Showing papers on "Electromagnetically induced grating published in 1995"

Electromagnetically induced transparency in a three-level Λ-type system in rubidium atoms

Abstract: An electromagnetically induced transparency is observed at one arm of a three-level \ensuremath{\Lambda}-type system in a rubidium D1 line (794.8 nm) with an 85% reduction in absorption, when a pumping field is present at the other arm. This reduction in absorption for the weak probe field is due to the atomic coherence produced by the strong pumping field. This experiment is done in a Rb vapor cell at room temperature with cw diode lasers for both pumping and probe beams in a Doppler-free configuration. A simple theoretical treatment including Doppler broadening is in good qualitative agreement with the experimental measurement.

Transient properties of an electromagnetically induced transparency in three-level atoms

Abstract: We describe the transient behavior of an electromagnetically induced transparency in three-level atomic systems. When the coupling f ield is switched on, the absorption for the probe field is oscillatorily damped to its new steady-state (transparent) value. Transient gain without population inversion and enhancement of dispersion are discussed. A realistic system for experimental testing of these effects is presented.

Novel and improved methods of writing Bragg gratings with phase masks

Abstract: The authors demonstrate the importance of spatial coherence of the writing beam for inscribing Bragg gratings with a phase mask. Bragg gratings were written using a normal excimer laser with the fiber placed at various distances away from the phase mask. This was repeated with an excimer laser that was modified to improve the beam spatial coherence showing dramatic improvement in the ability to inscribe gratings. Tunability of the inscribed Bragg grating wavelength, utilizing a single phase mask in conjunction with the improved spatial coherence of the excitation source, is demonstrated. >

Optical waveguide with diffraction grating and method of forming the same

Abstract: A method for forming an index grating in an optical waveguide, such as an optical fiber, with precise control over the grating's period, cross-sectional shape and length. A single writing beam is passed through an optical grating mask, such as a phase mask. A photosensitive waveguide is spaced from the optical grating by a distance that corresponds to an integer fraction of the Talbot self-imaging distance, so that the optical grating (or a desired transformation of it) is imaged in the waveguide core. The grating image has substantially the same cross-sectional shape, period and length as the portion of the optical grating that is illuminated by the writing beam. Thus, an index grating that substantially replicates the cross-sectional shape, period and length of the optical grating mask, which preferably has a substantially square-wave shaped cross-section, is written in the waveguide core. The substantially square-wave shaped cross-section results in higher reflectivity per unit length than prior waveguide gratings with sinusoidal cross-sections. As a result, a high order waveguide index grating may be formed over a waveguide length that is shorter than would previously be required. Alternatively, a low order grating may be formed that has higher reflectivity than a low order sinusoidal grating.

Nonlinear combinations of gratings in Bi 12 SiO 20 : theory and experiments

Abstract: Multiple optical interference patterns generate phase gratings in Bi12SiO20 as a result of the photorefractive effect. Because of the nonlinear response of the space-charge field to the optical interference pattern, these primary gratings interact and cause the formation of additional gratings at frequencies that are combinations of the primary grating frequencies. Only the diffusion regime is considered in the present investigation. In our setup, two primary gratings are induced as the result of interference among one reference and two closely situated object beams. The interference between the two object beams leads to a negligible photorefractive grating, because the photorefractive Bi12SiO20 crystal cannot respond to the corresponding large fringe spacing. We show theoretically that nonlinear combinations of gratings are introduced through this third interference pattern even though the corresponding grating itself is absent. We obtain analytical expressions for the fundamental frequency components of the space-charge field in the special case of a weak reference beam. In the general case, the frequency components of the space-charge field are calculated numerically by the use of Fourier transforms and are incorporated into the coupled-wave equations. The optical beam propagation method is then used for solving these equations numerically. The effect is probed in a three-wave mixing experiment that uses a sinusoidal phase modulation on one of the object beams to control the grating strength. With this technique one of the gratings is specifically canceled out, and the relative change of the diffraction efficiency in the remaining grating is measured. It is shown that nonlinear combinations of gratings may cause relative changes of more than 35% in the diffraction efficiency. The theoretical predictions are in excellent agreement with our experimental results and the cross talk observed in dynamic optical interconnects.

Writing and visualization of low-threshold type II Bragg gratings in stressed optical fibers.

Abstract: Type II phase gratings were produced by the use of holographic side writing in high-birefringence optical fibers with UV fluences of 0.06 J/cm2 over 10 times lower than that previously reported for standard fibers. The grating growth, transmission and reflection spectra, temperature response, short-wavelength light ejection, and high-resolution confocal microscopy images are reported. Diffraction theory is used to interpret the grating microstructure revealed by confocal microscopy. Each period of the grating is shown to consist of a plate of oriented cracks, and arguments relating to the arrangement of the cracks and crack growth are linked to the observed grating-growth dynamics.

