Abstract:
An optical sensor includes a support structure. An optical fiber has a plurality of bends arranged proximate the support structure. The optical fiber follows a circuitous path and is adapted to establish a detection zone.

Description:
BACKGROUND 
     This patent application relates to sensors and sensing systems, and more particularly to optical fiber sensors and systems. 
     Optical fiber cable can be used for sensing temperature, pressure, movement and/or vibration of the fiber. For example, light coupled into a multimode fiber travels along many modes in the optical fiber. In the case of coherent light, there is optical interference between the modes, resulting in a speckle pattern. Disturbances in the fiber result in strain that causes time-varying changes to the optical path lengths among the different modes. Because of the differential path lengths, disturbances of the fiber result in time variation in the speckle pattern. Thus, such a sensor works by monitoring the changes to the speckle pattern, and detecting the instances when the speckle pattern flickers. 
     Typical applications for optical fiber are found in security for perimeters, pipelines, rails, bridges, and other structures. In one approach, to sense motion or presence of the objects, such as unauthorized persons, a linear sensor including an optical fiber may be slotted into the concrete. However, such a linear sensor is susceptible to false events resulting from vibration caused by nearby aircraft and other heavy equipment. 
     In another approach, to sense motion or presence of unauthorized persons who try to illegally cross a rail station, for example, optical fibers including a vibration sensor may be attached underneath the pedestrian grating near the rail. This system has proven to be effective at detecting the presence of intruders who try to cross the rail station illegally. However, the system is susceptible to vibration noise caused by approaching and/or departing trains. Although the system can be tuned to reject vibrations caused by underlying noise, e.g., trains, the tuning may be costly, time consuming, and may require a trained technician. 
     Other situations related to motion detection include sudden infant death syndrome (SIDS). More babies fall victim to sudden infant death syndrome than the combined total of respiratory ailments, heart disease and cancer deaths. Despite extensive research, the exact cause of sudden infant death syndrome is not known. The only effective way of detecting SIDS is to monitor the baby&#39;s respiration and, if the baby stops breathing, awaken the baby. 
     Several monitors exist for alerting parents when a baby has stopped breathing. Some use a pad that detects movement, and others use a device that is attached to the baby&#39;s skin. However, these devices are typically not effective, they touch the baby directly and are cumbersome to use. 
     There is thus a need for improved sensors and sensor systems. 
     BRIEF SUMMARY 
     An embodiment of the current invention includes an optical sensor, having: a support structure; and an optical fiber having a plurality of bends arranged proximate the support structure. The optical fiber follows a circuitous path and is adapted to establish a detection zone. 
     Another embodiment of the current invention includes a fiber-optic detection system, having: a light source; an optical sensor in optical connection with the light source; and an optical detector in optical connection with the optical sensor. The optical sensor includes: a support structure; and an optical fiber attached to the support structure. The optical fiber has a plurality of bends which follow a circuitous path thus establishing a detection zone of the optical sensor. 
     Another embodiment of the current invention is a detection method, comprising: transmitting light into an optical sensor that has a plurality of fiber-optic bends which follow a circuitous path to establish a detection zone; receiving light reflected by the optical sensor indicative of one of a presence or an absence of an event; and detecting one of the presence or absence of the event within the detection zone based on the received light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described herein, by way of example only, with reference to the accompanying FIGURES, in which like components are designated by like reference numerals. 
         FIG. 1A  is a diagrammatic illustration of an optical detection system including an optical fiber arranged proximate a support structure; 
         FIG. 1B  is a diagrammatic illustration of another optical detection system including an optical fiber arranged proximate a support structure; 
         FIG. 2A  is a diagrammatic illustration of an optical detection system including optical sensors; 
         FIG. 2B  is a diagrammatic illustration of an optical detection system including optical sensors; 
         FIG. 3  is a diagrammatic illustration of an optical detection system including optical sensors; 
         FIG. 4  is a diagrammatic illustration of a vibration sensor; and 
         FIG. 5  is a diagrammatic illustration of an optical detection system including interferometers. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1A , a fiber-optic detection system  100  includes an optical sensor  110 , including an optical fiber  111  which is arranged about a support structure  112  such as a mat which, for example, may include a rubber mat, an ASTROTURF® mat, or a mat fabricated from any other appropriate flexible material. The optical sensor  110  may include a distributed optical fiber, for example, including multiple fiber-optic bends. In one embodiment, the fiber-optic bends follow a circuitous path. 
