Showing papers on "Diffraction grating published in 1994"

Electrically switchable volume gratings in polymer‐dispersed liquid crystals

Abstract: We report electrical switching of the diffraction efficiency in volume Bragg gratings written holographically in polymer‐dispersed liquid crystals (PDLCs). Scanning electron microscopy confirms the volume nature of the gratings and shows that they consist of periodic PDLC planes. The diffraction efficiency can be switched from a high value (∼50%) to a value near zero at fields ∼11 V/μm.

Phase-shifted fiber Bragg gratings and their application for wavelength demultiplexing

Abstract: It is shown that the transmission spectrum of a fiber Bragg grating can be tailored by incorporating single or multiple phase-shift regions during the fabrication process. Phase shifts open up narrowband transmission windows inside the stop band of the Bragg grating; transmitted wavelength can be changed by adjusting the amount of phase shift. As a specific application, we discuss how phase-shifted Bragg gratings can be used to make an all-fiber demultiplexer for multichannel lightwave systems. >

Algorithm for the rigorous coupled-wave analysis of grating diffraction

Abstract: Diffraction of light by periodic gratings is analyzed with a characteristic-matrix formalism based on a rigorous coupled-wave approach. This formalism is particularly convenient for modeling the diffraction by nonuniform periodic structures. In order to overcome numerical difficulties that are due to inhomogeneous eigenmodes, we propose a new algorithm that remains stable for gratings of any thickness. We obtain the stability by distinguishing in the computation the growing and the decaying inhomogeneous modes. Numerical examples and comparisons with previous results are given.

All-fibre Bragg grating strain-sensor demodulation technique using a wavelength division coupler

Abstract: A simple all optical fibre wavelength shift detection scheme, for use with fibre Bragg grating sensing systems, is described. The system is based on the use of a wavelength division coupler designed to exhibit a monotonic dependence of the splitting ratio on wavelength over a 50 nm range.

Holographic optical devices

Abstract: A holographic optical device (2) includes a light-transmissive substrate (2a), a first holographic optical element (Hs), and a second holographic optical element (Hr) that is laterally disposed on the substrate from the first holographic optical element. At least one of the holographic optical elements is a complex diffraction grating that can handle a multiplicity of plane and/or spherical waves arriving from a range of angles, and having a range of wavelengths. Applications for the device include a wavelength division multiplexer/demultiplexer, an image reconstructor, a beam expander/compressor, and a visor/head-up display.

Optical biosensor matrix

Abstract: A detection cell (2) which is used as a component of an optical biosensor comprises a transparent base plate (8) and a sample plate (4) on the base plate. The sample plate has a matrix of wells (6) extending through it to each receive a sample. The base plate includes a waveguiding film and a diffraction grating means to incouple an incident light field into the waveguiding film beneath a well to generate a diffracted light field to enable the detection of a change in the effective refractive index of the waveguiding film.

Homogeneous layer models for high-spatial-frequency dielectric surface-relief gratings: conical diffraction and antireflection designs

Abstract: The validity of various homogeneous layer models for high-spatial-frequency rectangular-groove (binary) dielectric surface-relief gratings is examined for both nonconical and conical diffraction. In each model the grating is described by a slab of uniaxial material with its optic axis parallel to the grating vector. The ordinary and principal extraordinary indices of the slab depend on the grating filling factor, the substrate and cover refractive indices, and the ratio of the wavelength to the grating period. These indices can be determined by solving two transcendental equations. Higher-order indices are defined as the exact solution to these equations. Second-order indices (second-order dependence on the wavelengthto- period ratio) and first-order indices (no dependence on the wavelength-to-period ratio) are defined by approximate solutions to these equations. Layer models using higher-order and second-order indices are shown to be accurate for high-spatial-frequency gratings, even at wavelength-to-period ratios near the onset of higher-order propagating diffracted waves. These models are used to design example antireflecting gratings on silicon substrates, including designs for conical incidence. All designs are evaluated and optimized by exact rigorous coupled-wave analysis.

