Showing papers on "Diffraction grating published in 1991"

Optical switch, multiplexer, and demultiplexer

Abstract: An N×N integrated optical interconnection apparatus capable of switching, multiplexing, or demultiplexing a large number of input and output wavelength channels achieves low levels of crosstalk and insertion loss. Two substantially identical N×M star couplers are connected by an optical diffraction grating comprising M unequal length waveguides spaced from one another by predetermined amounts. Each coupler comprises a dielectric slab defining a free space region between two periodic arrays of waveguides, each radially directed toward a virtual focal point. The arrays are configured so that their respective foci are located at a predetermined distance away from and outside the free space region to minimize phase errors caused by mutual coupling between adjacent waveguides. Specifically, the focal point of each array connected to each star coupler may be located so that it coincides with the phase center of the other array connected to each coupler. Residual phase errors may be reduced by appropriately setting the lengths of the waveguides in the optical grating between the two star couplers. The length difference between any two adjacent waveguides in the grating is not constant throughout the grating.

Artificial distributed-index media fabricated by zero-order gratings

Abstract: Zero-order gratings are grating structures with a period that is small compared with the wavelength of light. Only the directly transmitted or reflected light, the zero diffraction order, is nonevanescent and propagates in a distance from the grating. Thus the grating behaves like a slab of ordinary homogeneous material with an effective refractive index. By varying the material composition, i.e., by variation of the duty cycle of the grating, the effective refractive index can be changed. A grating with variable duty cycle therefore behaves like a material with distributed index. Based on such artificial materials, distributed-index elements are proposed. The physical principle is demonstrated with water waves.

Holographic diffraction grating patterns and methods for creating the same

Abstract: Decorative holographic diffraction grating patterns and methods of creating the same which provide low cost computer generation of two-dimensional holographic patterns and the generation of very large holographic patterns. In accordance with the method, the desired pattern is made up of a large plurality of individual spots, each spot comprising a holographic diffraction grating of a predetermined grating spacing and angular orientation for that spot. Variation of the angular orientation and/or grating spacing between spots and/or groups of spots provide the desired holographic effect. The spot locations may vary as desired, including locations drawing out a desired pattern, and two-dimensional orthogonal matrices in which a pattern is drawn by variations between spots or group of spots as in a raster scan type image. Various embodiments and methods for creating the same are disclosed.

Monolithic InP/InGaAsP/InP Grating Spectrometer for the 1.48-1.56 μm Wavelength Range

Abstract: We report a two‐dimensional grating spectrometer implemented in an InP/InGaAsP/InP planar waveguide for use in the low‐loss 1.5 μm wavelength fiber band. The spectrometer uses a single vertical‐walled focusing reflection grating to disperse 78 channels, spaced at 1 nm intervals, with diffraction‐limited resolution (∼0.3 nm) and a high channel isolation (≳19 dB). The spectrometer may be used such that it is insensitive to the state of the input polarization.

Fabrication of periodic domain grating in LiNbO3 by electron beam writing for application of nonlinear optical processes

Abstract: A novel technique for the fabrication of domain reversed gratings in LiNbO3 by means of direct electron beam writing at room temperature without any static bias is reported. The domain grating has been achieved not only at the – c surface of the crystal, but also for the entire region of the crystal. Such a structure should be useful for various nonlinear optic applications. A preliminary study on the second harmonic generation has also been demonstrated under the quasiphasematched condition with high efficiency.

Efficient optical elements to generate intensity weighted spot arrays: design and fabrication

Abstract: We report on the design and fabrication of fully 2-D surface relief diffraction elements that can split a single collimated beam into many beams in an arbitrary intensity distribution. These splitters were designed by computer using simulated annealing, and made into phase gratings by electron-beam lithography followed by plasma etching into quartz glass. Both two and four phase level gratings have been fabricated, allowing a wide range of uniform and weighted spot patterns to be generated. These grating elements have a measured diffraction efficiency of over 74%, with the beam intensity ratios accurate to within 1% of their target values.

Multiplexing fiber bragg grating sensors

Abstract: Bragg reflection gratings and out-coupling taps for sensors can be written holographically within the core of many commercial fibers available today. The gratings appear to be permanent and have been tested to temperatures in excess of 500°C. Quasi-distributed temperature, strain, pressure, chemical, and interferometric sensors can be made with the wavelength selective, reflection gratings, and taps. The fiber gratings, and the different types of sensors they can make, conveniently lend themselves to (wavelength-division multiplexing) WDM, (time-division multiplexing) TDM, and (frequency-division multiplexing) FDM types of multiplexing schemes. Instrumentation to detect the multiple sensors and measure their spectral shift for localized and quasi-distributed sensing is currently under development.

