Diffraction concentrator is an optical device, which provides angular inclination of optical rays in spacious by diffraction of them on the diffraction structure. The phenomenon of diffraction of radiation on a periodic spatial structure was also used earlier in spectral devices, in acousto-optic and electro-optical elements (Fig. 1).

      Holographic concentrator created on the basis of hologram is the most widespread type of diffraction concentrators. Under the hologram we will understand the diffraction structure registered on a certain registration material with the certain disposing of diffraction lines. In the publications by a hologram is named not only the three-dimensional record of wave front, that got through an object or was reflected back from an object, but also the different diffraction gratings and focusing zone plates, got both by interference method and by the computer synthesis (Fig. 2).

      Holographic concentrators can be classified in accordance with the following signs: in accordance with the form of wave front of the reconstructed beam; in accordance with the type of holograms; related to spectral composition of radiation; in accordance with the principle of interaction of optical radiation with a diffraction structure; in dependence with the type of photosensitive material and method of record of hologram; in accordance with the amount of holograms that are used for concentration of sun radiation and in accordance with the method of fabrication.

In accordance with the form of wave front of the reconstructed beam, which goes out from a hologram, the concentrators are divided into focusing and declining.

       Depending on the principle of interaction of optical radiation with a diffraction structure the holographic concentrators are divided onto the transmission and reflecting (Fig. 3).

       Relating to the type of holograms the optical concentrators are classified onto the axial and off-axis (Fig. 4).

       In accordance with the spectral composition of optical radiation, which is perceived by a holographic structure, the concentrators are classified into the monochromatic, ultraviolet, visible radiation, infra-red and multispectral or color (Fig. 5).

      Depending on the type of photosensitive material and method of record of hologram the concentrators are divided into the amplitude and phase elements.

        In accordance with the amount of holograms, which are located in one plane and used for concentration of sun radiation, the holographic concentrators are divided into single-element and multielement devices (Fig. 6).

       Depending on the method of fabrication the holographic concentrators are divided into interference and synthesized (geometrical) elements. Interference concentrators are created by photographing of objective and plane wave coherent beams on the photosensitive tape, synthesized – by cutting with the special equipment.

      To the basic parameters of holographic concentrators belong focal distance, optical diameter (optical aperture) and angular divergence. Diffraction efficiency is the important power parameter of holographic concentrators. It characterizes the efficiency of reflection or transmission of concentrators, and is determined as a relation of intensity of light beam that was diffracted in the first order to the intensity of light beam falling on concentrator.   

        As holograms of concentrators the linear regular diffraction gratings are used, along with the diffraction gratings with a variable step between diffraction lines, focusing Fresnel holograms and focusing aspheric (for example cylinder) holograms.

       Under the Fresnel hologram a diffraction structure is understood that consists of concentric transparent and opaque rings and provides the change of phase of wave front of light beam falling on this hologram. Depending on the structure of rings the focusing holograms are divided into the interference, geometrical (synthesized) and generalized, depending on the method of record – onto axial and off-axis. The generalized holograms are so-called Fourier transform holograms. The reference beam is added coherently to a Fraunhofer diffraction pattern of the object or formed by a lens.

        Axial focusing holograms are created by the record on photoregistrator material of interference pattern of spherical and flat wave fronts of coherent beams, here the optical axis of spherical beam is perpendicular to the photosensitive plate.   

        The off-axis holograms are made by the record of interference pattern created by the spherical wave front, inclined in relation to a photographic plate, and flat wave front perpendicular to the plane of photographic plate.

Synthesized «geometrical» holograms are used in sun concentrators that are working in the ultraviolet and infra-red ranges of optical radiation, for which high-quality photosensitive materials are absent. They are characterized by the less inclination angles and lower diffraction efficiency in comparison with interference holograms. Besides, the interference holograms are considerably cheaper.

       For the increase of diffraction efficiency of reflecting holograms the diffraction transparent or opaque elements are covered by thin metallic mirror film and that way multiply the intensity of the reflected light beam. During the process of development of holograms a photosensitive layer can be dissolved into a certain depth for creation of «brilliant» holograms, which demonstrate maximal diffraction efficiency at certain angle of incident radiation.  

       One of varieties of multispectral concentrators is a concentrator on the basis of three-color holograms formed by the successive record of the separate «colored» holographic components on the separate «colored» layer of the three-layered photosensitive structure. Each of the «colored» holograms is written by the radiation of certain wave-length. The removal of chromatic aberrations is achieved by using of the radiation of the same wavelength during processes of reconstruction and record. Other method of fabrication of the «colored» hologram foresees the use of technology of the interlaced holograms. In this case all three components of hologram are written consistently on one panchromatic tape. The use of multispectral concentrators, from one hand, diminishes the size of light spot in a focal plane, but, from other hand, lowers a general diffraction efficiency of concentrator.

       The multielement transmission holographic concentrators consist of a plenty of the small elementary focusing holograms located on the lengthened glass substrate, one near other. Each of holograms catches certain part of sun beam and focuses it on the photovoltaic receiver.

      As the photosensitive registrator the following materials are used: photoresists, haloids of silver, photochromic reversible materials, photopolymers, and photographic bechromic gelatin with the inclusions of silver and some other chemical elements.

       As the substrates the glass plates and polymeric optically transparent organic materials are used. 

       The main advantages of holographic concentrators of the second generation in comparison with ordinary refractive and mirror concentrators are small sizes (thickness), simplicity of production and cognation with integral technologies, which are used for production of semiconductor photovoltaic sun elements and, accordingly, low cost. An important advantage is also the possibility of the use of holograms in quality of spectral filters.

        Cognation with integral technologies and possibility of spectral and spatial filtrations permits to create the compact concentrating geterostructured photovoltaic sun modules, which connect in one construction the possibility of collection of large light beams, redistribution of these beams in accordance with the spectral characteristics and direction of them onto the separate compact photosensitive gratings. These gratings are built from the groups of elements, sensitized for perception of radiation of certain wave-length (energies).

       For wide introduction of holographic concentrators it is necessary to develop new effective technologies of removal of chromatic aberrations and to increase the diffraction efficiency of holographic elements, working in the wide spectrum of sun radiation.

Vasil Sidorov on May 18, 2010 from Technopark QUELTA

in Queltanews. sidorovvasil@gmail.com