Showing papers on "Diffraction grating published in 2009"

Off-chip beam steering with a one-dimensional optical phased array on silicon-on-insulator

Abstract: Optical phased arrays are versatile components enabling rapid and precise beam steering. An integrated approach is followed in which a 1D optical phased array is fabricated on silicon-on-insulator. The optical phased array consists of 16 parallel grating couplers spaced 2 mum apart. Steering in one direction is done thermo-optically by means of a titanium electrode on top of the structure using the phased array principle, while steering in the other direction is accomplished by wavelength tuning. At a wavelength of 1550 nm, continuous thermo-optical steering of 2.3 degrees and wavelength steering of 14.1 degrees is reported.

Complete discrimination of strain and temperature using Brillouin frequency shift and birefringence in a polarization-maintaining fiber

Abstract: This paper presents a novel method that realizes simultaneous and completely discriminative measurement of strain and temperature using one piece of Panda-type polarization-maintaining fibre. Two independent optical parameters in the fiber, the Brillouin frequency shift and the birefringence, are measured by evaluating the spectrum of stimulated Brillouin scattering (SBS) and that of the dynamic acoustic grating generated in SBS to get two independent responses to strain and temperature. We found that the Brillouin frequency shift and the birefringence have the same signs for strain-dependence but opposite signs for temperature-dependence. In experiment, the birefringence in the PMF is characterized with a precision of approximately 10(-8) by detecting the diffraction spectrum of the dynamic acoustic grating. A reproducible accuracy of discriminating strain and temperature as fine as 3 micro-strains and 0.08 degrees Celsius is demonstrated.

Designing periodic arrays of metal nanoparticles for light-trapping applications in solar cells

Abstract: The authors acknowledge the A. R. C. and NOW for research conducted at the FOM as a part of the Joint Solar Programme for financial support.

Waveguide grating coupler with subwavelength microstructures.

Abstract: We propose a silicon waveguide-fiber grating coupler that uses a subwavelength microstructure to achieve a continuously variable grating strength yet can be fabricated using only a single etch step. By adjusting the subwavelength microstructure at every point along the grating, the grating coupler can be optimized to give high field overlap with the optical fiber mode and also minimize backreflections along the incident waveguide path. Our design example is optimized for quasi-TM mode in a silicon photonic-wire waveguide, as required for waveguide evanescent-field-sensing applications. A field overlap of up to 94% with a standard single-mode optical fiber (SMF-28) is achieved by coupler apodization. Backreflection from the grating is reduced to ~0.1%, and the total predicted photonic wire to fiber coupling efficiency is 50%.

Fluid-Filled Solid-Core Photonic Bandgap Fibers

Abstract: We review the field of fluid-filled solid-core photonic bandgap fibers (SC-PBGF): we present the mechanisms and models of light guidance in these fibers, and how the guidance properties are sensitive to the refractive index of the infiltrated fluid. We discuss how this sensitivity can be used for creating tunable devices such as filters, delay lines and tunable non-linear pulse propagation experiments. We review refractive index sensors based on SC-PBGFs, including band edge sensing, SC-PBGF based long period grating sensing and new results on selectively filled SC-PBGFs which are the most sensitive microstructured fiber based sensors to date. We also discuss practical aspects of fluid filling.

Inverse geometry for grating-based x-ray phase-contrast imaging

Abstract: Phase-contrast imaging using conventional polychromatic x-ray sources and grating interferometers has been developed and demonstrated for x-ray energies up to 60 keV. Here, we conduct an analysis of possible grating configurations for this technique and present further geometrical arrangements not considered so far. An inverse interferometer geometry is investigated that offers significant advantages for grating fabrication and for the application of the method in computed tomography (CT) scanners. We derive and measure the interferometer’s angular sensitivity for both the inverse and the conventional configuration as a function of the sample position. Thereby, we show that both arrangements are equally sensitive and that the highest sensitivity is obtained, when the investigated object is close to the interferometer’s phase grating. We also discuss the question whether the sample should be placed in front of or behind the phase grating. For CT applications, we propose an inverse geometry with the sample ...

