Showing papers on "Diffraction grating published in 1992"

High-resolution fibre-grating based strain sensor with interferometric wavelength-shift detection

Abstract: A technique for the detection of dynamic strain induced wavelength shifts in fibre Bragg grating sensors is reported that is based on an unbalanced-interferometer wavelength discriminator, which is capable of subnanostrain resolution (0.6 ne/√(Hz) at 500 Hz) sensing.

A narrow‐band tunable diode laser system with grating feedback, and a saturated absorption spectrometer for Cs and Rb

Abstract: Detailed instructions for the construction and operation of a diode laser system with optical feedback are presented. This system uses feedback from a diffraction grating to provide a narrow‐band continuously tuneable source of light at red or near‐IR wavelengths. These instructions include machine drawings for the parts to be constructed, electronic circuit diagrams, and prices and vendors of the items to be purchased. It is also explained how to align the system and how to use it to observe saturated absorption spectra of atomic cesium or rubidium.

A passive wavelength demodulation system for guided-wave Bragg grating sensors

Abstract: A novel, passive, and self-referencing wavelength detection system (WDS) that measures the wavelength of the narrowband back-reflected spectrum of guided-wave Bragg gratings is described. This letter also reports on the use of such a detection system with fiber-optic Bragg gratings used as absolute strain sensors. The wavelength detection system demonstrated a 1% strain resolution of the total strain measurement range. >

Deformable grating optical modulator.

Abstract: A new type of light modulator, the deformable grating modulator, based on electrically controlling the amplitude of a micromachined phase grating is described. Mechanical motion of one quarter of a wavelength is sufficient for switching in this device. The small mechanical motion allows the use of structures with high mechanical resonance frequencies. We have developed a deformable grating modulator with a bandwidth of 1.8 MHz and a switching voltage of 3.2 V and have demonstrated modulation with 16 dB of contrast. Smaller devices with bandwidths of as much as 6.1 MHz and predicted switching voltages of less than 10 V were also fabricated.

Binary gratings with increased efficiency

Abstract: Coupled-wave analysis is used to design binary gratings with high efficiencies (70-80%). The binary designs have grating periods greater than one wavelength but use subwavelength structures within each period in order to achieve high efficiency.

Polarization-insensitive arrayed-waveguide grating wavelength multiplexer on silicon

Abstract: A wavelength-division multiplexer with no polarization dependence is demonstrated. It is based on an arrayed-waveguide grating fabricated with silica-based optical waveguides on a silicon substrate. The polarization dependence that is due to the birefringence of the waveguide is eliminated by inserting a quartz λ/2 plate in the middle of the arrayed waveguide. Moreover the grating is designed to operate in the 115th order for high wavelength resolution. The polarization insensitivity and the high resolution lead to the successful multiplexing of 13 wavelength channels with 1-nm spacing. A passband width of 0.3 nm and a cross-talk level of −30 dB are achieved.

Circularly symmetric operation of a concentric‐circle‐grating, surface‐ emitting, AlGaAs/GaAs quantum‐well semiconductor laser

Abstract: A surface‐emitting semiconductor laser that utilizes a concentric‐circle grating defined by electron‐beam lithography is observed to oscillate in a circularly symmetric fashion. The laser emits a circularly symmetric beam with a total beam divergence of less than 1°. Despite its broad‐area geometry, the laser shows no evidence of filamentation. The laser maintains a relatively narrow wavelength spectrum approximately 1 A in width.

Polymer dispersed liquid crystal panel with diffraction grating

Abstract: A layer having an irregular sectional surface is formed on the surface of at least one of a pixel electrode and confronting electrode of a liquid crystal layer so as to confront a liquid crystal layer. The liquid crystal layer is formed of a polymer dispersed liquid crystal. If the layer having an irregular sectional surface is formed in a convex and concave pattern regularly and cyclically, the thus formed layer serves as a diffraction grating. When the liquid crystal is in an OFF condition, the refractive index n of the diffraction grating is different from the refractive index of the liquid crystal layer. As a result, an incident light to the liquid crystal panel is scattered and at the same time, diffracted. When the liquid crystal in in an ON condition, there is no difference in the refractive index between the diffraction grating and the liquid crystal, and the incident light passes straight through. Using the liquid crystal panel as a light valve, the light from a light source is incident to the liquid crystal panel, and the image formed on the liquid crystal panel is projected through a projection lens onto a screen. In the case of a black display, the incident light is scattered or diffracted in the liquid crystal panel according to the projected image. The scattered or diffracted light is stopped by a light shielding part of a aperture, and thus does not arrive at the screen.

