Beam shaper for optical wireless power transmission system

Abstract

An optical wireless power transmitter adapted to transmit a laser power beam from a multimode diode laser to a photovoltaic cell located at up to a maximum operating range from the transmitter. The laser output has orthogonal beam components having different divergences emitted from the narrow and the broad dimensions of the laser cavity, and these two beam components originate from different axial positions in the laser cavity. In order to generate a well collimated and dimensionally uniform laser beam at up to the operating range, an optical beam shaper having both anamorphic power and astigmatic optical properties is used. The beam shaper may be constructed of an orthogonal pair of cylindrical lenses, in which the astigmatic properties are provided by selecting the different lens focal lengths or by positioning the lenses such that the different origins of the components generate a uniformly collimated beam.

Description

[0001] BEAM SHAPER FOR OPTICAL WIRELESS POWER TRANSMISSION SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to the field of wireless charging of electronic devices, especially for use in optimizing power conversion in a receiver such as a photovoltaic cell.

[0004] BACKGROUND OF THE INVENTION

[0005] Systems for optical transmission of power, such as those disclosed in US 5,925,271 , for “Laser Beam Shaping Device and Process including a Rotating Mirror”, to D. Pollack et al, and US 6,407,535, for “System for Beaming Power from Earth to a High Altitude Platform” to H.W. Friedman et al, and several patent applications having common inventors with the present application, such as US 9,312,660 for “Wireless Laser System Power Transmitter” to O. Alpert, et al, direct a laser beam towards a photovoltaic cell in order to transmit power wirelessly. Some prior art systems such as US 6,407,535 actively rotate the beam so that at long distances, its rotated profile matches the shape of a photovoltaic array, almost independently of its original shape. Other prior art systems, such as US 9,312,660, having a common inventor with the present application, include a diffuser at the receiving end, which is used to match the received beam shape to the photovoltaic cell.

[0006] In many prior art systems, the beam generally has a circular Gaussian distribution (TEMoo) or close to it, which is the optimum beam shape for achieving maximal range. Such beams are typically symmetrical if possible, as this is the most efficient way to design optics. The beam shape is typically the result of other considerations, such as the need to achieve a required Rayleigh length and hence a required range, such as in US US 9,312,660, and is optimized for maximal range with minimal optical losses, i.e. efficient power delivery, rather than for efficient power conversion in the photovoltaic cell. This resulted in less than optimal beam shapes at the receiving end for efficient power conversion, requiring the use of receiving end beam shapers to adapt the beam to the shape of the photovoltaic cell. As a result, such receivers were comparatively large and expensive to manufacture. The use of higher power laser diode sources, able to emit beams of sufficient power to provide useful output for wireless power transmission, complicates this situation, because of the more complicated emission patterns of the laser beam generated by such sources.

[0007] There therefore exists a need for a beam shaper that facilitates the ability to transmit the laser power over the desired range, and with efficient power conversion in the receiver, and which overcomes at least some of the disadvantages of prior art systems and methods.

[0008] The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.

[0009] SUMMARY

[0010] The present disclosure describes new exemplary systems for efficient power conversion in a receiver including a photovoltaic cell, even if at the expense of power delivery efficiency.

[0011] In order to provide useful power levels for optical wireless power transmission, lasers currently used in such systems are often multimode diode lasers, whose output beam has an asymmetrical shape and different divergences in the two orthogonal directions. In one axis, that of the thin gain medium layer generally normal to the semiconductor wafer’s layers and surface, the structure supports a single optical mode. Because of the small height of the emitting layer, which is comparable with the wavelength of the laser emission, the beam size as emitted from the diode is approximately 1 micron or even less, and has an angular divergence of typically between 30° and 50°, though it can also be outside of that range. In the other axis, having a waveguide many times wider than the wavelength of the laser emission, the waveguide can support multiple transverse optical modes, such that the beam output will not have a single mode output, but will contain additional multiple mode outputs. Depending on the width of the emitting facet of the laser diode, the beam’s initial dimension can range from 30 to 100 micron, though it can also be outside of that range, but it has a much more easily handled angular divergence of typically between 5° and 15°. The multi-mode output in the wide dimension of the lasing layer is useful in providing good power output to the beam, even if it generates greater difficulty in providing a collimated beam over the full required range of the system.

[0012] This asymmetry of the output beam shape, if not corrected, would result in an asymmetric beam shape on the photovoltaic cell, substantially broader in one direction than in the orthogonal direction, which is not conducive to optimum power absorption. Since laser diode beams are generally elliptic, the power conversion efficiency by a photovoltaic cell is reduced unless the beam shape is matched at the receiver to approximate the shape of the sensitive area of the photovoltaic cell to provide a good filling factor.