Method of creating bragg gratings in waveguides

Abstract: During the photo-induced creation of a Bragg grating in a photosensitive optical fibre using the interference fringe pattern generated in the near field of a phase grating, the spacing between fibre and phase grating is modulated.

Pick-up device for optical disk readout using waveguide gratings

Abstract: An optical device for detecting focus error, tracking error and recorded data in an optical head system has a grating plate having a plurality of gratings and waveguides. The gratings are fabricated by holographic exposure of a resist layer spin-coated onto a glass substrate. The gratings include a processing grating to efficiently input couple light into the waveguide and to effect a desired change in the input coupling efficiency with defocus of a beam of light at an optical disk. The optimum pattern of the processing grating is based on the location and intensity variation of diffraction orders in the beam returning from the optical disk and appodization of the beam. Unfocused beams and large active areas of the detectors relax the tolerance for detector placement so that placement is not critical. The four focus and tracking detectors and data detector needed for complete signal processing are compactly formed on single chip.

Opto-electronic component with axial grating period modulation for e.g. semiconductor laser

Abstract: The component has at least three layers several different semiconductor materials and an optical waveguide with a feedback grating with grating strips orthogonal to longitudinal direction. The inhomogeneous grating is combined with an arcuated waveguide. The inhomogeneous grating comprises several individual grating fields, fitted in longitudinal direction in tandem. The grating period within an individual grating field is constant and the grating period of adjacent fields in different and also differ from zero, meeting the specified relation.

Nonlinear combinations of gratings in drift-dominated recording in Bi 12 SiO 20

Abstract: Multiwave mixing in Bi12SiO20 causes the formation of multiple grating because of the photorefractive effect. These primary gratings interact and cause the formation of additional gratings at frequencies that are combinations of the primary grating frequencies. The nonlinear combinations of gratings stem from the nonlinear response of the space-charge field to the incident optical interference pattern through the generation of electrons into the conduction band. We investigate drift-dominated recording. The effects of nonlinear combinations of gratings are investigated quantitatively in a three-wave mixing configuration, in which one reference beam and two closely situated object beams induce three primary gratings. Using a sinusoidal phase modulation on one of the object beams provides absolute control of the primary grating strengths. Specifically, two of the primary gratings may be completely erased, and the nonlinear effect may be obtained as the relative change in the diffraction efficiency from the remaining grating. For drift-dominated recording an expression for the total space-charge field is derived, including multiple spatial frequencies. The derivation is based on the band-transport model. A numerical model is presented in which the relative change in diffraction efficiency is calculated from the corresponding change in the grating strengths. The grating strengths are found from the corresponding frequency components of the total space-charge field. The model is valid in the limit of low diffraction efficiencies and small coupling constants. The investigation is carried out with different values of the intensity ratio between the reference beam and the sum of the two object beams, the applied field to the crystal, and the separation angle between the two object beams. It is shown that relative changes of 200% in the diffraction efficiency occur. Thus the magnitude of the additional grating strengths may be substantial and even comparable with those of the primary grating. Comparing the predictions of the numerical model with the experimental data shows excellent agreement.

DIFFRACTiVE LIQUID CRYSTAL SPATIAL LIGHT MODULATORS WITH OPTICALLY INTEGRATED FINE-PITCH PHASE GRATINGS

Abstract: We describe opto-electronic LC spatial light modulators which allow the generation of alterable diffraction gratings down to 1 micrometers period. Optically defined binary and blazed phase gratings are used in which the diffraction is altered by a parallel aligned nematic LC phase modulation layer. This enables the creation of arbitrary fine-pitch gratings. Different types of modulators have been theoretically and experimentally investigated. The theoretical results show that in the case of small grating structures the exact wave theory has to be applied to calculate the diffraction properties. Then the grating modulation becomes dependent on the light polarization and the grating period. Modulators with gratings of 1 to 10 micrometers period have been fabricated. The modulator structure, the LC phase modulation layer, the problems of combining LCs with fine-pitch gratings, and the modulator properties are discussed. Good diffraction efficiencies and extinction ratios for the diffraction orders of the modulators for monochromatic light have been obtained as well as short switching times.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optoelectronic multi-wavelength component

Abstract: An optoelectronic multi-wavelength component is described for n waveguides with optical feedback gratings arranged side by side laterally to have different characteristic wavelengths. The multi-wavelength component is based on at least two optical waveguides arranged on a substrate so they intersect the grating lines of one or more grating fields at different angles. The grating lines of the feedback grating run at a tilted angle relative to a preferential direction. The angles between each individual waveguide and the grating lines of the respective grating are in the range between 40° and 140°. A controlled change in the effective corrugation period from one waveguide to the next is achieved by means of the individual dimensioning of the tilt angle between the waveguide and the grating lines of the respective feedback grating. The solution is used in photonic components that work on different waveguide channels and are based on DFB or DBR gratings or sampled gratings.

