Showing papers on "Diffraction grating published in 2006"

Efficiency enhancement in Si solar cells by textured photonic crystal back reflector

Abstract: An efficient light-trapping scheme is developed for solar cells that can enhance the optical path length by several orders of magnitude using a textured photonic crystal as a backside reflector. It comprises a reflection grating etched on the backside of the substrate and a one-dimensional photonic crystal deposited on the grating. Top-contacted crystalline Si solar cells integrated with the textured photonic crystal back reflector were designed and fabricated. External quantum efficiency was significantly improved between the wavelengths of 1000 and 1200nm (enhancement up to 135 times), and the overall power conversion efficiency was considerably increased.

Subwavelength waveguide grating for mode conversion and light coupling in integrated optics.

Abstract: We propose a new method for mode conversion and coupling between an optical fiber and a sub-micrometer waveguide using a subwavelength grating (SWG) with a period less than the 1st order Bragg period. The coupler principle is based on gradual modification of the waveguide mode effective index by the SWG effect that at the same time frustrates diffraction and minimizes reflection loss. We demonstrate the proposed principle by two-dimensional Finite Difference Time Domain (FDTD) calculations of various SWG structures designed for the silicon-on-insulator (SOI) platform with a Si core thickness of 0.3 microm. We found a coupling loss as small as 0.9 dB for a 50 microm-long SWG device and low excess loss due to fiber misalignment, namely 0.07 dB for a transverse misalignment of +/-1 microm, and 0.24 dB for an angular misalignment of +/-2 degrees. Scaling of the SWG coupler length down to 10 microm is also reported on an example of a 2D slab waveguide coupling structure including aspect ratio dependent etching and micro-loading effects. Finally, advantages of the proposed coupling principle for fabricating 3D coupling structures are discussed.

Nanolithography with coherent extreme ultraviolet light

Abstract: Extreme ultraviolet interference lithography (EUV-IL) is a newly developed technique for the production of periodic nano-structures with resolution below 20 nm. The technique is based on coherent radiation that is obtained from undulators at synchrotron radiation laboratories. The high resolution is afforded by small wavelength and practical absence of the proximity effect at this energy. The throughput of this parallel exposing method is much higher than that of the serial electron-beam lithography. Interference schemes based on both reflection (mirrors) and diffraction (gratings) optics have been realized. Both one-dimensional and two-dimensional patterns such as arrays of dots have been achieved. Achromatic interference schemes have been developed to make efficient use of the beam power available from the wideband sources in the extreme ultraviolet region. EUV-IL is used in a growing number of applications; examples include fabrication of self-assembly templates, magnetic nanodot arrays and nano-optical components.

Effective length of short Fabry-Perot cavity formed by uniform fiber Bragg gratings.

Abstract: In this paper, we describe the properties of Fabry-Perot fiber cavity formed by two fiber Bragg gratings in terms of the grating effective length. We show that the grating effective length is determined by the group delay of the grating, which depends on its diffraction efficiency and physical length. We present a simple analytical formula for calculation of the effective length of the uniform fiber Bragg grating and the frequency separation between consecutive resonances of a Fabry-Perot cavity. Experimental results on the cavity transmission spectra for different values of the gratings' reflectivity support the presented theory.

Fabrication of Transmission Color Filters Using Silicon Subwavelength Gratings on Quartz Substrates

Abstract: We investigate theoretically and experimentally transmission color filters using silicon subwavelength gratings on quartz substrates. Each grating area is 120 mum-square, which is suitable pixel size for displays and multichannel detectors. In the fabrication, electron beam lithography and fast atom beam etching are used. The grating periods are 400, 350, and 440 nm for the red, green, and blue filters, respectively. The transmission spectrum obtained from a coupling between an incident light and the submicrometer periodic grating matches with human color perception. The transmittances of 71.1%, 58.1%, and 59.3% are obtained for the red, green, and blue filters, respectively

Direct laser written waveguide-Bragg gratings in bulk fused silica.

Abstract: Optical waveguides that incorporate Bragg gratings have been written in bulk fused silica by using the femtosecond laser direct-write method and without the need for lithography or ion-beam techniques. A single manufacturing process is used to create waveguide-Bragg grating reflectors for operation in the C band.