Talbot-vonLau Atom Interferometry with Cold Slow Potassium

Abstract: A dc hot thermal and an ac-modulated cold slow potassium beam copropagate and pass through an atom de Broglie--wave interferometer, consisting of a sequence of three microfabricated diffraction gratings. Talbot-vonLau interference fringes are formed and sensed by measuring transmission with a hot wire as a function of grating position. The hot beam produces diffraction-limited shadow Moir\'e fringes, while the longer de Broglie wavelength slow beam produces interference fringes with high visibility at the fifth and sixth spatial harmonics of the shadow Moir\'e fringes.

Novel writing technique of long and highly reflective in-fibre gratings

Abstract: A novel method for writing fibre Bragg gratings is introduced. The method consists of translating a UV beam along the fibre through a phase mask. Because the phase mask and the fibre are held together, the phase of the interference pattern generated by the mask is not altered when the beam is translated along the mask. Gratings up to 1.5 cm long, limited by the mask dimensions, and with 98.5% reflection have been written in that way.

Wavelength multiplexer based on SiO/sub 2/-Ta/sub 2/O/sub 5/ arrayed-waveguide grating

Abstract: The characteristics of a wavelength multiplexer based on an arrayed-waveguide grating are carefully investigated by using the grating theory and related experiments. A 28-channel multiplexer is designed and fabricated as SiO/sub 2/-Ta/sub 2/O/sub 5/ waveguides on a 1 cm/spl times/2 cm substrate. The designed wavelength channel spacing of 1 nm is obtained. The crosstalk to an adjacent channel is -15 dB. The measured minimum loss is 4.2 dB, which is composed of 3.4 dB excess loss and 0.8 dB grating loss. >

Optical pulse amplification using chirped Bragg gratings

Abstract: Chirped Bragg gratings are used both for stretching and compressing of ultrashort optical pulses in a chirped pulse amplification system, so that even femtosecond pulses can be stretched and recompressed back to their initial shape and duration When used in chirped pulse amplification systems instead of bulk diffraction grating stretchers and compressors, Bragg gratings offer unprecedented compactness, robustness and system efficiency

A four-element fiber grating sensor array with phase-sensitive detection

Abstract: A four-element, time-division multiplexed, fiber grating sensor array operating between the wavelengths of 1280 and 1310 nm was constructed and tested. The array consists of a 1300-nm edge-emitting LED that illuminates four gratings spaced five meters apart. Each grating in the array reflects a spectrally narrow-band (0.3-0.5 mn) wavelength region. Measurand-induced changes in the grating period change the wavelength of the light reflected by each grating. The wavelength shifts are converted to phase changes by routing the reflected signal through a nearly path balanced fiber Mach Zehnder interferometer. The minimum detectable strain was measured to be as low as 2 nanostrain//spl radic/(Hz) for frequencies greater than 10 Hz. >

Fibre dispersion compensation using a chirped in-fibre Bragg grating

Abstract: A linearly chirped in-fibre Bragg grating has been used to compensate for the pulse broadening arising from negative group-delay dispersion in single mode optical fibre. Numerical simulations of the fibre and the Bragg grating produce good agreement with the experiment.< >

Multilayer-coated diffraction gratings: differential method of Chandezon et al. revisited

Abstract: I report significant improvements to the differential method of Chandezon et al. [ J. Opt. Soc. Am.72, 839– 846 ( 1982)]. The R-matrix propagation algorithm is used to remove completely the previously existing limitations on the total coating thickness and on the total number of coated layers. I analyze the symmetry properties of the eigenvalue problem that arises in the differential formalism and use them to speed up the numerical computation. The time needed for computing the eigensolutions of coated gratings in a conical mount is reduced to little more than what is needed for gratings in a classical mount. For gratings with dielectric coatings or gratings with symmetrical profiles the need to invert certain matrices appearing in the formalism is eliminated. Numerical results show that it is possible to make nearly 100% efficient surface-relief reflection gratings in a Littrow mount by the use of dielectric materials only.

