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 an optical fiber that ends with a phase modulator followed by a mirror. Environmental effects at the optical fiber impress relative phase differences between the counterpropagating light beams, which are detected from an interferometric signal that results therefrom.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/062,621 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 Sagnac sensing systems similar to those described in detail in U.S. Pat. No. 4,898,468 (E. Udd, Sagnac Distributed Sensor, Feb. 6, 1990), U.S. Pat. No. 4,976507 (E. Udd, Sagnac Distributed Sensor, Dec. 11, 1990), U.S. Pat. No. 5,046,848 (E. Udd, Fiber Optic Detection System Using a Sagnac Interferometer, Sep. 10, 1991), U.S. Pat. No. 5,402,231 (E. Udd, Distributed Sagnac Sensor Systems, Mar. 28, 1995), and U.S. Pat. No. 5,636,021 (E. Udd, Sagnac/Michelson Distributed Sensing Systems, Jun. 3, 1997) using a single installed optical fiber. Early work on using the Sagnac interferometer to detect time varying events can be found in U.S. Pat. No. 4,375,680 by Richard Cahill and Eric Udd, “Optical Acoustic Sensor” issued Mar. 1, 1983. 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 sensing and more particularly to use of the Sagnac interferometer to measure and locate a time varying event. In order to make a system of this type more practical and widely applicable for general use, a flexible, single fiber configuration is needed to support the base of currently installed optical fiber and to simplify future installations. 
     The Sagnac interferometer provides means to sense time varying events such as acoustic waves and vibrations with high sensitivity and unique optical filtering action as is described by Eric Udd in “Fiber Optic Sensors Based on the Sagnac Interferometer and Passive Ring Resonator”, Fiber Optic Sensors: An Introduction for Engineers and Scientists, E. Udd Editor, Wiley, 1991. These properties in combination with the ability of the Sagnac interferometer to be supported by low cost components, such as light emitting diodes, have allowed the usage of these devices as optical microphones, hydrophones and for intrusion sensing. 
     By combining properties of the Sagnac interferometers with color coded reflectors such as fiber gratings, it is possible to measure the presence of a time varying signal and localize it on a single optical fiber. These properties enable a system to be constructed that allows the transmission of information from multiple points along the single fiber that may be related to data transmission, sensing information, or a combination of both. 
     SUMMARY OF THE INVENTION 
     There is provided by this invention a Sagnac interferometer based sensing system that allows the measurement of the amplitude of a time varying event along a single fiber and with suitably placed reflectors, localization of the event. 
     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 single fiber that is the sensing leg of the system. The sensing leg has the ability to measure time varying signals such as acoustics or vibrations. By using a controlled device to induce time varying changes in optical pathlength on the fiber, data may be sent. Placing multiple colored reflectors in line allows environmental effects to be localized and data to be transmitted from multiple points. 
     Therefore, it is an object of the present invention to provide a single fiber Sagnac system that can be used to measure time varying events. 
     Another object is to enable the determination of the position of a time varying event along the single fiber line. 
     Another object is simultaneously measure the amplitude and location of a time varying event allowing information necessary for classification. 
     Another object of the invention is to allow the transmission of data via the simple attachment of a phase modulator to a sensing leg. 
     Another object of the invention is to allow the transmission of data from multiple points along a single fiber. 
     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 a schematic representation of a single fiber Sagnac sensor capable of measuring time varying events; 
     FIG. 2 is a schematic representation of a single fiber Sagnac interferometer showing the relationship between the position of a time varying event and pathlengths to the central beamsplitter; 
     FIG. 3 is a schematic representation of a single fiber Sagnac distributed sensor that uses fiber gratings to support localization of a time varying event; 
     FIG. 4 is a schematic representation of a single fiber Sagnac distributed sensor that includes a plurality of spaced phase modulators at different frequencies to support localization of a time varying event; 
     FIG. 5 is a schematic representation of a single fiber Sagnac distributed sensor that includes an unfolded Sagnac sensor with a separate light source to support localization of a time varying event; and 
     FIG. 6 is a schematic representation of a single fiber Sagnac distributed sensor that includes an unfolded Sagnac sensor with a shared light source to support localization of a time varying event. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a diagram of a basic single fiber Sagnac sensor system  9  that is capable of measuring a time varying event along its length. The time varying event could be an acoustic wave or a vibration. A light source  11  that may be a short coherence length light source such as a light emitting diode is used to couple light into the optical fiber end  13  generating the light beam  15 . The light beam  15  passes through a polarization scrambling element  17  that is used to reduce polarization preferences associated with the light source  11 . The light beam  15  continues to a central beamsplitter  19  where it generates counterpropagating light beams  21  and  23 . The light beam  21  propagates through the fiber leg  25  of length L 2  through the phase modulator  27  that is used to provide a dynamic bias to the system  9  optimizing sensitivity. The light beam  21  continues past another polarization scrambler  29  that is used to reduce polarization induced noise in the system  9 . The light beam  21  then passes through a second beamsplitter  31  where it is split into the light beams  33  and  35 . The light beam  33  propagates through the fiber leg  37  that is used for the sensing of time varying effects. The light beam  33  then reflects off the reflective element  39  that may be a dielectric mirror. The light beam  33  then returns to the beamsplitter  31  where it is split into the light beams  41  and  43 . The light beam  43  returns to the beamsplitter  19  via the fiber leg  25  via the polarization scrambler  29  and the phase modulator  27 . The light beam  41  propagates through the fiber leg  45  that is of length L 1  and returns to the first fiber beamsplitter  19 . 
