Stress formation during the recording and bleaching of photopolymer holograms results in distortion of the internal diffractive structure and causes many undesirable effects. This is especially important in the case of holographic solar concentrators and similar holographic optical elements that are supposed to meet stringent specifications. We report a new digital holographic method to measure and study stress formation in photopolymers during hologram recording. A glass substrate, with one of its sides diffusely reflecting, is effectively applied to facilitate the recording of good digital holograms captured using a CMOS camera. A green-sensitive photopolymer is used for the study. Digital holograms recorded in successive time intervals of exposure were overlaid and numerically reconstructed. High-contrast stress fringes were generated, and the preliminary results are presented.
A cost-effective and reliable liquid crystal spatial light modulator (SLM) system is developed for light modulation and holographic recording of variable data in photopolymer material. It is made by reverse engineering a mobile phone with a liquid crystal display (LCD) of resolution 84×48 pixels. The SLM is based on a detached LCD panel with drivers, a personal computer system, an interface to provide data, and the software for uploading data. The SLM works well in the transmissive mode, and the transmitted laser beam is modulated according to the electronic data input to the panel from a personal computer. The SLM is implemented with good transmission efficiency and image contrast for holographic data recording and related experiments, and is observed as a very cost-effective option for light modulation.
High efficiency solar cells are emerging as a viable tool for tapping solar energy. Such cells need cool light
to be concentrated on them for high conversion efficiency. Conventional refractive and reflective light
concentrators, with sun tracking systems, are bulky and heat the solar cell due to single focus convergence
of heat and light radiation. HOEs are emerging as an active candidate for making new generation light
concentrators. In this paper we report the design, development and characterization of a multi beam
reflection HOE for light concentration. The proposed design of single focus, multiple exposed reflection
element with wavelength selectivity can be used to avoid sun following system.
Photonic crystals are emerging as new generation devices with lots of promising applications. Holographic
fabrication of photonic crystals has become choice of many due to its associated flexibility. The paper
reports about our efforts on holographic recording of various 2-D periodic structures in photoresist material
and conversion of these to metallic structures, through electroforming. A He-Cd laser (442 nm) was used to
record the basic diffractive structures by two beam interference technique. These types of structures can
exhibit specific bandgap behavior and endurance in high power laser applications.
Photopolymer based microholograms are gaining much importance in the field of security imaging, product
authentication and prevention of document forgery. Security holograms, mass produced through soft or hard embossing,
from electroformed metal master holograms are not amenable to store and retrieve variable data. On the other side, rapid
developments in optical and digital technologies result in large scale counterfeiting of conventional security holograms
and look-alike holograms of great exactitude is becoming a real threat to original manufacturers. In contrast to
conventional recording materials, photopolymer holograms do not need wet processing. They are amenable to
replication and, at the same time, can hold variable data. This, apart from security at various levels, facilitates machine
readability, automation, easy tracking and effective maintenance of inventory. This paper presents design, development
and performance evaluation of a photopolymer based holographic variable data storage system for security applications.
A liquid crystal spatial light modulator (SLM) is used to create a modulated optical data beam that varies from hologram
to hologram. Photopolymer films in tape form are applied for continuous recording of micro-holograms, synchronous
with the variable data content. This is a novel, but simple data storage system and can be used to give added security, in
conjunction with conventional holograms. Easy and on site verification by applying special reading devices and
dedicated software is the other charm of the proposed system. Moreover, for added protection, variable key based data
encryption can be applied effectively. System parameters like diffraction efficiency, recording speed, preprocessing
requirements etc. are analyzed and the response of the photopolymer material is also evaluated.
Holographic Optical Elements (HOEs) are gaining much importance and finding newer and better applications in areas of optical fiber communication and optical information processing systems. In contrast to conventional HOEs, optical communication and information systems require smaller and efficient elements of desired characteristics and transfer functions. Such Micro Holographic Optical Elements (MHOEs) can either be an HOE, recorded with two narrow beams of laser light or a segment cut from a larger HOE (SHOEs), and recorded in the conventional manner. In this study, micro holographic couplers, having specific focusing and diffraction characteristics were recorded in different holographic recording media such as silver halide and dichromated gelatin. Wavelength response of the elements was tested at 633 nm and 442 nm. Variation in diffraction efficiency/coupling factor, and insertion loss of the elements were studied. The paper reports in detail about the above results and related design considerations.