Abstract:
Wavelength division multiplexed Sagnac distributed sensor systems that allow the determination of the position and amplitude of a frequency dependent environmental effect acting on the optical path of Sagnac loops. The sensor includes two Sagnac interferometers operating at different wavelengths and having optically spaced combining couplers and optical loops which include as portions thereof, the optical path. The outputs of the Sagnac interferometers are summed to determine the relative amplitude of the sensed effect and compared to determine its position on the optical path.

Description:
BACKGROUND OF THE INVENTION 
     In many applications, such as aerospace structures, it is desired to include a sensor system distributed along the structure to enable position and magnitude sensing of environmental effects in real time. This allows active damping of space structures and load monitoring and alleviation of aircraft structures to be built. The use of optical fibers as sensing elements is particularly desirable as they are light, tough and can be placed in or on a structure with a minimum of cost and degradation. These applications are discussed by E. Udd, in, &#34;Fiber Optic Smart Structures&#34;, Fiber Optic Sensors: An Introduction for Engineers and Scientists edited by E. Udd, Wiley, (1991). Dakin et al in, &#34;A Novel Distributed Optical Fibre Sensing System Enabling Location of Disturbances in a Sagnac Loop Interferometer&#34;, SPIE Vol. 838 Fiber Optic and Laser Sensors V (1987) pp. 325-326, describes a distributed fiber optic sensor based on the combination of a Sagnac interferometer and a Mach-Zehnder interferometer. The Sagnac interferometer has a position dependent response as described by E. Udd in, &#34;Acoustic Sensor Based on the Sagnac Interferometer&#34;, Proceedings of SPIE, Vol. 425, 1983, pp. 90-95. Dakin et al showed that by combining the output of the Sagnac interferometer in response to a frequency dependent environmental effect along with the response of a Mach-Zehnder interferometer (see A. Dandridge, &#34;The Mach-Zehnder and Michelson Interferometer&#34; in Fiber Optic Sensor and Introduction for Engineers and Scientists, edited by E. Udd, Wiley (1991)) and normalizing the result, the position and location of a frequency dependent environmental effect along an optical fiber could be determined. 
     One of the major disadvantages of the Dakin et al approach is that the requirements on the light source for optimum performance of a Mach-Zehnder and Sagnac interferometer are diametrically opposed. Specifically, a high performance Mach-Zehnder interferometer needs a long coherence length light source that is often very susceptible to feed back while the Sagnac interferometer performs best with a low coherence length light source. While it is possible to reduce these problems by redesigning the Dakin et al distributed sensor using wavelength division multiplexing techniques and an isolator, it is also possible to design a distributed sensor based solely on the Sagnac interferometer as described by E. Udd in U.S. Pat. Nos. 4,898,468, 4,976,507 and 5,046,848. 
     BRIEF DESCRIPTION OF THE PRESENT INVENTION 
     The present invention uses dual Sagnac interferometers operating in conjunction with one another on separate and distinct wavelengths to form a distributed sensor that can determine the position and amplitude of a frequency dependent environmental effect acting on an optical path that is common to the optical loops of the two Sagnac interferometers. The interferometers of the distributed sensor have optically spaced combining couplers that preferably are at opposite ends of the optical path. 
     A simple Sagnac interferometer includes a light source whose output light beam is passed through an optical splitter and propagated about an optical loop in opposite directions. Upon return, the beams are recombined so that the intensity of the combined beam depends on the relative phases of the two beams as they combine at the splitter. When an environmental effect, such as an acoustic wave, acts at a point a distance z along the optical loop, it produces phase modulation of the two beams which arrive progressively more out of phase at the splitter, the further distant the environmental effect is from the midpoint of the optical loop. This change in phase causes a proportional fringe shift dependent on the position and amplitude of the environmental effect on the optical path of the Sagnac loop, which can be converted by a detector into an electrical signal for further processing. A second Sagnac loop is operated in conjunction with the first Sagnac loop at a second wavelength so that the second loop in operated independently from the first, but shares a common optical path. The two Sagnac interferometers are arranged so that the outputs thereof vary in opposite directions with position of the effect, so that the outputs can be summed to determine the relative amplitude of the sensed effect and compared to determine its position on the optical path. Although slowly varying effects can be sensed, the system works best with rapidly varying environmental effects such as acoustics and vibrations. The resulting distributed sensor has applications in the field of fiber optic smart structures and also can be used to secure fiber optic communications. In the latter case, the sensor can be &#34;piggy backed&#34; on a communications link and be used to sense movement of the link or other effects indicative that someone is trying to tap into it to interfere or intercept communications thereon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a basic wavelength division multiplexed Sagnac distributed sensor; 
