Showing papers on "Electromagnetically induced grating published in 2004"

Stopping Light in a Waveguide with an All-Optical Analog of Electromagnetically Induced Transparency

Abstract: We introduce a new all-optical mechanism that can compress the bandwidth of light pulses to absolute zero, and bring them to a complete stop. The mechanism can be realized in a system consisting of a waveguide side coupled to tunable resonators, which generates a photonic band structure that represents a classical analogue of the electromagnetically induced transparency. The same system can also achieve a time-reversal operation. We demonstrate the operation of such a system by finite-difference time-domain simulations of an implementation in photonic crystals.

Optical waveguide grating coupler incorporating reflective optical elements and anti-reflection elements

Abstract: An optical wavelength grating coupler incorporating one or more distributed Bragg reflectors (DBR) or other reflective elements to enhance the coupling efficiency thereof. The grating coupler has a grating comprising a plurality of scattering elements adapted to scatter light along a portion of an optical path, and the one or more DBRs are positioned with respect to the grating such that light passing through the grating towards the substrate of the grating coupler is reflected back by DBRs toward the grating. The DBR comprises a multilayer stack of various materials and may be formed on the substrate of the grating coupler. The grating coupler may include a gas-filled cavity, where the cavity is formed by a conventional etching process and is used to reflect light toward the grating. The grating coupler may also incorporate an anti-reflection coating to reduce reflective loss on the surface of the grating.

The interrogation and multiplexing of long period grating curvature sensors using a Bragg grating based, derivative spectroscopy technique

Abstract: A long period grating is interrogated with a fibre Bragg grating using a derivative spectroscopy technique. A quasi-linear relationship between the output of the sensing scheme and the curvature experienced by the long period grating is demonstrated, with a sensitivity of 5.05 m and with an average curvature resolution of 2.9 × 10-2 m-1. In addition, the feasibility of multiplexing an in-line series of long period gratings with this interrogation scheme is demonstrated with two pairs of fibre Bragg gratings and long period gratings. With this arrangement the cross-talk error between channels was less than ± 2.4 × 10-3 m-1.

Simple fabrication of diffraction gratings by two-beam interference method in highly photosensitive hybrid sol-gel films.

Abstract: Sol-gel hybrid materials containing a large quantity of photoactive molecules exhibited large changes in both refractive index and volume by UV exposure The materials were used for the fabrication of diffraction gratings using the two-beam interference method With this technique, we could simply fabricate the diffraction gratings and easily control the grating periods The diffraction effects and efficiencies of gratings rely heavily on the UV doses and the fabricated diffraction gratings showed a good diffraction performance

Interference effects in lossy resonator chains

Abstract: We theoretically study properties of narrow spectral features occurring in chains of coupled optical resonators due to classical optical interference. We compare those properties with the properties of the phenomenon of electromagnetically induced transparency in quantum systems. Despite the difference in the underlying physics, we show that both types of systems show striking similarities in optical properties and in possible applications.

Diffraction by an optical fractal grating

Abstract: We report experimental and theoretical studies of Fraunhofer diffraction pattern of a 15-level H-fractal grating The diffraction pattern was found to exhibit self-similarity In particular, the diffracted light was found to be more intense at higher spatial frequencies than at lower frequencies, in stark contrast to the diffraction patterns of wire gratings and grid gratings Using Fourier transform theory, we show that this unusual behavior comes from the structural coherence of the H-fractal, which makes it favorable to use higher-order diffraction spectra for larger dispersion In addition, the fractal dimension of the grating is shown analytically and experimentally to be two

Optimization of diffraction grating profiles in fabrication by electron-beam lithography.

Abstract: We propose a new design method for periodic diffraction gratings to be fabricated with direct-writing electron-beam lithography. When the grating has a small period, the proximity effect of electron scattering restricts the grating profile after developing. Our design method optimizes the electron-dose profile and grating profile simultaneously to obtain the desired diffraction efficiency under the restriction of the proximity effect. The optimization is made with rigorous electromagnetic grating analysis and the resist development simulator. When we designed the diffraction grating with a period of 1.0 µm to obtain the highest efficiency of the first-order diffracted light of a 633-nm wavelength, the calculated grating profile was really different from the profile optimized only with rigorous electromagnetic grating analysis. Moreover, the diffraction grating of the electron-beam resist was fabricated according to the simulation result. The estimated diffraction efficiency was 82%, and the measured efficiency was 70%.

Optical grating coupler

Abstract: An optical grating is disposed on a waveguide to redirect light from the interior of the waveguide through the opposite side of the waveguide from the grating. In one embodiment the waveguide, the grating, and an optical sensor are combined in a single monolithic structure. In another embodiment, an absorbing layer is directly connected to the waveguide in the region of the grating. In still another embodiment, efficiency of the grating is improved by having a high index contrast between the refractive index of the grating and the refractive index of the cladding disposed over the grating, and by having an appropriately sized discontinuity in the grating.

