Dichromated Gelatin Holograms Research Articles

Ultra light-trapping filters with broadband reflection holograms

Significant optical absorption enhancement can be achieved by incorporating optical diffusers in the thin-film silicon photovoltaic (PV) cells. Absorption can be increased further by angular and spectral selective filters. In this work the properties of volume reflection holograms are examined for realizing ultra light-trapping filters for thin film silicon photovoltaic cell applications. The filter properties of reflection volume hologram are evaluated for this application. It is found that variation in the refractive index profile as a function of depth is an important factor. The optimized design is implemented in dichromated gelatin holograms and found to be in good agreement with predicted performance. The enhancement to the conversion efficiency of silicon PV cells are predicted with the PC-1D simulation tool and is found to be similar to that with an optimized Rugate filter. The simulated short circuit current density enhancement was found to be 8.2% for a 50 µm thick silicon PV cell and 15.8% for a 10 µm thick silicon PV cell.

Holographic optical elements recorded in silver halide sensitized gelatin emulsions. Part 2. Reflection holographic optical elements.

Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOEs). The drawback of DCG is its low energetic sensitivity and limited spectral response. Silver halide materials can be processed in such away that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-fine-grain silver halide (AgHal) emulsions. In particular, high spatial-frequency fringes associated with HOEs of the reflection type are difficult to construct when SHSG processing methods are employed. Therefore an optimized processing technique for reflection HOEs recorded in the new AgHal materials is introduced. Diffraction efficiencies over 90% can be obtained repeatably for reflection diffraction gratings. Understanding the importance of a selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOEs, also including high-quality display holograms of the reflection type in both monochrome and full color.

Holographic optical elements recorded in silver halide sensitized gelatin emulsions Part I Transmission holographic optical elements

Silver halide sensitized gelatin (SHSG) holograms are similar to holograms recorded in dichromated gelatin (DCG), the main recording material for holographic optical elements (HOE's). The drawback of DCG is its low sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-high-resolution silver halide emulsions. An optimized processing technique for transmission HOE's recorded in these materials is introduced. Diffraction efficiencies over 90% can be obtained for transmissive diffraction gratings. Understanding the importance of the selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOE's.

Dichromated gelatin hologram using digital image processing technology to optimize processing

In this paper a high-efficiency diffraction hologram fabricated using digital image processing technology in dichromated gelatin (DCG) is described. A charge coupled device (CCD) camera and frame capture card were used to obtain the holographic grating processing parameters using hologram manufacturing process procedures. With this image system the transmittance of the DCG solution can be measured, the uniformity and thickness of the emulsion on the glass plate can be controlled, the hardness of the prepared gelatin layer analysed and the variations in the hologram diffraction efficiency monitored. The experimental results verified that this image system provides an economical and effective method for producing a DCG hologram.

A Study on the Effect of Different Gelatins on the Performance of Dichromated Gelatin Holograms

This paper studies the effect of several different gelatins on the performance of dichromated gelatin holograms at the same conditions. We find that one gelatin differs greatly from another in the maximum diffraction efficiency and in the incident angle for reconstruction at the maximum diffraction efficiency. The highest diffraction efficiencies of dichromated gelatin holograms of active gelatins at optimum exposure are over 80 percent and that off PA modified gelatin is between 60 and 80 percent, whilst those of inert gelatins are below 50 percent; for PA gelatin, the incident angle for reconstruction is much larger than those of active or inert gelatins. In addition, the tendency of change of the maximum diffraction efficiency and incident angle for reconstruction with exposure has some differences among different gelatins, too. The dichromated gelatin hologram of PA gelatin is not as clear as those of active and inert gelatins.