A modal analysis of lamellar diffraction gratings in conical mountings

Abstract: A rigorous modal analysis of lamellar grating, i.e., gratings having rectangular grooves, in conical mountings is presented. It is an extension of the analysis of Botten et al. which considered non-conical mountings. A key step in the extension is a decomposition of the electromagnetic field in the grating region into two orthogonal components. A computer program implementing this extended modal analysis is capable of dealing with plane wave diffraction by dielectric and metallic gratings with deep grooves, at arbitrary angles of incidence, and having arbitrary incident polarizations. Some numerical examples are included.

Fiber-optic Bragg-grating differential-temperature sensor

Abstract: A differential temperature sensor based on fiber-optic Bragg-grating elements is described. A high sensitivity to thermally induced Bragg wavelength shifts is obtained using an interferometric detection approach. Results presented show a temperature resolution of >

Demonstration of a 15*15 arrayed waveguide multiplexer on InP

Abstract: By interconnecting two star couplers with a waveguide grating, the authors built a monolithic 15*15 multiplexer on InP. The grating order of 148 gives a free spectral range of 10.5 nm (1.3 THz) and a channel spacing of 0.7 nm (87 GHz) at 1550 nm wavelength. A crosstalk between adjacent channels of less than 18 dB and a residual crosstalk of less than 25 dB were obtained. The on-chip insertion loss is typically 2-4 dB. >

Display having diffraction grating pattern

Abstract: A display having a diffraction grating pattern includes: (a) a flat substrate; and (b) at least one dot formed on a surface of the flat substrate, the dot being formed by a diffraction grating pattern as an aggregate of a plurality of curves obtained by translating a curve.

Narrow-band optical channel-dropping filter

Abstract: Waveguide couplers are combined with lambda /4 shifted distributed feedback (DFB) resonators to produce narrowband channel dropping filters. The bandwidth of the filter can be made much narrower than the stopband of the grating. It is possible to remove the spurious responses of the grating filter by appropriate dispersion characteristics for the coupled waveguides. However, in some practical applications it may not be necessary to do this, if all channels can be accommodated within half the grating bandwidth of the filters. >

High-spatial-frequency binary and multilevel stairstep gratings: polarization-selective mirrors and broadband antireflection surfaces.

Abstract: High-spatial-frequency, surface-relief binary gratings have been shown to have diffraction properties that are similar to homogeneous layers of equivalent refractive indices, which depend on the grating characteristics, angle of incidence, and polarization. Thus these gratings in the long-wavelength limit could be used as equivalent thin-film coatings. Because of their polarization discrimination these gratings can function as polarization-selective mirrors. A procedure for designing these gratings to be antireflective for one polarization (TE or TM) and to maximize their reflectivity for the orthogonal polarization (TM or TE) is presented. Multilevel stairstep gratings can similarly exhibit characteristics that resemble those of multilayer antireflection coatings (quarter-wave impedance transformers), thus permitting a broader wavelength bandpass. A systematic procedure for designing multilevel stairstepgratings to operate as multilayer thin-film antireflection surfaces is presented. These design methods are valid for both TE and TM polarizations and for any angle of incidence. Example designs are presented, and the rigorous coupled-wave diffraction analysis is used to evaluate the performance of these gratings as functions of the ratio of their period to the incident wavelength. Comparisons are included with homogeneous layers that are equivalent to the gratings in the long-wavelength limit.

Grating spectrograph integrated with photodiode array in InGaAsP/InGaAs/InP

Abstract: A grating spectrograph integrated monolithically for the first time in the InGaAsP system with a photodiode array for a dense WDM application is presented for the 1.5 mu m wavelength region. The chip provides 42 wavelength channels with a spacing of 4 mm, a channel crosstalk of approximately -15 dB, an internal photodiode efficiency of 90%, and a photodiode capacitance of 0.33 pF. The chip needs no optical adjustments. It is therefore well suited for mass production. >

NxN Optical crossbar switch matrix

Abstract: An optical crossbar switch matrix for use in switching optical signals from a first set of optical fibers to a second set of optical fibers, in any order, which is characterized by having a matrix of rows and columns of diffraction gratings formed in a semiconductor heterostructure. Each grating is independently biased with either a forward or reverse bias voltage to switch the grating between a reflective state and a transmissive state. The gratings are oriented at an angle relative to the rows and columns so that when the Bragg condition for the light received from an optical film is met, a portion of the light is diffracted from the row in which it is propagating into a column toward another optical fiber. The heterostructure may include optical amplifiers to restore the optical signal to its original power level. Beam expanding, collimating and focussing optics may also be integrated into the heterostructure.

