Showing papers on "Diffraction grating published in 1990"

Guided-mode resonances in planar dielectric-layer diffraction gratings

Abstract: The guided-mode resonance behavior of the evanescent and propagating fields associated with an unslanted, planar diffraction grating is studied by means of the rigorous coupled-wave theory. For weakly modulated gratings, the condition on the guided-mode wave number of the corresponding unmodulated dielectric-layer waveguide may be used to predict the range of the incident angle or wavelength within which the resonances can be excited. Furthermore, the locations of the resonances are predicted approximately by the eigenvalue equation of the waveguide. As the modulation amplitude increases, the location and shape of the resonances are described in detail by the rigorous coupled-wave theory. The results presented demonstrate that the resonances can cause rapid variations in the intensity of the external propagating diffracted waves.

Arrayed-waveguide grating for wavelength division multi/demultiplexer with nanometre resolution

Abstract: A novel diffraction grating based on arrayed channel waveguides has been proposed for nanometre-spaced wavelength division multiplexing. A grating with a diffraction order of 20 has been fabricated using a composite glass waveguide. A wavelength resolution of 0.63 nm has been achieved in the 1.3 μm wavelength band.

Efficient mode conversion in telecommunication fibre using externally written gratings

Abstract: A novel approach to grating writing in optical fibres is described. This method involves writing the grating point by point through the side of the fibre using a UV laser source. Efficient mode conversion between forward propagating modes is demonstrated. The wavelength at which conversion occurs is determined by the periodicity of the grating. The grating can be designed to couple light at any wavelength of interest.

Theoretical analysis of waveguide second-harmonic generation phase matched with uniform and chirped gratings

Abstract: Theoretical analysis of second-harmonic generation (SHG) phase matched with gratings in a channel waveguide is presented from the viewpoint of device design. The analysis includes cases where efficiency is high and residual phase mismatch is involved. Nonlinear couples mode equations, with general expressions for effective SHG coefficients and coupling coefficients, are derived, solved, and discussed for uniform and chirped gratings. It was found that very high efficiency with broadened phase matching bandwidth can be simultaneously obtained with chirped gratings. The results are summarized in graphic data with normalized parameters, and guidelines and criterion for optimum design are given. >

All-fibre narrowband reflection gratings at 1500 nm

Abstract: The first demonstration of reflection gratings at around 1500 nm written into germanosilicate fibres using an ultraviolet laser is reported. Grating reflection coefficients up to 10% with linewidths around 1nm have been observed. By using one of these gratings, narrow linewidth operation of a 1500 nm Er3+ doped fibre laser has been achieved.

Laser‐induced surface acoustic waves and photothermal surface gratings generated by crossing two pulsed laser beams

Abstract: Gigahertz surface acoustic waves have been optically generated and detected on aluminum metallic films and silicon wafers using the laser‐induced grating technique. The acoustic velocities were in good agreement with those of Rayleigh waves. Photothermal surface gratings could also be detected as a nonpropagating component of the thermoelastic waves. The relaxation time of the grating was related to the thermal diffusivity of the material. A simplified theory for the generation of surface transient gratings containing acoustic waves is presented in addition to the experimental results.

Optical element and optical pickup device comprising it

Abstract: An optical element comprises a polarization diffraction element that is provided with diffraction gratings that are respectively mounted on different faces of a transparent substrate of a flat plate shape, which grating pitches are equal to each other and approximately equal to the wavelength of an incident light, and which grating lines are parallel. The thickness of the substrate is set such that the difference that occurs between the phases of the P and S polarizations of a light due to the grating, and the difference that occurs between the phases of the two polarizations when the light propagates through the substrate cancel out. The optical pickup device comprising the optical element includes a polarized light detecting unit provided with a diffraction grating which grating pitch is approximately equal to the wavelength of an incident light; a first photodetector that is mounted integrally and virtually in parallel with the substrate, and that receives a zero-order diffracted light produced by the diffraction grating; and a second photodetector that is located in the same plane as the first photodetector and mounted integrally with the first photodetector and the substrate, and that receives a first-order diffracted light produced by the diffraction grating.

