Patent Publication Number: US-7583865-B2

Title: Segmented fiber optic sensor and method

Description:
FIELD 
   This invention relates to optical sensors and more particularly to fiber optic sensors, which are configured to detect the presence of one or more analytes. 
   BACKGROUND 
   Optical fibers including materials which react in the presence of an analyte to alter the characteristics of light transmitted in the fiber are well known. U.S. Pat. No. 4,399,099 issued Aug. 11, 1983, for example, discloses fiber optic sensors for chemical and biochemical analysis, which employ an energy transmissive core with one or more coatings. The sensors are operative to modify energy passing through the core when an analyte is present. U.S. Pat. No. 4,834,496, issued May 30, 1989 also describes fiber optic sensors operating in a similar manner. 
   U.S. Pat. No. 6,205,263 issued May 20, 2001 describes a fiber optic sensor configured to exhibit uniform power loss along the length of the fiber optic sensors in order to achieve a predictable, preferably uniform, response to the presence of an analyte anywhere along the fiber length. 
   Fiber optic sensors designed to be operative over a long distance suffer increasing losses as a function of length. The high optical attenuation results in increasing difficulty in obtaining a usable signal indicative of the presence of an analyte. Therefore, there is a need in the art for fiber optic sensors that can provide signal levels at higher levels over longer distances. Also, if sensor surveillance is needed over a long continuous distance or at separated intervals over a long distance, it is desirable to be able to determine the location at which a sensing event has occurred. 
   SUMMARY 
   This summary is intended as an introductory statement and should not be taken as a recitation or an exact statement of all inventive aspects or of the content of each claim. 
   Embodiments of the present invention have one or more fiber optic sensing segments which are connected or spliced into or formed integrally with low attenuation lead portions to achieve practical signal levels. See the special definition of “optical fiber” and “fiber” as used herein. 
   In accordance with an embodiment of the present invention, a fiber optic sensor for the detection of an analyte comprises a plurality of optical fibers. Each optical fiber has a sensing segment which has a length that is a fraction of the total length of the optical fiber and the sensing segments are located in offset (see the special definition of this term) positions over the length of the sensor. The sensing segments are then deployed at positions where the detection of analyte is desired. 
   In one aspect the sensing segments are deployed to provide continuous detection of one analyte over a desired distance. For example, four sensing segments, each of length L/ 4 , may be disposed in contiguous (see the special definition of this term) positions to detect the presence of an analyte over the entire length L. Each of such segments is a fractional portion of an optical fiber of length L where the remaining 3L/ 4  length of the optical fiber, consisting of lead portions (see the special definition of this term) in each instance, has low attenuation compared to the attenuation of the analyte-sensing segment. The resulting bundle of (4) fibers is bundled and connected to a light source and a detector. 
   In a more general aspect of the invention the sensing segments may be of differing length or of the same length. For example the length of the sensing segments may be selected to accommodate the installation conditions. 
   In another broad aspect, the sensing segments for a given analyte are offset (see the special definition of this term). Such offset configuration therefore includes overlapping, contiguous and spaced apart. Where apparatus is configured for more than one analyte the sensing segments for different analytes need not be offset. 
   The sensing segments may also be deployed to provide detection of an analyte at selected and non-contiguous intervals over a desired distance, that is, they may be spaced apart. The sensing segments may have the same or different lengths or some of each. 
   The optical fibers may be supported by an elongated spine and protected by a cable sheath having openings such as a braid to allow an analyte to reach the fibers. 
   Light can be launched into one end of the fibers and received at the other end in transmission mode or the light may be launched and received at the same end of the fibers in reflection mode if a reflection element is disposed at the opposite end of the fibers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an illustrative embodiment of a segmented sensor according to the invention; 
       FIG. 2A  is a schematic diagram of an embodiment of the present invention in which sensing segments are contiguous and/or overlapping; 
       FIG. 2B  is a schematic diagram of an embodiment of the present invention where sensing segments are not contiguous; 
       FIGS. 3A-3F  depict alternative optical energy sources and optical energy detectors according to embodiments of the present invention; 
       FIG. 4A  shows an embodiment of the present invention where reflection elements are used to reflect light from light sources back to light detectors located at the same end of the fibers; 
       FIG. 4B  shows an embodiment of the present invention where a single light source is used to transmit light into fibers having reflection elements and the light is reflected back to a single detector located at the same end of the fibers as the light source; 
       FIG. 5  is a schematic cross-sectional representation an embodiment of the present invention having a plurality of sensors supported by fiber carriers; 
       FIG. 5A  shows that the embodiment depicted in  FIG. 5  may be scaled to have additional fiber carriers; 
       FIG. 5B  shows schematically a connectorized configuration of the fiber carriers. 