Mechanisms of ultrafast laser-induced deep-subwavelength gratings on graphite and diamond

Abstract: Deep-subwavelength gratings with periodicities of 170, 120, and 70 nm can be observed on highly oriented pyrolytic graphite irradiated by a femtosecond (fs) laser at 800 nm. Under picosecond laser irradiation, such gratings likewise can be produced. Interestingly, the 170-nm grating is also observed on single-crystal diamond irradiated by the 800-nm fs laser. In our opinion, the optical properties of the high-excited state of material surface play a key role for the formation of the deep-subwavelength gratings. The numerical simulations of the graphite deep-subwavelength grating at normal and high-excited states confirm that in the groove the light intensity can be extraordinarily enhanced via cavity-mode excitation in the condition of transverse-magnetic wave irradiation with near-ablation-threshold fluences. This field enhancement of polarization sensitiveness in deep-subwavelength apertures acts as an important feedback mechanism for the growth and polarization dependence of the deep-subwavelength gratings. In addition, we suggest that surface plasmons are responsible for the formation of seed deep-subwavelength apertures with a particular periodicity and the initial polarization dependence. Finally, we propose that the nanoscale Coulomb explosion occurring in the groove is responsible for the ultrafast nonthermal ablation mechanism.

Light trapping in thin-film silicon solar cells with integrated diffraction grating

Abstract: The optics of microcrystalline silicon thin-film solar cells with integrated light trapping structures was investigated. Periodic grating couplers were integrated in microcrystalline silicon thin-film solar cells and the influence of the grating dimensions on the short circuit current and the quantum efficiency was investigated by the numerical simulation of Maxwell’s equations utilizing the finite difference time domain algorithm. The grating structure leads to scattering and higher order diffraction resulting in an increased absorption of the incident light in the silicon thin-film solar cell. The influence of the grating period and the grating height on the short circuit current and the quantum efficiency was investigated. Enhanced quantum efficiencies are observed for the red and infrared parts of the optical spectrum. Optimal dimensions of the grating coupler were obtained.

Optical grating and method of manufacture

Abstract: A method and system for providing an optical grating are described. The optical grating is configured for light of a wavelength and includes a first optically transparent layer, a stop layer on the first optically transparent layer, and a second optically transparent layer on the stop layer. The first optically transparent layer is continuous and includes a material. The second optically transparent layer also includes the material. The second optically transparent layer also includes a plurality of discrete ridges spaced apart by a pitch. The stop layer is configured to be invisible to the light.

Analysis of Bloch-surface-wave assisted diffraction-based biosensors

Abstract: A systematic study of Bloch surface wave (BSW) properties and applications in diffraction-based biosensors is presented. The design of such devices starts with the calculation of the BSW dispersion relation for a semi-infinite one-dimensional photonic crystal. We propose an approach in which polarization and 1DPC termination effects are simply described. Since in a realistic device the number of periods is limited, we investigate the issues arising from finite size effects and the choice of a structure substrate. Diffraction efficiency is studied as a function index contrast, multilayer termination, grating thickness, and number of periods. Numerical examples for Si/SiO2 and a-Si1−xNx:H periodic dielectric stacks are presented, showing that BSW can be exploited for the realization of efficient diffraction-based biosensors from the infrared to the visible range.

Cladding-modulated Bragg gratings in silicon waveguides.

Abstract: A cladding-modulated Bragg grating implemented using periodic placements of cylinders along a waveguide is proposed in a silicon-on-insulator platform. The coupling strength is varied by changing the distance between the cylinders and the waveguide. This implementation enables precise control and a wide dynamic range of coupling strengths and bandwidths that can be practically achieved for applications with specific bandwidth requirements. Modeling results are verified experimentally, and we demonstrate coupling strengths differing by 1 order of magnitude (43 and 921 per cm) with bandwidths of 8 and 16 nm, respectively. This method scheme enables weakly coupled devices with high fabrication tolerance to be realized.