Holographic optical devices for the transmission of optical signals of a plurality of channels

Abstract: A holographic optical device includes a light-transmissive substrate, a first holographic optical element, and a second holographic optical element that is laterally disposed on the substrate from the first holographic optical element At least one of the holographic optical elements is a complex diffraction grating that can handle a multiplicity of plane and/or spherical waves arriving from a range of angles, and having a range of wavelengths Applications for the device include a wavelength division multiplexer/demultiplexer, an image reconstructor, a beam expander/compressor, and a visor/head-up display

Laser beam mode selection by computer generated holograms

Abstract: Preface Introduction Modes of Coherent Beams Modal Beams in Scalar Approximation Excitation of Modes in Optical Fibers and Laser Cavities Complex Eikonal Method Amplitude-and-Phase Relations for Free-Space Propagation of Mode Beams Gaussian and Bessel Modes Modans as the Computer Generated Holograms Physical Observability of Modes and the Concept of Modans Multifocal and Multibeam Holographic Optical Elements Modans on Crossed Diffraction Gratings Generating Functions Method for the Synthesis of Modans Digital Holography Methods for Generation of Modans Coherent Modes Selection Optical Stages for Mode Selection with the Aid of Modans Light Beam Formation with Required Composition of Modes The Analysis of Modal Composition in Coherent Light Beams Modal Spectral Transforms Modan's Performances Perturbations of Modal Functions Modal Composition in Presence of Perturbations Perturbation Matrix Evaluation in the Case of Sampling the Modal Functions Energy Efficiency of Modans Accuracy of Analysis or Formation of Coherent Beams Accuracy of Modal Spectral Transforms Modan's Development and Experimental Investigation Computer Generation of Modans by Diffractive Optics Technology Measurements of Efficiency and Operating Performances of Single-Beam Modans Correlative Investigations of Two Beam Modans Measurements of Modal Power Distribution in Graded-Index Fibers Mode-Selective Excitation of the Fiber with the Aid of Modans Tolerances for Launching Conditions in Modan-Aided Excitation of the Fiber Fiberoptic Sensors and Communications with Mode Selection Fiberoptic Communication Line with Modal Multiplexing Mode-Selective Fiber Optic Sensors Modan Perturbation Effects in Sensor Operation Fiber Optic Sensor of Pressure with Reflective Computer Generated Optical Elements Experimental Investigation of Microshift Sensor Employing Modans Appendices: The Proof of the Properties of Orthogonal Modes Sinusoidal Modes in a Real Form Analytical Equations for Gaussian Modes Excitation in Optical Fibers and Cavities Random-Fields Expansions with Decorrelated Coefficients References. Index.

Development of a prototype gas sensor using surface plasmon resonance on gratings

Abstract: A good basis has been established for the development of a prototype gas sensor using the phenomenon of surface plasmon resonance. By exciting a surface plasmon on a metallic diffraction grating that is twisted azimuthally so its grooves are not perpendicular to the plane of incidence, and with suitable choice of input and output polarization, a resonance maximum is detected (as opposed to the usual resonance minimum). the operation of the sensor is based on the measurement of this resonance maximum on a background of weak signal and incorporates a sensing head made remote from both the source and detector by means of fibre optics. Its use is demonstrated by sensing remotely the condensation of ≈0.9 nm of isopropyl alcohol onto a silver-coated grating surface.

