Aberrative retro-reflector for wireless power applications

Abstract

A wireless optical power transmission system, comprising a transmitter outputting an optical beam of power through an optical aperture, the transmitter comprising (a) a laser generating the optical beam of power, (b) a deflecting module for directing the optical beam of power, and (c) a controller adapted to adjust the deflecting module. The system also has a receiver comprising a retroreflector having a cat's eye configuration, adapted to return a retroreflected beam from at least a part of the optical beam of power incident thereon, back towards the transmitter optical aperture. The cat's eye retroreflector should have a level of optical performance that introduces aberrations into the retroreflected beam, the level of aberrations being such that the retroreflected beam has a divergence sufficiently large that the 1 / e2 diameter of the retroreflected beam at the optical aperture is larger than the optical aperture diameter.

Description

[0001] ABERRATIVE RETRO-REFLECTOR FOR WIRELESS POWER APPLICATIONS

[0002] FIELD

[0003] The present disclosure describes technology related to the field of wireless power transmission using an optical beam of power, and especially to the requirements of the retroreflector used by many such systems to ensure accurate locating and tracking of a receiver of the transmitted power.

[0004] BACKGROUND

[0005] An important feature of optical wireless power transmission systems is the need for the system to be able to locate receivers, track them and control the incidence of the optical beam of transmitted power on the element in a receiver which absorbs the transmitted power beam, which is usually a photovoltaic cell or an array of photovoltaic cells. This feature has a number of important functions in such an optical power transmission system.

[0006] In the first place, this feature is used in order to determine when a receiver has been detected and when the beam is preferably centered on the absorbing element, or at least that a major part of the beam power impinges on the power absorbing element. This task is an essential safety procedure and is generally performed by transmitting the power beam in a safe, low power mode to scan the area in which receivers are expected to be found, using a beam directing device such as a scanning mirror or a MEMS -based scanning device. This feature can be used to ensure that the beam has homed onto the receiver power absorber before the system is allowed to raise the power of the beam to a level enabling the transfer of useful power to the receiver, the higher level transmissions being typically made safe by more advanced mechanisms.

[0007] In addition, once the control system has enabled the wireless power beam to lock onto the receiver absorber, the beam aiming device may be controlled so as to follow the motion of the receiver around the area covered by the power transmitter, or to reduce the beam’s power or turn it off completely if the receiver has moved out of the service area of the system, in order to maintain power transmission and to maintain the safety of the system.

[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] SUBSTITUTE SHEET (RULE 26) SUMMARY

[0010] The present disclosure attempts to provide novel systems and methods for using a retroreflector to track the incidence of a beam of optical power on a receiver absorber element, that overcomes at least some of the disadvantages of prior art systems and methods.

[0011] A common method of enabling a wireless power system to lock the beam onto the receiver absorber, and to follow the motion of the receiver around the area covered by the power transmitter, or to reduce the beam’s power or turn it off for safety if the receiver has moved out of the service area of the system, is by using a retroreflector mounted on the receiver, which, when the transmitted beam impinges on it, reflects a beam back towards the transmitter, generally accurately collinear with the incident beam but in the reverse direction. That retroreflected beam is received at an optical aperture of the transmitter, and is used to provide the input to the control system, which determines such features as the direction in which to control the aim of the incident power beam, and the power which the transmitter is allowed to send towards the receiver. Since the retroreflector should be positioned at or near the center of the power receiving aperture, so that it can accurately determine when the transmitted power beam is incident on the power absorber element of the receiver, the retroreflector should have an area substantially smaller than that of the input aperture of the power receiving element, so that the major part of the incident power beam is absorbed by the power receiving element, in order to perform its primary function of transferring power to the receiver Only a minor portion of the incident beam is retroreflected back towards the transmitter. Such an arrangement is shown, for instance, in the PCT international patent application, published as WO 2016 / 125156, for “Distributed Optical Resonator with Thin Receiver Unit”, commonly owned by the present applicant and having a common inventor. In that reference, Figs. 5A to 5D show such a retroreflector covering only a small part of the input aperture of the photovoltaic absorber cell. Typically, the area of the retroreflector is such that it covers between 5% and 40% of the area of the power absorbing input aperture, though applications with an even smaller or larger percentage can be usefully deployed.

[0012] Because of the small size of the retroreflector, the beam retroreflected back towards the transmitter input aperture is very narrow, and can be as small as 1 mm or less in diameter for an incident power beam having a diameter of the order of a centimeter or more. For this reason, the retroreflectors generally used have high optical quality, with as low a level of optical aberrations as possible, in order to keep the retroreflected beam as collimated as possible. In order to keep the complexity and cost of the retroreflector within the limits required for a consumer product such as a wireless charging system for mobile devices like cellular phones or laptop computers, a simple spherical ball cat’s eye retroreflector, as is well known in the prior art, is often used as the retroreflector basis. The use of such a cat’s eye retroreflector, having low aberration, would seemingly enable optimum control over the size and divergence of the retroreflected signaling beam, thereby increasing aiming accuracy. Thus, for instance, in International Patent Publication WO 2012 / 172541 for “Spatially Distributed Laser Resonator”, commonly owned by the present applicant, there are described some of the problems to be contended with when using a beam with aberrations. Beam aberrations were therefore considered in prior art transmission systems, to be an unwanted hindrance to the operation of a laser power transmission system.