       FIG. 5C  shows a schematically a connectorized configuration of the fibers carriers with the sensing segments separated by non-sensing portions; 
       FIG. 5D  shows an open wrap disposed around the fiber carrier; 
       FIG. 6  shows a cross-section of an embodiment of the present invention having external analyte-sensing segments and embedded lead portions; 
       FIG. 7  shows a fiber carrier and fiber configuration according to an embodiment of the invention; 
       FIG. 8  is a schematic representation of the organization of sensors of the multi-carrier sensor system of  FIG. 7 ; and 
       FIG. 9  shows an embodiment of the present invention that provides for analyte detection in selected areas. 
       FIG. 10  shows a fiber carrier and fiber configuration according to an embodiment of the invention. 
       FIG. 11  shows a fiber carrier and fiber configuration according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   As used herein the following terms have the following meaning unless the context requires otherwise: 
   “optical fiber” and “fiber” refers to a functional length of one or more optical fibers that allows transmission of an optical signal from one end to another end or to a reflection element and back, as such it may be a continuous integral length of optical fiber and it may also be a series of connected or spliced lengths of optical fiber. An optical fiber determined by connectors or splicing is made up of portions of the optical fiber. 
   “segment”, “sensing segment”, “sensor segment”, “analyte-responsive segment” and “analyte-sensing segment” refer to a length of optical fiber that is rendered able to react to the presence of an analyte so as to alter an optical signal sent through the optical fiber. This term, when so indicated by the context can also refer to a sensing segment that responds to environmental conditions such temperature and humidity. 
   “lead portions” and “non-analyte sensing portions” means a length of optical fiber not treated to react to an analyte or to have any sensing chemistry. 
   “offset” with reference to the relative lengthwise relationship of sensing segments in different optical fibers means any such relationship that allows substantially continuous sensing surveillance over a distance, as such it includes any one or combination of sensing segments that are contiguous or overlapping, each of which is also defined herein. 
   “contiguous” with reference to the lengthwise relationship of sensing segments in different optical fibers means that one ends where another begins so as to provide substantial (not necessarily exact-see further) continuity of surveillance over a distance. Accordingly, the term contiguous is also intended to include the configuration where only a connector or fusion splice or other instrumentality creates a linear space between the end of one sensing segment on one fiber and the beginning of another sensing segment on another fiber such that the distance being surveilled is substantially continuous even though there is a nominal space caused by the connector or splice or other instrumentality thereby being effective for detection of an analyte that is relatively spatially spread in the local area. This is distinguished from spaced-apart segments where the place or distance being surveilled by different sensing segments is intended to be distinguishable and substantially separated as defined below. 
   “overlapping” with reference to the lengthwise relationship of sensing segments means that a portion of one occupies a distance that is also occupied by a portion of another but that each occupies a distance not occupied by the other. 
   “spaced apart” with reference to the lengthwise relationship of sensing segments means that there is a substantial distance between the end of one and the beginning of another so that each segment has surveillance of a discrete location; 
   “chemical” and “analyte” are synonymous when used to describe or to define material that can be sensed by a sensing segment and are not limited to any type of chemical. 
     FIG. 1  is a schematic representation of a segmented sensor in accordance with an embodiment of this invention. The sensor is designed to sense the presence of an analyte anywhere over an arbitrary distance L. In accordance with the invention, a plurality of sensor segments are arranged in offset positions such that N sensors, each of length L/N occupy the entire distance L, however, as will be further explained the sensor segments do not have to be of equal length. In the illustrative embodiment of  FIG. 1 , there are four fibers  10 ,  20 ,  30 ,  40  (i.e., N=4) and each fiber has an analyte-sensing segment  11 ,  21 ,  31 ,  41 . Each fiber  10 ,  20 ,  30 ,  40  also has one or more lead portions  15 ,  25   a ,  25   b ,  35   a ,  35   b ,  45 . Preferably (for this embodiment), the sensing segments  11 ,  21 ,  31 ,  41  are of equal length (L/ 4 ). The remainder (3L/ 4 ) (indicated by lead portions  15 ,  25   a ,  25   b ,  35   a ,  35   b ,  45  in  FIG. 1 ) of each fiber  10 ,  20 ,  30 ,  40  is preferably optimized for light transmission. The high attenuation analyte-sensing segments  11 ,  21 ,  31 ,  41  having lengths of L/ 4  may be spliced or otherwise coupled with the one or more lead portions  15 ,  25   a ,  25   b ,  35   