Abstract:
The system includes a generally broadband, low coherence length light source that injects light into a fiber beamsplitter that is used to generate counterpropagating light beams in a Sagnac loop. The loop includes two facing fiber beamsplitters connected together at differing length inner legs, with one of the output legs of the second beamsplitter usually being connected to a in place optical fiber that ends with a phase modulator followed by a mirror. Formatted data is transmitted by impressing relative phase differences between the counterpropagating light beams. Optimum performance depends on appropriate choices for critical lengths in the system.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/123,712 by Eric Udd et al., entitled, “Single Fiber Sagnac Interferometer Based Secure Communication System” which was filed on Mar. 10, 1999. 
    
    
     REFERENCE TO RELATED PATENTS 
     This disclosure describes means to provide the Sagnac secure fiber optic communication systems similar to those described in detail in U.S. Pat. Nos. 5,223,967, 5,274,488, 5,311,592, 5,422,772, and 5,455,698 using a single installed optical fiber. The teachings in those patents are incorporated into this disclosure by reference as though fully set forth below. 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to fiber optic communications and more particularly to Sagnac interferometer based fiber optic communication systems that utilize counterpropagating optical beams to impress data on a loop by means of phase modulation. In order to make a system of this type practical for general use, a flexible, single fiber configuration is needed to support the base of currently installed optical fiber. 
     The need for high bandwidth secure communication systems that are amenable to usage in networks and which minimize the need for encryption is becoming increasingly acute as more sophisticated systems come on line. Generally, encryption reduces the data throughput of a given system by an order of magnitude when compared to non-encrypted throughput. The need for additional data throughput is expected to continue into the indefinite future with networks supporting ever greater numbers of users demanding higher and higher bandwidth. The advent of fiber optic telecommunication systems has opened up a new era of low cost, high bandwidth systems that are enabling a host of new applications. Sagnac secure fiber optic communication systems offer the prospect of transmitting this data securely, but heretofore, such has not been adaptable to the installed single fiber, single light band links. 
     SUMMARY OF THE INVENTION 
     There is provided by this invention a Sagnac interferometer based secure fiber optic communication system that allows transmission of data securely along conventional telecommunication fiber cables. This invention is designed to include many of the advantages of the conventional loop configuration of Sagnac secure fiber optic communication systems while minimizing dispersion and fiber compatibility problems by using a single fiber configuration between the transmitting and receiving ends of the system. 
     The system includes a light source that is generally broadband with a low coherence length. The light source injects light into a fiber beamsplitter that is used to generate counterpropagating light beams in a Sagnac loop. The loop includes two facing fiber beamsplitters connected together at both inner legs, with one of the output legs of the second beamsplitter being connected to a transmission fiber line that ends with a phase modulator followed by a mirror at a fixed distance. Formatted data may be transmitted by impressing relative phase differences between the counterpropagating light beams in a manner similar to that described with respect to other secure Sagnac interferometric communication systems. Optimum performance depends on appropriate choices for critical lengths in the system. 
     Therefore, it is an object of the present invention to provide a secure fiber communications system that can use conventional fiber optic communications fibers as installed for non-secure telecommunications. 
     Another object is to essentially eliminate the need to install special optical fibers and links to install a secure fiber communications system. 
     Another object is utilize installed zero dispersion single mode fibers in a secure fiber communications system whether they be designed for 1300 or 1550 nm light. 
     These and other objects and advantages of the present invention will become apparent to those skilled in the art after considering the following detailed specification including the drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic representation of a typical prior art secure fiber optic communication system based on the Sagnac interferometer; 
     FIG. 2 is a schematic representation of a single fiber Sagnac interferometer secure communication system constructed in accordance with the present invention; and 
     FIGS. 3 and 4 are schematic representations of the single fiber Sagnac interferometer secure communication system of FIG. 2 showing the preferred relationships of the path lengths. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a prior art Sagnac secure fiber optic communication system  10  such as shown in U.S. Pat. No. 5,311,592. A light source  11  that may be spectrally broadband generates a light beam  12  that enters into an optical fiber  14 . A fiber depolarizer  16  may be used to scramble the polarization states of the light beam  12  and reduce residual preferential polarization. The light beam  12  is then split by a central beamsplitter  18  into the counterpropagating light beams  20  and  22 . The light beam  20  exits the receiver area  23 , passes through a fiber link  24  into the transmitter area  26  and reaches a phase modulator  28 . Formatted electrical data  30  is then converted to corresponding phase modulation on the light beam  20  by the modulator  28 . The light beam  20  then passes through a second polarization scrambler  32  used to reduce environmental effects, passes a second fiber link  34  and returns to the central beamsplitter  18 . The light beam  22  traverses the fiber link  34 , the polarization scrambler  32  and enters the phase modulator  28 . The phase modulator  28  is offset from the center  36  of the fiber loop  38 , which includes the lengths of fiber  24 ,  34  and the polarization scrambler  32  as well as the physical length of the phase modulator  28 . Thus, the time of arrival of the light beam  22  at the phase modulator  28  is different (earlier in this example) from the time of arrival of the light beam  20  and a net phase difference between the two beams  20  and  22  results through the action of the electrically formatted data signal  30  impressed on the phase modulator  28 . The light beam  22  then continues to circulate through the fiber loop  38  and returns via the fiber link  24  to the central beamsplitter  18 . 
