The versatility of Volume Holographic Optical Elements (vHOE) is high, especially because of their tunable angular and
spectral Bragg selectivity. Those unique lightweight, thin and flat optical elements are enabled by the new instant
developing photopolymer film Bayfol® HX technology, which allows to mass produce cost effective diffractive optics
due to its simplified and robust holographic recording process.
From a pure scientific point of view volume holography is well established. In practice though, commercially available
optical design software is not adapted to handle the specific characteristics of photopolymer diffractive optical elements
and their recording. To achieve high quality vHOE precision optics, the recording setup needs to accommodate several
aspects that will be covered in this paper. We report on means how to deal with photopolymer shrinkage and average
refractive index changes of the recording media. An important part in diffractive optics design is the compensation of
different conditions between the holographic recording setup and in a final product containing the vHOE. Usually
substrates might need to be changed (in material, in refractive index) as well the illumination sources are using
incoherent light having angular and spectral emission profiles with finite bandwidth.
Recently special in- and out-coupling vHOEs are becoming attractive e.g. in near eye displays and in compact lighting
devices. We will report on design considerations and adjustments to the recording condition for a specific in-coupling
vHOE and demonstrate the effects of pre-compensation on this example.
A hybrid system that allows a high quality (low distortion) imaging in strong off axis configuration (50°) is proposed and
analyzed in this paper. This imaging system is composed of a Bragg volume hologram (BVH) operating in catadioptric
conditions and a programmable transmission computer generated holographic optical element (PCG-HOE). The BVH is
used to produce a point by point virtual image in a sequential way by varying the angle of incidence of the reading wave.
The PCG-HOE provides sequential aberrations correction adapted to each point to improve the image quality. A method
to calculate the phase transfer function (PTF) to be implemented into the PCG-HOE to compensate for the aberrations of
each virtual image points is presented. By applying the proposed approach we first demonstrate that the compensation of
the aberrations is theoretically possible to a certain extent. In the last part of the paper, we discuss the constraining
experimental conditions which have to be met, as well as obstacles to be overcome in order to achieve the fabrication of
the BVH.
In digital holography, primary holographic fringes are recorded using a matricial CCD sensor. Because of the low spatial
resolution of currently available CCD arrays, the angle between the reference and object beams must be limited to a few
degrees. Namely, due to the digitization involved, the Shannon's criterion imposes that the Nyquist sampling frequency
be at least twice the highest signal frequency. This means that, in the case of the recording of an interference fringe
pattern by a CCD sensor, the inter-fringe distance must be larger than twice the pixel period. This in turn limits the angle
between the object and the reference beams. If this angle, in a practical holographic interferometry measuring setup,
cannot be limited to the required value, aliasing will occur in the reconstructed image. In this work, we demonstrate that
the low spatial frequency metrology data could nevertheless be efficiently extracted by careful choice of twofold, and
even threefold, undersampling of the object field. By combining the time-averaged recording with subtraction digital
holography method, we present results for a loudspeaker membrane interferometric study obtained under strong aliasing
conditions. High-contrast fringes, as a consequence of the vibration modes of the membrane, are obtained.
When using the interferometric techniques, the optical path changes induce the wavefront deformations that in turn cause
the appearance of fringes. One general characteristic of such an approach is the measurement sensitivity. The actual
sensitivity of a holographic interferometer is a function of, on one hand, the parameters of the measuring system (such as
the wavelength of the light used) and, on the other hand, the environmental conditions in which the measurements are
being made. The later depend predominately on statistical fluctuations inherent to the particular laboratory conditions. In
many applications the sensitivity is near the limits of the deformation detectability. In such cases, it is of vital interest to
increase the number of interferometric fringes thus improving the quality of the acquired data. In this paper, we give an
overview of the sensitivity increase through various experimental and numerical approaches. We also present a new
numerical iterative method in which every cycle doubles the number of interferometric fringes. The method has shown to
be especially useful in applications with sub-wavelength wavefront deformations.
Time-averaged holographic interferometry is a known technique frequently used for analyzing vibration properties of objects. The development of array photo-detectors allowing long integration times enabled the capture of time-averaged holograms. A new technique called 'subtraction digital holography' has been recently developed for suppressing the zero-order disturbance in off-axis digital holography. In this work, we combine the time-averaged principle with subtraction digital holography technique. Results for a torsional micro-electro-mechanical systems (MEMS) and an oscillating membrane demonstrate clear hologram reconstructions covered with high-contrast fringes that describe the vibration modes.