[0013] The present systems, unlike prior art systems, adapt the beam shape as emitted from the transmitter, to the shape of the photovoltaic cell, rather than trying to adapt the beam shape as received at the receiver, to that of the photovoltaic cell. This adaptation is performed by a static method, providing a cost efficient method of maintaining good power conversion efficiency. While this method may yield somewhat lower efficiency and less range compared to the more complex optimizations described in the prior art, it yields smaller and simpler receiver designs, which is important for a system intended to provide wireless power transmission to a consumer product, such as a mobile phone, or a mobile computer.

[0014] Remote wireless charging of a target photovoltaic cell in such devices, requires a laser generally positioned some meters away from the target. The prior art systems described in the Background hereinabove, having a comparatively symmetrical beam shape, typically use a simple lens, often a spherical lens, to collimate the laser beam. In this way, more of the power would fall inside the photovoltaic cell and the optical efficiency would improve. However, due to the very asymmetric nature of the laser diode emitter, such a simple lens would create a line shape on the photovoltaic cell, which would not fill the photovoltaic cell’s sensitive area well. Without a receiver beam shaper at the receiver end, the photovoltaic cell would only be partially illuminated resulting in inefficient conversion of the optical power to electrical power, as some parts of the photovoltaic cell would be flooded with high intensity illumination, while other parts would remain unilluminated. A suitable cell would therefore require to be adapted to convert a higher intensity than the averaged-out integrated intensity it actually receives, resulting in reduced conversion efficiency of the power actually received. This problem may be solved, as in the above mentioned previous references, by rotating either or both of the photovoltaic cell or the beam, or by use of an elongated photovoltaic cell, or by doing both.

[0015] The present disclosure describes an alternative and static optical system for shaping a laser beam having a non-circular profile, as emitted from a laser diode, for use with either a square or rectangular or circular photovoltaic cell located at a distance from the laser diode. This is performed by adapting the anisotropic shape of the beam as generated in the transmitter to that of the sensitive area of the photovoltaic cell. In order to overcome the difference in dimensions of the emitted beam between the shape of the beam in the narrow rapidly diverging axis, called the fast axis, and the wide, generally orthogonal, slowly diverging axis, called the slow axis, an anamorphic optical system is used. In the present disclosure a planned distorted image of the laser diode emitter, more accurately called a shape-corrected image of the laser diode emitter, is created at a distance from the transmitter. The planned distorted image results from providing a greater magnification in the fast beam axis direction, i.e. the narrow emitter dimension, than in the slow beam axis direction. For the extent of the distortion required, such an optical system typically requires between 3 and 6 optical surfaces to achieve the desired distorted image. The distorted image is chosen so that at any point along the operating range of the system, the beam is confined to a size smaller than the photovoltaic cell, typically, less than 80%, and typically 60% of the photovoltaic cell sensitive area, and the shape and intensity profile should be as uniform as possible over that area.

[0016] The distorted beam image size should be comparable to the size of the sensitive area of the photovoltaic target, so that, over the operating envelope of the system, the beam fits inside the outline of the target and essentially fills it. To achieve this aim for all reasonable angles of tilt of the photovoltaic cell, the distorted image size should preferably be no more than 60% of the target size, since 100% filling would not allow the full beam to be collected if the target was tilted. The present system thus adapts the beam shape to the shape of the photovoltaic cell, in the transmitter rather than in the receiver, and does so without any moving parts. While this method has lower optical efficiency and a shorter range compared to other optimizations, it also enables a smaller and simpler receiver design. An optically efficient method of providing a more uniformly collimated beam output of the asymmetric laser diode output, using a pair of cylindrical lenses to “circularize the beam”, is known in the prior art. A first cylindrical lens collimates the beam in one dimension only, with the propagation in the other dimension unaffected, while the second cylindrical lens, aligned orthogonally to the first cylindrical lens, then collimates the orthogonal dimension without affecting the already collimated first dimension. The ratio of the focal lengths of the two lenses in such prior art systems, is selected to be equivalent to the ratio of the beam divergences, such that each lens collimates the diversion of the beam component which the lens power is operating on, by the amount needed to compensate for the divergence of that beam component, and the lenses re positioned at distances equal to their respective focal lengths from the exit aperture of the laser.