     When the two light beams  20  and  22  recombine on the central beamsplitter  18 , their relative phase determines how much optical power is split into the fiber  14  and back to the light source  11  and how much is directed into the fiber  40  and onto the output detector  42 . If the two light beams  20  and  22  are completely in phase, all the optical power from their recombined beam is directed toward the light source  11 . If the two light beams  20  and  22  are 180° out of phase, all the light power is directed toward the output detector  42  via the fiber  40 . For situations where the phase difference between the beams  20  and  22  is between 0° and 180°, the combined light beam power is split. The result is an amplitude modulated light beam  44  directed to the output detector  42 . The amplitude modulated light beam  44  is converted by the output detector  42  to an electrical output signal  46 . 
     One of the major issues with the Sagnac secure fiber optic communication system  10  as shown in FIG. 1 is that a simplex link requires two fiber lines (the links  24  and  34 ). It is possible to use wavelength division multiplexing by interleaving two Sagnac loops at 1300 and 1550 nm to reduce the number of lines to two for full duplex. However there is a very large installed base of single mode fiber designed for zero dispersion at 1300 nm and a second large base optimized for 1550 nm. Unfortunately, dispersion is very high in these fibers at the wavelengths they are not designed for. The net result is that the choice then becomes that of implementing full duplex systems using four fiber lines using simplex links similar to that of FIG. 1 or going to limited length wavelength division multiplexed systems of 1300 and 1550 nm because of dispersion. 
     An alternative approach is to use a single fiber system. Early versions of single fiber systems were described in U.S. Pat. No. 5,223,967. The present invention includes how operation of single fiber systems can be optimized by appropriate choices of fiber lengths. Also methods to optimize performance and add cable monitoring and alarm systems are described. 
     FIG. 2 shows a schematic diagram of a single fiber Sagnac interferometer communication system  60 . Light from the light source  62  is coupled into the fiber  64  as the light beam  66 . A fiber scrambler  68  may be used to reduce residual polarization of the light beam  66 . The light beam  66  is then split by a central fiber beamsplitter  70  into the counterpropagating light beams  72  and  74 . The light beam  72  enters a fiber leg  76  that is of length L 1  and enters a second beamsplitter  78 . At the beamsplitter  78  the light beam  72  is split into the light beams  80  and  82 . The light beam  82  is directed toward a termination  84  that is designed to avoid back reflections and exits the system  60 . It is possible to use the port defined by the termination  84  to support a second transmitter as described in U.S. Pat. No. 5,223,967. 
     The light beam  80  enters a fiber leg  86  of length L and passes an alarm/cable monitoring system beamsplitter  88 . A portion of the light beam  80  is split off as the light beam  90 , which via the fiber  92  reaches an alarm/cable monitoring detector  94 . The electrical output  96  of the detector  94  can be used to support the monitoring of the cable  86  directly and/or via a ratio circuit  98 . 
     The other portion  100  of the light beam  80  passes though wavelength division multiplexing elements  102  and  103  with their included wavelength division multiplexing optical time domain reflectometer (OTDR) ports  104  and  105  respectively. The OTDR ports  104  and  105  are used to determine the location of a tap detected by the alarm/cable monitoring detector  94  or other detection device. The beam  100  then enters a phase modulator  106 , which impresses phase information on the light beam  100  corresponding to a formatted electrical data stream  108  (the length L being the distance from the beamsplitter  78  to the phase modulator  106  with L normally representing previously installed communication fiber). The light beam  100  then passes through a fiber leg  110  of length L 3  and is reflected off a mirror  112 . A portion  114  of the light beam  100  may pass through the mirror  112  and onto a cable monitoring, alarm detector  116 . Other components can be used to reverse the light flow such as short fiber loops and other beam turning devices 
     The reflected portion  118  of the light beam  100  passes back through the fiber leg  110  and through the phase modulator  106  where it is again phase modulated via the action of the electrical formatted data  108 . The light beam  118  then passes through the fiber  86  of length L, and the wavelength division multiplexing elements  104  and  102  and reaches the fiber beamsplitter  88  where it is split into the light beams  120  and  122 . The light beam  122  is directed to a detector  124 , whose output  126  is used to support a cable monitoring alarm system directly and/or via the output ratio circuit  98  which compares the electrical outputs  126  and  96 . 
