Showing papers on "Diffraction grating published in 2010"

Silicon-on-Insulator Spectral Filters Fabricated With CMOS Technology

Abstract: We give an overview of recent progress in passive spectral filters and demultiplexers based on silicon-on-insulator photonic wire waveguides: ring resonators, interferometers, arrayed waveguide gratings, and echelle diffraction gratings, all benefit from the high-index contrast possible with silicon photonics. We show how the current generation of devices has improved crosstalk levels, insertion loss, and uniformity due to an improved fabrication process based on 193 nm lithography.

High-efficiency fiber-to-chip grating couplers realized using an advanced CMOS-compatible Silicon-On-Insulator platform

Abstract: A new generation of Silicon-on-Insulator fiber-to-chip grating couplers which use a silicon overlay to enhance the directionality and thereby the coupling efficiency is presented. Devices are realized on a 200mm wafer in a CMOS pilot line. The fabricated fiber couplers show a coupling efficiency of −1.6dB and a 3dB bandwidth of 80nm.

Subwavelength grating periodic structures in silicon-on-insulator: a new type of microphotonic waveguide.

Abstract: We report on the experimental demonstration and analysis of a new waveguide principle using subwavelength gratings. Unlike other periodic waveguides such as line-defects in a 2D photonic crystal lattice, a subwavelength grating waveguide confines the light as a conventional index-guided structure and does not exhibit optically resonant behaviour. Subwavelength grating waveguides in silicon-on-insulator are fabricated with a single etch step and allow for flexible control of the effective refractive index of the waveguide core simply by lithographic patterning. Experimental measurements indicate a propagation loss as low as 2.1 dB/cm for subwavelength grating waveguides with negligible polarization and wavelength dependent loss, which compares favourably to conventional microphotonic silicon waveguides. The measured group index is nearly constant n(g) ~1.5 over a wavelength range exceeding the telecom C-band.

Refractive index engineering with subwavelength gratings for efficient microphotonic couplers and planar waveguide multiplexers

Abstract: We use subwavelength gratings (SWGs) to engineer the refractive index in microphotonic waveguides, including practical components such as input couplers and multiplexer circuits. This technique allows for direct control of the mode confinement by changing the refractive index of a waveguide core over a range as broad as 1.6–3.5 by lithographic patterning. We demonstrate two experimental examples of refractive index engineering, namely, a microphotonic fiber-chip coupler with a coupling loss as small as −0.9dB and minimal wavelength dependence and a planar waveguide multiplexer with SWG nanostructure, which acts as a slab waveguide for light diffracted by the grating, while at the same time acting as a lateral cladding for the strip waveguide. This yields an operation bandwidth of 170nm for a device size of only ~160μm×100μm.

Enhancement of optical absorption in thin-film organic solar cells through the excitation of plasmonic modes in metallic gratings

Abstract: We theoretically investigate the enhancement of optical absorption in thin-film organic solar cells in which the top transparent electrode is partially substituted by a periodic metallic grating. We show that the grating can result in broadband optical absorption enhancement for TM-polarized light, due to the large field enhancement in the vicinity of the strips of the grating, associated with the excitation of plasmonic modes. The overall optical absorption in the organic layers can be greatly enhanced up to ∼50% for such solar cell structures.

Apodized Waveguide Grating Couplers for Efficient Coupling to Optical Fibers

Abstract: We describe a shallow-etched diffractive waveguide grating coupler which achieves efficient coupling between single-mode fiber and a silicon-on-insulator optical waveguide. By the appropriate choice of waveguide/grating thicknesses and varying the coupling strength of the grating coupler via tailoring its fill factor to optimize the mode matching, a coupling loss of only 1.2 dB was obtained for each fiber/silicon waveguide interface. The coupling strength and effective refractive index of grating with different fill factors were also characterized to confirm the design simulations. The back reflection was suppressed by varying the coupling strength, leading to further enhancement of the coupling efficiency.