[0017] However, the prior art methods describing the use of such a simply arranged serial pair of cylindrical lenses does not succeed optimally with controlling the collimation of the two asymmetric beam components omitted from the type of high power laser diodes typically used in the laser wireless power transmission systems of the present disclosure. This problem arises primarily from the phenomenon associated with a difference between the origin points of the two asymmetric components of the laser diode output. The apparent emission point of the single mode beam component in the narrow, fast axis of the beam emitter, lies in the central volume of the gain medium waveguide, between the two end facets. On the other hand, the multiple mode laser output of the wide, slow axis of the waveguide appears to emerge from the end facet itself. This positional anomaly is independent of the widely different magnifications required for the two output modes, because of their widely different dimensions, and widely different divergences. Thus, in order to accommodate all of these impediments to the achievement of a well collimated beam of the desired shape and size over the intended range of the power transmission system, if a pair of cylindrical lenses are to be used for beam collimation, they must generate a combination of optical properties, not considered in prior art cylindrical lens combinations:

[0018] (i) The combined cylindrical lens pair must have anamorphic properties, i.e. the image magnification for the beam components in the fast and slow axes should be different, to take into account the different dimensions and the different divergences of the two beam components. The level of anamorphism required should be determined from the differences in the lateral elliptic dimensions of the two beam components and the difference in angular divergence of the two beam components, and the filling level desired of the photovoltaic cell, as compared to the beam output size. The required magnification will also be determined by the maximum range at which this beam size and shape is required. These effects are generally taken into account in prior art beach collimations arrangements.

[0019] (ii) However, the combined cylindrical lens pair must also have astigmatic properties, i.e. the focal points for the two orthogonal beam components in the fast and slow axes must be different, to take into account the different points of origin of the two beam components. This requirement distinguishes the systems of the present disclosure from those of the prior art, which only consider the above described (i) anamorphic properties. Without consideration of the compensation provided by the required astigmatic characteristics (ii), the provision of optimum efficiency of the power conversion of the beam at the photovoltaic cell is not enabled, since each of the first and second beam components would not undergo the level of collimation required to provide an optimally collimated beam simultaneously for both of the components.

[0020] To illustrate the quantitative importance of the astigmatic characteristic required of the cylindrical lens pair, it should be noted that the laser waveguide slab could be of the order of 2 to 4 mm. in length, such that the difference between the focal point of the cylindrical lens for collimating the single mode originating from the center of the slab length, as compared to that for collimating the multimode beam component originating from the end facet of the lasing slab, could be from 1 to 2 mm. When considering that the focal length of the lenses used may be 10mm. or less, the astigmatic correction would need to be 10 to 20% of the focal distance, which is a significant level, since not taking into account that 1 to 2mm difference in focal distance, will have a significant effect on the ability to collimate both components of the laser beam to the extent required. Consequently, the cylindrical lens pair should have astigmatic properties, which could be achieved either by changing the focal length of the lenses from that suggested by the difference in divergence of the two beam components, or by using the focal lengths of the lenses suggested by the difference in divergence of the two beam components, but adjusting the mutual position of the lenses such that they reflect the difference in the origin of the two distinct beam components.

[0021] Since the optical design of the beam shaper module of the present disclosure, is dependent not only on the optical parameters of each element making up the module, but also on the position of the optical elements in the beam shaping module, both relative to each other, and relative to the optical exit surface of the laser diode emitter, the optical characteristics of the beam shaping module are sensitive to temperature, which causes expansion or contraction of supporting structures in the module, and hence change in mutual position of the optical elements. Since the ultimate size and shape of the beam remote from the transmitter, is dependent on the optical characteristics of the beam shaping module, any change in those characteristics could generate change in the transmitted beam properties. Such changes could result in hazardous conditions of the system, since with change in temperature of the beam shaping module, the received beam may shift from its intended position and partly miss the target, or it could undergo different divergence, and spill over from the receiver into the room where the receiver is situated, and other such phenomena. Such errant beams could present a hazardous condition, and the system must be adapted to maintain safe operation by avoiding such conditions. The present system uses one or more temperature sensors located either on the laser diode itself , or on a part of the beam shaping module housing or on any one or more of the optical elements. A controller tracks the output signals of the temperature sensor or sensors, and if the signals indicate a temperature outside of the allowed temperature range of those measured components, the controller is configured to switch off the laser, or to reduce the power, or to stop the emitted beam from traversing into the environment around the receiver, such as by use of a shutter, or a beam diverter. In this way, the safety of the use of the beam shaping module is maintained.