     The light beam  120  continues onto the beamsplitter  78  and is split into the light beams  128  and  130 . The light beam  128  passes through the fiber leg  76  of length L 1  (from beamsplitter  70  to beamsplitter  78 ) onto the fiber beamsplitter  70 . The light beam  130  passes through a polarization scrambler  132 , which is part of a fiber leg  134  of length L 2 . The lengths L 1  and L 2  are chosen so that their difference exceeds the coherence length of the light source  62 , so that the beams  128  and  130 , when they combine on the fiber beamsplitter  70 , do not interfere, because during their two passes between the beamsplitters  70  and  78 , they end up traveling different path lengths whose difference is more than the coherence length, as do all beams except the light beams which are the data carrying light beams, described below. 
     The light beam  74  passes through the fiber leg  134  of length L 2  (from beamsplitter  70  to beamsplitter  78 ) and the polarization scrambler  132  to reach the fiber beamsplitter  78 , which splits the light beam  74  into light beams  136  and  138 . The light beam  138  exits the port  84 . The light beam  136  follows paths similar to that described in association with light beam  80 . A light beam  140  derived from the light beam  136  returns to the beamsplitter  78  and is split into the two light beams  142  and  144 . The light beam  142  propagates through the fiber leg  76  and returns to the beamsplitter  70 . The light beam  144  propagates through the fiber polarization scrambler  132  and the fiber leg  84  to return to the fiber beamsplitter  70 . The light beams  142  and  144  do not interfere on beamsplitter  70  because their paths  76  and  134  differ in length by more than the coherence length of the light source  62 . The only light beams of the set  128 ,  130 ,  142  and  144  that can interfere are light beams  130  and  142  because they have both traversed the paths of length L 1 +L 2 +2L+2L 3 . All other combinations of these light beams have a net path length difference of at least the absolute difference between L 1  and L 2  so there is no interference between them. Thus the two light beams  130  and  142 , when they are in phase with respect to each other, result in light being directed toward the light source  62 . When they are 180° out of phase, the combined light beam  146  is directed toward a detector  148  via the fiber leg  150 . For conditions in between, the split ratio varies depending on the phase difference. The net result is that an amplitude modulated light beam  146  is directed the output detector  148  that in turn coverts the modulated light beam  146  into an amplitude modulated electrical signal  152 , which contains the data of the electrical formatted data  108 . 
     There are special operating conditions associated with the single fiber configuration illustrated by FIG.  2 . FIG. 3 shows an unfolded version of the light paths of the system  60  followed by the light beams  130  and  142  originating and ending at the beamsplitter  70  that interfere with each other. Note that the two light beams traverse the phase modulator  106  in FIG. 3 twice. The exact amount of phase impressed on a light beam by this “double pass” system depends on the exact format of the electrical data stream driving the phase modulator and the length L 3  between the phase modulator  106  and the reflecting mirror  112 . There are two important special cases where the appropriate choice of the length L 3  greatly simplifies the requirements on the input electrical data stream  108 . The first is when the lengths L 1 , L 2  and L 3  are chosen so that one “position” of the phase modulator  106  is in the center of the “loop” path for the beams  130  and  142  (shown in dashed outline). This means that both beams  130  and  142  arrive at the phase modulator  106  simultaneously, resulting in no net phase difference. Looking at the “unfolded” schematic of FIG. 3, this can be the case when L 1 +2L 3 =L 2  or L 2 +2L 3 =L 1 . Both of these situations place one of the positions of the phase modulator  106  in the two pass system at the “center” of the unfolded Sagnac loop of FIG.  3 . The &#39;second important case is when L 3  equals zero as shown in FIG.  4 . This situation can be accomplished by putting the mirror  112  directly adjacent the phase modulator  106 , effectively resulting in one phase modulator position instead of two.