Aztec surface-relief volume diffractive structure

Abstract: Surface-relief diffractive optical elements made interferometrically typically are recorded with the off-axis technique, for which the fringe maxima lie on planes that are predominantly perpendicular to the recording surface and with an intensity variation that is sinusoidal. Such a structure can be readily replicated by mechanical means. Volume diffractive elements, on the other hand, which result from beams propagating in opposite directions, have fringe planes that are predominantly parallel to the surface and, as such, cannot be mechanically replicated. A new type of surface-relief diffractive structure called Aztec is discussed here; it combines features of both off-axis and volume recording geometries, with the result being a phase-quantized, or terraced, surface-relief pattern. The groove profile, instead of being sinusoidal, resembles a stepped pyramid. This structure has been replicated by metal mastering and molding into plastic in the same manner as conventional embossed surface-relief elements, but the diffraction characteristics are typical of volume phase reflective structures. Light of a given wavelength is resonantly diffracted from steps that are a half-wavelength apart and with a bandwidth that is inversely proportional to the number of steps. Color control has been achieved by overcoating the step structure with a clear dielectric that shifts the resonant wavelength to a new value, depending on the index of refraction of the dielectric. Information content is less for the single-layered, but stepped, Aztec structure than for the usual multilayered volume diffractive element. Deep Aztec stepped gratings have also been fabricated by optical lithography, using multiple-mask techniques.

Analysis of grating surface emitting lasers

Abstract: An approach to the analysis of grating-coupled semiconductor lasers is presented. It is shown that there are only two resonant solutions when the grating has infinite extent. The solutions are either symmetric or antisymmetric about the center of the longitudinal coordinate system where the antisymmetric solution is nonradiating. The field in the grating layer is expressed in terms of grating eigenfunctions and rigorously matched to the boundary conditions at the waveguide interface. Solutions to the finite-length grating problem are expressed as linear combinations of the infinite-length solutions. It is shown that the two diffraction parameters in the coupled-wave equations are composed of sums and differences of the eigenvalues from the infinite-length problem. >

Method and an apparatus for measuring a displacement between two objects and a method and an apparatus for measuring a gap distance between two objects

Abstract: A method and an apparatus for measuring a displacement between two objects. Corresponding pairs of regions of the two objects each have at least one diffraction grating which generate two-dimensionally distributed diffracted light beams. These light beams are diffracted and caused to interfere with each other in the paired regions, whereby two-dimensionally distributed diffracted interference light beams are emitted. A light beam of a specific order is detected from each of these diffracted interference light beams, and is converted into a beat signal. The displacement is obtained in accordance with the phase difference between these beat signals.

Three-dimensional (vector) rigorous coupled-wave analysis of anisotropic grating diffraction

Abstract: The rigorous coupled-wave analysis of diffraction by grating(s) formed in general anisotropic media is reviewed and extended. The method is first applied to a single slanted phase and/or amplitude grating with general three-dimensional incidence of a plane wave. The regions external to the grating can be isotropic, uniaxial, or biaxial anisotropic. The cases of gratings in isotropic media and of the grating vector lying in the plane of incidence (scalar analysis) are obtained as limiting cases of this general analysis. Coupling between the two orthogonal polarizations vanishes in these limiting cases. The Bragg conditions for various combinations of ordinary (for isotropic and uniaxial) and extraordinary (for uniaxial) polarized waves are quantified. The analysis is then extended to multiple cascaded gratings and to volume-superposed gratings. Sample calculations are presented for single anisotropic gratings (a lithium niobate photorefractive hologram in air and an interdigitated-electrode-induced grating in an electro-optic crystal), for multiple cascaded gratings (a lithium niobate hologram with grating strength varying with thickness), and for superposed gratings (multiplexed hologram storage). Applications for this analysis include optical storage, switching, modulation, deflection, optical interconnects, beam splitting, beam combining, and data processing.

