Showing papers on "Diffraction grating published in 1986"

Laser-Induced Dynamic Gratings

Abstract: The invention ofthe laser 25years ago resulted in powerfullight sources which led to the observation of unexpected and striking phenomena. New fields of science such as holography and nonlinear optics developed constituting the basis of this volume. The classical principle of linear superposition of light wavesdoes not hold anymore. Two laser beams crossing in a suitable material may produce a set of new beams with different directions and frequencies. The interaction of light waves can be understood by considering the optical grating structures which develop in the overlap region. The optical properties of matter become spatially modulated in the interference region of two light waves. Permanent holographic gratings have been produced in this way by photographic processes for many years. In contrast, dynamic or transient gratings disappear after the inducing light source, usually a laser, has been switched off. The grating amplitude is controlled by the light intensity. Dynamic gratings have been induced in a large number ofsolids, liquids, and gases, and are detected by diffraction, 'forced light scattering' of a third probing beam, or by self-diffraction of the light waves inducing the grating. The combined interference and diffraction effect corresponds to four-wave mixing (FWM) in the language of nonlinear optics. The process is called degenerate ifthe frequenciesofthe three incident wavesand the scattered wave are equal. Degenerate four-wave mixing (DFWM) is a simple method to achieve phase conjugation, i.e. to generate a wave which propagates time reversed with respect to an incident wave.

Rigorous coupled-wave analysis of metallic surface-relief gratings

Abstract: A rigorous coupled-wave analysis for metallic surface-relief gratings is presented. This approach allows an arbitrary complex permittivity to be used for the material and thus avoids the infinite conductivity (perfect-conductor) approximation. Both TE and TM polarizations and arbitrary angles of incidence are treated. Diffraction characteristics for rectangular-groove gold gratings with equal groove and ridge widths are presented for free-space wavelengths of 0.5, 1.0 and 10.0 μm for all diffracted orders as a function of period, groove depth, polarization, and angle of incidence. Results include the following: (1) TM-polarization diffraction characteristics vary more rapidly than do those for TE polarization, (2) 95% first-order diffraction efficiency occurs for TM polarization at 10.0 μm, (3) 50% absorption of incident power occurs at 0.5 μm, and (5) the perfect-conductor approximation is not valid for TM polarization at any of the wavelengths and is not valid for TE polarization at 0.5 μm.

Optical sensor for selective detection of substances and/or for the detection of refractive index changes in gaseous, liquid, solid and porous samples

Abstract: The optical sensor for the detection of specific substances and refractive index changes in gaseous, liquid, solid and porous samples is composed of integrated optical elements. It consists of a waveguiding film (1) equipped with a diffraction grating (4) and applied onto a substrate (2). For the selective detection of a specific substance contained in the sample (3) a selectively chemisorbing additional layer (5) covers the film (1) at least in the grating region. The sample (3) is applied at least in the grating region either directly onto the waveguiding film (1) or onto the selectively chemisorbing additional layer (5). Chemisorption of the substance, which is contained in the sample (3) and is to be detected, to the selectively chemisorbing additional layer (5) results in a further adlayer (6) coating the waveguiding film (1). The principle of the sensor is as follows: Due to chemisorption of a further adlayer (6) and/or due to a refractive index change of the sample (3) a change in the effective refractive index N of a mode guided in the waveguiding film (1) is effected. For the detection of this change the set-up (1,2,3,4,5) described above is used as grating coupler or Bragg reflector.

Integrated optics components and devices using periodic structures

Abstract: The selected research activities on integrated optics components and devices using periodic structures are reviewed, with emphasis on the authors' works employing the electron-beam writing technique. The periodic structures include static gratings and dynamic ones produced through acoustooptic (AO) and electrooptic (EO) effects. They provide a variety of passive functions and effective means for guided-wave control. The review is made from the integration point of view, including the most recent results. First, the theoretical fundamentals are outlined and the electron-beam writing techniques, including the writing system, are discussed. Next, passive components (grating deflectors, filters, lenses, couplers, etc.) and elements for guided-wave controlling and detecting (AO and EO grating elements and photodetectors) are described. Then, integrated optic devices, i.e., wavelength demultiplexers, RF spectrum analyzers, optical disk pickup, etc., are presented. Finally, the possibility of future applications is discussed.

