Original Grating Research Articles

Wavelength-interleaved OADMS incorporating optimized multiple phase-shifted fbgs for fiber-radio systems

We demonstrate that a single fiber Bragg grating (FBG)-based optical add-drop multiplexer can be used to improve the optical spectral efficiency in a mm-wave WDM-fiber wireless network. The technique employs wavelength interleaving in conjunction with optical single-sideband modulation, together with a single multiphase-shifted fiber Bragg grating. Wavelength optical add-drop features are also demonstrated using the grating, and our results show the performance of the novel grating in terms of channel isolation and add-drop functionality. We perform simulations to optimize placement of the phase shifts when an apodization profile is applied to the grating, in an attempt to minimize in-band and out-of-band transmission-amplitude ripples. Experiments are performed using cascade configurations both with the apodized gratings with optimal phase-shift positions and with the original unoptimized gratings. The experimental results demonstrate how these novel gratings can be used in fiber-radio networks incorporating multiple optical add-drop multiplexers.

Action from tunable periodic structures. II. Experimental observation of electric field-induced diffraction peaks.

As previously predicted [Appl. Opt. 40, 5583 (2001)], we have now observed electric field-induced diffraction peaks in transmission and reflection experiments by use of a LiNbO3 sample with interdigital planar electrodes that serve as a diffraction grating. The magnitudes of the new peaks in the reflection experiments are ten times larger than those in the transmission experiments. We interpret these effects in terms of a field-induced refractive-index change produced by the linear electro-optic effect. The positive and negative changes in the refractive index produce two diffraction gratings that are period doubled with respect to the original grating and that have a phase difference between them. The superposition of the diffracted light from these gratings is shown to account for the new peaks. From the relative magnitude of the new peak to that of the central peak, we estimate the refractive-index change to be 0.004.

Moiré beating digital technique to collimation testing

A digital collimation technique based on the self-imaging phenomenon is proposed. Basically, a moiré pattern is observed by viewing the Fresnel image of an original self-imaged grating through a second grating. Decollimation of the laser beam causes a variation in the self-image period. In our method, the moiré beating is produced through a digital subtraction operation of a replica of the original grating and the self-images provided by a decollimated beam. The variation of the moiré period is related to the amount of decollimation of the illuminating beam. Lenses of different focal lengths were used in the collimation procedure, as well as gratings with different periods. The validity of the approach is experimentally demonstrated and setting sensitivities are compared.

Profiled hologram diffraction gratings based on As2Se3 inorganic photoresist

We devised a method of producing blaze hologram diffraction gratings by transforming symmetric grooves of the original grating into asymmetric ones using an additional oblique irradiation by monochromatic or polychromatic light and a repeated chemical etching of the additionally irradiated grating. Photosensitive As2Se3 film layers are used as a photoconductive material for recording gratings. A numerical computer modeling of the formation of asymmetric grooves is conducted. The calculated shape of the groove profile is in good agreement with experimental data. The manifestation of asymmetry of the profile shape of the produced gratings in angular and spectral relations for their diffraction efficiency is considered. It is shown that by varying the parameters of producing the original gratings and the conditions of their additional treatment it is possible to obtain hologram gratings with the required profile shape and blaze angle.

Photorefractive shooting stars

Under certain conditions the direction of a beam diffracted from a photorefractive grating wanders as the grating decays. This phenomenon is due to competition between the original grating and those formed spontaneously by the Fabry-Perot modes produced by the crystal's surfaces.

Efficient Coupling into Polymer Waveguides by Gratings

Investigations of highly efficient grating couplers for polymer slab and strip waveguides fabricated by electron-beam lithography are reported. A maximum input efficiency of 67% is achieved. The electron-beam direct-writing technique allows one to replicate the original gratings into polymer substrates by embossing. An all-polymeric optical chip with efficient grating couplers is demonstrated. Waveguide grating couplers with blazed profile and variable grating depth are investigated. Thus, the intensity distribution of the outcoupled light is matched to a Gaussian-like profile. A focusing blazed grating that couples the light with an efficiency of 42% into a polymer strip waveguide is reported. A curvature correction of the grating lines allows one to improve the focusing properties.

Generation of Lohmann images from binary-phase Talbot array illuminators

A systematic analysis has been performed that predicts the existence of 36 cases in which 100% modulated, square-wave irradiance distributions can be generated in the Fresnel regime by simple binary-phase gratings. These types of distributions, referred to here as Lohmann images, have been previously predicted by researchers studying phase gratings known as Talbot array illuminators. Twenty of the cases are reported, to the best of my knowledge, for the first time. Sixteen of these new cases result in Lohmann images with twice the spatial frequency of the original grating. Experimental verifications of the theoretical predictions are presented.