[0013] However, it has been found in practice, that optical wireless power transmission systems with low aberration retroreflectors, such as those using a cat’s eye retroreflector, do not function well enough to locate and track a receiver with the high accuracy desired, nor can they be used to precisely fulfill the safety device requirements of such systems. Unlike the wide beam illumination with RF powering systems, optical wireless power systems illuminate the absorber target on the receiver, typically a photovoltaic cell, with a very narrow beam emitted from the transmission lens in an optical exit aperture of the transmitter unit. That part of the incident beam reflected from the optical wireless power system retroreflector, which is usually an even narrower beam than the incident power beam, is collected by the transmitter, in order to enable the system to determine the precise angular location of the receiver’s absorbing photovoltaic cell, in some cases also to assist in laser formation and in wavelength tuning, and to provide a safety indication in the event of an object entering the beam path causing a change in the reflected optical signal, as described hereinbelow. This feature of keeping the power beam centered on the power absorbing device in the receiver enables a measurement of the difference between (i) the power of the beam when transmitted from the transmitter and (ii) the power of the beam received back at the transmitter after being retroreflected from the receiver, to provide information as to whether any object has intruded into the beam path, which would be indicative of a serious beam hazard situation. Only if the control system can maintain the incident beam at a fairly constant spatial position on the absorber device, does such a measurement have any reliable meaning.

[0014] While locating the direction of a receiver may be done at the transmitter using a different lens from that used to transmit the incident power beam, the safety feature of verifying that no object is present in the beam path is best implemented if collection of the retroreflected light is done using the same lens and specifically, using the same clear aperture portion of the lens for the outgoing and incoming beams. This can be achieved by using a beamsplitter behind the beam aperture of the transmitter, to divert the returning retroreflected beam to a power meter where its power level can be monitored, and to a position detector behind a pinhole to determine where the retroreflected beam is impinging, the control system adjusting the beam steering mirror until the retroreflected beam is at its predetermined central position. This predetermined central position then also enables accurate monitoring for beam intrusion, since the retroreflected beam always impinges at the same position on the input aperture of the transmitter.

[0015] It is conjectured that the use of retroreflectors having low optical aberrations and high beam quality, generating narrow and close-to-collimated retroreflected beams, may be the cause for some of the problems arising in previously available optical wireless power transmission systems. Such problems arise from the comparative ease by which the retroreflected beam can miss falling within the input aperture of the transmitter lens as a result of causes such as beam deviations, or vibration or small or fast motions of the receiver incorporating the retroreflector, as is expected, for instance, from a hand-held device. In order to overcome these problems, such previously described systems need to use a comparatively large transmitter optical aperture, in order to accept the retroreflected beam into the aperture, even if the retroreflected beam is not incident on the optical axis of the aperture, because of potential deviation of the retroreflected beam.

[0016] The presently described optical wireless power transmission systems differ from previously available systems, in that the retroreflector used to return the retroreflected location indicating beam to its detector within the transmitter optical aperture, is either or both of (i) a purposely defocused retroreflector or (ii) a retroreflector having an intentionally increased level of optical aberration, over that of a retroreflector having a reflected beam with an optimum optical quality, as is generally used in currently available power transmission systems. The increased level of aberration may advantageously be an increased level of spherical aberration, but the addition of any higher order rotationally symmetric aberrations can also be used to degrade the optical quality of the retroreflector. Any of the above mentioned degradations of the retroreflector result in a retroreflected beam having an increased divergence, rather than the beams returning from the previously preferred retroreflectors, which have optical characteristics optimized to provide the best beam collimation possible. As a result, the reflected beam has a larger area when it reaches the transmitter entrance aperture, and the motion or vibration of the retroreflected beam does not prevent the detection of such a larger sized beam by the detector in the optical aperture of the transmitter. Once at least a part of the retroreflected beam is incident within the optical aperture of the transmitter, it then becomes possible to capture the position of the axial center of the retroreflected beam, and using a position detector such as a quadrant detector behind a pin-hole beyond the beam aperture, to provide a control signal to the beam transmitting scanning device, to center the beam on the receiver input aperture, and thereby to track the position of the receiver as it moves. Such a pinhole position sensor is only a simple version of a position tracking configuration, and more complex optical systems may be used. As an alternative to the use of a pinhole for selecting a small portion of the retroreflected beam for position tracking, a beam splitter may be used for deflecting a small part of the retroreflected beam to a position detector.

[0017] Use of a less than optimally designed retroreflector, resulting in a more diverging retroreflected beam means that the increased size of the retroreflected beam at the transmitter is such as to enable the use of a smaller optical aperture of the transmitter, since even if the retroreflected beam is off axis relative to the input aperture, the edge of the larger divergent beam diameter will enable at least part of the beam to be detected. Furthermore, this can be achieved using a smaller diameter optical aperture A prior art system with a narrow retroreflected beam transmitted at the same angle of deviation from the transmitted beam axis as a more divergent beam, would require a larger input aperture in order to be detected. The presently described system, using a more divergent beam, can use smaller diameter optics, both for the beam transmission and for detecting the retroreflected beam, such use of smaller diameter optical elements representing a significant cost saving. The optimum extent of the planned divergence of the retroreflected beam is dependent on such factors as the range of the system, the sensitivity of the detector used to detect the returned beam, and the level of stray light projected onto the detector, either as a result of radiation from external sources such as the sun, but primarily as a result of internal reflections of the laser beam within the transmitter. The range of the system is understood to be the maximum range out to which the system is intended to be able to provide a power beam to the receiver.

[0018] A common cat’s eye retroreflector structure of a positive lens disposed in front of a reflective concave mirror, is most easily implemented by using a glass sphere, or two hemispherical glass elements bonded together, and with a reflective rear surface, such that the focal point of the lens and of the rear concave mirror are confocal. The spherical aberration of the retroreflector can be adjusted by using a slightly aspherical form of the incident lens surface of the retroreflector, or by using an aspherical reflective rear surface of the retroreflector, or by using an optical material having a slightly different refractive index from that required to provide optimum optical performance, or by using a slightly different radius of one of the hemispherical elements making up the cat’s eye retroreflector, or a combination of more than one method. Whatever method is used, it is important to keep non-spherical aberrations to as low a level as is possible, to ensure the ease and reliability of tracking the position of the retroreflected beam. In this context, it is disadvantageous to use comer cube retroreflectors in the presently described systems, since they may introduce higher levels of non-spherically symmetric phase and positional aberrations.