Subwavelength grating crossings for silicon wire waveguides

Abstract: We report on the design, simulation and experimental demonstration of a new type of waveguide crossing based on subwavelength gratings in silicon waveguides. We used 3D finite-difference time-domain simulations to minimize loss, crosstalk and polarization dependence. Measurement of fabricated devices show that our waveguide crossings have a loss as low as -0.023 dB/crossing, polarization dependent loss of < 0.02 dB and crosstalk <-40 dB.

High contrast gratings for integrated optoelectronics

Abstract: We discuss a new concept of dielectric subwavelength grating leveraging high index contrast to manipulate light to achieve ultra broad band reflector, high-Q resonator, planar high numerical aperture lens and reflector, and hollow-core integrated optics.

Point-by-point written fiber-Bragg gratings and their application in complex grating designs

Abstract: The point-by-point technique of fabricating fibre-Bragg gratings using an ultrafast laser enables complete control of the position of each index modification that comprises the grating. By tailoring the local phase, amplitude and spacing of the grating’s refractive index modulations it is possible to create gratings with complex transmission and reflection spectra. We report a series of grating structures that were realized by exploiting these flexibilities. Such structures include gratings with controlled bandwidth, and amplitude- and phase-modulated sampled (or superstructured) gratings. A model based on coupled-mode theory provides important insights into the manufacture of such gratings. Our approach offers a quick and easy method of producing complex, non-uniform grating structures in both fibres and other mono-mode waveguiding structures.

Highly efficient nonuniform grating coupler for silicon-on-insulator nanophotonic circuits

Abstract: We present design, fabrication, and characterization of a silicon-on-insulator grating coupler of high efficiency for coupling between a silicon nanophotonic waveguide and a single mode fiber. By utilizing the lag effect of the dry etching process, a grating coupler consisting of nonuniform grooves with different widths and depths is designed and fabricated to maximize the overlapping between the upward wave and the fiber mode. The measured waveguide-to-fiber coupling efficiency of 64% (-1.9 dB) for the transverse electric polarization is achieved by the present nonuniform grating coupler directly defined on a regular silicon-on-insulator wafer.

The Promise of Diffractive Waveplates

Abstract: Diffractive waveplates exhibit the high diffraction efficiency of Bragg gratings in micron-thick material layers.

Wide Bandwidth Silicon Nitride Grating Coupler

Abstract: We demonstrate a one-dimensional grating coupler in silicon nitride with a 67-nm 1-dB bandwidth, the largest reported for a grating coupler butt-coupled to standard single-mode fiber. The peak coupling efficiency is -4.2dB. It requires no partial etch and requires only 0.6-μm fabrication resolution.

Non-linear optical grating

Abstract: A method and system for providing an optical grating are described. The optical grating is configured for light of a wavelength. The optical grating includes a top cladding, a core, and bottom cladding. The core resides between the bottom cladding and the top cladding. The core includes a plurality of discrete ridges spaced apart by a nonlinear pitch. The light traverses the top cladding before the core and has a plurality of angles of incidence with the core. The nonlinear pitch of the core is larger for a larger angle of incidence of the plurality of angles of incidence.

Double optical grating

Abstract: A method and system for providing an optical grating are described. The optical grating is configured for light of a wavelength. The optical grating includes a top cladding, a first plurality of discrete ridges forming a first grating, a core, a second plurality of discrete ridges forming a second grating, and a bottom cladding. The first plurality of discrete ridges are spaced apart by a first pitch. The second plurality of discrete ridges are spaced apart by a second pitch. The core has a top side adjacent to at least a portion of the top cladding and a bottom side. The bottom cladding is adjacent to at least a portion of the bottom side of the core. The second grating resides between the bottom cladding and the core.

Extending the detection range of optical vortices by Dammann vortex gratings

Abstract: We report a 2D static binary phase Dammann vortex grating that combines the features of a conventional vortex grating and a Dammann grating. This grating uniformly distributes energies among the diffraction orders, so the low-efficiency problem at higher diffraction orders of conventional vortex gratings is resolved and the detection range of the optical vortices (OVs) is greatly increased. Experimental results of OV detection using a fabricated 5×5 Dammann vortex grating are given, and the topological charge detection range from −12 to +12 is achieved. The potential applications of such gratings include transmitting, receiving, and multiplexing OV beams in optical communication systems.