     FIG. 2 is a graph of response vs. length of the optical sensor path for both of the interferometers of FIG. 1; 
     FIG. 3 is a diagram of the basic wavelength division multiplexed Sagnac distributed sensor of FIG. 1 configured to sense variations in strain in an aircraft wing; 
     FIG. 4 is a diagram of the basic wavelength division multiplexed Sagnac distributed sensor of FIG. 1 using 3 by 3 couplers to optimize sensitivity to low level signal response; 
     FIG. 5 is a diagram of a modified wavelength division multiplexed distributed Sagnac sensor using fiber coating techniques to optimize sensitivity; 
     FIG. 6 is a diagram of a modified sensor system using offset fiber coils to optimize sensitivity of the secure link; and 
     FIG. 7 is a diagram of a wavelength multiplexed distributed Sagnac sensor configured in a package with a sensitive fiber loop. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings more particularly by reference numbers, number 21 in FIG. 1 refers to a basic wavelength division multiplexed Sagnac distributed sensor system 21 including a first Sagnac sensor 22. In the sensor 22, light 23 from a light source 25 operating at a center wavelength π 1 , is coupled into the end 27 of an optical fiber 29. The light source 25 may be a spectrally broad based light source such as a light emitting diode or laser diode. After entering the fiber 29, preferably the light 23 is has its polarization scrambled by an intensity maintenance device 31 so that the magnitude of the light 23 is not severely reduced by random polarizations throughout the sensor 22. The intensity maintenance device 31 shown is a Lyot fiber depolarizer consisting of two lengths 33 and 35 of bifringent polarization preserving fiber, having stretched glass, with the major axes thereof spliced at 45° with respect to each other. The use of the depolarizer 31 enables the use of conventional single mode fiber rather than polarization preserving fiber throughout, lowering the cost of the sensor 22. 
     The depolarized light beam 37 then enters a fiber optic coupler 39 where it is split into clockwise and counterclockwise propagating light beams 41 and 43. The coupler 39 is connected to a first Sagnac loop 45 constructed from optical fiber 46 which includes, along its length, two spaced pairs of wavelength division multiplexers 47 and 49, and 51 and 53, and a second fiber depolarizer 55 positioned between the pair of wavelength division multiplexers 51 and 53. The clockwise light beam 41 circulates about the Sagnac loop past the wavelength division multiplexers 47 and 51, the fiber depolarizer 55, and the wavelength division multiplexers 53 and 49 before returning to the coupler 39. The counterclockwise light beam 43 circulates in the opposite direction about the Sagnac loop fiber 46 passing through the wavelength division multiplexers 49 and 53, the fiber depolarizer 55, and the wavelength division multiplexers 51 and 47 and returns to the coupler 39. The clockwise and counterclockwise light beams 41 and 43 interfere with each other at the coupler 39 to form a combined light beam 57. If the light beams 41 and 43 are in phase with respect to each other, the combined light beam 57 is directed by the coupler 39 toward the light source 25. If the light beams are 180° degrees out of phase, all of the combined light beam 57 is directed by the coupler 39 toward a detector 59. 
     When a frequency dependent environmental effect, such as a vibrational stressing, acts on a optical fiber section 61, in this case located between multiplexers 49 and 53 of the Sagnac loop 45, it induces an optical path length modulation locally in the optical fiber section 61 at that frequency. The amplitude of the resulting oscillation depends on the strength of the environmental effect and the response of the optical fiber section 61 to it. The response of the Sagnac sensor 22 to the environmentally induced oscillation depends on the position of the frequency dependent environmental effect and its amplitude. If the effect occurs near the wavelength division multiplexers 51 and 53, which is close to the center of the Sagnac loop 45, both the clockwise and counterclockwise propagating light beams 41 and 43 arrive nearly simultaneously and the induced phase difference between the two beams moves toward zero. As the frequency dependent environment effect acts on a portion of the Sagnac fiber 46 closer to the coupler 39, the difference in arrival time between the clockwise and counterclockwise propagating light beams increases and as long as the frequency of the environmental signal is small compared to the characteristic frequency f cl  of the Sagnac loop 45 (f cl  =c/Ln where c is the speed of light in vacuum, L is the length of the loop and n is the index of refraction) the amplitude of the resultant signal on the detector 59 will increase linearly. 
     The response of sensor 22 to a position dependent environmental effect is shown as a solid line on the graph of FIG. 2. 