Grating and prism compressors in the case of finite beam size

Abstract: The grating and prism compressors are studied for the case of finite beam size by using the Kirchhoff–Fresnel integral and an amplitude transfer operator for the grating and prism. The results show that, even for well-collimated beams, pulse broadening may occur (mainly because of the wavelength dependence of the lateral walk-off). Methods to avoid the different types of distortions appearing in these systems are discussed.

Stimulated Wood's anomalies on laser-illuminated surfaces

Abstract: Spontaneous, highly periodic, often permanent surface gratings or "ripples" can develop on the surface of almost any solid or liquid material illuminated by a single laser beam of sufficient intensity, under either pulsed or CW conditions. The grating periods are such that the incident laser beam is diffracted into a tangential wave which skims just along or under the illuminated surface. These spontaneously appearing surface ripples are generated by a runaway growth process analogous to stimulated Brillouin or Raman scattering or small-scale self focusing, but having many of the same properties as Wood's anomalies in diffraction gratings. Hence, it seems appropriate to refer to these spontaneous surface structures as "stimulated Wood's anomalies."

Zero-reflectivity high spatial-frequency rectangular-groove dielectric surface-relief gratings.

Abstract: Using rigorous coupled-wave analysis, high spatial-frequency rectangular-groove surface-relief phase gratings are shown to be capable of exhibiting zero reflectivity. Thus these corrugated surfaces may act as antireflection coatings in a variety of applications. The diffraction characteristics of rectangular-groove surface-relief gratings are presented for several ratios of incident wavelength to grating period as a function of filling factor, groove depth, angle of incidence, and polarization. The conditions for zero reflectivity are identified. Results are compared with single-homogeneous-layer approximate theory results. In the limit of long wavelengths for an electromagnetic wave in a dielectric of refractive index n1 normally incident on a dielectric of index n2, it is determined that for antireflection behavior, the grating groove depth should be λ/4(n1n2)1/2 and the filling factor should be n1/(n1 + n2) or n2/(n1 + n2) for the electric field perpendicular or parallel to the grating vector, respectively. The spectral and angular responses of these gratings are like those of single-homogeneous-layer antireflection coatings. These gratings also exhibit birefringent retardation.

Resonances of two-dimensional particle gratings in surface-enhanced Raman scattering

Abstract: Dipolar interactions between silver particles arranged on a square two-dimensional grating give rise to characteristic features in the wavelength dependence of surface-enhanced Raman scattering from adsorbed molecules. The interactions produce two pronounced effects: first, they shift and broaden the plasmon resonance corresponding to the isolated particle, and second, they induce an additional sharp resonance feature at lower energy. The two enhancement maxima are separated by a minimum (cusp), which corresponds to the wavelength at which a grating order changes from radiating to evanescent in character. Dispersion relations describing the grating-induced resonances are presented. The consequences of the simultaneous enhancement of both the input and output electric fields are discussed, and the effects of imperfections in the particle shape are considered. The wavelength dependence of Raman scattering from p-nitrobenzoic acid adsorbed on a 380-nm-period two-dimensional array of silver spheroids was measured by means of an internal standard. Close agreement between experimental results and theoretical predictions was obtained.

Holographic multiplexer/demultiplexer and its manufacturing method

Abstract: A holographic multiplexer and holographic demultiplexer, which has a diffraction grating formed of a hologram with an interference pattern produced by interference of two wave fronts, of which at least one wave front is of an aspherical wave and the other is of, for example, a spherical wave, and its manufacturing method are disclosed. The aspherical wave is used for correction of aberration and provided by obtaining the phase function φ G of the aspherical wave using a computer generated hologram and through an optical method or an electron-beam direct drawing method. The hologram serving as the diffraction grating for the holographic multiplexer/demultiplexer is produced by a two-beam interference method of an electron-beam direct drawing method. As the result, a holographic multiplexer/demultiplexer having a high degree of multiplexing, which is compact in size, and exhibiting low loss of light can be provided.

Narrowband grating resonator filters in InGaAsP/InP waveguides

Abstract: We report the first demonstration of efficient narrowband optical wavelength filters using InGaAsP/InP passive waveguide grating resonators. Filter bandwidths as narrow as 1 A, centered about λ=1.55 μm with excess resonator loss as low as 1 dB, have been achieved.

