Abstract:
Embodiments of the invention include a power beam receiver that will not reflect light beyond the regulatory limits for human exposure, except along paths known to be without people. In one embodiment, a baffle is used to trap reflections from surfaces of the receiver. In a second embodiment, the power beam receiver is arranged so that reflections are reflected to another surface of the receiver. These surfaces may be designed as a retroreflector. In a third embodiment, an intentional scattering medium is added to the power beam receiver so that parallel light rays incident on the front surface of the power beam receiver are scattered through a series of angles. As a result, any light escaping the system is diffused.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 60/866,807 filed Nov. 21, 2006, which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to the receive portion of a power beam system. More specifically, it relates to a power beam receiver that limits reflection of the incident radiation for increased safety. 
         [0004]    2. Description of the Related Art 
         [0005]    Prior power beaming systems are unsafe for use around people not wearing eye protection. A human proximate to a power beaming system can be hurt in two ways. First, a person can receive power directly from the transmitter—a person could look into the beam. The reader should assume that the incident beam path is protected from intrusion. In a power beam system where the beam path is not protected from intrusion, a power beam exceeding human exposure limits is unsafe. Second, a person can receive unsafe levels of light reflected from a surface in the path of the beam. That surface might be accidentally inserted in the beam path, or it might be part of the power beaming receiver. Even a power beaming receiver with anti-reflection coated surfaces is potentially a source of unsafe reflections because it is subject to contamination with water, oil, or other reflective material. Power beaming systems are not currently designed to limit reflections to be within regulatory limits for human exposure. For example, U.S. Pat. Nos. 5,982,139, 6,114,834, 6,792,259, and 7,068,991 all by inventor Ronald J. Parise, describe remote charging systems for vehicles and electronic devices, but do not treat reflections that will occur nor discuss methods of reducing reflections. 
         [0006]    The laser power beaming systems for the NASA aircraft experiment at Huntsville, Ala., and all entrants in the NASA space elevator competitions, as well as other systems described in patent filings, have a power conversion element perpendicular to the incident radiation.  FIG. 1A  illustrates an example of this arrangement. The power receiving element  10  is perpendicular to incident light  11 . This method is efficient, but it is generally unsafe. There is no control over where the reflections go. If the power conversion element is at even a small angle to the incident light, the light is likely to reflect in an unsafe direction. 
         [0007]    Free space optical telecommunication systems, such as those that were made by Terabeam, Inc. of San Jose, Calif., use a perpendicular conversion element. Because these systems are designed to be mounted up high, far from people, and because they can have a long baffle on the front of the receiver, it is very unlikely that any human will receive radiation beyond the regulatory limits, despite the use of a perpendicular power conversion element. Generally these systems use small photodiodes. To collect light onto them, they use large front lenses.  FIG. 1B  illustrates an example of this arrangement. Light  11  is focused onto power receiving element  10  by lens  90 . This approach might not be safe in a situation where people were nearby. Moreover, it requires sufficient depth to allow for the lens to concentrate the light on the photodiode. The angle between the incident light and the first surface (the lens) must be closely controlled, presumably perpendicular. 
       SUMMARY 
       [0008]    Embodiments of the invention include a power beam receiver that will not reflect light beyond the regulatory limits for human exposure, except along paths known to be without people. Even when the first surface that the power beam impinges on (the “front surface”) is contaminated with water, oil, or other reflective material, the power beam receiver will not reflect light such that a human exposure exceeds regulatory limits. 
         [0009]    In one embodiment, the power beam receiver is arranged so that any part of a power beam within an acceptance cone that is reflected from the front surface or secondary surfaces of the receiver is trapped by a baffle. 
         [0010]    In a second embodiment, the power beam receiver is arranged so that any part of a power beam incident from any angle within an acceptance cone that is reflected is reflected to another surface of the photoreceiver. These surfaces may be designed as a retroreflector. 
         [0011]    In a third embodiment, an intentional scattering medium is added to the power beam receiver so that parallel light rays incident on the front surface of the power beam receiver are scattered through a series of angles. As a result, any light escaping the system is diffused. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1A  shows a prior art a power conversion element perpendicular to the incident light, which is assumed to be collimated. 
           [0014]      FIG. 1B  shows a prior art a power conversion element behind a concentrating lens perpendicular to the incident light, which is assumed to be collimated. 
           [0015]      FIG. 2A  is an illustration of a power beam receiver where the power conversion elements are arranged to reflect incident light into a baffle, in accordance with one embodiment. 
