Recording high-dispersion spherical holographic gratings in a modified Rowland mounting by use of a multimode deformable mirror

To reduce the uncorrected higher-order aberrations for holographic gratings requiring an extreme dispersion, we have modified the Rowland mounting by moving the recording laser sources away from the grating. Then, with a multimode deformable plane mirror to record the grating, the correction of all the aberrations up to the fourth order inclusive is found sufficient to obtain a high-quality image. Applied to the FUSE-LYMAN grating, with a groove density of as much as 5740 grooves/mm, for which a resolution of 30,000 was required, this new recording device produces a resolution from 139,000 to 222,000 over the spectral range.     

Comparison of mechanically ruled versus holographically varied line-spacing gratings for a soft-x-ray flat-field spectrograph

Recent progress in the design of aspheric wave-front recording systems has permitted the manufacture of holographic gratings with highly variable groove densities that are suitable for flat-field spectrographs. A holographic grating thus recorded was processed to produce a laminar profile by use of reactive-ion etching. Measurements are reported of the absolute diffraction efficiency of this grating and of a comparable mechanically ruled grating. It is found that the holographic grating is much more effective in suppressing the higher orders. The spectral resolution was determined by use of a carbon Kalpha x-ray generator and a spectrograph with an imaging detector. The spectral resolution of the holographic grating was approximately 3 times worse than that of the ruled grating.     

Holographic recording characteristics of an acrylamide-based photopolymer

Recent research on an acrylamide-based photopolymerizable holographic recording material is presented. The recording characteristics of the material are discussed in detail in terms of sensitivity, diffraction efficiency, recording linearity, resolution limit, and sources of noise. Although the resolution is not sufficient for reflection holography, the recording characteristics are excellent for transmission gratings. The material was found to suffer no shrinkage during recording, and high-diffraction-efficiency slanted gratings were made. Finally, the suitability of this self-developing material to both double-exposure and real-time holographic interferometry is demonstrated.     

Universal method for holographic grating recording: multimode deformable mirrors generating Clebsch-Zernike polynomials

Recording methods for making aberration-corrected holographic gratings are greatly simplified by use of a plane multimode deformable mirror (MDM) upon one of the two recording beams. It is shown that MDM compensators easily provide the superposition of many interesting active optics modes, which we have named Clebsch-Zernike modes. When we apply only a uniform loading or no loading at all onto the rear side of the MDM clear aperture, the available Clebsch-Zernike modes are made to belong to a subclass of the Zernike modes that includes the three modes of the third-order aberration theory as well as a well-defined part of the Zernike higher-order modes. Such a recording method is considered to be universal, since it does not require the use of a sophisticated optical system such as a compensator. Active optics 12-arm MDM's in the vase form have been designed from the elasticity theory. The design of six-arm MDM's is currently carried out with theoretical results. As an example of the method, the recording of three holographic gratings of the Hubble Space Telescope Cosmic Origins Spectrograph has been investigated. Substantial improvements in image quality have been found by use of a six-arm MDM as recording compensator. The result is that aberrations of much higher order can simultaneously be corrected so that the residual blur images of the spectra occupy areas approximately 10 (direction of dispersion) x 3 (cross dispersion) = 30 times smaller-also in terms of pixel number-than those obtained by our American colleagues. Therefore the active optics recording method appears to provide substantial gains in resolving power and in sensitivity: (i) For all three gratings the spectral resolution would be increased by a factor of 10, and (ii), in addition, for the two higher dispersion gratings, the limiting magnitude on the sky appears to be increased by a magnitude of approximately 1-1.2.     

Copying low spatial frequency diffraction gratings in photopolymer as phase holograms

Abstract

Photopolymers are studied as holographic recording materials for recording computer-generated phase holograms. The recording was performed using a holographic copying process, in which the master is produced by means of a high resolution graphic device. To our knowledge it is the first time a photopolymer has been used to obtain copies with this technique. As it does not work in real time, it is possible to store the diffractive optical element for a long time because the dye is finished during exposition and no wet processing is needed after exposition. The photopolymer used in these experiments was composed of acrylamide and triethanolamine as the co-initiator, photo-initiated with a dye, yellow eoxine. Components were supported by a film of poly-(vinylalcohol) (PVA). The resulting thickness of the film was 70 & plusmn; 5 μm, symmetrical and asymmetrical diffraction gratings were obtained as phase holograms by index and thickness modulation, monitored with an electron microscope, and the low spatial frequency response of the photopolymer was studied. Diffraction efficiency achieved for copied gratings was about 30%. With this study we get a complete characterization of the photopolymer for the full range of the typical spatial frequencies that can be recorded with the high resolution graphic device used.     

