Patent Publication Number: US-2004052446-A1

Title: Optical data interface for communication devices

Description:
FIELD OF THE INVENTION  
       [0001] This invention relates to communication systems. In particular, the present invention relates to a system and method for transferring data between communication devices and other electronic devices.  
       BACKGROUND OF THE INVENTION  
       [0002] Data transmitted by communication systems can represent many different types of information, including, for example, voice channels, full motion video, and computer data. Parallel to the developments in the communication technologies, the increase in computing power and high-speed data processing has spurred the need to share information among computers. Computer users constantly need to share information and services while allowing full mobility.  
       [0003] Many different communication technologies support different services to their users, using a wide range of protocols and transport options. Cellular and cordless phones, pagers and mobile radio units, among the rest use radio frequencies for transport. Infrared, ultrasonic communication and other technologies also play a role as transport methods.  
       [0004] The use of communication devices for audio and video data streaming, Internet communications transmission and other media types requires high bandwidth, to support fast and reliable data transmission. However, wide bandwidth wireless communication devices are usually characterized by the emission of electromagnetic radiation; the wider the bandwidth, the higher the frequency, and the higher the amplitude, the greater is the damage caused by the electromagnetic radiation emitted from the device. There is increasing evidence that this electromagnetic radiation affects and damages biological tissue. This radiation also creates electromagnetic interference, disturbing other electronic devices adjacent to the wireless device.  
       [0005] Furthermore, by increasing the amplitude, it is possible to achieve a clearer signal, thereby achieving better processing. However, as before, the higher the amplitude, the greater is the damage caused by the electromagnetic radiation.  
       [0006] Therefore it is often considered preferable to keep wireless devices as distant as possible from the human body during communication. Also it is considered preferable to keep wireless communication devices as distant as possible from other electronic devices.  
       [0007] Communication devices can be coupled to various end units, such as computers, video monitors, audio devices (e.g. earpieces and/or microphones) etc. However, embedding the communication device in the end units results in the above limitations of electromagnetic radiation and interference, thus impacting on the performance and/or safety of the end units. In order to overcome the above limitation and avoid the need to embed the communication devices within the end units, they can be coupled to the end units, most commonly, through metallic conductors. However, such cables do not solve the radiation problem: to the contrary, the metallic conductor actually becomes an antenna, conducting, and probably also amplifying the emitted radiation.  
       [0008] WO 01/13661 (Qualcomm Incorporated) describes a system for connecting a wireless phone to an external circuit. The system includes a first optical data interface adapter connected to the wireless phone and a second optical data interface adapter, connected to the external circuit. A fiber optic cable connects the first optical data interface adapter to the second optical data interface adapter.  
       [0009] Such a system transmits the data received by the wireless telephone to the external circuit “as is”, without modification. Therefore, if such a system is used with a high bandwidth telephone transmitting a large amount of data, the system requires the use of a high diameter optical fiber cable, supporting a wide enough bandwidth, or it will suffer from delays and loss of data. Optical fiber cables are expensive resources, and their cost increases as their diameter increases. Therefore, from purely cost considerations it is preferable to use as low diameter fibers as possible. Moreover, low diameter fibers are suitable for some applications, such as optical microphones (Phone-or&#39;s LiteMic is an example for a commercially available optical microphone model), which would be impractical to implement using high gauge cables. However, the use of low diameter optical fiber cables militates against their use for high bandwidth data transmission, which is becoming increasingly required. It would clearly be desirable to modify the system described in WO 01/13661 so as to allow high bandwidth communication albeit using low diameter optical fiber cables.  
