Efficiency of volume phase reflection holograms recorded in an attenuating medium

The effect of attenuation when recording a volume phase hologram has been treated by a number of authors who neglect the effect of the varying average permittivity. It is shown here that such a variation, and the consequent violation of the Bragg conditions, leads to lower efficiency, the effect being more detrimental for greater attenuation and greater beam ratio. The mathematical solution is obtained with the aid of a generalisation of the coupled wave approach from which a set of numerical results is derived.     

The diffraction efficiency of hologram grating recorded in an absorptive medium

The profile of the hologram grating is described considerating the absorption of the recording medium and its diffraction efficiency is calculated for special cases using the coupled wave theory. The maximum diffraction efficiency obtainable is not affected by the absorption of the recording medium in the case of transmission holograms, while it decreases with increasing absorption in the case of absorptive reflection holograms.     

Shifting selectivity of collinear volume holographic storage

A paraxial equation for calculating pixel-based 2-D shifting selectivity of collinear holographic storage system is proposed. The equation shows that the broader reference pattern makes high shifting selectivity. In addition, both amplitude and phase modulation make no obvious change in shifting selectivity so shifting selectivity is not affected through advanced design of reference pattern. Besides, a simplified formula based on a point signal is proposed, which shows a clear physical insight that the shifting selectivity is the square of Fourier transform of the intensity distribution of reference pattern and does not depend on thickness of the disc when a point signal is applied.     

Use of holographic interferometer to investigate local deformations of adaptive mirrors

A method is described for using and monitoring local deformations of adaptive mirrors by means of a holographic interferometer. Interference patterns of the mirror surface are obtained and permit qualitative and quantitative analysis of the deformations.Translated from Voprosy Metrologicheskogo Obespecheniya Izmereniya Parametrov Tekhnologicheskikh Lazerov, pp. 46–48, 1984.     

Total internal reflection holographic gratings recorded in polymer-dispersed liquid crystals

We report diffraction gratings recording in polymer dispersed liquid crystals (PDLC) material with a beam suffering total internal reflection from the sample air surface. The real time diffraction efficiency kinetics as well as the polarization, voltage behavior and angular selectivity of the gratings are studied at different reconstruction geometries. An increase of three times of the diffraction efficiency in case of applied voltage is observed for one of these geometries in opposite to typical PDLC switchable gratings. Such dependence appears in most of the geometries and is connected to the specific formation of two gratings by the Stetson scheme.     

Calculation of shear angles in holographic gratings recorded in bleached silver-halide emulsions

Shearing phenomena in holographic gratings formed in bleached silver-halide emulsions due to chemical processing are evaluated. These effects are introduced because of the inability of emulsion-thickness changes to explain the anomalous detuning of the peak diffraction efficiency from the expected Bragg condition. Experimental results obtained with transmission and reflection gratings are presented and discussed considering shearing effects, and the values of the shear angles calculated from the Bragg-angle deviation measurements are also provided. In order to separate the shearing phenomenon and the shrinkage of the emulsion layer, a rehalogenating bleach without fixing is used because this bleach bath introduces a minimal change in the thickness of the emulsion. The influences of exposure, spatial frequency and slant angle on shear angles are briefly analyzed and discussed.     

Polarization holographic gratings with high diffraction efficiency recorded in azopolymer PAZO

Azopolymers are one of the most efficient materials able to record the polarization state of light. They have numerous applications, such as data storage and diffractive optical elements with unique polarization properties. An essential parameter for each diffractive element is its diffraction efficiency η. In order to optimize the recording conditions and obtain high-efficient polarization holographic gratings, in the present work we study the dependence of the diffraction efficiency on the recording angle and thickness of a series of azopolymer layers. Three recording angles are used ??10°, 20° and 30° and three series of thin films with thicknesses 470, 850 and 2400 nm from the water-soluble azopolymer PAZO. The gratings are inscribed by two plain waves with left and right circular polarization from a He-Cd gas laser (442 nm). The diffraction efficiency of the gratings is probed with a right hand circularly polarized beam from a probe laser with wavelength 635 nm. The kinetics of diffraction efficiency η(t) in the +?1 diffraction order are presented and compared. Our experimental results indicate that highest diffraction efficiency (more than 40%) is obtained for the sample with thickness 2400 nm and for recording angle 10°. As the holographic recording in azopolymers is usually accompanied by formation of surface relief gratings, the surface topography of the recorded samples is also investigated by atomic force microscopy.     

Bifurcation in reflection spectra of holographic diffraction grating recorded on dicromated pullulan

Volume diffraction gratings were created holographically on a dichromated pullulan (polysaccharide doped with chromium). Specific and very rare multi-peak structure in the reflection spectra is observed. It was found that multiple peaks are consequence of non-ideal structure of volume diffraction grating. Theoretical analysis and numerical simulations confirm that the observed behavior arise from a slight randomness of grating parameters: refractive index, grating period, film thickness. The results of calculation are in agreement with experimental results.     