Recording of concave diffraction gratings in counterpropagating beams using meniscus blanks

The most technological way of recording blazed concave holographic gratings is by using the direct- and back-reflected beams. Usually plano-concave blanks are used for diffraction grating fabrication. To compensate the refraction at the plane back surface of the blank, one has to use additional elements or two-step recording mountings. Another solution is to use diffraction grating blanks having zero optical power. A number of gratings with different groove frequencies have been fabricated using concave—convex blanks. The theoretical investigation of the aberration properties of the recording mountings, and the experimental study of the grating properties are discussed. Using the CODE V program it is shown that a significant reduction in the aberration size of the virtual recording source can be achieved by optimization of the radius of curvature of the blank back surface. The experimental results confirm the possibility of achieving gratings that can be used in flat-field spectrometers with a limit of resolution of about 2–3 nm even in the case where an aberrated virtual recording source is used.     

Recording of holographic diffraction gratings in photopolymers: Theoretical modelling and real-time monitoring of grating growth

Abstract

Recording of holographic volume diffraction gratings in Du Pont's photopolymer HRS-150 is studied theoretically and experimentally. Particular attention is paid to the dynamics of the recording process. The temporal evolution of holographic gratings is monitored for a range of illumination intensities in two ways: (i) by means of a multiple-exposure approach when intensity profiles of recording beams are uniform, many holographic exposures are necessary and each exposure is carried out for a particular value of the total illumination intensity; (ii) by means of a single-exposure approach when intensity profiles of recording beams are strongly Gaussian and a volume grating with spatially distributed diffraction efficiency arises, depending on a local (average) value of the total illumination intensity. The second approach proves to be a useful tool providing us very quickly with qualitative information about the dynamics of the recording process while the first one, which is much more time-consuming, is more reliable for quantitative evaluations. The grating evolution is analysed theoretically by means of the well known photopolymerization-diffusion model. Discrepancies are found between the model and experimental results for low illumination intensities while qualitative agreement is found for higher intensities. For a given grating period, the discrepancies are interpreted in terms of insufficient change of the refractive index due to photopolymerization for low illumination intensities.     

Photopolarimeter based on two gratings recorded in thin organic films

A photopolarimeter based on two different kinds of diffraction gratings (a two-grating photopolarimeter) has been developed for real-time measurements of the four elements of the Stokes vector. The main elements of the device are a pure polarization grating and an ordinary transmission grating, both recorded by means of holographic techniques in thin films of organic materials. The first one consists of a diffraction grating recorded by two interfering opposite circularly polarized beams in a Langmuir-Blodgett film of an azo-compound material. The second component is a grating recorded by two interfering parallel circularly polarized beams in a thin film of a photosensitive polymer. Both gratings offer long time stability and good diffraction efficiency. Four photodiodes collect the first-order diffracted beams from these gratings, the output signals of which are read through an analog-to-digital converter by a PC. The optical alignment of the device is easy and the calibration is realized in a one-step procedure.     

Holographic sensor for water in solvents

The diffraction color of a gelatin holographic diffraction grating changed as a function of the water activity when immersed in a "wet" hydrophobic liquid. Quantification of the absorption maximum of the diffracted light showed that it was related, after calibration, to either the water content or the water activity of the solvent. The holographic diffraction grating measured water contents of hydrocarbon solvents at sensitivities comparable to that of the Karl Fischer coulometric titrator and over a wide range of water contents. A grating immersed in xylene revealed a visible color change when the water content was increased from 47 to 120 ppm. Conversely, the holographic grating responded to ethanol in water in the range 0-1% (w/w). The inexpensiveness and simplicity of silver halide holographic reflection gratings, combined with their relatively high sensitivity, suggests that these devices might find widespread application as immersible water activity sensors for hydrophobic liquids.     