       [0010] Signal conversion units are now on the market that directly convert an optical signal to a desired output, or vice versa, i.e. convert any kind of input, such as electromagnetic signals to optical signals. For example, optical CD players and recorders (an example of a commercially available optical CD player and recorder is model DN-C550R manufactured by Demon) convert an optical signal directly to audio data and vice versa without any need to convert first to a corresponding electrical signal. However, if such signal conversion units are used in the system described in WO 01/13661, the optical signal propagated through the optical fiber cable is converted to an electrical signal that is fed to a computer. It would therefore be necessary to couple a third signal conversion unit to the computer for reconverting the signal back to an optical signal. The computer, as well as the third signal conversion unit, both introduce delays as well as introducing components that are redundant and increase the cost of the system with no benefit.  
       [0011] It is further necessary to understand that during propagation of a signal along an optical fiber cable, a carrier light wave is propagated along the optical fiber, which carries a data component. The carrier is characterized by a deflection angle, characterizing its deflection when impinging on to the optical fiber&#39;s wall. The carrier is used to carry modulated optical data components.  
       SUMMARY OF THE INVENTION  
       [0012] In order to address the above-mentioned drawbacks, there is provided in accordance with a first aspect of the invention a system for communicating a signal between a communication device and an external circuit, the system comprising:  
       [0013] a signal conversion unit connected to said communication device responsive to a received electromagnetic signal for producing a corresponding optical signal component, and  
       [0014] an optical fiber cable coupling said signal conversion unit to the external circuit for propagating said optical signal component to the external circuit;  
       [0015] characterized in that:  
       [0016] a signal filtering processor is coupled between said communication device and said signal conversion unit for processing the electromagnetic signal prior to converting to the optical signal component.  
       [0017] The invention further provides for a system for communicating a signal between a communication device and an external circuit, the system comprising:  
       [0018] a signal conversion unit connected to said communication device responsive to a received electromagnetic signal for producing a corresponding optical signal, and  
       [0019] an optical fiber cable coupling said signal conversion unit to the external circuit for propagating said optical signal to the external circuit;  
       [0020] characterized in that:  
       [0021] a signal filtering processor is coupled between at least one said communication device and said signal conversion unit for identifying and processing the electromagnetic signal prior to converting to the optical signal, and  
       [0022] an optical filter is coupled between the signal conversion unit and the optical fiber for receiving the optical signal and producing at least one corresponding filtered optical signal component.  
       [0023] Still further, the invention provides for a method for communicating a signal between a communication device and an external circuit, the method comprising:  
       [0024] One) processing an electromagnetic signal output by the communication device for automatically identifying a respective communication protocol thereof,  
       [0025] Two) filtering the electromagnetic signal prior to converting to an optical signal component,  
       [0026] Three) converting the electromagnetic signal to the optical signal component, and  
       [0027] One) conveying the optical signal component via an optical fiber cable to the external circuit.  
       [0028] The invention supports the use of low diameter optical fiber cables by processing the data received at the communication device, before converting the electromagnetic signal to optical signals, and performing signal filtering on the received signal. The signal filtering is performed by a signal filtering processor coupled between the communication device and a signal conversion unit. The signal conversion unit may include photodetectors and transmitters as is known in the art and converts the filtered signals into optical signals, and possibly propagates more than one optical signal along the optical fiber, such that each optical signal is characterized by a unique deflection angle. To this end, the filtered signals can be suitable for specific end-units such as CD players in the case of audio data. Therefore, in the event that the end-units are optical (e.g. optical CD players) they may be coupled directly to the optical fiber without requiring intermediate conversion to and from an electrical signal. This provides a mechanism that prevents the delay characteristic of the system described in WO 01/13661.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0029] In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:  
     [0030]FIG. 1 is a schematic representation of one proposed embodiment of the invention.  
     [0031]FIG. 2 is a schematic representation of another proposed embodiment of the invention, supporting many different communication devices.  
     [0032]FIG. 3 schematically illustrates time slot division.  
     [0033]FIG. 4 is a schematic representation of yet another proposed embodiment of the invention, including an optical filter.  
     [0034]FIG. 5 is a flow chart illustrating the functionality of the signal filtering processor.  
     [0035]FIG. 6 illustrates filtering an optical signal component characterized by a certain wavelength out of an optical fiber.  
