Effect of polarization on efficiency of volume Bragg grating filter

<正>Diffraction efficiency of volume Bragg grating,whose period is in the same order as the incident wavelength, is related to the polarization direction of the incident linear polarized beam.When two linearly polarized recording beams with the same polarization direction are used for recording volume Bragg gratings in a photopolymer with diffusion amplification,the azimuth of polarization of the reconstruction beam influences the diffraction efficiency of the grating.When the probe beam is linearly polarized and oriented orthogonally to the grating vectors,the±1-order diffraction beams are also linearly polarized with polarization direction parallel to that of the probe beam.According to the results,a two-dimensional nonspatial optical filter consisting of the volume Bragg gratings would achieve significantly higher efficiency.     

Characteristics of fourier phase holograms recorded on photopolymers

The results of the investigation of recording Fourier phase holograms on a self-developing photopolymer photosensitive in the range λ=400–515 nm are presented. It has been found that, due to the transient energy transfer between the beams, noise gratings are recorded, and a corresponding sharp reduction in the signal-to-noise ratio occurs, while the diffraction efficiency of the hologram as a whole remains relatively high (above 50%). It has been found that the noise-grating recording can be substantially suppressed by increasing the intensity of the reference beam relative to the intensity of the object beam. In this way, the signal-to-noise ratio has been considerably improved for Fourier holograms of binary phase masks: at a reference to object beam intensity ratio R=26, Fourier phase holograms are recorded with a diffraction efficiency η=15% and signal-to-noise ratio N=20 dB.     

Holographic recording on the basis of forbidden singlet-triplet electronic transitions

The possibility of recording holograms in thick layers of a photopolymeric material on the basis of a forbidden singlet-triplet transition of a sensitizing dye, erythrosine, is theoretically substantiated and experimentally confirmed. A single hologram and 16 superimposed holograms are recorded with a low-power He-Ne laser (632 nm) at high concentrations of the sensitizing dye and high optical densities in the range of its main absorption. The diffraction efficiency of the single hologram is ～50%. The dependence of the increase in the diffraction efficiency of a transmission grating in samples of photopolymeric material on the intensity of incident radiation of a Kr⁺ laser (647 nm) is studied. The observed linear dependence of the maximal rate of increase in the diffraction efficiency in the photopolymer on the incident light intensity is explained by the manifestation of a two-step excitation of the dye (T ₁←S ₀, T ₂←T ₁) in the samples studied.     

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.     

Hologram multiplexing in acrylamide hydrophilic photopolymers

Unslanted diffraction gratings are recorded in a 900 μm thick acrylamide photopolymer by means of peristrophic multiplexing. A solid state Nd:YAG (λ = 532 nm) laser is used as the recording beam, with a total incident intensity of 5 mW/cm², and a He-Ne laser as the reconstruction beam. The dye concentration in the photopolymer is optimized so that it does not limit the dynamic range. Nine holograms are recorded using constant exposure time scheduling and variable exposure time scheduling. From the results obtained it may be deduced that optimization of the dye allows us to work in the linear response region of the photopolymer and at the same time obtain high values of diffraction efficiency for each hologram. An exponential increase in exposure time as the number of holograms increases enables the values of diffraction efficiency to be homogenized with regard to the case of constant exposure scheduling. In this way, better use is made of the dynamic range of acrylamide hydrophilic photopolymer.     

Reconstruction of parasitic holograms to characterize photorefractive materials

Photoinduced light scattering is a serious drawback that limits the applicability of thick holographic recording media but provides valuable information on the recording medium. As long as there is no correlation between the scattering centers in the crystal, photoinduced light scattering may be explained to result from the interference pattern of the incident beam and the field scattered from a single point-like scattering center. The hologram of this ellipsoidally scattered wave field will have practically the same structure in the reciprocal space modified by a response function which reflects the anisotropic properties of the recording medium. We studied photoinduced light scattering in LiNbO₃:Fe, a model system for photorefractive materials. The transmitted intensity in the stationary state of the scattering process is investigated as a function of the reconstruction angle at different wavelengths and polarizations of the reconstructing beam. The experimental results are analyzed by a simple phenomenological model based on the Ewald construction and can be used to choose suitable conditions at which holographic scattering can be minimized as well as to extract some physical parameters of the crystal. Received: 27 September 2000 / Revised version: 1 December 2000 / Published online: 21 February 2001     

Hologram recording in glass and direct reconstruction using visible-wavelength laser beam

We propose a method of hologram recording in a glass plate by corona charging. The holographic recording materials used in this study are conventional soda-lime glass and azobenzene polymer film. A Fourier transform hologram on an azobenzene polymer film coated on the glass plate is recorded in the glass plate by corona charging. After removing the polymer film, the hologram recorded in the plate can be reconstructed using a visible-wavelength laser beam. The first-order diffraction efficiency of the hologram at a wavelength of 532 nm is 0.03%; the efficiency depends on the depth of the surface relief structure on the azobenzene polymer film and the corona-charging time. The hologram recorded in the glass plate has high environmental resistance.     