     The light beam  35  propagates through the fiber leg  47  and exits the system via the terminated end  49  that is designed to avoid back reflection. Alternatively, the leg  47  could be arranged to be a second sensing leg by positioning a reflective element on its end similar to that associated with the fiber  37  and reflective means  39 . 
     The light beam  23  propagates along the fiber leg  45  and reaches the fiber beamsplitter  31  where it is split into the light beams  51  and  53 . The light beam  53  propagates along the fiber leg  47  and exits the system via the terminated end  49 . The light beam  51  propagates along the fiber leg  37  and is reflected off the reflective element  39  and returns to the beamsplitter  31 . The light beam  51  is then split into the light beams  55  and  57 . The light beam  55  is directed to the beamsplitter  19  by the fiber leg  45 . The light beam  57  is directed to the beamsplitter  19  by the fiber leg  25  via the polarization scrambler  29  and the phase modulator  27 . The polarization scrambler  29  could also be placed in the leg  45  instead of the leg  25  and still act effectively. It has been found experimentally that placing polarization scramblers in both legs  25  and  45  is not as effective nor is placing the polarization scrambler in the fiber sensing leg  37 . Optimally the polarization scrambler  29  should be placed in either the leg  25  or the leg  45 . In the case of FIG. 1, it has been shown in the fiber leg  25 . 
     At this point four light beams  41 ,  43 ,  55  and  57  have all returned after passing through the system to the fiber beamsplitter  19 . In order for the light beams to interfere, they should have passed through optical pathlengths that are different by less than the coherence length of the light source  11 . In particular the light beam  41  has passed though the optical lengths L 2 , L 1  and  2 L associated with legs  25 ,  45  and twice through  37 . The light beam  57  has passed through the lengths L 1 , L 2  and  2 L associated with the legs  45 ,  25  and twice though  37 . Thus the light beams  41  and  57  traverse nearly identical paths with environmental effects being primarily responsible for any net difference in net optical pathlength. The light beam  43  propagates through a length equal to  2 L 2  and  2 L while the light beam  55  propagates through a path equal to  2 L 1  and  2 L. Because the difference in pathlength between L 1  and L 2  is arranged to be much larger than the coherence length of the light source  11  there is no interference between the light beams  43  and  55  and any of the other light beams, only  41  and  57  interfere with each other. 
     When the relative phase between  41  and  57  is equal, the two light beams interfere constructively and all the light is directed toward the light source  11 . When the light beams  41  and  57  are 180° out of phase with respect to each other, all the light is directed in a light beam  58  toward an output detector  59  via the fiber leg  61 . Now consider a time varying environmental effect  63  that interacts with the sensing leg  37  a distance y from the beamsplitter  31 . 
     FIG. 2 is used to illustrate the action of the time varying environmental effect on the net phase difference between the counterpropagating light beams  41  and  57  associated with FIG.  1 . Referring to the diagram the points  101  and  103  correspond to the position of the beamsplitter  31  of FIG.  1 . The points  105  and  107  correspond to the locations where the time varying environmental effect is acting on the fiber leg  37 . In order to induce a net phase difference between the two light beams  41  and  57 , the position of the time varying environmental effect  63  must be offset from the center  109  of the fiber loop  110  since both beams  33  and  51  arrive at this point simultaneously. In order for the net induced phase difference between the light beams  41  and  57  to be additive, the positions of  107  and  105 , contrary to what is shown in FIG. 2, need to be on the same side of the center point  109 . Otherwise, the induced phases are opposite and they subtract reducing the net overall effect. From this it is evident that the difference in pathlength between L 1  and L 2  should be chosen so that the entire length L of the sensing leg  37  is on one side or the other of the center point of the loop  39 . 