     The optical sensor  110  may be attached to a lower surface  120  of the mat  112 . In one embodiment, the optical sensor  110  may be embedded in the support structure  112 . Intruders who step on the optical sensor  110  cause disturbances in the optical sensor  110  that affect optical properties of the optical sensor  110 . As described in detail below, such resulting optical irregularities may be sensed and used to generate an alarm. The support structure or mat may be attached to the optical sensor  110  to detect intruders along a perimeter at one or multiple locations. For example, the support structure  112  substantially isolates the optical sensor  110  from the underlying vibrations caused by, for example, nearby equipment to prevent sensing of the false events, while providing a coupling between actual events and the optical sensor  110 . 
     With continuing reference to  FIG. 1A  and further reference to  FIG. 1B , in this embodiment, the support structure  112  includes first and second portions  130 ,  132 , between which the optical sensor  110  may be disposed. 
     The optical sensor  110  is optically coupled to an optical transmitter or light source  138  via a first connector  140  to receive light pulses. The transmitter  138  may include a pulsed laser, a continuous wave laser modulated, for example, to produce a series of pulses, or any other light source which generates suitable time-varying signals to provide light signals to the optical sensor  110 . In one embodiment, in which the optical sensor  110  includes a multimode optical fiber, the transmitter  138  includes a 1300 nm laser or a single-mode laser. In this embodiment, the sensing is modalmetric. Of course, it is contemplated that the sensing may be other than modalmetric, and the laser may have some other wavelength such as 850 nm, for example. The sensing could, for instance, be polarimetric. Generally, the terms transmitter and light source are used herein to refer to any suitable source of electromagnetic radiation, regardless of whether it is in the visible range or not. 
     An optical receiver  150  is optically coupled to the optical sensor  110  through a second connector  152  to receive optical signals from the optical sensor  110 . The receiver  150  may include an optical detector such as a photodiode or any other suitable detector to convert the optical signals received from the optical sensor  110  into electrical signals. A processor  160  receives electrical signals from the receiver  150  and analyzes the received electrical signals to determine, for example, the presence of an event sensed by the optical sensor  110 . Based on the analysis, an alarm generating mechanism  170  may generate an alarm  172  such as an audible alarm, a visual alarm, a text message to be displayed at a remote station, and the like. 
     In one embodiment, the optical sensor  110  may be disposed along the sides of the tracks in a subway station or train station to detect the presence of people who fall or climb near the tracks. 
     In one embodiment, the optical sensor  110  may be disposed on an airport tarmac, surrounding a protected perimeter, to visually remind of the protected zone and sound an alarm upon detection of an intruder. 
     The optical sensor  110  may be rolled up for transportation and un-rolled on site. 
     With reference to  FIG. 2A , a fiber-optic detection system  200  includes a spatially distributed detection system. The fiber-optic detection system  200  includes an optical transmitter  202  optically coupled to an optical conduit  210  to transmit light signals. First and second optical sensors  212 ,  214  each includes a corresponding first or second optical fiber  220 ,  222 , each disposed proximate a respective first, second support structure  224 ,  226  and optically coupled to the optical conduit  210  to receive light signals from the transmitter  202 . An optical receiver  230  is optically coupled to the optical conduit  210  to receive optical signals from the first, second sensors  212 ,  214 . Although not shown in  FIG. 2A , the fiber-optic detection system  200  may include three, four or even up to fifty or more optical sensors each optically coupled to a different respective position of the optical conduit  210 , such as at a first tap coupler  234 , a second tap coupler  236 , and so forth. 
     With continuing reference to  FIG. 2A  and further reference to  FIG. 2B , in this embodiment, the detection system  200  includes the first and second optical sensors  212 ,  214  disposed about a single support structure  237 . The first and second optical sensors  212 ,  214  are optically coupled to the optical conduit  210 . Although not shown in  FIG. 2B , the detection system  200  may include three, four or even up to fifty or more optical sensors, disposed proximate the support structure  237 , each optically coupled to the optical conduit  210  at a different respective position of the optical conduit  210 , such as at the first tap coupler  234 , the second tap coupler  236 , and so forth. 