The effect of nonlinearity of the recording media response on the properties of thick phase holograms

The effect of nonlinearity of a photosensitive medium response on the diffraction properties of thick phase holograms is analyzed on the basis of coupled-wave equations. It is shown that, for a typical dependence of the refractive index on the exposure, the response nonlinearity lowers the diffraction efficiency of a hologram illuminated by an s-polarized light. For the p polarization of the reading beam, the response nonlinearity reduces the hologram efficiency when the angle between the beams diffracted into the zero and second orders (into the minus first and first orders) is smaller than 90°, increases the efficiency for angles larger than 90°, and hardly affects the diffraction efficiency when this angle is close to 90°. The results for p polarization are obtained, to the best of our knowledge, for the first time. The analysis of angular selectivity of the gratings has shown that the effect of nonlinearity becomes particularly important for light waves incident on the hologram at angles close to the Bragg one. It is also found that, for any thickness of the photosensitive medium, there exists an optimum value of the maximum change in the refractive index, for which the diffraction efficiency may be close to 100% in a wide range of exposures. In this case, the nonlinearity for s polarization stabilizes, to some extent, the dependence of the diffraction efficiency on the exposure.     

Optimization of exposure time in linear hologram recording by pre-exposure or post-exposure

Using a given intensity of the laser beam, the optimum hologram recording time is discussed when the recording is performed on a pre-exposed or post-exposed medium, retaining the reconstruction efficiency the same as that in the corresponding conventional recording. It is assumed that a variable ratio, lossless beam splitter is used for the beam division process.     

Polarization properties in two wave mixing with moving grating in photorefractive BSO crystals. Application to dynamic interferometry

The polarization properties of the volume gratings in photorefractive B₁₂SiO₂₀ crystals permit to increase the contrast and signal to noise ratio in the image plane of a large size vibrating object structure. The experiment is based on the nearly degenerate two wave mixing of a pump beam with a low intensity signal beam due to the object structure (moving grating recording with an externally applied field in the 110 crystallographic direction). For a correct orientation of the incident recording beam polarizations, the pump beam emerging from the crystal is linearly polarized, while the signal, after two beam coupling is elliptically polarized. The selective attenuation of the direct pump and scattered beams is obtained with a polarizer. Improved contrasts in the interferogram plane of the vibrating object are demonstrated.     

局域体全息光栅的衍射特性

基于三维耦合波理论，研究了两束有限宽度的任意偏振平面波干涉产生的局域体全息光栅的衍射问题。以单位均匀振幅的任意偏振平面波为例，给出了透射波和衍射波振幅的主分量和交叉分量的解析表达式，讨论了记录过程和再现过程中入射的参考光波的偏振角对透射波和衍射波振幅的影响。计算结果表明，记录过程中的偏振角越小，形成的光栅内的耦合越强，衍射效率越高，但光束质量越差；再现过程中不同的偏振角，由于入射波和衍射波的电矢量的不同方向的耦合强弱不同，透射波和衍射波振幅的主分量和交叉分量的振幅变化行为不同。     

Permanent holographic recording in indium oxide thin films using 193 nm excimer laser radiation

Permanent holographic recording in sputtered indium oxide (InO_x) thin films is demonstrated, using ultraviolet radiation at 193 nm emitted by an ArF excimer laser. Steady-state refractive index changes of up to 5×10^-3 are calculated from the measured diffraction efficiency of a HeNe laser probe beam. The recorded gratings exhibit a dynamic behaviour that relaxes to a steady-state value that depends on the oxygen partial pressure used during growth and on the recording beam intensity. The observed behaviour is explained in terms of laser-induced structural changes. Received: 12 October 1998 / Accepted: 8 March 1999 / Published online: 4 August 1999     

Influence of the divergence of a light beam on the characteristics of collinear diffraction

Two types of anisotropic collinear diffractions of light on a diffraction grating are theoretically investigated, including that where the light is scattered forward in the direction of the incident radiation and that where the light is scattered backward in the direction toward the incident radiation. For both types, two-dimensional transfer functions are calculated, and the character of their transformation upon variation of the light wavelength and the period of the diffraction grating is analyzed. The dependence of the integrated diffraction efficiency and transmission band of diffraction filters on the divergence angle of the light beam is studied.     