Until recently, display holography was usually associate with 3D imaging. After the appearance of color holography it has become possible, however, to record holographic images of 2D objects, such as, for example, oil paintings. The realistic-looking virtual image recorded in a Denisyuk reflection hologram is the most suitable for such reproductions. A holographic contact recording of a painting reproduces the painting with all its texture details preserved, such as brush strokes, the painter's signature, etc. This means that an exact copy of the painting can be made, which can then be displayed at art exhibitions, museums, etc., when the original is not available. If an expensive painting is concerned, possessing an exact copy of the painting may also be important for insurance purposes, in case the painting is stolen or damaged. The advantage of a color contact hologram is that the hologram reconstruction process can be relaxed, as there is no need of spatial coherence of the white light source used to illuminate the hologram. In addition, no depth distortions are introduced as a function of the light source's distance from the plate. Only the angel of illumination is of primary importance if good color reproduction is to be obtained. The paper discusses the rendition of color in hologram, which is extremely important in this case. The holographic reproduction process of an oil painting is also described, and the major advantages of holographic reproduction are discussed together with its limitations.
Quantitative measurements of shapes, displacements and deformations of opaque objects, as well as of refractive properties of transparent media, through spatial and temporal fringe patterns analysis is done by applying two basic techniques. The first one is the phase modification technique. The second one is the Fast Fourier Transform technique (FFT), assisted by some sort of heterodyning. FFT is also the choice for a single frame, not-modified, interferogram analysis. In this case however, because the lack of a spatial carrier, the sign of the phase cannot be determined. A solution to this problem is a technique that requires only two phase-shifted interferograms. In this paper we propose a holographic interferometry method based on wavelength multiplexing in which no spatial carrier neither second, phase shifted interferogram is required. Using the Lippmann-Denisyuk single beam holographic setup, an interferogram form a deformable object is recorded with multiple laser wavelengths in a panchromatic emulsion. A reconstruction of the 3D object in colors is observed, superimposed with a multicolor fringe pattern. The object phase is then computed from the multiple single-wavelength fringe patterns, allowing the measurement of the object deformation. This work is aiming at quantitative analysis of highly dynamic objects.
The recording of high-quality color holograms is the only way in which holography can be made truly useful and totally acceptable in the world of business and everyday life. This possibility has been now opened by the introduction of panchromatic, single-layer ultra-high- resolution silver-halide emulsions which make it possible to obtain high-quality, large-format color reflection holograms. The use of three laser wavelengths on a single-layer emulsion in the recording process (Lippmann holography) makes the holographic recording technique similar to the early Lippmann photography of the last century. That combination promotes not only good color rendition but, additionally, due to wavelength multiplexing, the image resolution is improved as compared to monochrome holographic recordings. This fact is important in holographic microscopy and endoscopy, where high-resolution color images are particularly important.
A comparison between Western commercial silver-halide emulsions and the Russian ultra-fine- grained emulsions for holography has been performed. Single-beam reflection holograms of the Denisyuk type were recorded using continuous-wave lasers. This recording scheme was selected since it represents a rather simple and stable holographic setup, but an extremely demanding one on the recording materials. Various processing schemes were applied to obtain high-quality holograms. Phillips' three-step processing technique, fixation-free rehalogenating bleaching, reversal bleaching, and solution-physical development were all investigated. In particular, diffraction efficiency, image resolution, scattering, and signal-to-noise ratio were studied as a function of emulsion and processing method. For comparison, some recordings were also made on other popular materials in common use in holography.
The processing procedures for surface relief holograms recorded on silver halide emulsions are presented. The most promising methods have been investigated and compared. The aim is to develop and optimize methods for obtaining relief structures on silver halide gelatin emulsions for holography at rather high spatial frequencies. Such a relief pattern can then be used for making a metallic matrix. The main advantage of using a silver halide material, instead of photoresist material, is the favorable sensitivity characteristics over the whole visible spectrum of silver materials.
New experimental set-ups are presented which allow multidirectional analysis of inhomogene - ous phase objects by high resolution holographic interferometry. The applicability of these arrangements is tested in a first step. After this check of the method different kinds of plasmas are investigated. In order to obtain more sensitivity the techniques of resonance heterodyne holographic interferometry are applied. Knowing the relations between refractive index in the vicinity of a resonance line and the corresponding number density of ground state atoms this number density can be determined. 1.
Investigations of transparent objects like stationary plasmas on the basis of hologra- phic interferometry are difficult because of the relatively small fringe shift caused by these phase objects. For tomographic reconstruction procedures the phase shifts derived from the interferograms have to be particularly accurate , otherwise the reconstruction of the refrac- tive index distribution of the inhomogeneous plasmas is not reliable. The highly resolved phase profiles can be obtained by the use of spatial heterodyning, providing an accuracy of 2n/20 in the determination of the phase. A test of the optical arrangement and the computer-tomographic reconstruction algorithms is performed. The method is applied to diagnostics of different plasmas, such as high frequency plasmas and dc-glow-discharges. A special feature of these investigations is the use of resonance interferometry, in order to determine spatial distributions of particle densities of certain atoms in distinct states.
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