Abstract:
An optical switching device that routes an optical data packet using an all optical architecture signal detection and switching system. The packet includes header bits, data bits and a reset bit. The header bits identify the switch state for routing the data packet and the specific routing information for distinct portions of the data packet. The header bits are transmitted at an optical carrier frequency different than the carrier frequency of the data bits. The reset bit resets the switch element processor to enable it to process and route the next data packet. The frequency of a particular header bit affects the index of refraction of a Bragg grating of a detector and the output of the detector is provided to a switch that determines the routing path of the packet. A return command resets the diffraction grating so that it does not affect subsequent header bits.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to an optical switching system for routing a digital optical data packet in the optical domain and, more particularly, to an optical switching system for routing a digital optical data packet in the optical domain, where the data packet includes header bits distinguishable from each other and from data bits by their carrier frequency to identify the routing path.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Optical communications networks exist in the art that transmit information from one location to another location along fiber optical cables. In these types of optical communications networks, optical signals are modulated by RF signals to define the information being transmitted. The information is identified by optical digital bits, where groups of bits are transmitted as packets including a predetermined number of bits. Typically, these packets will include several hundred bits. The packets of bits include several header bits that identify the address location to which the information is being routed. Further, overhead bits are usually provided in the packet that are used for system maintenance purposes. The remaining bits which make up the bulk of the packet are the data bits that identify the specific information being transmitted. The packets are routed through a routing network that includes nodes or switches that read the header bits to direct the packets to the desired destination.  
           [0005]    The known optical systems that route packets of optical digital data typically include devices that convert the bits of data to a representative electrical signal using an array of photodiodes. The equivalent electrical signal is sent to a buffer where the header bits are read, and depending on the header bit information, the electrical information packet is routed to the desired location. Usually, the electronic signal switching hardware has the same length header and the same data bit length to be read before the information packet is routed so that there is no selection time saving when the data bits are already used up. Once the routing of the packet is determined, the electrical signal is then converted back to an optical signal for transmission along the next link of fiber optical cable.  
           [0006]    Optical routing devices that convert the optical signal to an electrical signal prior to being read suffer from a number of drawbacks and disadvantages through the switch. Further, because the incoming optical signals are converted to electrical signals, stored in a buffer to be read, and then read, the process is relatively time consuming. Further, the conversion hardware adds significant complexity to the system. Also, it is difficult to vary the speed of the conversion process for different data rates.  
           [0007]    It would be beneficial in terms of speed and efficiency to eliminate the electrical digital conversion step in an optical data routing system, and perform all signal processing and switching in the optical domain. It is therefore an object of the present invention to provide an optical switching system that routes optical data packets in the optical domain.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the teachings of the present invention, an optical switching system is disclosed that routes an optical data packet using an all-optical architecture. The packet includes header bits, data bits and a reset bit. The header bits identify the switch state for routing the data packet and the specific routing information for distinct portions of the data packet. The header bits are transmitted at an optical carrier frequency different from the carrier frequency of the data bits. The number of header bits determines the total number of data packet destinations. It is also possible to use the header bits to identify the form of the incoming data, and to determine synchronization data for the packet that allows for true, all optical ATM packet switching. The reset bit at the end of the packet resets the switching system to enable it to process and route the next data packet. This allows variable length data packets to be sent in the same transmission because the location and the value of the reset bit determines the packet size.  
           [0009]    In one embodiment, the incoming data packet is applied to a series of detectors including Bragg diffraction gratings. The frequency of a particular header bit determines which Bragg grating couples light out of the fiber. The output of a corresponding detector is provided to a switch that determines the routing direction of the packet. A return command causes the diffraction grating to be de-tuned so that it does not affect subsequent header bits having that carrier frequency. A reset detector is provided to detect the reset bit so that the switching system is ready for the next data packet.  
           [0010]    Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is an optical data packet showing the various bits in the packet;  
         [0012]    [0012]FIG. 2 is an optical data packet showing various data transmission protocols that can be used with the switching system of the present invention;  
         [0013]    [0013]FIG. 3 is a schematic block diagram of a switching system for routing an optical data packet in an all optical domain, according to an embodiment of the present invention; and  
         [0014]    [0014]FIG. 4 is a plan view of a header bit detector used in the switching system shown in FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    The following discussion of the preferred embodiments directed to a switching system for routing an optical data packet completely in the optical domain is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.  