Diffraction properties of volume holographic gratings for an ultrashort pulsed beam with different polarization states readout

The diffraction properties of volume holographic gratings are studied when the gratings are illuminated by an ultrashort pulsed beam with different polarization states. The developed coupled wave theory of Kogelnik is used. Considering the dispersion effect of the grating media, solutions for the diffracted and transmitted intensities, diffraction efficiencies and the bandwidths of the gratings are given in transmission volume holographic gratings and reflection volume holographic gratings. The bandwidths of the gratings are reduced by the dispersion effect of the grating media. They also have different influences on the diffraction of an ultrashort pulsed beam with different polarization states. For different values of the ratio of the spectral bandwidth of the input pulse to that of the grating, the changes of the spectral and temporal distributions of the diffracted intensities, as well as the diffraction efficiencies of the gratings are shown.     

Finite-number-of-periods holographic gratings with finite-width incident beams: analysis using the finite-difference frequency-domain method

The effects of finite number of periods (FNP) and finite incident beams on the diffraction efficiencies of holographic gratings are investigated by the finite-difference frequency-domain (FDFD) method. Gratings comprising 20, 15, 10, 5, and 3 periods illuminated by TE and TM incident light with various beam sizes are analyzed with the FDFD method and compared with the rigorous coupled-wave analysis (RCWA). Both unslanted and slanted gratings are treated in transmission as well as in reflection configurations. In general, the effect of the FNP is a decrease in the diffraction efficiency with a decrease in the number of periods of the grating. Similarly, a decrease in incident-beam width causes a decrease in the diffraction efficiency. Exceptions appear in off-Bragg incidence in which a smaller beam width could result in higher diffraction efficiency. For beam widths greater than 10 grating periods and for gratings with more than 20 periods in width, the diffraction efficiencies slowly converge to the values predicted by the RCWA (infinite incident beam and infinite-number-of-periods grating) for both TE and TM polarizations. Furthermore, the effects of FNP holographic gratings on their diffraction performance are found to be comparable to their counterparts of FNP surface-relief gratings.     

Dark self-enhancement in dichromated poly(vinyl alcohol) gratings: a detailed study

Experimental results concerning the real-time hologram recording process in dichromated poly(vinyl alcohol) are presented. Self-enhancement, the increase in diffraction efficiency of the holographic gratings in the dark after recording, is also presented. The influence of the recording parameters (pH, exposure energy, dichromate concentration) on the self-enhancement gain is shown. Maximum gain was 6, and self-enhancement occurred during the 3 days following the exposure. The results and a model for the reduction of chromium ions corresponding to the formation of the grating are discussed.     

Improvement of sensitivity and refractive-index changes in LiNbO3:Ce:Cu crystals with UV light

A nonvolatile recording scheme is proposed using LiNbO₃:Ce:Cu crystals and modulated UV light to record gratings simultaneously in two centres and using red light to bleach the grating in the shallow centre to realize persistent photorefractive holographic storage. Compared with the normal UV-sensitized nonvolatile holographic system, the amplitude of refractive-index changes is greatly increased and the recording sensitivity is significantly enhanced by recording with UV light in the LiNbO₃:Ce:Cu crystals. Based on jointly solving the two-centre material equations and the coupled-wave equations, temporal evolutions of the photorefractive grating and the diffraction efficiency are effectively described and numerically analysed. Roles of doping levels and recording-beam intensity are discussed in detail. Theoretical results confirm and predict experimental results.     

Plane gratings for high-resolution grazing-incidence monochromators: holographic grating versus mechanically ruled varied-line-spacing grating

Comparative studies have been made on the holographic plane grating and the ruled varied-line-spacing (VLS) plane grating designed for two kinds of objective Monk-Gillieson type high-resolution grazing incidence monochromator, I and II. The ray-traced performance of monochromator types I and II on a synchrotron radiation beam line was evaluated in terms of resolving power and spectral purity by the introduction of new concepts of effective Gaussian line and purity profiles. The resolving power defined on the basis of the effective Gaussian profile is consistent with the spectral purity of the beam emerging from the exit slit and is more realistic as compared with those defined in the conventional manner, especially when spectral images have asymmetric profiles. It is concluded that holographic plane gratings recorded with a spherical and an aspheric wave front are capable of providing high resolution with high spectral purity and are fully interchangeable with the corresponding ruled VLS plane gratings. This interchangeability provides more flexibility for users in choosing a proper grating for a high-resolution grazing incidence monochromator of the Monk-Gillieson type.     

