy arises from energy along the vertical direction in the global Fourier transform of the pre-corrugated pattern. (5) Local orientation changes across lines of minimum curvature can be also critical for conveying shape. (6) These correlated orientation changes along lines of maximum and minimum curvature are entirely lost in parallel projection. Hence texture is a useful cue for shape if the image is a perspective projection. (7) Only some natural textures will provide sufficient monocular cues to support veridical shape inferences, and this can be predicted from their global Fourier transforms.