Irina V. Semenova

Very thick holographic nonspatial filtering of laser beams

Philip Hemmer, MEMBER SPIE Rome Laboratory RL/EROP, 63 Scott Road Hanscom Air Force Base, Massachusetts 01731 Abstract. A novel device, the nonspatial filter, is described for laser beam cleanup. It is based on the Bragg selectivity of thick holograms. Unlike pinhole and fiber spatial filters, which employ lenses and apertures in the transform plane, nonspatial filters operate directly on the laser beam. This eliminates the need for laser beam focusing, which is the source of many of the material and alignment instabilities and laser power limitations of spatial filters. Standard holographic materials are not suitable for this application because differential shrinkage during processing limits the maximum Bragg angle selectivity attainable, and because they are generally too thin. New technologies that eliminate the problem of differential shrinkage are described. These technologies are based either on the use of a rigid porous substrate material, such as porous glass, filled with a light-sensitive material, such as holographic photopolymers or dichromated gelatin, or on the use of a thick photopolymer with diffusion amplification (PDA). We report results of holographic nonspatial filtering of a laser beam in one dimension, with an angular selectivity of better than 1 mrad.

The optimization of a holographic system for solar power generation

Abstract A holographic device has been developed that greatly improves the efficiency of solar energy conversion. The single-element hologram focuses light, spectrally splits it and diverts unwanted infrared heat away from the solar cells. The output appears as a thin concentrated line, focused perpendicular to the hologram and displaced to the side. Solar cells are placed along this line such that each cell absorbs only the wavelengths which it can efficiently convert to electric power. The theoretical and experimental development of this system are discussed, as well as its application in space and on Earth. The system is excellent for space applications since the holograms are single element, very lightweight, and require minimal cooling. For terrestrial purposes, the projected costs of the system are nearly a factor of two lower per kWh than other solar concentrator systems; thus it is competitive with conventional power generation systems. Other state of the art holographic solar power generation systems are also discussed.

Holographic nonspatial filter

The present paper deals with new results ont he development of a holographic nonspatial filter to be used for laser beam clean up. An analysis of thick holographic materials suitable for recording of such elements is carried out. The experimental setups for hologram recording and evaluation are described. The results on measurements of angular selectivity contour of such holographic filters are presented.

Porous-matrix holography for nonspatial filtering of lasers

A novel technique is described for laser beam cleanup, the nonspatial filter, which is based on the Bragg selectivity of thick holograms. Unlike pinhole and fiber spatial filters, which employ lenses and apertures in the transform plane, nonspatial filters operate directly on the laser beam. This eliminates the need for laser beam focusing, which is the source of many of the alignment instabilities and laser power limitations of spatial filters. Standard holographic materials are not suitable for this application because differential shrinkage during processing limits the maximum Bragg angle selectivity attainable. This paper describes a new technology which eliminates the problem of differential shrinkage. This technology is based on the use of a rigid porous substrate material, such as porous gas, filled with a light sensitive material, such as holographic photopolymers or dichromated gelatin. We report preliminary results of holographic nonspatial filtering of a laser beam in one dimension, with an angular selectivity of less than 1 milliradian.

Rigid polymer materials with hologram enhancement by molecular diffusion

The principle of diffusional enhancement has been embodied in the rigid glassy polymer with phenanthrenequinone able to photochemically attach to surrounding macromolecules, thus forming a permanent grating. Owing to material stiffness, it does not suffer from shrinkage and can be made very thick; serving a basis for very stable spectrally selective elements. Replacement of commonly used acrylic glass by polycarbonate ensures further significant improvement of performance and stability of 3D holographic optical elements and memories.

Hologram development by diffusion in a polymer glass

Thick rigid polymer media with diffusive development of gratings are suitable for archive information storage, and especially for 3D holographic optical elements. Diffusion of unreacted molecules of photosensitive dye ensures postexposure growth of diffracted light, which can be followed either by secondary growth, or by some decay caused by displacement of chromophore groups photochemically attached to polymer chains (photoproduct). In a long run, extremely slow, though still finite, diffusion of macromolecules leads to destruction of a holographic grating. Not only the rate, but also the shape of postexposure kinetics noticeably depends on the choice of particular polymer, its degree of polymerization, temperature of processing and thermal history of material: in aged samples, gratings appear more efficient.

Holographic optical elements with high spectral and angular selectivity

It is well known that very thick volume holograms both transmission and reflective type being illuminated by light indicate very high angular (less than milliradian) and spectral (less than nanometer) selectivity. Such highly selective holograms can find wide application in different fields. The variety of diffraction elements: filters, angular and spectral selectors, etc. can be designed basing on very thick holograms. The main problem in manufacturing of thick holograms (of about millimeter thickness) is in developing of very thick photosensitive media, in which this grating can be recorded without shrinkage and distortions. The overview of Russian materials suitable for the recording of thick volume holograms is made in this paper. We demonstrate the experimental results of the recording of high efficient (90%) transmission angular selector (with the selectivity bandwidth of about milliradian) and reflection spectral selector (bandwidth of about nanometer) using PDA material (photopolymer with diffusive amplification). Such very high selective optical elements can find application in spectroscopy, space communication, lidar sensing, wavelength demultiplexing, optical storage, and others.

Narrowband holographic spectral filters: principles, manufacturing, and applications

In this paper we present an analysis of hologram parameters required to obtain an extremely narrowband holographic spectral filters, operating in reflection configuration. The spectral selectivity of such gratings is calculated as a function of the layer thickness and the recording geometry. The experimental results on the recording of spectral filters in photopolymer with diffusive amplification are presented. The current and potential applications are discussed.

Very selective volume holograms for spatial and spectral filtering

We discuss the application of narrowband holographic filters for aerosense technique including laser radars, communication devices, etc. It is well known that very thick volume holograms both transmission and reflective type being illuminated by light indicate very high angular and spectral selectivity. The variety of sensor require such selection. We report the experimental result of recording of selectors with nanometric bandwidth of spectral selectivity and milliradian bandwidth of angular selectivity recorded in photopolymer with diffusive amplification of about millimeter thickness. We investigated the affect caused by introduction of holographic filter into receiving signal: the influence of filter selectivity property on the spatial frequency distribution of light passed through the atmosphere. We analyzed the possibility to vary the shape of the output power spectral density by means of hologram apodization.

Two-dimensional holographic nonspatial filters

Holographic nonspatial filters designed to clean up the output of a laser have been shown to be a great improvement over conventional spatial filters. This paper successfully addresses several major problems or shortcomings of the nonspatial filter. The problems were in the use of two filters together for cleaning up a laser beam in both dimensions. Polarization and orientation effects made the system complicated an inconvenient. A simple compound element consisting of a sandwich of two identical holograms is shown to solve these problems.