Secondary grating formation by readout at Bragg-null incidence

We show that when a dynamic hologram is read out by illumination at the Bragg nulls of a previously recorded grating the diffracted beam inside the medium can result in the recording of two secondary gratings that alter the final selectivity curve. This is confirmed experimentally. This effect can cause cross talk in hologram multiplexing that is stronger than interpage cross talk when a small number of holograms with high diffraction efficiencies are multiplexed.     

Spherical beam volume holograms for spectroscopic applications: modeling and implementation

The spherical beam volume hologram, recorded by a plane wave and a spherical beam, is investigated for spectroscopic applications in detail. It is shown that both the diffracted and the transmitted beam can be used for spectroscopy when the hologram is read with a collimated beam. A new method is introduced and used for analysis of the spherical beam volume hologram that can be extended for analysis of arbitrary holograms. Experimental results are consistent with the theoretical study. It is shown that the spherical beam volume hologram can be used in a compact spectroscopic configuration when the transmitted beam is monitored. Also, on the basis of the properties of the spherical beam hologram, the response of a hologram recorded by a plane wave and an arbitrary pattern is predicted. The information can be used to optimize holographic spectrometer design.     

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.     

Two-channel self-focusing matched filter

Abstract

A two-channel self-focusing matched filter produced by a computer generated hologram is proposed in this paper. The filter function is synthesized for two different objects to be recognized. The composite filter function consists of two terms, one of which is formed by adding a negative quadratic phase factor into the conjugate Fourier spectrum of one object, and another, which is adding a positive quadratic phase factor into the Fourier spectrum of another object. The composite function is then encoded into a computer-generated hologram to fabricate a matched filter. When performing pattern recognition, the filter will produce two correlation signals of two objects in the first positive order and the first negative order of diffraction, respectively. The diffracted light can be self-focused in the plane at a designed distance behind the filter. The optical experiments have shown the capability of the filter for simultaneously recognizing two different objects.     

Experimental analysis in recording transmission and reflection holograms at the same time and location

Holographic recording media with a reflection layer are useful because they make it possible to maintain backward compatibility with CDs and DVDs, and a conventional servo system is easily attachable. The incident beam is fed back to the recording layer by the reflection layer, so there are four beam pairs to record the transmission and reflection holograms. We analyze the basic property of the transmission and reflection holograms and evaluate the problem when the transmission and reflection holograms are recorded at the same time. It is shown that the shrinkage in the photopolymer medium has a different effect on each hologram, so the readout image from the two holograms is misaligned. Those diffraction beams make the interference pattern, and the signal-to-noise ratio (SNR) of the output image decreased. Taking into account the difference in wavelength selectivity between the transmission and the reflection holograms, we propose a way to select one hologram to get the diffraction beam and eliminate the interference pattern using the tuning readout wavelength. By using this method, we can eliminate the diffraction beam from the reflection hologram and keep a high SNR.     

Anti-phase domain in GaP grown on Si by MBE

Anti-phase domain structures in GaP thin film grown on Si with (001) and (011) orientations by molecular beam epitaxy (MBE) have been studied using transmission electron microscopy. The structural integrety of APDs has been analyzed using a polarity of (111) diffracted discs in the convergent beam electron diffraction (CBED) pattern. The crystallographic reflections of APDs in the present non-centro symmetry crystal were then identified through an observation of the non-polarity in the diffraction intensity of ±111 discs of CBED projection. This result suggests that a CBED pattern may become a simple tool for the determination of a complex APD present in a spharelite structure.     

Secure holographic memory by double-random polarization encryption

A novel optical encryption based on polarization is proposed and applied to a holographic memory system. Original binary data are described as two orthogonal linear polarization states. These input polarization states can be modulated by use of two polarization-modulation masks located at the input and the Fourier planes. Each modulation mask can convert an input polarization state into a random polarization state. Once encrypted, the polarization state is recorded as a hologram. For the decryption, the hologram can generate a vector phase-conjugate beam. When the same polarization-modulation masks are used, the vector phase-conjugate readout can cancel the polarization modulation at each mask, and the original polarization state can be recovered. The encryption of the proposed method is evaluated numerically. We also present experimental results by demonstrating holographic recording in a bacteriorhodopsin film.     

