PHOTOVOLTAIC CELL FOR LASER BEAM POWER DETECTION

Abstract

A wireless optical power transmission system comprising a transmitter and receiver, the transmitter comprising a laser emitting a beam, a scanning mirror for steering the beam towards said receiver and a control unit receiving signals from a detection unit on the receiver and controlling the beam power and the scanning mirror. The receiver has a photovoltaic cell having a bandgap energy of 0.75-1.2 e V, with a plurality of conductors on abeam receiving surface. A cover layer of material blocking illumination of wavelengths outside that of the laser, is disposed on the photovoltaic cell. The cover layer may have anti-reflective coatings on its top and bottom surfaces. The detection unit thus generates a signal representing the power of the laser beam impinging upon the receiver, independent of illuminations other than that of said laser beam. The control unit thus can maintain the laser power impinging on the receiver.

Claims

1. A power converting device for converting optical power into electrical power adapted for optical wireless power transmission using a laser beam, said power converting device comprising: a photovoltaic cell having a plurality of conductors on a surface adapted to receive said laser beam, said photovoltaic cell having at least one junction having a bandgap energy between 0.75 eV and 1.2 eV; and a cover layer disposed upon said photovoltaic cell, said cover layer comprising a material adapted to restrict transmission, by either absorption or reflection, of illumination having wavelengths outside of the range of the wavelength of said laser beam, and to transmit said laser beam towards said photovoltaic cell; wherein: said laser beam has a wavelength between 700 nm and 1500 nm; said wavelengths of said illumination outside of the range of said wavelength of said laser beam fall within the range of 550 nm to 700 nm; and the transmission of said cover layer for said wavelength of said laser beam is at least 50% higher than its transmission for wavelengths within the range of 550 nm to 700 nm, such that said power converting device has an efficiency of conversion to electrical power at said wavelength of said laser beam at least 2.5 times higher than its efficiency at a wavelength of 550 nm.

2. The power converting device according to claim 1, wherein said bandgap energy is tuned to said wavelength of said laser such that the efficiency of conversion of optical power into electrical power for any wavelength longer by at least 25% than said wavelength of said laser, is more than four times less than the efficiency of conversion at the laser wavelength.

3. The power converting device according to claim 1, wherein said cover layer further comprises at least one of: a first anti-reflective coating disposed upon the surface of said cover layer remote from said photovoltaic cell, said first anti-reflective coating adapted to reflect illumination having wavelengths outside of the range of the wavelength of said laser beam, and to transmit said laser beam into said cover layer, or a second anti-reflective coating disposed between the surface of said photovoltaic cell and said cover layer, said second anti-reflective coating adapted to reflect illumination having wavelengths outside of the range of the wavelength of said laser beam, and to transmit said laser beam into said photovoltaic cell.

4. A safety system for a wireless optical power transmission system comprising a transmitter, a receiver and a control unit, wherein: (i) said transmitter comprises: a laser adapted to emit a beam; and a scanning mirror adapted for steering said beam towards said receiver; (ii) said receiver comprises: a photovoltaic cell having a plurality of conductors on a surface adapted to receive said laser beam, said photovoltaic cell having at least one junction having a bandgap energy between 0.75 eV and 1.2 eV; and a cover layer disposed upon said photovoltaic cell, said cover layer comprising a material adapted to restrict by either absorption or reflection, illumination having wavelengths outside of the range of the wavelength of said laser beam, and to transmit said laser beam towards said photovoltaic cell; and (iii) said control unit is adapted to receive first data representing the position of said scanning mirror, and to receive second data from said transmitter representing the power of said beam emitted by said laser, and to determine from said first and second data, the expected power incident on said photovoltaic cell, and to compare said expected power with said power of said laser beam impinging upon said receiver, as measured by said photovoltaic cell, and to indicate a potential safety problem if said expected power deviates from said measured power by more than a predetermined level.