[0019] There is thus provided in accordance with an exemplary implementation of the systems and devices described in this disclosure, a wireless optical power transmission system, comprising:

[0020] (i) a transmitter outputting an optical beam of power through an optical aperture, the transmitter comprising:

[0021] (a) a laser generating the optical beam of power,

[0022] (b) a deflecting module for directing the optical beam of power,

[0023] (c) a controller adapted to adjust the deflecting module, and

[0024] (ii) a receiver comprising a retroreflector having a cat’s eye configuration, adapted to return a retroreflected beam from at least a part of the optical beam of power incident thereon, back towards the transmitter optical aperture, wherein: the cat’s eye retroreflector has a level of optical performance that introduces aberrations into the retroreflected beam, the level of aberrations being such that the retroreflected beam has a divergence sufficiently large that the 1 / e2diameter of the retroreflected beam at the optical aperture is larger than the optical aperture diameter.

[0025] In any such a wireless optical power transmission system, the 1 / e2diameter of the retroreflected beam should be at least 1.5 times the optical aperture diameter. Alternatively, it should be at least twice the optical aperture diameter.

[0026] Furthermore, the major part of the aberrations may comprise radially symmetric aberrations or defocus, and the major part of the aberrations may be spherical aberrations. In any such wireless optical power transmission systems, the cat’s eye retroreflector may comprise a transparent sphere having a refractive index configured to be 1 + n, where n is the refractive index of the medium from which the optical beam of power is incident, the sphere having a reflective coating on the surface opposite to that on which the optical beam of power impinges. In such a system, if the medium from which the optical beam of power is incident is air, the refractive index of the transparent sphere should be 2.

[0027] In any of the above described wireless optical power transmission systems besides those comprising a transparent sphere, the cat’s eye retroreflector may comprise a pair of separate optical elements, the element on which the optical beam of power impinges having a positive focal length and a second element having a reflective surface positioned at the focal plane of the first element. In such systems, the cat’s eye retroreflector may comprise a pair of hemispherical spheres in optical contact on their plane faces, each of the hemispheres having different radii. In such systems, the hemisphere on which the optical beam of power impinges should have a radius smaller than that the hemisphere opposite to that on which the optical beam of power impinges, the ratio of the radii of the hemispheres being equal to n - 1, where n is the refractive index of the material of the hemispheres.

[0028] Additionally, in any of the above described wireless optical power transmission systems, the aberrations introduced into the retroreflected beam may be such that the retroreflected beam has a divergence sufficiently large that the optical aperture captures at least a part of the retroreflected beam, even in a situation when, if the retroreflector had had optimally minimal aberrations, no part of that retroreflected beam would be captured by the optical aperture.

[0029] Furthermore, the level of optical performance of the retroreflector should be less than that of a retroreflector having an optimally minimum aberration level.

[0030] Additionally, in any of the above described wireless optical power transmission systems, the retroreflector is the sole retroreflector on the receiver.

[0031] There is further provided in accordance with another exemplary implementation of the systems and devices described in this disclosure, a cat’s eye retroreflector of a wireless power transmission system, adapted to be illuminated by a first beam transmitted through an aperture of the transmitter, and configured to retroreflect a second beam essentially back towards the aperture, wherein the retroreflector has an aberration that imparts to the second beam an aberration level comprising at least 65% of at least one of radially asymmetric aberrations or defocus, such that the 1 / e2diameter of the second beam at the transmitter is larger than the aperture of the transmitter. The aberration level of the retroreflector may be selected intentionally for that purpose.

[0032] According to yet another exemplary implementation of the systems of the present disclosure, there is further provided a cat’s eye retroreflector for wireless optical power transmission system, adapted to be illuminated from a transmitter by a first optical beam emitted through an optical aperture of the transmitter, and retroreflecting a second beam essentially back towards the aperture, wherein when the cat’s eye retroreflector is off-axis of the first beam, the retroreflected second beam does not fully overlap the first beam and is only partially transmitted through the optical aperture of the transmitter, and wherein the extent of overlap is dependent at least in part on the level of aberration induced into the retroreflected beam by the cat’s eye retroreflector, and wherein the cat’s eye retroreflector has an aberration level greater than that of a second retroreflector that would generate an optimally collimated retroreflected beam, such that the optical aperture of the transmitter would capture the retroreflected second beam from the cat’s eye retroreflector for a greater off-axis distance of the retroreflector than for the optimally collimated retroreflected beam of the second retroreflector.

[0033] Using such a cat’s eye retroreflector, for the same off-axis distance of the cat’s eye retroreflector as the second retroreflector generating the optimally collimated retroreflected beam, the aperture size of the transmitter can be made smaller than that of a system using the second retroreflector that would generate an optimally collimated retroreflected beam.

[0034] Additional inventions that may be implemented in systems or devices described in the present disclosure, may include at least some of:

[0035] A retroreflector for wireless power transmission system, which, when illuminated by an off- axis location of a first beam transmitted from a source aperture, retroreflects a second beam back towards the source aperture, wherein the aberrations induced to the retroreflected beam by the retroreflector are such that when the diameter of the retroreflector is less than 50% of the l / e2 diameter of the first beam, the retroreflected beam is still detected at the source, even if the retroreflector is positioned at least one diameter of the retroreflector from the axis of the first beam, but not more than 4 diameters of the retroreflector from the axis of the first beam.

[0036] A retroreflector for wireless power transmission system, which, when illuminated by an off- axis location of a first beam transmitted from a source aperture, retroreflects a second beam back towards the source aperture in a direction not counter propagated to the first beam, wherein the aberrations induced to the retroreflected beam by the retroreflector are such that when the diameter of the retroreflector is less than 50% of the l / e2 diameter of the first beam, the retroreflected beam is still detected at the source, even if the retroreflector is positioned at least one diameter of the retroreflector from the axis of the first beam, but not more than 4 diameters of the retroreflector from the axis of the first beam.

[0037] In such a retroreflector, the source aperture and the transmitted beam are of sizes such that the aperture transmits less than 98% of the source beam incident on it.