Review of Materials for Holographic Optics

The success of applications involving holographic optical elements depends on the performance of the recording materials used to form the elements. Selection criteria of a recording material must include not only the usual optical considerations such as achievable diffraction efficiency and optical quality, but also the environmental stability and the ease and cost of manufacture of the elements. Three materials are in widespread use and development for holographic optics applications: dichromated gelatin, photopolymer, and photoresist. Dichromated gelatin forms very high-quality holograms, but is relatively difficult to produce and must be protected from moisture. Dichromated gelatin holograms are in use as head-up display combiners, narrowband filters, and diffraction gratings. Photopolymer is generally easier to use, typically does not require wet processing, and usually has good environmental stability. Photopolymer holograms are in use or under development for several applications including laser eye protection filters, automotive lighting devices, and security holograms. Photoresist forms surface relief holograms that can be replicated by epoxy or, for large production runs, by embossing techniques. Photoresist holograms are used as diffraction gratings for scientific applications, as patterns for fabrication of photonic devices, and as master holograms for security applications such as credit card holograms.

Methylene-blue-sensitized dichromated gelatin holograms with antihumidity polymer coatings

An efficient method of resisting humidity for methylene-blue-sensitized dichromated gelatin (MBDCG) holograms is reported. The method uses a viscous liquid pure poly(styrene) solution with a low degree of polymerization as the coating material. On the basis of this coating, the diffraction efficiency of treated MBDCG holograms exposed to a high-humidity (relative humidity of 80-85%) environment was not obviously changed during observation over 3 mon.

Multiband wavelength-division demultiplexing with a cascaded substrate-mode grating structure

A multiband wavelength-division-demultiplexing (WDDM) structure, which incorporates cascaded substrate-mode holograms, is presented. The method can be used to design a WDDM device that consists of two or more layers of fundamental units (i.e., substrate-mode holograms). The fundamental unit is based on a diffracted grating and a substrate that include angular dispersion, wavelength bandwidth, and total internal reflection, which can be used to separate optical signals of different wavelengths. We have designed and built a multiband WDDM device, incorporating cascaded substrate-mode holograms in dichromated gelatin.

Comparison of methods for determining the bias index of a dichromated gelatin hologram

Determination of the bias refractive index of a holographic emulsion before exposure and after development is an important factor in the design of holographic optical elements. Several experimental methods are discussed for determining the bias index of a volume hologram in dichromated gelatin, and the results for each technique are presented. It is shown experimentally that these measurement methods yield different results for the same hologram, and the cause of the differences is proposed to be a variation of the bias index with depth in the hologram. An index measurement technique is also presented that accounts for variation in the bias index and is shown to yield an accurate value for the bias index.

Double-layer method for increasing the bandwidth of UV spectrally responsive holograms in dichromated gelatin

We present a method for increasing the bandwidth of reflection holograms in dichromated gelatin. The recording plate has two dichromated-gelatin layers of different hardness so that their shrinkage at the end of processing is different. We use this method for doubling the bandwidth of holographic mirrors that are designed for solar UV energy concentration. We also study the superposition of two double-layer holograms, which results in UV holographic mirrors with a bandwidth of 60 nm.

Dichromated gelatin holography: an investigation into laser-induced damage

An investigation into the occurrence of laser-induced damage in a sealed dichromated gelatin hologram is presented. Our research shows that damage appears when the laser beam heats the gelatin above a certain critical temperature. As a way to relate this to the optical power, a model predicting the temperature profile generated by a Gaussian beam is developed and used to calculate the optical damage threshold of a dichromated gelatin hologram.

Ultraviolet spectrally responsive holograms in dichromated gelatin

A method for making ultraviolet-responsive holograms in dichromated gelatin by using the 488-nm radiation from an argon laser is presented. We obtain the main shift toward short wavelengths by swelling the gelatin layer in a high-humidity atmosphere before exposure, and we adjust the final reconstruction wavelength (355 nm) by controlled heating of the holograms. These holograms, which exhibit the high performance of dichromated-gelatin holograms, are quite uniform compared with those made with other swelling agents. The swelling effects are discussed, and some procedures for improving efficiency are given.

Parallel optoelectronic interconnections with high packing density through a light-guiding plate using grating couplers and field lenses

Optoelectronic interconnections between boards at 1–100 cm distances are proposed, which are based on a typically some 5– 10 mm thick light-guiding plate for confinement and transport of the optical signals with low loss, on the order of 0.1 dB/cm. The plate is equipped with grating couplers fabricated as volume phase holograms in dichromated gelatin with insertion loss as low as 0.5 dB/coupler. The apertures of the optoelectronic transmitters are imaged onto the apertures of the receivers by aspheric field lenses or mirrors mounted on the plate. Such optical systems render packing densities as high as 1000 channel/cm 2 possible that are required for the application of densely packed integrated arrays of semiconductor lasers and detectors. Grating dispersion, axial astigmatism, geometrical distortion and coma are eliminated in the proposed optical setup. Results from ray tracing and an experiment are presented.