5. A wireless optical power transmission system comprising a transmitter and receiver, said transmitter comprising: a laser adapted to emit a beam; a scanning mirror adapted for steering said beam towards said receiver; and a control unit adapted to receive signals from a detection unit on said receiver, and to control at least one of (i) the power of said beam emitted by said laser and (ii) a position of said scanning mirror: said receiver comprising: a photovoltaic cell having a plurality of conductors on a surface adapted to receive said laser beam, said photovoltaic cell having at least one junction having a bandgap energy between 0.75 eV and 1.2 eV, said photovoltaic cell adapted to detect said power of said laser beam reaching said photovoltaic cell; wherein: said receiver comprises a cover layer disposed upon said photovoltaic cell, said cover layer comprising a material adapted to absorb or reflect illumination having wavelengths outside of the range of the wavelength of said laser beam, and to transmit said laser beam towards said photovoltaic cell; and at least one of: (i) a first anti-reflective coating disposed upon the surface of said cover layer remote from said photovoltaic cell, said first anti-reflective coating adapted to reflect illumination having wavelengths outside of the range of the wavelength of said laser beam, and to transmit said laser beam into said cover layer; and (ii) a second anti-reflective coating disposed between the surface of said photovoltaic cell and said cover layer, said second anti-reflective coating adapted to reflect illumination having wavelengths outside of the range of the wavelength of said laser beam, and to transmit said laser beam into said photovoltaic cell; wherein, said detection unit generates a signal representing the power of said laser beam impinging upon said receiver, independent of illuminations of other wavelengths other than that of said laser beam, and said control unit is adapted to control at least one of (i) said beam and (ii) said position of said scanning mirror in order to maintain said power impinging on said receiver.

6. A power converting device for converting optical power into electrical power adapted for optical wireless power transmission using a laser beam, comprising: a power converting device having at least one junction having a bandgap energy between 0.75 eV and 1.2 eV, and having an external layer through which laser light is transmitted towards said at least one junction, said external layer being configured to transmit at least a first wavelength into said at least one junction with at least 80% efficiency when illuminated through said external layer from any direction between ±20° to the normal to the surface of said external layer; wherein at least one of: the conversion efficiency of said power converting device for said first wavelength being at least 30%, said first wavelength being a near infra-red wavelength between 700 nm and 1500 nm; said power converting device external layer being configured to reflect or absorb a portion of incident illumination at a second wavelength between 550 nm and 700 nm, so that less than 60% of the illumination at said second wavelength reaches the at least one junction when illuminated through said external layers from any direction between ±20° to the normal to the surface of said external layer(s) and said power converting device conversion efficiency for said second wavelength is at below 20%; said power converting device external layer being configured to absorb or reflect at least a third wavelength between 300 nm and 550 nm, so that at least 50% of the power of said third wavelength is absorbed before reaching said at least one junction, when illuminated through said external layer from any direction between ±20° to the normal to the surface of said external layer, and said power converting device conversion efficiency for said third wavelength being less than 10%; and said power converting device conversion efficiency for a fourth wavelength between 1500 nm and 2000 nm being below 5%.

7. A power converting device for converting optical power beam to electrical power comprising: a semiconductor device having a p-n junction adapted to absorb said optical power beam, top and bottom conductors in electrical contact with said semiconductor device, said top conductor covering a portion of a top surface of said semiconductor device; and an optical layer disposed on said top surface of said semiconductor device, said optical layer comprising a top volume and a bottom volume, said bottom volume being in optical contact with said top surface of said semiconductor device, and with said top conductor of said semiconductor device, and said top volume being in optical contact with air; wherein: said top conductor is adapted to reflect at least 30% of the light impinging upon it; said optical layer has an optical density for said optical power beam of less than 2; and said top conductor is adapted to direct at least 25% of the light reflected by it into angles greater than sin.sup.−1(1/the refractive index of said bottom volume).

8. The power converting device according to claim 7, wherein at least a portion of said light reflected by said conductors is reflected at angles which undergo total internal reflection from the top surface of said top volume.

9. The power converting device according to claim 7, wherein said top volume of said optical layer is an anti-reflective coating adapted to reduce the reflections of said optical power beam coming from a medium having a refractive index of approximately 1.

10. The power converting device according to claim 7, wherein said top volume of said optical layer is a scratch resistive coating.

11. The power converting device according to claim 9, further adapted to reduce reflections of said optical power beam over angles between at least −10 degrees and +10 degrees to the normal to said top surface.