[0038] A retroreflector for use with a wireless power transmission system, which, when illuminated by a first beam transmitted through a source aperture of the transmitter, retroreflects a second beam towards the source aperture of the first beam, wherein the retroreflector is an aberrative element, introducing into the second beam at least one of radially symmetric aberrations and defocus.

[0039] A retroreflector for use with a wireless power transmission system, which, when illuminated by a first beam transmitted through a source aperture of the transmitter less than 98% of the beam, retroreflects a second beam towards the source aperture of the first beam, wherein the retroreflector is an aberrative element, introducing into the second beam at least one of radially symmetric aberrations and defocus.

[0040] A retroreflector for wireless power transmission system, illuminated by a first beam through an aperture, the aperture transmitting less than 98% of the source beam incident thereon, and retroreflecting a second beam towards the source of the first beam, wherein the second beam does not fully overlap with the first beam and is only partially transmitted through the aperture.

[0041] In such a retroreflector, at least 1% of the second beam is transmitted through the aperture. In such a retroreflector, at least 10% of the second beam is transmitted through the aperture.

[0042] A detector for a retroreflected beam in a wireless power transmission system transmitting a laser beam through an aperture which allows less than 98% of the beam power to pass through it, the laser beam being directed towards a remote point a predetermined longitudinal distance from the transmitter, where a receiver having a retroreflector is likely to be located, wherein the laser axis does not impinge directly on the retroreflector, but only an edge of the laser beam impinges on the retroreflector, the retroreflector having a defocusing effect on its retroreflected beam, and wherein the retroreflected beam passes through the aperture where it is detected by the detector, the detector being configured to detect the signal retroreflected from the defocusing retro reflector when the defocusing retroreflector is positioned at least 0.25*r from the laser beam’s axis, where r is the 1 / e2radius of the laser beam when exiting the transmitter, but wherein the detector is configured to modify or reduce or terminate the power of the laser beam when the retroreflector is 2.5*r or more from the axis of the laser beam.

[0043] A detector for a retroreflected beam in a wireless power transmission system transmitting a laser beam through an aperture which allows less than 98% of the beam power to pass through it, the laser beam being directed towards a remote point a predetermined longitudinal distance from the transmitter, where a receiver having a retroreflector is likely to be located, wherein the laser axis does not impinge directly on the retroreflector, but only an edge of the laser beam impinges on the retroreflector, the retroreflector having a defocusing effect on its retroreflected beam, and wherein the retroreflected beam passes through the aperture where it is detected by the detector, the detector being configured to detect the signal retroreflected from the defocusing retro reflector when the defocusing retroreflector is positioned at least 0.25*r from the laser beam’s axis, where r is the 1 / e2radius of the laser beam when exiting the transmitter, wherein the detection of the retro reflected signal enables the wireless power transmission system to improve the aim of the laser beam onto the receiver.

[0044] BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Fig.lA illustrates schematically a prior art laser-based wireless power transmission system, in which a well collimated incident beam from the transmitter is shown impinging on the photovoltaic cell and retroreflector in the receiver;

[0046] Fig. IB shows the same system as that of Fig. 1A, but includes a pinhole located behind the transmitter output lens, which samples the retroreflected beam and uses a position detector to provide information about the spatial orientation of the receiver and its retroreflector;

[0047] Figs. 2A and 2B illustrate an optical wireless transmission system retroreflecting a beam from the incident optical power beam, which converges towards the beam waist, and then diverges beyond the waist; Fig. 2A shows the case of the receiver being positioned after the beam waist, and Fig. 2B shows the case of the receiver being positioned before the beam waist;

[0048] Fig. 3A shows a prior art retroreflector, generating a well collimated retroreflected beam, with the result that when the beam axis is offset from the retroreflector, a large transmitter working aperture with a larger diameter lens, is required in the transmitter in order to increase the likelihood of capturing the retroreflected beam;

[0049] Fig. 3B shows the same situation as that of Fig. 3A, but using a smaller diameter lens, as a result of which, the retroreflected beam of Fig. 3A is not captured;

[0050] Fig. 3C shows the situation where a more aberrated retroreflected beam is used, enabling capture of the larger diameter of the more divergent retroreflected beam, with a smaller input aperture, and hence a less expensive lens system;

[0051] Fig. 4 illustrates a prior art retroreflector with no intentionally generated defocusing effects, having a front sphere that generates a focal point on the reflective rear surface of the back sphere;

[0052] Fig. 5 illustrates schematically a plot of the percentage of optical power transmitted by a lens as a function of the relative area of the lens; and

[0053] Fig. 6 illustrates a graphical example of the use of a ball retroreflector positioned off- axis from a transmitted beam, the graph showing the extent of deviation from the center of the transmitter aperture of the retroreflected beam spot, and the retroreflected beam spot size, as a function of the distance of the retroreflector along the beam path.

[0054] DETAILED DESCRIPTION

[0055] Reference is first made to Fig. 1A, which illustrates schematically a prior art laser-based wireless power transmission system, in which a well collimated incident beam 11 from the transmitter 10 is shown impinging on the photovoltaic cell 12 and the retroreflector 13 in the receiver. The retroreflector 13 may be positioned on the edge of the region of impingement of the incident power beam, such that the retroreflected beam 14, which is reflected generally parallel but not overlapping the incident power beam, and which is much weaker than the incident beam, may be near the edge of the clear aperture of the transmitter optical system, such that part of it may be truncated and lost to the position detector behind the optical system clear aperture. Furthermore, any small movement of the receiver may cause the retroreflective beam to completely miss the input aperture of the transmitter, causing a break in the transmission of power to the receiver, and the need for the transmitter beam directing system, to search for the receiver again with a lowered power scanning beam.

