Controllable Fabry-Perot interferometer based on dynamic volume holograms

We present experimental results for a high-resolution interferometer based on dynamic volume holography in photorefractive materials. The proposed interferometer is able to detect small angular and phase deviations separately. It has no free spectral range limitations and is comparatively insensitive to light intensity fluctuations.     

Peculiarities of the angular selectivity of 3D holograms in the direction of nondispersive angular mismatch

Influence of the angular mismatch between the hologram and the reconstructing beam in the direction perpendicular to the plane of dispersion on the efficiency of diffraction was studied. It is demonstrated that angular detuning in this direction can lead to substantial increase of the intensity of the diffracted beam if the Bragg conditions are met. The results of calculation of the “anomalous” behavior of the diffraction efficiency are in good agreement with the experimental data for holograms recorded in the layers of a photopolymer.     

Transient volume holograms in polymethylmethacrylate

New transient volume holographic recording materials are described in this report. These consist of either coronene or 1,2;5,6-dibenzanthracene incorporated into polymethylmethacrylate. The largely amplitude, volume holograms formed as a result of triplet-state absorption have lifetimes of 6.4 and 0.9 s respectively and diffraction efficiencies close to the predicted values of 10⁻⁴. Production of permanent holograms has been prevented by careful control of the polymerization conditions to produce a matrix of sufficient rigidity to prevent reactions between the triplet states and the polymer. By using spectrally well separated write and read wavelengths, erasure of the holograms by the read beam is avoided.     

Model for the effects of material shrinkage on volume holograms

A model for analyzing the effects of material shrinkage on volume holograms is presented. This model is based on the fringe-plane rotation model used for describing the effects of plane-wave grating holograms that undergo shrinkage. A computer was used to exercise the model for a simple input object typical of those used in digital holographic memory applications and stored as a Fourier-transform hologram. The theoretical formulation of the model is presented as well as the results of the numerical analysis.     

Chromatic dispersion correction in planar waveguide using one-layer volume holograms based on three-step exposure

We propose a novel method to correct the chromatic dispersion in a planar waveguide with volume holograms fabricated by the three-step exposure technique. The 532 nm green laser is used to illuminate the holographic plate in three groups of different angles for achieving the desired holograms. When it is used in the planar waveguide, the chromatic dispersion of the original display can be corrected and an image with the real color can be obtained. The experiments are performed, and the results are in good agreement with the theory. It is believed that this technique is a good way to correct the chromatic problems in the display systems in the future.     

Polarization-selective volume holograms: general design

General design of polarization-selective volume holograms is discussed in detail. The required diffraction angles for these highly polarization-selective elements are calculated. An effective indexmodulation parameter is defined and used to calculate the required index-modulation value at any specified operating wavelength. Design examples of operating wavelengths at 780, 830, 1050, 1300, and 1550 nm are given. Highly polarization-selective substrate-mode grating pairs for 780-nm operation were fabricated to verify the idea. These elements are suitable for applications in optical switching networks and magneto-optic data storage systems.     

Effect of a random pattern through a multimode-fiber bundle on angular and spatial selectivity in volume holograms: experiments and theory

We demonstrate a multimode-fiber bundle reference scheme in a photorefractive volume hologram and discuss its angular/spatial selectivity both experimentally and theoretically. We measure the angular/spatial selectivity, changing distances between the photorefractive crystal and the output facet of the multimode-fiber bundle. The distance variation leads to a beam-size change in the reference beam, however, the object-beam size was fixed in all cases. The dependence of the angular/spatial selectivity for random-pattern (RP) referencing on a hologram dimension was examined and the characteristics compared with those derived from Gaussian referencing. Experimental results show that the RP referencing makes little contribution to the enhancement in angular selectivity, even though it leads to a considerable enhancement in spatial selectivity. The angular selectivity is mainly dependent on the dimension of a volume hologram, even with RP referencing. It is also noteworthy that the use of an RP beam can cause some high-frequency noise in the angular-multiplexing regime, which should be avoided in a hologram memory system. Nonetheless, the spatial selectivity of the RP referencing shows nearly no dependence on hologram dimension, and is mainly dependent on the transversal speckle size of the random-pattern beam. These characteristics can also be verified by the numerical results presented here, which are in good agreement with the experimental results.     

Holographic memory with localized recording

We experimentally demonstrate and characterize a memory module that features selective page erasure and readout persistence using the localized recording method in doubly doped LiNbO(3). Pages of information can be selectively erased without partially erasing the whole memory. Data pages can be written over erased pages multiple times. Information is read millions of times before refreshing is required. We quantify the optical quality of the holograms by measuring their signal-to-noise ratio for a memory size up to 100 holograms. A compact phase-conjugate readout architecture is also presented and experimentally demonstrated.     

Pixel-size-maintained image reconstruction of digital holograms on arbitrarily tilted planes by the angular spectrum method

We present an effective method for the pixel-size-maintained reconstruction of images on arbitrarily tilted planes in digital holography. The method is based on the plane wave expansion of the diffraction wave fields and the three-axis rotation of the wave vectors. The images on the tilted planes are reconstructed without loss of the frequency contents of the hologram and have the same pixel sizes. Our method shows good results in the extreme cases of large tilting angles and in the region closer than the paraxial case. The effectiveness of the method is demonstrated by both simulation and experiment.     