12. The power converting device according to claim 7, wherein said portion of coverage of said top surface covered by said top conductor is at least 4%.

13. The power converting device according to claim 7, wherein said conductors are made of metal.

14. The power converting device of claim 13 wherein said conductors comprise at least partially of aluminum, gold, silver or copper.

15. The power converting device according to claim 7, wherein the area of the geometric projection of the portions of said conductors aligned at an angle of at least sin.sup.−1(1/said refractive index of said bottom volume) onto said top surface of said semiconductor device is at least 25% of the area of said semiconductor device multiplied by said portion of coverage of said top surface covered by said top conductor.

16. The power converting device according to claim 7, configured such that laser reflection from said power converting device is diffused.

17. The power converting device according to claim 16 wherein said diffused reflection from said power converting device has an angular subtence of at least 1.5 millirad.

18. The power converting device according to claim 7, wherein the area of the semiconductor device measured in Meter2 times the bandgap of the junction, measured in Joules squared, times the designed maximal electrical power of the cell, measured in watts to the third power, is less than 214*10 30, such that P3*(bandgap)2A<214*10-30.

19. The power converting device according to claim 7, wherein said top conductor comprises a conducting grid having a finger-shaped profile.

20. The power converting device according to claim 7, wherein said top conductor comprises a conducting grid having a triangular shaped profile.

21. A power converting device for converting optical power into electrical power adapted for optical wireless power transmission using a laser beam, said power converting device comprising: a photovoltaic cell having a plurality of conductors on a surface adapted to receive said laser beam, said photovoltaic cell having at least one junction having a bandgap energy between 0.75 eV and 1.2 eV, and having a cover layer disposed upon it; wherein: said cover layer comprises a material adapted to at least one of absorb or reflect illumination having wavelengths within the range of 550 nm to 700 nm, and to transmit said laser beam towards said photovoltaic cell, and said laser beam having a wavelength between 700 nm and 1500 nm; and the transmission of said cover layer for said wavelength of said laser beam being at least 50% higher than its transmission for wavelengths within the range of 550 nm to 700 nm, such that said power converting device has an efficiency of conversion to electrical power at said wavelength of said laser beam at least 2.5 times greater than its efficiency at a wavelength of 550 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0104] The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

[0105] FIG. 1 shows schematically a typical structure of a prior art photovoltaic cell;

[0106] FIG. 2 shows schematically the first two orders of Bragg reflection of an incident beam on a prior art rectangular grid photovoltaic cell of the type shown in FIG. 1;

[0107] FIG. 3 shows a representation of a typical multiple order reflection from the surface of a photovoltaic cell, with at least 20 orders of reflection being visible;

[0108] FIGS. 4A, 4B, and 4C illustrate schematically alternative grid profiles used in PV cells, and a schematic representation of the reflective beam patterns generated by each profile;

[0109] FIG. 5 illustrates the features of a photovoltaic cell relevant for the present disclosure, illustrating the terms used;

[0110] FIGS. 6A and 6B show the effect of a high index cover layer on the incidence of the incoming beam angle impinging on a photovoltaic cell;

[0111] FIG. 7A shows the effect of a cover layer and the shape of the conductor fingers on representative rays of light impinging upon a photovoltaic cell according to the present disclosure;

[0112] FIG. 7B shows an implementation of a photovoltaic cell according to the present disclosure, with the addition of anti-reflection coatings on either side of the cover layer;

[0113] FIGS. 7C and 7D illustrate schematically the difference in angular subtence between a collimated light beam and a diffuse light beam impinging on the lens of an eye;

[0114] FIGS. 8A and 8B schematically show reflections from a photovoltaic cell and follow the path of a reflected collimated beam as it impinges on a human eye;

[0115] FIG. 8C schematizes the paths of a few representative, reflected diffuse beams that escape the cover layer of a photovoltaic cell in an embodiment of the present disclosure; and

[0116] FIG. 9 shows a safety control system for providing a warning of a situation in which the laser beam is supposedly directing a beam of a certain power in the direction of a receiver, and yet the receiver is not receiving all or any of that power, indicating that the beam is being diverted into a non-intended direction.