Optical properties of deep lamellar Gratings: A coupled Bloch-mode insight

Abstract: A coupled-Bloch-mode model for explaining various optical properties of strongly modulated subwavelength lamellar gratings is proposed. The model evidences the key role played by propagative Bloch modes, which bounce inside the grating and couple at the grating interfaces. Many optical properties of these gratings, such as their broadband reflectance, their polarization effect, and the presence of sharp anomalies, are understood as resulting from vertical resonances of the coupled-resonator modes. The approach provides a new insight on the physical mechanisms or optical behaviors observed, which is different from the classical study of the poles and zeros of the scattering operator.

Dual-wavelength erbium-doped fiber laser with a simple linear cavity and its application in microwave generation

Abstract: A single-longitudinal-mode (SLM) dual-wavelength erbium-doped fiber (EDF) laser based on a simple linear cavity is proposed and demonstrated. The SLM operation is achieved by incorporating a dual-phase-shift fiber grating with two ultranarrow transmission bands. Due to the gain grating produced by spatial hole-burning in the EDF, the proposed linear cavity supports dual-wavelength oscillation at room temperature with a wavelength interval of 27 pm. The laser output is heterodyned on a photodetector and the generated microwave signal has a linewidth <20 kHz without any feedback.

Highly efficient liquid crystal based diffraction grating induced by polarization holograms at the aligning surfaces

Abstract: Low-scattering and highly efficient orientational gratings are obtained, exploiting polarization holography to modulate the in-plane anchoring axis at both surfaces of a planar nematic cell. Polarization-sensitive azo-dye-doped polyimide films are used as aligning layers. For proper values of cell thickness and spatial periodicity of the gratings, the director configuration in the nematic bulk is a perfect replica of the polarization gratings recorded on the aligning layers. High first-order diffraction efficiency, up to 98%, is observed in thin grating regime. External ac voltage allows to adjust the efficiency over the whole range for arbitrary cell thickness.

Theory of the transmission properties of an optical far-field superlens for imaging beyond the diffraction limit

Abstract: A conventional optical superlens for imaging beyond the diffraction limit produces images only in the near-field zone of the superlens. In contrast, an optical far-field superlens (FSL) device has a remarkable transmission property that leads to a one-to-one relationship between the far-field and the near-field angular spectra. This property makes the device suitable for imaging beyond the diffraction limit from far-field measurement. This specific FSL is composed of a properly designed periodically corrugated metallic slab-based superlens. Through the numerical design and parameter study, we show that the transmission property of this FSL is based on a specific strong-broadband wavenumber excitation of surface-plasmon polaritons supported by the nanostructured metallic grating.

External cavity quantum-cascade laser tunable from 8.2to10.4μm using a gain element with a heterogeneous cascade

Abstract: A heterogeneous quantum-cascade structure based on two bound-to-continuum designs emitting at 9.6 and 8.4μm is presented. Its spontaneous emission spectrum at room temperature has a full width at half maximum of 350cm−1 and shows a variation of intensity of less than 20% over more than 200cm−1. External cavity lasers using a grating in Littrow configuration and antireflection coated chips with this active region could be tuned over 265cm−1 from 8.2to10.4μm, that is, over 24% of the center wavelength.

Design of midinfrared photodetectors enhanced by surface plasmons on grating structures