     [0036]FIG. 7 illustrates combining two different optical signal components to activate a single external circuit. 
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS  
     [0037] In the following description, components that are common to different embodiments are referenced by identical reference numerals.  
     [0038]FIG. 1 shows a schematic representation of a system  100  according to a first embodiment of the invention. A communication device  101  is connected to a signal flittering processor  102 . Downstream a signal conversion unit  103  translates the electromagnetic signal received from the communication device and the signal flittering processor to an optical signal, which further propagates along the optical fiber cable  104 , and vice versa, the system being bi-directional. Optical signals propagate along the optical fiber cable  104 , and impinge on the signal conversion unit  103 , where they are converted to electromagnetic signals. The electromagnetic signals propagate towards the signal filtering processor  102  and the communication device  101 , which further transmits them according to the communication protocol in use.  
     [0039] Any type of communication device can be connected to the system  100  in accordance with the invention. Many communication devices characterized by different communication protocols and transports for wireless and wired data transmission exist in the market. For example, regular telephones, cellular TDMA, cellular CDMA, satellite and infrared communication, Bluetooth devices etc are know. Each of these communication devices uses different communication channels or protocols to transmit data. To avoid the need to manufacture different signal filtering processors for different communication devices, the signal filtering processor described by the invention provides the ability to identify the transmission method, therefore allowing the use of a single signal filtering processor for any communication device.  
     [0040] Furthermore, the system  100  enables using a single processor for receiving data from more than one device at the same time, as shown in FIG. 2, showing schematically a system  200  according to a second embodiment of the invention, supporting many different communication devices. This embodiment utilizes the optical fiber cable&#39;s ability to conduct higher capacities of data, compared to a metal conductor.  
     [0041] Thus, as shown in FIG. 2, the single signal filtering processor  102  connects to four different communication devices: a cellular CDMA telephone  201 , a cellular GSM telephone  202 , a wireless infrared (IR) communication device  203  and a regular telephone  204 . The filtered electromagnetic signals propagate to the signal conversion unit  103 , where they are converted to optical signals, which propagate through the optical fiber cable  104 . By such means, the optical fiber cable  104  can conduct data received by more than one communication device according, to its higher conductance capacity.  
     [0042] As in the system  100  shown in FIG. 1, also in the system  200  illustrated in FIG. 2, the signals can propagate in the opposite direction, upon being transmitted by the appropriate communication device according to criteria stored in the signal filtering processor.  
     [0043] Use of the system  200  will now be described by way of example. A doctor is carrying a laptop computer for typing his conclusions (textual data). The doctor uses an EKG monitor to perform EKG tests on his patients. In urgent cases he transmits the EKG results (the EKG graphs&#39; data) to a remote medical center, while at the same time also speaking with personnel situated at the medical center (audio data) via an optical microphone, to give his indications orally. The laptop, the EKG monitor and the optical microphone all have optical interfaces and are connected to the optical fiber cable. The doctor can configure the signal filtering processor to transmit the EKG graphs&#39; data through the cellular GSM mobile telephone  202 , the textual data through the IR communication device  203 , and the audio data through the regular telephone  204 .  
     [0044] While transmitting data through a communication channel, the data occupies a certain fraction (or the whole) of the available bandwidth. Many sources of data, e.g. a full motion video, require a higher bandwidth than the available capacity, which in turn requires a wider range of frequencies dedicated for the transmission for increasing the bandwidth. The increased range of frequencies involves higher radiation emitted from the wireless communication devices. To avoid this limitation, or to improve the performance, it is possible to use digital compression. However, simple digital compression may still result in data requiring a high bandwidth channel for propagation and therefore more advanced compressions are preferred.  