Tunable optical gratings based on buckled nanoscale thin films on transparent elastomeric substrates

Abstract: This letter reports a tunable optical grating based on buckled thin film with periodic sinusoidal patterns on a transparent elastomeric substrate. Submicron scale sinusoidal gratings have been fabricated with nanometer thick Gold/Palladium film coated on 30% pretensioned polydimethylsiloxane substrates. Due to competition between the soft elastomeric substrates and relatively stiff films, periodic wavy profiles are created upon releasing the pretension. The buckling profiles can be easily tuned by mechanically stretching or compressing. Optical transmittance diffraction testing has been conducted, and 85 nm peak wavelength shifts of the first order diffraction have been achieved by stretching the grating up to 30% of its original length.

High-efficiency, large-bandwidth silicon-on-insulator grating coupler based on a fully-etched photonic crystal structure

Abstract: A grating coupler for interfacing between single-mode fibers and photonic circuits on silicon-on-insulator is demonstrated. It consists of columns of fully etched photonic crystal holes, which are made in the same lithography and etching processes used for making the silicon-on-insulator wire waveguide. The holes have a diameter of around 143 nm, and are defined with electron-beam lithography. A peak coupling efficiency of 42% at 1550 nm and 1 dB bandwidth of 37 nm, as well as a low back reflection, are achieved. The performance of the proposed fully etched grating coupler is comparable to that based on the conventional shallowly etched grating, which needs additional fabrication steps.

A grating-based single-shot x-ray phase contrast and diffraction method for in vivo imaging

Abstract: Purpose: The purpose of this study is to develop a single-shot version of the grating-based phase contrastx-ray imaging method and demonstrate its capability ofin vivo animal imaging. Here, the authors describe the principle and experimental results. They show the source of artifacts in the phase contrast signal and optimal designs that minimize them. They also discuss its current limitations and ways to overcome them. Methods: A single lead grid was inserted midway between an x-ray tube and an x-raycamera in the planar radiography setting. The grid acted as a transmission grating and cast periodic dark fringes on the camera. The camera had sufficient spatial resolution to resolve the fringes. Refraction and diffraction in the imaged object manifested as position shifts and amplitude attenuation of the fringes, respectively. In order to quantify these changes precisely without imposing a fixed geometric relationship between the camera pixel array and the fringes, a spatial harmonic method in the Fourier domain was developed. The level of the differential phase (refraction) contrast as a function of hardware specifications and device geometry was derived and used to guide the optimal placement of the grid and object. Bothex vivo and in vivoimages of rodent extremities were collected to demonstrate the capability of the method. The exposure time using a 50 W tube was 28 s. Results: Differential phase contrastimages of glass beads acquired at various grid and object positions confirmed theoretical predictions of how phase contrast and extraneous artifacts vary with the device geometry. In anesthetized rats, a single exposure yielded artifact-free images of absorption, differential phase contrast, and diffraction. Differential phase contrast was strongest at bone-soft tissue interfaces, while diffraction was strongest in bone. Conclusions: The spatial harmonic method allowed us to obtain absorption, differential phase contrast, and diffractionimages, all from a single raw image and is feasible in live animals. Because the sensitivity of the method scales with the density of the gratings, custom microfabricated gratings should be superior to off-the-shelf lead grids.

High-power widely tunable thulium fiber lasers

Abstract: Applications requiring long-range atmospheric propagation are driving the development of high-power thulium fiber lasers. We report on the performance of two different laser configurations for high-power tunable thulium fiber lasers: one is a single oscillator utilizing a volume Bragg grating for wavelength stabilization; the other is a master oscillator power amplifier system with the oscillator stabilized and made tunable by a diffraction grating. Each configuration provides >150W of average power, >50% slope efficiency, narrow output linewidth, and >100nm tunability in the wavelength range around 2μm.