           [0016]      FIG. 2B  is an illustration of a power beam receiver where an off-axis parabolic mirror is used to concentrate incident light on a power conversion element, and where reflections from the power conversion element are trapped by a baffle in accordance with one embodiment. 
           [0017]      FIG. 2C  is an illustration of the system in  FIG. 2B  where the power conversion element is angled in accordance with one embodiment. 
           [0018]      FIG. 2D  is an illustration of an off-axis parabolic mirror used in  FIG. 2B . 
           [0019]      FIG. 2E  is a perspective view of an off-axis parabolic mirror of  FIG. 2D . 
           [0020]      FIG. 2F  illustrates a plurality of parabolic mirrors mounted in an assembly, in accordance with one embodiment. 
           [0021]      FIG. 3A  is an illustration of a power beam receiver where the surfaces on which the incident light impinges are arranged to reflect incident light onto other surfaces of the receiver, in accordance with one embodiment. 
           [0022]      FIG. 3B  is an illustration of a power beam receiver where the surfaces on which the incident light impinges are arranged to reflect incident light onto other surfaces of the receiver, in accordance with one embodiment. 
           [0023]      FIG. 3C  is an illustration of a power beam receiver where the front surface comprises corner cube retroreflectors. 
           [0024]      FIG. 4A  is an illustration of a power beam receiver where an intentional dispersion medium is inserted to increase the angles of the incident light upon reflection, in accordance with one embodiment. 
           [0025]      FIG. 4B  is a view of the arrangement of  FIG. 4A  showing a border around the receiver. 
       
    
    
       [0026]    The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
       DETAILED DESCRIPTION 
       [0027]      FIG. 2A  is an illustration of one power conversion element  10  of a power beam receiver arranged to reflect incident light  11  into a baffle  20 , in accordance with one embodiment. In this embodiment, power receiving element  10  is tilted with respect to the incoming beam  11 . For illustration purposes, only a single power conversion element  10  with a single baffle is shown, but multiple power conversion elements arranged at the same or different angles with multiple baffles can be included in a power beam receiver, for example in a line or grid pattern. 
         [0028]    For many practical power beaming systems, power receiving element  10  will be one or more photodiodes. All light reflected  12  from its surface is trapped by a baffle  20 . Baffle  20  can be made of any material that overwhelmingly absorbs light at least at the wavelength at which the system operates. Example materials include black anodized aluminum or a rigid material covered in a light-absorptive cloth. In  FIG. 2A , there is no lens in front of power receiving element  10 . Alternatively, an angled and baffled optic can be placed there. In the arrangement shown in  FIG. 2A , if the front element were flat, slightly angled, or gently rounded, reflection from the surface might escape and cause a safety problem. Even if the surfaces were anti-reflection coated, a practical power beaming system is likely to be used in a situation where dust, water, grease, or other contamination causes reflection.  FIG. 2A  shows the angle of the tilt of the power receiving element  10  as 45 degrees, but other angles can also be used. The power receiving element  10  can be made in many sizes, and generally smaller is better because the smaller the power receiving element  10 , the shorter the baffle  20  and the thinner the receiver. The downside to this is that the thickness  22  of baffle  20  represents lost area coverage, and the greater the number of baffles  20 , the greater the lost area, and therefore the less efficient the system. The optimum sizes depend heavily on the requirements of the application. This embodiment is preferred when the light is from a known direction, preferably straight-on as illustrated by incident light  11  which is at 45 degrees to the power receiving element  10  shown in  FIG. 2A . If the power beam  11  enters at an angle, the baffles must be taller, and they begin to mask the power receiving elements  10 . The power receiving elements  10  will usually have surface coatings  40  (not shown in  FIG. 2A ), as described below. 
         [0029]    Although the arrangement of  FIG. 2A , as shown, requires approximately 1.4 times as much surface area for the same effective area of the power receiving element  10 , it is safe from reflection. The perpendicular method illustrated in  FIG. 1A  uses less material, and, if the beam is perpendicularly incident, the reflection from the surface will be back to the transmitter (ignoring diffraction), which is assumed to be a safe path, provided the incident angle is guaranteed to great precision. For example, assume the power beam is incident from 20 meters, so the total optical path will be 40 meters from the power beam transmitter to the receiver and back. Assume the power beam has a width of 100 mm. Assume the transmitter has a width of 250 mm (the extra width might be for any reason, including to baffle the reflections from the power beam receiver). However, if the angle exceeds 0.001875 rad (0.10743 degrees), the reflection will not be baffled by the transmitter. This 0.001875 rad tolerance includes tolerance for diffraction, for the non-ideal characteristics of the lens train, and for the mechanical tolerances related to manufacturing spread, thermal creep, lash, and operation tolerances. Even assuming one could account for all these variables, the transmitter still must be designed not to re-reflect the retroreflected light to unanticipated positions. A perpendicular power conversion element with a curved lens in front would have the potential advantage of reflecting through a series of angles, which would tend to reduce the power density of the reflected beam. However, at the same time, it would increase the amount of light scattered outside the beam path. Moreover, as the focal length became shorter, the lens would become more highly curved, increasing this effect. The arrangement of  FIG. 2A  is a simpler solution for assuring that reflections are safely treated. 