Steady-state population inversion by multiphoton electromagnetically induced transparency

Abstract: We show that electromagnetically induced transparency suppresses nonlinear absorption of all orders in a multilevel atomic system and leads to selective, multiphoton excitation of resonantly coupled atomic states. Under appropriate conditions, higher-order nonlinear absorption becomes dominant and selective steady-state population inversion is created among the resonantly coupled states.

Diffraction of optical communication Gaussian beams by volume gratings: comparison of simulations and experimental results

Abstract: The diffraction effects induced by a thick holographic grating on the propagation of a finite Gaussian beam are theoretically analyzed by means of the coupled-wave theory and the beam propagation method. Distortion of the transmitted and diffracted beams is simulated as a function of the grating parameters. Theoretical results are verified by experimentation realized by use of LiNbO3 volume gratings read out by a 1550-nm Gaussian beam, typical of optical fiber communications. This analysis can be implemented as a useful tool to aid with the design of volume grating-based devices employed in optical communications.

Optical filter and method of manufacturing thereof

Abstract: Novel structure of the optical elements (i.e. filter) to be operated in the long, mid, and near infrared wavelengths of lights is provided. The filter can offer very narrow linewidth, and high reflectivity (or transmissivity) at the peak wavelength. The optical element consists of the substrate, first diffraction grating and single uniform surface, and the second grating. Alternatively, the optical element again consists of the substrate, single uniform surface and the diffraction grating on the top of it. Alternatively, filter may also consist of number of sequence of layers, wherein each sequence comprises the single uniform layer sandwiched by the two diffraction grating layers. Filter again alternatively consists of the number of sequences wherein each sequence comprises the single uniform layer and the single diffraction grating. Diffraction grating may be two-step grating or multilevel grating with synchronously or nonsynchronously samples diffraction gratings.

Volume diffraction gratings for optical telecommunications applications: design study for a spectral equalizer

Abstract: The main characteristics required for a diffraction grating used for (de)multiplexing functions in spectral equalizing systems are investigated, both theoreticaly and experimentally. We show that volume-phase holographic (VPH) gratings can be used as dispersive elements instead of classic reflection surface-relief gratings presently employed in most optical telecommunication devices. A design method for this type of diffraction grating and experimental results are presented, confirming that VPH gratings are well suited for such applications.

Optical Multi-wave Mixing Process Based on Electromagnetically Induced Transparency

Abstract: In this paper, we propose and analyze an optical multi-wave mixing scheme for the generation of coherent light in a five-level atomic system in, the context of electromagnetically induced transparency A detailed semiclassical study of the propagation of generated mixing and probe fields is demonstrated The analytical dependence of the generated mixing field on the probe field and the respective detuning is predicted Such a nonlinear optical process can be used for generating short-wavelength radiation at low pump intensities

A novel wavelength-division multiplexer using grating-assisted two-mode interference

Abstract: A novel design of wavelength-division multiplexer based on two-mode interference assisted by a highly dispersive grating structure is two-dimensionally simulated and analyzed using finite-difference time-domain method. We show that the designed grating waveguide provides a mode-dependent reflection spectrum which can be used to modify the beat lengths of the two selected wavelengths 1342 and 1560 nm, so that they can be divided in a very short distance about 48 /spl mu/m. Simulation results show that contrasts /spl sim/12 dB and insertion losses about 2 dB could be achieved.

Grating apodization technique for diffused optical waveguides

Abstract: An optical channel waveguide having a reflection grating and a related method for its fabrication. The grating is apodized to provide a desired reflection or transmission spectral characteristic, by varying the grating width along the length of the grating in the direction of light propagation. The grating has multiple parallel elements extending across the waveguide channel width, and apodization is effected by appropriate selection of the width of each element relative to the width of the channel, without varying the grating duty cycle or other parameters.

Electromagnetically Induced Absorption and Transparency Spectra of Degenerate Two-Level Systems with a Strong Coupling Field in Cs Vapour

Abstract: The electromagnetically induced absorption and electromagnetically induced transparency spectra of degenerate two-level systems with a strong coupling laser were observed The frequency detuning and intensity effect of the coupling laser were demonstrated simultaneously A dispersion-like spectrum can be obtained when the coupling laser is situated at blue-side detuning The absorption inversion was realized when the coupling laser intensity is small The coherent resonance has a linewidth much narrower than the natural linewidth of the optical transitions

Theory of two-photon interference in an electromagnetically induced transparency system

Abstract: We examine the possibility of storing and retrieving a single photon using electromagnetically induced transparency. We consider the theory of a proof-of-principle two-photon interference experiment, in which an atomic vapor cell is placed in one arm of a two-photon interferometer. Since the two-photon state is entangled, we can examine the degree to which entanglement survives. We show that while the experiment might be difficult, it should be possible to perform. We also show that the two-photon interference pattern has oscillatory behavior.