     In one embodiment, a first mirror  238  is disposed at a first end  250  of the first optical fiber  220 , while a second mirror  252  is disposed at a second end  254  of the second optical fiber  222 . The first, second mirror  238 ,  252  may be either formed on the respective end  250 ,  254  of the first, second optical fiber  220 ,  222 , may be a component that is attached to the end  250 ,  254  of the respective first, second optical fiber  220 ,  222  or may be a reflective layer formed on the end  250 ,  254  of the respective first, second optical fiber  220 ,  222 . A first, second inline polarizer  256 ,  258  each is optically coupled to a corresponding portion  260 ,  262  of the first, second optical fiber  220 ,  222 . For example, the inline polarizer  256 ,  258  is a separate component that is attached to the respective portions  260 ,  262  of the first, second optical fiber  220 ,  222 . 
     The transmitter  202  may include a depolarizer to provide a depolarized, time-varying source of light indicated by an arrow  264 . The first and second tap couplers  234 ,  236  each splits off a portion of light transmitted in the optical conduit  210  to direct the light into the respective optical sensor  212 ,  214 . If a large number of optical sensors are coupled to the optical conduit  210  of the detection system  200 , each tap coupler may split off small portions of the transmitted light that reaches it. For example, the first tap coupler  234  may split off between approximately 2% and approximately 5% of the transmitted light  264  into the first optical sensor  212 . 
     The receiver  230  may be optically coupled to the optical conduit  210  via a splitter  270 . A processor  272  receives electrical signals from the receiver  230  and processes the received electrical signals to determine, for example, the presence of an event sensed by one or more optical sensors  212 ,  214 . 
     In operation, a pulse of light from the transmitter  202  travels past the splitter  270  along the optical conduit  210  as the depolarized pulse  264  of laser light. When the light pulse  264  reaches the first tap coupler  234 , a portion of the light is split off into the first optical sensor  212  and substantially the rest of the light indicated by an arrow  274  travels further in the optical conduit  210 . The light, split off at the first tap coupler  234 , is directed into the first optical sensor  212  at the portion  260 . The light passes through the first inline polarizer  256  and travels along the length of the first optical fiber  220  to the first mirror  238 . The light reflects back off the first mirror  238 , passes again through the first inline polarizer  256  and into the optical conduit  210  through the first tap coupler  234  as a first return light  275 . At the splitter  270 , the return light  275  is split off to the receiver  230 . 
     The portion  274  of the light pulse transmitted from the transmitter  202  continues beyond the first tap coupler  234  into the second optical sensor  214 . The process described above in relationship to the first optical sensor  212  may be repeated for the second optical sensor  214  and other optical sensors which may be disposed along the optical conduit  210 . Correspondingly, a respective first, second return pulse  275 ,  276  of light is received from each optical sensor  212 ,  214  for a given light pulse from the transmitter  202 . For example, in an embodiment in which there are fifty optical sensors, fifty return light pulses are received for each transmitted light pulse. In one embodiment, the processor  272  includes a variety of algorithms to positively identify a location of each optical sensor, such as delaying the transmission of additional signal pulses from the transmitter  202  until after all pulses have been received by the receiver  230  after returning from all optical sensors. 
     As long as the optical sensor  212 ,  214  remains undisturbed, the amount of the light  275 ,  276  returned from substantially equal successive pulses remains substantially constant. If the first or second optical sensor  212 ,  214  is disturbed, for example, by being moved in some way, the birefringence of the fiber may change and lead to a change in the amount of light directed back from the disturbed optical sensor into the optical conduit  210 . In one embodiment, the optical conduit  210  includes an optical fiber. Since the light is depolarized, the optical conduit  210  is insensitive to being disturbed. For example, the first optical sensor  212  provides a measure of disturbance localized between the first tap coupler  234  and the end  250  of the first optical fiber  220  at the first mirror  238 . Based on the information about the time for the pulse to travel from the transmitter  202  to the optical sensor and then back to the receiver  230 , the processor  272  determines the position of disturbance along the optical conduit  210 , e.g., which of the optical sensors is disturbed. An alarm generating mechanism  280  may generate an alarm  282 , similar to the embodiment of  FIG. 1A . 