Optical head comprising a diffraction grating for directing two or more diffracted beams to optical detectors

Abstract: For use in combination with an optical source for generating a coherent beam along a main optical axis, an optical head device comprises a diffraction grating having a plurality of grating regions which are responsive to the coherent beam for directing a zeroth-order diffracted beam to a focussing lens and are responsive to an optical beaem reflected from an optical recording medium for directing sidewards diffracted beams to an optical detector assembly. An effective area of a diameter of about 5 mm is sufficient for the diffraction grating. Each sidewards diffracted beam may form an angle of 20° with the main optical axis. The optical detector assembly is used in detecting focussing and tracking errors for the recording medium and can be used in reproducing optical information from the recording medium. A six-partitioned optical detector assembly is preferred when the grating regions are four in number. Either a four-partitioned or a three-partitioned optical detector assembly is preferred when the grating regions are two in number. Depending on the grating regions, the optical detector assembly preferably has two optical detector elements which are placed on different levels relative to the diffraction grating.

External-cavity semiconductor laser with 15 nm continuous tuning range

Abstract: A 1.26 μm optical amplifier with a facet modal reflectivity below 0.0001 has been used in a 58 mm-long grating external cavity. The lasing wavelength has been continuously tuned without mode hopping over a range of 15 nm by combined translation-rotation of the diffraction grating. A linewidth of 20 kHz has been derived from heterodyne beat frequency measurements between two tunable external-cavity lasers.

Diffraction grating structures

Abstract: A diffraction grating responds to incoming radiation incident thereon within a given range of incidence angles and re-directs such incident radiation from the structure in a selected direction within relatively limited confines. A liquid crystal material is positioned in contact with the diffracting surface of said at least one diffraction structure, the liquid crystal material, when inactivated, having a refractive index substantially the same as that of the diffraction structure. Activation means place the liquid crystal material in an activated state so that the refractive index thereof is substantially different from that of the diffraction structure whereby incoming radiation within a given range of incidence angles is transmitted through the structure and exits in the selected direction.

Coherent summation of laser beams using binary phase gratings.

Abstract: A method for coherent addition of lasers is presented that uses binary phase-only gratings. It is shown that a grating that splits a single beam into N equal orders with high efficiency can be used in reverse to convert N laser beams into a single beam with the same efficiency. Experiments to demonstrate the conversion of seven beams to one beam are performed with a resulting conversion efficiency of 75%. An experimental apparatus is described that adds the power from both two and three He–Ne lasers (wavelength, 3.39 μm) with efficiencies as high as 83%.

Light valve for use in a color display unit with a diffraction grating assembly included in the valve

Abstract: In a display unit for use in projecting projection light onto a screen to display a picture, a light valve comprises a diffraction grating assembly for selecting one of chrominance components included in an incident light given through one surface of the light valve The diffraction grating assembly is divided into a plurality of grating elements arranged to give different colors corresponding to the chrominance components In addition, a control beam, such as a laser, is incident onto the other surface of the light valve to selectively scan the grating elements and to heat selected ones of the grating elements through an energy converting film Thus, the incident light is subjected to modulation on the grating elements in accordance with heat energy and emitted through a liquid crystal as controlled light from the light valve The controlled light is sent as the projection light to the screen

Position detection apparatus

Abstract: An apparatus includes a means for providing a predetermined frequency difference between two light beams and generating an optical beat with respect to interference between first and second diffracted light beams from a diffraction grating formed on a substrate, and a means for detecting a phase difference between the optical beat and a reference signal having a frequency corresponding to the frequency difference between the two light beams, and detects a position of the substrate based upon the phase difference in accordance with an optical heterodyne interference method.

Laser for providing rapid sequence of different wavelengths

Abstract: The optical cavity of the present laser has a folded optical path with a pair of reflective diffraction gratings and a gain cell in it. A disk with radially distributed perforations rotates in the optical path between the gratings and the gain cell. The laser wavelength depends upon which perforation is aligned with the slot in a slotted plate also in the optical path. The laser output consists of a sequence of different wavelength but colinear rays.