Grating efficiencies comparison study: calculations versus metrology for various types of high-groove density gratings at VUV-UV wavelengths

Abstract: The complex study of a ultra-fine (4000 to 5870 grooves/mm) holographic gratings designed for Near Ultra Violet (NUV) channel of Cosmic Origin Spectrograph (COS) are presented. The gratings underwent a comprehensive program of efficiency and scatter characterization at flight-like environment conditions. Initial tests revealed significant departures of grating efficiencies from the values predicted. Effects of profile non-replication (layers nonconformity) for multi-layer coated grating surfaces are investigated. The rigorous efficiency modeling based on groove profiling atomic force microscopy (AFM) data for both multi-layer conformal and non-conformal coated gratings in both polarizations allowed to identify the problem as a leaky mode anomalies for dielectrically coated gratings. Both shift in peak efficiencies and wide absorption band are observed to be critically dependant of grating groove shape and reflective coating thickness. Grating depth/profile topography changes induced by coating process are observed and called for groove profile measurements at all stages of the gratings production line. Grating scatter characterizations data for COS and SORCE/SOLSTICE gratings at UV-VUV wavelength are presented. The wavelength scaling at VUV-UV waveband (140 nm to 442 nm) for holographically ruled 3600 gr/mm grating are also reported.

A proposal for high resolution photolithography using optical limiters

Abstract: The main problem of the traditional photolithography in microelectronics and photonics Engineering (minimum line-width) can be relaxed using optical limiters. In this work, nonlinear Bragg Grating (Kerr-Like non-linearity) is used as an optical limiter. Also, a suitable setup configuration for high-resolution photolithography based on optical limiter is proposed. The minimum line-width for integrated systems with our proposed configuration depends on the Grating parameters. We show that, the linear and nonlinear index of refractions profile, and the number of layers in the proposed grating, determines the minimum line-width.

Focusing grating coupler for blue laser light

Abstract: A focusing grating coupler (FGC) for use with a blue laser of 400-nm wavelength as a light source was fabricated for the first time. The FGC was designed to have a numerical aperture of 0.48 and a minimum period of 0.2 /spl mu/m when the focal length and the grating area were 900 /spl mu/m and 1/spl times/1 mm/sup 2/, respectively. Grating pattern of minimum line of 0.1 /spl mu/m was fabricated on a waveguide based on the boron phosphor-silicate glass material by electron-beam lithography process using the vector scan method. The spot size at the full width (1/e/sup 2/) was measured as 0.92 and 0.85/spl mu/m in x and y directions and was nearly the same as the diffraction limit.

Nonlinear cascade-configuration multi-wave mixing scheme based on electromagnetically induced transparency

Abstract: A nonlinear optical cascade-configuration multi-wave mixing (CCMWM) scheme is presented and analysed for the generation of coherent light in a six-level atomic system in the context of electromagnetically induced transparency (EIT) A detailed semi-classical study of the propagation of the generated mixing and probe fields is demonstrated We show by numerical simulations that EIT is capable of suppressing linear and nonlinear photon absorption The analytical dependence of the generated mixing field on the probe field and the respective detuning is also predicted Such a nonlinear optical process can be used for generating coherent short-wavelength radiation

Thermomechanical modification of diffraction gratings.

Abstract: The most accurate approaches to fabrication of diffraction gratings are known to be the lithographic and holographic methods The lithographic methods allow fabrication of arbitrarily chirped gratings whose performance, however, is degraded by stitching errors The holographic methods are free from stitching errors; however, they are limited in the achievable spatial variations of their grating periods We suggest a method of diffraction grating modification by nonuniform heating and stretching that is much more flexible than the holographic approach and does not suffer from the problem of stitching error We demonstrate our approach for quartz phase masks that have a characteristic grating period of 1 µm and a length of several centimeters Our approach allows the grating periods of the phase masks to vary in a range from a few picometers to a few nanometers and a spatial resolution of a few millimeters It is shown that the grating period can be modified with a negligible effect on the profile of the gratings

Displacement measurement of a grating using moiré modulation of an optical spectrum

Abstract: A new method for remote displacement measurement of a grating is proposed, which utilizes moire fringes of two gratings that modulate a spectrum of white light coming from an optical fibre. The amount of displacement of the sensing grating is calculated with Fourier coefficients of the spectrum of light detected by an arrayed spectrometer. The displacement of the spectral profile is proportional to that of the sensing grating. Parameters of the measurement system are analysed theoretically. Experimental results are given for two experimental set-ups—one for light being transmitted through gratings and the other for light reflecting from a grating and mirror. Sensitivity for the light-transmitting set-up reached 1/710 of the spacing of the grating, which corresponds to 14 nm for a grating of 10 µm spacing.

