Showing papers on "Diffraction grating published in 2002"

Space-variant Pancharatnam-Berry phase optical elements with computer-generated subwavelength gratings.

Abstract: Space-variant Pancharatnam-Berry phase optical elements based on computer-generated subwavelength gratings are presented. By continuously controlling the local orientation and period of the grating we can achieve any desired phase element. We present a theoretical analysis and experimentally demonstrate a Pancharatnam-Berry phase-based diffraction grating for laser radiation at a wavelength of 10.6microm.

Radially and azimuthally polarized beams generated by space-variant dielectric subwavelength gratings.

Abstract: We present a novel method for forming radially and azimuthally polarized beams by using computer-generated subwavelength dielectric gratings. The elements were deposited upon GaAs substrates and produced beams with a polarization purity of 99.2% at a wavelength of 10.6 µm. We have verified the polarization properties with full space-variant polarization analysis and measurement, and we show that such beams have certain vortexlike properties and that they carry angular momentum.

Formation of helical beams by use of Pancharatnam-Berry phase optical elements.

Abstract: Spiral phase elements with topological charges based on space-variant Pancharatnam-Berry phase optical elements are presented Such elements can be achieved by use of continuous computer-generated space-variant subwavelength dielectric gratings We present a theoretical analysis and experimentally demonstrate spiral geometrical phases for infrared radiation at a wavelength of 106microm

Overlay alignment metrology using diffraction gratings

Abstract: Alignment accuracy between two or more patterned layers is measured using a metrology target comprising substantially overlapping diffraction gratings formed in a test area of the layers being tested. An optical instrument illuminates all or part of the target area and measures the optical response. The instrument can measure transmission, reflectance, and/or ellipsometric parameters as a function of wavelength, polar angle of incidence, azimuthal angle of incidence, and/or polarization of the illumination and detected light. Overlay error or offset between those layers containing the test gratings is determined by a processor programmed to calculate an optical response for a set of parameters that include overlay error, using a model that accounts for diffraction by the gratings and interaction of the gratings with each others' diffracted field. The model parameters might also take account of manufactured asymmetries. The calculation may involve interpolation of pre-computed entries from a database accessible to the processor. The calculated and measured responses are iteratively compared and the model parameters changed to minimize the difference.

Light Propagation in Periodic Media: Differential Theory and Design

Abstract: General Properties Basic Principles of the Differential Theory of Gratings Stacks of Gratings Fast Fourier Factorization (FFF) Method Maxwell Equations in Truncated Fourier Space Rigorous Coupled Wave (RCW) Method Coordinate Transformation Methods Gratings Made of Anisotropic Materials Crossed Gratings Photonic Crystals X-Ray Gratings Transmission Gratings Grating Couplers and Resonant Excitation of Guided Modes Differential Theory of Non-Periodic Media Fourier Factorization of Maxwell Equations in Nonlinear Optics Appendix I: The z-Dependence of the Total Field in Conical Diffraction Appendix II: Some Formulas about Toeplitz Matrices Appendix III: Expression of the Transverse Components of the Field in Terms of the Axial Ones Appendix IV: The Shooting Method in Matrix Notations List of Notations Index

Aspects of basic OCT engine technologies for high speed optical coherence tomography and light source and other improvements in optical coherence tomography

Abstract: An optical coherence tomography (OCT) system including an interferometer provides illuminating light along a first optical path to a sample and an optical delay line and collects light from the sample along a second optical path remitted at several scattering angles to a detector. In one embodiment, illuminating light is directed along a number of incident light paths through a focusing lens to a sample. The light paths and focusing lens are related to the sample and to both the incident light source and the detector. In another embodiment, a focusing system directs light to a location in the sample. A transmission grating or acousto-optic modulator directs light from the sample at an angle representative of the wavelength of the incident light on the transmission grating or acousto-optic modulator.

Electrically tunable collective response in a coupled micromechanical array

Abstract: We employ optical diffraction to study the mechanical properties of a grating array of suspended doubly clamped beams made of Au. The device allows application of electrostatic coupling between the beams that gives rise to formation of a band of normal modes of vibration (phonons). We parametrically excite these collective modes and study the response by measuring the diffraction signal. The results indicate that nonlinear effects strongly affect the dynamics of the system. Further optimization will allow employing similar systems for real-time mechanical spectrum analysis of electrical waveforms.