     [0045] On the other hand, some data do not utilize all the available bandwidth of a communication channel. In such case, while transmitting data through a communication channel, it is also possible to transmit from numerous sources in parallel (the signal from each source constituting a signal component), coding their data altogether to generate a composite signal. For example, medical data such as the representation of EKG graphs can be transmitted together with audio signals and textual data. The composite signal is characterized by a better utilization of the available bandwidth, as opposed to the regular, single signal transmission.  
     [0046] The composite signal may be produced in three ways. According to a first approach, a fraction of the available bandwidth is utilized for transmitting a first analog signal, while adding a second analog signal to the unutilized bandwidth. The first analog signal is referred to as the first analog component of the composite signal, while the second signal is referred to as the second analog component of the composite signal. In this case a conventional receiver can always receive all the analog components of the signal. According to a second approach, the second signal is a digitally compressed signal, referred to as the second digital component of the composite signal, which is added to the first analog component. In this case, a conventional receiver can always receive the first analog component of the signal, but to receive the second digital component, the receiver must have a suitable decoder, able to decode digital data. According to a third approach, both signals are compressed digitally requiring the receiver to have a suitable decoder in order to receive and decode any one of the received components.  
     [0047] To receive and decode the digital components of the signal, both the transmitter and the receiver must be equipped with a respective encoder/decoder using an identical communication protocol.  
     [0048] According to the invention, the signal filtering processor detects an incoming signal and identifies whether the signal is a composite electromagnetic signal or a simple one. When the incoming signal is found to be a composite signal, the signal filtering processor detects and decodes the constituent signals of the transmitted composite signal, splitting it into electromagnetic signal components. In the case of a composite incoming signal, the total bandwidth occupied together by all the split signal components is higher than the bandwidth occupied by the composite signal. The signal filtering processor recognizes and identifies different signals using known headers characteristic of each, as known to those versed in the art.  
     [0049] The connection between the signal filtering processor and the signal conversion unit has a limited bandwidth, i.e. a limited data capacity. Moreover, the same limitation exists for the optical fiber cable, which must be kept as narrow as possible. To further convey the composite signal&#39;s optical components (or the simple signal, in the case that the received signal was detected as such) over as narrow a bandwidth channel as possible, the signal filtering processor may perform time slot division multiplexing. In time slot division multiplexing, each of the signal&#39;s electromagnetic components may be conveyed by the signal filtering processor during a respective time slot. Downstream, the signal conversion unit converts each electromagnetic component, carried by a dedicated time slot, to the equivalent optical signal component prior to being transmitted over the optical fiber cable. To enable real time transmission, the time slots must altogether occupy no more than a predetermined time period (for example 125 microseconds in TDMA).  
     [0050]FIG. 3 schematically illustrates time slot division, splitting a composite electromagnetic or optical signal, composed of EKG graphs&#39; data component  301 , audio data component  302  and textual data component  303  into three time slots occupying altogether a 12 microsecond cycle  304 . The cycles&#39; composition repeats itself as long as the three components coexist and compose together the composite signal. Moreover, the different components may occupy time slots characterized by a different duration. In the example illustrated in FIG. 3, the EKG component  301  occupies time slots of four microseconds, the audio data component  302  occupies time slots of six microseconds and the textual data component occupies time slots  303  of two microseconds.  
     [0051] A total duration of the time slots longer than the predetermined time period (12 microseconds in the example illustrated by FIG. 3) results in a delayed transmission, and probably also in data loss. However, it may happen that the nature of the received signals requires more than the predetermined time period altogether. For example, in the case of a predetermined time period of 12 microseconds, if the received composite transmission contains two video signal components, splitting them into two time slots of 6 microseconds each will not suffice for a quality video transmission. Therefore, the signal filtering processor must determine which of the two is more important and transmit it first, dedicating enough bandwidth for it. Identification and priority determination in this case can be based on the transmission&#39;s source. In the above example of a doctor communicating with a remote medical center or hospital, a video transmitted by the hospital&#39;s information departments is more important than a video transmitted to him by any other source. The less important signal component, in this example, can be processed in different ways. For example, the signal filtering processor can record the signal component for a later playback, or ignore it and discard the data. A different approach may be to transmit both signal components while reducing the respective qualities of both. In the above example, 6 microseconds will be dedicated for each signal component, whereby both video signals will reach their destination, albeit with reduced quality.  