         [0030]      FIG. 2B  shows another embodiment of the invention wherein concentration is used. Parabolic reflector  91  focuses incident light  11  onto power conversion element  10 . Light  12  reflected from the surface of power conversion element  10  is trapped by Baffle  20 . As with  FIG. 2A , all reflected light can be captured. 
         [0031]    The main advantage of the system described in  FIG. 2B  over  FIG. 2A  is economy: It requires a lot less material for the power conversion element  10 . Specifically, InGaAs diodes operating at 1450 nm operate with concentrations of 500 suns. Both systems require that the light be incident at a known angle. Parabolic reflectors  91  can be on-axis or off-axis. The choice mostly relates to convenience, although there are efficiency issues as well. Off-axis parabolic reflectors, such as those made by Janostech Technology, Inc. of Keene, N.H., can be bought in 30 degree, 60 degree, and 90 degree variants. In production volumes, one can use a metalized injection molded plastic part which is both cheap and convenient. The advantages of a parabolic reflector over a lens are particularly profound from 1400 nm to 1500 nm, where most plastic lenses absorb heavily. The reflector is cheaper than glass lenses and 99 percent efficient. Moreover, there is much less concern with contamination than with a lens. If the parabolic reflector  91  is contaminated by something reflective and conformal, there is no harm. The same cannot be said of a lens as described in  FIG. 1B . An example of a suitable parabolic reflector  91  is illustrated in  FIGS. 2D and 2E . A plurality of parabolic reflectors  91 A-D mounted in an assembly is illustrated in  FIG. 2F . 
         [0032]      FIG. 2C  shows a version of  FIG. 2B  where the power conversion element  10  is set at an angle so that is not parallel to the incident radiation  11 . This can reduce the length of the top baffle  20 A at the cost of requiring a bottom baffle  20 B to absorb the light  13  that twice reflects from the parabolic reflector  91 . Specifically, some portion of the incident light  11  first reflects from the parabolic reflector  91 , then reflects  12  from the power conversion element  10 , and reflects again 13 from the parabolic reflector  91 . Because any incident light  11  that hits the power conversion element  10  on a perpendicular will be reflected back where it came, it is important to choose the angle of power conversion element  10  with this in mind. 
         [0033]    It should be recognized by one of ordinary skill in the art that the arrangement of an on-axis parabolic reflector  91  with a power conversion element  10  at 45 degrees to the incident light  11  will perform substantially similarly to the system described in  FIG. 2A . The optical path is just being concentrated, and there is some small masking due to the size of the power conversion element  10  and its mechanical support (not shown). Likewise, the systems described in  FIG. 2B  and  FIG. 2C  operate with the same optical elements. The optical elements are simply moved and altered for convenience and efficiency. 
         [0034]      FIG. 3A  is an illustration of a power beam receiver with the front surfaces arranged such that all incident radiation from within the receiver&#39;s acceptance angle that reflects from one surface is guaranteed to impinge upon a second surface, in accordance with one embodiment. In this figure, these surfaces are power conversion elements, but the arrangement can be used more generally. For example, the front surface might be an optic, which reflects onto a detector, as in  FIG. 2B  and  FIG. 2C . In this embodiment, two power receiving elements  10  are angled toward each other. Any beam of light that reflects from the first surface will hit the second, regardless of which is the first surface. Anti-reflection coatings, such as those by Edmund Industrial Optics of Barrington, N.J., have approximately 2% reflection at 45 degrees. Any ray that hit the first surface, reflected, hit the second surface, and reflected back out, would be attenuated to 0.04%. Potential limitations to this system are the awkwardness of fixing power receiving elements  10  at right angles to each other and the risk for contamination of the surfaces. The resulting device may be thicker than is acceptable. Also, if water or oil accumulates on the surfaces of the power receiving elements  10 , the reflectivity would increase. However, the arrangement illustrated in  FIG. 3A  is useful in reducing the total amount of reflections with which humans may come into contact. 
