SYSTEM FOR OPTICAL WIRELESS POWER SUPPLY

Abstract

A system incorporating safety features, for optical power transmission to receivers, comprising an optical resonator having end reflectors and a gain medium, a driver supplying power to the gain medium, and controlling its small signal gain, a beam steering apparatus and a controller to control at least the beam steering apparatus and the driver. The controller responds to a safety risk occurring in the system, by outputting a command to change at least some of the small signal gain of the gain medium, the radiance of the optical beam, the power supplied by the driver, the scan speed or the scan direction and position of the beam steering apparatus, or to register the scan pose which defines the location of said optical-to-electrical power converter. The controller may also ensure a high overall radiance efficiency, and may warn of transmitted power not received by a targeted receiver.

Claims

1. A system for optical wireless power transmission to at least one power receiving apparatus, said system comprising: an optical resonator having end reflectors and adapted to emit an optical beam; a gain medium comprising either (i) a semiconductor device, or (ii) a solid host doped with Neodymium ions and in optical communication with a filter attenuating radiation for at least one frequency having a wave number in the range 8,300 cm.sup.−1 to 12,500 cm.sup.−1, said gain medium being positioned inside said optical resonator and having a first bandgap energy, said gain medium being thermally attached to a cooling system and configured to amplify light passing through it; a driver configured to supply power to said gain medium, and enabling control of the small signal gain of said gain medium; a beam steering apparatus configured to direct said optical beam in at least one of a plurality of directions; an optical-to-electrical power converter located in said at least one power receiving apparatus and configured to convert said optical beam into electrical power having a voltage, said optical-to-electrical power converter having a second bandgap energy; a detector configured to provide a signal indicative of said optical beam impinging on said optical-to-electrical power converter; and a controller adapted to control at least one of the status of said beam steering apparatus and said driver, said controller receiving a control input signal at least from said detector, wherein said optical beam has a radiance of at least 8 kW/m.sup.2/Steradian and the overall radiance efficiency of the transmission between said transmitter and said at least one power receiving apparatus is at least 20%.

2. A system according to claim 1, further including a voltage converter connected to the output of said optical-to-electrical power converter.

3. A system according to claim 2 wherein said voltage converter is configured to track the maximum power point of said optical-to-electrical power converter.

4. A system according to claim 2 wherein said voltage converter is a DC/DC boost voltage converter.

5. A system according to claim 1, wherein said resonator comprises at least one dielectric mirror.

6. A system according to claim 1, wherein said optical-to-electrical power converter is a photovoltaic cell.

7. A system according to claim 6 wherein said photovoltaic cell comprises a III-V semiconductor material.

8. A system according to claim 1, further including an inductor and an energy storage device that may be a capacitor or a rechargeable battery.

9. A system according to claim 8 wherein said inductor has an inductance between: $L=\frac{1}{1.28*{10}^{-40}*f}*E_{\mathrm{gain}}^2*\frac{1-\frac{E_{\mathrm{gain}}}{5*{10}^{-19}*V_{\mathrm{output}}}}{P_{{\mathrm{laser}}_{—}.Math.\mathrm{driver}}}$ $\mathrm{and}$ $L=\frac{1}{3*{10}^{-38}*f}*E_{\mathrm{gain}}^2*\frac{\left(1-\frac{E_{\mathrm{gain}}}{4*{10}^{-20}*V_{\mathrm{output}}}\right)}{P_{{\mathrm{laser}}_{—}.Math.\mathrm{driver}}}$ where f in the switching frequency measured in Hertz, E.sub.gain is the bandgap energy of the gain medium measured in Joules, V.sub.output is the output voltage of the DC/DC converter in volts and P.sub.laser.sub._.sub.driver is the power measured in watts, supplied to the gain medium by the laser driver.

10. A system according to claim 1, wherein said system is configured to receive information from said power receiving apparatus.

11. A system according to claim 10 wherein said information includes at least one of battery status, device identification, power needs, voltage needs and a key.

12. A system according to claim 1, further comprising a sensor for determining the temperature of said optical-to-electrical power converter.

13. A system according to claim 12 configured to modify the power of said optical beam in response to changes in said temperature of said optical-to-electrical power converter.

14. A system according to claim 1, further comprising an optical window positioned between said photovoltaic optical-to-electrical power converter and said beam steering apparatus, said window having a refractive index of at least 1.5.

15. A system according to claim 1, wherein said second bandgap energy is smaller than said first bandgap energy.

16. A system according to claim 1, wherein said controller is adapted such that said beam steering apparatus directs said optical beam onto said at least one power receiving apparatus.

17. A system for optical wireless power transmission to at least one power receiving apparatus, said system comprising: an optical resonator having end reflectors and adapted to emit an optical beam; a gain medium comprising either a semiconductor device or a solid host doped with Neodymium ions and in optical communication with a filter attenuating radiation for at least one frequency having a wave number in the range 8,300 cm.sup.−1 to 12,500 cm.sup.−1, said gain medium being positioned inside said optical resonator and having a first bandgap energy, said gain medium being thermally attached to a cooling system and configured to amplify light passing through it; a driver configured to supply power to said gain medium, and enabling control of the small signal gain of said gain medium; a beam steering apparatus configured to direct said optical beam in at least one of a plurality of directions; an optical-to-electrical power converter located in said at least one power receiving apparatus, and configured to convert said optical beam into electrical power having a voltage, said optical-to-electrical power converter having a second bandgap energy; a detector configured to provide a signal indicative of said optical beam impinging on said optical-to-electrical power converter; and a controller adapted to control at least one of the status of said beam steering apparatus and said driver, said controller receiving a control input signal at least from said detector, wherein said controller is configured to respond to an indication of a safety risk occurring in the system, by outputting a command to result in at least one of: causing said driver to change the small signal gain of the gain medium; changing the radiance of said optical beam; changing the power supplied by said driver; changing the scan speed of said beam steering apparatus; changing the pose of said beam steering apparatus; and recording the scan pose which defines the location of said optical-to-electrical power converter.

18. A system according to claim 17 wherein said indication of a safety risk occurring in the system is obtained at least from said signal generated by said detector configured to provide a signal indicative of said optical beam impinging on said optical-to-electrical power converter, and from a signal generated by the level received at said resonator of said beam reflected from said at least one power receiving apparatus.

19. A system according to claim 17, further including a voltage converter connected to the output of said optical-to-electrical power converter.

20. A system according to claim 19 wherein said voltage converter is configured to track the maximum power point of said optical-to-electrical power converter

21. A system according to claim 19 wherein said voltage converter is a DC/DC boost voltage converter.

22. A system according to claim 17 wherein said optical-to-electrical power converter is a photovoltaic cell.

23. A system according to claim 22 wherein said photovoltaic cell comprises a III-V semiconductor material.

24. A system according to claim 22, further including an inductor and an energy storage device that may be a capacitor or a rechargeable battery.

25. A system according to claim 24 wherein said inductor has an inductance between: $L=\frac{1}{1.28*{10}^{-40}*f}*E_{\mathrm{gain}}^2*\frac{1-\frac{E_{\mathrm{gain}}}{5*{10}^{-19}*V_{\mathrm{output}}}}{P_{{\mathrm{laser}}_{\mathrm{driver}}}}$ $\mathrm{and}$ $L=\frac{1}{3*{10}^{-38}*f}*E_{\mathrm{gain}}^2*\frac{\left(1-\frac{E_{\mathrm{gain}}}{4*{10}^{-20}*V_{\mathrm{output}}}\right)}{P_{{\mathrm{laser}}_{—}.Math.\mathrm{driver}}}$ where f in the switching frequency measured in Hertz, E.sub.gain is the bandgap energy of the gain medium measured in Joules, V.sub.output is the output voltage of the DC/DC converter in volts and P.sub.laser.sub.—driver is the power measured in watts, supplied to the gain medium by the laser driver.

26. A system according to claim 17, wherein said system is configured to receive information from said at least one power receiving apparatus said information including at least one of battery status, device identification, power needs, voltage needs and a key.

27. A system according to claim 17, further comprising a sensor for determining the temperature of said optical-to-electrical power converter.

28. A system according to claim 27, said system configured to modify the power of said optical beam in response to changes in said temperature of said optical-to-electrical power converter.

29. A system according to claim 17, further comprising an optical window positioned between said photovoltaic optical-to-electrical power converter and said beam steering apparatus, said window having a refractive index of at least 1.5.