     A second Sagnac interferometer is set up to act as sensor 63 in a similar manner. A light source 65 produces light 67 at a center wavelength λ 2  that is separable by the wavelength division multiplexers 47 and 49, and 51 and 53, from light from the light 23 of source 25 operating at λ 1 . The light 67 from the source 65 is coupled into a fiber end 69 and depolarized by a depolarizer 71 similar to depolarizer 31. The output light 73 whose polarization is scrambled is split into clockwise and counterclockwise beams 75 and 77 by a coupler 79. The beams 75 and 77 are coupled into the Sagnac fiber loop 45 by the wavelength division multiplexers 51 and 53 and circulate around the Sagnac loop 81 of the second Sagnac interferometer by means of the wavelength division multiplexers 47 and 49 between which the light beams 75 and 77 are cross coupled through a third fiber depolarizer 83 and through wavelength division multiplexers 51 and 53 to fiber coupler 79. The light beams 75 and 77 interfere and are directed toward the light source 65 or a second detector 85 dependent upon whether they are in phase or 180° out of phase, respectively. 
     The response of sensor 63 to a position dependent environmental effect is shown in dashed line on the graph of FIG. 2. As can be seen, it is the reverse of the response of sensor 22 although the magnitudes from the sensors may be different. 
     The signal outputs 87 and 88 from the detectors 59 and 85 of sensors 22 and 63 from the frequency dependent environmental effect are fed into a signal processor 89. The sum of the signal outputs 87 and 88 from the detectors 59 and 85 is then used to produce signal on an amplitude output 90 representative of the amplitude of the frequency dependent environmental effect and the ratio between the signal outputs 87 and 88 from the detectors 59 and 85 is used to measure its location on the loops 45 and 81, which should be somewhere on the optical fiber section 61. In order to assure the light sources 25 and 63 do not add excess noise to the system 21, they are stabilized via the drive circuitry 92 and 93. 
     The system 21 is shown in FIG. 3 arraigned with physically adjacent sensors 22 and 66 whose common sensing section 61 is embedded in the skin of an aircraft wing 95. Any flexure of the wing 95 results in strain in the skin 94 and the fiber section 61, whose location and magnitude can be determined with the sensor system 21. Preferably, the length of the fiber section 96 between the multiplexers 47 and 51 is similar to the length of the fiber section 61, although offsets can be accommodated in the signal processor 89. Similar applications can involve embedding the fiber section in buildings, bridges, and highways. 
     For weak signals, Kjell Krakanes and Kjell Blotekjar (Optics Letters, Vol. 14, p. 1152, 1989) have demonstrated the ability to bias a Sagnac acoustic sensor system using a 3 by 3 coupler. Distributed Sagnac acoustic sensor 97 of FIG. 4 illustrates how 3 by 3 couplers can be substituted for the couplers 39 and 79 in sensor system 21. Like sensor system 21, sensor system 97 has a pair of sensors 98 and 99 that sense from different directions over a common optical fiber run 100. A light source 101 that operates about a center wavelength λ 1  couples light into the fiber end 103. The resulting light beam 105 then passes through a polarization scrambler 107 that acts to depolarize the light beam 105. The light beam 105 then enters the 3 by 3 coupler 109 where it is split into three light beams, a clockwise propagating light beam 111, a counterclockwise propagating light beam 113 and a light beam 115. The light beam 115 propagates to the fiber end 117, which includes an optical termination 118 to avoid back reflection into the system 97, and is lost. As an example, the termination 118 may be constructed by crushing the fiber end 117 and covering it with index matching cement (see E. Udd and R. E. Wagoner, Method of Terminating an Optical Fiber, U.S. Pat. No. 4,834,493, May 30, 1989 for additional examples). The light beam 111 propagates about the Sagnac loop 119 of the sensor 98 through wavelength division multiplexing elements 120 and 121 and the polarization scrambler 123, returning to the 3 by 3 coupler 109 via wavelength division multiplexing elements 125 and 127. The counterclockwise propagating beam 113 circulates through the Sagnac loop 119 in the opposite direction through the elements 127, 125, 123, and 120 before returning to the 3 by 3 coupler 109. When the two counter propagating light beams 111 and 113 return to the 3 by 3 coupler 109, they interfere with each other and depending upon their relative phase, will be directed toward a detector 129, a detector 131 or the light source 101 since 3 by 3  couplers with equal power splitting have the characteristic of shifting an input signal from output to output by 120° of phase. For a frequency dependent environmental signal 133 applied to the upper fiber leg 134 of the Sagnac loop 119, this will result in corresponding amplitude modulated signals 135, 136 and 137 being directed toward the detectors 129 and 131 and the light source 101 that are 120° out of phase with respect to each other (as opposed to 180° out of phase as is the case for a 2 by 2 coupler). The result is that the signals 135, 136 and 137 induced by the frequency dependent environmental effect 133 have significant first harmonic content that is fed as the outputs 138 and 139 from the detectors 129 and 131 into a signal processor 140. 
