Abstract:
A method of focusing radiation with an array of lenses, which includes modifying the angles of propagation and convergence and/or divergence with transmission and reflection holographic optical elements, and coupling the radiation into an optical fiber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/476,388, filed on Apr. 18, 2011, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to collecting broadband optical radiation and injecting it into an optical fiber or waveguide, from the side of said fiber or waveguide, and more particularly, wherein the injection relates to concentrating solar radiation into the fiber or waveguide for its use as a solar concentrator. 
       BACKGROUND 
       [0003]    Photovoltaic solar cells can be improved through the use of concentrators, which collect sunlight in a large area and present it in a much smaller area. This can both increase the efficiency of the solar cells and reduce the required area of the cells. 
         [0004]    The simplest solar concentrator is a focusing lens or mirror, which increases the light intensity on the solar cell. Traditional optics limits the distance from the focusing lens or mirror to the focal spot to be no less than approximately the characteristic dimension of the focusing element (for example, the diameter of a lens with circular aperture). Large concentration, then, results in a large concentrator using traditional optics. 
         [0005]    Accordingly, there is a need for optical concentrators that are thinner (smaller in the direction perpendicular to the concentrator surface) and lighter than permitted by traditional optics. 
       SUMMARY 
       [0006]    In accordance with a preferred implementation, the system comprises: an array of lenses (or lenslets) focusing solar radiation to a point; a broadband transmission holographic optical element to partially collimate and/or redirect the radiation; a broadband reflection holographic optical element to partially collimate and or redirect the radiation; and an optical fiber to transmit the radiation away from the lens array. 
         [0007]    In accordance with an exemplary embodiment, the broadband holographic optical element is a transmission holographic optical element and/or a reflection holographic optical element. In addition, multiple layers or a plurality of holographic optical elements can be used. In accordance with an embodiment, the holographic optical elements operate at different wavelengths. 
         [0008]    In accordance with another exemplary embodiment, the optical fiber is a plurality of optical fibers. The optical fiber is composed of a core, a cladding, and an optional transparent sheath. Alternatively, the optical fiber is composed of bare core. In accordance with an exemplary embodiment, the optical fiber is an optical waveguide. The optical fiber is preferably a graded-index fiber or a step-index fiber. The optical fibers are preferably bundled at the output. In addition, the optical fibers can be embedded in a medium. For example, the medium in which the fibers are embedded can be used as a cooling system. 
         [0009]    In accordance with an exemplary embodiment, the array of lenses is monolithic. Alternatively, the array of lenses consists of independent lenses. The array of lenses can have spherical surfaces, aspherical surfaces, and/or can be flat. In accordance with an exemplary embodiment, the lenses are Fresnel lenses. In accordance with another exemplary embodiment, the array of lenses is holographic or diffractive optical elements. 
         [0010]    Because the lens array consists of a plurality of individual lenses with short focal length, the distance to maximum concentration is much shorter than when an individual focusing lens or mirror of the same size as the lens array is used. The invention, then, is much thinner than a solar concentrator using traditional optics and having the same level of concentration. 
         [0011]    In order for the focused radiation to be captured by the optical fiber, the set of rays representing this radiation must fall within the angle defined by the limiting numerical aperture of the fiber: 
         [0000]      NA=n core sin θ,
 
         [0012]    where n core  is the refractive index of the fiber core and θ is half the maximum cone angle of the rays. For a fiber whose cladding refractive index is n clad , the limiting numerical aperture is 
         [0000]        NA   max =√{square root over ( n   core   2   −n   clad   2 )}.
 
         [0013]    Note that the holographic optical elements are assumed to have significant spectral bandwidth. 
         [0014]    In accordance with another exemplary embodiment, a method of focusing radiation with an array of lenses, comprises: modifying the angles of propagation and convergence/divergence with transmission and reflection holographic optical elements; and coupling it into an optical fiber. 
         [0015]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
           [0017]      FIG. 1  shows a side view of the preferred implementation. 
           [0018]      FIG. 2  shows a front view of the preferred implementation. 
           [0019]      FIG. 3  shows the general operation of the invention. 
           [0020]      FIG. 4  shows operation of the invention using only the reflection holographic optical element. 
           [0021]      FIG. 5  shows operation of the invention using only the transmission holographic optical element. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0023]      FIG. 1  shows a side view of the preferred implementation of a system for collecting broadband optical radiation and injecting it into an optical fiber or waveguide. Layer  101  is an optical lens array, layer  102  is a broadband transmission holographic optical element, layer  103  is an optical fiber, and layer  104  is a broadband reflection holographic optical element. As shown in  FIG. 1 , the optical fiber  103  may extend outward (i.e., a long way out) from the lens array. 
         [0024]      FIG. 2  shows a front view of the preferred implementation. The same four layers are identified as in  FIG. 1 : layer  201  is the optical lens array,  202  is the transmission holographic optical element,  203  is the optical fiber (drawn in this Figure as including just a core and cladding), and  204  is the reflection holographic optical element. 