30. A system according to claim 17, wherein said second bandgap energy is smaller than said first bandgap energy.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0156] The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:

[0157] FIG. 1 shows the energy density of various battery chemistries;

[0158] FIG. 2 shows the maximal permissible exposure value for lasers for various exposure times, according to the US Code of Federal Regulations, title 21, volume 8, (21 CFR §8), revised on April 2014, Chapter I, Subchapter J part 1040;

[0159] FIG. 3 shows an example of a warning sign for a class IV laser product;

[0160] FIGS. 4, 5, 6, 7, 8, and 9 show examples of the chemical composition of various commonly used transparent polymers;

[0161] FIG. 4 shows a Poly-methyl methacrylate (PMMA) chain;

[0162] FIG. 5 shows the structure of a polycarbonate;

[0163] FIG. 6 shows the polystyrene structure;

[0164] FIG. 7 shows the structure of nylon 6,6;

[0165] FIG. 8 shows a polypropylene chain structure;

[0166] FIG. 9 shows the polyethylene chain structure;

[0167] FIG. 10 shows the IR absorption bands for common organic chemical bonds;

[0168] FIG. 11 shows the IR absorption spectrum of polyethylene;

[0169] FIG. 12 shows the overtone absorption bands for some common organic chemical bonds;

[0170] FIGS. 13A and 13B show different electronic configurations for converting the output voltage of a photovoltaic cell to a different voltage;

[0171] FIG. 14 shows the power reflected per square meter by a mirror, when a beam of radiance 8 kW/m.sup.2/steradian is focused upon it, as a function of numerical aperture;

[0172] FIGS. 15A, 15B, and 15C show schematic drawings of exemplary apparatus according to the present disclosure, for avoiding unsafe reflections from the front surface of a receiver being illuminated by a transmitter of the present disclosure;

[0173] FIG. 16 shows a schematic diagram showing a more detailed description of the complete optical wireless power supply system of the present disclosure;

[0174] FIG. 17 is a graph showing the change in power transmission of the system of FIG. 16, as a function of the angle of tilt of the beam steering mirror;

[0175] FIG. 18 shows a schematic representation of a cooling system for the gain medium of the system of FIG. 16;

[0176] FIG. 19 is a schematic diagram showing a detailed description of the system of FIG. 16, but further incorporating a safety system;

[0177] FIG. 20 is a schematic view of the optical-to-electrical power converter of the systems shown in FIGS. 16, 19;

[0178] FIG. 21 shows a block diagram view of the safety system of FIG. 19;

[0179] FIG. 22 shows an output laser beam of the system of FIG. 19, deflected by a mirror rotating on a gimbaled axis, or on gimbaled axes;

[0180] FIG. 23 shows the mirror of FIG. 22 rotated so that the beam is deflected at a larger angle than that shown in FIG. 22;

[0181] FIG. 24 shows a schematic representation of the intensity profiles of a typical deflected beam;

[0182] FIG. 25 shows a side view of a laser diode from a direction perpendicular to the fast axis of the laser, and a lens for manipulating the beam; and

[0183] FIG. 26 shows a block diagram of the complete laser protector

DETAILED DESCRIPTION

[0184] In view of the above described considerations, one exemplary implementation of the optical wireless power supply systems of the present disclosure could be a system tuned to work in between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1. Such overtone bands are less known bands, containing much less chemical information, and arise from essentially forbidden quantum mechanical transitions, and are only allowed due to complex mechanisms. Consequently, they provide wide, weak absorption bands, exactly as preferred for this application, but have found significantly less use in analytical chemistry. The broad nature of the bands allows for detecting various different polymer compositions, while the weak absorption allows the system to continue operation even in the vicinity of organic dirt and fingerprints. This makes these lines significantly less useful for typical uses of absorption measurements, but ideal for the present task. Another advantage of these lines is that there are no commonplace absorption lines directly positioned at the same frequencies, so that changing chemical composition of the materials will not alter the measurement results strongly. Many such overtone bands are illustrated in the chart of FIG. 12.

[0185] Electro-optical components that operate in that band are scarce and hard to source, probably since both diode lasers and diode-pumped, solid state (DPSS) lasers are significantly less efficient at those frequencies, and only lower power lasers are currently commercially available. Since lasers at the preferred frequencies, with the desired parameters, are, not currently available, a laser suitable for this use has to be designed from the ground up. The resonator and gain medium have to be designed. A laser with the selected frequency and a radiance value sufficient to facilitate a roughly collimated or nearly collimated beam must be constructed. To achieve good collimation of the beam, a radiance of at least 8 kW/m.sup.2/Steradian is needed, and even 800 kW/m.sup.2/Steradian may be needed for higher power systems for efficient power transmission. For small systems working at long distances, much higher radiance (up to 10 GW/m.sup.2/Steradian) may be designed in the future, according to similar principles. Receivers for use with radiance of less than that level need to be too large, which would make the system cumbersome.

[0186] Different mirror setups for the resonator have been used, specifically good quality metallic mirrors made of Gold, Silver or Aluminum. These are found to reduce the lasing efficiency significantly. Much better results are achieved with dielectric material mirrors. Alternatively, Fresnel mirrors have one advantage in that they are low cost. Other mirrors that may be used are Bragg mirrors (which may be dielectric). The mirrors need to be positioned so as to form a stable, or a nearly stable resonator, or a resonator where photons are confined in space by a barrier inside the laser (such as in a fiber or diode laser) and a gain medium should be placed in the resonator between the mirrors in a position allowing the gain medium to amplify the beam resonating inside the resonator, such that it has a radiance of at least 8 kW/m.sup.2/Steradian.

[0187] If the gain medium is capable of lasing at more than one wavelength, the dielectric mirrors can be selected to limit that wavelength to a specific value. Alternatively, a filter can be used to fix the lasing frequency.

[0188] Specifically, it is better if the mirrors have high reflectivity for at least one wavelength between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1.

[0189] Three different approaches may be used for the gain medium:

[0190] 1. DPSS Design [0191] In the DPSS design, the gain medium may be a Nd-doped YAG crystal, though YVO.sub.4 crystals, GGG crystals and Glasses are also options for a clear host. Neodymium is most suitable for operation between the first overtone of the C—H band and the second overtone of the C—H band since Nd has a transition near ˜7450 cm.sup.−1. The Nd ions need to be excited by absorbing radiation, typically from 808 nm laser diodes, although other wavelengths may be used. A Nd-based gain medium tends to lase at a much higher frequency unless a filter blocking the transition around 9400 cm.sup.−1 is added inside the resonator, or unless the unwanted radiation from the resonator is otherwise extracted. When such a filter is added, lasing commences at 7440-7480 cm.sup.−1. Such filter action can be achieved using a prism or a grating, instead of a filter or by proper chromatic design of the laser resonator.

[0192] 2. Semiconductor Laser

[0193] As an alternative, a semiconductor-based design may be proposed. It is possible to tune the wavelength of semiconductor lasers by altering the lasing bandgap of the semiconductor used. Semiconductors, especially those of the III-V type and more especially, though not exclusively, quantum dot types, having bandgaps of the order of 1 eV, emit light at the desired frequencies between 6900 cm.sup.−1 and 8200 cm.sup.−1. Specifically bandgaps between 0.8 eV and 1.1 eV yield good results and are absorbed, at least partially, by essentially all commonly used polymers. [0194] 3. Various alternative designs may also be used in the systems described in this disclosure, such as Nd doped fiber lasers, that may include Bragg mirrors and/or fiber loop mirrors. Alternatively Raman shifted fiber lasers may also be used.

[0195] During operation, the gain medium heats up, and should be cooled to prevent wavelength shift and efficiency degradation. If the gain medium is properly cooled, then it is possible to increase the pump power or current until a beam having radiance of at least 8 kW/m.sup.2/Steradian is emitted, having a frequency between 6900 cm.sup.−1 and 8200 cm.sup.−1. Such a beam can be nearly collimated and will be attenuated by most organic materials, including polymers, allowing detection. However, it will not be strongly absorbed by contaminations such as fingerprints.

[0196] The laser gain medium is typically configured to work at a temperature below 150 degrees centigrade. If its temperature exceeds a level, typically around 250 centigrade, a number of problems may arise.

[0197] Firstly, the efficiency of light emission may drop significantly, due to population of lower level excited states, especially in 3- and 4-level lasers, and also due to thermal recombination of charge carriers in semiconductors.

[0198] Secondly, the soldering of the gain medium, if such a thermal attachment method is used, may be damaged.

[0199] Thirdly, thermal aberrations may occur which may cause beam degradation

[0200] Fourthly, the thermal expansion of the laser gain medium may be different from that of its surroundings, which may cause mechanical stress or even warping and fracture of the gain medium.

[0201] For those reasons, inter alfa, the gain medium has to be thermally attached to a cooling system. Typically the gain medium emits between 0.1 and 100 W of heat from a surface that is between 1 mm.sup.2 and 40 mm.sup.2. In order for the temperature of the gain medium to be maintained at less than 150 degrees, the cooling system of the gain medium needs to have a thermal resistance of less than 200 Kelvin per Watt, and for systems transmitting higher powers, typically arising from more than 10 W of electrical power input, the thermal resistance should be significantly lower, and in many cases thermal resistance lower than 0.05 Kelvin/Watt is needed.

[0202] The surface of the cooling system is attached to the gain medium, typically using a third material such as solder or adhesive, which must have an expansion coefficient that is compatible to both the expansion coefficient of the gain medium itself and to the expansion coefficient of the front surface of the cooling system.

[0203] Typically such cooling systems may be either a passive heat sink, a heat sink with a fan, a Peltier element connected to a heat sink with or without a fan, or a liquid cooled cooling system. Alternatively, use may be made of a stand-alone liquid circulating cooling system with active circulation based on a circulating pump, or with passive circulation, based on heat pipes.