     Thus far the present single fiber Sagnac sensor has been described configured to detect a time varying signal along its length without determining location. In order to determine location, some indicator of position is necessary. FIG. 3 illustrates a system  199  that uses fiber gratings to achieve localization. The light source  201  may be a broadband low coherence length light source. The polarization scramblers  203  and  205  can be used to reduce polarization sensitivity associated with the system  199  as described earlier. The beamsplitters  207  and  209 , the phase modulator  211  and the fiber legs  213  and  215 , perform functions analogous to those described in association with FIG.  1 . In the single fiber sensing leg  216 , color reflecting elements that may be fiber gratings,  217 ,  219  and  221  are positioned between the beamsplitter  209  and the terminating end  223  that acts to remove any light entering into it. The color reflecting elements  217 ,  219  and  221  centered at wavelengths λ 1 , λ 2 , and up to λ n  respectively are used to define single fiber Sagnac interferometers operating at different and separate wavelength bands defined by the reflectors  217 ,  219  and  221 . Each of these single fiber Sagnac interferometers can be monitored separately via the action of the wavelength division multiplexing element  225 , which acts to split the operating wavelength bands onto the output detectors  227 ,  229  and  231 . When a time varying event occurs, its location will be defined by which segments provide an output signal. All segments farther from the fiber beamsplitter  209  relative to the point at which the environmental effect occurs will carry the signal while nearer segments will not allowing the signal to be localized. 
     In some cases it is highly desirable to be able to transmit data from multiple points on a single fiber line. An example would be transmitting data back from an oil or gas well. FIG. 4 illustrates how this could be accomplished by using a slightly modified single fiber Sagnac sensor system  297 . Along the sensing fiber  299  are place a series of phase modulators  301 ,  303  and  305  operating at the carrier frequencies ω 1 , ω 2 , and up to ω n . The sensing fiber is terminated by a reflecting element  306 . The amplitude of the phase modulated signal at these carrier frequencies could be modulated or the frequency varied along the carrier band to allow for data transmission at multiple points. The outputs from the various phase modulators  301 ,  303  and  305  are read out via the detector  307  whose output electrical signal is separated by a demodulator element  309  into the various carrier frequency outputs  311 ,  313  and  315 . 
     The response of the single fiber Sagnac loop is flat over the sensing fiber leg. This response is different from an unfolded Sagnac loop, which has no sensitivity in the center of the loop and increasing sensitivity near the central fiber beamsplitter. This situation allows for distributed fiber sensing to occur where the amplitude of a time varying event may be measured by the single fiber Sagnac sensor and the position can be monitored by the ratio of the open and single fiber responses. FIG. 5 illustrates this embodiment  399 . 
     The input end  401  of the single fiber Sagnac sensor  399  is used to support the single fiber sensing leg  403  terminated by the reflecting element  405 . The response of the time varying environmental effect  407  acting on the fiber leg  403  results in a position independent output on the single fiber Sagnac sensor detector  409 . A second open loop Sagnac sensor system  410  has no response to a time varying environmental effect at the center  411  of its loop  412  . Over the fiber segment  413 , which is shared with the single fiber Sagnac fiber leg  403 , the response of the open loop Sagnac sensor system  410  increases linearly as the position of the time varying environmental effect  407  moves toward the beamsplitter  415 . Taking the ratio of the output of the detectors  409  and  417  allows the position of the time varying environmental event to be located, as is described in the earlier cited patents. It is possible to run the system  399  at a single wavelength by using fiber beamsplitters at the common wavelength. Alternatively by using wavelength division multiplexing elements the single fiber Sagnac  401  and loop Sagnac  410  can be run independently. As an example the light source  11  could operate at wavelength λ 1  which could be  1300  nm and the light source  423  could operate at the wavelength λ 2 , which might be 1550 nm. The wavelength division multiplexing elements  425  and  427  could then operate to let λ 1  pass straight through and cross couple λ 2  for optimal operation. 
     It is possible to operate both the single fiber Sagnac sensor system  399  and an unfolded Sagnac loop system  410  with a single light source sacrificing optical power. FIG. 6 shows a system  499  with a light source  501  that couples light into the fiber end  503  to form the light beam  505 . A polarization scrambler  507  is used to reduce residual polarization. The light beam  505  is split by the beamsplitting element  509  into the light beam  511  that is used to support the single fiber Sagnac sensor  515  and the light beam  516  that is used to support the unfolded Sagnac sensor  517 . The output of the single fiber Sagnac sensor system  515  from the detector  519  can be used in combination with the output from the unfolded Sagnac sensor detector  521  to determine the amplitude and location of a time varying environmental event along the fiber leg  523 . For optimum performance, the beamsplitters  525  and  527  would be approximately 50/50 if no additional spectral separation provision were made to effectively force the single light source  501  to act like two in analogy to FIG.  5 . The pathlengths within the single fiber Sagnac sensor system  515  and the unfolded Sagnac loop system  521  are different so that the light beams circulating within one system do not interfere in the other. 
     Thus there has been shown and described novel fiber optic secure communication systems which fulfill all of the objects and advantages sought therefor. Many changes, modifications, alterations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.