[0022] There is therefore provided, in accordance with one exemplary implementation of the present invention, an optical wireless power transmitter adapted to transmit a laser power beam to a photovoltaic cell located at up to a maximum operating range from the transmitter, the transmitter comprising:

[0023] (i) a multimode laser diode having an emitter capable of emitting a laser beam comprising a first axis beam component having first beam divergence parameters, and diverging from a first axial location of the emitter, and a second axis beam component having second beam divergence parameters, and diverging from a second axial location of the emitter,

[0024] (ii) an optical beam shaper having:

[0025] (a) astigmatic optical power such that the first beam component diverging from the first axial location in the emitter, and the second beam component diverging from the second axial location in the emitter achieve predetermined levels of collimation at the maximum operating range of the transmitter, and

[0026] (b) anamorphic optical power such that the first axis beam component having the first beam divergence parameters and the second axis beam component having second beam divergence parameters undergo different magnifications, such that they attain predetermined levels of beam size at the maximum operating range of the transmitter,

[0027] (iii) at least one temperature sensor attached to at least one of the multimode laser diode and the beam shaping module,

[0028] (iv) a controller, adapted to receive an output of at least one temperature sensor, and

[0029] (v) at least one control element configured to control the beam output of the optical wireless power transmitter according to an instruction received from the controller, wherein the controller is configured to issue the instruction when at least one temperature sensor indicates that the temperature of either the laser diode or the beam shaping module are outside of a predetermined range.

[0030] In such an optical wireless power transmitter, the emitter of the multimode laser diode may have the form of a narrow slab whose end facet emits a laser beam having larger divergence in the direction of the narrow side of the emitter slab, than the laser beam emitted from the wider dimension of the emitter slab, and wherein the anamorphic optical power of the optical beam shaper is adapted to provide a greater level of collimation to the beam emitted from the narrow dimension of the slab, than the beam emitted from the wider dimension of the emitter slab. In such an optical wireless power transmitter, the astigmatic optical power of the optical beam shaper may be adapted to provide a similarly collimated level to the laser beam components emitted from the different axial locations in the emitter slab.

[0031] Furthermore, in any of such optical wireless power transmitters, the optical beam shaper having astigmatic optical power and anamorphic optical power may comprise a pair of sequentially located cylindrical lenses, aligned such that their cylindrical axes are perpendicular.

[0032] Additionally, the astigmatic optical power and the anamorphic optical power of the optical beam shaper may have levels such that the first axis beam component and the second axis beam component of the emitted laser beam generate a beam at the maximum operating range of the transmitter, having a predetermined shape and size, in accordance with the size and orientation of the photovoltaic cell when located at the maximum operating range of the transmitter. In such an optical wireless power transmitter, the predetermined shape and size of the beam is such that the beam provides a filling factor of the photovoltaic cell providing a desired level of conversion efficiency of the laser beam to electrical power.

[0033] There is further provided, according to yet another exemplary implementation of the present invention, an optical wireless power transmitter having an operating range such that it is adapted to transmit a laser power beam to a photovoltaic cell remotely located from the transmitter, the transmitter comprising:

[0034] (i) a multimode laser diode having an emitter capable of emitting a laser beam having a first axis of divergence and a second axis of divergence and a first beam shape,

[0035] (ii) an optical beam shaper, adapted to convert the divergence in the first axis, and the divergence in the second axis such that the beam acquires a second beam shape, having second divergence parameters, the second beam shape being larger than the first beam shape and the second divergence parameters being smaller than the divergences in the first axis and in the second axis, the optical beam shaper comprising optical elements configured to have different optical power in the first and second axes, such that the optical beam acquires similar dimensions in the first and second optical axis directions, and is adapted to create an enlarged image of the emitter at the operating range of the transmitter, and wherein the optical beam shaper has a level of astigmatism such that different virtual axial focal points of different components of the laser beam are collimated to the same extent,

[0036] (iii) at least one temperature sensor attached to at least one of the multimode laser diode and the beam shaping module,

[0037] (iv) a controller, adapted to receive an output of at least one temperature sensor, and

[0038] (v) at least one control element configured to control the beam output of the optical wireless power transmitter according to an instruction received from the controller, wherein the controller is configured to issue the instruction when at least one temperature sensor indicates that the temperature of either the laser diode or the beam shaping module are outside of a predetermined range. In such an optical wireless power transmitter, the at least one control element may be adapted to switch the laser diode off, or to reduce the laser power, or to divert or block the laser beam. In such an optical wireless power transmitter, an image of the emitter may be formed at a distance of at least 1 m from the optical beam shaper. Furthermore the optical magnification in the first axis may be different from the optical magnification in the second axis.