a ,  35   b ,  45  comprising low attenuation lead portions having total lengths of 3L/ 4  to provide the desired sensing length L. In the embodiment of the present invention depicted in  FIG. 1 , the analyte-sensing segments  11 ,  21 ,  31 ,  41  are shown as equal length and additive, that is contiguous, to extend to length L. However, alternative embodiments of the present invention may have analyte-sensing segments  11 ,  21 ,  31 ,  41  with unequal lengths. Other embodiments of the present invention may have analyte-sensing segments  11 ,  21 ,  31 ,  41  that overlap one or more other sensing segments of other fibers  10 ,  20 ,  30 ,  40 . Such an overlap is indicated by the segment  33  in  FIG. 1 . As discussed below, still other embodiments of the present invention may have equal length or unequal length analyte-sensing segments spaced apart from one another. As also shown in  FIG. 1 , the fibers  10 ,  20 ,  30 ,  40  are connected between a light source  110  under the control of a signal generator  112  and a detector  120  operative to transmit to a data acquisition system  122 . A processor module  124  determines from the detected signal whether there is indication that an analyte has reacted with the detection chemistry of any of the sensor segments. A common type of response is calorimetric. This is preferably done by comparing the received optical energy that has passed through the sensor segments with the source optical energy. It is also possible to measure the received optical energy with a previously established baseline to determine change. Those skilled in the art will recognize that other devices or means may also be used to provide optical energy to the fibers  10 ,  20 ,  30 ,  40  and/or receive optical energy from the fibers  10 ,  20 ,  30 ,  40 . It should be understood that the sensor system can be operated in reflection mode as well as in transmission mode that is illustrated and described. That is, optical energy may be transmitted and received at the same end of an optical fiber due to the reflection of the optical energy as it propagates within the fiber.  FIGS. 4A and 4B  show embodiments of the present invention using optical energy reflection. 
     FIG. 2A  illustrates the fibers of  FIG. 1  in a bundled configuration, at least at the ends thereof being adapted for ease of connection to the light source  110  and detector  120 .  FIG. 2A  also shows that the analyte-sensing segments  11 ,  21 ,  31 ,  41  are disposed in a contiguous manner. Also, an overlap is illustrated at  33 . Consequently, analyte-sensing segments  11 ,  21 ,  31 ,  41  are present at each point along the entire distance L of the bundled configuration, which provides the ability to detect analyte presence anywhere along the bundle and to spatially resolve it to the portion of the distance L occupied by the sensing segment or segments that have been exposed to the analyte.  FIG. 2B  shows an embodiment of the present invention in which the analyte sensing segments  11 ,  21 ,  31 ,  41  are non-contiguous such that there are gaps between the analyte sensing segments  11 ,  21 ,  31 ,  41  from fiber to fiber. In the configuration depicted in  FIG. 2B , analyte presence can be located at the discrete spaced-apart areas in which each of the sensing segments  11 ,  21 ,  31 ,  41  are deployed. 
   Alternative embodiments of the present invention may use different apparatus to implement the light source  110  and the detector  120  shown in  FIG. 1 .  FIGS. 3A-3F  depict some of the alternatives that may be used to provide the light source  110  and/or the detector  120 .  FIGS. 3A-3F  show an exemplary four fiber embodiment of the present invention, but those skilled in the art will understand that other embodiments may have less than four fibers or more than four fibers and also that the sensing segments may be offset, including contiguous or overlapping or they may be spaced-apart or any combination thereof as described above. In  FIGS. 3A-3F , each optical fiber  10 ,  20 ,  30 ,  40  has a sensing segment  11 ,  21 ,  31 ,  41  and one or more lead portions  15 ,  25 A,  25   b ,  35   a ,  35   b ,  45 . 
     FIG. 3A  shows an embodiment of the present invention in which each fiber  10 ,  20 ,  30 ,  40  is illuminated by a dedicated light source  111  and the optical output is measured by a dedicated photodetector  121 . As can be seen from  FIG. 3A , N optical fibers will then require N light sources and N photodetectors. 
     FIG. 3B  shows an embodiment of the present invention in which a series of optical splitters  141  couples light from a single light source  111  into N optical fibers  10 ,  20 ,  30 ,  40 . The optical outputs from the N optical fibers  10 ,  20 ,  30 ,  40  are measured by N dedicated photodetectors  121 . An alternative embodiment may use an optical switch (not shown) in place of the optical splitter. 
     FIG. 3C  shows an embodiment of the present invention in which N light sources  111  provide light to N optical fibers  10 ,  20 ,  30 ,  40 . The outputs from the N optical fibers are combined using an N×1 optical coupler  151 , which directs the combined optical signal to a single photodetector  121 . Preferably, the optical signals from the light sources are separated using Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM). 