A polydimethylsiloxane (PDMS) deformable diffraction grating for monitoring of local pressure in microfluidic devices

Abstract: In this paper, a novel optical method for monitoring of local pressure in microfluidic devices using a deformable diffraction grating is presented. A test device was fabricated with transparent silicone elastomer - polydimethylsiloxane (PDMS) - using the replica moulding technique. The moulded PDMS chip and a flat glass plate have a bonding interface, which defines a 2 mm×2 mm diffraction grating and a 200 µm wide, 20 µm deep microchannel. The grating consists of 5 µm wide, 2 µm deep rectangular grooves arrayed with a period of 10 µm. All the grooves are connected to the microchannel, and deformed by internal pressure. The optical response of the device to pressure ranging from -80 to 100 kPa is presented and compared with the theoretical prediction. It is also demonstrated that the test device can be used for measurement of air flow rates ranging from 0 to 0.3 ml min-1. The major advantages of this method are simple design and inexpensive fabrication. This method is not only desirable for flow characterization of microfluidic devices, but also opens up the possibility of producing new types of fibre-optic pressure sensor and pressure-driven optical modulator.

["Nematic liquid-crystal polarization gratings by modification of surface alignment"]

Abstract: The stylus of an atomic force microscope is used to scribe preferred directions for liquid-crystal alignment on a polyimide-coated substrate. The opposing substrate that comprises the liquid-crystal cell is rubbed unidirectionally, resulting in a twisted nematic structure associated with each micrometer-sized pixel. The polarization of light entering from the uniformly rubbed substrate rotates with the nematic director by a different amount in each pixel, and each of the two emerging polarization eigenmodes interferes separately. Two examples are discussed: a square grating that allows only odd-order diffraction peaks and a grating that combines rotation with optical retardation to simulate a blazed grating for circularly polarized light. The gratings can be electrically switched if used with semitransparent electrodes.

Long-period gratings in planar optical waveguides

Abstract: We present a theoretical analysis of light propagation in a four-layer planar waveguide that consists of a long-period grating (LPG) having a period of the order of 100 µm. By means of the coupled-mode theory, we show that such a structure is capable of coupling light from the fundamental guided mode to the cladding modes at specific wavelengths (resonance wavelengths) and thus results in sharp rejection bands in the transmission spectrum of the waveguide. Our numerical results show that the resonance wavelengths as well as the transmission spectrum can be significantly changed with the waveguide and grating parameters. A waveguide-based LPG should provide a useful approach to the design of a wide range of integrated-optic devices, including wavelength-tunable filters, switches, and environmental sensors.

Conformal grating electromechanical system (GEMS) for high-speed digital light modulation

Abstract: A diffractive optical MEMS device for spatial and temporal light modulation is described that is capable of high-speed digital operation. The device contains electromechanical ribbons suspended flat above a silicon substrate by a periodic series of intermediate supports. When actuated electrostatically, the ribbons conform around the support substructure to produce a grating. The device has optical switching times of less than 50 nsec, sub-nanosecond jitter, high optical contrast and efficiency, and reliable actuation in contact mode. The fine gray levels needed for digital imaging systems are produced by pulse width modulation.

Phenomenological theory of filtering by resonant dielectric gratings

Abstract: Using a phenomenological theory of diffraction gratings made by perturbing a planar waveguide allows us to deduce important properties of the sharp filtering phenomena generated by this kind of structure when the incident light excites a guided wave. It is shown that the resonance phenomenon occurring in these conditions acts on one of the two eigenvalues of the Hermitian reflection matrix only. As a consequence, we deduce a mathematical expression of the reflectivity and demonstrate that high-efficiency filtering of unpolarized light requires the simultaneous excitation of two uncoupled guided waves. Numerical examples are given.

Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings. I. Theoretical model

Abstract: Polarization properties and electro-optical switching behavior of holographic polymer-dispersed liquid-crystal (HPDLC) reflection and transmission gratings are studied. A theoretical model is developed that combines anisotropic coupled-wave theory with an elongated liquid-crystal-droplet switching model and includes the effects of a statistical orientational distribution of droplet-symmetry axes. Angle- and polarization-dependent switching behaviors of HPDLC gratings are elucidated, and the effects on dynamic range are described. A new type of electro-optical switching not seen in ordinary polymer-dispersed liquid crystals, to the best of the author’s knowledge, is presented and given a physical interpretation. The model provides valuable insight to the physics of these gratings and can be applied to the design of HPDLC holographic optical elements.