     [0052] According to the invention, it is possible to configure the signal filtering processor, in a way that grades, or prioritizes the signals according to their types and according to their relative importance. For example, EKG graphs may be more important than audio data, which in turn may be more important than video signals and so on. The signal filtering processor may be responsive to the grading, thus applied so as to allocate component signals within the time slots according to their relative importance so that a signal component of high importance is given preference to one of lesser importance. The signal filtering processor&#39;s configuration also allows a received signal having an unrecognized or unwanted signal type to be automatically discarded. Upon receipt of an unrecognized signal type, the processor can transmit it “as is” or ignore it. While transmitting this signal, as the signal filtering processor does not know what is the minimal required time slot, it can delay the signal, and dedicate all the predetermined time period (in the above example 12 microseconds) to it, when it is available. According to the invention, it is possible to define the signal types known by the signal filtering processor, and introduce new signal types when needed.  
     [0053] It should be noted that the terms “signal component”, “component signal” and “data component” are used interchangeably, and they all mean the same.  
     [0054] Another embodiment of the invention relates to a signal filtering processor that performs frequency and phase division multiplexing on the composite signal, instead of time slot division. The components of the electromagnetic composite signal are split, and converted by the signal conversion unit to predetermined frequencies and phases characterizing the corresponding optical signal components. The carrier can then carry multiple optical data components, either having a different wavelength, and/or having the same wavelength, but being mutually orthogonal. It is also possible to have multiple carriers propagating along the optical fiber, each carrying single or multiple optical data components. A different embodiment relates to the usage of a bundle of optical fibers, forming together an optical fiber cable, on each fiber of which a carrier is forwarded. A combination of the described embodiments is also possible. It will be understood that the above embodiments of frequency and phase division are merely representative, non-limiting examples.  
     [0055] The signal filtering processor can command the signal conversion unit to propagate several optical signal components at the same time along the optical fiber cable, thus distinguishing the current invention over the invention described in WO 01/13661. According to one embodiment, the signal conversion unit may contain a prism, such as a pentaprism (a commercially available example being that manufactured by Oriel under catalog number 46200), propagating different optical signal components with different deflection angles. Other embodiments may use other optical means such as mirrors, filters or combinations thereof Controlling the signal conversion unit by the optical filtering processor is known to those versed in the art.  
     [0056] Note also that time slot division, frequency and phase division or a combination of both can be performed for multiplexing several components of a composite electromagnetic signal received from a single communication device, or for multiplexing several simple electromagnetic signals received from multiple communication devices, or for a combination of both.  
     [0057] In all the previously described embodiments, the signal filtering processor performs signal manipulations (such as time slot division or frequency and phase division) on the received electromagnetic signals, which are then converted to optical signals by the signal conversion unit. According to the invention, it is possible to obtain further improvement by means of a system  400  according to another embodiment of the invention illustrated in FIG. 4. The system  400  includes an optical filter  401  coupled between the signal conversion unit  103  and the optical fiber cable  104 . The signal filtering processor  102  controls the optical filter  401  by an appropriate control signal  402 , allowing it to perform optical manipulations (such as time slot division or frequency and phase division) to produce filtered optical signal components. It is possible to achieve an optical filter  401  for example by using a micro video imaging leans, and by controlling the lens&#39;s angle relative to the signal conversion unit  103  and to the optical fiber cable  104 . An example of a commercially available lens can be found in Edmund Industrial Optics, catalog number N001B.  
     [0058] Note that the filtered optical signal components are equivalent to the previously described optical signal components.  