     FIG. 3D  shows an embodiment of the present invention in which a single light source  111  and a single photodetector are used  121 . A 1×N optical splitter  141  (a series of 1×2 optical splitters may be used, as in  FIG. 3B ) is used to couple the single light source  111  to N optical fibers  10 ,  20 ,  30 ,  40  and an N×1 optical coupler  151  is used to combine the outputs of the optical fibers  10 ,  20 ,  30 ,  40  into a combined optical signal for the single photodetector  121 . Preferably, an optical switch  161  is used to switch the optical signals in the N optical fibers  10 ,  20 ,  30 ,  40  to limit the photodetector  121  to receiving the optical signal from only one optical fiber at a time. 
     FIG. 3E  shows an embodiment of the present invention in which N highly sensitive Mach-Zehnder interferometers are used to detect the change in the optical signal from a coherent light source. The embodiment comprises a single coherent light source  111  and N photodetectors  121 . A 1×(N+1) splitter  143  splits the light from the coherent light source  111  into the N optical fibers  10 ,  20 ,  30 ,  40  and a reference optical fiber  90 . The reference optical fiber  90  may have a length different than that of the N optical fibers  10 ,  20 ,  30 ,  40 . The output of the reference fiber  90  is split by a 1×N splitter  145  and directed to N 2×1 optical couplers  153 . Each 2×1 optical coupler  153  combines the output from one optical fiber  10 ,  20 ,  30 ,  40  and the reference optical fiber  90  and directs the combined signal to a photodetector  121 . The combination of the 2×1 optical coupler  153  and the photodetector  121  acts as a Mach-Zehnder interferometer to detect changes in the optical signal directed through the optical fiber  10 ,  20 ,  30 ,  40 . Other embodiments of the present invention may use other types of interferometers. For example, a sensing segment can comprise a component inside a Fabry-Perot cavity or become a branch of a Michelson interferometer. 
     FIG. 3F  shows an embodiment of the present invention in which changes in the optical signal from a coherent light source are detected, but no separate reference optical fiber is used. As shown in  FIG. 3F , a 1×N splitter  141  is used to direct the light from a coherent light source  111  to N optical fibers  10 ,  20 ,  30 ,  40 . The optical signals from a pair of non-adjacent optical fibers ( 10  and  30 , or  20  and  40 ) are combined using 2×1 optical couplers  155  and the combined signal is directed to a photodetector  121 . In this embodiment, one member of the pair serves as a reference for the other, for example fiber  30  serves as a reference for fiber  10  and fiber  40  serves as a reference for fiber  20 . The embodiment in  FIG. 3F  uses only N/2 optical couplers and N/2 photodetectors. In this embodiment, it is assumed that the phase of the optical signal in non-adjacent segments does not change simultaneously, while the phase in adjacent segments may change. Hence, the output of each photodetector  121  will produce a signal that has a magnitude that reflects the strength of the optical signal in each fiber  10 ,  20 ,  30 ,  40  while the polarity of the photodetector output will indicate the fiber in which the change has occurred. 
   Another embodiment of the present invention uses balanced detection from two non-adjacent sensing segments on different fibers. This embodiment is intensity-based. However, since the information about the phase change is unavailable, this embodiment detects change in at least one of the fibers, but does not identify the affected segments. 
   As indicated above, alternative embodiments of the present invention may have the light sources and the light detectors located at the same end of the fibers, operating in reflection mode.  FIG. 4A  shows an embodiment of the present invention similar to the embodiment depicted in  FIG. 3A .  FIG. 4A  shows four fibers  10 ,  20 ,  30 ,  40 , where each fiber  10 ,  20 ,  30 ,  40  is illuminated by a dedicated light source  111  and the optical output is measured by a dedicated photodetector  121 . However, optical circulators  171  are used to transmit light from each light source  111  into the fibers  10 ,  20 ,  30 ,  40 . Reflection elements  173  disposed at the end of each fiber  10 ,  20 ,  30 ,  40  reflect light back through the fibers  10 ,  20 ,  30 ,  40  towards the optical circulators  171 . The optical circulators then direct the reflected light to the photodetectors  121 . 
   In the embodiment depicted in  FIG. 4A , light is transmitted twice through the sensing segments  11 ,  21 ,  31 ,  41 , once during forward transmission and once during reflected transmission. Hence, the embodiment depicted in  FIG. 4A  may provide twice the attenuation of light than that of a similar embodiment in which the light sources  111  and the photodetectors  121  are disposed at opposite ends of the fibers  10 ,  20 ,  30 , and  40  (e.g., the embodiment depicted in  FIG. 3A ) and consequently provide increased sensitivity and an improved signal to noise ratio. 