[0056] Fig. IB shows the same system as that of Fig. 1A, but includes the pinhole 16 located behind the transmitter output lens, which samples the retroreflected beam 14, and passes the signal to a position detector 17, which provides information about the spatial orientation of the receiver 12 and its retroreflector 13 to a controller 15, which then inputs a direction control signal to a beam scanning mirror 19 to direct the power beam onto the position of the receiver determined by the position detector. Fig. IB also shows the concave form 18 which the waveform of the transmitted beam can take, depending on whether the receiver is before or after the transmitted beam waist, as will be further explained in connection with Figs. 2 A and 2B.

[0057] Reference is now made to Figs. 2A and 2B, which illustrate this effect schematically, for a system with a single retroreflector having good optical quality, used to retroreflect a signal from a conventional collimated optical beam having a diameter converging towards the beam waist, and then diverging beyond the waist. If the retroreflector is not centered relative to the incident beam axis, this being a common situation because of vibrations or motion of the receiver 21, the retroreflector may not send the retroreflected signal back to the transmitter, because the retroreflected wave is directed back in the direction perpendicular to that section of the wavefront falling on the retroreflector, and not perpendicular to the overall beam axis direction. In this situation, the retroreflector effectively “samples” only a small lateral part of the beam, and retroreflects that part of the a beam in a direction that is perpendicular to that part of the wavefront. If the sampled portion is not in the center of the original beam, i.e. on- axis, the reflected beam in the direction that is perpendicular to that sampled portion of the wavefront, is not parallel to the incident beam axis

[0058] Referring now to the details of Fig. 2A, there is shown schematically, the exit lens 28 of a transmitter 20 transmitting a forward beam 23, converging through a beam waist 26, which is the narrowest part of the focused beam and then diverging towards the receiver 21, which has a ball retroreflector 22. The wavefront profiles 25 of the incident optical power beam 23 are shown, being concave towards the receiver 21 up to the beam waist 26, and then concave towards the transmitter 20 beyond the beam waist.

[0059] The receiver 21 includes a single retroreflector 22, which is used to send a retroreflected beam back to the transmitter 20, indicating impingement of the forward laser beam 23 on the retroreflector, this returned beam being used as feedback input to control the aim of the optical beam 23 at the retroreflector 22, and as a feedback for safety interlocks. As shown in Fig. 2A, assuming that the retro reflector 22 is not at the waist of the incident beam 23, but is within the lateral limits of the beam width at another point of the beam length, (beyond the beam waist in Fig. 2A), it samples only a small part of the wavefront 25 of beam 23 falling on it, and retroreflects a beam 24 whose axis is perpendicular to the small sampled wavefront 25 in the vicinity of retroreflector 22. Since the small sampled wavefront 25 is not perpendicular to the axial direction of the forward transmitted beam 23, the retroreflected beam 24 is directed in a direction perpendicular to the wavefront 25, which is not co-axial with the forward transmitted beam 23, and, as is shown in the example of Fig. 2, even though it has the expected slight divergence of the retroreflected incident wave, it may miss the optical aperture 28 of the transmitter altogether, thereby negating its entire function to provide feedback for control and safety of the system.

[0060] The extent of the situation in which part or even all of the retroreflective beam misses the clear aperture of the transmitter is dependent on whether the receiver is closer to the transmitter than the focal waist of the incident power beam or is further away than the focal waist of the incident power beam. The failure to capture all or part of the retroreflective beam is increased if the retroreflector is closer to the transmitter than the focal waist of the beam, and is decreased if the retroreflector is further away than the focal waist of the beam. Looking at the example shown in Fig. 2A, where the receiver 21 is located beyond the beam waist 26, it is clear that as the retroreflector moves to the right R, the section of the wavefront of the incident beam impinging on the retroreflector, is returned back in a direction perpendicular to the wavefront at the retroreflector, towards the transmitter, and because of the concave shape of the wavefront beyond the beam waist, the retroreflected direction has an inclination towards the left hand side L of the beam axis. In other words, as the receiver moves to the right, away from the beam axis and the position of the transmitter clear aperture, the retroreflective beam moves to the left, thereby compensating, at least partly, for the right hand movement of the retroreflector. On the other hand, referring now to Fig. 2B, where the receiver 21 is shown located before the beam waist, the curvature of the wavefront before the waist is such that as the receiver moves to the right R, the retroreflected beam also moves to the right R, thereby increasing the probability that the retroreflective beam will miss hitting the lens in the clear aperture 28 of the transmitter. This situation is more difficult to correct than the situation of Fig. 2 A, since as the receiver moves off-axis, the retroreflected beam diverges even more from the axis of the transmitter beam aperture, and in the same direction as the receiver movement. In practice, such an increased divergence of the retroreflected beam in the region before the beam waist can be compensated for in the control system by applying a minus 1 digital multiplication to the image processing beam position detection stage such that the diverging beam image can be handled by the detection system.

[0061] To summarize this effect, the retroreflective beam will return directly to the transmitter clear aperture only if the receiver is located with the retroreflector exactly on axis of the transmitted incident beam. If the receiver moves such that the retroreflector is no longer on axis, the retroreflective beam will no longer hit the center of the clear aperture of the transmitter, the extent of the departure therefrom being dependent on whether the receiver is located before or after the beam axis. With comparatively little motion of the receiver, the retroreflected beam from a well collimated incident beam will completely miss the clear aperture of the transmitter, thereby causing the system to lose track of the position of the receiver, requiring the beam scanning facility in the transmitter to again begin a search for the receiver and to lock onto the position of the receiver once found again.