     The situation for the Sagnac interferometer sensor 99 supported by the light source 141 operating at the wavelength λ 2  is analogous. Light is coupled into the fiber end 143 and the resultant light beam 145 passes through a polarization scrambler 147. The light beam 145 is then split by a 3 by 3 coupler 149 into three light beams, beam 151, counterclockwise beam 153, and clockwise beam 155. The light beam 151 exits the fiber end 156 that is optimized to reduce back reflection and is lost. The clockwise counter propagating light beam 155 transverses the Sagnac loop 157 of the sensor 99, being cross-coupled by the wavelength division multiplexing elements 125 and 127 into polarization scrambler 158 and cross-coupled back toward the 3 by 3 central coupler 149 by the wavelength division multiplexing elements 120 and 121. The counterclockwise propagating light beam 153 traverses the Sagnac loop 158 in the opposite direction before returning to the 3 by 3 coupler 149. The light beams 153 and 155 interfere and output beams 159 and 160 that are 120° out of phase with respect to each other, are directed toward the output detectors 161 and 162. The outputs 171 and 173 of the detectors 161 and 162 are then directed into the signal processor 140 which in turn uses the sum and ratio of the signals from the two Sagnac interferometers 98 and 99, operating independently on wavelengths λ 1  and λ 2 , respectively to calculate the amplitude output signal 175 of the environmental signal, and the location output signal 177. In order to assure that the light sources 101 and 141 do not add excess noise to the sensor system 97, closed loop light source drivers 179 and 181 may be employed as before. 
     A typical electronics support set forming the signal processor 140 is shown in FIG. 4. The outputs 138 and 139 from the detectors 129 and 131 are fed into a demodulator 182 for the Sagnac sensor 98 operating at λ 1 . These outputs 138 and 139 are used to extract the amplitude and frequency content of the environmental signal 133 acting on the Sagnac sensor 98. This information is conveyed via the communication link 183 which could be a fiber optic or electrical link to a wavelength division multiplexed Sagnac distributed sensor signal processor 184. Similarly, the outputs 171 and 173 of the detectors 161 and 162 are fed into a demodulator 185 for the Sagnac sensor 99 operating at λ 2 . These outputs 171 and 173 are used to extract the amplitude frequency content of the environmental signal 133 acting on the Sagnac sensor 99. This information is conveyed via the communication link 186 to the signal processor 184. By combining the signals delivered via the communication links 183 and 186, the signal processor 184 determines the signal on the amplitude output 175 and by taking the normalized ratio, the signal on the position output 177 is determined so that electrical signals representative of the location and amplitude of the environmental signal 133 are produced. 
     For many situations, it would be expected that the two fibers 134 and 187 used to form the Sagnac loops will be located in the same fiber cable or closely adjacent to each other where they are subjected to the same environmental signal 133. If the two fibers 134 and 187 are symmetrically located and are both responsive to the environmental signals 133, the Sagnac signals will to first order cancel these effects. To avoid this situation two approaches are shown in FIGS. 5 and 6. 
     FIG. 5 illustrates a modified sensor system 200 where one of the fibers 202 in a cable 204, connecting the two multiplexing elements 125 and 127, is acoustically isolated, by means of a coating 205 placed on it, relative to the acoustically sensitive fiber 206. In this manner, an acoustic signal 208 impinging on the fiber cable 204 will result in a greater optical path length change in the acoustically sensitized fiber 206 relative to the acoustically desensitized fiber 202 and a larger signal will be generated for the Sagnac distributed sensors 98 and 99 to process. As an example, the fiber 202 may be desensitized from acoustic sensitivity by coating it with a metal such as aluminum and the other fiber 206 may be optimized for acoustic sensitivity by using a soft compliant coating 210. These and other coatings, and processes for their application are commercially available. 
     FIG. 6 illustrates another modified sensor system 220 employing an alternative approach to the usage of fiber coatings. In system 220, offset fiber coils 222 and 224 are used to cause the two fibers 226 and 228 in the fiber cable 230 to be offset in distance by the length of the offset coils 222 and 224. The result is that the fiber cable 230 will be acoustically sensitive along its entire length. In general the offset coil length can be increased to increase sensitivity with a decrease in sensitivity occurring only when the characteristic frequency of the offset coil, c/Ln where c is the speed of light, L is length of the offset coil and n is the index of refraction, begins to be approached by the frequency of an environmental signal 232. Optional fiber coils 234 and 236 can be added between wavelength division multiplexing elements 120 and 127, and 121 and 125. If the coils 222, 224, 234 and 236 are about the same length as the fibers 226 and 228, there is no overlap in optical position between the fibers 226 and 228. 