[0204] If the cooling system comprises a heat sink with a fan, its thermal resistance should be less than 0.1° Kelvin per Watt

[0205] If the cooling system is a passive heat sink, its thermal resistance should be less than 0.3° Kelvin per Watt

[0206] If the cooling system is a Peltier element, it needs to generate at least 5 degrees temperature difference ΔT.

[0207] If the cooling system is an active liquid cooled cooling system, it should be able to cover the entire span of thermal resistances mentioned here.

[0208] A passive heat sink is preferred in systems designed for low cost and quiet operation, while a liquid cooled system is preferred for high-power systems. A heat sink with a fan or a fluid pump is used for systems typically having more than 1 W electrical output and a transmitter having a small volume, such as less than approximately 1 liter.

[0209] The gain medium is typically driven by a driver, supplying it with power, which may be provided as electrical power as in the case of some semiconductor gain media, or optical as in the case of other semiconductor gain media or DPSS systems, or chemical or other forms of energy. The amount of power supplied by the driver determines the small signal gain achieved, which determines the working conditions and emission of the laser, while the saturated gain of the gain medium is generally a function of the material selected for the gain medium, though not always in a simple linear fashion, and ultimately, the radiance emitted from the laser. Such a laser driver might have two or more operational states, one used for power transmission, and the others used for other functions of the system, such as target seeking, setup, and information transmission. It is important that the laser driver produces stable emission (with regards to power and beam parameters) in both working conditions, although stable operation during power transmission is more important.

[0210] To convert the optical beam into electricity again, so that useful power is delivered, an optical-to-electric power converter, typically a photovoltaic cell, should be used. As with the lasers, suitable photovoltaic cells tailored to the frequency of the beam used, are not commonly available as off-the-shelf components, and a custom cell is required. The bandgap of the photovoltaic semiconductor should be slightly smaller than the bandgap of the gain medium used, so that the beam frequency is absorbed efficiently by the semiconductor. If not, the conversion efficiency will be very poor. On the other hand, if the bandgap used is too small, then a poor efficiency system is achieved. Also the conductors on the photovoltaic cell need to be tailored to the radiance of the beam used—the higher the radiance, the thicker the conductors needed.

[0211] Since the bandgap of the laser gain medium should be in the range 0.8-1.1 eV, and the bandgap of the photovoltaic cell used must be lower, and since a single junction photovoltaic cell typically produces a voltage that is about 60-80% of the bandgap energy divided by the electron charge, a single junction cell tailored to the laser frequency yields a very low voltage, typically 0.3-0.8V, and typically a high current, assuming an output power of a few watts, as required by a practical system. The conductors on the semiconductor need to be thick enough to carry the generated current without significant (e.g. >5%) losses. Typically the series resistance of the conductors needs to be below 1 Ohm, or even better, below 0.1 Ohm, and the heat generated should be efficiently extracted from the photovoltaic cell as its efficiency generally decreases with temperature.

[0212] This combination of low voltage combined with high power cannot be easily converted to the higher voltages required to charge portable devices, typically 3.3 or 5V. Furthermore, some systems, such as communication systems, may require voltages such as −48V, 12V or 3.8V. The system needs to supply a stable voltage, and at higher levels than the output voltage expected from the photovoltaic cells. A typical method to increase the voltage of photovoltaic cells is to connect them in series, such as is described in U.S. Pat. No. 3,370,986 to M. F. Amsterdam et al, for “Photovoltaic Series Array comprising P/N and N/P Cells”, which shows a typical configuration for yielding a higher voltage, while utilizing almost the same amounts of semiconductor and no additional components, and is therefore the typically chosen solution.

[0213] However this solution is not suitable for systems such as those described in the present application, in which a laser having a radiance as high as 8 kW/m.sup.2/Steradian is used, especially since such a laser typically does not have a uniform shaped beam. Furthermore, its beam shape may be variable in time and the pointing accuracy may be less than optimally desired. In such a situation it is virtually impossible to design a compact and efficient system that will illuminate all the cells uniformly. If the photovoltaic cells connected in series are not uniformly illuminated, they do not produce the same current. In such a case the voltage will indeed be increased to the desired level but the current would drop to the current generated by the cell producing the least current, usually the cell least illuminated. In such a situation efficiency will be very poor. There is thus a need for an improved alternative method to increase the voltage.

[0214] One method of increasing the voltage of a single cell may be by charging capacitors in parallel, and then discharging them in series. This method yields good results for low currents, but when current is increased beyond a certain level, the switching time becomes a dominant factor, influencing efficiency, which degrades with increasing switching time.

[0215] If the energy is converted to AC using a fast, low resistance, switching mechanism, that AC current can be amplified using coupled inductances and then converted to AC again. The increased voltage AC can be converted to DC using a diode bridge and an energy storage device, such as a capacitor or a battery. Such systems have advantages when the voltage needs to be increased beyond twenty times that of the photocell voltage. Another advantage of such a system is that the switching can be done from the transmitter using the laser, thus saving receiver cost and complexity. Such systems have disadvantages when the voltage needs to be increased by a factor of less than 10 or when size and volume limitations are critical to the application.

[0216] Reference is now made to FIG. 13A, which shows a method of voltage conversion that is efficient and simple. In the configuration of FIG. 13A, a single inductor may be used with a low resistance switching mechanism and an energy storage device to increase the voltage of the photovoltaic cell. In FIG. 13A, the square on the left is the photovoltaic cell, the switch S, is a low resistance switch, such as a MOSFET, JFET, BJT, IGBT or pHEMT, the inductance L is connected to the output of the photovoltaic cell and the capacitor C acts as an energy storage device.

[0217] The following description assumes for simplicity the use of components with zero resistance. Taking resistance losses into account complicates the calculations, and is explained in a later section of this disclosure. The switching mechanism cycles the inductor between two primary operating phases: charging phase and discharging phase. In the charging phase the inductor is connected in parallel with the photovoltaic cell, by the closing of switch S. During this phase the inductor is being charged with the energy converted by the photovoltaic cell. The inductor energy increase is given by:

ΔE.sub.L CH=Vpv*I.sub.L*T.sub.CH

[0218] Where: [0219] Vpv is the output voltage of the photovoltaic cell; [0220] I.sub.L is the average inductor current; and [0221] T.sub.CH is the duration of the charging phase.

[0222] In the discharging phase, the inductor is connected between the photovoltaic cell and the load by the opening of switch S. During this phase, the energy delivered from the inductor to the output energy storage device is given by the inductor energy decrease:

ΔE.sub.C=Vo*I.sub.L*T.sub.DIS

[0223] Where: [0224] Vo is the voltage of the energy storage device, which is typically very close to the desired output voltage of the device, and can therefore be approximated as the output voltage of the system; [0225] I.sub.L is the average inductor current; and [0226] T.sub.DIS is the duration of the discharging phase.

[0227] The energy delivered from the photovoltaic cell to the inductor during that phase is given by: ΔE.sub.L.sub._.sub.DIS=Vpv*I.sub.L*T.sub.DIS.

[0228] The change in the inductor energy during that phase is the difference between the incoming and outgoing energy:

ΔE.sub.L.sub._.sub.DIS=Vpv*I.sub.L*T.sub.DIS−Vo*I.sub.L*T.sub.DIS.

[0229] In steady state operation, the energy of the inductor at the end of the cycle returns to the same value it was at the beginning of the cycle yielding

ΔE.sub.L.sub.CH=−ΔE.sub.L.sub._.sub.DIS,

Which, after substitution, yields:

Vo=Vpv*(1+T.sub.CH/T.sub.DIS).

[0230] The voltage at the energy storage device is thus defined by the photovoltaic cell voltage and the ratio of the charging and discharging phase durations.

[0231] In the present system, however, the parasitic characteristics and other aspects of the components might have a significant impact on conversion operation and efficiency and care should be taken into account in selecting and using the right components, in order to allow the system to operate efficiently. These elements are now considered, one by one:

Inductor

[0232] The inductance of the inductor defines the rate of change of the inductor current due to applied voltage, which is given by dI/dt=V/L, where dI/dt is the rate of current change, V is the voltage applied across the inductor and L is the inductance. In the context of the current system, V is determined by the gain medium in the transmitter. Selection of a different gain medium causes change in the photon energy, which mandates consequent changes in the photovoltaic bandgap, and hence a change in the photovoltaic voltage. This then calls for selection of a different inductor and/or switching frequency. The switching rate must be fast enough to allow the inductor current to respond to changes in the incoming power from the transmitter through the optical-to-electrical power converter, and slow enough to avoid high-magnitude current ripple which contributes to power loss, input voltage ripple and output voltage ripple. The optimal value of the inductor should yield ripple current which is between 20% and 40% of the maximum expected input current, but systems may be operable between 10% and 60%. Rigorous analysis of the circuit parameters shows that in order to achieve this objective, the value L, of the inductor measured in Henries, must be within the limits:

[00007]$L<\frac{1}{1.28*{10}^{-40}*f}*E_{\mathrm{gain}}^2*\frac{1-\frac{E_{\mathrm{gain}}}{5*{10}^{-19}*V_{\mathrm{output}}}}{P_{{\mathrm{laser}}_{—}.Math.\mathrm{driver}}}$$L>\frac{1}{3*{10}^{-38}*f}*E_{\mathrm{gain}}^2*\frac{\left(1-\frac{E_{\mathrm{gain}}}{4*{10}^{-20}*V_{\mathrm{output}}}\right)}{P_{{\mathrm{laser}}_{—}.Math.\mathrm{driver}}}$

[0233] Where: [0234] f is the switching frequency measured in Hz; [0235] E.sub.gain is the bandgap of the gain medium, measured in Joules; [0236] V.sub.output is the output voltage from the voltage converter, measured in Volts; and [0237] P.sub.laser.sub._.sub.driver is the power pumped by the laser driver into the gain medium, measured in Watts.