Compact Beam Expander with Linear Gratings

Abstract: Novel compact beam expanders that could be useful for applications such as providing light to flat panel displays are presented. They are based on a planar configuration in which three spatially linear gratings are recorded on one transparent substrate, so as to expand a narrow incoming beam in two dimensions. We present the design and recording procedures along with results, showing a relatively uniform intensity of the wide output beam. Such expanders can serve for illuminating flat panel displays.

Fabrication of dot matrix, comb, and nanowire structures using laser ablation by interfered femtosecond laser beams

Abstract: A line of periodic dot structure with an interval of 6.25 μm on a gold thin film was fabricated with a single shot of interfered femtosecond laser beams split by a diffraction beam splitter. The total length of the structure was 6 mm. In addition, dot matrix and comb structures were fabricated with transportation of samples at an arbitrary speed during the process. The samples worked as transmission and reflection gratings. In addition, nanowires were fabricated by peeling the comb structure, of which the thickness was 200 nm.

Temperature compensation of long-period fiber grating for refractive-index sensing with bending effect

Abstract: In this letter, we show that by properly mounting a bent long-period fiber grating (LPFG) on a suitable material, the temperature dependence of the resonance wavelength of the LPFG can be largely eliminated. With this method, we reduced the temperature sensitivity of the resonance wavelength of a typical LPFG by two orders of magnitude from -0.933 nm//spl deg/C to 0.008 nm//spl deg/C. The principle of using this mounted LPFG as a temperature-insensitive chemical concentration or refractive-index sensor is demonstrated.

Stable and efficient bloch-mode computational method for one-dimensional grating waveguides.

Abstract: We present a stable and efficient method for the Bloch-mode computation of one-dimensional grating waveguides. The approach uses the Fourier modal method and the S-matrix algorithm to remove numerical instabilities. The use of perfectly matched layers provide a high accuracy. Numerical results obtained for different lamellar grating waveguides and for both TE and TM polarizations illustrate the performance of the approach.

Design of a widely tunable modulated grating Y-branch laser using the additive Vernier effect for improved super-mode selection

Abstract: We present the design of a modulated grating Y-branch semiconductor laser containing a splitter and two multi-peak reflectors. High selectivity is achieved using the additive Vernier effect.

High efficient light-emitting diodes with antireflection subwavelength gratings

Abstract: A two-dimensional subwavelength grating (SWG) has been fabricated on a GaAlAs light-emitting diode (LED). The SWG is patterned by electron beam lithography and etched by fast atom beam with Cl/sub 2/ and SF/sub 6/ gases. The fabricated grating has 200 nm period and the tapered grating shape with aspect ratio of 1.38 to prevent reflection in the spectral region including 850 nm light emission. The emission is increased by 21.6% at the normal emission angle. The total emittance is increased by 60% with the SWG in comparison with that of the flat surface.

Photonic crystals for the visible range fabricated by autocloning technique and their application

Abstract: We fabricate photonic crystals for the visible range by the ‘autocloning’ technique, in which multilayers are stacked by an appropriate combination of sputter deposition and sputter etching. TiO2/SiO2 and Ta2O5/SiO2 are chosen as materials since they are transparent in the range and give a high contrast of refractive indices. The fabrication technique has flexibility regarding materials and size and is very reliable and reproducible even if the pitch is less than 0.2 μm. We also study the application of photonic crystals to birefringent elements such as waveplates and polarization selective gratings and experimentally verify that they are useful for optical pick-up systems.

Gain-flattening filter using long-period fiber gratings

Abstract: We propose a novel structure for a gain-flattening filter, in which a conventional long-period fiber grating (LPFG) and a phase-shifted LPFG are written closely. The transmission characteristics are investigated in the case of a closely arranged configuration. For the fabrication of a phase-shifted LPFG, a UV-trimming method to control the amount of the phase shift is examined theoretically and experimentally. Finally, a 67-mm-long LPFG gain-flattening filter designed for an erbium-doped fiber amplifier is fabricated and the measured spectrum is shown.

Finite-number-of-periods holographic gratings with finite-width incident beams: analysis using the finite-difference frequency-domain method.