Efficient reflective multiplexer arrangement

Abstract: The efficiency and freedom from cross-talk achieved by transmissive multiplexer/demultiplexer devices is achieved in a reflective multiplexer/demultiplexer device. The reflective geometry avoids the large size devices necessitated by transmissive geometries handling a large number of optical channels. The reflective geometry also avoids long bends in waveguides used in optical gratings in high efficiency transmissive geometries. Integrated optical multiplexers/demultiplexers in accordance with this invention comprise a plurality of waveguides for carrying unmultiplexed optical signals interleaved with a plurality of waveguides for carrying multiplexed optical signals. The two pluralities of waveguides are connected to the boundary of a free space region. The pluralities of waveguides communicate through the free space region with an optical grating comprising a plurality of waveguides each terminated in a reflective element. The length of each waveguide in the grating differs from the lengths of adjacent waveguides in the grating by a predetermined amount to introduce predetermined path length differences for the optical signals traveling in the waveguides of the grating. Unmultiplexed optical signals, introduced into one or more of the input waveguides for carrying unmultiplexed optical signals, travel through those waveguides into the optical grating and are reflected towards one or more of the output waveguides for carrying multiplexed optical signals. When the device is used as a multiplexer in this fashion, it causes the optical signals introduced into the device to be multiplexed together and appear in a predetermined one or more of the waveguides for carrying multiplexed signals. Multiplexed optical signals introduced into one or more of the waveguides for carrying multiplexed signals are directed through the free space region to the optical grating and are reflected toward one or more of the output waveguides for carrying unmultiplexed optical signals. When the device is used as a demultiplexer in this manner, input signals which comprise a mixture of optical frequencies are demultiplexed so that the individual optical frequencies are separated from one another and appear at one or more predetermined waveguides for carrying unmultiplexed optical signals.

Digitally tunable channel dropping filter/equalizer based on waveguide grating router and optical amplifier integration

Abstract: We demonstrate a novel and powerful device that permits individual and simultaneous control of all the wavelength channels in a WDM system. The device is based on the monolithic integration of two identical waveguide grating routers with semiconductor optical amplifiers. By biasing appropriately the individual amplifier, each WDM channel can be amplified, detected or modulated. The device exhibits a channel bandwidth of 60 GHz, a channel spacing of 195 GHz and a crosstalk of /spl minus/19 dB. >

Programmable fiber optic delay line

Abstract: This paper describes a novel programmable delay line that generates up to 50 ns of true time-delay in discrete 10 ns intervals. The delay line consists of an externally modulated wavelength tunable fiber laser and a six-element wavelength multiplexed fiber Bragg grating array, with the grating spacing set to yield the desired delay. >

Narrow bandwidth volume holographic optical filter operating at the Kr transition at 1547.82 nm

Abstract: We describe and characterize a narrow bandwidth volume holographic optical filter operating at the Krypton transition line (1s2‐2p8) at 1547.82 nm, which corresponds to the center wavelength of the proposed International Telecommunications Union wavelength standard. A reflectivity of 98% and a bandwidth full width at half‐maximum of 0.18 nm are measured. The filter exhibits clean sideholes with a −20 dB optical response 0.5 nm away from the peak. The filter can be fabricated and operated with an absolute wavelength precision better than 0.005 nm.

Analysis of rectangular waveguide-gratings for amplifier applications

Abstract: A slow-wave structure composed of a grating inside a rectangular waveguide is analyzed. This type of slow-wave structure is examined for use in a low-voltage amplifier application with a sheet electron beam. Dispersion curves, mode field profiles, and taper designs for the waveguide-grating are presented. The amplifier application places stringent requirements on the taper sections that match the smooth waveguide to the waveguide-grating with minimal reflection. >

Optically generated liquid crystal gratings

Abstract: Optically controlled planar orientation of liquid crystal molecules with polarized light is used to make phase gratings in liquid crystal media. The electro‐optical properties of liquid crystal linear and chirped gratings are discussed.

12 frequency WDM laser based on a transmissive waveguide grating router

Abstract: A digitally tunable laser that is based on a 12*12 waveguide grating router integrated with semiconductor optical amplifiers is demonstrated. The device emits on 12 wavelengths spaced exactly by 3.2 nm. ASE and sidemodes are suppressed by more than 40 dB. This laser is suitable for simultaneous multifrequency operation.

Secure optical identification method and means

Abstract: A method of providing secure identification for an article, including: providing on the article a diffraction grating strip including a pattern of a series of diffraction grating elements, each the diffraction grating element to diffract light, from a light source, in one of at least three selected different planes; serially illuminating the diffraction grating elements, detecting changes in plane of diffracted light as the diffraction grating elements are serially illuminated, and generating first information representative of the changes in plane; storing the first information representative of the changes in plane; subsequently, serially illuminating the diffraction grating elements, detecting changes in plane of diffracted light as the diffraction grating elements are serially illuminated, and generating second information representative of the changes in plane; and then, comparing the first and second information to determine the authenticity or not of the article. A coded pattern may be placed on the diffraction grating strip, with one of the coded pattern and the pattern of diffraction grating elements precessing with respect to the other and the coded pattern is read, stored, and compared as part of the first and second information, as above.