   An alternative embodiment of the present invention similar to that shown in  FIG. 3D  is shown in  FIG. 4B .  FIG. 4B  shows a single light source  111  and a single photodetector  121  coupled to a single optical circulator  171 . The optical circulator  171  transmits light from the light source into an optical switch  175 . The optical circulator  171  also transmits light from the optical switch  175  to the photodetector  121 . The optical switch  175  directs light from the light source  171  to a selected one of the fibers  10 ,  20 ,  30 ,  40 . The light then radiates down the selected fiber  10 ,  20 ,  30 ,  40 , through the corresponding sensing segment  11 ,  21 ,  31 ,  41 , and is reflected by the reflection element  173  disposed at the end of each fiber  10 ,  20 ,  30 ,  40  back through the corresponding analyte responsive segment  11 ,  21 ,  31 ,  41  and into the optical switch  175 . The optical switch  175  is preferably controlled such that the timing relationship between light launched into a selected fiber  10 ,  20 ,  30   40  and the light received at the photodetector  121  is known (i.e., Time Division Multiplexing) so that the response seen at the photodetector  121  can be related to the corresponding fiber  10 ,  20 ,  30 ,  40 . The embodiment shown in  FIG. 4B  also provides the advantage of effectively doubling the attenuation provided by the analyte responsive segments  11 ,  21 ,  31 ,  41 . 
   Other embodiments of the present invention may adopt architectures similar to those shown in  FIGS. 3B-3C  and  FIGS. 3E-3F , except that the light sources  111  and photodetectors  121  are placed at the same end of each fiber and a reflection element  173  is used at the opposite end of each fiber to reflect light back towards the end having the light sources  111  and photodetectors  121 . 
     FIGS. 4A and 4B  show a reflection element  173  disposed at the end of the fibers  10 ,  20 ,  30 ,  40 . Those skilled in the art understand that the reflection element  173  may be provided by many elements and devices known in the art. Further, the reflection element  173  may be simply provided by terminating (e.g. cutting) the fiber  10 ,  20 ,  30 ,  40  or terminating the fiber  10 ,  20 ,  30 ,  40  and polishing and/or further shaping the fiber end to improve its reflection characteristics. 
     FIGS. 4A and 4B  also show that the sensing segments  11 ,  21 ,  31  may be followed by the one or more lead portions  15 ,  25   b ,  35   b  and the reflection element  173  deployed at the distal end of each fiber  10 ,  20 ,  30 ,  40 . However, according to some embodiments of the present invention, light transmission through the lead portions  15 ,  25   b ,  35   b  that are located on the other side of the sensing segments  11 ,  21 ,  31  from the light source  111  and photodetector  121  may not be required. Therefore, according to some embodiments of the present invention, the reflection element  173  may be simply disposed at the end of each sensing segment  11 ,  21 ,  31 ,  41 , that is, immediately after each sensing segment  11 ,  21 ,  31 ,  41 . This configuration may decrease the light attenuation caused by light propagation through the lead portions of the fibers. 
   In all of the foregoing descriptions and figures sensing segments are shown as being separate from lead portions before and after them. The means to separate analyte-sensing segments from lead portions is by connection such as by use of connectors or fusion connection or other connection means known in the art. However, alternative embodiments of the present invention may comprise a fiber in which the analyte-sensing segment is formed integrally with an adjacent lead portion albeit with some length of gradual transition due to the fiber manufacturing process to account for the transition from an analyte-sensing segment to a lead portion. It may ultimately be practical to apply sensing chemistry as a fiber is being manufacturing, which is likely to include a transition distance so that an optical fiber as contemplated by this description could be made from a single continuous fiber. 
   Therefore, in the broadest sense, an analyte-sensing segment may be either attached by connection or be integrally made with associated lead portions. 
   Fibers having analyte-sensing segments may be arranged in a cable assembly or harness as shown in  FIG. 5 . Specifically,  FIG. 5  shows a cross-section of a cable assembly  200  having four segmented sensors such as those shown in  FIGS. 1 ,  2 A- 2 B,  3 A- 3 F,  4 A- 4 B. Each of the four fibers  10 ,  20 ,  30 ,  40  is disposed on a fiber carrier comprising a cylindrical slotted spine  213 ; A central strength member  211  may be embedded within the slotted spine  213  in case the material of the slotted spine  213  is not considered to be sufficiently strong. The slotted spine  213  is preferably constructed such that the fibers  10 ,  20 ,  30 ,  40  disposed within the slots  217  of the slotted spine  213  will be exposed to the environment in which analytes are to be detected. The combination of the slotted spines  213  and the fibers  10 ,  20 ,  30 ,  40  in the slots  217  defines a segmented cable or segmented optical fiber assembly also referred to as segmented sensor  214  in which the fibers  10 ,  20 ,  30 ,  40  are disposed lengthwise adjacently. A reticulated or foraminous covering  215  that permits passage of any analyte from the outside environment to the fibers  10 ,  20 ,  30 ,  40  may be used to retain the fibers  10 ,  20 ,  30 ,  40  in the slotted spine  213 . An exemplary covering  215  is an open braid comprising glass yarn. 