     With reference again to  FIG. 1A  and continuing reference to  FIGS. 2A and 2B , in one embodiment, the optical detection system  100 ,  200  includes the optical sensor  110 ,  212 ,  214  including a web of optical fiber  111 ,  220 ,  222  woven into the support structure  112 ,  224 ,  226 , such as a pad, to detect motion or lack of motion of a subject. For example, the optical sensor  110 ,  212 ,  214  may be placed in the baby&#39;s crib, under a sheet, as a detection mechanism for sudden infant death syndrome (SIDS). Movements by the baby may be converted to the signals that are detected by the processor  160 ,  272 . If the processor  160 ,  272  does not detect the optical signal for a predetermined period of time, the alarm generating mechanism  170 ,  280  may generate the alarm  172 ,  282  such as an audible alarm, a visual alarm, a text message to be displayed at a remote station, and the like. In one embodiment, the optical detection system  100 ,  200  includes multiple optical sensors  110 ,  212 ,  214 , for example, including multimode fiber in a modalmetric configuration. In such configuration, motion by the subject causes bending in a fiber which causes differential delay between different modes, resulting in a time-varying speckle pattern. A stable speckle pattern indicates no motion by the subject. In another embodiment, a polarimetric method is used, in which motion by the subject causes stress that changes the state of polarization in the fiber. The change in the state of polarization is converted to a variation in intensity through an in-line polarizer. Other optical systems are contemplated in which fiber-optic interferometers, such as Sagnac interferometer, Mach Zehnder interferometer, Michelson interferometer, etc., are used. 
     With reference to  FIG. 3 , a detection system  300  includes first and second optical sensors  302 ,  304  such as stingers optically coupled to the transmitter  306  and receiver  308  via an optical conduit  310  at corresponding first and second tap couplers  314 ,  316  as described in detail above regarding the embodiment of  FIG. 2A . Each optical sensor  302 ,  304  includes at least one length of first, second optical fiber  318 ,  320 . Similarly to the embodiment of  FIG. 2A , first, second mirror  328 ,  332  may be disposed at end  330 ,  334  of corresponding first, second optical fiber  318 ,  320 . First, second inline polarizer  336 ,  338  each may be optically coupled to portion  340 ,  342  of the respective first, second optical fiber  318 ,  320 . 
     As described above, the transmitter  306  may include a depolarizer to provide a depolarized, time-varying source of light indicated by an arrow  344 . The first and second tap couplers  314 ,  316  each splits off a portion of light transmitted in the optical conduit  310  to direct the light into the respective optical sensor  302 ,  304 . The receiver  308  may be optically coupled to the optical conduit  310  via a splitter  350 . A processor  360  receives electrical signals from the receiver  308  and processes the received electrical signals to determine, for example, the presence or absence of an event sensed by one or more optical sensors  302 ,  304 . 
     When the light pulse  344  reaches the first tap coupler  314 , a portion of the light is split off into the first optical sensor  302  and the rest of the light indicated by an arrow  352  travels further in the optical conduit  310 . The light, split off at the first tap coupler  314 , is directed into the first optical sensor  302  at the portion  340 . The light passes through the first inline polarizer  336  and travels along the length of the first optical fiber  318  to the first mirror  328 . The light reflects back off the first mirror  328 , passes again through the first inline polarizer  336  and into the optical conduit  310  through the first tap coupler  314  as a first return light  354 . At the splitter  350 , the return light  354  is split off to the receiver  308 . The portion  352  of the light pulse transmitted from the transmitter  306  continues beyond the first tap coupler  314  into the second optical sensor  304 . The process described above in relationship to the first optical sensor  302  may be repeated for the second optical sensor  304  and other optical sensors which may be disposed along the optical conduit  310 . Correspondingly, a respective first, second return pulse  354 ,  356  of light is received from each optical sensor  302 ,  304  for a given light pulse from the transmitter  308 . In one embodiment, a processor  360  includes a variety of algorithms to positively identify a location of each optical sensor, such as delaying the transmission of additional signal pulses from the transmitter  306  until after all pulses have been received by the receiver  308  after returning from all optical sensors. 