Method of creating a Bragg grating in a photosensitive optical waveguide

Abstract: of EP0684491During the photo-induced creation of a Bragg grating in a photosensitive optical fibre (66) using the interference fringe pattern generated in the near field of a phase grating (65), the spacing between fibre and phase grating is modulated.

White-light spatial frequency multiplication using soft x-rays

Abstract: The authors have patterned a 0.25-micron period grating in SAL-601 photoresist using soft x-ray white-light spatial frequency multiplication. The configuration is that of a grating interferometer using two transmission gratings having the same period ({Lambda} = 0.5 micron) and fabricated by electron beam lithography and lift-off. The first transmission grating splits an incoming x-ray beam into two paths and the second grating, operating in higher order, combines the two beams. A standard wave pattern is obtained at the intersection of the two beams and recorded by a photoresist coated substrate. This patterning technique has the advantage of multiplying the spatial frequency of the interferometer gratings by an even integer factor. Furthermore, the recording geometry is insensitive to both the longitudinal and transverse coherence of the illumination. Synchrotron bending magnet radiation from the Advanced Light Source located at the Lawrence Berkeley National Laboratory was used as the source. The grating interferometer geometry has been used din the past to record white-light interference fringes using visible and ultraviolet light sources. They have used a two-grating interferometer to provide an initial demonstration of white-light spatial frequency doubling at soft x-ray wavelengths. By using this technique with shorter period parent gratings, it should bemore » possible to pattern gratings with higher resolution than electron beam lithography.« less

Excimer laser irradiated phase masks for grating formation

Abstract: Excimer laser irradiated phase masks provide a convenient and effective method for writing submicron gratings for optoelectronic device applications including optical fibre gratings. We have analysed the interference field produced by a periodic mask and have assessed the near-field energy density and fluence distribution for varying degrees of order content when exposed using an excimer laser with finite spatial and temporal coherence. Comparisons are made between theory and experimental findings for gratings produced on ablated polymers and in optical fibres.

White-light spacial frequency multiplication using soft x rays

Abstract: We have patterned a 0.25-micron period grating in SAL-601 photoresist using soft x-ray white-light spatial frequency multiplication. The configuration is that of a grating interferometer using two transmission gratings having the same period ((Lambda) equals 0.5 micron) and fabricated by electron beam lithography and lift-off. The first transmission grating splits an incoming x-ray beam into two paths and the second grating, operating in higher order, combines the two beams. A standing wave pattern is obtained at the intersection of the two beams and recorded by a photoresist coated substrate. This patterning technique has the advantage of multiplying the spatial frequency of the interferometer gratings by an even integer factor. Furthermore, the recording geometry is insensitive to both the longitudinal and transverse coherence of the illumination. Synchrotron bending magnet radiation from the advanced light source located at the Lawrence Berkeley National Laboratory was used as the source. The grating interferometer geometry has been used in the past to record white-light interference fringes using visible and ultraviolet light sources. We have used a two-grating interferometer to provide an initial demonstration of white-light spatial frequency doubling at soft x-ray wavelengths. By using this technique with shorter period parent gratings, it should be possible to patten gratings with higher resolution than electron beam lithography.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Gray-level image generation by multiphase diffraction grating

Abstract: The diffraction gratings which split a incoming beam into multiple spots are useful optical devices for the generation of regular array in the Fourier plane. The structure of the grating can be divided into rectangular cells such that each cell imparts a phase delay of O/spl sim/2/spl pi/[rad] to the incident wave front. So, this pixelated grating is called multiple-beam hologram or computer-generated hologram, and it can produce a desired diffraction pattern at the arbitrary positions. In this paper, we have designed and implemented the diffraction gratings written by laser pattern generator for obtaining a images that have weighted intensity values.

Radiation produced by relativistic electrons moving over a diffraction grating

Abstract: The characteristics of the emission spectrum produced by a single electron traversing a diffraction grating are explored. Emphasis is on the limit where the electron energy is relativistic.