Showing papers on "Diffraction efficiency published in 1986"

Rigorous coupled-wave analysis of metallic surface-relief gratings

Abstract: A rigorous coupled-wave analysis for metallic surface-relief gratings is presented. This approach allows an arbitrary complex permittivity to be used for the material and thus avoids the infinite conductivity (perfect-conductor) approximation. Both TE and TM polarizations and arbitrary angles of incidence are treated. Diffraction characteristics for rectangular-groove gold gratings with equal groove and ridge widths are presented for free-space wavelengths of 0.5, 1.0 and 10.0 μm for all diffracted orders as a function of period, groove depth, polarization, and angle of incidence. Results include the following: (1) TM-polarization diffraction characteristics vary more rapidly than do those for TE polarization, (2) 95% first-order diffraction efficiency occurs for TM polarization at 10.0 μm, (3) 50% absorption of incident power occurs at 0.5 μm, and (5) the perfect-conductor approximation is not valid for TM polarization at any of the wavelengths and is not valid for TE polarization at 0.5 μm.

Theoretical Study of the Anomalies of Coated Dielectric Gratings

Abstract: The zero-order diffraction efficiency anomalies of a corrugated dielectric waveguide are studied theoretically in detail. A new and surprising phenomenon is observed: the efficiency changes from 0 to 100 per cent in the vicinity of the excitation of guided waves. The fundamental parameters of the system are found in the case where only one order is propagating and some of their properties are shown. The behaviour of the efficiency curves is explained by a phenomenological theory and a comparison with numerical rigorous results is made.

Polarization properties of photorefractive diffraction in electrooptic and optically active sillenite crystals (Bragg regime)

Abstract: The polarization properties of diffraction from volume phase gratings in photorefractive sillenite crystals such as bismuth silicon oxide (Bi12SiO20), bismuth germanium oxide (Bi12GeO20), and bismuth titanium oxide (Bi12TiO20) are strongly modified by the presence of concomitant natural optical activity and electric-field-induced linear birefringence. A set of coupled-wave equations that characterize the Bragg regime has been derived for the 〈110〉 and the 〈001〉 crystallographic orientations typically employed in volume holographic storage and multiwave-mixing applications. The predicted anisotropic behavior of the grating diffraction is experimentally confirmed, and a significant efficiency improvement is shown to occur for proper choice of the operating mode and the probe beam polarization in a given configuration.

Zero-reflectivity high spatial-frequency rectangular-groove dielectric surface-relief gratings.

Abstract: Using rigorous coupled-wave analysis, high spatial-frequency rectangular-groove surface-relief phase gratings are shown to be capable of exhibiting zero reflectivity. Thus these corrugated surfaces may act as antireflection coatings in a variety of applications. The diffraction characteristics of rectangular-groove surface-relief gratings are presented for several ratios of incident wavelength to grating period as a function of filling factor, groove depth, angle of incidence, and polarization. The conditions for zero reflectivity are identified. Results are compared with single-homogeneous-layer approximate theory results. In the limit of long wavelengths for an electromagnetic wave in a dielectric of refractive index n1 normally incident on a dielectric of index n2, it is determined that for antireflection behavior, the grating groove depth should be λ/4(n1n2)1/2 and the filling factor should be n1/(n1 + n2) or n2/(n1 + n2) for the electric field perpendicular or parallel to the grating vector, respectively. The spectral and angular responses of these gratings are like those of single-homogeneous-layer antireflection coatings. These gratings also exhibit birefringent retardation.

Fourth-order partial-coherence effects in the formation of integrated-intensity gratings with pulsed light sources

Abstract: We performed theoretical calculations of the relative diffraction efficiency of partially coherent light-induced integrated-intensity gratings using pulsed sources, paying particular attention to thermal gratings. We provided a simple intuitive picture of the phenomenon and then calculated exact expressions that, unlike instantaneous-intensity-grating results, necessarily require the use of fourth-order coherence functions. Assuming several radiation models, we evaluated these expressions and found that the results proved to be insensitive to the specific radiation model assumed. The application of these results to a previously performed pulsed-laser experiment yielded a better fit to the data than an expression involving only second-order coherence, which is valid only in the cw limit. We included the effects of grating decay and, in addition, compared the use of integrated-intensity gratings for ultrashort-pulse-length measurement with standard techniques. Finally, we calculated expressions for the relative diffraction efficiency of integrated-intensity gratings created with excitation beams from two separate and independent sources of different frequency, and we report an experiment whose results were found to agree with this theory.