     In addition to passive biasing as with the wavelength division multiplexed Sagnac distributed sensor system 220, it is possible to use dynamic biasing techniques such as those employed in association with the fiber optic gyros. FIG. 7 illustrates a wavelength division multiplexed Sagnac distributed sensor system 240 of this type. Although two separate light sources can be used as before, since the light source can be one of the most expensive components of the system 240, a single broadband light source 242, such as an LED operating over wavelengths λ 1  and λ 2  is positioned to couple light into the fiber end 244 of a multiplexing element 246. The multiplexing element 246 produces two resultant light beams, one 248 having a center wavelength λ 1  and the other 249 having a center wavelength λ 2 . The resulting light beam 248 passes through a polarization scrambler 250 and is split by a central coupler 252 into counter propagating light beams 254 and 256 for travel around a Sagnac loop 258. The clockwise propagating light beam 254 passes wavelength division multiplexing elements 260 and 262 that are designed to pass light centered about the wavelength λ 1  straight through, to a polarization scrambler 264. The light beam 254 then passes through wavelength division multiplexing elements 266 and 268 to a phase modulator 270 and returns to the central coupler 252. The counterclockwise propagating light beam 256 traverses the Sagnac loop 258 through the phase modulator 270, the elements 268 and 266, the scrambler 264, and the elements 262 and 260 before returning to the central coupler 252. The phase modulator 270 is driven by an oscillator 272 with a sinusoidal output 274 to introduce an oscillating non-reciprocal phase shift between the counter propagating light beams 254 and 256. The oscillator 272 also provides the same sinusoidal signal as an output 276 to a synchronous demodulator 278. 
     When there is no frequency dependent environmental effect acting on the Sagnac loop 258, the two counter propagating light beams 254 and 256 mix and produce an amplitude modulated signal 280 that is directed to the output detector 282 and whose content is largely second and higher order even harmonics of the sinusoidal drive signal applied to the phase modulator 270. When a frequency dependent environmental signal 284 hits the Sagnac loop 258, the amplitude modulated signal 280 will contain first harmonics of the drive signal of the phase modulator 270. The amplitude of the first (and higher order odd) harmonic will be proportional to the amplitude of the environmental effect 284 and its location and the resultant electrical signal output 288 of the detector 282 is synchronously demodulated at the drive frequency of the sinusoidal oscillator 272. The resulting output 286 of the synchronous demodulator 278 is then fed into a signal processor 290. The second Sagnac loop 292 is supported by the light beam 249 operating about a center wavelength λ 2 . The light beam 249 propagates through a polarization scrambler 302 and is split by a central coupler 304 into counter propagating light beams 306 and 308. The counterclockwise propagating light beam 306 is cross-coupled by the wavelength division multiplexing elements 262 and 260 to a polarization scrambler 310. The beam 306 then is cross-coupled by the wavelength division multiplexing elements 268 and 266 to a phase modulator 312 and returns to the central coupler 304. The clockwise propagating light beam 308 traverses the Sagnac loop 292 by means of the phase modulator 312, the elements 266 and 268, the scrambler 310, and the elements 260 and 262 before returning to the central coupler 304. An oscillator 314 applies a sinusoidal signal to the output 316 to the phase modulator 312. The action of the phase modulator 312 in turn is used to induce a sinusoidally varying phase shift between the counter propagating light beams 306 and 308 for demodulation purposes. When the two beams 306 and 308 recombine after circulating through the Sagnac loop 292, they interfere with each other and the resultant amplitude modulated signal 318 is directed toward an output detector 320. The output 322 from the detector 320 is directed to a synchronous demodulator 324 which in turn receives a sinusoidal drive signal on output 326 from the oscillator 314 for demodulation purposes. The resulting output 328 is directed to the signal processor 290, which uses the outputs 288 and 328 to produce a location output 330 and an amplitude output 332 of the environmental effect 284. The technique of using dynamic biasing and a single light source to implement a wavelength division multiplexed Sagnac distributed sensor may be applied in analogous fashion to all the embodiments described herein. 
     Thus there has been shown and described novel Sagnac distributed sensor systems which fulfill all the objects and advantages sought therefor. Many changes, modifications, variations, uses and applications of the subject invention will however will become apparent to those skilled in the art after considering this specification and the accompanying drawings. All such 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.