         [0025]      FIG. 3  shows the general operation of the invention. Solar radiation  301  is focused by each individual lens (only one is shown in use) within the lens array  302 . This radiation is adjusted by the transmission holographic optical element  303 , passes through the optical fiber cladding  304  and core  305 , and is adjusted again by the reflection holographic optical element  306 . The radiation is then conditioned such that it enters the core of the optical fiber  305  completely within the limiting numerical aperture of the fiber. The radiation then transmits through the fiber to leave the end  307 . 
         [0026]      FIG. 4  shows operation of the invention using only the reflection holographic optical element. Solar radiation  401  is focused by each individual lens (only one is shown in use) within the lens array  402 . This radiation passes through the optical fiber cladding  403  and core  404 , and is adjusted by the reflection holographic optical element  405 . The radiation is then conditioned such that it enters the core of the optical fiber  404  completely within the limiting numerical aperture of the fiber. It then transmits through the fiber to leave the end  406 . 
         [0027]      FIG. 5  shows operation of the invention using only the transmission holographic optical element. Solar radiation  501  is focused by each individual lens (only one is shown in use) within the lens array  502 . This radiation is adjusted by the transmission holographic optical element  503 , and then passes through the optical fiber cladding  504  and core  505 . The radiation is then conditioned such that it enters the core of the optical fiber  505  completely within the limiting numerical aperture of the fiber. It then transmits through the fiber to leave the end  506 , which may be part of a fiber bundle. 
         [0028]    In the ideal implementation, the invention is embodied in four sections: (1) a preformed, monolithic lenslet array to focus and concentrate the incoming solar radiation; (2) a broadband transmission holographic optical element, which partially recollimates and redirects the focused radiation; (3) a broadband reflective optical element, which also partially recollimates and redirects the focused radiation (which may have been recollimated and redirected by the transmission holographic optical element); and (4) a number of optical fibers, which take this recollimated and redirected radiation and deliver it to the location at which it is needed. 
         [0029]    As an example of how this could work in an exemplary implementation, the lenslet array could be aimed towards the sun. An array covering an area of 1 m 2  on the surface of the Earth would then receive approximately 1,000 W of solar radiation (this is described in optical terms as insolation of 1,000 W/m 2 ). The array of lenslets can have average spacing of 5 mm, in which case there would be 40,000 such lenslets in an array  200  lenslets on a side. If the optical fibers are laid out as shown in  FIG. 2 , with one fiber per row of lenslets, the 1,000 W collected by the lenslet array could be coupled into 200 optical fibers. If the core of each fiber is 0.25 mm diameter, the total area of the fiber output is then about 10 mm 2 . The concentration factor of this invention, then, could be as large as 10 5 , or 100,000 times. In that case, the output irradiance would be 10 8  W/m 2 , or in more commonly used units  10  kW/cm 2 ; this is generally far below the optical damage threshold of the fibers. It can be appreciated, that in accordance with an exemplary embodiment, the losses in this paragraph are ignored, which results in extraordinarily optimistic values. 
         [0030]    The method of operation of this invention is shown in  FIG. 3 . Solar radiation is focused by the lens array. These lenses may be individual or monolithically formed with a single substrate. Since each lens is much smaller than the total array, the focal length can also be small (for example, with lenslets spaced at 5 mm, the focal length can be 5 mm, compared to the 1,400 mm focal length of a 1 m square lens). 
         [0031]    Without further optics, the radiation would be brought to a focus, then diverge at the same rate as it had converged. For the low focal ratios (focal length divided by lens diameter) which are expected to be used, the full angle of the cone containing the rays being focused is around 90° (the angle of rays is ±45° with respect to the optical centerline. There are two important points about this: (1) the numerical aperture (in air) of this propagating pattern is 0.71, well beyond the typical limiting numerical aperture of an optical fiber; and (2) the direction of propagation (the optical centerline) is perpendicular to the centerline of the optical fiber. Thus, the cone of radiation must be partially recollimated and partially redirected, so that it lies within the acceptance angle of the optical fiber. 
         [0032]    The holographic optical elements perform this task. As more fully set forth and described in the Claims, there may be one or two such elements, and wherein each element may be a single layer or multiple layers. A single element can achieve both recollimation and redirection. It can be appreciated that there are reasons, however, to use multiple elements and/or multiple layers as set forth herein. 
         [0033]    For example, one reason to use multiple elements is that holographic optical elements usually have a tradeoff between angular acceptance and wavelength acceptance. Thus, a broadband element would work better as a collimator or a beam director, rather than both. Alternatively, the transmission element can be a collimator with a small amount of beam direction, and the reflection element can be a beam director with only a small amount of recollimation. 
         [0034]    The second reason to use multiple elements or layers is the broadband nature of solar radiation. Since it can be difficult to make the holographic optical element work at high efficiency over a large bandwidth, in accordance with an embodiment, it may be necessary to have three or four layers of such elements, or have the transmission element work in one wavelength range and the reflection element work in another wavelength range. 
         [0035]    It will be understood that the foregoing description is of the preferred embodiments, and is, therefore, merely representative of the article and methods of manufacturing the same. It can be appreciated that many variations and modifications of the different embodiments in light of the above teachings will be readily apparent to those skilled in the art. Accordingly, the exemplary embodiments, as well as alternative embodiments, may be made without departing from the spirit and scope of the articles and methods as set forth in the attached claims.