High-reflectivity monomode-fibre grating filters

Abstract: The fabrication and characterisation of efficient, narrowband fibre-optic filters utilising surface-relief gratings etched into monomode fibre are described. A device with 92% reflectivity and 1·8 nm bandwidth at 1·3 μm wavelength using a first-order Bragg grating with a high-index overlayer is reported.

Picosecond holographic grating generation of ultrasonic waves

Abstract: The picosecond holographic grating generation and detection of ultrasonic waves are described. Two mechanisms, a heating mechanism and a stimulated Brillouin scattering mechanism, are described. These permit the generation of tunable ultrasonic waves in the MHz to GHz range. It is demonstrated that the two mechanisms can be distinguished by the distinct time dependences of the diffraction of a variably delayed probe pulse from the acoustic gratings. In addition, the acoustic waves can be optically manipulated, i.e., amplified, cancelled, and phase shifted. The use of the technique for the detection of weak ground state absorptions and excited state-excited state absorptions is illustrated. Mixed excited state and acoustic gratings are discussed. The phase grating contribution to diffraction from an excited state grating is displayed, and the interference between the acoustic phase grating and the excited state phase grating is described and experimentally illustrated. Contributions to the acoustic grating diffraction from spectral shifts when the probe is near a strong, narrow transition are considered. Grating experiments on various phases of liquid crystal thin films are presented. It is shown that in addition to an acoustic grating, a Kerr effect grating is produced. The Kerr grating can be separated from the acoustic grating using polarization grating excitation. The Kerr grating reveals nonexponential relaxation of orientation order in liquid crystals on a short time scale (<1 ns).

Fourth-order partial-coherence effects in the formation of integrated-intensity gratings with pulsed light sources

Abstract: We performed theoretical calculations of the relative diffraction efficiency of partially coherent light-induced integrated-intensity gratings using pulsed sources, paying particular attention to thermal gratings. We provided a simple intuitive picture of the phenomenon and then calculated exact expressions that, unlike instantaneous-intensity-grating results, necessarily require the use of fourth-order coherence functions. Assuming several radiation models, we evaluated these expressions and found that the results proved to be insensitive to the specific radiation model assumed. The application of these results to a previously performed pulsed-laser experiment yielded a better fit to the data than an expression involving only second-order coherence, which is valid only in the cw limit. We included the effects of grating decay and, in addition, compared the use of integrated-intensity gratings for ultrashort-pulse-length measurement with standard techniques. Finally, we calculated expressions for the relative diffraction efficiency of integrated-intensity gratings created with excitation beams from two separate and independent sources of different frequency, and we report an experiment whose results were found to agree with this theory.

Absolute soft x-ray measurements with a transmission grating spectrometer

Abstract: Absolute measurements of the soft x-ray emission (50 eV < hv < 1000 eV) from laser plasmas were made with transmission gratings, including pinhole-grating combinations for spatially resolved spectroscopy. Kodak 101–01 film was absolutely calibrated as a function of wavelength with the help of a bolometer, the laser plasma being used as a source. An example of a quantitatively analyzed, space-resolved pinhole grating spectrum recorded on film will be shown.

Embossing technique for fabricating surface relief gratings on hard oxide waveguides

Abstract: Surface relief gratings with 1200, 2400, and 3600 lines/mm on planar SiO2–TiO2 waveguides were fabricated by a combination of a dipcoating method (sol gel process) with an embossing technique. The grating is embossed into a deformable gel film, which by subsequent heat treatment is transformed into an oxide waveguiding film. During the heat treatment the film thickness shrinks to about one-fourth of its value directly after the embossing. Values of the process parameters, e.g., of the embossing pressure, are given with which input and output grating couplers of good quality were fabricated. Measurements of input and output efficiencies are reported; incoupling efficiencies up to ∼28% were obtained.