         [0016]    [0016]FIG. 1 is a representation of an optical digital data packet  10  that is transmitted through an optical communications system from an optical source to an optical destination where the digital data is processed. The various bits in the packet  10  are identified by modulating a carrier wave, where the modulation may be intensity, phase, frequency or polarization modulation. Optical communication networks of this type are well known to those skilled in the art, and may implement various protocols and optical switching devices.  
         [0017]    The optical data packet  10  includes a header portion  12  and a data portion  14  that are made up of a plurality of optical header bits  16  and data bits  18 , respectively. In this example, the header portion  12  includes eleven header bits  16  that identify the location or destination of the packet  10 , and provide 2 11  such destinations. The bulk of the packet  10  is the data portion  14  which is separated into different data sections  20 , where each section  20  includes a plurality of the data bits  18 . The header bits  16  typically come at the beginning of the packet  10 , and a reset bit  24  comes at the end of the packet  10  to identify the end of the packet  10 . The main purpose of the reset bit  24  is to let the system know not to look for any additional header bits in the packet  10 . FIG. 2 illustrates an optical data packet including a variety of optical formats that can be transmitted through the optical network, including analog, OOK and DPSK, where a reset bit  26  is not positioned at the end of the packet in this example.  
         [0018]    In the conventional system, each bit in the data packet would be transmitted at the same carrier frequency f. The packet would be processed in a serial manner where a certain number of consecutive header bits would identify an address or routing location. According to the present invention, for reasons that will become apparent from the following discussion, different optical carrier wave frequencies are used for the data bits  18  and the header bits  16 . This allows the packet  10  or the header portion  12  to be processed in a parallel manner. For example, each of the bits  18  in the data portion  14  may be transmitted at a carrier frequency f 0 , and the header bits  16  may be transmitted at a different frequency f 1 . Additionally, different bits  16  within the header portion 12  may be transmitted at different carrier frequencies and at different frequencies than the bits  18  in the data portion  14 . For example, the data bits  18  may be at frequency f 0 , the first header bit may be at a frequency f 1 , the second header bit may be at frequency f 2 , the third header bit  16  may be at frequency f 3 , etc. Further, the reset bit  24  may be at yet another frequency f 4 . Different systems will incorporate different carrier frequencies for the various header bits  16  and data bits  18 , within the scope of the present invention.  
         [0019]    The number of header bits  16  determines the total number of data packet destinations. For example, a two-bit header means the data goes to one of four possible locations, and a one-bit header means the data goes to one of two locations. Therefore, not only does the system use the carrier frequency of the header bits to determine the location, but also uses the information in those bits to determine the location. The header bits  16  are encoded with optical intensity frequency, phase or polarization information that can be used to decode and to determine where the data should be routed.  
         [0020]    It is also possible to use the header bits  16  to identify the form of the incoming data. For example, the frequency of the carrier wave for the header bit  16  could be used to determine whether the incoming data is in an analog phase-modulated format, phase shift-keyed digital format, or double side-band suppressed carrier format. The header bits  16  can also be used to determine synchronization data for the packet  10  that allows for true, all optical ATM packet switching.  
         [0021]    [0021]FIG. 3 is a schematic block diagram of an optical signal routing system  30  that routes optical signals to one of several locations, according to an embodiment of the present invention. The routing system  30  receives the optical packet  10  on an input fiber optical cable  32 , and outputs the optical packet  10  on either an output fiber optical cable  34  or an output fiber optical cable  36  depending on the carrier frequency of the header bits  16 . As will be appreciated by those skilled in the art, the system  30  can include other outputs for directing the packet  10  to other destinations within the scope of the present invention. The packet  10  is received by a reset detector system  40  that detects the presence of the reset bit  24  to identify the end of the packet  10 . The operation of the reset detector system  40  will be described in more detail below.  