[0039] Additionally, in such optical wireless power transmitters, if the photovoltaic cell has a sensitive area A, the lateral dimension of the image of the emitter in any direction may be between 0.2*A0 5and 1.2* A0 5. Furthermore, the optical beam shaper should have an exit clear aperture of at least VA.

[0040] Finally, in any of the above described optical wireless power transmitters, the optical components of the optical beam shaper, their coatings and their clear apertures may be selected so that the transmission efficiency of the optical beam shaper is at least 60%. Additionally, the second divergence parameters in both the slow axis and the fast axis may advantageously be less than 20 milliradians.

[0041] There is further provided in accordance with another exemplary implementation of the presently described disclosure, a method of transmitting a laser power beam to a photovoltaic cell located at up to a maximum operating range from an optical wireless power transmitter, the method comprising the steps of:

[0042] (i) providing a multimode laser diode having an emitter capable of emitting a laser beam comprising a first axis beam component having first beam divergence parameters, and diverging from a first axial location of the emitter, and a second axis beam component having second beam divergence parameters, and diverging from a second axial location of the emitter; and

[0043] (ii) directing the laser power beam through an optical beam shaper positioned at the laser beam output, the optical beam shaper providing:

[0044] (a) astigmatic optical power such that the first beam component diverging from the first axial location in the emitter, and the second beam component diverging from the second axial location in the emitter achieve predetermined levels of collimation at the maximum operating range of the transmitter; and

[0045] (b) anamorphic optical power such that the first axis beam component having the first beam divergence parameters and the second axis beam component having second beam divergence parameters undergo different magnifications, such that they attain predetermined levels of beam size at the maximum operating range of the transmitter.

[0046] In such a method, the emitter of the multimode laser diode may have the form of a narrow slab whose end facet emits a laser beam having larger divergence in the direction of the narrow side of the emitter slab, than the laser beam emitted from the wider dimension of the emitter slab, and the anamorphic optical power of the optical beam shaper may be adapted to provide a greater level of collimation to the beam emitted from the narrow dimension of the slab, than the beam emitted from the wider dimension of the emitter slab. In such a case, the astigmatic optical power of the optical beam shaper may be adapted to provide a similarly collimated level to the laser beam components emitted from the different axial locations in the emitter slab.

[0047] In any of the above described methods, the optical beam shaper having astigmatic optical power and anamorphic optical power may comprise a pair of sequentially located cylindrical lenses, aligned such that their cylindrical axes are perpendicular.

[0048] Furthermore, In any of the above described methods, the astigmatic optical power and the anamorphic optical power of the optical beam shaper may be selected to have levels such that the first axis beam component and the second axis beam component of the emitted laser beam generate a beam at the maximum operating range of the transmitter, having a predetermined shape and size, in accordance with the size and orientation of the photovoltaic cell when located at the maximum operating range of the transmitter. In such a case, the predetermined shape and size of the beam may be such that the beam provides a filling factor of the photovoltaic cell providing a desired level of conversion efficiency of the laser beam to electrical power.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The presently claimed invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

[0051] Fig. 1A and Fig. 1 B show the placing of optical elements in the beam path from the laser diode, so that the beam profile is altered to fit the size and shape of the receiver target, and Fig. 2 is a schematic diagram of the control functions used to ensure the safety of a laser system using a beam-forming module of this disclosure, when the laser diode and its associated optics undergo temperature changes.

[0052] DETAILED DESCRIPTION

[0053] Reference is first made to Fig. 1A and Fig. 1 B which illustrate schematically an exemplary optical system for optically processing the anisotropic shape of the output beam of a multimode diode laser, typically having a fast axis divergence angle (at the 1 / e2point) of 30-50° and a slow axis divergence angle of 5-15° (also at the 1 / e2point). The emitter is typically less than a micron thick in the fast axis and 30-80 micron wide in the slow axis, and generates a multimode beam. Fig. 1A shows the optical system viewed from the narrow, side dimension of the laser slab, showing the optical manipulation of the beam emitted in the fast axis, while Fig. 1 B shows the optical system viewed from the top, broad dimension of the laser slab, showing the optical manipulation of the beam emitted in the slow axis,

[0054] A first cylindrical lens 10 is placed close to the emitter, generating an image of the emitter at a range R, of approximately 10m, this being the typical range limit of the beam transmission for a domestic or commercial setting. Immediately after the first lens, an asymmetric, approximately rectangular image of the emitter is formed. A second cylindrical lens 11 , aligned with its cylindrical axis perpendicular to that of the first lens, then converts the approximately rectangular image into an image having a shape closer to a square shape, with similar x and y dimensions, as required to match the general shape of the photovoltaic cell, whether square or circular.