Computer-generated holograms of three-dimensional objects synthesized from their multiple angular viewpoints

Synthesizing computer-generated holograms (CGHs) of a general three-dimensional (3D) object is usually a heavy computational task. We propose and demonstrate a new algorithm for computing CGHs of 3D objects. In our scheme, many different angular projections of computer-designed 3D objects are numerically processed to yield a single two-dimensional complex matrix. This matrix is equivalent to the complex amplitude of a wave front on the rear focal plane of a spherical lens when the object is located near the front focal point and illuminated by a plane wave. Therefore the computed matrix can be used as a CGH after it is encoded to a real positive-valued transparency. When such CGH is illuminated by a plane wave, a 3D real image of the objects is constructed. The number of computer operations are equivalent to those of a two-dimensional Fourier CGH. Computer and optical constructions of 3D objects, both of which show the feasibility of the proposed approach, are described.     

Duplication of phase key for random-phase-encrypted volume holograms

Holographic memory encrypted by an optical random-phase key and decrypted by either the original phase key or a duplicate key is proposed and demonstrated. The duplicate key is made by recording the encryption wave front with angle multiplexing during writing of the hologram. The amount of three-dimensional shifting that is tolerable in the duplicate key is analyzed.     

Electric-field multiplexing of volume holograms in paraelectric crystals

Electric-field multiplexing (EFM) results from the tuning of the effective wavelength of the light beam inside a photorefractive crystal. This tuning results from the application of an external electric field to the crystal during holographic recording. We demonstrate the high Bragg selectivity of this multiplexing technique in paraelectric crystals and compare it with the selectivity obtained in the ferroelectric phase. The effects of the two major physical parameters of working in the paraelectric phase, the temperature and the external electric field applied during the writing stage, are investigated. Experimental results of the EFM of three image-bearing holograms recorded in reflection geometry are presented along with a qualitative analysis of the Bragg selectivity in paraelectric crystals.     

Content-addressable data storage by use of volume holograms

Data stored as volume holograms-optical interference patterns imprinted into a photosensitive storage material-can be accessed both by address and by content. An optical correlation-based search compares each input query against all stored records simultaneously, a massively parallel but inherently noisy analog process. With data encoding and signal postprocessing we demonstrate a holographic content-addressable data-storage system that searches digital data with high search fidelity.     

Recording volume holograms in dye-activated aqueous gelatin gel

Holographic characteristics of a thick photosensitive material based on a dye-activated aqueous gelatin gel have been studied. Holograms recorded using second harmonic radiation of YAG:Nd³⁺ laser with a pulse duration of 17 ns showed a diffraction efficiency of ～70% and an angular selectivity of ～20′.     

Thick-layer glycerin-containing bichromated gelatin for recording volume holograms

A thick-layer light-sensitive material consisting of glycerin-containing bichromated gelatin has been prepared and used to record real-time volume holograms with 0.44 μm helium-cadmium laser radiation. The thickness of the layers is 400–600 μm. The holographic sensitivity of the material is 1 J/cm². The highest diffraction efficiency of holograms recorded using a symmetric system with parallel beams is 32%. Pis’ma Zh. Tekh. Fiz. 23, 62–65 (April 12, 1997)     

On-axis computer-generated holograms based on centrosymmetric partitioning of the pixel

A novel, to our knowledge, configuration for the design and fabrication of zero-order computer-generated holograms in which each pixel is split into two centrosymmetric equal-sized regions is proposed and tested. In a manner similar to other approaches this configuration also permits the encoding and the reconstruction of a complex function that exhibits phase as well as amplitude variations by use of a phase-only filter. A detailed mathematical analysis is followed by evaluation of the error of the encoding approach, which is calculated and compared with the error exhibited by other approaches. Computer simulations as well as optical experiments demonstrate the capabilities of this novel configuration.     

Three-dimensional shifting selectivity of random phase encoding in volume holograms

We analyze and demonstrate the three-dimensional shifting selectivity of volume holograms based on random phase encoding with ground glass. Under weak coupling, the diffraction characteristic is caused by the phase difference between the reference and the reading light. We find that the shifting selectivity is different for different shifting directions, which include laterally horizontal, laterally vertical, and longitudinal directions. The shifting selectivity depends on the diameter of the region of illumination on the random phase plate, the thickness of the hologram, and the distance between them.     

Polarization-controlled multistage switch based on polarization-selective computer-generated holograms

We describe a polarization-controlled free-space optical multistage interconnection network based on polarization-selective computer-generated holograms: optical elements that are capable of imposing arbitrary, independent phase functions on horizontally and vertically polarized monochromatic light. We investigate the design of a novel nonblocking space-division photonic switch architecture. The multistage-switch architecture uses a fan-out stage, a single stage of 2 x 2 switching elements, and a fan-in stage. The architecture is compatible with several control strategies that use 1 x 2 and 2 x 2 polarization-controlled switches to route the input light beams. One application of the switch is in a passive optical network in which data is optically transmitted through the switch with a time-of-flight delay but without optical-to-electrical conversions at each stage. We have built and characterized a proof-of-principle 4 x 4 free-space switching network using three cascaded stages of arrayed birefringent computer-generated holographic elements. Data modulated at 20 MHz/channel were transmitted through the network to demonstrate transparent operation.     

Enhancement of the angular selectivity in encrypted holographic memory

Enhancement of the angular selectivity in a double random-phase encoded holographic memory by use of reference plane waves is presented. In the storage algorithm, the angular selectivity is improved with the effect of random-phase masks. We show that the angular selectivity achieved by this scheme is more sensitive than that for Bragg angle detuning in a 90 degrees geometry. Both theoretical and experimental results are presented and analyzed.