Efficient holographic recording in novel azo-containing polymer

We report on periodic structures inscribed through holographic recording by both intensity light patterns and polarization patterns in novel azo-containing polymer poly[oxyethylene({6-[4-(biphenyl-4-azo)phenoxy]hexyl}imino) ethylene-carbonylimino(2-methyl-1,3-phenylene)iminocarbonyl] abbreviated as A-TDI. The kinetic of grating recording and diffraction efficiency of recorded gratings using polarized cw Ar⁺ laser light have been measured for s–s, p–p and s–p polarization configurations. The translation grating technique applied during the light self-diffraction process allowed for investigations of phase and amplitude gratings contributions to light diffraction in the studied polymer. Full reversibility of polarization (s–p) grating recording and observation of a half-period structure development during optical erasure process makes this material a good candidate for further studies as a potential material for dynamic holography purposes.     

Multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals: static and dynamic studies

The optimization of the experimental parameters of two multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals is investigated. Two methods are used to record the holograms: simultaneous and sequential multiplexing. These two processes are optimized to produce two multiplexed Bragg gratings that have the same and the highest possible diffraction efficiencies in the first order. The two methods show similar results when suitable recording parameters are used. The parameters of the recorded gratings (mainly the refractive-index modulation) are retrieved by use of an extension of the rigorous coupled-wave theory to multiplexed gratings. Finally, the response of the holograms to an electric field is studied. We demonstrate few coupling effects between the behavior of both gratings, and we expect a possibility of switching from one grating to the other.     

First-generation holographic, grazing-incidence gratings for use in converging, extreme-ultraviolet light beams

We present two holographic recording solutions that produce gratings suitable for use at grazing incidence in the extreme ultraviolet. The rulings are formed when the interference pattern of two spherical wave fronts is recorded on a planar substrate. Each grating is designed to minimize or eliminate the dominant aberration terms in order to maximize the spectral and spatial resolution of the system. In the first design, the dominant astigmatism term in a power-series expansion of the light path function is eliminated; in the second design, the dominant comatic terms are minimized. Each grating is placed directly in a converging light beam at grazing incidence to provide high system efficiency in the extreme ultraviolet. The aberration control afforded by both recording solutions is excellent, providing detector-limited spatial and spectral resolution over much of the usable bandpass. Furthermore, the aberration control is maintained over a wide range of beam speeds and off-axis angles, thereby outperforming conventional varied line-space gratings for use in the extreme ultraviolet. We discuss the methodology used to develop the recording solutions, model and compare the performance of the gratings, and discuss possible space-based applications for these gratings.     

Modeling and design of planar slanted volume holographic gratings for wavelength-division-multiplexing applications

Holographic gratings are modeled and designed for path-reversed substrate-guided-wave wavelength-division demultiplexing (WDDM) as a continuation of earlier research [Appl. Opt. 38, 3046 (1999)]. An efficient and practical method is developed to simulate the slanted volume holographic gratings. The trade-off between dispersion and the bandwidth of the holograms is analyzed. A 60 degrees (incident angle of the grating)/60 degrees (diffraction angle of the grating in air) grating structure is selected to demultiplex optical signals in the 1555-nm spectral region, and a 45 degrees /45 degrees grating structure is chosen for the spectral region near 800 nm. Experimental results are consistent with the simulation results for these two WDDM devices. A four-channel WDDM is also demonstrated at a center wavelength of 1555 nm and with a channel spacing of 2 nm.     

Varied line spacing plane holographic grating recorded by using uniform line spacing plane gratings

Uniform line spacing plane gratings are introduced into a recording system to generate aspherical wavefronts for recording varied line spacing plane holographic gratings. Analytical expressions of groove parameters are derived to the fourth order. A ray-tracing validation algorithm is provided based on Fermat's principle and a local search method. The recording parameters are optimized to record a varied line spacing plane holographic grating with the aid of derived analytical expressions. A design example demonstrates the exactness of the analytical expressions and the superiority of recording optics with auxiliary gratings.     

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

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.