DETAILED DESCRIPTION

[0117] Reference is now made to FIG. 1, which shows schematically a typical structure of a photovoltaic cell, made from p-type semiconductor 13 attached, or sometimes in close proximity but still separated by another layer, to n-type semiconductor 11, forming a p-n junction 12. A full metallic contact is positioned on the back of the cell 14 and a partial metallic contact in the form of a grid (or similar structure) 15 is positioned on the front of the cell allowing light to penetrate through the spaces between conductors. Incident illumination generates a current 16 through the load.

[0118] Reference is now made to FIG. 2, which shows schematically the first two orders of Bragg reflection of an incident beam 23 on a prior art rectangular grid photovoltaic cell 21. An incoming beam 23 impinging on the metal grid 22 of photovoltaic cell 21, some 2-10% of the power in beam 21 is reflected by conductor grid 22 in a Bragg pattern including several orders such as zero order 24 and 1st order 25 shown in FIG. 2.

[0119] Reference is now made to FIG. 3 which shows a representation of such a typical multiple order reflection 31 from the surface 32 of a prior art photovoltaic cell, with at least 20 orders of reflection being visible. The power in each “order” is very much dependent on the angle of illumination, causing safety and aiming problems. The total reflection spans some 5 degrees from the zero order reflection and the zero order reflection is the strongest reflection. Higher orders of reflection include much less power compered to lower orders. This is unlike the current cells, which prefer reflection into higher order modes, specifically above the critical angle of the optical cover layer.

[0120] Reference is now made to FIGS. 4A, 4B and 4C, which illustrate schematically alternative grid profiles used in PV cells. FIG. 4A shows the reflection from a grid of rectangular conductors, the reflection being generated at an angle opposite to the incoming beam angle. FIG. 4B shows a reflectance from a triangular reflector, which divides the beam into 2 wide angle beams and works well for small fields of view (FOV). FIG. 4C shows the reflection from a rounded conductor which is spread over a wide angle. A preferred implementation (not shown in FIGS. 4A to 4C) is a conductor covered with a diffusive top layer, which may spread the radiation in a very wide pattern.

[0121] The structures shown in FIG. 4B and FIG. 4C as well as the diffusive coating on the conductors enhance the high order reflections from the grid and therefore enhance the “recycling” of the reflected light. It is to be understood that perfect geometrical shapes are typically impossible to manufacture, and such illustration serves as a simplistic example. It is preferred to choose a pattern that will spread the light over a wide FOV such as a diffusive, round or triangular shape. Shapes which reflect a significant percentage of the light into wide angles are preferred, such as triangles with reflector incline greater than a threshold angle. The conductors are typically made of reflective materials such as metals, reflecting at least 30% of the light falling on them, typically a material reflecting at least 90% of the light is chosen, such as aluminum, silver or gold. The conductors may be coated with a diffusive coating.

[0122] Reference is now made to FIG. 5, which illustrates the terms used in the present disclosure. It shows the angular dependence of the reflected light as a function of the incident light for finger-shaped conductors on a PV 51, having a back electrode, and p-n junctions, besides the conductors 52. The angle 55 is the local angle formed by the intersection of a line 57 tangential to the surface of the finger conductor at a point 56 on the conductor, with the normal 58. The area projected onto the upper surface 59 of the PV by the conductors 52 is shown below the PV and is numbered 53. The area projected onto the PV surface by the part of the conductors 52 wherein the angle is less than a threshold is shown below the PV and is numbered 54.

[0123] Reference is now made to FIGS. 6A and 6B, which illustrate the difference between the incoming beam angle 61a of a known PV cell 62 without a high index cover layer, and incoming beam angle 61b of a cell 64 with a high index cover layer 63, as proposed in the current application. Each is illuminated by a beam of light, either beam 61a in FIG. 6A, or beam 61b in FIG. 6B. Beam 61b passes through the cover layer 63, which is a transparent coating layer, and is referred to as the optical cover layer. The top of optical cover layer 63 may be anti-reflective coated to reduce the reflection of beam 61b and allow most of it to enter optical cover layer 63. The bottom surface 65 of optical cover layer 63 is in contact with both the top of the PN junction of cell 64 as well as with the conductors (not shown) on the cell 64. There is an anti-reflective coating between the bottom 65 of optical cover layer 63 and cell 64, which is adapted to reduce the reflection of light beam 61b when traversing from the refractive index of the bottom part 65 of optical cover layer 63 to the top part of cell 64. Optical cover layer 63 is typically of high index, preferably has an optical density that allows at least 80% of the power to transit through it, and has an optical density<2 when measured against optical beam 61b. Optical beam 61b reaches the top surface of cell 64 at a reduced angle of incidence compared to the angle at which beam 61a, which is parallel to 61b, hits cell 62, due to the high index of coating layer 63. The thickness of coating layer 63 should be less than the width of cell 64.

