In recent years mobile devices have found widespread popular use. In many cases, these devices use a battery to provide electric power; in other cases they draw their power from another device such as a computer via an electrical cable, through a USB port for example. One of the most common drawbacks of such devices is the need for recharging the battery, in the case of a rechargeable battery, or replacement of the battery in the case of a non-rechargeable battery, or the inconvenience of requiring a cable connection. It would be desirable if such devices were able to receive their power, or at least part of it, wirelessly from a remote source, thus reducing the need for recharging cycles or change of batteries, while not limiting the device to be bound to a fixed location limited by a cable connection to a power source.
In published International Patent Application No. WO/2007/036937, entitled “Directional Light Transmitter and Receiver”, to one of the inventors in the present application, and incorporated herewithin by reference in its entirety, there is described a directional light transmitter and receiver that may be used to transmit power to such a mobile device. A transmitter, fixed for example at the ceiling of a room, contains an amplifying laser medium, and this together with a retroreflector in the receiver, forms a laser resonator. When lasing sets in, the receiver can extract some optical power through an output coupler and convert it to electrical power such as in a photovoltaic cell. The resonator described in WO/2007/036937 may be either a ring cavity or a regular cavity formed between the 6 mirrors of the two retroreflectors.
In order to ensure safe operation of the apparatus described in WO/2007/036937, a system is needed to ensure that the amounts of power that can be emitted outside the confines of the laser resonator in a manner that may pose risk to the neighboring environment, such as to people, animals or equipment, or to components of the system itself, is minimal and complies with the safety limitations in operation to prevent such a danger.
In a system such as the one described in WO/2007/036937, power is emitted to the destination device through the output coupler on the receiver. To ensure safe operation, the system must be designed and constructed in such a way that minimal power is dissipated by the system other than to the power destination. This is especially true for optical power, as it poses a bigger risk to persons and the environment than thermal loss, which is usually locally confined. The system described in WO/2007/036937 relies on the cessation of lasing in the event that an obstruction diverts power from the resonator. However, although such cessation of lasing will generally take place, the need for a high level of safety necessitates more comprehensive safety means to prevent leakage of power in unintended directions. However, the beam may be partially blocked by a transparent object, such as a window, causing a part of the power to be reflected outside the beam's path, but without stopping the lasing. Such a beam may pose a risk to persons or objects in its vicinity, and will also reduce the overall efficiency of transmission, such that there is a need to detect such an occurrence.
In published U.S. patent application number US 2008/0130124 there are described safety methods for preventing unintentional dispersion of a laser beam transmitted from a self-contained laser transmitter to a receiver. In published U.S. patent application number US 2008/0084596, there are described motorized mirror assemblies for use with such a laser beam transmission system. Other systems include a subsidiary beam sent from the receiver to the transmitter to verify that the line of sight between transmitter and receiver is not broken by an intruding object.
Other prior art patents of relevance to the techniques used in this field of interest include U.S. Pat. No. 5,553,088 for “Laser amplifying system” to A. Giesen, and U.S. Pat. No. 5,048,051 for “Optically-stabilized plano-plano optical resonators” to J. J. Zayhowski
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.