[0238] In order to successfully integrate the inductor into a mobile client, the inductance should typically be smaller than 10 mH, as inductors that are suitable for the current required by mobile client charging and having volume limitations suitable for a portable application are typically well below this value. Also inductors having inductances too small, such as 10 nH, will require such a high switching frequency that it will severely limit the availability of other components in the system such as the switch, and the switching loss caused by such a high frequency may be higher than the amount of power delivered by the photovoltaic cell.

[0239] The serial resistance of the inductor, R.sub.parasitic, should be as low as possible to minimize the conduction power loss: Typically, a value which yields less than 10% efficiency drop is chosen: the serial resistance of the inductor, measured in Ohms should be less than

[00008]$R_{\mathrm{parasitic}}\le \frac{1}{2*{10}^{-40}}*\frac{E_{\mathrm{gain}}^2}{P_{{\mathrm{laser}}_{—}.Math.\mathrm{driver}}}$

[0240] Where: [0241] E.sub.gain is the bandgap of the gain medium, measured in Joules; and [0242] P.sub.laser.sub._.sub.driver is the power pumped by the laser driver onto the gain medium, measured in Watts.

[0243] In a typical system the inductor serial resistance would be less than 10Ω. The saturation current of the inductor is usually chosen to be higher than the expected inductor peak current, given by:

I.sub.SAT>I.sub.PEAK=Im+Vpv*(1−Vpv/Vo)/(2*L*f)

[0244] For extracting more than 10 mW of power from a single junction photovoltaic cell, the saturation current must be higher than 10 mW/0.8v=12.5 mA.

[0245] For reliable operation the inductor shall be rated at a higher current than the expected maximum input current. For extracting more than 10 mW of power from a single junction photovoltaic cell, the inductor rated current must be higher than 10 mW/0.8 v=12.5 mA.

Switching Mechanism

[0246] The switching mechanism is usually made of two or more devices. The first device, a main switch, when conducting, sets the inductor into the charging phase. The second device can be either a diode (as in FIG. 13A) or a switch whose function is to connect the inductor to the load or output energy storage device, during the discharging phase, and to disconnect it from the load during the charging phase.

[0247] The switching mechanism should have low switch node capacitance to minimize switching losses:

P.sub.SW2=0.5*Csw*Vo.sup.2*f

[0248] For extracting more than 50% of the laser power, the switch node capacitance should be less than:

[00009]$\mathrm{Csw}\le \frac{P_{\mathrm{laser}.Math..Math.\mathrm{driver}}}{V_o^2*f}.$

[0249] In a typical system switch node capacitance would be less than 100 nF and more than 10 pF.

[0250] The serial resistance of the main switch in the switch node, that switch being either that connecting the inductor to the ground or that connecting the optical-to-electrical power converter to the inductor, should be less than:

[00010]$R\le \frac{E_{\mathrm{gain}}^2}{2*{10}^{-40}*P_{\mathrm{laser}.Math..Math.\mathrm{driver}}}$

In a typical system the switch serial resistance would be less than 10Ω.

Energy Storage Device

[0251] The energy storage device can be either a capacitor or a battery or both.

[0252] The energy storage device is required to maintain the output voltage during the charging phase, when the inductor is disconnected from the output. The capacitance of the storage device is chosen based of the switching frequency, laser power and the desired output ripple voltage:

C.sub.OUT>P.sub.LASER DRIVER/(f*Vo*ΔVo)

Where ΔVo is the desired output ripple voltage.

[0253] The energy storage device can also supply power to the load during temporary interruption of the optical path. For uninterrupted power supply, the energy storage device should be able to store at least the amount energy equal to minimal operational output power (P.sub.OUT.sub._.sub.MIN) multiplied by the interruption time interval (T.sub.INT):

E.sub.OUT.sub._.sub.MIN≧P.sub.OUT.sub._.sub.MIN*T.sub.INT

[0254] If a capacitor is used as the energy storage device, the capacitance should be larger than: C.sub.OUT≧2*E.sub.OUT/V.sub.OUT.sup.2.

[0255] For uninterrupted operation at minimal operational output power larger than 10mW and interruption time interval longer than 100 ms the stored energy has to be larger than 1 mJ and the capacitance larger than 80 μF (assuming V.sub.OUT=5V).

[0256] In some cases, the capacitor may serve as the energy storage device for the client application. In such cases, the client application may be designed without any secondary energy storage device (the conventionally used battery installed in the mobile device), and the energy storage device of the presently described systems would have to store enough energy to supply the power needs of the client device until the next charging event. In such cases, super capacitors having a capacitance at least 0.5 F, and even above 10 F, may be used. In other cases, where the power requirement of the client device is low, or when it has an independent energy storage device such as the battery internally installed in the device, or if the device does not need to operate when no power is supplied, the capacitor used would typically be well beyond 1 F. If a rechargeable battery is used as the energy storage device, then, similar to the capacitor logic above, if the battery is used only as means of regulating the voltage, but not as the means for maintaining power supply to the client device between charging events, then the energy capacity of the battery may advantageously be up to 100 times the energy supplied during 100 cycles of the switch (typically below 0.1 Wh), this level being determined according to the volume budget and cost effectiveness of the battery. On the other hand, if the battery is also used to power the client device between charging events, its capacity should be at least large enough to store the energy needed by the client device between charging events—typically above 0.1 Wh in the case of a cellular phone. Batteries also have a volume limitation depending on the product in which they are intended to be used. Thus, the battery of a product that has some volume V, if incorporated externally to the device, would typically be limited to up to times the volume of the device, i.e. 3V. As an example of this rule of thumb, a battery used to power a cellular phone of 100 cc. volume would typically be limited to less than 300 cc. in volume. Such a battery would typically have a capacity of below 300 Wh. because of the above mentioned limitation.

[0257] The circuit in FIG. 13A is not the only possible topology. FIG. 13B shows a different design that can achieve similar performance characteristics. The components roles, constraints and expected values for FIG. 13B are the same as those listed for the circuit in FIG. 13A. The primary difference is that the positive and negative terminals of the output voltage are reversed.

[0258] In some applications the energy storage device may be preferably located inside the device which is intended to use the power received. In other applications, specifically those applications where short term operation is anticipated, and which does not require a regulated voltage, the energy storage device may even be eliminated.

Point of Regulation

[0259] The power output of the photovoltaic cell depends on the incoming optical power and load applied to it. The optimal loading condition will yield the maximal output power from the photovoltaic cell, therefore, the control mechanism of the voltage converter must regulate the loading point. The control mechanism can be either designed to maintain constant voltage between the cell terminals, which is known to be maximum power operating point for most conditions, or it can track the maximum power operating point by measuring the cell output power and seeking the optimal cell voltage under any operating condition. The first approach is simpler; the second is more power efficient.

[0260] The generated laser beam needs to be directed towards the receiver. In order to direct the beam towards the receiver, a beam steering apparatus should be used. Some beam steering sub-systems that could be used include a moving mirror, a moving lens, an electro-optical modulator, a magneto-optical modulator, a set of motors moving the whole transmitter system in one or more directions, or any other suitable beam deflection device.

[0261] The beam steering apparatus should be controlled by a controller, most conveniently the same controller used to control the laser driver.

[0262] The beam steering apparatus is configured to direct the >8 kW/m.sup.2/Steradian beam in any of a number of directions.

[0263] The damage threshold of the beam steering apparatus needs to be able to withstand the beam's radiance.

[0264] For example, if the beam is focused on a mirror using a focusing mechanism having a numerical aperture of 0.5, the mirror needs to withstand a power density of at least 6.7 kW/m.sup.2 for a beam having 8 kW/m.sup.2/steradian. If a beam having higher radiance is used the mirror should be chosen so that it would have a higher damage threshold correspondingly.

[0265] FIG. 14 shows the power reflected per square meter by a mirror when a beam of 8 kW/m.sup.2/steradian is focused upon it as a function of numerical aperture.

[0266] If a higher radiance beam is used, then the power reflected by the mirror increases correspondingly in a linear manner.

[0267] Since the beam may be far from uniform, “hotspots”, sometimes having 10× irradiance compared to the beam average, may be generated.

[0268] Hence, mirrors should have a damage threshold which is at least as large and preferably at least 10× that shown in FIG. 14, scaled to the actual beam irradiance and numerical aperture of the focusing mechanism on the mirror.