Conical diffraction mounting generalization of a rigorous differential method

Abstract: A generalization of the method of J. Chandezon et al. (1982) is presented for conical diffraction mounting. The applicability of the invariance theorem for real metal and dielectric gratings is discussed and the existence of some polarization effects is demonstrated. A formulation of the reciprocity theorem in conical diffraction mounting is proposed, valid not only for the efficiencies of the diffracted orders but for the amplitudes, too.

Diffraction characteristics of planar corrugated waveguides

Abstract: The diffraction efficiency curves are calculated as a function of the angle of incidence for planar waveguides with corrugation on the waveguide-air boundary or on the waveguide-substrate boundary, respectively. It is shown that the reflectance of the system is increased up to 100% because of the excitation of the waveguide mode. A detailed phenomenological study is carried out taking into account the influence of the waveguide thickness and the corrugation depth. Possible applications of these waveguides as a narrow-band reflection filter and selective mirror are discussed.

Time-resolved photorefractive four-wave mixing in semiconductor materials

Abstract: The time responses of the photorefractive effect in semi-insulating InP doped with Fe and CdTe doped with In are measured. Using microjoule infrared pulses, gratings can be written and erased in less than a microsecond and probably much faster. Through use of a pulsed applied field, good diffraction efficiencies are achieved along with fast dark relaxation times of the order of 10−4 sec. These factors indicate that semiconductors have promise for photorefractive image-processing applications.

Convergence of Rayleigh-Fourier Method and Rigorous Differential Method for Relief Diffraction Gratings

Abstract: The Rayleigh-Fourier method and the method of Chandezon et al. for the calculation of diffraction efficiency of sinusoidal metal, dielectric and coated dielectric gratings are compared with respect to the threshold truncation value and the thickness of the coating layer. For shallow gratings the convergence of the two methods is practically one and the same. However, for deeper gratings the method of Chandezon et al. is more powerful. It is shown that for coated dielectric gratings the thickness of the layer imposes a limit on the truncation value, the limitations being weaker for the method of Chandezon et al.

Optically induced variable light deflection by anisotropic Bragg diffraction in photorefractive KNbO 3

Abstract: Photoinduced anisotropic Bragg diffraction in photorefractive coefficient KNbO3 using the electro-optic r42 = 380 pm/V has been studied. It has been demonstrated that, by using a noncritical configuration, a He–Ne laser beam with fixed incidence angle can be deflected within a range of 5.67 deg by changing the wavelength λr of the Ar+ laser beams used for recording the photorefractive grating (wavelength tuning range; 457.9 nm < λr < 514.5 nm). It is shown that for the reported interaction angles between recording and diffracted beams, the Bragg condition is noncritically fulfilled for the whole deflection range without any beam tilting being required.

Volume reflection holograms with multiple gratings: an experimental and theoretical evaluation.

Abstract: A coupled wave model for two multiplexed reflection holograms recorded in a volume material is presented. Both simultaneously and sequentially formed gratings are evaluated. The effects of a strong intermodulation grating, the thickness change with emulsion processing, and the construction geometry on the desired diffraction orders are investigated over a range of reconstruction angles. The model results are compared with experimental data from holograms formed in bleached photographic emulsions.

Holographic optical display system with optimum brigtness uniformity

Abstract: (D An optical display system (10) employs a holographic optical element (20 or 22) that has a holographic fringe pattern which is coordinated with the phosphor emission peak (32) of a cathode ray tube (24) to eliminate perceptible variations in image brightness. The holographic optical element has a reflection characteristic (34) that defines two diffraction efficiency peaks (36 and 38) which are resolved by a low diffraction efficiency dip (40) that is interposed between them. The optimum wavelength spacing between the two diffraction efficiency peaks for a given wavelength spacing is determined by computing for all observer (28) head positions and look angles of concern the differences among the areas under the intergrated efficiency characteristics (42) for the reflection characteristic (30) holographic optical element and the phosphor emission characteristic of the image-producing cathode ray tube. The optimum wavelength spacing is that which provides the desired variation among the computed difference values, which represent the display brightness uniformity. The center dip reflection characteristic increases the spectral bandwidth of the holographic optical element and thereby increases its reflectivity to promote good contrast with the use of a cathode ray tube operating at reduced beam current levels. When installed as a combiner (12 or 14) in a head-up display system (10) for aircraft, the holographic optical element superimposes the image on an outside scene without introducing significant discoloration of the scene.