10 GHz spacing optical frequency division multiplexer based on arrayed-waveguide grating

Abstract: 11 channel optical-frequency-division multiplexing with a 10 GHz channel spacing is achieved with an arrayed-waveguide grating consisting of SiO2–GeO2/SiO2 waveguides on a silicon substrate.

Demonstration of broadband tunability in a semiconductor laser using sampled gratings

Abstract: We propose and demonstrate a new monolithic laser structure in which the fractional tuning range Δλ/λ can be more than an order of magnitude larger than the fractional index change Δμ/μ in one section. The key idea involves using grating mirrors with grating elements removed in a periodic fashion. These ‘‘sampled gratings’’ have reflection spectra with periodic maxima. By using two such mirrors with identical grating pitch but mismatched sampling periods, it is possible to tune among the various reflectivity maxima. Our initial experimental results show 29.3 nm of tuning in an InGaAsP laser.

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 degree(s)C. Quasi-distributed temperature, strain, pressure, chemical, and interferometric type 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 WDM, TDM, and 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.

Diffraction Properties of Stratified Volume Holographic Optical Elements

Abstract: We present a unified treatment of the diffraction properties of stratified volume holographic optical elements (SVHOE’s). We show that the relative phasing of the diffraction orders as they propagate from layer to layer gives rise to a unique notched diffraction response of the +1 order (for the case of Bragg incidence) as a function of the normalized buffer-layer thickness, the grating spatial frequency, and the readout wavelength. For certain combinations of these parameters Bragg diffraction behavior characteristic of volume holographic optical elements (VHOE’s) is observed, whereas for other combinations pure Raman–Nath behavior periodically recurs. By using these same relative-phasing arguments, the principal features of the periodic angular sensitivity of the +1 and −1 orders can be predicted. In addition to examining the fundamental aspects of SVHOE diffraction behavior, we discuss several possible applications, including optical array generation, spatial frequency filtering, and wavelength notch filtering. With the use of the SVHOE concept, holographic materials with otherwise exemplary characteristics that are currently available only in thin-film form can be used in structures designed either to access unique SVHOE diffraction properties or to emulate conventional VHOE’s.

Observations of changes in UV absorption bands of singlemode germanosilicate core optical fibres on writing and thermally erasing refractive index gratings

Abstract: Refractive index gratings have been formed in singlemode germanosilicate core fibres via UV side-writing at 242 nm. Direct measurements of the fibre core UV transmission spectrum show a reduction in the intensity of the GeO band at 241 nm and the growth of a short wavelength absorption edge whose origin appears to lie below 200 nm. Thermal erasure of the grating results in a recovery of the 241 nm GeO band and the disappearance of the short wavelength edge absorption.

Diffraction grating driven linear frequency chirped laser

Abstract: The present invention provides various embodiments of laser systems for generating "chirp" signals In the broadest sense, a diffraction grating is placed within a laser cavity on the face of a carrier and the motion of the carrier in its own plane causes the laser to chirp Typically, the diffraction grating is placed in the end-reflector position of the optical path of the resonant cavity associated with the laser By putting the diffraction grating on the outer peripheral rim of a wheel and rotating the wheel, a sequence of either up-chirp or down-chirp signals can be generated continuously Similarly, the desired "chirped" signals can be generated by using a diffraction grating oriented radially on the face of a rotating wheel The system also contemplates the simultaneous generation of both an up-chirp signal of one polarization and a down-chirp signal of an orthogonal polarization by using a pair of lasers and a single wheel carrying the diffraction grating on either its outer peripheral rim or its face disposed in the end-reflector positions of the lasers for simultaneously generating both the up-chirp and down-chirp signals An optical system including a reflective mirror, a one-half waveplate, and a polarizing beamsplitter are provided at the output of the two laser system for combining the up-chirp signal and down-chirp signal for simultaneous outputting

A combined FEM/MoM approach to analyze the plane wave diffraction by arbitrary gratings

Abstract: The diffraction of TE- and TM-polarized plane waves by planar gratings is numerically analyzed using a combined finite-element-method/method-of-moments (FEM/MoM) algorithm based on the generalized network formulation. The interior region, treated using the FEM, is truncated to a single unit cell with the introduction of an exact periodic boundary condition, which is enforced as a natural boundary condition. Using the FEM to compute the fields within the periodic structure allows gratings of arbitrary cross section and material composition to be efficiently modeled. >

New theoretical and experimental results in fresnel optics with applications to matter-wave and X-ray interferometry