Spatial heterodyne spectroscopy: interferometric performance at any wavelength without scanning

Abstract: Spatial heterodyne spectroscopy (SHS) employing a two-beam dispersive interferometer producing a Fizeau fringe pattern having wavelength-dependent spatial frequencies is presented. The pattern is recorded on an imaging detector and Fourier transformed to recover the input stream. It is pointed out that spectrometers operating on the SHS principle can achieve the theoretical resolution limit of the gratings without scanning, retaining at the same time the large angular input tolerance and multiplexing properties of conventional scanning Fourier-transform spectrometers. Additionally, broad spectral coverages can be achieved, and field widening can be accomplished without moving parts.

Array generation with multilevel phase gratings

Abstract: The efficiency of phase gratings used for array illuminators can be improved by increasing the number of phase levels in computer-generated diffraction gratings. This is of interest to provide power to two-dimensional arrays of optical logic devices that are used for optical computing purposes. The theory and fabrication techniques are described, and the experimental performance of a four-level grating that produces a 5 × 5 array is presented.

Extended length embedded Bragg grating manufacturing method and arrangement

Abstract: An arrangement for forming an extended length light redirecting embedded grating in an elongated solid material optical waveguide includes a source that directs a coherent light beam of a frequency in the ultraviolet range in a primary path transversely toward the waveguide. A section of a diffraction grating extends through the primary path at a spacing from the waveguide, and the diffraction grating has a dimension normal to the primary path that exceeds the corresponding dimension of the primary path. Relative movement is effectuated between the waveguide and diffraction grating in unison, and the primary path, so that the light beam is diffracted at the diffraction grating into two mutually frequency shifted partial light beams propagating in diverging secondary paths. The partial light beams are caused to travel toward a shared location of the waveguide where they form an interference pattern that moves longitudinally of the waveguide but respective high intensity fringes of which extend through the waveguide at respective positions that are stationary relative to the waveguide to effect refractive index changes at such positions along an extended length of the waveguide.

Single-mode behavior of a circular grating for potential disk-shaped DFB lasers

Abstract: The reflectivity and the resonance condition of a circular grating for a disk-shaped distributed-feedback (DFB) laser are calculated. The periodicity and the position of the grating are so chosen that all of the reflections from each refractive index step are superimposed in-phase so as to be consistent with the resonant behavior of the fundamental mode wave. The calculated wave impedance (E/H) is almost purely imaginary in the central region of the grating for the higher modes. >

Semiconductor laser device

Abstract: PURPOSE:To manufacture a semiconductor laser for dynamic-single-longitudinal mode oscillations having optical wave guides of low loss which are suitable for integration by providing two channels of optical waveguide layers which are combined optically by diffraction gratings and performing current injection from a transverse direction to an active layer. CONSTITUTION:At least the 1st and 2nd optical waveguides 2 and 5 which are used for emitting light and for combining optically with an active layer 4 respectively are provided and both of them are combined optically by diffraction gratings 21 and 22 in which their degree are high, i.e., more than second order. In a dynamic-single-longitudinal mode oscillation laser that is manufactured in this way, carriers are injected horizontally from a transverse direction to the active layer 4 and then the carriers combine each other again in an active waveguide to emit light. Distributed feedback of light emitted is performed by the diffraction gratings of the 2nd optical waveguides 5 and its light combines optically with the 1st optical waveguide 2. Then it is induced and emitted in an active region by being reflected in the form of distribution. The waveguides of oscillating light become uniform and even when amplitude modulation takes place, they may oscillate at a stable, dynamic-single-mode.