[0062] In order to reduce this occurrence, prior art systems with well collimated retroreflected beams therefore require a clear aperture sufficiently large that the system keeps track of the retroreflected beam for a predetermined deviation of the receiver from the incident beam axis, up to the level of deviation expected because of receiver movement. On the other hand, if, in accordance with the presently suggested methods and systems, the retroreflected beam is intentionally generated with an increased aberration level over that of an optimally collimated retroreflected beam, the retroreflected beam will diverge sufficiently that the likelihood of at least an outer part of the retroreflected beam entering the clear aperture of the transmitter is increased. In order to take advantage of this phenomenon, the clear aperture of the transmitter may then be made smaller than that of a system with an optimally collimated retroreflected beam, while still maintaining the same likelihood of capturing the retroreflected beam as in the well collimated case. The generation of a retroreflected beam having a higher divergence is most conveniently achieved by using a retroreflector having a controlled level of defocusing, namely of spherical aberration, or of higher order aberrations. Spherical aberration is the preferred aberration, since it maintains a rotational symmetry of the beam. An aberration not maintaining a reasonable level of spherical symmetry may cause the retroreflected beam to acquire a shape so far from a reasonably circular shape, such as an elongated elliptical shape, that it would be difficult for the system to provide meaningful beam tracking.

[0063] The use of a more divergent retroreflected beam means that the beam can be detected over a wider lateral range from the transmitted beam axis, than a system using a retroreflector having optimum optical quality. Therefore, instead of using a large optical aperture in order to capture the low divergence retroreflected beams of previously used systems, it becomes possible to use a smaller optical aperture to capture the more widely diverging retroreflected beams of the presently described systems, with the concomitant reduction in system cost and size.

[0064] This feature of the systems and methods of the present application are illustrated in. Fig. 3A to Fig. 3C. Fig. 3A shows the situation at the optical exit / entrance aperture 30 of the transmitter, using a prior art retroreflector, generating a well collimated retroreflected beam from the transmitted beam 31. Since the retroreflector itself generally has small dimensions, so as not to use too much of the received transmitted power, the retroreflected beam is also small in dimensions, and even allowing for the expected divergence even if well collimated, and also the natural diffraction effects of a beam coming from a very small source, it will have a small size when arriving back at the transmitter exit / entrance aperture. As a result, when the beam axis is offset from the retroreflector, due, for instance, to movement of the receiver, the well- collimated retroreflected beam, having a comparatively small footprint 33 at the aperture, may be significantly diverted from the aperture center, requiring a large transmitter working aperture 30, with a larger diameter lens, in order to increase the likelihood of capturing the well collimated retroreflected beam 33. The large working aperture 30 has to accommodate both the outgoing power beam 31 and the retroreflected returned beam 33, and hence requires a more costly lens. The transmitted beam itself 31, requires a clear aperture 32, only slightly larger than the diameter of the transmitted beam 31. In Fig. 3 A, the lateral deflection at the transmitter exit / entrance aperture of the retroreflected beam is given by the distance between the centers of the beams 31 and 33, and this deflection is used also in the examples of Figs. 3B and 3C below, in order to compare the situations. For the case shown in Fig. 3A, with a large entrance aperture 30, the retroreflected beam 33 is shown well detected, even if at the edge of the entrance aperture. Fig. 3B now shows the effect of trying to use a smaller transmitter entrance aperture 36, generally in order to use a smaller and lower cost lens. In this situation, for a retroreflected collimated beam of the same size as that of Fig. 3A, and having the same lateral deflection from the center of the exit / entrance aperture as that in Fig. 3A, the retroreflected beam is shown as having completely missed entering the exit / entrance aperture 36 of the transmitter, such that the movement of the receiver has resulted in the transmitter losing its detection of the beam, and hence losing track of the receiver. This situation illustrates the problem in prior art systems using well collimated retroreflected beams, which the systems and methods of the present disclosure attempt to solve.

[0065] On the other hand, Fig. 3C now shows the situation when the less well collimated retroreflected beam of the present disclosure is used in the system. Because of the larger diameter of the more divergent retroreflected beam 37, the same level of handling a certain receiver displacement can be obtained with a smaller input aperture, and hence a less expensive lens system. In Fig. 3C, for the same transmitted beam 31, the smaller lens diameter 36 can be used, while still capturing the edges of the more divergent retroreflected beam 37 at the same deflection distance from the optical axis as that shown in Figs. 3A and 3B.As is seen in Fig. 3C, the same situation as that shown in Fig. 3B, a part of the larger divergent collimated beam 37 does enter the entrance aperture 36, and this can continue to be tracked by the system. The overall effect of the presently described use of a retroreflector having an aberration level greater than that of beam having well corrected aberration levels, is that a smaller and hence less costly clear aperture optic 36 of the transmitter can be used while achieving the same effect as that of the larger clear aperture optic 30 of prior art systems.

[0066] Component selection

[0067] In order to implement a laser power transmission system with an increased aberration retroreflected beam, as described in this disclosure, a number of features are now presented to assist in calculating the numerical values of components which will enable such implementation.

[0068] Retroreflector design

[0069] Fig. 4A illustrates a prior art cat’s eye retroreflector with no intentionally generated defocusing effects, having a precision front hemisphere 40 having a radius that generates a focal point 41 on the reflective rear surface of the back sphere 42. Two different radii are needed to achieve retroreflection. The conventional geometrical optics lens-maker’s equation may be used to calculate the radii of both half spheres, resulting in the expression that: r2 = n / (n - 1) where n is the radius of the hemisphere acting as the beam input lens of the retroreflector, and T2 is the radius of the hemisphere acting as the reflector of the beam, and n is the refractive index of the material of the hemispheres.

[0070] The above equation assumes that the refractive index of the air in which the retroreflector is situated is 1. Using more exact formalism, the value of the refractive index should take this difference into account.

[0071] Often, some corrections are also needed to the results of this equation, as the numerical aperture for such applications is typically high.

[0072] It is advantageous to use the same refractive index for both half spheres, to avoid the need for an anti-reflection coating between the two parts, but different refractive indexes for both parts may be used. The desired level of aberration of the retroreflected beam may be achieved by slightly modifying either or both of the radius or the refractive index of one or both half spheres.

[0073] From the above equation, it is noted that when n = 2, then n = r2, meaning that the cat’s eye retroreflector can be formed from a single sphere, which is a simpler and more practical than cementing two hemispheres together. Fig. 4B shows such a single sphere cat’s eye retroreflector 43 having a refractive index as close as possible to the value 2, and a radius that generates a focal point 44 on the reflective rear surface 45 of the sphere.