Fabrication method and equipment for diffraction gratings

Abstract: A method and equipment for fabricating two diffraction gratings having the same period but phases that are shifted by λ/4 on a work to be treated. A laser beam is divided into two light beams and one of the beams is delayed over a portion of the light beam. The divided light beams are then mixed together so that they interfere. Using the known photolithography technology, the interference patterns are formed into diffraction gratings.

Guided-wave acousto-optic interaction in proton-exchanged Y-cut LiNbO3

Abstract: We report for the first time results for acousto‐optic interaction in proton‐exchange waveguides in Y‐LiNbO3. The value of acousto‐optic diffraction efficiency was as high as 72% for an electrical input power of 540 mW at 425 MHz.

Thin film waveguide type optical diffraction element

Abstract: PURPOSE:To improve the diffraction efficiency by forming a grating by providing periodically a groove in an optical waveguide layer on the surface of a substrate, and embedding a substance whose refractive index difference to the waveguide layer is at least >=0.5, into the groove part of the lattice. CONSTITUTION:An optical waveguide layer 3 of a glass film, etc., formed by a sputtering vapor-deposition, etc., is provided on an Si wafer, etc., as an optical circuit board 5. Subsequently, a diffraction grating 2 is formed by drawing a grating pattern having periodical unevenness on the optical waveguide layer 3 by using a photolithography, and embedding a substance whose refractive index difference to the waveguide layer is at least >=0.5 into a groove of a recessed part. In such a way, the refractive index difference between the embedded part 2 and the optical waveguide layer 3 can be enlarged, and also, the boundary of the embedded part becomes clear, and the diffraction grating having a sudden refractive index variation can be formed, and a large diffraction efficiency is obtained.

Study on the pH dependence of diffraction efficiency of phase holograms in dye sensitized dichromated gelatin

Abstract: Dichromated gelatin is thought to be a good substitute for photographic emulsions in some uses. The results of a systematic study of the effect of the pH of the developer on the diffraction efficiency of volume holographic gratings recorded in dye sensitized dichromated gelatin are presented.

Spectroscopic measurement system

Abstract: A spectroscopic measurement system comprises at least two kinds of diffraction gratings (1,2) whose grating surfaces are in line, an exchange device (3,4) for ex­changing the positions of the two kinds of diffraction gratings (1,2) in connection with incident light to be measured while the two kinds of diffraction gratings (1,2) are placed in a predetermined rotation angle, at least two kinds of detectors (5,6) having characteristics corresponding to those of the two kinds of diffraction gratings (1,2), respectively, a light path switch (7) for switching a path of diffraction light toward either of the two kinds of detectors (5,6), and a switch circuit (9) for switching the detection output of the two kinds of detectors (5,6) in synchronization with the exchange ope­ration of the two kinds of diffraction gratings (1,2).

Diffractive Infrared Filters Fabricated by Electron-beam Lithography

Abstract: With the increasing number of optical systems for use in the near- and middle-infrared spectral regions, the demand for optical elements that exhibit predetermined spectral and polarizing characteristics has increased also. Although continuous thin films are capable of producing a wide variety of optical characteristics, the use of metal meshes adds another dimension to spectral and polarization selectivity. Metal meshes are essentially amplitude diffraction gratings whose periodicity is less than the wavelength of the incident radiation. As such, only a single propagating mode exists, all others being evanescent. Diffractive structures of this type have been shown to produce spectral and polarizing properties that depend on the geometry of the mesh. Microlithographic techniques were used to fabricate diffractive patterns of aluminum and gold, with minimum feature sizes less than 0.25 p,m for use as mesh filters with near and middle-infrared.

Improvement Of Photothermoplastic Devices For Their Application To Optical Switching

Abstract: The cycling properties of photothermoplastics based on Staybelite Ester 10 can be improved to several thousands of recording cycles. A chemical purification of as-received Staybelite Ester 10 and the use of relatively thick thermoplastic and photoconductive layers are needed in order to preserve both high diffraction efficiency and good cycling properties. Such devices are especially suited to variable beam steering and optical switching.

Tomographic reconstruction of objects by grating interferometer.