     [0059] Filtering by optical means is more efficient than performing filtration of the electromagnetic waves by the signal filtering processor. The signal filtering processor directs the optical filter to perform time slot division and/or frequency and phase division in order to multiplex several components of a composite signal, in order to multiplex several simple signals received from multiple communication devices, or in order to multiplex a combination of both.  
     [0060] It should be noted that whereas the embodiments described so far relate to systems having only one signal conversion unit, one optical filter and one optical fiber cable, if desired, multiple signal conversion units and/or multiple optical filters and/or multiple optical fiber cables may also be used.  
     [0061]FIG. 5 is a flow chart illustrating the operation of the signal filtering processor. On startup denoted by step  501 , the processor reads the priority levels set by the user, according to which a received signal will later be graded. Priority can be inserted by the user on the fly, or it may be read from memory, such as non-volatile memory. At step  502 , the signal filtering processor then reads the communication volumes supported by the different components of the system, such as communication devices and optical fiber cable, in order to later be able to filter and route communicated data. Finally at step  503 , the signal filtering processor detects the communication protocol of the connected communication devices. Now the signal filtering processor is ready to start receiving and/or transmitting data as designated by step  504 , i.e. the signal filtering processor is ready to start receiving data from the communication devices or from the signal conversion unit. At step  505 , the processor detects the data type and compares it to the priority levels read at step  501 , and, at step  506 , transmits (to the signal conversion unit or to the communication devices) the highest priority data according to the available communication volume and priority levels. If at step  507  the supported data volumes suffice and as long as step  508  determines that there is more communication data to transmit, the signal filtering processor continues to transmit the highest priority data which is currently all the data it has to transmit. However, if step  507  determines that there is insufficient data volume supported by the communication devices to transmit all the received data, then at step  509 , the signal filtering processor stores lower priority data for later transmission. In the case where the signal filtering processor is embedded in a portable computer, lower priority data can be stored, e.g. on the computer hard disk. In another example, the processor can be embedded in an ASIC carrying also memory for data storage, this ASIC being installed in a dedicated case where required connectors are also installed. As long as the highest priority data transmission does not terminate at step  510 , the signal filtering processor branches to step  506  and continues to transmit the highest priority data and possibly to store the lower priority data at step  509 . However, when at step  510  the highest priority communication terminates, the signal filtering processor starts reading the stored lower priority data at step  511 , and again branches back to step  506  where it starts to transmit lower priority data, which, being now the only data, constitutes the instant highest priority data. If there is still insufficient data volume to transmit all the previously stored lower priority data, the flow continues. The signal filtering processor terminates when all data is transmitted, that is when there is no more “highest priority data” to transmit at step  508 .  
     [0062] The various embodiments described so far, with or without the optical filter, all result in optical signal components carried by carriers propagating along an optical fiber cable, the signals being characterized by specific amplitudes, which correspond to the nature of the propagating signal. The carrier is characterized by a certain deflection angle.  
     [0063] Being characterized by a known deflection angle, it is possible to compute the exact location where a certain carrier (carrying one or more specific optical signal components) would impinge on the optical fiber&#39;s wall. According to one embodiment, a holographic notch filter (commercially available examples being manufactured by Oriel under catalog numbers 53680-53686) is located at the computed location. Such a filter permits a signal component with a specific wavelength (carried by the impinging carrier and characteristic of the used filter) to continue propagating further on, without deflecting back into the optical fiber&#39;s interior. By such means, the signal component is extracted from the optical fiber toward a respective path, creating another signal component.  
     [0064]FIG. 6 illustrates a system  600  for filtering an optical signal component characterized by a specific wavelength so as to exit an optical fiber  601 . A carrier  602  propagates along the optical fiber  601  and carries two optical signal components  603 ,  604 . A filter  605  for a certain wavelength is disposed on an external wall of the optical fiber  601  at a computed location as explained above. When the carrier  602  impinges on the filter  605 , the optical signal component  604  is extracted from the optical fiber  601 , further creating another optical signal component  606 , which propagates over an auxiliary optical fiber  607 . The carrier  602  continues to propagate along the optical fiber  601 , carrying optical signal component  603 .  