Abstract: We present new methods and formulae for calculating the image amplitude and image spatial power spectral density produced by monochromatic point-source illumination of a finite (and/or infinite) periodic complex transmission grating. At specific finite-width resonances the image amplitude is seen to display periodic complex amplitude self-imaging of the grating, with interlaced alias images. The finite width grating resonances (as a function of spatial frequency) are broadened (from zero width) and displaced in frequency relative to those produced by an infinite grating, although the finite resonance width relative to illumination wavelength variation persists with infinite gratings. In the Fresnel domain the self images are generalizations of the Talbot and von Lau effects, while in the Fraunhofer to Fresnel transition domain, our formulae demonstrate the formation of these structures from Fraunhofer diffraction order side-lobes. Using these results, design criteria are provided for constructing lens-free three-grating interferometers with spatially diffuse illumination and detection. Such interferometers have a wide variety of applications for both X-rays and matter-waves, including a phase sensitive imaging device and/or narrow-band interference filter. For wavelengths in the Angstrom to sub-Angstrom range they feature high throughput and ease of fabrication. Experimental results using light with such an interferometer are presented. Our results conclusively demonstrate interference and image aliasing in such a device with spatially diffuse illumination and detection. The experiment is readily reproducible in any undergraduate physics laboratory.

Optical spectrophotometer having a multi-element light source

Abstract: A spectrophotometer (10) includes an array of light emitting diodes (12) configured for activation in successive Hadamard encodement patterns, a diffraction grating (14), an optical slit (16), a detector (18) and electronic controls including a computer. In operation, the diffraction grating (14) disperses and collimates radiation from the array (12) and directs selected spectral components through the slit (16) onto the detector (18) whereupon the computer performs a Hadamard analysis on the detector signals.

Multiple wavelength operation of an erbium-doped fiber laser

Abstract: Wavelength-locked, six-channel, colasing operation using a single gain medium is reported for the first time. The system is an all-fiber, erbium-amplifier-based design that uses a grating wavelength division multiplexer with a fixed channel spacing of 4.8 nm for frequency selection. The authors investigate two possible configurations for the laser cavity. >

Synthetic diffractive optics in the resonance domain

Abstract: Resonance-domain diffractive optics covers the region where the characteristic feature sizes in the surface-relief modulation structure of the diffractive optical element are comparable to the wavelength of light; it may be viewed as a bridge between synthetic holography and the electromagnetic theory of diffraction gratings and rough surfaces. We consider the problem of synthesizing, in the framework of the electromagnetic theory, various types of periodic resonance-domain diffractive optical elements that utilize several diffraction orders. Parametric optimization is used to design one-to-N fan-out elements, N-to-N star couplers, and polarization-controlled optical beam splitters and switches with close to 100% efficiencies and no undesired diffraction orders in the image half-space. Reflection-type fan-out gratings with six and seven output beams are demonstrated experimentally at λ = 10.6 μm.

Luminescent color image generation on porous silicon.

Abstract: Black and white images were projected onto n-type silicon (100) wafers during a photoelectrochemical etch to produce a color image that photoluminesces. The photoluminescence originates from a thin layer of luminescent porous silicon that is produced in the photoetch, and the colors that appear in the etched image arise from thin-film optical interference. A diffraction grating was also photoetched into the substrate, demonstrating simultaneous encoding of a gray-scale image into thin-film interference, luminescence, and diffraction phenomena.

Buried-oxide silicon-on-insulator structures. II. Waveguide grating couplers

Abstract: For pt.I, see ibid., vol.28, no.1, p.157-63 (1992). Grating couplers formed in buried-oxide silicon-on-insulator structures are analyzed using both a convergent Block wave approach and a simple approximate method. Strong interface reflections that occur during grating coupling can cause interference effects which result in variations in coupling efficiency and coupling length by an order of magnitude when varying grating period and film thickness parameters. Results indicate that proper coupler design is essential in order to obtain efficient coupling. >

Semiconductor distributed feedback lasers with quantum well or superlattice gratings for index or gain‐coupled optical feedback

Abstract: We introduce a semiconductor distributed feedback (DFB) in which the grating is fabricated out of quantum well (QW) or superlattice multilayers. This approach provides a very simple and effective scheme for achieving gain (loss)‐coupled DFB lasers. The present idea was successfully demonstrated with a 1.55‐μm wavelength 6‐QW In0.6Ga0.4As (5 nm)/InGaAsP (band‐gap wavelength=1.25 μm, 18.6 nm) separate confinement heterostructure DFB laser utilizing only a 2‐QW In0.62Ga0.38As (4 nm)/InP (9.3 nm) as the grating.