[0269] There is typically an optical front surface in the receiver, positioned near the photovoltaic cell and between the photovoltaic cell and the transmitter, through which the beam enters the receiver, and which is needed in order to protect the typically delicate structure of the photovoltaic cell, and in many cases, in order to match the exterior design of the device where the power receiver is integrated in. The front surface may have a coating protecting it from scratches, such as Corning Gorilla Glass®, or similar, or may be treated to make it better withstand scratches. It may be also be treated to reduce the levels of contaminants, such as fingerprints and dust which may settle on it, or to reduce their optical effect, or it may be coated with an anti-reflection coating to reduce the level of light reflected from it. The front surface of the photovoltaic cell may also be coated. In some cases the front surface would be part of the structure of the photovoltaic cell itself or coated on the photovoltaic cell.

[0270] While in some situations, it may be possible to reduce the amount of reflection from the surface to below the safety threshold, by choosing a very low reflection anti-reflective coating, should the coating be contaminated or covered by either a liquid spilled on it, or a fingerprint, such anti-reflective coating would be ineffective in reducing the amount of reflection, and typically, 3-4% of the incident light will be reflected in an uncontrolled direction. If such a reflection is reflected in a diverging manner, its power density would soon drop to safe levels. However, should the reflection be focused, the power density may increase to unsafe levels. For this reason, it is important that the ROC (radius of curvature) of such a surface, at any point on it, should not be less than a predetermined value. In general, the reflection from the surface is intended to be only a small part of the incident light, thereby reducing the danger of any significant beam reflections, regardless of what nature or of what form the surface curvature takes. The level of reflected light may be variable, since even the ˜4% reflection from an untreated glass surface may be increased, if a layer of extraneous contaminant material on the surface generates increased reflectivity. However, that reflection is expected not exceed 20%, and will generally be substantially less than the 4% of untreated glass, such as in the case of AR coated glass, where reflectivities of 0.1% or even less are common. Therefore, the surface is described in this disclosure, and is thuswise claimed, as having properties which reflect a small part of the incident light, this description being used to signify less than 20% of the incident light, and generally less than the 4% of untreated glass.

[0271] Reference is now made to FIGS. 15A to 15C, which illustrate schematically methods of avoiding the above-mentioned unsafe reflections, even for the small part of the incident light which may be reflected from the surface. FIG. 15A shows a situation where the surface is a concave surface, FIG. 15B shows a situation where the surface is a convex surface, and FIG. 15C shows the situation where the surface is a diffusive surface. In FIG. 15A, an incident beam 110, having at least 8 kW/m.sup.2/Steradian radiance, is directed towards photovoltaic cell 112, passing through a front surface 111, which may be the front surface of the photovoltaic cell. The front surface 111 reflects some of beam 110 creating a focused beam 113 with a focal point 114 some distance from the surface. In order to ensure that focal point 114 does not present any danger to an eye or skin, or other objects, the Radius of Curvature (ROC) of the surface 111 must be such that the beam is focused with low numerical aperture, as in FIG. 15A, or that it be defocused, as in FIG. 15B, or that it be diffused, as in FIG. 15C. To achieve these limitations, if the surface is concave looking from the transmitter towards the photovoltaic cell, as in FIG. 15A, its ROC must be larger than 1 cm, and if higher power systems are used, typically above 0.5 W of light, it should be greater than 5 cm. Alternatively, the surface ROC can be negative, as in FIG. 15B, but the ROC cannot be in the range 0-1 cm. These limitations will ensure that the reflected beam of light has a focal point which is either virtual, i.e. associated with a diverging reflected beam, or at least 1 cm in front of the surface, such that the risk generated by the focus is significantly reduced. The surface may also have numerous regions with smaller curvatures, creating a diffusive surface, as in FIG. 15C, which significantly helps reducing the risk of a dangerous focal point. In such a case, the radius of curvature of each sub section of the surface may be smaller than 1 cm without creating a focal point. Furthermore, if the surface is split into multiple zones, each zone may have smaller curvature.

[0272] In order to operate safely, the system also needs to be able to direct the power beam to the photovoltaic cell so that it is blocked by it, and not be directed at some unsafe region. In order to accomplish that, a detector should be positioned to provide indication of the impingement of the beam on the receiver. Such a detector should typically be positioned in the receiver, but configurations where such a detector is located in the transmitter are also possible, in which case the detector should be responsive to a phenomenon occurring due to the impact of the beam on the receiver. Such a transmitter-associated system may include image acquisition and processing of optical information received from the receiver, such as the reflection of the beam from a barcode printed on the receiver, so that the transmitter may detect the barcode's illumination pattern. Reflections from a retro reflector or retro reflectors or arrays or patterns thereof may be positioned on the receiver and such reflection may be detected in the transmitter, either by way of image processing, by measuring back reflection or by measuring coherence effects of the reflection. The detector may be a current or voltage sensor positioned in the receiver, a photodiode in the receiver or in the transmitter, or an imaging device which may be either in the transmitter or the receiver. A retro-reflector in the vicinity of the photovoltaic cell may also be used, in combination with an additional detector in the transmitter, detecting light reflected from the retroreflector.

[0273] The detector, upon detecting the beam of light impinging on the photovoltaic cell, sends a signal accordingly to the system controller. If the detector is in the receiver, such signalling may be done wirelessly, using a communication channel which may be RF, IR, visible light, UV, modulation of the beam, TCP/IP, or sound. The system controller is usually located in the transmitter, but may also be located in a main control unit, which may even be on a computer network from the transmitter. On receipt of the signal, the controller responds by performing at least one of: [0274] (a) Changing the state of the laser driver; or [0275] (b) Changing the operational properties of the beam steering apparatus, such as the direction to which it directs the beam, or the speed in which such direction is changed.

[0276] Reference is now made to FIG. 16, which is a schematic diagram showing a detailed description of the complete system. The system comprises transmitter 21 and receiver 22. In general, the transmitter and receiver will be located remotely from each other, but are shown in FIG. 16, for convenience, to be close to each other. Beam 15 transfers power from transmitter 21 to receiver 22.

[0277] On the receiver 22, the front surface 7 reflects a small part of incident beam 15 as a reflected beam 16, while either diffusing it or creating a virtual focal point behind front surface 7, or a real focal point at least 1 cm in front of surface 7. After transmission through the at least partially transparent surface 7, beam 15 impinges on the optical-to-electrical power converter 1.

[0278] The optical-to-electrical power converter 1 may be enclosed in a package that may have a front window, which may be surface 7 or a separate window. It may also be coated to have an external surface adapted to function as an interface with the air, or the adhesive or the glass surrounding it. In a typical configuration, the optical-to-electrical power converter 1 could be a junction of semiconductor layers, which typically have conductors deposited on them. In many embodiments surface 7 would be coated on, or be the external surface of one of these semiconductor layers.

[0279] Signalling detector 8 indicates that beam 15 is impinging on photovoltaic cell 1 and transmits that information to the controller 13, in this example system, located in the transmitter 21. The control signal is transmitted by a link 23 to a detector 24 on the transmitter.

[0280] Electrical power converter 1, has a bandgap E8 and typically yields a voltage between 0.35 and 1.1V, though the use of multi-junction photovoltaic cells may yield higher voltages. Power flows from the photovoltaic cell 1 through conductors 2a and 2b, which have low resistance, into inductor 3 which stores some of the energy flowing through it in a magnetic field.

[0281] Automatic switch 4, typically a MOSFET transistor connected to a control circuit (not shown in FIG. 16), switches between alternating states, allowing the current to flow through the inductor 3 to the ground for a first portion of the time, and for a second portion of the time, allowing the inductor to emit its stored magnetic energy as a current at a higher voltage than that of the photovoltaic cell, through diode 5 and into load 6, which can then use the power.

[0282] Automatic switch 4 may be operating at a fixed frequency or at variable frequency and/or duty cycle and/or wave shape which may either be controlled from the transmitter, or be controlled from the client load, or be based on the current, voltage, or temperature at the load, or be based on the current, voltage or temperature at automatic switch 4, or be based on the current, voltage or temperature emitted by the optical-to-electrical power converter 1, or be based on some other indicator as to the state of the system.

[0283] The receiver may be connected to the load 6 directly, as shown in FIG. 16, or the load 6 can be external to the receiver, or it may even be a separate device such as a cellphone or other power consuming device, and it may be connected using a socket such as USB/Micro USB/Lightning/USB type C.

[0284] In most cases there would also be an energy storage device, such as a capacitor or a battery connected in parallel to load 6, or load 6 may include an energy storage device such as a capacitor or a battery.

[0285] Transmitter 21 generates and directs beam 15 to the receiver 22. In a first mode of operation, transmitter 21 seeks the presence of receivers 22 either using a scanning beam, or by detecting the receiver using communication means, such as RF, Light, IR light, UV light, or sound, or by using a camera to detect a visual indicator of the receivers, such as a retro-reflector, or retro-reflective structure, bar-code, high contrast pattern or other visual indicator. When a coarse location is found, the beam 15, typically at low power, scans the approximate area around receiver 22. During such a scan, the beam 15 impinges on photovoltaic cell 1. When beam 15 impinges on photovoltaic cell 1, detector 8 detects it and signals controller 13 accordingly.