Design of binary-phase and multiphase Fourier gratings for array generation

Abstract: A review and comparison of design procedures for binary-phase and multiphase Fourier gratings used as array generators is presented. Grating structures include one- and two-dimensional binary-phase Dammann gratings, general binary-phase gratings, quarternary-phase Dammann gratings, and kinoforms for which coherent and incoherent designs are considered. One coherent method of design is that due to Dammann, which involves the solution of a set of N nonlinear equations in N unknowns. Although Dammann’s method generates little error, it does not permit the explicit maximization of diffraction efficiency. To increase diffraction efficiency, Dammann’s method is modified such that diffraction efficiency is a design parameter. To ensure the existence of a grating that has high diffraction and generates the desired source array, the number of grating parameters are increased, and an incoherent design is considered. Simulated annealing is applied to the solution of this problem. Examples of grating design using the different techniques are presented.

All-optical switching of grating transmission using cross-phase modulation in optical fibres

Abstract: All-optical modulation of the transmission of a grating by picosecond laser pulses (1064 nm) in an optical fibre is demonstrated. The photosensitive grating is reflecting in the visible (514.5 nm) and the modulation is achieved by changing the refractive index through cross-phase modulation.

Miniaturized spectrometer employing planar waveguides and grating couplers for chemical analysis.

Abstract: Polymeric and metal oxide planar waveguides were used to demonstrate the potential of a miniature spectrometer. Multiwavelength light was transmitted through the substrate and coupled into the waveguide through a diffraction grating located at the substrate/waveguide interface. A second diffraction grating spatially dispersed the light propagated through the waveguide into component wavelengths for rapid analysis with a photodiode array detector. These results suggest that planar waveguides can be used to perform attenuated total internal reflection measurements in the visible and near-IR regions for chemical analysis of weak vibrational overtones and combination modes with effective path lengths of several millimeters.

Optical system with aberration suppression and optical head utilizing the same

Abstract: An optical system, that includes a light source having sufficient coherency to provide a predominate operating wavelength and a diffraction grating lens system to focus the light from the light source to an image point on the optical axis of the optical system, provides for change of position of the image point along the optical axis over a wide focusing range with suppression of generated aberration. In particular, the optical system comprises a light source both movable in the direction of the optical axis and capable of variation in wavelength and a diffraction grating lens system that condenses the beam from the light source onto an image point on and along the optical axis, which system is effective in cancelling out aberration generated due to movement of the light source. The spherical aberration associated with the movement of the image spot brought about by the movement of the light source or its equivalent in the optical system is cancelled out by the chromatic aberration generated by the diffraction grating created by adjustment the light source wavelength.

Diffraction grating apparatus and method of forming a surface relief pattern in diffraction grating apparatus

Abstract: A phase diffraction grating apparatus which is usable to generate an array of N spots, where n is an even integer, and the N spots are substantially equally spaced. The generation of an even number of spots is achieved using a translation symmetry in the grating design. Illustratively, the intensities of the N spots are substantially equal. Binary, multi-level, and continuous phase grating embodiments are disclosed. In the case of the multi-level and continuous embodiments, the uniform intensity is obtained using a reflection symmetry in the grating design.

Phase-shifted Moire grating fibre resonators

Abstract: A novel technique for fabricating fibre grating resonators is reported. Using this technique, a resonant filter response with a bandwidth of 0.04 nm (∼5 GHz) centred at 1535.1 nm and with a finesse of 20 has been obtained.

The Light Sword Optical Element-a New Diffraction Structure with Extended Depth of Focus

Abstract: This letter introduces a novel diffraction structure, the light sword optical element, with extended depth of focus.

High-efficiency single-order waveguide grating coupler.

Abstract: High-efficiency single-order grating coupling between an optical waveguided mode and a volume wave is achieved by means of a double-sided corrugated cover film that has phase-shifted undulations. A greater than 90% outcoupling ratio into air is demonstrated for weakly guiding ion-exchanged waveguides. The conditions for efficient coupling and the tolerances on them are established.