     [0065] It should be noted that in order to extract a signal component from within the optical fiber, other optical means beside a filter can be used. For example, a mirror may be used to deflect the carrier, or a beam splitter may be used such as a polka dot beam splitter (commercially available examples being Oriel&#39;s polka dot beam splitters, catalog numbers 38105-38106). Both allow propagation of a first component of the carrier along the optical fiber while extracting a second component of the same carrier. Thus, assuming the carrier originally carries two optical signal components, which may be characterized, for example by different wavelengths, one optical signal component may continue to propagate along the optical fiber carried by the first carrier component, while the other optical signal component may be deflected out of the optical filter by the beam splitter, when carried on the second carrier component. In the case of two different optical signal components characterized by the same wavelength but being orthogonal in phase, a prism may be used to separate the two waves and orient them towards different respective paths. Note also that any combination of optical means can also be used, i.e. a mirror and a prism.  
     [0066] One or more external circuits  608  can be coupled to the optical fiber  607 , to be activated by the optical signal components  606 . For example, assume that the optical signal component  604  carries audio data. By deflecting it into another optical signal component  606 , and by coupling to it an external circuit  608  in the form of a CD player, it is possible to directly play the audio data represented by the optical data component  606  by the optical CD player without needing first to convert it to electromagnetic data, as would be required using a system designed according to WO 01/13661. There might also be two external circuits (not shown) coupled to the optical fiber, both activated by the same optical component. In the example above, two CD players might play the same audio data.  
     [0067]FIG. 7 illustrates a system  700  for combining two different optical signal components propagating through an optical fiber  701  in order to activate a single external circuit disposed external to a wall of the optical fiber. Two optical signal components  702 ,  703 , carried by two respective carriers, can be extracted from the optical fiber  701  by two different optical means  704  and  705 , thereby generating two other optical signal components  706  and  707  respectively. It is possible to further deflect the two optical signal components  706  and  707 , for example by using prisms  708 ,  709 , so to direct the optical signal components towards a common location where a single external circuit  710  is located. It should be noted that there may propagate along the optical fiber  701  other optical signal components (not shown), which are not conveyed to the single external circuit  710 .  
     [0068] This embodiment may be useful mainly when the data required to activate the external circuit is too large to transmit by a single optical (and possibly also electromagnetic) wave component. Consider, for example, that a movie, composed of a succession of video images and of audio, must be transmitted to a video projector connected through a mobile telephone, and possibly also by multiple mobile telephones. The movie is too large to be transmitted as a single unified signal to a single mobile telephone. Therefore, it is preferable to split the movie into two signal components, the first signal component carrying the succession of video images and the second signal component carrying the audio component. The two signal components can form a single composite signal having lower bandwidth than the bandwidth occupied by the original movie. Thus, this composite signal can be transmitted toward a single mobile telephone. However, it is also possible to transmit the two signal components to two different mobile telephones.  
     [0069] By receiving the two signal components and converting them to two optical signal components, the invention overcomes the large bandwidth problem. However, it is now required to re-combine both optical signal components to form together the full motion video at the video projector, which is the external circuit according to this example. This can be performed by the system  700  described above with reference to FIG. 7 of the drawings.  
     [0070] In the method claims that follow, alphabetic characters and Roman numerals used to designate claim steps are provided for convenience only and do not imply any particular order of performing the steps.  
     [0071] The present invention has been described with a certain degree of particularity, and accordingly those versed in the art will readily appreciate that various alterations and modifications may be carried out without departing from the scope of the following claims. In saying this, it will be appreciated by those skilled in the art that the features defined by the subsidiary claims may be combined. Thus, for example, a system according to the invention may include multiple communication devices and/or multiple external circuits, each having the relevant features set out in the appended claims relating to a system having a single communication device and a multiple external circuit only.