Abstract: The authors propose to exploit the unique properties of surface plasmons to enhance the signal-to-noise ratio of midinfrared photodetectors. The proposed photodetector consists of a slit in a metallic slab filled with absorptive semiconductor material. Light absorption in the slit is enhanced due to Fabry-Perot resonances. Further absorption enhancement is achieved by surrounding the slit with a series of periodic grooves that enable the excitation of surface plasmons that carry electromagnetic energy towards the slit. Using this scheme, they design and optimize a photodetector operating at 0=9.8 m with a roughly 250 times enhancement in the absorption per unit of volume of semiconductor material compared to conventional photodetectors operating at the same wavelength. © 2006 American Institute of Physics. DOI: 10.1063/1.2360896 Midinfrared photodetectors and imaging systems operating in the vicinity of 10 m wavelength are important in applications ranging from night vision to astronomy research. 1 To suppress noise caused by thermal fluctuation, usually these detection systems have to be cooled, 1‐3 which greatly increases the cost and limits the portability. To improve signal/noise ratio, it is desirable to reduce the active detector volume from which the noise arises without affecting the absorbed light power. In this letter, we computationally explore the use of surface plasmons in these systems to improve the signal/noise ratio. The proposed structure, shown in Fig. 1a, consists of a metallic slit, surrounded by a linear grating structure. The entire structure is placed on top of an insulating oxide. The slit is filled with absorptive semiconductor material, and has a width that is far smaller than the operating wavelength. The metal regions serve both as electrical contacts and as a concentrator that funnels light into the deep-subwavelength slit. 4‐6 To maximize the concentration effects, we combine two distinct mechanisms that relate to the presence of the slit and the grating structure. 7 A metal slit supports a propagating TE mode, even when the width of the slit is at deepsubwavelength scales. Moreover, due to the strong impedance mismatch between the modes in the slit and free-space propagating waves, strong reflection can occur at the entrances to the slit. Consequently, with an appropriate choice of the length, the slit forms a Fabry-Perot resonator, and light absorption in the slit is resonantly enhanced. The grating, on the other hand, enhances the light absorption in the slit by converting incident electromagnetic EM waves into surface plasmons on the metal surface that can be funneled into the slit. 8 The area from which EM energy is collected can be substantially larger than the slit width and gives rise to significant enhancements in the absorbed energy. With a careful choice of the geometric parameters of the structure, these two mechanisms can be made to occur at the same wavelength, resulting in an optimized efficiency of the devices. To simulate the response of this structure for a normally incident EM plane wave, we use a two-dimensional finitedifference frequency-domain method, 9,10 which directly uses experimental data for the frequency-dependent dielectric constant of metals such as gold, 11 including both the real and imaginary parts, with no further approximation. For the detective materials in the slit, we have chosen to use HgCdTe MCT with a frequency independent dielectric permittivity of =12.5+i1.75. 2,11 The substrate is a low-index oxide

Photonic crystal laser lift-off GaN light-emitting diodes

Abstract: We report on the fabrication and study of laser lift-off GaN-based light-emitting diodes, thinned down to the microcavity regime, incorporating two-dimensional photonic crystal diffraction gratings. Angle-resolved measurements reveal the photonic behavior of the devices, which strongly depends on the GaN thickness. Data point out the detrimental role of metal absorption. We explore theoretically the possibility to limit this loss channel.

Polymeric, electrically tunable diffraction grating based on artificial muscles.

Abstract: We demonstrate a low-cost, electrically tunable diffraction grating that is driven by a dielectric elastomer actuator. The angular tuning range of the polymer-based device is up to 118 mrad for the first diffracted order. The achievable grating period change of 32% is an improvement by more than a factor of 150 compared with existing analog tunable diffraction gratings based on hard materials. We show that in combination with a white light source, the presented diffraction grating can be used as a wavelength-adjustable luminous source. Such an illuminant has a potential application in inexpensive color displays that could reproduce all perceivable colors.

Optical inclinometer based on a single long-period fiber grating combined with a fused taper.

Abstract: A new concept to measure rotation angles based on a fiber-optic modal Mach-Zehnder interferometer is demonstrated by using a nonadiabatic taper cascaded with a long-period fiber grating. Information about the magnitude of the rotation angle can be obtained from the measurement of the interference pattern visibility, and under certain conditions it is also possible to obtain the sign of the rotation angle from the induced phase variation in the fiber interferometer.

Optic sensors of high refractive-index responsivity and low thermal cross sensitivity that use fiber Bragg gratings of >80° tilted structures

Abstract: For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with >80° tilted structures nave been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), >80° tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340 nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs. © 2006 Optical Society of America.