Narrow line semiconductor laser using fibre grating

Abstract: A narrow linewidth, low chirp, wavelength selectable, packaged alternative to current DFB structures has been developed which employs a fibre reflection grating incorporated in the fibre pigtail. The wavelength of the packaged device can be determined by the appropriate choice of fibre grating coupled to the antireflection coated facet of a 1.55μm BH laser. Linewidths of less than 50 kHz, sidemode suppression of more than 30 dB and chirp when modulated at 1.2 Gbit/s of <0.5 MHz have been demonstrated.

Polarization conversion from diffraction gratings

Abstract: If a surface-modulated diffraction grating is arranged so that the scattering vector does not lie in the plane of incidence of the illuminating radiation, then polarization conversion can occur. This conversion can be strongly enhanced if a surface mode, such as a surface-plasmon-polariton or guided mode, is excited at the grating surface. In this paper we develop a mathematical description of the polarization conversion process, using a method that transforms the grating surface into a planar system. The predicted polarization conversion is compared very favorably with data for surface-mode-enhanced conversion.

Optical frequency shifting by means of a rotating diffraction grating

Abstract: The doppler frequency shift produced in the various diffraction orders of a rotating radial diffraction grating allows such a device to be used as an optical modulator. The theory and performance of such devices is presented.

Multicolor projector employing diffraction grating type liquid crystal light modulators

Abstract: A projector capable of compactizing the structure and improving the efficiency of light utilization is disclosed. The projector has an illuminating system, a light modulation device and a projection optical system. A first and a second lights of mutually different colors are emitted from the illumination optical system. The light modulating device has a first light modulator for modulating the first light according to a first image signal and a second light modulator for modulating the second light according to a second image signal. Each of the first and second light modulators is provided with a grating, a liquid crystal filled in the grooves of the grating and a driver for driving the molecules of the liquid crystal. The driver is adapted to drive the molecules of the liquid crystal in such a manner that the liquid crystal and the grating have mutually substantially matching refractive indexes in a certain state but have mutually unmatched refractive indexes in another state, thereby generating or substantially erasing a diffraction grating by the grating and the liquid crystal. A first and a second images of mutually different colors are projected on a screen by the first and second lights from the first and second light modulators.

Simple CD measurement of periodic structures on photomasks

Abstract: For critical dimension (CD) metrology of photomasks, a laser scatterometer linewidth measurement tool provides noncontact rapid, and nondestructive measurement of linewidth. Calculation of the linewidth is based on a rigorous theoretical model, thus eliminating the need for calibrations. A chrome-on-glass diffraction grating is illuminated with a laser. A photodetector mounted behind the photomask measures the scattered power in each diffracted order. This provides data for the rigorous theoretical model which provides a relationship between the linewidth of the photomask grating and the fraction of total power diffracted into the transmitted zero-order. This scatterometer linewidth measurement technique provides a simple, rapid, nondestructive, and noncontact method of linewidth determination which takes into account the effect of the glass substrate on which the grating is placed. This technique is insensitive to variations in angle of incidence, spot size, position of the spot on the grating, polarization and wavelength.

Projector utilizing liquid crystal light-valve and color selection by diffraction

Abstract: A projection system which produces a color image on a screen corresponding to an applied color signal. In a preferred embodiment, light beams from a source of white light are separated by a dichroic mirror into two beams, one containing green and the other containing blue and red. The beam containing green illuminates a first active matrix liquid crystal panel which contains electrodes which define a diffraction grating having parameters corresponding to green, and the beam containing blue and red illuminates a second active matrix liquid crystal panel which contains electrodes which form two separate diffraction gratings, one having parameters corresponding to red and the other having parameters corresponding to blue. As each beam passes through its respective LC panel its transmittance is modulated in accordance with video information signals applied to the panels, and the red and blue colors are selected by diffraction. The LC panels are embodied in a schlieren optical system for converting modulations of the LC panels and the colors selected by diffraction into an image.

Grating spectrograph in InGaAsP/InP for dense wavelength division multiplexing

Abstract: A grating spectrograph in InGaAsP/InP suitable for use in the wavelength region from 1.2 to 1.6 μm is presented. Experiments for devices with a channel spacing of 3.7 nm and more than 30 channels between 1.48 and 1.59 μm are described. The measured cross talk level is below −25 dB. The devices have only very low polarization sensitivity. This spectrograph is suited for monolithic integration with photodiodes, laser diodes, or optical amplifiers on a single chip.