   As shown in  FIG. 5 , the assembly  200  may dispose a plurality of segmented optical fiber assemblies  214  around a cable assembly central strength member  231 . Hence, the cable assembly  200  may comprise multiple segmented cables  214 A,  214 B,  214 C,  214 D each of which may carry multiple fibers. Therefore the cable assembly  200  is made up of a plurality of segmented optical fiber assemblies that extend in a parallel bundle. Therefore, the cable assembly  200  may provide the capability to sense an analyte over a longer distance or with a finer distance resolution by reason of the greater number of sensing segments; and both benefits can be realized. The cable assembly  200  may also be used to detect multiple analytes by using analyte responsive segments that are responsive to different analytes. This enables a plurality of analyte sets of segmented optical fibers to be deployed where each analyte set has sensor segments responsive to a particular analyte and each analyte set is responsive to an analyte different from the other analyte sets so that surveillance for several different analytes can be established by segmented deployment. Preferably each of the segmented optical fiber assemblies  214  A-D is equipped with sensing segments in its optical fibers for the same analyte and each of them has sensing segments that differ from the others. That is, for example, segmented optical fiber assembly  214 A may have optical fibers whose sensing segments are all responsive to a first analyte while segmented optical fiber assembly  214 B has optical fibers whose sensing segments are all responsive to a second analyte. The cable assembly  200  may also be constructed such that the analyte responsive segments (indicated by the hatching in  FIG. 5 ) appear at different rotational orientations of the slotted spines  213  by twisting the segmented cables around the strength member  231 . 
   In an exemplary embodiment, the cable assembly  200  depicted in  FIG. 5  may have central strength members  211  comprising epoxy glass rods that each has a nominal diameter of 1.2 mm. The optical fibers  10 ,  20 ,  30 ,  40  may have outside nominal diameters of 250 μm and the fiber carriers  213  may have outside diameters of about 5.0 mm. The covering  215  such as braided glass yarn around each slotted spine  213  would then have an outside diameter of about 5.5 mm. The cable assembly central strength member  231  may also have a central core comprising an epoxy glass rod or a rod made of glass fibers. An open wrap  235  may then be used around all of the slotted spines  213  to hold all of the fibers and the segmented cables together.  FIG. 5D  shows how the open wrap  235  comprising, for example, a glass or plastic line may be wrapped lengthwise around the cable assembly  200 . 
   The embodiment depicted in  FIG. 5  may be scaled to have any number of segmented cables  214 .  FIG. 5A  shows how the embodiment can be scaled to have six segmented cables  214 . 
   In a preferred embodiment, each of the four segmented cables  214 A-D in  FIG. 5  is dedicated to a specific analyte such as hydrogen cyanide, hydrogen sulfide, chlorine gas, and nerve agent. In this case each optical fiber in one of the segmented cables (four segmented cables being illustrated) has a sensing segmented covering its own designated distance. The configuration of  FIG. 5A  can be similarly equipped with additional sensing cables for general environmental conditions such as temperature and humidity. 
   In a further embodiment of the invention as hereinbefore described, for each segmented cable ( 214  A-D in  FIG. 5 ), each fiber&#39;s sensing segment is in a fiber length bounded by connectors, while the other fibers in that segmented cable are lead portions. This is illustrated in  FIG. 5B  in which a segmented cable  400  has connectors  401 ,  403 ,  405 ,  407  and  409 . Fibers  10 ,  20 ,  30  and  40  run from connector  401  to connector  409  although they do so in discrete lengths A, B, C and D between the connectors. In each of the discrete lengths one of the fibers has its sensing segment as at  11 ,  21 ,  31 , and  41 , while the other fibers are lead portions. In this configuration, the sensing segments being substantially contiguous, a continuous distance can be under surveillance (it is discontinuous only to the extent of the distance occupied by the connectors), with the ability to determine if the analyte is present at one or more of the areas A, B, C, and D. In the case where a plurality of analytes is of interest, a plurality of segmented cables  400  is deployed, each being dedicated to a specific analyte. 
     FIG. 5C  shows a similar connectorized set-up 500 but with the sensing discrete lengths A, B, C and D being spaced-apart by any selected distance by the insertion of a wholly passive discrete length as, X, Y, and Z 
   The connectorized configurations of  FIGS. 5B and 5C  can be applied to the arrangements of  FIGS. 5 and 5A  such that each segmented cable is bounded by connectors or splices, as  401  and  409  in  FIG. 5A  representing the ends of sensing cable  214 A. Similarly each of the plurality of sensing cables  214  A-D would have ends bounded by connectors or splices with additional intermediate connectors representing each segmented distance. In the configurations of  FIGS. 5-5C  the sensing segments of each analyte set are grouped sequentially so that over a distance the first sensing segment for each analyte set is grouped spatially with the first sensing segment of the other analyte set(s) and so on for each sensing segment in spatial order. 