Light injection in SOI microwaveguides using high-efficiency grating couplers

Abstract: An experimental characterization of the grating couplers for sub-micrometer silicon-on-insulator (SOI) waveguides is presented. The grating couplers have been designed, realized, and characterized for the +1 diffraction order at an operating wavelength of 1.31 mum for TE polarization. At the resonant angle, a coupling efficiency higher than 55% has been measured. The angular coupling range and the wavelength tolerance have been evaluated to 3deg and 20 nm, respectively. The grating coupler is followed by a taper, and about 50% of the input power at 1.31 mum is coupled into sub-micrometer rib and strip SOI waveguides. The ration between light power decoupled toward the cladding and light power decoupled toward the substrate is about three

Compact high-resolution spaceborne echelle grating spectrometer with acousto-optical tunable filter based order sorting for the infrared domain from 2.2 to 4.3 μm

Abstract: A new compact spaceborne high-resolution spectrometer developed for the European Space Agency's Venus Express spacecraft is described. It operates in the IR wavelength range of 2.2 to 4.3 μm and measures absorption spectra of minor constituents in the Venusian atmosphere. It uses a novel echelle grating with a groove density of 4 lines/mm in a Littrow configuration in combination with an IR acousto-optic tunable filter for order sorting and an actively cooled HgCdTe focal plane array of 256 by 320 pixels. It is designed to obtain an instrument line profile of 0.2 cm−1. First results on optical and spectral properties are reported.

Resolution of 90 nm (λ/5) in an optical transmission microscope with an annular condenser

Abstract: Resolution of 90 nm was achieved with a research microscope simply by replacing the standard bright-field condenser with a homebuilt illumination system with a cardioid annular condenser. Diffraction gratings with 100 nm width lines as well as less than 100 nm size features of different-shaped objects were clearly visible on a calibrated microscope test slide. The resolution increase results from a known narrower diffraction pattern in coherent illumination for the annular aperture compared with the circular aperture. This explanation is supported by an excellent accord of calculated and measured diffraction patterns for a 50 nm radius disk.

Simultaneous Measurement for Strain and Temperature Based on a Long-Period Grating Combined With a High-Birefringence Fiber Loop Mirror

Abstract: This work presents an alternative solution for simultaneous measurement of strain and temperature. The sensing head is formed by a long-period fiber grating combined with a high-birefringence fiber loop mirror resulting in a configuration capable of temperature and strain discrimination. These optical devices have opposite sensitivity responses when a variation of temperature and/or strain is applied. Maximum errors of plusmn0.8degC and plusmn21muepsiv are reported over 60 degC and 700-muepsiv measurement ranges, respectively

Direct inscription of Bragg gratings in coated fibers by an infrared femtosecond laser.

Abstract: Inscription of fiber Bragg gratings through the coating of a nonphotosensitized standard telecommunication fiber is demonstrated for what is believed to be the first time. Highly reflective gratings were produced by direct point-by-point writing with an infrared femtosecond laser. The length of the gratings presented ranged from 5 to 26mm. The technique does not require a special coating, as standard coatings are transparent to infrared radiation. Inscription through the coating improves the mechanical strength of the processed segment of fiber.

Inscription of fiber Bragg gratings with femtosecond pulses using a phase mask scanning technique

Abstract: We report on the fabrication of highly efficient fiber Bragg gratings (FBG) in non-photosensitive fibers based on nonlinear absorption of fs laser light. Up to 40 mm long gratings with a transmission of T=-25 dB at the Bragg reflection wavelength were obtained and their coupling constant determined by spectral analysis. Therefore, a phase mask scanning technique with appropriate control of the focus was established.

Polarization-controlled switching between diffraction orders in transverse-periodically aligned nematic liquid crystals

Abstract: Transverse-periodic-oriented nematic liquid crystals (LCs) are a special type of optical axis grating that are capable of very high efficiency diffraction (theoretically, 100%) in thin layers of materials with thickness comparable to the radiation wavelength. In particular, they fully diffract linearly polarized input beam into circularly polarized +1st and −1st diffraction orders. We experimentally demonstrate switching between diffraction orders of such gratings when the polarization of the incident beam changes from right-circular to left-circular and vice versa with the aid of an electrically controlled LC phase retarder. Such a setup in which the diffraction efficiency and direction are controlled externally, without application of an electric field to the transverse-periodic grating, provides additional control opportunities and does not compromise the quality of the grating. The grating used in the experiment was 1.5μm thick and had a period of 4μm. The contrast ratio of switching between the +1st and −1st orders was as high as 267:1 for a He-Ne laser beam with a switching time of 6.6ms.