Colour and the Optical Properties of Materials: An Exploration of the Relationship Between Light, the Optical Properties of Materials and Colour

Abstract: Preface. 1 Light and Colour. 1.1 Colour and light. 1.2 Colour and energy. 1.3 Light waves. 1.4 Interference. 1.5 Light waves and colour. 1.6 Black body radiation and incandescence. 1.7 The colour of incandescent objects. 1.8 Photons. 1.9 Lamps and lasers. 1.10 Vision. 1.11 Colour perception. 1.12 Additive coloration. 1.13 The interaction of light with a material. 1.14 Subtractive coloration. 1.15 Electronic "paper". 1.16 Appearance and transparency. Appendix 1.1 Definitions, units and conversion factors. Further reading. 2 Colours due to Refraction and Dispersion. 2.1 Refraction and the refractive index of a material. 2.2 Total internal reflection. 2.3 Refractive index and polarisability. 2.4 Refractive index and density. 2.5 Invisible animals, GRINS and mirages. 2.6 Dispersion and colours produced by dispersion. 2.7 Rainbows and halos. 2.8 Halos. 2.9 Fibre optics. 2.10 Negative refractive index materials. Further reading. 3 The Production of Colour by Reflection. 3.1 Reflection from a single surface. 3.2 Interference at a single thin film in air. 3.3 The colour of a single thin film in air. 3.4 The reflectivity of a single thin film in air. 3.5 The colour of a single thin film on a substrate. 3.6 The reflectivity of a single thin film on a substrate. 3.7 Low-reflection and high-reflection films. 3.8 Multiple thin films. 3.9 Fibre Bragg Gratings. 3.10 "Smart" windows. 3.11 Photonic engineering in nature. 3.12 Further reading. 3.13 Problems and exercises. Appendix 3.1 The colour of a thin film in white light. Further Reading. 4 Polarisation and crystals. 4.1 Polarisation of light. 4.2 Polarisation by reflection. 4.3 Polars. 4.4 Crystal symmetry and refractive index. 4.5 Double refraction: calcite as an example. 4.6 The description of double refraction effects. 4.7 Colour produced by polarisation and birefringence. 4.8 Pleochroism and dichroism. 4.9 Nonlinear effects. 4.10 Frequency matching and phase matching. 4.11 More on second harmonic generation. 4.12 Optical activity. 4.13 Liquid crystals. Further reading. 5 Colour due to Scattering. 5.1 Scattering and extinction. 5.2 Tyndall blue and Rayleigh scattering. 5.3 Blue skies, red sunsets. 5.4 Scattering and polarisation. 5.5 Mie scattering. 5.6 Blue eyes and some blue feathers. 5.7 Paints, sunscreens and related matters. 5.8 Multiple scattering. 5.9 Gold sols and ruby glass. 5.10 The Lycurgus Cup. Further reading. 6 Colour due to Diffraction. 6.1 Diffraction and colour production by a slit. 6.2 Diffraction and colour production by a rectangular aperture. 6.3 Diffraction and colour production by a circular aperture. 6.4 The diffraction limit of optical instruments. 6.5 Colour production by linear diffraction gratings. 6.6 Two-dimensional gratings. 6.7 Estimation of the wavelength of light by diffraction. 6.8 Diffraction by crystals and crystal-like structures. 6.9 Disordered diffraction gratings. 6.10 Diffraction by sub-wavelength structures. 6.11 Holograms. Further reading. 7 Colour from Atoms and Ions. 7.1 The spectra of atoms and ions. 7.2 Terms and levels. 7.3 Atomic spectra and chemical analysis. 7.4 Fraunhofer lines and stellar spectra. 7.5 Neon signs and early plasma displays. 7.6 The helium-neon laser. 7.7 Sodium and mercury street lights. 7.8 Transition metals and crystal field colours. 7.9 Crystal field splitting, energy levels and terms. 7.10 The colour of ruby. 7.11 Transition-metal-ion lasers. 7.12 Emerald, alexandrite and crystal field strength. 7.13 Crystal field colours in minerals and gemstones. 7.14 Colour as a structural probe. 7.15 Colours from lanthanide ions. 7.16 The neodymium (Nd3+) solid state laser: a four level laser. 7.17 Amplification of optical fibre signals. 7.18 Transition metal and lanthanide pigments. 7.19 Spectral hole formation. 7.20 Further reading. 7.21 Problems and exercises. Appendix 7.1 Electron configurations. Appendix 7.2 Terms and levels. Further Reading. 8 Colour from Molecules. 8.1 The energy levels of molecules. 8.2 The colours arising in some simple inorganic molecules. 8.3 The colour of water. 8.4 Chromophores, chromogens and auxochromes. 8.5 Conjugated bonds in organic molecules: the carotenoids. 8.6 Conjugated bonds circling metal atoms: porphyrins and phthalocyanines. 8.7 Naturally occurring colorants: flavonoid pigments. 8.8 Autumn leaves. 8.9 Some dyes and pigments. 8.10 Charge transfer colours. 8.11 Colour change sensors. 8.12 Dye lasers. 8.13 Photochromic organic molecules. Further reading. 9 Luminescence. 9.1 Luminescence. 9.2 Activators, sensitizers and fluorophores. 9.3 Atomic processes in photoluminescence. 9.4 Fluorescent lamps. 9.5 Plasma displays. 9.6 Cathodoluminescence and cathode ray tubes (CRTs). 9.7 Field emission displays (FEDs). 9.8 Phosphor electroluminescent displays. 9.9 Upconversion. 9.10 Quantum cutting. 9.11 Fluorescent molecules. 9.12 Fluorescent nanoparticles. 9.13 Fluorescent markers and sensors. 9.14 Chemiluminescence and Bioluminescence. 9.15 Triboluminescence. 9.16 Scintillators. Further reading. 10 Colour in Metals, Semiconductors and Insulators. 10.1 The colours of insulators. 10.2 Excitons. 10.3 Impurity colours in insulators. 10.4 Impurity colours in diamond. 10.5 Colour centres. 10.6 The colours of semiconductors. 10.7 The colours of semiconductor alloys. 10.8 Light emitting diodes (LEDs). 10.9 Semiconductor diode lasers. 10.10 Semiconductor nanostructures. 10.11 Organic semiconductors and electroluminescence. 10.12 Electrochromic films. 10.13 Photovoltaics. 10.14 Digital photography. 10.15 The colours of metals. 10.16 The colours of metal nanoparticles. 10.17 Extraordinary light transmission and plasmonic crystals. Further Reading. Index.