DFB Quantum Cascade Laser Arrays

Abstract: DFB quantum cascade laser (DFB-QCL) arrays operating between 8.7 and 9.4 mum are investigated for their performance characteristics-single-mode selection of the DFB grating, and variability in threshold, slope efficiency, and output power of different lasers in the array. Single-mode selection refers to the ability to choose a desired mode/frequency of laser emission with a DFB grating. We apply a theoretical framework developed for general DFB gratings to analyze DFB-QCL arrays. We calculate how the performance characteristics of DFB-QCLs are affected by the coupling strength kappaL of the grating, and the relative position of the mirror facets at the ends of the laser cavity with respect to the grating. We discuss how single-mode selection can be improved by design. Several DFB-QCL arrays are fabricated and their performance examined. We achieve desired improvements in single-mode selection, and we observe the predicted variability in the threshold, slope efficiency, and output power of the DFB-QCLs. As a demonstration of potential applications, the DFB-QCL arrays are used to perform infrared absorption spectroscopy with fluids.

All-optical Hilbert transformer based on a single phase-shifted fiber Bragg grating: design and analysis

Abstract: A simple all-fiber design for implementing an all-optical temporal Hilbert transformer is proposed and numerically demonstrated. We show that an all-optical Hilbert transformer can be implemented using a uniform-period fiber Bragg grating (FBG) with a properly designed amplitude-only grating apodization profile incorporating a single π phase shift in the middle of the grating length. All-optical Hilbert transformers capable of processing arbitrary optical waveforms with bandwidths up to a few hundreds of gigahertz can be implemented using feasible FBGs.

Dispersive element, diffraction grating, color display device, demultiplexer, and diffraction grating manufacture

Abstract: In a transmission grating as a dispersive element, diffraction efficiency is enhanced and manufacturing costs are considerably reduced. A dispersive element includes resin members for forming a diffraction grating, being composed of a plurality of diffraction grating members having a cross-sectional shape respectively surrounded by two straight lines such as a triangular shape, and metal members as light-shielding members each being formed on corresponding one of the diffraction grating members at one side of the diffraction grating member along any of the straight line and the curved line of the cross-sectional shape of the diffraction grating member formed by the resin member. The metal members are configured to reduce zero-order transmitted light with respect to incident light, and to enhance diffraction efficiency of first-order transmitted light.

High-Contrast Grating VCSELs

Abstract: Recent advances in a single-layer 1-D high-index-contrast subwavelength grating structure are reviewed. Its incorporation into a vertical-cavity surface-emitting laser (VCSEL) structure enabled simple fabrication, lithographically defined polarization control and large aperture, single-transverse-mode control. Extraordinarily large fabrication tolerance is demonstrated with plusmn20% variation of the high-contrast grating (HCG) critical dimension. Emission wavelength of HCG-VCSEL varied 0.2% with a 40% change in lithography linewidth. Tunable VCSELs are fabricated using HCG, which led to a 8000 times reduction in the tunable mirror size and 160 times improved tuning speed of 63 ns. This configuration will open the door for a wide spectrum of optoelectronic devices in large wavelength regimes.

Focusing Polarization Diversity Grating Couplers in Silicon-on-Insulator

Abstract: We report on compact focusing polarization diversity grating couplers in silicon-on-insulator, which can be used to overcome the polarization dependence of nanophotonic integrated circuits. The minimum fiber-to-fiber polarization dependent loss is 0.4 dB and the focusing grating couplers are as performant as standard 2-D-grating couplers without focusing. In addition, the focusing property of the gratings results in an 8-fold length reduction of the coupling structure as compared to standard nonfocusing 2-D-grating versions.

Linear-wavenumber spectrometer for high-speed spectral-domain optical coherence tomography

Abstract: An equidistant (in the wavenumber) spectrometer based on a diffraction grating, a compensation prism, and a CCD linear array is developed and implemented for spectral-domain optical coherence tomography. A criterion is introduced for estimating the level of residual nonequidistance. This criterion allows one to determine the threshold compensation level necessary for obtaining the spectrally limited spatial resolution. The method is tested in spectral-domain optical coherent tomography systems at wavelengths of 1270 and 830 nm.

Utilization of phononic-crystal reflective gratings in a layered surface acoustic wave device

Abstract: In this paper, a design that combines two-port surface acoustic wave (SAW) devices and phononic crystals (PCs) acting as reflected gratings is demonstrated. Finite-difference time-domain method is used to analyze SAWs encountering the PC and optimize the design. A layered ZnO/Si SAW device and a square lattice PC composing of cylindrical holes on silicon were fabricated. With the PC of 15-layer cylinders, experimental insertion loss shows a 7 dB improvement at 212 MHz at central frequency. In addition, the size of gratings is reduced significantly as compared to the traditional gratings with hundreds of metal strips.