[0124] Reference is now made to FIG. 7A, which shows a photovoltaic cell 71, similar to that shown above in FIG. 5 with the addition of the cover layer 78. This drawing shows the different paths which incident beams can take when impinging on the PV structure. Beams 79a, 79b, 79c and 79g are shown absorbed by the cell 71 as they would be on a known PV cell with the same conductor coverage. Beam 79d, on the other hand, is shown reflected by the side of conductor 72, and is immediately absorbed by the cell 71. Beam 79e is shown reflected by the top of the conductor 72, and escaping from the device, in a similar manner to what would occur if it were to impinge on a prior art rectangular conductor. It is thus lost and not converted to electrical energy. Beam 79f is shown reflected off the conductor at an angle which, in a prior art PV cell, would have resulted in the beam being lost. However, because the current PV is covered by the cover layer 78, beam 79f now impinges on the inner side of the top surface of the cover layer 78 at an angle of incidence greater than the critical angle for that upper surface interface, and is thus reflected as beam 79f′ back towards the PV surface, and, instead of being lost, is absorbed by the cell, even after additional reflections as typically illustrated in FIG. 7A. Optical conductors 72 are wider than similar conductors in a prior art cell and therefore have lower resistance and higher reflections compared with prior art conductors. The shape of optical conductors 72 is designed to maximize the projected area of the portions of the conductors which are tilted at an angle above 80% of the critical angle, as calculated based on the refractive index of the bottom part of optical cover layer 78. Reflection at such angles has a high probability of eventually reaching the cell again and being absorbed by it and converted to electrical power. Conductors 72 typically have reflectivity of greater than 80 to 90%, but in some cases they may be less reflective. Conductors 72 may be made of metals such as aluminum, silver, gold, molybdenum, copper, nickel, or tungsten, and may be coated with diffusive coating, such as opal coating. Alternately, the conductors may have small reflective structures or particles deposited on them. Conductors 72 are spaced so that the reflection from them is maximized to orders above the 80% of the critical angle as calculated based on the refractive index of the bottom part of optical cover layer 78. Such spacing is typically more than 0.5 wavelengths/refractive index and less than 100 wavelengths.

[0125] Although the conductors in FIG. 7A, and in previous FIG. 5 and subsequent FIGS. 7B and 8C are shown as finger shaped, it is to be understood that they could also have any other suitable profile to provide the multiple internal reflections of the beam within the cover layer 78, such as the triangular shape of FIG. 4B.

[0126] Reference is now made to FIG. 7B, which shows the PV cell of FIG. 7A, according to the present disclosure, with the addition of anti-reflection coatings 73, 74 on the upper and lower surfaces of the cover layer 78.

[0127] Reference is now made to FIG. 7C, which illustrates the significance of the angular subtence of an apparent source, this being the angle subtended by an apparent source as viewed from a point in space. Specifically, the resolution of the human eye is such that a typical retinal photoreceptor views an angular subtence of the surrounding environment of approximately 1.1 millirad. This calculation derives from the fact that a single retinal photoreceptor cell outer segment is ˜25 micron in radius and the effective focal length of the human eye is ˜17 millimeter in a water-based environment, equivalent to ˜22 millimeter in air. A light source with an angular subtence of 1.1 millirad may be focused on a single retinal cell and the entire power from the source may thus be absorbed by the same cell. A light source with a smaller angular subtence would still be absorbed by a single photoreceptor. However, a source with higher angular subtence, resulting from poor optical quality, cannot be focused on a small spot. Thus, it would be impossible to focus such a source on a single biological cell, and as a result a source with higher angular subtence would pose lesser danger. In other words, a single retina cell occupies ˜4*10.sup.−6 steradians. The radiation from a diffused source having angular subtence of 1.5 steradians is expected to be distributed over at least 2 retinal cells and therefore carries approximately half of the retinal risks and is safer.