[0286] Controller 13 responds to such a signal by either or both of instructing laser driver 12 to change the power P it inputs into gain medium 11 and or instructing mirror 14 to alter either its scan speed or direction it directs the beam to or to hold its position, changing the scan step speed. When gain medium 11 receives a different power P from the laser power supply 12, its small signal gain—the gain a single photon experiences when it transverses the gain medium, and no other photons traverse the gain medium at the same time,—changes. When a photon, directed in a direction between back mirror 10 and output coupler 9 passes through gain medium 11, more photons are emitted in the same direction—that of beam 15—and generate optical resonance between back mirror 10 and output coupler 9.

[0287] Output coupler 9 is a partially transmitting mirror, having reflectance R, operating at least on part of the spectrum between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1, and is typically a multilayer dielectric or semiconductor coating, in which alternating layers of different refractive index materials are deposited on a substrate, which is typically glass, plastic or the surface of gain medium 11. Alternatively Fresnel reflection can be used if the gain medium is capable of providing sufficient small signal gain or has a high enough refractive index, or regular metallic mirrors can be used. A Bragg reflector may also be used, should the gain medium be either a semiconductor or a fiber amplifier. Output coupler 9 may also be composed of a high reflectance mirror combined with a beam extractor, such as a semi-transparent optical component that transmits a part of the light and extracts another part of the light from the forward traveling wave inside the resonator, but typically also a third portion extracted from the backwards propagating wave inside the resonator.

[0288] Back reflector 10 should be a high reflectance mirror, although a small amount of light may back-leak from it and may be used for monitoring or other purposes, working at least on part of the spectrum between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1. It may typically be constructed of alternating layers of different refractive index materials deposited on a substrate, usually glass, metal or plastic. Alternatively Fresnel reflection can be used if the gain medium is capable of providing sufficient small signal gain, or regular metallic mirrors can be used. A Bragg reflector may also be used should the gain medium be either a semiconductor or a fiber amplifier.

[0289] Gain medium 11 amplifies radiation between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1, although not necessarily over the whole of this spectral range. It is capable of delivering small signal gain larger than the loss caused by output coupler 9 when pumped with power P by laser driver 12. Its area, field of view, and damage thresholds should be large enough to maintain a beam of at least 8 kW/m.sup.2/Steradian/(1−R), where R is the reflectance of output coupler 9. It may be constructed of either a semiconductor material having a bandgap between 0.8-1.1 eV or of a transparent host material doped with Nd ions, or of another structure capable of stimulated emission in that spectral range. Gain medium 11 is positioned in the optical line of sight from the back reflector 10 to output coupler 9, thus allowing radiation reflected by the back reflector 10 to resonate between the back reflector 10 and the output coupler 9 through gain medium 11.

[0290] For the exemplary implementation where the gain medium 11 is a semiconductor having a bandgap between 0.8-1.1 eV, it should be preferably attached to a heat extracting device, and may be pumped either electrically or optically by laser driver 12.

[0291] In the exemplary implementation where the gain medium 11 is a transparent host, such as YAG, YVO.sub.4, GGG, or glass or ceramics, doped with Nd ions, then gain medium 11 should preferably also be in optical communication with a filter for extracting radiation around 9400 cm.sup.−1 from that resonating between back mirror 10 and output coupler 9.

[0292] The beam steering apparatus 14 is shown controlled by controller 13. It can deflect beam 15 into a plurality of directions. Its area should be large enough so that it would contain essentially most of beam 15 even when tilted to its maximal operational tilt angle. Taking a simplistic 2D example, if beam 15 were to be a collimated 5 mm diameter (1/e.sup.2 diameter) Gaussian beam, and the beam steering apparatus were to be a single round gimballed mirror centered on the beam center, and if the maximal tilt required of the mirror is 30 degrees, and assuming that beam steering apparatus 14 has no other apertures, then if the mirror has a 5 mm diameter like that of the beam, it would have an approximately 13% loss at normal incidence to the beam, but approximately 60% loss at 60 degrees tilt angle. This would severely damage the system's performance. This power loss is illustrated in the graph of FIG. 17.

[0293] At the beginning of operation, controller 13 commands laser driver 12 and mirror 14 to perform a seek operation. This may be done by aiming beam 15, with the laser driver 12 operating in a first state, towards the general directions where a receiver 22 is likely to be found. For example, in the case of a transmitter mounted in a ceiling corner of a room, the scan would be performed downwards and between the two adjacent walls of the room. Should beam 15 hit a receiver 22 containing an optical-to-electric power converter 1, then detector 8 would signal as such to controller 13. So long as no such signal is received, controller 13 commands beam steering 14 to continue directing beam 15 in other directions, searching for a receiver. If such a signal is received from detector 8, then controller 13 may command beam steering 14 to stop or slow down its scan to lock onto the receiver, and to instruct laser driver 12 to increase its power emission. Alternatively controller 13 may note the position of receiver 22 and return to it at a later stage.

[0294] When laser driver 12 increases its power emission, the small signal gain of gain medium 11 increases, and as a result beam 15 carries more power and power transmission begins. Should detector 8 detect a power loss greater than a threshold, which may be pre-determined or dynamically set, and which is typically at a level representing a significant portion of the maximal permissible exposure level, and which is also typically greater than the system noise figure, these conditions implying either that beam 15 is no longer aimed correctly at the optical-to-electrical power converter 1, or that some object has entered the beam's path, or that a malfunction has happened, controller 13 should normally command laser driver 12 to change its state, by reducing power to maintain the required safety level. If another indication of safe operation is present, such as an indication from the user as to the safety of transmission, which may be indicated by a user interface or an API, or an indication of safe operation from a second safety system, the controller may command the laser to increase power to compensate for the power loss. The controller 13 may also command the beam steering assembly 14 to perform a seeking operation again.

[0295] There may be two different stages in the seek operation. Firstly, a coarse search can be performed using a camera, which may search for visual patterns, for a retro reflector, for high contrast images, for a response signal from receivers or for other indicators, or by using the scanning feature of beam steering 14. A list of potential positions where receivers may be found can thus be generated. The second stage is a fine seek, in which the beam steering mirror 14 directs beam 15 in a smaller area until detector 8 signals that beam 15 is impinging on an optical-to-electrical power converter 1.

[0296] Reference is now made to FIG. 18, shows an example cooling system for the gain medium 11 of the system of FIG. 16. Although the reflectors 9, 10 are shown as separate optical elements, it is to be understood that one or both of them may be coated directly on the gain medium end faces for simplifying the system. Gain medium 11 converts the power it receives from the laser driver 12 into both heat and photons, and would typically degrade the system performance if the gain medium were to be heated above a certain temperature.

[0297] For that reason, gain medium 11 is attached to heatsink 34 using a bonding agent 33 which is preferably a heat conducting solder having low thermal resistance. Bonding agent 33 may also be a conductive adhesive. Bonding agent 33 may have a thermal expansion coefficient which is between that of gain medium 11 and heat sink 34. Heat sink 34 may typically be a low thermal resistance heatsink made out of metal, which may be equipped with fins for increasing its surface area or an external fluid pumping system such as a fan or a liquid pump 35.

[0298] Reference is now made to FIG. 19, which is a schematic diagram showing a detailed description of the system of FIG. 16, but further incorporating a safety system 31, constructed and operable according to the methods and systems described in the present application. Although shown in FIG. 19 as a separate module, in order to show the additional inputs provided thereto, the safety system can be incorporated into the controller 13, and is generally described and may be claimed thereas. As described above, the system comprises transmitter 21 and receiver 22. In general, the transmitter and receiver will be located remotely from each other, but are shown in FIG. 19, for convenience, to be close to each other. Beam 15 transfers power from transmitter 21 to receiver 22.

[0299] On the receiver 22, the front surface 7 reflects a small part of incident beam 15 as a reflected beam 16, while either diffusing it or creating a virtual focal point behind front surface 7, or a real focal point at least 1 cm in front of surface 7. After transmission through the at least partially transparent surface 7, beam 15 impinges on the optical-to-electrical power converter 1 having a semiconductor layer having thickness T and an absorption coefficient to said optical beam 15. The thickness of the layer is dependent on the designed wavelength of the beam, and, when measured in cm, should be greater than 0.02 times the reciprocal of the absorption coefficient of the optical beam in the semiconductor layer, as described in further details on FIG. 20 below.

[0300] The optical-to-electrical power converter 1 may be enclosed in a package that may have a front window, which may be surface 7 or a separate window. It may also be coated to have an external surface adapted to function as an interface with the air, or the adhesive or the glass surrounding it. In a typical configuration, the optical-to-electrical power converter 1 could be a junction of semiconductor layers, which typically have conductors deposited on them. In many embodiments surface 7 would be coated on, or be the external surface of one of these semiconductor layers.