   In another embodiment for use in a system set up for surveillance for a plurality of analytes, analyte sets of segmented fibers, each analyte set having sensing segments for a particular analyte are installed on a fiber carrier. In general fiber carriers have a portion on which the sensing segments are carried such as a surface that is available to be deployed so that it will encounter an analyte the presence of which it is in surveillance for and there is also a portion for carrying the lead portions of the fibers. The latter portion can be a surface or conduit that is located on the fiber carrier in a place that is convenient to carry all the lead portions and it need not be and preferably is not positioned to encounter the analyte but rather is positioned to allow a clear and unobstructed deployment of the sensing segments. In one embodiment, the fiber carrier has an external structure on which the first sensing segments, in sequential order, for each analyte set are disposed adjacently. The fiber carrier also has a passive structure (surface or conduit or the like) on which all the lead portions are disposed. The external structure is exposed to the environment in order that the sensing segments be exposed to an analyte when present while the passive structure can be hidden or on a reverse side where exposure to the analyte is irrelevant.  FIG. 6  depicts an example of this embodiment.  FIG. 6  depicts a cross-section of an assembly  300  comprising a slotted fiber carrier  393  with lead fiber portions  312 ,  313 ,  314 ,  322 ,  323 ,  324 ,  332 ,  333 ,  334 ,  342 ,  343 ,  344  contained inside the slotted fiber carrier  393  in an interior conduit  345  and sensing segments  311 ,  321 ,  331 ,  341  located in slots  395  on the external structure  399  of the fiber carrier  393 . A braid  396  or other means can be used to retain the sensing segments in the slots  395 . The embodiment shown in  FIG. 6  may have a smaller diameter than the embodiment shown in  FIG. 5 , since the embodiment of  FIG. 6  only uses a single fiber carrier for surveillance for a plurality of analytes. Note also that the embodiment of  FIG. 6  allows the sensing segments  311 ,  321 ,  331 ,  341  to be more completely exposed to the ambient environment, which may facilitate the detection of chemical agents (i.e., analytes) and decrease the response time. Although slots have been shown for retaining and positioning the fibers on the carrier periphery, it will be apparent to those skilled in the art that other retaining means could be employed, for example, spaced apart pairs of posts could be embedded or molded into the carrier or simple ties could be used to secure the fibers. 
     FIG. 7  depicts how the transition from a sensing segment  311  located on the periphery of the slotted spine carrier  393  to an internally located lead portion  312  may be handled.  FIG. 7  depicts a first fiber assembly  301  coupled to a second fiber assembly  302 , each of which has the structure of the assembly  300  shown in  FIG. 6 .  FIG. 7  shows a sensing segment  311  located on the periphery of the first fiber assembly  301  coupled to a lead portion  312  located within the conduit  345  in the slotted spine carrier  393  of the second fiber assembly  302 . Similarly,  FIG. 7  shows a first lead portion  334  and a second lead portion  314 , both located within the conduit  345  in the slotted spine carrier  393  of the first assembly  301 , coupled to a sensing segment  331  and a second sensing segment  311 , respectively, both located on the periphery of the second assembly  302 . 
   As shown in  FIG. 7 , only the sensing segment (depicted by hatching) of each fiber is exposed, while the remainder of the fiber is buried within the slotted spine carrier  393  in the conduit  345 . Thus,  FIG. 7  depicts four simultaneously sensing fiber segments, each responsive to a different sensing chemistry. 
   It is to be noted that the first and second fiber assemblies  301 ,  302  are linearly adjacent so that the sensing segments on the surface of first assembly  301  are exposed the whole length of first assembly  301 ; and then their lead portions are concealed in the conduit  345  in the second assembly  302 . Additional such assemblies can be fitted so that sensing segments for each sensing chemistry on different fibers are exposed on each assembly thereby allowing for location of analytes. Hence, fiber assemblies of optimum length may be connected together by standard optical connectors (represented by  381  in  FIG. 7 ). For example, each fiber assembly may be a certain length and connected together in groups of four to provide a sensor having a length additive of their individual lengths (disregarding small distances occupied by connectors). 