Boundary-element analysis of plane-wave diffraction from groove-type dielectric and metallic gratings

Abstract: A numerical approach based on the boundary-element method (BEM) is described for the analysis of plane-wave diffraction from groove-type gratings. First, the diffraction problem is exactly analyzed as a two-medium boundary-value problem. Further, for metallic gratings, a simple method in which an approximate boundary condition using the surface impedance is combined with the BEM is proposed. Both cases of the TE- and TM-wave incidences are systematically formulated. Numerical examples are presented for dielectric holographic gratings and metallic Fourier gratings, and the validity of the BEM and the effectiveness of the surface-impedance approximation are confirmed.

Constant-dispersion grism spectrometer for channeled spectra

Abstract: A new spectrometer design is presented in which the angular dispersion with respect to wave number is nearly constant. The spectrometer is a type of grism, a series combination of grating and prism, in which the constant parts of the dispersion terms add to one another but the slopes of the dispersions tend to cancel one another. A systematic method is presented for optimizing the grating and prism parameters. A cross-dispersion technique is presented, eliminating overlapping grating orders. A design example is given for the visible region from 0.45 to 0.80 micron with essentially constant wave-number dispersion and a peak transmission of approximately 95 percent. This grism is well suited for measuring channeled spectra as generated by an optical stellar interferometer.

X-ray/VUV transmission gratings for astrophysical and laboratory applications

Abstract: This paper describes the techniques used to fabricate deep-submicron-period transmission gratings for astrophysical and laboratory applications, with special attention given to the major steps involved in the transmission grating fabrication. These include the holographic lithography procedure used to pattern the master transmission grating, the fabrication of X-ray mask, the X-ray lithography step used to transfer the X-ray mask pattern into a substrate, and the electroplating of the substrate to form the final grating pattern. The various ways in which transmission gratings can be used in X-ray and VUV spectroscopy are discussed together with some examples of experiments reported in the literature.

Viscoelastic spatial light modulators and schlieren-optical systems for HDTV projection displays

Abstract: Thin metallized viscoelastic layers coated by a mirror electrode and addressed by an active matrix are proposed for ue in schlieren-optical light-valve projection systems for high-definition television (HDTV). Previously, the deformation behavior of such viscoelastic spatial light modulators (VSLMs) was analyzed extensively by use of the theories of electrostatics, linear viscoelasticity, and plate bending. In the meantime, most of the theoretical results could be qualitatively confirmed by interferometric deformation measurement. Of the many predictions derived from the theoretical treatment, only the deformation patterns to be expected with one bright line and with two bright lines separated by a dark line are demonstrated here. Furthermore, the number of grating periods per picture element as well as the ratio of electrode width and grating period are discussed in view of resolution limits. In addition, three concepts of full-color schlierenoptical projection systems for the above reflective VSLMS are described and compared. All three systems contain a light source, three spatial light modulators (one for each primary color), and a projection lens. The first concept is based on a very small light source and a correspondingly small stop; in the second concept, mirror bars as in the well-known Eidophor projector are employed; and parallel light is assumed in the third concept.

Laser catheter having diffraction grating for beam shaping

Abstract: A laser catheter includes an elongated tubular catheter body, one or more optical fibers extending through the catheter body and a catheter window mounted at the distal end of the catheter body. The optical fibers direct laser energy through the catheter window for removal of biological material. A diffraction grating is formed on the proximal surface of the catheter window in alignment with laser energy from the optical fibers. The laser energy is modified by the diffraction grating to provide a desired spatial distribution pattern. Preferably, the diffraction grating provides a diverging beam suitable for forming a hole in an obstruction, the hole having a diameter that is approximately the same as the diameter of the catheter. The diffraction grating enables formation at the distal end of the catheter of a beam pattern having an elliptical or other noncircular cross section. In another embodiment, an optical fiber with a noncircular cross section is used to shape the laser radiation emitted from the laser catheter.

External grating tunable MQW laser with wide tuning range of 240 nm

Abstract: A very wide tuning range in an external grating tunable InGaAs/InGaAsP MQW laser is demonstrated. Second subband recombination occurs in MQW diode at high current density injection. This contributes to gain expansion and a tuning range of 240 nm was obtained when operated continuously (CW) at room temperature.