High-power widely tunable Tm:fibre lasers pumped by an Er,Yb co-doped fibre laser at 1.6 mum.

Abstract: High-power and widely tunable Tm-doped silica fibre lasers cladding-pumped and core-pumped by a 1565 nm Er,Yb fibre laser are reported. Output power up to 19.2W was generated from the cladding-pumped cavity configuration for ~38.2W of launched pump power and with slope efficiency up to ~72% with respect to absorbed pump power. Wavelength tuning was realized by use of an external cavity containing a diffraction grating. A maximum output power of 17.4 W at 1941 nm was generated for 38.2 W of launched pump power and the operating wavelength could be tuned over 202 nm from 1859 to 2061 nm. In the core-pumped configuration, a maximum output power of 12.1 W was generated at 1851 nm for 23.1 W absorbed pump power using a simple free-running cavity configuration with only ~24 cm of Tm-doped fibre. By employing a tunable cavity configuration, the operating wavelength of the core-pumped Tm:fibre laser could be tuned over 250 nm from 1723–1973 nm at multi-watt power levels.

Monochromator beamline for FLASH

Abstract: The design of a high resolution monochromator for the vacuum ultraviolet free electron laser at Hamburg (FLASH), DESY, is described. The monochromator is constructed as a plane grating monochromator using collimated light. Modifications have been made to take into account the free electron laser (FEL) beam characteristics, in particular, the extremely high peak power density of the radiation. Ray tracing simulations yield a resolving power in the range of 10 000–70 000 depending on the photon energy and the grating in use. Our monochromator is equipped with a 200line∕mm grating for the energy range of 20–200eV—the operation regime of FLASH—and a high resolution 1200line∕mm grating for the energy range of 100–600eV, covering the higher harmonic radiation of the FEL.

Self-phase modulation-based integrated optical regeneration in chalcogenide waveguides

Abstract: We demonstrate integrated all-optical 2R regenerators based on Kerr optical nonlinearities (subpicosecond response) in chalcogenide glass waveguides with integrated Bragg grating filters. By combining a low loss As/sub 2/S/sub 3/ rib waveguide with an in-waveguide photo-written Bragg grating filter, we realize an integrated all-optical 2R signal regenerator with the potential to process bit rates in excess of 1 Tb/s. The device operates using a combination of self phase modulation induced spectral broadening followed by a linear filter offset from the input center wavelength. A nonlinear power transfer curve is demonstrated using 1.4 ps pulses, sufficient for suppressing noise in an amplified transmission link. We investigate the role of dispersion on the device transfer characteristics, and discuss future avenues to realizing a device capable of operation at subwatt peak power levels.

Solid-state laser spectral narrowing using a volumetric photothermal refractive Bragg grating cavity mirror

Abstract: Dramatic spectral narrowing of two normally broadband lasers, Ti:sapphire and Cr:LiSAF, was achieved by simply replacing the output mirror with a reflective, volumetric Bragg grating recorded in photothermal refractive glass. The output power of each laser was unchanged from that obtained using dielectric coated output mirrors with the same output coupling as the Bragg grating while spectral brightness increased by 3 orders of magnitude.

Optical transmission through double-layer metallic subwavelength slit arrays.

Abstract: We present measurements of transmission of infrared radiation through double-layer metallic grating structures. Each metal layer contains an array of subwavelength slits and supports transmission resonance in the absence of the other layer. The two metal layers are fabricated in close proximity to allow coupling of the evanescent field on individual layers. The transmission of the double layer is found to be surprisingly large at particular wavelengths, even when no direct line of sight exists through the structure as a result of the lateral shifts between the two layers. We perform numerical simulations using rigorous coupled wave analysis to explain the strong dependence of the peak transmission on the lateral shift between the metal layers.

Wavelength Filters in Fibre Optics

Abstract: Optical Filters in Wavelength-Division Multiplex Systems.- Phase Characteristics of Optical Filters.- Diffraction Gratings WDM Components.- Arrayed Waveguide Gratings.- Fibre Bragg Gratings.- Fabry-Perot Interferometer Filters.- Dielectric Multilayer Filters.- Ring-Resonator-Based Wavelength Filters.- Interleavers.