2D back-side diffraction grating for improved light trapping in thin silicon solar cells

Abstract: Light-trapping techniques can be used to improve the efficiency of thin silicon solar cells. We report on numerical investigation of a light trapping design consisting of a 2D back-side diffraction grating in combination with an aluminum mirror and a spacing layer of low permittivity to minimize parasitic absorption in the aluminum. The light-trapping design was compared to a planar reference design with antireflection coating and back-side aluminum mirror. Both normally and obliquely incident light was investigated. For normal incidence, the light trapping structure increases the short circuit current density with 17% from 30.4 mA/cm2 to 35.5 mA/cm2 for a 20 µm thick silicon solar cell. Our design also increases the current density in thinner cells, and yields higher current density than two recently published designs for cell thickness of 2 and 5 µm, respectively. The increase in current may be attributed to two factors; increased path length due to in-coupling of light, and decreased parasitic absorption in the aluminum due to the spacing layer.

Photocurrent increase in n-i-p thin film silicon solar cells by guided mode excitation via grating coupler

Abstract: Angle resolved measurements of the external quantum efficiency of n-i-p single junction amorphous solar cell deposited on a grating structure show clearly defined peaks of enhanced photocurrent in the weakly absorbing region between 1.6 and 2.15 eV. We explain these absorption phenomena and their angular variation with the excitation of guided modes via grating coupling. Calculation using an equivalent flat multilayer system permits to relate the theoretical values with the experimental data.