[0055] The typical dimension in the x and y directions of the beam, is adjusted to be approximately 60% of the lateral dimensions of the photovoltaic cell, whether approximately square or circular, so that the beam has the following properties:

[0056] 1. At maximal beam diameter (typically at maximum range) and at maximal operational tilt of the receiver with respect to the beam, the beam width is approximately equal to the photovoltaic cell dimensions.

[0057] 2. At minimal tilt of the receiver with respect to the beam, i.e. at normal incidence, and at minimal beam diameter, the beam width is approximately 60% of the photovoltaic cell dimensions. In these beam properties, it is to be understood that the terms beam “width” and cell “width” are intended to be illustrative of the lateral dimensions of the beam and cell respectively, whether considered as the width or the height or the diameter of the beam and cell dimensions respectively.

[0058] Details of one exemplary implementation of such an optical beam shaper is shown in Figs. 1A and 1 B, viewed respectively from the directions parallel and perpendicular to the fast axis of the laser diode. Fig. 1 A shows a view of the imaging effect of the optical system on the divergence in the fast axis, while Fig. 1 B shows a view of the imaging effect of the same optical system on the divergence in the slow axis. A first fast axis cylindrical collimator lens 10, is placed close to the emitter 15, followed by a slow axis perpendicularly aligned cylindrical collimator lens 11 , which is placed at a further distance, such that the each lens generates collimation of the beam from its diverging form, in the direction of the cylindrical power of the lenses. The combined result of these lenses is to generate an image of the emitter at a distance R of up to several meters, (more accurately, at a distance typically in the order of, or slightly greater than the system’s half range), having the approximate dimensions of the target, as detailed below.

[0059] The beam profile leaving the emitter is a thin line. At a distance of between half of the system range and three times the system range, the two or more lenses act together as an anamorphic and astigmatic, compound lens system, that converts the long rectangular emitter shape, typically of the order of 1 micron x 50 micron, into an image, having an approximately square shape 26, with similar x and y dimensions. The typical x or y dimensions of the beam are adjusted to be approximately no more than 60% of the dimension of the photovoltaic cell, which is typically also approximately square. The term “approximately square” or similar terms used in this disclosure, is intended to mean a rectangular type of shape having an aspect ratio at least approximating that of a square, for instance, of between 0.5 to 2. This broad definition of a “square-like” shape is intended to indicate the great difficulty of designing an optical system which can take a rectangular beam having an aspect ratio of the order of 50, and with different divergences in its two orthogonal directions, and originating from different axial locations relative to the lens assembly, to convert it at a remote distance into a near square shape, where near square means anything rectangular or elliptical, or roundish, having an aspect ratio approximately as stated above. This optical arrangement results in the typically rather square shape of the beam, close to the transmitter’s emitter, gradually converting into a more closely circular beam shape close to the half way point to the image, and a squarish shape again at the image point.

[0060] Since the compound lens should be astigmatic, as well as anamorphic, the “focused” image of the fast axis does not necessarily occur in the same distance as the “focused” imgage of the slow axis. The word image is used herein to indicate the position of the image as can be determined by optical calculations for both the fast and the slow axis. The astigmatism of the lens assembly considered as a single optical module, can be generated by positioning the lenses such that their virtual focal points generating a collimated beam, are not at the same output aperture position of the laser diode, but that the lens collimating the fast axis mode is positioned such that its virtual focus is situated within the axial center of the lasing slab. Thus, even though each lens may be a regular cylindrical lens, the lens pair combination can be considered to be an astigmatic beam shaper, since as a single optical module, it focusses light at different focal lengths depending on the orientation of the light passing therethrough.

[0061] In conclusion, the optical system is adapted to create a corrected image of the laser diode at a remote distance, typically up to the maximum operational range of the system. By virtue of the optical system having predetermined values of astigmatism and anamorphism, the resulting compound lens appears as having different position, magnification, and / or focal length in the fast axis compared to the position, magnification, and / or focal length in the slow axis. This results in an image of the laser diode emitter being “stretched” more in the fast axis, than in the slow axis. The values of anamorphism are chosen such that the resulting image of the laser diode emitter is approximately square-shaped, such that it matches the shape of the photovoltaic cell area, while the level of astigmatism is chosen to ensure that each component of the lasing beam is collimated to the same final beam shape despite the two components not originating from the same axial position in the laser source.