A distributed feedback ridge waveguide quantum well heterostructure laser

Abstract: A method for incorporating distributed feedback in a ridge waveguide laser by means of lateral gratings and a single growth step is discussed. The necessary Bragg condition for distributed feedback is satisfied by etching gratings along the ridge in the top confining layer of the laser on either side of the contact stripe. Both Fabry-Periot modes and a single emission peak away from the peak of the gain profile are observed in lasers with cleaved facets. The Bragg reflection emission peak does not shift with increasing drive current, which is characteristic of a distributed-feedback (DFB) laser. >

Large photoinduced birefringence in an optically nonlinear polyester polymer

Abstract: A nonlinear optical polyester with disperse red side groups exhibited a stable photoinduced birefringence, from 0.14 at 850 nm to 0.21 at 633 nm, after exposure to linearly polarized short‐wavelength visible or ultraviolet light. Thin‐film wave plates, birefringent diffraction gratings and waveguides were written in this polymer using photoexposure. Thin‐ film wave plates of <1 μm thickness showed ≳π/4 phase delay and good temporal stability. A 9:1 diffraction efficiency ratio of the two polarization eigenmodes was measured from thin birefringent gratings. This ratio is in good agreement with the theoretical prediction obtained using a simple three‐level model.

Optical modulation element having two-dimensional phase type diffraction grating

Abstract: An optical modulation element characterized in that a two-dimensional phase type diffraction gratings, whose refractive index changes for each predetermined period along two different directions in a predetermined plane, is adopted, and a changing state of the refractive index is controlled to change characteristics of the diffraction grating, so that a diffraction phenomenon of light incident on the element is controlled.

LiNbO(3) waveguide second-harmonic-generation device phase matched with a fan-out domain-inverted grating.

Abstract: A LiNbO(3) waveguide optical second-harmonic-generation (SHG) device, consisting of a channel waveguide array and a ferroelectric domain-inverted grating in a fan pattern for quasi-phase matching, is proposed. The structure has redundancy to absorb the residual phase mismatch caused by design uncertainties and fabrication errors. A SHG device of 3-mm interaction length was fabricated by domain inversion by Ti indiffusion and channel waveguide formation by annealed proton exchanging and reactive ion etching. A normalized SHG efficiency of 17%/W was experimentally obtained for pumping by a cw Nd:YAG laser.

82 nm of continuous tunability for an external cavity semiconductor laser

Abstract: A wavelength tunability of 82 nm without mode hopping has been obtained with an external cavity semiconductor laser emitting around 1540 nm by combined rotation/translation of a diffraction grating. This demonstrates for the first time the tunability of an optical oscillator having a spectral purity better than 100 kHz over more than 10,000 GHz.

Tunable diffraction grating

Abstract: A diffraction grating (10) can be tuned by varying the spacing (16) of the rows (12, 14) of the diffraction grating (10). The diffraction grating (10) is comprised of rows (12, 14) of an electrically conductive material disposed on a deformable material (11). The deformable material (11) is transparent to a light (19) of numerous wavelengths. A means for applying a voltage (20, 21) on the rows (12, 14) of the electrically conductive material is provided to change the spacing (16, 17) of the rows, thus changing which wavelength of light (19) is diffracted.

Optical apparatus using wavelength selective photocoupler

Abstract: The present invention relates to optical apparatus such as a photosensor, a semiconductor laser, an optical amplifier in which a wavelength selective photocoupler is used so as to couple two waveguides through a diffraction grating. A photosensor which is one of the optical apparatus according to the present invention comprises a substrate, a first waveguide layer formed on the substrate, a second waveguide layer formed on the first waveguide layer to be stacked in a direction of thickness and which has a guided mode difference from that of the first waveguide layer, a diffraction grating formed on an overlapping region of the guided modes of the first and second waveguide layers and which couples light components of a specific wavelength range of light propagating through the first waveguide layer to the second waveguide layer, a light absorption layer for absorbing at least some light components of the light components coupled to the second waveguide layer, and an electrode for converting the light components absorbed by the light absorption layer into an electrical signal and outputting the electrical signal.

Method for fabricating an angled diffraction grating

Abstract: A method for fabricating an angled diffraction grating (12) is provided. An optical medium (13) with fine lines and spaces that are defined in an etch mask is provided. Transferring the defined lines and spaces of the etch mask into the optical medium (13) at a desirable angle is achieved by an etching means, thereby resulting in an angled diffraction grating (12).

Array generation with lenslet arrays.