Suppression of the zero-order diffracted beam from a pixelated spatial light modulator by phase compression

Phase compression is used to suppress the on-axis zero-order diffracted (ZOD) beam from a pixelated phase-only spatial light modulator (SLM) by a simple modification to the computer generated hologram (CGH) loaded onto the SLM. After CGH design, the phase of each SLM element is identically compressed by multiplying by a constant scale factor and rotated on the complex unit-circle to produce a cancellation beam that destructively interferes with the ZOD beam. Experiments achieved a factor of 3 reduction of the ZOD beam using two different liquid-crystal SLMs. Numerical simulation analyzed the reconstructed image quality and diffraction efficiency versus degree of phase compression and showed that phase compression resulted in little image degradation or power loss.     

Optical monitoring of the end point in thin film plasma etching

An in-process optical technique is described for accurately monitoring the end point in plasma etching processes. A grating pattern is lithographed somewhere in the film to be etched. The grating modulation decreases as the film is etched out and the process may be monitored by measuring the diffraction of a low power He-Ne laser beam aimed at the grating. The etching end point is accurately detected by the disappearance of all diffracted orders. The laser beam does not need to be directed at normal incidence and so any available plasma etching equipment is suitable. The detection is carried out with low cost photovoltaic detectors but simple visual inspection is satisfactory also.Comparative experimental results are presented.     

Holographic imaging and interferometry with non-Bragg diffraction orders on volume and surface-relief gratings in lithium niobate and photo-thermoplastic materials

The recording of holographic volume and surface-relief gratings in a photorefractive crystal using a photo-thermoplastic (PTP) holographic camera with an image-bearing signal beam leads to the appearance of two Bragg and two or more non-Bragg diffracted beams that show the transformed images in each beam (rotation and angular amplification of images). Using this real-time mode of interferometry, the hologram is retrieved with a deformed object beam, resulting in the appearance of fringes with a proper phase shift in each of four diffracted beams. This one-shot (one-exposure) phase-shifting interferometry results in clarification of the object wave-front information (for example, from surface deformation) and solution of the sign ambiguity problem. This procedure demonstrates that high-resolution holographic imaging of the PTP holographic camera static deformations in the order of ～0.1?mm can be revealed on the diffusion reflection surface. In addition, it was demonstrated that using the PTP materials could achieve holographic recording and imaging through phase aberration, with the image appearing in the non-Bragg diffraction order.     

Spectrograph based on a single diffractive element for real-time measurement of circular dichroism

In this study, a novel and simple diffractive spectrographic method for real-time measurements of circular dichroism (CD) is considered from a theoretical and experimental approach. A demonstrator prototype of the CD spectrograph has been developed and its performance has been compared with a commercial phase-modulation CD spectrometer. The main element of the device is a polarization holographic grating, recorded in a thin photosensitive organic film, by two interfering opposite circularly polarized beams. A peculiarity of this grating is that the amplitude of the +1 (-1) order of diffraction is proportional to the right (left) circular polarization component of the incoming beam. Here we demonstrate that the CD spectrum of a specimen can be easily evaluated from the intensities of the diffracted beams. A white light beam passing through the specimen is diffracted from the grating and the intensities of the +/-1 orders of diffraction are measured. Due to the spectral selectivity of the grating, the CD at each wavelength can be evaluated at the same time using two linear array detectors.     

Folding mirror schlieren diffraction interferometer

We demonstrate the use of a mirror as a viewing diaphragm to generate a built-in diffracted reference beam in schlieren diffraction interferometry (SDI). The use of a mirror edge as a diffracting element instead of a conventional knife edge considerably enhances the contrast of the schlieren pattern, and it is shown to be equal to that of a phase knife edge. This increase in contrast is due to the fact that the otherwise unutilized diffracted beam in SDI is recombined in the described folding mirror geometry.     

Modeling the grating-formation process in thick bacteriorhodopsin films

We model the grating-formation process in bacteriorhodopsin films for the interference of two plane waves. We simulate the temporal dependence of grating recording and readout, and we examine the behavior of the diffraction efficiency with respect to exposure, write and read wavelengths, and film parameters such as initial optical density and lifetime of the upper state. Gratings written in thick bacteriorhodopsin films are generally nonuniform and nonsinusoidal owing to the absorption and saturation properties of the material. The simulations also show that one can often obtain optimization of hologram recording and readout by writing and reading at wavelengths far off the peak of the ground-state absorbance spectrum, especially for films with high values of the peak optical density.     