[0062] Reference is now made to Fig. 2, which illustrates schematically an additional and optional feature of a laser system having the beam shaping module of the present disclosure, in order to deal with a potential safety issue arising specifically from the use of such a beam shaping module. The operating laser diode 15 generates heat, which increases the temperature of the whole beam shaping module 20. As is known in the art, a change in the laser diode temperature may change the wavelength of the beam. But besides that effect, and of particular importance to the effect of temperature on the beam shaping modules described in the present disclosure, the heat may also cause the expansion or contraction of the distances between the optical components themselves of the beam shaping module, and of the distance between the module and the laser diode. Changes such as these may affect the collimation of the components of the laser beam, may affect the position and quality of the image, and may result in beam shapes which the system was not adapted to take account of. As a result, the beam may become hazardous, and the safety of system may be compromised. This can be a result of excessive heating or cooling of the laser beam shaping module and its components.

[0063] Therefore, in order to handle the possible outcomes of these features, the currently described system may be equipped with one or more temperature sensors 21 , and one or more switches or control elements 25, allowing automatic shutdown of the laser diode, or at least a reduction in the power of the beam emitted, as a function of the measured laser diode temperature, or the temperature of any other part of the beam shaping module, if those measured temperatures depart from an allowed predetermined range.

[0064] Referring now to Fig. 2, the laser diode beam emitter 15 is shown transmitting its beam through the lens system 10, 11 , for processing the optical beam shape, and the amended beam is transmitted downrange R to the photovoltaic cell 26 in the remote receiver. The beam shaping module 20 is shown in Fig. 2 as having succeeded in forming a beam which essentially covers a large part of the sensitive area of the photovoltaic cell 26.

[0065] The laser diode 15 is fitted with at least one temperature sensor 21 , shown in Fig. 2 as attached to the laser diode, which provides a signal input to the controller 22, reflecting the temperature of the laser diode, and hence of the interior of the beam forming module 20. Temperature sensors (not shown in Fig. 2) may alternatively or additionally be disposed directly on the one or more parts or components of the beam shaping module itself. The controller 22 is configured to output at least one and preferably at least two command signals, which activate switches 25 to cause the laser diode to cease emitting its beam, orto safely control the beam level or beam direction. In the exemplary system of Fig. 2, one such command signal is operative on the laser diode power supply 23, causing it to reduce or to terminate the laser beam. An alternative method may be for a controller output signal to open the switch 25 supplying the mains supply 24 to the power supply, such that laser operation is interrupted. In Fig. 2, this switch is shown schematically as a circuit breaker, but any electronically controlled switch could also be used. Furthermore, the safety operations are shown with the controller terminating or reducing the laser beam power, but it is to be understood that any other method of ensuring safety if the temperature shifts out of the permitted range, may also be used, such as a controller operated a shutter, or a controlled beam deflector that directs the beam to a beam dump. In this respect, the term “switch” is used in this disclosure not only in the conventionally understood ON-OFF type of operation, but also for any mechanism which “switches” or changes the condition, level, transmission direction, or any other similar characteristic of the beam.

[0066] When the temperature measured by the sensor 21 is above (or below) a certain operating threshold, the switches shown in Fig. 2 disconnect the laser diode to prevent the system from operating with an unplanned, and possibly hazardous beam shape. The provision of two or more switches or methods of controlling the emitted beam, enables fail-safe disconnection of the laser diode, as is mandated in many regulatory references.

[0067] Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. Furthermore, 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.

Claims

CLAIMSWe claim:1 . An optical wireless power transmitter adapted to transmit a laser power beam to a photovoltaic cell located at up to a maximum operating range from the transmitter, the transmitter comprising: a multimode laser diode having an emitter capable of emitting a laser beam comprising a first axis beam component having first beam divergence parameters, and diverging from a first axial location of the emitter, and a second axis beam component having second beam divergence parameters, and diverging from a second axial location of the emitter; an optical beam shaper having: astigmatic optical power such that the first beam component diverging from the first axial location in the emitter, and the second beam component diverging from the second axial location in the emitter achieve predetermined levels of collimation at the maximum operating range of the transmitter; and anamorphic optical power such that the first axis beam component having the first beam divergence parameters and the second axis beam component having second beam divergence parameters undergo different magnifications, such that they attain predetermined levels of beam size at the maximum operating range of the transmitter; at least one temperature sensor attached to at least one of the multimode laser diode and the beam shaping module; a controller, adapted to receive an output of at least one temperature sensor; and at least one control element configured to control the beam output of the optical wireless power transmitter according to an instruction received from the controller; wherein the controller is configured to issue the instruction when at least one temperature sensor indicates that the temperature of either the laser diode or the beam shaping module are outside of a predetermined range.

