Dichromated Gelatin Films Research Articles

Bandwidth in Dichromated Gelatin Holographic Filters

Refractive volume gratings in dichromated gelatin film have been studied. The swelling of the emulsion during developing leads to displacement of the interference fringes from the positions observed for the exposure set-up and the fringes are smoothly curved rather than irregular. The effects of the displacement are studied in exposed and developed holograms by examining the reconstruction angle and spectral bandwidth when using white light reconstruction. The influence of construction geometry and absorption on spectral bandwidth is illustrated with experimental results.

Real-time holography with undeveloped dichromated gelatin films

Holographic characteristics of undeveloped dichromated gelatin films, working in real-time configurations, are described on the basis of experimental data. Applications in double-exposure holography, character recognition, edge enhancement, image subtraction, and phase conjugation are presented.

Waveguide holograms in thin dichromated gelatin films

Thin dichromated gelatin films were used to record waveguide holograms directly inside the film. Information was stored and retrieved by light of the same wavelength. The waveguides were characterized by optical attenuation < 1 dB/cm and this value was retained after the film sensitization, exposure, and development cycle, so that holograms could be recorded. Sinusoidal gratings were formed and these were used to couple radiation into thin-film waveguides and also as waveguide Fourier holograms of two-dimensional images. The efficiency of a sinusoidal grating as a coupling component was 60%. A Fourier hologram reconstructed by the waveguide method with a diffraction efficiency of 11% was characterized by a recorded data density of ~ 105 bit/ mm2 over an area of 1 mm2.

Multifocus dichromated gelatin hololens

A method of making dichromated gelatin holographic lenses (hololens) with many equal intensity foci and some applications of the lens are described. The method employs a specially designed contact screen in a coherent optical spatial filtering system with the resulting holographic lens recorded in a dichromated gelatin film. The principle of the design and a detailed discussion of the various space–bandwidth products of the optical system are discussed. For demonstration, a 25-focus holographic lens has been made. As examples of the application of the multifocus hololens, the experimental results of its usages in the reproduction of 25 replicas of an image and in a real-time Vander Lugt matched filter pattern recognition system are presented.

Efficient and flexible laser beam scanners constructed from volume holograms

We combine the scan flexibility of computer-generated holographic laser beam scanners with the high diffraction efficiency of volume phase holograms to produce a new type of “holo-scanner”. The scanner-hologram consists of numerous, small, volume hologram facets recorded in dichromated gelatin films and produces an arbitrary output pattern with greater than 90% light efficiency. Experimental results from discrete point, continuous line, and 3-D scanners are shown. Multi-wavelength scanners are also considered. This paper is based on the concepts presented in the preceding paper on “computer-interferometric holograms used for laser beam scanners”.

In-Line Holographic Lenses of High Numerical Aperture

In-line holographic lenses of numerical aperture of 0.4 and 0.55 were successfully fabricated for the purpose of the application to an optical disk player with a He-Ne laser. They were recorded at 6471 Å in the methylene-blue sensitized dichromated gelatin film of 15 µm in thickness. The special design of the optical recording system and the careful processing of the recording materials were required. The evaluations of the modulation transfer function, the focused beam profile, the average diffraction efficiency and the stability test were performed. Bragg and Raman Nath diffractions occurred in the hologram were also investigated to explain the observed results.

Laser Beam Scanners Constructed From Volume Holograms

We combine the scan flexibility of computer-generated holographic laser beam scanners with the high diffraction efficiency of volume phase holograms to produce a new type of holo-scanner. The scanner-hologram consists of numerous, small, volume hologram facets recorded in dichromated gelatin films and produces an arbitrary output pattern with greater than 90% light efficiency. Experimental results for discrete point, continuous line, and 3-D scanners are shown. Multiwavelength and materials processing scanners are also considered.

Holographic total-internal-reflection selector for tunable lasers

A new holographic selector, operating on the basis of total internal reflection, is described. An analysis shows that this selector has a higher angular dispersion than the devices available at present and this is combined with lower losses inside a laser resonator. A method is given for the calculation of the parameters of a selector operating in the range of wavelengths (0.2–1.2μ) emitted from tunable lasers. This selector uses holographic volume phase gratings recorded in dichromated gelatin films whose optical strength is 2–4 J/cm2 at λ = 1.06μ for pulses of τp = 10 nsec duration. Frequency selection and wideband tuning are reported for a neodymium-glass laser. The width of the spectrum does not exceed 0.7cm–1 in the spiky regime. Single-frequency emission, tunable in a range exceeding 200cm–1, can be obtained in the Q -switched regime (switching by a passive shutter).

Index modulation and spatial harmonic generation in dichromated gelatin films

A model for volume hologram formation in thick dichromated gelatin films is given. Using the model and a few basic film measurements, we can quantify the index modulation vs. exposure curve. The effects of saturation and dynamic absorption increase during exposure are considered in the generation of spatial harmonics. The effects of exposure with non-unity beam-ratio and multiple incoherent exposures are predicted and experimentally verified. The thick transmission grating diffraction efficiencies resulting from first-and higher-order index modulations are calculated and compared to experiments. The methods and conclusions are applicable to other phase materials.

Efficient and Aberration-Free Wavefront Reconstruction from Holograms Illuminated at Wavelengths Differing from the Forming Wavelength

Recording materials producing higher efficiency, low absorption holograms are usually photosensitive only to blue and uv light. If a hologram is formed at a blue wavelength in such a material and is illuminated at a red wavelength, generally the reconstructed wavefront is aberrated, and the resolution of the image is reduced. A method of hologram formation is described that can alleviate this problem. In this method, a hologram H(1) is formed first at the red wavelength lambda(1) in a photographic emulsion. This hologram is then illuminated at the blue wavelength lambda(2) The diffracted wave from H(1) is used as the subject wave for forming the desired high efficiency hologram H(2). If certain requirements are met, illumination of H(2) at lambda(1) can produce an aberration-free reconstruction of the original subject wave. Experimental results of forming H(2) of a point source on dichromated gelatin film are presented. The hologram was formed at lambda(2) = 4800 A but could reconstruct an aberration-free wavefront at lambda(1) = 6328 A. The hologram had about 80% diffraction efficiency and essentially no absorption loss.

Preparation of Dichromated Gelatin Films for Holography

Preparation of Dichromated Gelatin Films for Holography

Hologram Formation in Hardened Dichromated Gelatin Films

Hardened gelatin films sensitized with ammonium dichromate can be utilized to record high quality holograms. The maximum diffraction efficiency of the hologram approaches 90%. The light scattering from the hologram is so low that under ordinary light the hologram plate appears almost indistinguishable from a clear glass plate. Either a transmission or a reflection hologram can be recorded. Linear recording range of light amplitude is large. A practical method of preparing and processing the film is described, and the exposure characteristics are presented.