Abstract: Lenslet arrays can be used as phase gratings, having many diffraction orders with equal intensity. Applications are multiple imaging and illumination of arrays of optical or optoelectronic devices in digital optics. The homogeneity of the intensities within the array can be improved by using field lenslets. The basic theory as well as experiments with diffractive and with graded index lenses are shown.

A multichannel grating cavity laser for wavelength division multiplexing applications

Abstract: A novel form of wavelength selectable diode laser source is proposed for wavelength division multiplexing (WDM) applications. The source may be used to operate at specific wavelengths across the complete gain spectrum of the laser diode material, but does not require any moving parts. Theoretical simulations indicate that the device can be expected to operate in a single longitudinal mode for channel separations on the order of 1 nm. The potential of using the device to generate parallel streams of WDM data is considered. >

Magneto-optical recording/reproducing apparatus with light beam splitting means

Abstract: In an apparatus for recording and reproducing optical information, a light beam splitter is disposed on an optical path between a laser beam source and an optical information recording medium. The light beam splitter has a three-beam forming diffraction grating on one face of this light beam splitter on a light-emitting side thereof. The light beam splitter also has a light branching diffraction grating on another face thereof. A light beam emitted from the laser beam source is divided into three beams by the three-beam forming diffraction grating. The light branching diffraction grating is used to branch light reflected from the optical information recording medium. A polarizing/separating diffraction grating is formed in the light beam splitter on the same plane as the three-beam forming diffraction grating. The polarizing/separating diffraction grating is used to polarize and separate a light beam branched by the light branching diffraction grating.

Grating line shape characterization using scatterometry

Abstract: Identification of dimensional parameters of an arbitrarily shaped grating using scatter characteristics is presented. A rigorous diffraction model is used to predict the scatter from a known grating structure, and utilizing this information we perform the inverse problem of predicting line shape from a measurement of the scatter.

Analysis of the electromagnetic scattering by thick gratings using a combined FEM/MM solution

Abstract: The problem of diffraction by a thick, conducting grating situated in an inhomogeneous dielectric slab is investigated using the generalized network formulation. This formulation combines the method of moments and the finite-element method, permitting the treatment of periodic elements of arbitrary cross section and inhomogeneous profiles. Solutions are presented for both transverse electric (TE) and transverse magnetic (TM) polarizations. Transmission gratings composed of rectangular conductors filled with dielectric materials of arbitrary profiles are studied. >

Position detector and method of measuring position

Abstract: According to the principles of the invention, an improved system and method of accurately measuring the position of an object to high resolution are provided. A read head is positioned adjacent a grating. The read head emits light onto the grating. The light is diffracted into two light beams by the grating. The light beams are reflected back towards the grating, to be diffracted a second time and combined into a single beam. The polarization of the respective light beams is modified before being diffracted the second time. The polarization component of the beam parallel to the diffraction grating grooves is rotated perpendicular to the diffraction grating grooves and the component of the beam perpendicular to the diffraction grating grooves to be rotated parallel to the diffraction grating grooves. The effects of the diffraction on perpendicular or parallel polarized light are canceled because the same light impinges at two different polarizations, each the opposite of the other, prior to being combined into a single beam.

Holographic transmission elements using improved photopolymer films

Abstract: Volume phase holographic gratings provide the high diffraction efficiency often required from holographic optical elements. In order to provide wide angular bandwidth, high index modulation is also required. A new addition to the family of Du Pont holographic photopolymer films has significantly higher index modulation for the grating spacings typical of holographic transmission elements. In this paper, the recording of volume phase transmission gratings in this holographic photopolymer is described. The dependence of holographic properties on exposure conditions, grating characteristics, and processing are discussed. The performance characteristics of holographic optical elements produced in this material are also presented.

Characteristics of coherent two-dimensional grating surface emitting diode laser arrays during CW operation

Abstract: Recent progress in the development of monolithic two-dimensional coherent grating surface emitting (GSE) laser arrays is presented. Such GSE arrays have operated continuously to more than 3 W/surface and pulsed to more than 30 W/surface. They have achieved continuous wave (CW) threshold current densities of under 140 A/cm/sup 2/ with CW differential quantum efficiencies of 20-46% per surface. Linewidths in the 40-100 MHz range were obtained with output powers of 100-300 mW per surface. The arrays typically consist of 10-30 mutually injection coupled gain sections with 10 laterally coupled ridge-guided lasers in each gain section. A single GaInAs strained-layer quantum well with a graded-index separate confinement heterostructure (GRINSCH) geometry allows junction down mounting with light emission through the transparent GaAs substrate. A surface relief grating is used for feedback and outcoupling. >