2. An optical wireless power transmitter according to claim 1 , wherein the emitter of the multimode laser diode has the form of a narrow slab whose end facet emits a laser beam having larger divergence in the direction ofthe narrow side of the emitter slab, than the laser beam emitted from the wider dimension of the emitter slab, and wherein the anamorphic optical power of the optical beam shaper is adapted to provide a greater level of collimation to the beam emitted from the narrow dimension of the slab, than the beam emitted from the wider dimension of the emitter slab.

3. An optical wireless power transmitter according to claim 2, wherein: the astigmatic optical power of the optical beam shaper is adapted to provide a similarly collimated level to the laser beam components emitted from the different axial locations in the emitter slab.

4. An optical wireless power transmitter according to any of the previous claims, wherein the optical beam shaper having astigmatic optical power and anamorphic optical power comprises a pair of sequentially located cylindrical lenses, aligned such that their cylindrical axes are perpendicular.

5. An optical wireless power transmitter according to any of the previous claims, wherein the astigmatic optical power and the anamorphic optical power of the optical beam shaper have levels such that the first axis beam component and the second axis beam component of the emitted laser beam generate a beam at the maximum operating range of the transmitter, having a predetermined shape and size, in accordance with the size and orientation of the photovoltaic cell when located at the maximum operating range of the transmitter.

6. An optical wireless power transmitter according to claim 5, wherein the predetermined shape and size of the beam is such that the beam provides a filling factor of the photovoltaic cell providing a desired level of conversion efficiency of the laser beam to electrical power.

7. An optical wireless power transmitter having an operating range such that it is adapted to transmit a laser power beam to a photovoltaic cell remotely located from the transmitter, the transmitter comprising:a multimode laser diode having an emitter capable of emitting a laser beam having a first axis of divergence and a second axis of divergence and a first beam shape; an optical beam shaper, adapted to convert the divergence in the first axis, and the divergence in the second axis such that the beam acquires a second beam shape, having second divergence parameters, the second beam shape being larger than the first beam shape and the second divergence parameters being smaller than the divergences in the first axis and in the second axis, the optical beam shaper comprising optical elements configured to have different optical power in the first and second axes, such that the optical beam acquires similar dimensions in the first and second optical axis directions, and is adapted to create an enlarged image of the emitter at the operating range of the transmitter, and wherein the optical beam shaper has a level of astigmatism such that different virtual axial focal points of different components of the laser beam are collimated to the same extent; at least one temperature sensor attached to at least one of the multimode laser diode and the beam shaping module; a controller, adapted to receive an output of at least one temperature sensor; and at least one control element configured to control the beam output of the optical wireless power transmitter according to an instruction received from the controller; wherein the controller is configured to issue the instruction when at least one temperature sensor indicates that the temperature of either the laser diode or the beam shaping module are outside of a predetermined range.

8. An optical wireless power transmitter according to claim 7, wherein the at least one control element is adapted to switch the laser diode off.

9. An optical wireless power transmitter according to claim 7, wherein the at least one control element is adapted to reduce the laser power.

10. An optical wireless power transmitter according to claim 7, wherein the at least one control element is adapted to divert or to block the laser beam.

11. An optical wireless power transmitter according to any one of claims 7 to 10, wherein an image of the emitter is formed at a distance of at least 1 m from the optical beam shaper.

12. An optical wireless power transmitter according to any one of claims 7 to 11 , wherein the optical magnification in the first axis is different from the optical magnification in the second axis.

13. An optical wireless power transmitter according to any one of claims 7 to 12, wherein the photovoltaic cell has a sensitive area A, and the lateral dimension of the image of the emitter in any direction is between 0.2*A°5and 1 .2* A05.

14. An optical wireless power transmitter according to any one of claims 7 to 13, wherein the photovoltaic cell has a sensitive area A, and the optical beam shaper has an exit clear aperture of at least 4.

15. An optical wireless power transmitter according to any one of claims 7 to 14, wherein the optical components of the optical beam shaper, their coatings and their clear apertures are selected so that the transmission efficiency of the optical beam shaper is at least 60%.

16. An optical wireless power transmitter according to any one of claims 7 to 15, wherein the second divergence parameters in both the slow axis and the fast axis are less than 20 milliradians.

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