[0128] Referring now to the details of FIG. 7C, a PV 71a reflects a portion of the light illuminating it in a diffuse manner. Lens 75, positioned 100 mm from the surface of the window in front of PV 71a, converges the beam of light to a point of light 77 at the focal point of the lens. Should PV 71a reflect a high quality optical beam, similar to a TEM.sub.00 laser, lens 75 will focus the light to a diffraction limited spot. The angle 70 subtending between the top of the image, the center of the lens, and the bottom of the image would be close to 0.

[0129] Another possible scenario is shown in FIG. 7D, where PV 71a is reflecting diffusively image instead of preserving the optical quality of the original laser beam. In such a case, lens 75 would not create a diffraction limited spot, but would rather create an image 76 of PV 71a, and the angle 70 would obviously be bigger.

[0130] Typically lens 75 is moved around, closer and farther from the PV cell, to locate the point of minimal angular subtence of the beam to determine the angular subtence of the beam at a certain distance.

[0131] On top of the above requirements, it is important that the PV would be responsive to changes in illumination levels. The higher the beam power, the more responsive the photovoltaic cell must be in order to allow a safety system to be based on detecting the light levels. It has been found that for the PV to be responsive, the cell's structure must be adapted to match the intended power level according to the following formula:

[00001]${10}^{27}*{\left(\mathrm{bandgap}\right)}^2\frac{A}{d}<\frac{214}{P^3}$

[0132] where d is the thickness of the layer absorbing beam's photons in the photovoltaic cell measured in meters

[0133] bandgap is the bandgap energy of the p-n junction measured in Joules

[0134] A is the area of the photovoltaic cell measured in Meter.sup.2.

If too large a cell is used, then the responsiveness of the cell drops and it cannot react fast enough to changes in illumination levels.

[0135] Since d is usually less than 300 micron thick, and always less than 1000 micron thick, this can be simplified to

[00002]${10}^{30}*{\left(\mathrm{bandgap}\right)}^{2}A<\frac{214}{P^3}$

[0136] Or in a more convenient form

P.sup.3*(bandgap).sup.2A<214*10.sup.−30

[0137] Reference is now made to FIGS. 8A and 8B, in which FIG. 8A illustrates a reference PV cell 81a with known characteristics, and FIG. 8B shows the effect of reflected, collimated laser light on a human eye. In FIG. 8A, the light beam 84 illuminates the PV cell 81a, and while most of the light is absorbed by the cell 81a, some of the incident beam is reflected by the cell towards eye 85. The cell 81a, reflects typically some 2-10% of the light impinging upon it, mostly into the first order reflection, in which the light hitting a flat surface produces a mirror like reflection, the angle of incidence being equal to the angle of return. The reflected light is collected by the lens 88 of the human eye 85 and forms a few discrete small lines, these being an image of the conductors on the retina 86.

[0138] As shown in FIG. 8B, the cornea and lens may focus the strip of light 89 from the collimated beam to small point, with the potential to cause greater focal damage. As the reflection from a collimated beam would diverge minimally, and as the beam is refracted by both the cornea 87 and the lens 88, the result is a focusing of a powerful beam of light energy, as illustrated in FIG. 7C, that may converge onto sensitive ocular tissue. Such a focused beam has a significant potential to cause damage 80 at the point 80 where the beam impinges on the retina 86.

[0139] Reference is now made to FIG. 8C, schematically illustrating a PV cell 81b according to an implementation of the current disclosure. In contrast to PV cell 81a, PV cell 81b uses non-rectangular, highly convex profile conductors 82b and a cover layer 83. The cell 81b comprises conductors 82b, which reflect generally up to about 10% of the light, as the conductors are wider than a typical rectangular conductor, but since the conductors 82b reflect the beam at an angle, the majority of the first order reflections are above the total internal reflection angle of the optical cover layer and are thus reflected back towards the cell, where approximately 90% of it is absorbed. About 0.04%-2% of the original beam is reflected a second time by the conductors and these rays 89b may be reflected outside the optical cover layer 83, but are now reflected in a diffused manner. Eye lens 88 collects a small amount of the diffuse emitted light 89b and may form an image of the cell 81b on the retina. However, that image has significantly less power per unit area, and therefore poses a significantly smaller risk to the retina. The highest collection of light by the eye would be at the smallest distance. Since the minimal focal distance of the human eye is ˜100-150 mm, the riskiest collection position would be at approximately 100-150 mm distance from the cell.