[0074] A glass needed to provide a refractive index of exactly 2 may not be readily available, such that the prior art optimally perfect aberration correction using available glasses with refractive index around 2, may still have a very small residual aberration. However, a deviation from the exact refractive index value 2 can be used in the presently described retroreflectors in order to adjust the level of deviation of aberrations of the retroreflector, from the optimal aberration level achievable, in order to provide a retroreflector having the aberrative properties required according to the present invention.

[0075] Furthermore, it is to be noted that the value of n - 1 in the above equation is only an approximation, since the value 1 is intended to be the refractive index of the medium, in this case air, around the retroreflector, and if the refractive index of the air is different from the value 1, then the equation and the resulting proposals for the retroreflectors of the present disclosure, should be accordingly amended, even if very slightly. Since such an amendment is so generally so small, it is ignored in the present description. However, the effect is pointed out in order to clarify that the unity value cited in the claims when reciting the refractive index of the cat’s eye retroreflector materials is understood to be an approximation for the true refractive index of the air surrounding the retroreflector.

[0076] Transmitter beam optics

[0077] The transmitted beam in a typical optical wireless power transmission system for use in a domestic setting, is emitted from a clear aperture having a diameter of up to a very few centimeters, typically up to 2 cm. The beam needs to be beamed over a range of ten or more meters, which is a factor of the order of 1,000 larger than the exit aperture. This means that the focusing of the beam at the far extent of its range, can only be done with a lens having a numerical aperture of the order of 0.001, and this task becomes close to impossible with a lens having a numerical aperture of more than 0.01.

[0078] In order to achieve commercial acceptability, there is typically a need to reduce the transmitter’s overall size and in particular, the size of the optical system, whose cost is very dependent on its diameter. Practically, the clear aperture of the optical system should be chosen to transmit at least about 90% of the transmitted power beam, or even somewhat more, up to about 98%, but a requirement to transmit close to 100% of the beam is shown to be a task of diminishing return.

[0079] To illustrate this, reference is now made to Fig. 5, which illustrates schematically a plot of the percentage of optical power transmitted by a lens as a function of the relative area of the lens. As is observed from the graph, the lens area required to transmit 99% of a Gaussian profile beam is twice as large as the lens area required to transmit 90% of the same beam, and in order to transmit 99.99% of the power, a lens having four times the area of the 90% lens, and typically 8 times the weight because of the need for a larger lens thickness, and hence a substantially higher cost, will be required. Consequently, the need to use a transmission level of more than about 90% or slightly more, leads to an inordinate increase in size and cost of the optical system, without a significant increase in the transmitted power level. However, while the power gain from increasing the aperture size is very marginal, safety requirements dictate the need to collect the retroreflected beam from the receiver through the same aperture, in order to ensure that the system can detect that no object is within the line of sight between the transmitter and the receiver.

[0080] When the incident power beam is indeed centered on the retroreflector, this aim is achieved automatically, and the retroreflected beam is counter-propagated with the transmitted beam directly into the transmitter optical aperture. However beam alignment with the retro reflector cannot always be perfect, due to beam deviations, vibration of the system, movement of the receiver, and other factors. As a result, when the incident power beam is not perfectly directed at the retro reflector, the beam may not return into the limited diameter beam aperture and may miss being collected at all. This results in a conflict between the wish to keep the lens diameter, and hence lens cost down, while at the same time maintaining a large enough clear aperture that will enable input of the returning retroreflected beam, without compromising the safety need to transmit and receive the incident and retroreflected beams through the same lens. Thus, prior art systems generally use a transmission lens having a clear aperture larger than that required by the need to transmit an acceptable percentage of the laser power capability.

[0081] The spot size d, created by focusing the transmitted beam is given approximately by the well- known expression: d = 1.22* NA where is the wavelength of the optical beam, and NA is the numerical aperture of the focusing lens.

[0082] Using a beam having a wavelength of the order of 1pm, and using the small angle approximation for the sine and tangent functions, the NA is given by: sin(tcm

[0083] Entering the approximate values given above, it can be seen that the spot size is roughly 2500*k, which roughly equals half the clear aperture of a very few centimeters, the value used above to describe the typical domestic installation, as illustrated in Fig. 3A, where the clear aperture used in such prior art systems is generally slightly more in order to increase the level of receiver motion permissible without “losing” the retroreflected beam. Reference is now made to Fig. 6, which illustrates a graphical example of the result of a ball retroreflector positioned 4mm. off-axis from a transmitted beam, the graph showing the extent of deviation from the center of the transmitter aperture of the retroreflected beam spot, and the retroreflected beam spot size, as a function of the distance of the retroreflector along the beam path, for a beam focused to have its waist at a distance of 7m.

[0084] The retroreflected beam spot size increases continually with increasing retroreflector distance down-range, as shown by the solid line indicating the retroreflected signal spot size on, or at least near to the transmitter beam aperture. On the other hand, the distance of the center of the returned beam spot from the optical center of the clear aperture, is shown by the dashed line in Fig. 6. It can be seen that the beam may shift up to approximately 5mm from the center of the emitter for positions of the retroreflector before the beam waist, while for the retroreflector positioned after the beam waist, the self-compensating effect described in Fig. 2A comes into effect, causing the deviation to decrease as the distance of the retroreflector increases downrange. However, for a large part of the useful range of the system, a retroreflector generating a well-collimated beam would result in the retroreflected beam missing the clear aperture, and the system thus losing track of the receiver, which is the very problem which is solved in the systems and methods of the present application using a less well collimated retroreflected beam, which therefore still enables at least a peripheral part of the purposely widened beam spot to enter the clear aperture, even when deviated from the central axis of the clear aperture, and to correct the beam aiming direction to keep track of the receiver. Another alternative result of the systems of the present disclosure is to maintain the same beam detection criteria as those of the prior art optimally focused retroreflected beam, but to achieve that with a significantly smaller optical element in the transmitter, thus reducing the cost of the transmitter.