         [0035]      FIG. 3B  shows an improvement on the arrangement of  FIG. 3A . In  FIG. 3B , a series of small, hollow, anti-reflection coated corner cube reflectors  50  is placed before the power receiving element  10 .  FIG. 3C  is an illustration of a power beam receiver where the front surface comprises corner cube retroreflectors. 
         [0036]    Corner cubes are easy to make in plastic—bicycle reflectors are one example. A molded plastic piece can be made. If a finer scale is desired, a grayscale photolithographic process such as those used to make microlenses CCDs and CMOS imagers can be used. If the power beaming system uses a wavelength to which plastic is opaque, cast glass can be used. A reasonable thickness for the corner cubes is 1 mm, although many thicknesses can be used. When choosing the thickness of the corner cubes, considerations include making sure the corner cubes cannot easily be filled with liquid and sizing them such that they tend not to retain dust and dirt. A surface coating  40 , such as an anti-reflection coating should be used on every exposed surface—the purpose of the structure is to reflect as little light as possible, but to be certain that any light reflected is back along the beam path. An additional type of surface coating  40  may also be used, such as an anti-scratch coating, as is commonly used on prescription eyeglasses. Note, that in this embodiment, the reflector is a hollow corner cube. A filled corner cube, such as would be obtained by cutting the corner off a glass cube, may be subject to contamination. 
         [0037]    Note that in the embodiment shown in  FIG. 3B , the power receiving elements  10  can now be laid flat, not angled, and that corner cube reflectors  50  can be quite thin. It is also safe against contamination. If water accumulates on both surfaces and the reflectance is very high, the beam would be reflected back along the path from which it came (except for some dispersion due to diffraction). Thus, in one embodiment, the transmitter is also designed not to reflect incident radiation unsafely, for example by use of baffles and/or anti-reflection coatings. 
         [0038]    The embodiment of  FIG. 2A  may be superior when the light comes from a fixed position such that the beam is incident at a controlled angle, preferably perpendicular to the power beam receiver (which would be 45 degrees to front surface shown in  FIG. 2A ). The embodiment of  FIG. 3B  is advantageous when the angle of the light cannot be conveniently fixed. 
         [0039]      FIG. 4A  is an illustration of a power beam receiver where an intentional dispersion element  70  is inserted to increase the angles of the incident light upon reflection, in accordance with one embodiment.  FIG. 4A  shows one position for a dispersion element  70 . The dispersion element  70  can be a roughness present on or intentionally added to any surface. Alternatively, it can be extra material added between elements. Further alternatively, it can be within an element, such as glass balls molded into a plastic lens. One way to make the roughness is with a mechanical process, like sanding or grinding. Another way is to use a photoetch step on the surface of an element, such as a power receiving element  10 . Still another way is to intentionally mark or scratch the mold or die from which a molded or cast part is made. The design of these scratches is often non-critical as long as they are not too deep. A more accurate method, like a photoresist method, can put features designed for diffraction into the optical system. The main design consideration for these defects is the tradeoff between efficiency—getting the light to where it will be converted to electricity—and safety. Any system where light propagates across regions with index differences is subject to Fresnel reflection, and so there will be an efficiency loss due to back reflection. 
         [0040]      FIG. 4B  shows a receiver with a dispersion element  70  and a border  80 . When using a dispersion element  70 , in one embodiment, a border  80  around the dispersion element  70  is used to guarantee that there is a minimum distance between a human eye or other human tissue and the surface from which the light is scattered. Because the beam path and the border are assumed to be protected, the closest a person can get to the light is the width of the border  80 . Assume that a 32 mm×32 mm square has normal incident light at 1 mW/sq. mm. Assume that the reflection from the surface is 10% with equal scattering through a hemisphere (2π steradians). Assume that a person&#39;s pupil is 7 mm, and that their head cannot interfere with the beam but rather must be outside the border, which is 10 mm wide. The greatest amount of light that a 7 mm pupil could receive under these conditions is 0.016 mW, which is well within the regulatory exposure limits. 
         [0041]    For efficiency, the front of the intentional dispersion element  70  should be anti-reflection coated, and it should be index-matched to the power conversion device  10 . It can be best to have the dispersion elements exposed, as shown, so that contamination causing reflection will cause dispersed reflection. Power conversion device  10  is shown supported by a substrate, which forms border  80 . 
         [0042]    Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed in detail above. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention. Therefore, the scope of the invention should be determined by the appended claims and their legal equivalents.