Abstract: The effects of finite number of periods (FNP) and finite incident beams on the diffraction efficiencies of holographic gratings are investigated by the finite-difference frequency-domain (FDFD) method. Gratings comprising 20, 15, 10, 5, and 3 periods illuminated by TE and TM incident light with various beam sizes are analyzed with the FDFD method and compared with the rigorous coupled-wave analysis (RCWA). Both unslanted and slanted gratings are treated in transmission as well as in reflection configurations. In general, the effect of the FNP is a decrease in the diffraction efficiency with a decrease in the number of periods of the grating. Similarly, a decrease in incident-beam width causes a decrease in the diffraction efficiency. Exceptions appear in off-Bragg incidence in which a smaller beam width could result in higher diffraction efficiency. For beam widths greater than 10 grating periods and for gratings with more than 20 periods in width, the diffraction efficiencies slowly converge to the values predicted by the RCWA (infinite incident beam and infinite-number-of-periods grating) for both TE and TM polarizations. Furthermore, the effects of FNP holographic gratings on their diffraction performance are found to be comparable to their counterparts of FNP surface-relief gratings.

1-GHz-spaced 16-channel arrayed-waveguide grating for a wavelength reference standard in DWDM network systems

Abstract: We fabricated a 1-GHz-spaced 16-channel arrayed-waveguide grating (AWG) by using a new AWG configuration where the path of each arrayed waveguide winds backward and forward across a 4-in diameter wafer without crossing any other waveguides. The ultra-narrow (< 1 GHz) and stable transmission bands of this AWG can be used to construct a wavelength reference standard covering the S, C, and L bands in the dense wavelength-division-multiplexing network systems whose frequency deviation is /spl plusmn/160 MHz.

Large-scale resonance amplification of optical sensing of volatile compounds with chemoresponsive visible-region diffraction gratings.

Abstract: Micropatterning of the vapochromic charge-transfer salt, [Pt(CNC6H4C10H21)4][Pd(CN)4], on transparent platforms yields transmissive chemoresponsive diffraction gratings. Exposure of the gratings to volatile organic compounds (VOCs) such as chloroform and methanol leads to VOC uptake by the porous material comprising the grating lattice or framework, and a change in the material's complex refractive index, n. The index change is accompanied by a change in the degree of index contrast between the lattice and the surrounding medium (in this case, air), and a change in the diffraction efficiency of the grating. When a monochromatic light source that is not absorbed by the lattice material is employed as a probe beam, only changes in the real component of n are sensed. Under these conditions, the grating behaves as a nonselective, but moderately sensitive, sensor for those VOCs capable of permeating the porous lattice material. When a probe color is shifted to a wavelength coincident with the vapochromic charge-transfer transition of the lattice material, the sensor response is selectively amplified by up to 3.5 orders of magnitude, resulting in greatly enhanced sensitivity and some degree of chemical specificity. On the basis of studies at four probe wavelengths, the amplification effect is dominated by resonant changes in the imaginary component of the refractive index. The observed wavelength- and analyte-dependent amplification effects are quantitatively well described by a model that combines a Kramers-Kronig analysis with an effective-medium treatment of dielectric effects.

Rigorous electromagnetic analysis of dipole emission in periodically corrugated layers: the grating-assisted resonant-cavity light-emitting diode.

Abstract: We study the grating-assisted light-emitting diode, an LED design for high brightness based on a resonant cavity containing one- or two-dimensionally periodically corrugated layers (grating). We give in detail a generally applicable electromagnetic analysis based on the rigorous coupled-wave theory to calculate the extraction efficiency of spontaneous emission in a periodically corrugated layer structure. This general model is then specified on the grating-assisted resonant-cavity LED, showing simulated efficiencies of more than 40%.

Concept of multiorder multimode resonant optical filters

Abstract: The idea of implementing optical filters by coupling evanescent waves from several diffracted orders into multiple leaky waveguide modes is studied theoretically. Using a dielectric thin-film structure consisting of a coupling grating placed between adjacent waveguides, guided-mode resonance filters exhibiting multiple reflection peaks within a specified wavelength range can be obtained. These peaks originate in the resonant waveguide modes that are excited by the diffracted waves dispersed by the grating. It is shown that this device can be used to realize multiwavelength as well as wide-band spectral filters.

Piecewise linear spatial phase modulator using dual-mode micromirror arrays for temporal and diffractive fourier optics

Abstract: One embodiment of the present invention provides a spatial light phase modulator, which can perform piecewise linear phase modulation of a light beam. This spatial light phase modulator includes an array of movable micromirrors and an array of actuators. Each actuator of the array of actuators is movably coupled to one micromirror of the array of movable micromirrors and can move the micromirror both vertically and rotationally. Additionally, the present invention provides an optical function generator that is a femtosecond pulse shaper. This optical function generator includes a diffraction grating that disperses an input pulse into a dispersed spectrum, a lens assembly to focus the dispersed spectrum onto a micromirror array, and the micromirror array to provide spatial filtering to the dispersed spectrum to provide the filtered spectrum.