[0085] To provide some exemplary quantitative estimates, while the lens area required to transmit 95% of the laser beam power towards the receiver with a numerical aperture of 0.001, is of the order of 3mm2, in order to capture the returned beam, a lens is needed having a diameter of at least 2*(5mm shift+4 mm reflected beam radius,) which is approximately 18mm. Such a lens would have an area of more than 250mm2and would significantly increase the size and cost of the transmitter optical system. If the smaller aperture, sufficient just to transmit the beam had been used, the retroreflected signal would not be in a position to enter the lens aperture, and would result in loss of tracking of the receiver. These quantitative estimates provide the basis for the solution disclosed in this application for using a defocusing retroreflector instead of one with an optimal optical design, or a retroreflector with intentionally increased spherical aberration, or a combination of the two. Besides use of increased spherical aberration, i.e. use of a non-optimal focal length retroreflector, it is also possible to use other rotationally symmetric aberrations to achive the desired deviation of the retroreflected beam.

[0086] 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 have been 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. A wireless optical power transmission system, comprising: a transmitter outputting an optical beam of power through an optical aperture, the transmitter comprising: a laser generating the optical beam of power; a deflecting module for directing the optical beam of power; a controller adapted to adjust the deflecting module; and a receiver comprising a retroreflector having a cat’s eye configuration, adapted to return a retroreflected beam from at least a part of the optical beam of power incident thereon, back towards the transmitter optical aperture, wherein: the cat’s eye retroreflector has a level of optical performance that introduces aberrations into the retroreflected beam, the level of aberrations being such that the retroreflected beam has a divergence sufficiently large that the 1 / e2diameter of the retroreflected beam at the optical aperture is larger than the optical aperture diameter.

2. A wireless optical power transmission system according to claim 1 wherein the 1 / e2diameter of the retroreflected beam is at least 1.5 times the optical aperture diameter.

3. A wireless optical power transmission system according to claim 1 wherein the 1 / e2diameter of the retroreflected beam is at least twice the optical aperture diameter.

4. A wireless optical power transmission system according to, any of the previous claims, wherein the major part of the aberrations comprises radially symmetric aberrations or defocus.

5. A wireless optical power transmission system according to any of the previous claims, wherein the major part of the aberrations are spherical aberrations.

6. A wireless optical power transmission system according to any of the previous claims, wherein the cat’s eye retroreflector comprises a transparent sphere having a refractive index configured to be 1 + n, where n is the refractive index of the medium fromwhich the optical beam of power is incident, the sphere having a reflective coating on the surface opposite to that on which the optical beam of power impinges.

7. A wireless optical power transmission system according to claim 6, wherein the medium from which the optical beam of power is incident is air, such that the refractive index of the transparent sphere should be 2.

8. A wireless optical power transmission system according to any of claims 1 to 5, wherein the cat’s eye retroreflector comprises a pair of separate optical elements, the element on which the optical beam of power impinges having a positive focal length and a second element having a reflective surface positioned at the focal plane of the first element.

9. A wireless optical power transmission system according to any of claims 1 to 5, wherein the cat’s eye retroreflector comprises a pair of hemispherical spheres in optical contact on their plane faces, each of the hemispheres having different radii.

10. A wireless optical power transmission system according to either claim 8 or claim 9, wherein the hemisphere on which the optical beam of power impinges has a radius smaller than that the hemisphere opposite to that on which the optical beam of power impinges, the ratio of the radii of the hemispheres being equal to n - 1, where n is the refractive index of the material of the hemispheres.

11. A wireless optical power transmission system according to any of the previous claims, wherein the aberrations introduced into the retroreflected beam are such that the retroreflected beam has a divergence sufficiently large that the optical aperture captures at least a part of the retroreflected beam, even in a situation when, if the retroreflector had had optimally minimal aberrations, no part of that retroreflected beam would be captured by the optical aperture.

12. A wireless power transmission system according to any of the previous claims, wherein the level of optical performance of the retroreflector is less than that of a retroreflector having an optimally minimum aberration level.

13. A wireless power transmission system according to any of the previous claims, wherein the retroreflector is the sole retroreflector on the receiver.

14. A cat’s eye retroreflector of a wireless power transmission system, adapted to be illuminated by a first beam transmitted through an aperture of the transmitter, and configured to retroreflect a second beam essentially back towards the aperture, wherein the retroreflector has an aberration that imparts to the second beam an aberration level comprising at least 65% of at least one of radially asymmetric aberrations or defocus, such that the 1 / e2diameter of the second beam at the transmitter is larger than the aperture of the transmitter.

15. A cat’s eye retroreflector according to claim 14, wherein the aberration level of the retroreflector is selected intentionally.

16. A cat’s eye retroreflector for wireless optical power transmission system, adapted to be illuminated from a transmitter by a first optical beam emitted through an optical aperture of the transmitter, and retroreflecting a second beam essentially back towards the aperture, wherein when the cat’s eye retroreflector is off-axis of the first beam, the retroreflected second beam does not fully overlap the first beam and is only partially transmitted through the optical aperture of the transmitter, and wherein the extent of overlap is dependent at least in part on the level of aberration induced into the retroreflected beam by the cat’s eye retroreflector, and wherein the cat’s eye retroreflector has an aberration level greater than that of a second retroreflector that would generate an optimally collimated retroreflected beam, such that the optical aperture of the transmitter would capture the retroreflected second beam from the cat’s eye retroreflector for a greater off-axis distance of the retroreflector than for the optimally collimated retroreflected beam of the second retroreflector.

17. A retroreflector according to claim 16, wherein for the same off-axis distance of the cat’s eye retroreflector as the second retroreflector generating the optimally collimated retroreflected beam, the aperture size of the transmitter can be made smaller than that of a system using the second retroreflector that would generate an optimally collimated retroreflected beam.

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