[0301] Signaling detector 8 indicates that beam 15 is impinging on photovoltaic cell 1 and transmits that information to the controller 13, in many cases it also transmits other data such as the power received, the optical power received, identification information, temperatures of the receiver and photovoltaic as well as information relayed from the client device, which may be control information. In this example, system controller 13 is located in the transmitter 21, but may also be located remotely therefrom. The control signal is transmitted by a link 23 to a detector 24 on the transmitter.

[0302] Safety system 31 receives information from various sources, further detailed in FIG. 21 below, and especially may receive information from a small portion of the beam 15 coupled out by beam coupler 32, and from the signaling detector 8, usually through a data channel between the power receiver and the power transmitter. Safety system 31 outputs safety indications to control unit 13.

[0303] Electrical power converter 1, has a bandgap E8 and typically yields a voltage between 0.35 and 1.1V, though the use of multi-junction photovoltaic cells may yield higher voltages. Power flows from the photovoltaic cell 1 through conductors 2a and 2b, which have low resistance, into inductor 3, which stores some of the energy flowing through it in a magnetic field.

[0304] Automatic switch 4, typically a MOSFET transistor connected to a control circuit (not shown in FIG. 19), switches between alternating states, allowing the current to flow through the inductor 3 to the ground for a first portion of the time, and for a second portion of the time, allowing the inductor to release its stored magnetic energy as a current at a higher voltage than that of the photovoltaic cell, through diode 5 and into load 6, which can then use the power.

[0305] Automatic switch 4 may operate at a fixed frequency or at variable frequency and/or duty cycle and/or wave shape, which may be controlled either from the transmitter, or be controlled from the client load, or be based on the current, voltage, or temperature at the load, or be based on the current, voltage or temperature at automatic switch 4, or be based on the current, voltage or temperature emitted by the optical-to-electrical power converter 1, or be based on some other indicator as to the state of the system.

[0306] The receiver may be connected to the load 6 directly, as shown in FIG. 16, or the load 6 can be external to the receiver, or it may even be a separate device such as a cellphone or other power consuming device, and it may be connected using a socket such as USB/Micro USB/Lightning/USB type C. The receiver typically further comprises a load ballast used to dissipate excess energy from the receiver, which may not be needed by the client.

[0307] In most cases, there would also be an energy storage device, such as a capacitor or a battery connected in parallel to load 6, or load 6 may include an energy storage device such as a capacitor or a battery.

[0308] Transmitter 21 generates and directs beam 15 to the receiver 22. In a first mode of operation, transmitter 21 seeks the presence of receivers 22 either using a scanning beam, or by detecting the receiver using communication means, such as RF, Light, IR light, UV light, or sound, or by using a camera to detect a visual indicator of the receivers, such as a retro-reflector, or retro-reflective structure, bar-code, high contrast pattern or other visual indicator. When a coarse location is found, the beam 15, typically at low power, scans the approximate area around receiver 22. During such a scan, the beam 15 should impinge on photovoltaic cell 1. When beam 15 impinges on photovoltaic cell 1, detector 8 detects it and signals controller 13 accordingly.

[0309] Controller 13 responds to such a signal by either or both of instructing laser driver 12 to change the power P input into gain medium 11 and or instructing mirror 14 to alter either its scan speed or direction it directs the beam to or to hold its position, changing the scan step speed. When gain medium 11 receives a different power P from the laser power supply 12, its small signal gain—the gain a single photon experiences when it transverses the gain medium, and no other photons traverse the gain medium at the same time—changes. When a photon, directed in a direction between back mirror 10 and output coupler 9 passes through gain medium 11, more photons are emitted in the same direction—that of beam 15—and generate optical resonance between back mirror 10 and output coupler 9.

[0310] Output coupler 9 is a partially transmitting mirror, having reflectance R, operating at least on part of the spectrum between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1, and is typically a multilayer dielectric or semiconductor coating, in which alternating layers of different refractive index materials are deposited on a substrate, which is typically glass, plastic or the surface of gain medium 11. Alternatively, Fresnel reflection can be used if the gain medium is capable of providing sufficient small signal gain or has a high enough refractive index, or regular metallic mirrors can be used. A Bragg reflector may also be used, should the gain medium be either a semiconductor or a fibre amplifier. Output coupler 9 may also be composed of a high reflectance mirror combined with a beam extractor, such as a semi-transparent optical component that transmits a part of the light and extracts another part of the light from the forward traveling wave inside the resonator, but typically also a third portion extracted from the backwards propagating wave inside the resonator.

[0311] Back reflector 10 should be a high reflectance mirror, although a small amount of light may be allowed to back-leak from it and may be used for monitoring or other purposes. These optical characteristics should operate at least on part of the spectrum between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1. It may typically be constructed of alternating layers of different refractive index materials deposited on a substrate, usually glass, metal or plastic. Alternatively, Fresnel reflection can be used if the gain medium is capable of providing sufficient small signal gain, or regular metallic mirrors can be used. A Bragg reflector may also be used should the gain medium be either a semiconductor or a fiber amplifier.

[0312] Gain medium 11 amplifies radiation between the first overtone of the C—H absorption at 6940 cm.sup.−1 and the second overtone of the C—H absorption at 8130 cm.sup.−1, although not necessarily over the whole of this spectral range. It is capable of delivering small signal gain larger than the loss caused by output coupler 9 when pumped with power P by laser driver 12. Its area, field of view, and damage thresholds should be large enough to maintain a beam of at least 8 kW/m.sup.2/Steradian/(1−R), where R is the reflectance of output coupler 9. It may be constructed of either a semiconductor material having a bandgap between 0.8-1.1 eV or of a transparent host material doped with Nd ions, or of another structure capable of stimulated emission in that spectral range. Gain medium 11 is positioned in the optical line of sight from the back reflector 10 to output coupler 9, thus allowing radiation reflected by the back reflector 10 to resonate between the back reflector 10 and the output coupler 9 through gain medium 11.

[0313] For the exemplary implementation where the gain medium 11 is a semiconductor having a bandgap between 0.8-1.1 eV, it should preferably be attached to a heat extracting device, and may be pumped either electrically or optically by laser driver 12.

[0314] In the exemplary implementation where the gain medium 11 is a transparent host, such as YAG, YVO4, GGG, or glass or ceramics, doped with Nd ions, then gain medium 11 should preferably also be in optical communication with a filter for extracting radiation around 9400 cm.sup.−1 from the radiation resonating between back mirror 10 and output coupler 9.

[0315] The beam steering apparatus 14 is shown controlled by controller 13. It can deflect beam 15 into a plurality of directions. Its area should be large enough so that it would contain essentially most of beam 15 even when tilted to its maximal operational tilt angle. Taking a simplistic 2D example, if beam 15 were to be a collimated 5 mm diameter (1/e.sup.2 diameter) Gaussian beam, and the beam steering apparatus were to be a single round gimballed mirror centered on the beam center, and if the maximal tilt required of the mirror is 30 degrees, and assuming that beam steering apparatus 14 has no other apertures, then if the mirror has a 5 mm diameter similar to that of the beam, it would have an approximately 13% loss at normal incidence to the beam, but approximately 60% loss at 60 degrees tilt angle. This would severely damage the system's performance. This power loss is illustrated in the graph of FIG. 17, and in FIGS. 22 and 23.

[0316] At the beginning of operation, controller 13 commands laser driver 12 and mirror 14 to perform a seek operation. This may be done by aiming beam 15, with the laser driver 12 operating in a first state, towards the general directions where a receiver 22 is likely to be found. For example, in the case of a transmitter mounted in a ceiling corner of a room, the scan would be performed downwards and between the two adjacent walls of the room. Should beam 15 hit a receiver 22 containing an optical-to-electric power converter 1, then detector 8 would signal as such to controller 13. So long as no such signal is received, controller 13 commands beam steering 14 to continue directing beam 15 in other directions, searching for a receiver. If such a signal is received from detector 8, then controller 13 may command beam steering 14 to stop or slow down its scan to lock onto the receiver. Controller 13 then waits for safety system 31 to generate a signal indicating that it is safe to operate, and once such a safety signal is received from safety system 31, controller may instruct laser driver 12 to increase its power emission. Alternatively, controller 13 may note the position of receiver 22 and return to it at a later stage, which may be done even without the presence of a safety signal.

[0317] When laser driver 12 increases its power emission, the small signal gain of gain medium 11 increases, and as a result, beam 15 carries more power and power transmission begins. Should detector 8 detect a power loss greater than a certain threshold, safety system 31, may report such a situation to controller 13, which should normally command laser driver 12 to change its state, by reducing power to maintain the required safety level. Such a power loss threshold may be pre-determined or dynamically set, and is typically at a level representing a significant portion of the maximal permissible exposure level, and is also typically greater than the system noise figure. Such conditions imply either that beam 15 is no longer aimed correctly at the optical-to-electrical power converter 1, or that some object has entered the beam's path, or that a malfunction has happened. If another indication of safe operation is present, such as an indication from the user as to the safety of transmission, which may be indicated by a user interface or an API, or an indication of safe operation from a second safety system, the controller may command the laser to increase power to compensate for the power loss. The controller 13 may also command the beam steering assembly 14 to perform a seek operation again.