Sub-nm resolution cavity enhanced microspectrometer.

Abstract: A novel on-chip spectrometer device using combined functionalities of a micro-ring resonator and a planar diffraction grating is proposed. We investigate the performance of this architecture by implementing it in a silicon-on-insulator platform. We experimentally demonstrate such a device with 100 channels, 0.1 nm channel spacing and a channel crosstalk less than -10 dB. The entire device occupies an area of less than 2 mm2. Based on our initial results we envision that this device enables the possibility of the realization of low-cost and high-resolution ultra-compact spectroscopy.

Electromagnetically-Induced Phase Grating

Abstract: Exploiting the giant Kerr nonlinearity associated with electromagnetically induced transparency, an atomic phase grating created with arbitrarily weak fields is proposed. Almost 30% of a probe beam can be diffracted into the first diffraction order.

Polarization diffraction grating produced by femtosecond laser nanostructuring in glass

Abstract: We demonstrate polarization sensitive diffractive optical element fabrication by femtosecond direct writing in the bulk of silica glass. Modulation of the anisotropic properties is produced by controlling light-induced self-assembled nano-gratings.

Decomposition of radially and azimuthally polarized beams using a circular-polarization and vortex-sensing diffraction grating

Abstract: Both radially polarized and azimuthally polarized beams can be decomposed into linear combinations of circularly polarized vortex beams having opposite vortex charges. We show experimental evidence for this decomposition using a specially designed vortex sensing diffraction grating that generates multiple vortex patterns having different senses of circularly polarization in the different diffracted orders. When this grating is illuminated with a radially or azimuthally polarized beam, the grating separates the components into different diffracted orders. Experimental results are shown.

All-fiber temporal photonic fractional Hilbert transformer based on a directly designed fiber Bragg grating

Abstract: An all-fiber temporal photonic fractional Hilbert transform (FHT) based on a fiber Bragg grating (FBG) is proposed and investigated, for the first time to our knowledge. The photonic FHT is designed based on the discrete layer peeling method, which enables the FBG to have a strong strength. Numerical results show that the FBG can be used to efficiently and accurately implement broadband all-optical FHT for an arbitrary optical waveform with a bandwidth up to hundreds of gigahertz.

High-temperature multiparameter sensor based on sapphire fiber Bragg gratings.

Abstract: We present, for the first time to our knowledge, a dual strain/temperature sapphire fiber Bragg grating sensor. Temperature and strain coefficients of the grating are evaluated. By recording the blackbody radiation level above 650°C, wavelength shifts due to temperature can be decoupled from those due to strain.

Planar optical system for wide field-of-view polychromatic imaging

Abstract: A planar optical system for wide field-of-view polychromatic imaging includes a planar waveguide including two plane parallel faces, an entry coupler including a first diffraction grating, and an exit coupler including a second diffraction grating. The diffraction gratings are low line density diffraction gratings that have a pitch greater than the wavelength of use such that the grating is adapted to couple an entry beam having a mean angle of incidence i0 ranging between 30 to 60 degrees into the waveguide by positive first order (+1) diffraction, the coupled beam defining an internal angle of incidence greater than the angle of total internal reflection and less than γ=80 degrees, and the second grating is adapted to receive the coupled beam and to diffract it out of the waveguide by negative first order (−1) diffraction at a mean exit angle i1 ranging between 30 to 60 degrees.

Silicon-photonics light source realized by III–V/Si-grating-mirror laser

Abstract: A III–V/Si vertical-cavity in-plane-emitting laser structure is suggested and numerically investigated. This hybrid laser consists of a distributed Bragg reflector, a III–V active region, and a high-index-contrast grating (HCG) connected to an in-plane output waveguide. The HCG and the output waveguide are made in the Si layer of a silicon-on-insulator wafer by using Si-electronics-compatible processing. The HCG works as a highly-reflective mirror for vertical resonance and at the same time routes light to the in-plane output waveguide. Numerical simulations show superior performance compared to existing silicon light sources.