         [0022]    The packet  10  propagates through the detector system  40  and is received by a header bit detector system  42 . The header bit detector system  42  detects the header bits  16  depending on their carrier frequency. A sequence of serial header bits  44  (arranged serially in time) and parallel header bits  46  (arranged in WDM fashion within the same time slot) are represented at the different carrier frequencies f 1 -f 3  that are acted on by the detector system  42  either in a serial or parallel manner.  
         [0023]    The header detector system  42  includes a plurality of optical detectors, where each detector detects one of the frequencies f 1 -f 3  of the header bits  16 . The number of detectors is determined by the number of carrier frequencies of the header bits  16 . FIG. 4 shows a representative example of one of the plurality of detectors  52  in the header detector system  42 . The detector  52  includes a Bragg diffraction grating  54  formed in a fiber optical cable  56  through which the packet  10  propagates. The diffraction grating  54  includes a series of diffraction lines  58  that are formed in waveguide, and change the index of refraction of the optical medium of the cable  56 . The diffraction grating  54  operates in such a manner that if the light has a particular frequency, it will be diffracted in a desirable manner by the lines  58  as it propagates through the cable  56 . In other words, if the grating  54  is tuned to a particular frequency, the light at that frequency will be directed out of the cable  56  to be received by a fiber optical cable  62  that is optically coupled to the cable  56 . That header bit  16  is then removed from the packet  10 . The use of the diffraction grating  54  is by way of a non-limiting example in that other optical detectors or gratings that detect optical frequencies suitable for the purposes described herein can be used.  
         [0024]    The light received by the fiber optical cable  62  is detected by a photodetector  64  which generates an electrical signal indicative of the optical signal received. For reasons that will become apparent from the discussion below, the diffraction grating  54  receives a control signal that de-tunes the grating lines  58  so that the particular carrier frequency is not diffracted and the light can propagate through the cable  56  without being directed to the photodetector  64 . A separate detector  52  is provided for each of the frequencies so that the processor uses these frequencies to determine the routing of the packet  10 .  
         [0025]    The plurality of signals from the various detectors  52  and the header detector system  42  are applied to a processor  70 . Photodetector  64  in the detector system  42  provides outputs to the processor  70  which determines the command on line  72  applied to an optical switch  74 . Because the data bits  18  in the packet  10  will have a frequency that is not removed by the detectors  52  in the header detector system  42 , they will propagate unimpeded through the detector system  42 . The packet  10  propagates from the detector system  42  on fiber optical cable  76  through a delay  78  provided for timing purposes. The switch  74  provides one of two outputs for the packet  10  depending on the command on the line  72 . Because the switch  74  is being activated based on information in the header portion  12  of the packet  10 , it also acts to resynchronize the data for timing purposes. In different embodiments, multiple commands may be coming from the processor  70  to the switch  74  for those embodiments with more than two output paths.  
         [0026]    The reset detector system  40  also includes a Bragg diffraction grating detector, such as detector  52 , or some other suitable frequency detection device. When the reset detector system  40  detects the reset bit  24 , it sends a signal to the processor  70  that the end of the packet  10  has been received. The processor  70  then sends a signal to the header detector system  42  that is the control signal to the various Bragg gratings  54  for tuning purposes, as discussed above. Once the processor  70  receives the commands from the header detector system  42  identifying the routing of the switch  74 , it can send a signal back to the header detector system  42  to de-tune the various Bragg gratings  54  so that they do not remove subsequent header bits  16  that may have the same carrier frequency that the Bragg gratings  54  were tuned to, so that the these header bits can be used in subsequent routing systems to route the packet  10 . Once the processor  70  receives the signal from the reset detector system  40  indicating that the end of the packet has passed, the signal from the processor  70  to the header detector system  42  can then retune the Bragg gratings  54  to the original frequencies.  
         [0027]    The removed header bits  16  can be reinstated or the remaining header bits can be modified depending on how the data packet  10  is to be routed from the system  30 . An output from the processor  70  is applied to a modifier  82  or  84  that, depending on which output from the switch  74  the data packet  10  is on, will reinsert the header bits  16  removed by the header detector system  42 , or change the carrier frequency of the header bits  16  to adjust the routing of the data packet down the road.  
         [0028]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.