[0318] There may be two different stages in the seek operation. Firstly, a coarse search can be performed using a camera, which may search for visual patterns, for a retro reflector, for high contrast images, for a response signal from receivers (such as a blinking light from a LED or other light source), or for other indicators, or the coarse search may be performed by using the scanning feature of beam steering unit 14. A list of potential positions where receivers may be found can thus be generated. The second stage is a fine seek, in which the beam steering mirror 14 directs beam 15 in a smaller area until detector 8 signals that beam 15 is impinging on an optical-to-electrical power converter 1.

[0319] Reference is now made to FIG. 20, which is a schematic view of the optical to electrical power converter, marked as item 1 in FIGS. 16, 19. Beam 15 impinges on photovoltaic cell 106, which is thermally connected to heat removal system 107. Beam 15 is absorbed by absorbing layer 108 causing a current to flow in conductors 111, the current being normally collected by a bus. The optical power absorbed by absorbing layer 108 is typically converted into electrical power and into heat. The electrical power is transferred through conductors 111 and a bottom electrode, while the thermal energy is evacuated mostly through a cooling system 107. Conductors 111 cast a shadow on absorbing layer 108 decreasing its efficiency, and it should therefore be made of a high conductivity material, such as materials having less than 3 E-6 Ohm*Meter specific electrical resistance. It can be shown that such conductors should have a thickness in meters that is at least

[00011]$\frac{0.034*P.Math..Math.\rho }{V^2*\chi }.Math.m,$

[0320] Where: [0321] P is the power absorbed by the photovoltaic, measured in watts; [0322] ρ is the specific electrical resistance of the conductors; [0323] V is the voltage emitted by the photovoltaic cell at its maximal power point; and [0324] χ is the fraction of the area of the absorbing layer covered by conductors.

[0325] The absorbing layer also needs to be thick enough to absorb most of beam 15 impinging on it. In order to do so, the thickness of absorbing layer 108 measured in meters, needs to be at least 0.02/μ.sub.10, where μ.sub.10 is the decadic attenuation coefficient measured in 1/m.

[0326] Reference is now made to FIG. 21, which shows a block diagram view of the safety system 31 of FIG. 19. Safety system 31 receives inputs from various sensors and sub-systems and sends output to controller 13, in those situations where the safety system is not an integral part of the controller 13, or when parts of the safety system are in an external control unit. Safety system 13 can also sometimes receive inputs from those various sensors and sub-systems. Such inputs can be from wavelength sensor 407, which monitors primarily the beams wavelength, in order to provide information needed for estimating the safety limits associated with the beam. It may also receive information from a beam analyzer (401) which may monitor the beam's properties such as shape, M.sup.2, symmetry, polarization, power, divergence, coherence and other information related to the beam and to the above parameters. It usually also receives information measured by external sub-systems through RF link 402. Temperature measurement of various components in the transmitter, receiver and surrounding area can be provided by temperature sensor(s) 403. It may receive an image from camera 404, which may be visible, thermal, IR or UV, and from power meter 406 measuring the beam's power at various positions. In many cases, the primary sensors connected to safety system 31 may be intrusion sensors (405) which monitor the beam for foreign objects traversing or approaching the beam path or its surroundings. It may also receive inputs from other sensors such as current, voltage, smoke, humidity and other environmental sensors. Upon reception of those inputs, or at a prescheduled time, safety system 31 assesses the potential for a security breach and issues a notification to controller 13 if that assessment exceeds a predetermined threshold.

[0327] Reference is now made to FIG. 22 showing a beam deflected by a mirror rotating on a gimbaled axis, or on gimbaled axes. Beam 15 impinges on mirror 332 rotating around 2 axes in two dimensions. Beam 15 forms a spot 333 on mirror 332 and is deflected in a different direction. The importance of selecting the proper center of rotation and mirror dimensions becomes clearer by referring to FIG. 23. In FIG. 23, the mirror 332 has now rotated so that beam 15 is now deflected at a larger angle compared to FIG. 22. Due to the increased angle, spot 333 now forms a projection on the mirror surface longer than the effective length of mirror 332, so that a significant portion of beam 15, that portion being marked 333A, is now spilled around mirror 332. This spilling reduces the brightness of beam 15, both by reducing its power and by cutting off its edges, which in most cases, degrades the beam quality in the far field. Typically the beam diameter is reduced in the near field close to the mirror, or on images of the near field, and increased in the far field. In order to achieve a minimally dimensioned system, working at relatively high efficiency, it is important to maintain the brightness as high as possible. This can be done by reducing the brightness loss experienced by beam 15, across all angles within the field of view of the system. This may be done by mounting the mirror so that its center of rotation is essentially close to the beam's center, measured either by a weighted average of the beam's intensity, or by a cross section of the beam's diameter at a certain intensity, or by the center of an elliptic aperture through which the beam passes. It is noted that, in contrast to the length projection, the width of the beam projection on the mirror is unchanged with impingement angle.

[0328] FIG. 24 shows a schematic representation of an intensity profile of a typical beam, contour 1 marks the 90% line of the maximum intensity, contour 2 marks the 80% of the maximum intensity line, contour 3 the FWHM (Full Width at Half Maximum) intensity line, contour 4 the 1/e intensity line, contour 5 the 1/e.sup.2 intensity line, and contour 6 the 1/e.sup.4 intensity line. Point 231 is approximately at the weighted average point of the beam, point 232 is at the center of the first contour and point 233 is at the center of the 6.sup.th contour, all being valid points at which to place the center of rotation of the mirror. However, placing the center of rotation beyond such points will require a larger mirror in order to maintain high radiance efficiency of the gimballed mirror.

[0329] Maintaining high radiance efficiency for other components is also of importance, although the gimballed mirror and the first lens following the laser are typically the limiting components for the radiance efficiency.

[0330] FIG. 25 shows a schematic side view of a laser diode from a direction perpendicular to the fast axis of the laser, also showing a lens 242 for manipulating, and usually nearly collimating the fast axis. In most cases lens, 242 is a compound lens, comprising several optical elements. Laser 241 is connected to heat sink 243 and emits beam 15, into interface layer 244, which has a refractive index n for the wavelength associated with beam 15. The value of n is 1.000293 in the case of an air interface at 532 nm, and higher in the case of oil or optical cement. Beam 15 has a divergence in at least one direction. The FWHM contour of beam 15 at the front surface of lens 242 has a diameter d, defined as the maximal distance between any two points on the FWHM contour. In order to have high radiance efficiency, lens 242 should have a numerical aperture NA with respect to the emitter of laser 241, of at least:

[00012]$\mathrm{NA}>\frac{0.36*d}{\mathrm{BFL}.Math.\sqrt{1+{\left(\frac{d}{2*\mathrm{BFL}*n.Math.}\right)}^2}}$

[0331] Where: [0332] d is the FWHM diameter measured in mm between the two furthest points on the beam's FWHM contour on the lens front surface; [0333] BFLis the back focal length of the lens measured in mm; and [0334] n is the refractive index of the interface layer between the laser and the lens.

[0335] If a lens having a smaller Numerical Aperture is used, the radiance of the beam is reduced by the lens resulting in either poor efficiency of the system, or in larger receiver, which may be disadvantageous in many situations. Using a smaller NA will also result in heating of the lens holder, which may cause two harmful effects—firstly it may thermally expand and move the lens from its optimal position and secondly, it may apply force to the lens and cause it to distort thus reducing its optical quality and as a result reduce the radiance of the beam. Furthermore, a small NA may result in reflections towards the laser, which might interfere with the laser mode and further reduce the original beam's radiance, which may further harm the radiance of the emitted beam. The light emitted from the edges of the lens, if a small NA lens is used, may interfere with the operation of other parts of the system, such as beam monitors, a tracking servo or other optical elements in the system, or may cause excessive heating to other portions of the system, which may interfere with their operation.

[0336] Reference is now made to FIG. 26 showing a block diagram of the laser protector 251. As mentioned above, safety system 31 assesses the potential for a safety breach and notifies controller 13 in case such potential exceeds a threshold. Controller 13 may then command laser driver 12 to terminate or reduce the power supplied to laser 252, which may be the laser which is emitting beam 15, or it may be the laser pumping the gain medium which is used to generate beam 15. Such termination of power may need to be very fast. If the power supplied is cut or reduced suddenly, negative voltages may develop in the conductors carrying the laser driver current (if those are electrical conductors), which may damage laser 252. To prevent such damage to the laser 252, laser protector 251 is connected between laser driver 12 and the laser 252, typically close to the laser 252. Laser protector 251 protects the laser 252 from negative voltages, typically by connecting a diode, or an equivalent circuit/component such as a Zener diode, a varistor or a circuit designed to drain such excessive negative voltage quickly, between the current conductors, such that when a negative voltage exists between the conductors, current flows through the protective diode or equivalent circuit, causing a fast decay of the voltage to safe levels. Laser protector 251 can also be used to protect the laser from overheating, or from current waves, by attenuating the power sent to laser 252 when over-temperature or overcurrent is sensed.

[0337] 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 sub-combinations 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.

[0338] The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.