Complex Optical Surfaces Formed by Replica Molding Against Elastomeric Masters

Complex, optically functional surfaces in organic polymers can be fabricated by replicating relief structures present on the surface of an elastomeric master with an ultraviolet or thermally curable organic polymer, while the master is deformed by compression, bending, or stretching. The versatility of this procedure for fabricating surfaces with complex, micrometer- and submicrometer-scale patterns was demonstrated by the production of (i) diffraction gratings with periods smaller than the original grating; (ii) chirped, blazed diffraction gratings (where the period of a chirped grating changes continuously with position) on planar and curved surfaces; (iii) patterned microfeatures on the surfaces of approximately hemispherical objects (for example, an optical surface similar to a fly's eye); and (iv) arrays of rhombic microlenses. These topologically complex, micropatterned surfaces are difficult to fabricate with other techniques.

Two-dimensional Talbot array illuminators with single-step phase grating

The purpose of the paper is to pursue solutions to the two-dimensional (2D) Talbot array illuminators (AIL’s). In order to pave the way for the 2D AIL’s, we reshape the formulation for the one-dimensional AIL’s so that it can be referred to for the 2D AIL’s. It is found that there are eight types of 2D single-step phase grating, which lead to eight types of 2D AIL, and they form four conjugate pairs, each of which satisfies the conjugate requirement of diffraction. In practice, we need to consider only one AIL of each conjugate pair, because the given AIL automatically involves the other, owing to the specific structure of the original grating. Moreover, each AIL represents a series of 2D AIL’s, because we are free to assign the structure ratios of the original phase grating. However, the complexity of the 2D AIL offers more design flexibility. We show that some of the data that are simply worthless in the 1D case turn out to be useful in the 2D case. We do not claim to have exhausted the case of 2D AIL’s, as we have confined ourselves only to the case in which the 2D AIL’s have the same periodic structures as those of the original single-step phase gratings.

Optical fringe multiplication in moiré interferometry

An optical method for multiplication of moiré fringes is proposed to increase the sensitivity of moireé interferometry. The process involves two recording steps. In the first step, a traditional moiré interferometry setup is used. The moiré pattern containing carrier fringes and load fringes is recorded onto a glass-based holographic plate. The carrier frequency is much lower than that of the original specimen grating. The plate is then developed. In the second step, the holographic plate, regarded as a distorted specimen grating, is further examined by a similar moiré interferometry system. The frequency of the second virtual grating is arranged to be 2n times that of the carrier fringes contained in the recorded plate. As a result, the load fringes are revealed with a multiplication factor of 2n. The interpretation of the optical multiplication method from wave-front interference theory is given and an experiment is conducted.

Super-resolution through illumination by diffraction-born evanescent waves

A diffraction grating in silicon with 50 nm lines and spaces is illuminated with diffraction-born evanescent waves from its transparent replica. A rotation φ between the gratings results in a set of fringes with period λf that is larger than the period of the original gratings by a magnification factor M. These fringes are induced and observed with a photon tunneling microscope having an incident illumination of 650 nm in air. This super-resolution by what may be considered a form of optical heterodyning with evanescent waves is discussed.

Construction of constant-deviation constant-length spherical grating monochromator at UVSOR

A constant-deviation constant-length spherical grating monochromator was constructed at the bending-magnet beamline 8B1 of the UVSOR. The monochromator has a simple scanning mechanism with a fixed position of the entrance and exit slits, as well as fixed directions of incident and exit photon beams. The monochromator was designed to cover the photon energy of 31–620 eV with three interchangeable laminar gratings (1080 lines/mm: R=15 m, 540 lines/mm: R=15 m, 360 lines/mm: R=7.5 m). All gratings are original gratings fabricated on synthetic quartz substrates and coated with Au. The resolving power evaluated by ray tracing at the lowest energy of each grating with 10 μm slit width is ∼4400 for two high energy gratings and ∼7000 for a low energy grating. In the preliminary performance check, it was found that the output spectrum of the monochromator with G1 grating extends up to 870 eV with resolving power of ∼4000 at 400 eV with 10 μm slits.

Dynamics of a composite grating in photorefractive crystals for memory application

The dynamics of a composite grating that consists of an original grating and a newly superimposed grating are investigated for memory applications by solution of coupled-wave equations and photorefractive-material equations with the following initial conditions: the original grating has an arbitrary refractive index modulation, an arbitrary phase shift exists between the original and the superimposed grating, and there is an externally applied dc electric field. The effect of beam coupling, including fringe bending on the composite grating, is investigated by numerical simulation. Simplified analytical solutions that neglect beam coupling are derived, showing good agreement with experimental results. The investigation also shows that a selective erasure process in the presence of an external electric field is made possible by provision of an appropriate constant phase shift. Analytical solutions of the composite-grating dynamics are used to analyze both scheduled and incremental recording dynamics with an external electric field. It is shown that scheduled recording results in a nonuniform relative phase distribution among multiplexed gratings. In contrast, the incremental recording converges to a uniform relative phase distribution and indicates that selective erasure may be easily implemented for fast memory update.