Resonance properties of dielectric waveguide gratings: theory and experiments at 4-18 GHz

Abstract: The guided-mode resonance characteristics of dielectric waveguide gratings are presented. The sharp resonance features predicted theoretically are experimentally observed in the microwave region (4-18 GHz) and are shown to be in agreement with theory. >

High-Energy Transmission Grating Spectrometer for the Advanced X-ray Astrophysics Facility (AXAF)

Abstract: The High Energy Transmission Grating Spectrometer (HETGS) is one of the scientific instruments being developed for NASA's Advanced X-ray Astrophysics Facility (AXAF), scheduled for launch in 1998. The HETGS will be capable of measuring spectra with high resolution and sensitivity from a variety of compact and slightly extended cosmic X-ray sources. In this paper we describe the overall design of the HETGS and its expected scientific performance. The HETGS consists of two arrays of gold grating elements (High-Energy gratings [HEGs] and Medium-Energy Gratings [MEGs] which are optimized for the energy ranges 0.8-10 keV [HEG] and 0.4-5 keV [MEG]). The details of the grating elements and their fabrication methods are described in Schattenburg et al. (this conference). The gratings are mounted on a support plate which can be inserted immediately behind the AXAF telescope assembly. X-rays diffracted by the gratings are dispersed onto the focal plane detector strips which are components of either of the two AXAF imagers (the HRC or ACIS). The two kinds of gratings are oriented at a slight angle with respect to each other so that the dispersed spectra form a shallow 'X' on the readout device. The gratings and detectors are mounted on a Rowland torus to correct for most of the optical aberrations. The grating-detector combination achieves resolving powers (E/(Delta) E) as high as 1000 at some energies, and has significant effective area (10-200 square cm) for all energies 400 eV < E < 10 keV.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical transmission and receiving module and optical communication system using the same

Abstract: A bidirectional optical transmission and receiving module of a simplified structure and an optical communication system using the same. The optical transmission and receiving module includes a light emitting device and a light receiving device disposed with in a same package having an opening covered with a cover glass sheet, and a holographic diffraction grating provided on a top or lower surface of the glass sheet. In transmitting operation, a light beam emitted from the light emitting device passes through the diffraction grating to be concentrated onto an end face of an optical fiber by a lens. In receiving operation, a received light beam emanated from the end face of the optical fiber reaches the diffraction grating via the lens to be thereby diffracted. A plus-sign primary diffracted light beam resulting from the diffraction is concentrated onto a light detecting surface of the light receiving device. Signal as transmitted through the optical fiber in the form of signal light can thus be received.

Diffraction from optically written persistent plasma gratings in doped compound semiconductors

Abstract: We propose and demonstrate a new type of optical nonlinearity based on the properties of the DX center in doped compound semiconductors. We report measurements on samples of AlGaAs:Si which were exposed to interfering laser beams and find diffraction from a large, persistent refractive index change associated with the well‐known persistent photoconductivity effect in this material. The new effect is shown to exhibit a refractive index change 30 times larger than that of conventional photorefractive materials. We explain the origin of the refractive index change in terms of the plasma effect and show that its expected magnitude is consistent with our observations.

Coupling coefficient modulation of waveguide grating using sampled grating

Abstract: A new method of modulating the coupling coefficient along the waveguide is proposed. The effect of our method, which uses a nonperiodic sampled grating, is demonstrated both theoretically and experimentally. A Bragg filter is fabricated on an InGaAsP/InP wafer utilizing the method, and the sidelobe level is shown to improve by 7 dB compared with a conventional filter with a uniform coupling coefficient. The experimental results are found to be in good agreement with calculations. >