Visual cortex operations and their implementation using the incoherent optical neuron model

The incoherent optical neuron model uses two different device responses, an inhibitory response and a nonlinear output response, to realize a complete neuron unit that has both inhibitory and excitatory inputs. We describe its use to implement a model of simple cells of the visual cortex. Such simple cells perform the operations of edge detection, orientation selection, and in the case of moving objects, direction and speed selection. Experiments are described that utilize two Hughes liquid-crystal light valves to perform the functions of input transduction and optical neuron unit-array implementation. A multiplexed dichromated gelatin hologram serves as a holographic optical element that forms space-invariant (but otherwise arbitrary) point-spread functions for the network interconnections. Changing the holographic interconnection pattern permits implementation of different simple cells performing, e.g., transient response, edge detection, orientation preference, and direction and speed preference. Experimental results of these operations are presented.

Reflection display holograms on dichromated gelatin

Procedures for making dichromated gelatin holograms are described that reliably produce high-quality holograms of uniform brightness. The methods are unique and incorporate techniques that promote uniform development.

Diffractive beam splitter for laser Doppler velocimetry

A miniaturized sensor head for the optical measurement of velocities of fluids based on laser Doppler velocimetry is demonstrated. Holographic optical elements mounted on a glass substrate are used for beam splitting and deflection. Volume holograms in dichromated gelatin exhibit good optical efficiency (75% transmission of a cascade of two holographic optical elements). With diffractive devices one can achieve achromatic behavior that makes the sensor insensitive to wavelength drifts or mode hopping of a semiconductor laser.

Guided-wave planar optical interconnects using highly multiplexed polymer waveguide holograms

An intraplanar interconnection scheme using substrate guided modes in conjunction with a highly multiplexed waveguide volume hologram is proposed. Acoustooptically addressed 1-to-50 passive and 1-to-2-to-100 reconfigurable interconnections with a fan-out diffraction efficiency of 55% at 514-nm wavelength are demonstrated. A coordinate transformation converts the 3-D diffraction problem into a 2-D one, which significantly simplifies the theoretical calculation. Intraplane massive fan-out optical interconnection using substrate guided mode provides both collinear and coplanar fan-out capability for data and clock signals. Colinearity of signal distribution allows the 2-D input signal array to be processes. The laminated waveguide device containing a highly multiplexed dichromated gelatin (DCG) hologram has been evaluated. >

Broadband reflection holograms in dichromated gelatin

The spectral response of broadband reflection holograms in dichromated gelatin is studied; conditions for obtaining 100-nm bandwidth and 80% diffraction efficiency holograms are given. The structure of these holograms is discussed, and the scanning electron microscope is used to investigate fringe-spacing variations.

Holographic Optical Beam Splitters in Dichromated Gelatin

Abstract Holographic beam splitters can split an incoming beam into several partial beams travelling at different angles with arbitrary intensities, thus offering high design flexibility. In this paper the fabrication of beam splitters as multiplex holograms in dichromated gelatin is reported. Shrinking or swelling of the gelatin was avoided by careful process control, reciprocity law failure (as expected from silver halide holograms) was not observed, and, for example, a 1 : 2 beam splitter diffracting 47·4% and 47·0% of the incoming light into two higher orders was realized.

Hybrid kinoform fanout holograms in dichromated gelatin

The technique for fabricating hybrid holograms, introduced by Bartelt and Case [Appl. Opt. 21, 2886 (1982)], is applied here to record extremely high-efficiency (>90%) space-invariant fanout holograms in dichromated gelatin. The object wave front corresponding to that produced by a kinoform with a continuous-phase profile is derived by appropriate spatial filtering from a binary computer-generated hologram fabricated using electron-beam lithography.