Polarization recording and reconstruction in Disperse Red 1 -doped cellulose acetate

Cellulose acetate films doped with the azo dye Disperse Red 1 were prepared by dip coating. This matrix provides holograms of comparable efficiency for gratings recorded by intensity and polarization modulations. We used this matrix to store and successfully reconstruct the polarization state of an elliptically polarized object beam. A single plane-polarized reference beam was used for both recording and reconstruction.     

Influence of two-dimensional bragg diffraction on the competition of the waveguide TE<Subscript>0</Subscript> and TM<Subscript>0</Subscript> modes during formation of spontaneous gratings in AgCl-Ag thin films

An intricate space-time instability of patterns of small-angle scattering and diffraction of a laser beam on the structures appearing in AgCl-Ag films under exposure to the same beam is investigated. The instability is related to the formation of spontaneous gratings in the film resulting from the interference of the incident beam with the waveguide modes scattered in the film. The existence of a two-dimensional Bragg diffraction on the TE₀ and TM₀ modes with subsequent formation of secondary spontaneous gratings is revealed and is theoretically substantiated. It is established that the difference in the indicatrices of the radiation scattered into the TE₀ and TM₀ modes for the s-and p-polarizations manifests itself in different kinds of diffraction and small-angle scattering patterns at the output of illuminated samples.     

Effect of intensity quantization level in parallel phase-shifting digital holography

Parallel phase-shifting digital holography (PPSDH) enables the instantaneous recording of three-dimensional fields. The three-dimensional field can be reconstructed using a computer by numerical scalar wave propagation. In PPSDH, we record a space-division multiplexed hologram that includes the required phase retardation in the spatial distribution of the reference wave and then interpolate the data at blanked pixels for each phase retardation to obtain the complex amplitude distribution of an object wave. The recorded quality of the multiplexed hologram influences the reconstruction quality. In this study, we investigate the effect of the intensity quantization of a multiplexed hologram on the reconstruction quality. We compare the influence of intensity quantization in PPSDH with that in the conventional phase-shifting method. Random noise is also added to the multiplexed hologram. The required intensity quantization level is helpful for selecting a digital image sensor.     

Periodische Schwankungen der Röntgen-Reflexionsintensität von optischen Liniengittern mit der Einstrahlrichtung

Ruled gratings consisting of interrupted metallic films on glass show a periodical change of X-ray reflection power with the glancing angle of the incident beam. This oscillation is interpreted as an effect of interference between the radiation scattered in different depth of the grating lines, a kind of ”?transverse structure factor”?.     

Liquid crystal polarization rotator: Optical element for intermodulation spatial-bandwith reduction in holography

A liquid crystal polarization rotator (LCPR) to reduce the intermodulation spatial-bandwidth in offset-reference holography is proposed. The LCPR is divided into two portions which rotate the direction of polarization of the incident light beam differently. The rotation angle is set so that the direction of polarization of the transmitted light beams make an angle 90° with each other, so that object beams through the two portions do not interfere with each other. This rotator reduces the spatial frequency range of hologram intermodulation noise to 1/2 that of the conventional hologram recording method.     

Spontaneous periodic structures induced in thin AgCl-Ag films by the continuum in the case of optical rotation of the plane of polarization

The formation of spontaneous periodic structures in thin AgCl-Ag films subject to the continuum (λ≈450–700 nm) in the case of rotation of the plane of polarization with the help of crystalline quartz is studied. The periodic structures are formed due to the excitation of waveguide TE₀ modes in spontaneous Rayleigh scattering of radiation with a wavelength λ incident on the film. The structure periods are smaller than λ and vary in proportion with λ. After special treatment of the irradiated sample (fixing and deposition of an Al layer), periodic structures form a thin reflection hologram capable of reproducing the direction of linear polarization, the angle of optical rotation, and the color of the spectrum recorded. The study of diffraction patterns and small-angle scattering from small portions of a hologram, which were exposed to light with various wavelengths in the recording process, with the help of a probing UV beam (λ_p=337 nm), made it possible to reveal special features of the manifestation of Rayleigh and Wood anomalies associated with the domain structure of PS.     

Dynamic Light Diffraction by Holographic Gratings in Photopolymeric Materials

A theoretical model of light diffraction by tilted holographic gratings in photopolymeric materials in the nonstationary regime of reading, which describes the dynamic process of optical hologram amplification with allowance for the photopolymerization and diffusion mechanisms of recording, is developed. For the given model, the dynamics of the diffraction efficiency and selective properties of hologram reading are modeled numerically when Bragg's conditions are satisfied and violated. It is demonstrated that the diffraction characteristics of tilted holograms are spatially noninvariant for the reading beam direction. The optimal conditions of reading of tilted holograms are established.