Multilayer film laminar grating with uneven spacing groove and grating spectral device

Abstract: PROBLEM TO BE SOLVED: To provide a grating having high diffraction efficiency and resolution in a 0.2 to 2 mm wavelength region where spectroscopy using a grating is difficult because of a decrease in the efficiency, and to provide a spectral device using the grating. SOLUTION: A conditions for manufacturing a wavelength scanning mechanism and a grating of a spectral device simultaneously satisfies the following four conditions without depending on wavelengths in order to highly efficiently extract soft X-rays spectrally resolved by a spectral device using a grating. The four conditions are: (1) a condition expressed by grating formula specifying the correlation between directions of incident light and diffracted light and a grating constant; (2) a condition about the groove depth and a ratio of the ridge width to the grating constant (duty ratio) of a laminar grating which gives the maximum reflectance; (3) an expanded Bragg condition of a multilayer film; and (4) a condition to keep a specified focal length while making uneven spacing of grooves. COPYRIGHT: (C)2006,JPO&NCIPI

Grating element for filtering wavelengths < 100 nm

Abstract: There is provided a grating apparatus for filtering wavelengths ≦100 nm. The grating apparatus includes multiple individual grating elements having grating lines. The individual grating elements are positioned one behind another on a curved support surface in relation to a plane spanned by the grating apparatus in a direction of beans of a light bundle that is incident on the grating apparatus.

Recording of concave diffraction gratings in a two-step process using spatially incoherent light

Abstract: An original optical design for two-step recording of holographic diffraction gratings is presented. It combines the merits of the two previously developed methods. On the one hand, due to the two-step recording one can use wide possibilities for the aberrations compensation. On the other hand, one can record both the grating objective and the final grating in spatially incoherent light. This simplifies the technology of the holographic recording. Previously, one must use spatially coherent light for at least one of two recording steps. Our method does not require the use of additional optics. Both surfaces of the grating objective blank have a spherical form. The blank of the final grating can be plane concave, which is usual for diffraction gratings. Since the blaze angle of the grating recorded in the counterpropagating beams is determined by the recording geometry, and the efficiency of the grating depends on the polarization of the incident light, the transmission calculations taking into consideration the polarization are also provided. The transmission of the system is optimal for the s-polarized light, additional increasing of the transmission can be achieved by use of an immersion grating objective.

Recording of concave diffraction gratings in a two-step process using spatially incoherent light

Abstract: The original optical design for two steps recording of the holographic diffraction gratings is presented. It gives the combination of the merits of two previously developed methods in one. From one hand, due to recording in two steps, we can use wide possibilities of the aberrations compensation. From another hand, we can record both the gratings - objective and the final grating in spatially incoherent light. This simplifies the technology of the holographic recording. Previously per limit one of the two steps required using of the spatially coherent light. Method does not require using of the additional optics. Both surfaces of the grating - objective blank have spherical form. The blank of the final grating can be plane - concave, which is usual for diffraction gratings. Since the blaze angle of the grating, recorded in the counterpropagating beams is determined by the recording geometry, and the efficiency of the grating depends on the polarization of the incident light, the transmission calculations with taking into consideration the polarization also have been provided. The transmission of the system is optimal for the S polarized light, the addition increasing of the transmission can be achieved by using of the immersion grating - objective.

Electromagnetically induced transparency and lasing without inversion in three-level atoms imbedded in a frequency-dependent environment

Abstract: The response of a three-level atomic system driven by a resonant coherent field acting on a transition near the photonic band-edge of a photonic band-gap material as well as the general case of a frequency-dependent reservoir is studied. The strong frequency dependence of the radiation mode spectral density on the scale of the driving field Rabi frequency is shown to lead to essential and controllable changes in the refractive index, as well as to effects of electromagnetically induced transparency and lasing without inversion. Such an effective dynamic control of the atomic response enables for applications in nonlinear optics and optical computing and communications.

Continuous-wave four-wave mixing with linear growth based on electromagnetically dual induced transparency

Abstract: Using Schrodinger-Maxwell formalism, we propose and analyze a continuous-wave four-wave mixing (FWM) scheme for the generation of coherent light in a six-level atomic system based on electromagnetically dual induced transparency We derive the corresponding explicit analytical expressions for the generated mixing field We find that the scheme greatly enhances FWM production efficiency and is also capable of inhibiting and delaying the onset of the detrimental three-photon destructive interference by choosing the proper decay rate in the second electromagnetically induced transparency (EIT) process In addition, such an optical process also provides possibilities for producing short-wave-length coherent radiation at low pump intensities