Experimental study of multiplexed holographic gratings recorded in a photopolymerizable silica glass

Recently the capability of photopolymerizable sol–gel materials to angular multiplex holographic gratings has been demonstrated [1, 2]. In this study, holographic transmission gratings were recorded using angular multiplexing in a silica glass doped photopolymer holographic recording material. The time interval between exposures and the angular selectivity were analyzed, in particular their effects on the dynamic range. By using the optimal parameters gratings of similar response were achieved with values of M/#=4.8, as well as good values of diffraction efficiency with high photosensitivity. PACS 42.40.Pa; 42.70.Ln     

Spectral selectivity of radiation-and tunnel-coupled waveguide structures

The spectral transmission profile of systems of radiation-and tunnel-coupled waveguides is studied. It is determined how a smooth variation of the thickness of the buffer layer in a structure consisting of a pair of tunnel-coupled waveguides with strongly varying parameters will affect the spectral properties of the system as a whole. A simple physical treatment of the operation of a number of integrated and fiber-optic devices is given, and the prospects for using the proposed structure of two tunnel-coupled waveguides to realize a microlaser with waveguide output is demonstrated. Zh. Tekh. Fiz. 67, 59–65 (February 1997)     

Spectral photosensitivity and holographic performance of europium-doped fluorophosphate glass

Spectral investigation of europium-doped flourophosphate glass shows photosensitivity in the range of 244 nm to 351 nm. Holographic investigations of the dependence of the achievable refractive-index changes indicate a strong impact of the doping concentration. The refractive-index changes increase with europium concentrations up to 1.0×10²⁰ Eu/cm³; for higher doping levels the effect becomes smaller. Based on these results, refractive-index changes in excess of 10^-3 for optimized material and holographic recording are predicted. About 23% of the initially recorded holographic grating decay in a double-exponential way with time constants of 7 h and 240 h, but the other 77% of the primary refractive-index changes are persistent. The spatial resolution of this material is better than 1 m. PACS 42.40.Lx; 42.40.Pa; 42.70.Ln     

Optical characteristics of a turbid medium between two mirrors

Using the one-dimensional Boltzmann equation, we examine the optical scattering properties of a turbid medium that is located between two mirrors with controllable reflectivity. We examine how the mirrors can be used to enhance the total transmission of an intensity modulated laser beam through this system. The analytical results show that, for certain modulation frequencies, the total transmission can be increased if the laser source is placed between the mirrors. This finding could improve diffusive imaging for those highly scattering media that are so thick that the laser light would not penetrate sufficiently deep in the absence of any mirrors.     

Design and performance of two-channel EUV multilayer mirrors with enhanced spectral selectivity

In this paper, we present a study on two-channel multilayer mirrors which can operate at two wavelengths in Extreme Ultraviolet (EUV) spectral range. We propose a new method to design two-channel EUV multilayer mirrors with enhanced spectral selectivity. The mirror structure is a stack of two periodic multilayers separated by a buffer layer. We have defined the main parameters which allow adjustment of the distance between different order Bragg’s peak and of wavelength positions of reflectivity minima. Two mirrors have been designed and deposited for solar EUV telescope applications by using this method. The first mirror reflects Fe IX–X line (17.1 nm) and Fe XVI (33.5 nm) lines with attenuation of the He II line (30.4 nm). The second mirror reflects Fe IX–X and He II lines with attenuation of Fe XV (28.4 nm) and Fe XVI lines. Measurements with synchrotron radiation source confirm that, in both cases, for these mirrors, we are able to adjust reflectivity maxima (Bragg peak position) and minima. Such multilayers offer new possibilities for compact design of multi-wavelength EUV telescopes and/or for high spectral selectivity.     

Spectral selectivity of nickel and chromium rough surfaces

The reflectance of metal surfaces with sinusoidal roughness of different periods and depths has been investigated experimentally. The results show clearly that the surface structure can be used to modify the optical properties of metal surfaces at different wavelengths. With a proper choice of groove depth to period ratio, nickel or chromium coatings on gratings have low reflectance in the short wavelength region, but achieve high reflectance in the infrared region. A solar absorptance as high as 93% has been obtained from such a surface. The surfaces are thought to be representative for randomly rough surfaces provided proper correlation length and height variation are chosen. Further, as the absorber is made of a single metal surface, it could be highly temperature resistant.     

Optimum holographic optical element lens recorded by visible laser beams for an infrared two-dimensional vertical-cavity surface-emitting laser array

We have designed an optimum holographic optical element (HOE) lens recorded by visible laser beams for an infrared two-dimensional vertical-cavity surface-emitting laser (VCSEL) array. The hologram computer-aided design tool that we have developed is used for the optimum design. The optimum HOE has both high light efficiency and a small amount of aberration. An Nd:YAG laser operated at 532 nm and an 8 x 8 VCSEL array operated at 850 nm are used for HOE recording and reconstruction, respectively. The designed lens is experimentally demonstrated, and the experimental results of the lens almost agree with the numerical results.     

Stability of optical laser resonators with mirrors of gaussian reflectivity profiles,which contain an active medium

The stability of laser resonators which contain a medium with a gain profile is investigated. Such resonators are unstable when the resonator mirrors are large (infinite aperture approximation), and the gain is lowest on the optical axis and increases with radial distance. It is shown that such resonators have well-defined regions of stability if the mirrors have gaussian reflectivity tapers.