Abstract: A theory of diffraction tomography using incoherent radiation is presented. Diffraction gratings are used in the formation of Fourier components of the object distribution.

Crystallographic fringe orientation diffraction efficiency in bismuth silicon oxide

Abstract: We report the first experimental measurement of fringe-angle applied electric field scaling of space charge growth and of crystallographic orientation effects in the initial development of the diffraction efficiency of thick holograms produced by the photorefractive effect in a bismuth silicon oxide (BSO) crystal. Diffraction efficiencies of holograms made by interfering two plane waves on the [¯110] face are measured as a function of the angle between the fringe pattern and the applied electric field. As the crystal is rotated relative to the interference fringes, the applied field may be scaled to yield identical space charge growth. Polarization-dependent diffraction measurements agree with the theory of a birefringent grating when optical activity is included as a separate, serial effect. Both the rotation-scaled applied electric fields and the crystallographic variations in the birefringent diffraction grating are consistent with charge transport processes in which the initial space charge fields are perpendicular to the interference fringes over growth times extending nearly into the steady state regime.

Holographic Grating Spectrometers: Wood's Anomalies, Spectral Response Functions, and Their Dependence on Groove Density

Abstract: Presented is a study of plane holographic gratings with sinusoidal groove profiles, focusing attention on the dependence of grating efficiency on groove density. Experimental spectral response curves are given for a number of gratings with different groove densities in an additive-dispersion double monochromator designed for work at visible wave-lengths. The Wood's anomalies, both order-passing and resonance type, are characterized in each case. Simple equations are derived for predicting order-passing wavelengths given a fixed angle of deviation between the incident beam and the observed diffraction order. The trend in the strength of the anomalies is compared with theoretical calculations.

Holographic Recording Material Containing Poly-N-vinylcarbazole

Abstract: A volume phase holographic recording material with high diffraction efficiency and high durability against humidity has been developed. The holographic material consists of Poly-N-vinylcarbazole (PVCz) as a base polymer, camphorquinone as an initiator and thioflavine-T as a sensitizer. This film is sensitive to argon ion laser light, and exposure energy of 500 mJ/cm2 is required to realize high diffraction efficiency. After recording a latent image of a fringe pattern by exposure, a hologram was developed by swelling and shrinking of the film with two sorts of solvent. The thickness of the hologram could be reduced to 2.5 μm, because the PVCz hologram has a large amplitude of the refractive index modulation related with the crystallinity modulation. The high diffraction efficiency coupled with the thin layer made an incident light angle wide enough to maintain a high diffraction efficiency around the Bragg angle.

Optical-Heterodyne Detection of Mask-to-Wafer Displacement for Fine Alignment

Abstract: The lateral displacement between a mask and a wafer was measured by a newly developed optical-heterodyne method using three symmetrically-arranged gratings. This method is based on variation of the diffraction efficiency of a grating with the polarization of the incident light. With a 0.72 µm-period-grating system and a He–Ne transverse-mode Zeeman laser (wave-length=0.6328 µm), sensitivity better than 1° /0.01 µm was obtained and displacement smaller than 0.005 µm was detected independent of the mask-wafer gap variations. The principle and experimental results are described.

Vacuum UV performance of a new 6.65-m concave diffraction grating with 4800 grooves/mm

Abstract: A series of new high ruling density (4800-groove/mm) concave gratings for 6.65-m spectrometers is being produced expressly for use in experiments using synchrotron radiation in combination with high-resolution spectroscopy. We report here the results obtained from extensive testing of a gold coated replica (125- × 110-mm ruled area) of the first grating in this series. This grating is blazed near 900 A in first order, and the quality of the ruling is excellent. Comparisons with results obtained from other high-quality 6.65-m gratings of lower ruling densities are presented.

Experimental investigations of thermally stimulated degenerate four-wave mixing

Abstract: Results of experimental investigations aimed at testing efficiency-related aspects of thermally induced degenerate four-wave mixing in absorbing solutions are reported. The effects of fluorescence, read beam time delay, and forward beam angle variations are measured and shown to strongly influence the achievable thermal grating diffraction efficiency. In particular, a strong decrease of conjugation efficiency is observed as a function of increasing angle between the forward beams. Conjugate beam diffraction efficiencies as high as 17 percent were measured for the dye RhB in acetone. Based on our observations, we predict that still higher conjugation efficiencies will be feasible under optimal conditions with the thermal grating mechanism.