Light field image sensors based on the Talbot effect

Abstract: We present a pixel-scale sensor that uses the Talbot effect to detect the local intensity and incident angle of light. The sensor comprises two local diffraction gratings stacked above a photodiode. When illuminated by a plane wave, the upper grating generates a self-image at the half Talbot depth. The second grating, placed at this depth, blocks or passes light depending upon incident angle. Several such structures, tuned to different incident angles, are sufficient to extract local incident angle and intensity. Furthermore, arrays of such structures are sufficient to localize light sources in three dimensions without any additional optics.

Voltage-programmable liquid optical interface

Abstract: Recently, there has been intense interest in photonic devices based on microfluidics, including displays1,2 and refractive tunable microlenses and optical beamsteerers3,4,5 that work using the principle of electrowetting6,7. Here, we report a novel approach to optical devices in which static wrinkles are produced at the surface of a thin film of oil as a result of dielectrophoretic forces8,9,10. We have demonstrated this voltage-programmable surface wrinkling effect in periodic devices with pitch lengths of between 20 and 240 µm and with response times of less than 40 µs. By a careful choice of oils, it is possible to optimize either for high-amplitude sinusoidal wrinkles at micrometre-scale pitches or more complex non-sinusoidal profiles with higher Fourier components at longer pitches. This opens up the possibility of developing rapidly responsive voltage-programmable, polarization-insensitive transmission and reflection diffraction devices and arbitrary surface profile optical devices. Voltage-programmable liquid surface profiles with large amplitudes resulting from dielectrophoresis are demonstrated. The oil interface formed can be ultrastable and static, or rapidly switchable, as shown in the case of a modulated diffraction grating. The scheme provides the possibility for responsive and polarization-insensitive transmission and reflection devices, and for optical interfaces with arbitrary surface profiles.

Polarization insensitive imaging through polarization gratings.

Abstract: Liquid crystal polarization gratings exhibit high diffraction efficiency (~ 100%) in thin material layers comparable to the radiation wavelength. We demonstrate that they can be combined for polarization-insensitive imaging and optical switching applications. A pair of closely spaced, parallel oriented, cycloidal polarization gratings is capable of canceling the diffractive property of an individual grating. As a result, the phase of the beam is not distorted, and holographic images can be formed through them. An anti-parallel arrangement results in a broader effective diffraction band and doubles the diffraction angle. Broadband diffraction spanning from 480 nm to beyond 900 nm wavelengths has been obtained for a pair of gratings with 500 nm and 633 nm peak diffraction wavelengths. Liquid crystal polymer cycloidal gratings were used in the study showing 98% diffraction efficiency over a large area, and allowed for the use of laser beams expanded to 25 mm. The characteristics of combined cycloidal gratings were tested with laser beams at both UV and red wavelengths.

Vortex sensing diffraction gratings

Abstract: We demonstrate a specially designed vortex sensing diffraction grating that generates multiple vortex patterns in the different diffracted orders. When this grating is illuminated with a separate vortex beam, the sign and order of the topological charge of the incident beam can be easily detected. Experimental results are shown for a variety of vortex beams including fractional values of the topological charge, and where both the diffraction grating and incident vortex illumination beam are generated by two different liquid crystal displays (LCDs). The programmability offered by the LCDs offers extremely convenient flexibility.

Colored images generated by metallic sub-wavelength gratings.

Abstract: Gratings with periods smaller than visible wavelengths in ambient white light will exhibit enhanced colors if the profile is designed so that resonant light interaction occurs in the visible range. Resonances have a frequency-selective influence to the grating diffraction inducing colors in transmittance and reflectance, respectively. Apart from the well-known surface-plasmon polariton excitations and cavity resonances, newly discovered resonances in TE-polarization can be exploited for colorizing wire-gratings, when simply illuminated by unpolarized white light. Colors can be laterally tuned by varying the grating profile. The capability of generating images by sub-wavelength gratings is exemplified by a metallic wire grating embedded in a plastic foil with a lateral variable modulation depth. This method for producing colored images is predestined for industrial mass production and will have ever more practical applications such as for security features.