     As long as the optical sensor  302 ,  304  remains undisturbed, the amount of the light  354 ,  356  returned from substantially equal successive pulses remains substantially constant. If the first or second optical sensor  302 ,  304  is disturbed, for example, by being moved in some way, the birefringence of the fiber may change and lead to a change in the amount of light directed back from the disturbed optical sensor into the optical conduit  310 . For example, the first optical sensor  302  provides a measure of disturbance localized between the first tap coupler  314  and the end  330  of the first optical fiber  318  at the first mirror  328 . Based on the information about the time for the pulse to travel from the transmitter  306  to the optical sensor and then back to the receiver  308 , the processor  360  determines the position of disturbance along the optical conduit  310 , e.g., which of the optical sensors is disturbed. An alarm generating mechanism  370  may generate an alarm  372  indicative of the detected presence or absence of an event. 
     With continuing reference to  FIG. 3  and further reference to  FIG. 4 , the detection system  300  may include a vibration sensor  402  used in place of one or more of the optical sensors  302 ,  304 . As shown, the vibration sensor  402  is disposed along the length of the first optical fiber  318 . For example, the first optical fiber  318  is separated into first and second portions  406 ,  408  to define a gap  410  between opposing ends  412 ,  414  of the first and second portions  406 ,  408 . The first portion  406  of the optical fiber  318  may be secured to a first mount  430 . The second portion  408  of the optical fiber  318  may be secured to a second mount  432 . In an undisturbed state, the first and second portions  406 ,  408  are aligned to couple light traveling between the first and second portions  406 ,  408 . A change in the optical alignment between the first and second portions  406 ,  408  due, for example, to vibrations or displacement, leads to a change in the amount of optical coupling between the ends  412 ,  414 , thus leading to a change in the strength of the pulses received at the receiver  308 . 
     With reference to  FIG. 5 , a detection system  500  includes first and second optical sensors  502 ,  504  each including a Sagnac interferometer coupled to an optical conduit  505 . In this embodiment, each Sagnac interferometer  502 ,  504  includes an optical fiber loop  506 ,  508  optically coupled to a corresponding first or second coupler  510 ,  512  such as, for example, a 50/50 optical coupler. Light  513 , transmitted from a transmitter  514 , splits off from the optical conduit  505  at a first tap coupler  524  and travels through a portion  540  of the first optical sensor  502 . After traveling past the first coupler  510 , the light splits to travel in first and second directions  544 ,  546  around the interferometer loop  506 . The counter-rotating beams of light come together at the first coupler  510  and interfere either constructively or destructively with one another while being coupled back into the optical conduit  505  to travel back as a return light  554  to a receiver  556  to be received and processed by a processor  558 . 
     Similarly, for the second optical sensor  504 , a portion of the light  562 , transmitted from the transmitter  514 , splits off from the optical conduit  505  at a second tap coupler  566  and travels through a fiber portion  572  of the second optical sensor  504 . After traveling past the second coupler  512 , the light splits to travel in the first and second directions  544 ,  546  around the interferometer loop  508 . The counter-rotating beams of light come together at the second coupler  512  and interfere either constructively or destructively with one another while being coupled into the optical conduit  505  to travel as a return light  576  to the receiver  556  to be received and processed by a processor  558 . Disturbances of each Sagnac interferometer  502 ,  504 , such as movement, rotation, etc., lead to a change in the interference of the counter-rotating beams at the corresponding coupler  510 ,  512  and thus lead to a change in the signal returned to the receiver  556 . The processor  558  analyzes signals received from the receiver  556  and determines presence or absence of an event. An alarm generating mechanism  580  may generate an alarm  582  as described above. 
     It is contemplated that optical fibers that change their optical properties in the presence of certain chemical agents may be used in the embodiments described above. For example, optical fibers that change their optical density in the presence of certain chemical agents may be used. Optical fibers that darken, i.e., increase attenuation, in the presence of chlorine gas may be used as another example. For example, changes in the optical density of one or more of the optical sensors due to the presence of a chemical agent lead to the detected changes in the received pulses. 
     Many modifications and alternatives to the illustrative embodiments described above are possible without departing from the scope of the current invention, which is defined by the claims.