[0140] Comparing the reflected light from a reference PV cell in FIG. 8A with an implementation of the present disclosure schematically depicted in FIG. 8C, beam 84 approaches cell 81b from the same angle as it was shown impinging on cell 81a. By contrast, in FIG. 8C, the beam traverses cover layer 83 before being reflected by conductors 82b in angles above 80% of the critical angle, as calculated based on the reflective index of the bottom of cover layer 83. The rays that reflect are absorbed by the cell 81b at a higher efficiency compared with cell 81a, and the percentage of beams eventually emitted through the top of optical cover layer 83 is lower compared to the percentage reflected by conductors 82a. Further, the rays 89b that are emitted from the top of optical cover layer 83 are diffuse, each emitted in a different direction. Therefore, the power of light impinging on the eye 85 in FIG. 8C is lower compared to the power of light collected by the eye 85 in FIG. 8A. In FIG. 8C, because the rays are not traveling in the same direction, they will not converge on the same position, and they thus form a diffuse image on the retina of the eye 85, as illustrated in FIG. 7D, that poses a much lower risk to the retina.

[0141] The currently described devices typically allow for a cell of 1 cm by 1 cm size, to reflect diffused back-reflection, such that the reflection from the conductors of the beam of a TEM.sub.00 laser would form a minimal image when focused by a f=25 mm lens placed 100 mm from the surface of the cell, subtending at its 1/e diameter, at least 1.5 mRad, and typically much more, therefore posing much less risk to the retina. Furthermore, by diffusing the beam, the current cell configuration allows for the typically center-weighted beam received from the laser source, to be less centered and more uniform. Improved uniformity of illumination thus allows for a more complete utilization of the cell by allowing current to flow from a shorter distance to the current collectors at the edge of the cell. The current cell also allows for thicker metal conducting fingers, which result in lower Ohmic losses, an advantageous feature in the case of a high optical flux, as is the case with most laser power converters.

[0142] Reference is now made to FIG. 9, which shows a safety control system 90 configured to provide a warning in case of a situation in which the transmitter 95, typically a laser, is supposedly directing a beam of a certain power in the direction of a receiver 91, and yet the receiver is not receiving all or any of that power, indicating that the beam is being diverted to a non-intended direction. The control system 90 provides an indication or warning that the transmitted laser beam may present a hazard to the surroundings, since it is not reaching its intended receiver, or is reaching the correct receiver but with excessively reduced power. The controller unit 97, which is most advantageously located in the transmitter 95, but could also be positioned elsewhere in the system, as shown in FIG. 9, or in the space being serviced by the transmitter, receives a signal input 92 from a detection unit 91 in the receiver, which is generally the PV cell in the receiver. The signal 92, which can be sampled from the electrical power output 93 from the PV cell, represents that portion of the power level falling on the PV cell, of illumination having the laser wavelength, even in situations in which significant levels of power from other sources, such as sunlight 94, may fall on the PV cell 91. The control unit 97 is configured to also receive signals from the laser power supply 95 in the transmitter, indicating the power of the laser beam emitted by the laser and from the position of the scanning mirror 96, both of the settings of these components of the system being determined by the operational requirements for supplying beamed laser power to the receiver. Should the laser power setting and the mirror scan position show that a certain power level is expected from the detector unit, and the actual input to the control unit shows a power level below that level expected by more than a predetermined amount, it is assumed that part of the beam is being obstructed, and is not reaching its intended receiver target, thus triggering a safety warning by the system.

[0143] It is to be understood that the control system can also operate in its conventional manner, i.e. in the reverse direction, in order to optimize the scanning mirror setting, to keep the laser beam centered on the receiver PV, and to control the laser to supply the intended laser power, according to the power measured by the detector unit.

[0144] It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.