Soft x‐ray beam line for surface EXAFS studies in the energy range 60≤hν≤11 100 eV at the Daresbury SRS

Abstract: The characteristics of the beam line used for surface EXAFS (extended x‐ray absorption fine structure) studies at the Daresbury Synchrotron Radiation Source (SRS) are described. Monochromatic photons are available in the energy range 60≤hν≤11 100 eV. This has been achieved using a monochromator with a combination of crystal and grating optics. The radiation is focused through the monochromator by a toroidal premirror at 0.5° grazing angle. Three pairs of crystals and a plane grating with focusing mirror are available which can be interchanged under ultrahigh vacuum (UHV). The plane grating/mirror monochromator has two ranges of zero‐order angles, 6.4° and 2°, giving photon energy ranges of 60–550 eV and 600–2000 eV, respectively, with a 1200 l mm−1 grating. The crystals in use are InSb(111), Ge(111), and Ge(220) having photon energy ranges of 1745–7360 eV, 2000–8430 eV, and 3260–11 100 eV. The performance of the gratings and crystals in respect to intensity, resolution, scattered light, and higher orders ...

An ultrahigh vacuum reflectometer/goniometer for use with synchrotron radiation

Abstract: An ultrahigh vacuum (UHV) reflectometer/goniometer has been designed and built for the Naval Research Laboratory beamline at the National Synchrotron Light Source at Brookhaven National Laboratory. This beamline contains the Grating/Crystal Monochromator [1] that has a wavelength range from 2000 to 5 A, eventually to be extended to 2.5 A. The combination of these facilities permits reflectometry over a very wide spectral range for measurements of optical constants and reflectances of multilayer coatings, scattering, and simple diffraction grating efficiency measurements.

Picosecond multiple-pulse experiments involving spatial and frequency gratings: a unifying nonperturbational approach

Abstract: The concept of a grating in real and frequency space is examined in the context of a three-pulse optical excitation cycle applied to a pseudo two-level model system. The calculations are done analytically using the Liouville-operator formalism in matrix form. It is shown that a continuous transition occurs from a grating in real space to a grating in frequency space when the first two excitation pulses separate in time. During this transition, the role of the population-relaxation time constant (T1) is taken over by the dephasing time constant (T2) bringing out the irreversible nature of the loss of coherence in an excited state. The underlying space-time transformation when moving from a grating in real space to a grating in frequency space further clarifies the loss in symmetry of the scattering pattern induced by a probe pulse by attributing it to the law of causality. It is finally concluded that the generalized grating concept is a powerful means of analyzing or predicting the effects of multiple-pulse multicolor optical-coherence experiments.

Magnetic recording media and servo systems using light-transmitting optical gratings

Abstract: Magnetic recording media, and magnetic recording systems using such media, are disclosed which incorporate a light-transmitting optical grating at least substantially coextensive with the magnetic recording area. A second light-transmitting optical grating is associated with the magnetic transducer or read/write head. A moire pattern is produced by light transmitted through both optical gratings, and changes in the moire pattern resulting from changes in the alignment of the optical gratings resulting from movement of the read/write head relative to the media are utilized to provide servo information to keep the magnetic transducer aligned with the magnetic track. Higher magnetic recording capacities may be obtained using such media and systems, since none of the magnetic recording area is used for servo information.

Kinetics for optical erasure of sinusoidal holographic gratings in photorefractive materials

Abstract: The kinetics of erasure of sinusoidal holographic gratings in photorefractive materials has been investigated. A general solution applying to arbitrary transport lengths in comparison to the grating period has been obtained. The proposed method permits a good insight into the involved physical mechanism and reveals some interesting features of the general solution such as the constant dephasage between the donor and carrier gratings. On the other hand, it can account for the role of the oxidation/reduction state of the sample, which very markedly influences the kinetic behavior. In particular, it has been shown that the short transport length approximation rapidly loses its validity with the level of reduction. The model has been applied to LiNbO 3 and used to discuss some recent experimental erasure data obtained by Tyminski and Powell.

Wavelength division optical multiplexer/demultiplexer

Abstract: A wavelength division multiplexing or demultiplexing optical coupler of the diffraction grating type includes a pure fused silica optical element having a convex spherical surface on one end and a diffraction grating on a portion of its other end The remaining portion of its other end receives a multiple fiber array for transmitting and receiving the light to be multiplexed or demultiplexed

Single-mode behaviour of a multimode 1.55 μm laser with a fibre grating external cavity

Abstract: A 1.55 μm DCPBH multimode laser has been made to oscillate in a single longitudinal mode by feedback from a fibre grating which acts as an external cavity for line narrowing and as a frequency-selective component to give single-mode behaviour. Linewidths of less than 10kHz and side-mode rejection ratios of 320 have been achieved.