Optical time-domain measurement of Brillouin dynamic grating spectrum in a polarization-maintaining fiber.

Abstract: We demonstrate the distributed measurement of Brillouin dynamic grating spectra in a polarization-maintaining fiber based on time-domain analysis. Local reflection spectra of the Brillouin dynamic grating are acquired by synchronized propagation of the pump and the probe pulses based on the map of the Brillouin frequency distribution. Large temperature sensitivity as high as −50.9 MHz/°C is observed with 2 m spatial resolution in 100 m range.

Broadband astigmatism-free Czerny-Turner imaging spectrometer using spherical mirrors

Abstract: We describe the elimination of the astigmatism of a Czerny-Turner imaging spectrometer, built using spherical optics and a plane grating, over a broad spectral region. Starting with the principle of divergent illumination of the grating, which removes astigmatism at one chosen wavelength, we obtain design equations for the distance from the grating to the focusing mirror and the detector angle that remove the astigmatism to first order in wavelength. Experimentally, we demonstrate near diffraction-limited performance from 740 to 860 nm and over a 5 mm transverse spatial extent, while ray-tracing calculations show that barring finite-aperture and detector size limitations, this range extends from 640 to 900 nm and over 10 mm transversely. Our technique requires no additional optics and uses standard off-the-shelf components.

Polarization properties of chiral fiber gratings

Abstract: Recent experiments (Kopp et al 2007 J. Opt. Soc. Am. B 24 A48) have demonstrated that the polarization sensitivity of chiral fiber gratings depends strongly on the grating symmetry: double-helix fibers are polarization sensitive while single-helix fibers are not. A coupled-mode perturbation theory is developed and used to explain the polarization properties of chiral fiber gratings. Features of the transmission spectrum such as multiple dips in the spectrum and circular dichroism are also derived and attributed to chiral Bragg scattering of the core modes into the cladding modes of the fiber.

Nanoholes Grating Couplers for Coupling Between Silicon-on-Insulator Waveguides and Optical Fibers

Abstract: Simulation and experimental results of grating couplers composed of arrays of nanoholes are presented. The use of an array of holes instead of a conventional waveguide grating provides an additional degree of freedom in the design of the coupler, thus enabling fabrication using the same photolithography mask and etching process as used for the silicon-on-insulator (SOI) waveguides. A grating coupler with coupling efficiency as high as 34% for coupling between the TE mode of the silicon nanophotonic wire waveguide and a single-mode optical fiber and with 3-dB bandwidth of 40 nm was fabricated. A theoretical model is presented, and 3-D finite-difference time-domain simulations are used to optimize the coupler design.

Silicon-on-Insulator Echelle Grating WDM Demultiplexers With Two Stigmatic Points

Abstract: We present ultracompact integrated optical echelle grating wavelength-division-multiplexing (de)multiplexers for on-chip optical networks, fabricated using high-index-contrast silicon-on-insulator photonic waveguide technology. These devices are based on a design with two stigmatic points, which enables compact geometries with reduced aberrations. In the example presented here, this design allows us to achieve an eight-channel (de)multiplexer with 3.2-nm channel spacing, within an ultracompact footprint of 250 times 200 mum. The channel-to-channel isolation of the devices is 19 dB. The minimum insertion loss, relative to a straight waveguide, is 3 dB with a channel-to-channel variation of 0.5 dB.

Corrugated-sidewall interband cascade lasers with single-mode midwave-infrared emission at room temperature

Abstract: We report interband cascade lasers operating in a single spectral mode (λ≈3.6 μm) at −5–30 °C. A corrugated pattern etched into both sidewalls of the 6- and 9-μm-wide ridges serves to suppress higher-order lateral modes by increasing their loss, and also provides a fourth-order distributed-feedback grating for longitudinal mode selection. Despite the grating’s weak coupling strength, the 9 μm ridge produced up to 12 mW per facet of single-mode cw output power at 25 °C, with a side-mode suppression ratio of >30 dB.