Reflection volume holographic scanners with field-curvature corrections

A new type of polygonal holographic scanner that combines a reflection volume hologram with a computer-generated hologram (CGH) is described. The scanner is free from the aberration of field curvature. Such a scanning system can allow for a compact folded version of the scanner and Bragg diffraction into only a single order. The equations expressing the spatial-variable image distance are derived and are fit to the phase function designated by polynomials incorporated into a CGH in terms of the least-squares method. A reflection scanner with field-curvature correction is made by interfering a diffracted wave front from this CGH with a spherical wave front having scanning focal power through a second plane hologram. Experiments demonstrating the feasibility of this scanner are presented. Raster-scan patterns using a multifaceted scanner are shown. Helpful data on the diffraction efficiency and the spectrally diffracted intensity of reflection holograms are also presented.     

Off-Bragg analysis of the diffraction efficiency of transmission photorefractive holograms

Formulas for calculating the diffraction efficiency of gratings recorded in a photorefractive medium are given. The analysis uses coupled-wave theory for photorefractive hologram gratings and takes into account the photorefractive phase shift and fringe-bending effect. General solutions for diffracted (signal) and undiffracted (reference) waves are derived in a closed-form expression. By use of the derived formulas the diffraction efficiency for angle mismatch from the Bragg condition can easily be evaluated as compared with numerical-analysis methods. The diffraction efficiency is also quantified in terms of medium parameters and recording and reconstruction conditions.     

Fringe pattern of the field diffracted by axicons

The far-field intensity pattern of laser beams diffracted by axicons is extensively characterized both theoretically and experimentally. The regular structure of the pattern, consisting of high-contrast fringes, is explained. The experimental results have been interpreted by representing the diffracted field as generated by an extended virtual source shaped as a circle centered on the optical axis of the incident laser beam. The simulations include modifications to the diffraction pattern arising from the laser radiation diffraction limit at the axicon tip, and they reproduce well the measured intensity profile at different distances from the axicon.     

Spatial-phase code-division multiple-access system with multiplexed Fourier holography switching for reconfigurable optical interconnection

A new, to our knowledge, space-variant optical interconnection system based on a spatial-phase code-division multiple-access technique with multiplexed Fourier holography is described. In this technique a signal beam is spread over wide spatial frequencies by an M-sequence pseudorandom phase code. At a receiver side a selected signal beam is properly decoded, and at the same time its spatial pattern is shaped with a Fourier hologram, which is recorded by light that is encoded with the same M-sequence phase mask as the desired signal beam and by light whose spatial beam pattern is shaped to a signal routing pattern. Using the multiplexed holography, we can simultaneously route multisignal flows into individually specified receiver elements. The routing pattern can also be varied by means of switching the encoding phase code or replacing the hologram. We demonstrated a proof-of-principle experiment with a doubly multiplexed hologram that enables simultaneous routing of two signal beams. Using a numerical model, we showed that the proposed scheme can manage more than 250 routing patterns for one signal flow with one multiplexed hologram at a signal-to-noise ratio of ~5.     

Characterization and integration of polymeric and microdiffraction specimens using a two-dimensional position-sensitive detector

An automated x-ray diffraction system configured with a two-dimensional position sensitive detector (PSD) was used to characterize both polymeric and microdiffraction specimens. Pinhole collimation and an incident beam monochromator were used for the polymer applications, while dual nickel coated x-ray mirrors were used to focus and monochromatize the x-ray beam for microdiffraction experiments. Radial and azimuthal integration techniques automatically convert the two-dimensional PSD data into a traditional powder pattern for data manipulation.

The complete or partial Debye ring(s) of a diffraction pattern can be imaged simultaneously with the Siemens two-dimensional PSD. For polymeric specimens this allows data to be collected rapidly (in seconds) and in digital form eliminating cumbersome film techniques. The integration over a large section of the Debye ring(s) also enhances the diffracted beam detectability making this system sensitive for microdiffraction experiments.     

Encoding amplitude information onto phase-only filters

We report a new, to our knowledge, technique for encoding amplitude information onto a phase-only filter with a single liquid-crystal spatial light modulator. In our approach we spatially modulate the phase that is encoded onto the filter and, consequently, spatially modify the diffraction efficiency of the filter. Light that is not diffracted into the first order is sent into the zero order, effectively allowing for amplitude modulation of either the first-order or the zero-order diffracted light. This technique has several applications in both optical pattern recognition and image processing, including amplitude modulation and inverse filters. Experimental results are included for the new technique.     

Encoding amplitude and phase information onto a binary phase-only spatial light modulator

We report what to our knowledge is a new technique for encoding both amplitude and phase information onto a single binary-valued spatial light modulator. In our approach, we spatially modulate the diffraction efficiency of the filter. Light that is not diffracted into the first order is sent into the zero order, effectively allowing amplitude modulation of either the first-order or zero-order diffracted light. This technique has applications in both optical pattern recognition and image processing. Experimental results are included.