What part of a vernier stimulus determines performance?

Vernier acuity was measured using a pair of long, abutting vertical sinusoidal luminance gratings. Performance was also measured for pairs of thin horizontal strips of grating whose vertical separation was varied in order to represent different parts of the original grating. At small strip separations, performance was equivalent to that for the long gratings only at high spatial frequencies. At moderate spatial frequencies, intermediate strip separations resulted in equivalent performance. This suggests that different parts of a vernier stimulus can mediate performance depending upon the underlying spatial frequency content. Implications for the effect of contrast upon different vernier configurations are examined.

Chemical Cleaning of Gratings in Distributed Feedback InP Lasers

We have studied three chemical solutions for cleaning gratings prior to epitaxial growth for distributed feedback laser applications. These chemicals are concentrated and mixed solutions of where . On substrates, the etching rate of solutions was about 2–3 times faster than that of concentrated. The solutions etched the grating structure isotropically and preserved the original grating morphology. However, tended to remove the sharp ridges of the original grating and reduce the distributed feedback effect. We have also found that the solutions are much more effective than in removing carbon contaminants, which could provide nucleation sites for dislocation loops and adversely affect the laser diodes' reliability.

Binary Phase Gratings for Materials Processing

The feasibility of using a diffractive optical element, a phase grating, as a beam handling device for use in industrial laser material processing is examined. The binary phase grating is an efficient beam splitting device and is useful in many materials processing operations (welding, cutting, soldering, drilling) where a high degree of parallelism could significantly improve process throughput. Overall efficiency of etched quartz binary gratings agrees well with published predictions (nominal 70–90%). Copies of the grating formed in PMMA (acrylic) survived preliminary life tests (more than 1000 9-J pulses from a commercial multimode Nd-Glass laser) with no measurable change in performance. Conventional Small Outline (SO) IC electronic packages were both laser soldered and welded to substrates with one laser pulse. Beam splitting uniformity of desired images is within 10% for the original quartz gratings and for some PMMA copies. Conventional doublet achromat and planoconvex lenses are adequate for maintaining 10% uniformity of intensity. Splitting uniformity is experimentally evaluated with energy and focused spot (burn) tests on kapton (polyimide) and thin and thick metal samples using a pulsed industrial Nd laser.

Fresnel Images of a Binary Diffraction Grating with Open Ratio Less than 0·5

Abstract An analysis of Fresnel images of the amplitude binary diffraction grating with open ratio α/d < 0·5 is presented. Particular attention is paid to images appearing behind the self-imaging grating at distances z = 0 to d 2/λ. An attempt is made at a systematic examination of Fresnel images that have the form of binary gratings with ratio α′/d′ different from that of the original grating.

X-ray diffraction gratings for synchrotron radiation spectroscopy: a new fabrication method

Silicon dioxide lamellar gratings fabricated by a new technique combining holographic exposure and reactive ion-beam etching are found experimentally to be appropriate to and useful for synchrotron radiation spectroscopy. Methods to improve edge roughness and to control the land-to-groove width ratio are also investigated. The fabrication technique developed here is relatively simple in comparison with the ruling technique which needs delicate control. Therefore, it is a promising way to employ these bakable original gratings for synchrotron radiation spectroscopy.

Performances of the lure rowland circle monochromator using gas phase photoelectron spectroscopy up to a photon energy of 280 eV

The performance of the LURE Rowland circle grazing incidence monochromator has been determined up to a photon energy of 280 eV using gas phase photoelectron spectrometry with the PAX method. Suitable converters such as He 1s, Ne 2p, Ar 2p and Ar 3p have been used to study the resolution, the intensity, the contribution of higher orders and the level of stray light. The results obtained with various sorts of gratings (blazed and holographic, 600 lines/mm to 2400 lines/mm, replica and original gratings) are presented. The superiority of the holographic Jobin-Yvon gratings in the high energy region is demonstrated.

Wire Diffraction Gratings

In the summer of 1972, I attended the workshop on recreating classic experiments in physics at Barnard College.1 This was developed by Samuel Devons, and it was a defining experience that set me toward a research career involving early physics teaching apparatus. During the course of the workshop, I became curious about the original diffraction gratings developed by Fraunhofer and built a wire diffraction. A short note about the gratings was published in the American Journal of Physics the next year.2