     FIG. 8  shows schematically the organization of a sensor of the type shown in  FIG. 7 . Specifically, the schematic of  FIG. 8  is based on the assumption that four different sensing chemistries are used with segments  311 ,  321 ,  331  and  341  of  FIG. 7 . Sixteen fibers are required for four segmented sensors for four analytes. Connector  381  of  FIG. 8  corresponds in position to the space between assembly  301  and assembly  302  of  FIG. 7 .  FIG. 8  shows that the analyte-sensing segments for the first sensing chemistry are denoted as element  311 . The left most fiber assembly shown in  FIG. 8  corresponds to the first fiber assembly  301  shown in  FIG. 7 .  FIG. 8  shows that the analyte-sensing segment  311  in this left most assembly couples through the connector  381  to lead portions  312  in the next fiber assembly. This next fiber assembly corresponds to the second fiber assembly  302  in  FIG. 7 .  FIG. 7  shows that the lead portion  312  is embedded within the conduit  345 . Two additional connectors, connector  383  and connector  385 , are required to connect the four fiber assemblies into a complete four segment sensor. For example, each fiber assembly may be 20 meters, so that the total length of the sensor is 80 meters. 
   The different analyte sets of  FIGS. 6-9  are like those of  FIGS. 5-5C  sequentially grouped so that along a distance over which they are deployed a spatially first sensing segment for each group is on the same fiber carrier and consequently is surveilling the same partial length of the distance under surveillance and similarly the next sensing segments in spatial order for each analyte set are grouped and so on for all the sensing segments, in spatial order. 
   In an experimental set up, four chemistries were used to sense the presence of four different analytes, each requiring light of a different wavelength. The blocks (outlined) to the left as viewed in  FIG. 8  represent the optical light source input; the blocks to the right represent the corresponding photodetectors measuring responses to individual chemistries. 
   Advantageously, the optical fibers are twisted by a relative rotation of the carriers with respect to one another to avoid possible stress on the system if the apparatus is placed along arcuate paths. This rotation is represented by the position of the optical fibers in the space between the first fiber assembly  301  and the second fiber assembly  302  of  FIG. 7 . 
   Fiber carriers according to embodiments of the present invention are not limited to carriers with circular cross-sections.  FIG. 10  shows an alternative embodiment of the present invention with a fiber carrier having a non-circular cross-section.  FIG. 10  shows a first fiber assembly  303  coupled to a second fiber assembly  304 , each assembly  303 ,  304 , having a fiber carrier  305  with a rectangular cross-section. The fiber carrier  305  has an external structure  398  on which the analyte-responsive segments  311 ,  321 ,  331 ,  341  are disposed. The external structure  398  proves for exposure of the analyte-responsive segments  311 ,  321 ,  331 ,  341  to the environment in which analytes are to be detected. Lead portions  312 ,  322 ,  332 ,  342  are disposed in a passive structure  394  which is a cavity on the bottom of the fiber carrier  305 . Other embodiments according to the present invention may disposed the passive structure on a surface of the fiber carrier  305 . While  FIG. 10  shows a fiber carrier  305  with a rectangular cross-section, other fiber carriers of other embodiments according to the present invention may have cross-sections with different shapes. 
     FIG. 10  also shows the transitions from the analyte-responsive segments  311 ,  321 ,  331 ,  341  to the lead portions  312 ,  322 ,  332 ,  342 .  FIG. 10  shows the analyte-responsive segment  311  of the first assembly  303  coupled to lead portion  312  of the second fiber assembly  304 . Similarly,  FIG. 7  shows the portion  312  of the first assembly  303  coupled to the analyte-responsive segment  311  of the second assembly  302 . The coupling of segments of the first assembly  303  and the second assembly  304  may be accomplished with coupling means  381  known in the art, such as fiber couplers connectors, fiber bonding, etc. 
     FIG. 11  shows an alternative embodiment of the present invention with a ribbon-like fiber carrier.  FIG. 11  shows a first fiber assembly  306  coupled to a second fiber assembly  307 , each assembly  306 ,  306 , having a fiber carrier  308  generally shaped like a ribbon. The fiber carrier  308  has an external structure  399  on which both the analyte responsive segments  311 ,  321 ,  331 ,  341  and the non-analyte-responsive segments  312 ,  322 ,  332 ,  342  are disposed. The analyte responsive segments  311 ,  321 ,  331 ,  341  and the non-analyte-responsive segments  312 ,  322 ,  332 ,  342  are disposed generally parallel to each other on the external structure  399 . The external structure  399  proves for exposure of the analyte responsive segments  311 ,  321 ,  331 ,  341  to the environment in which analytes are to be detected and also provides a carrying surface for the non-analyte responsive segments  312 ,  322 ,  332 ,  342 . 
     FIG. 11  also shows the transitions from the analyte responsive segments  311 ,  321 ,  331 ,  341  to the non-analyte responsive segments  312 ,  322 ,  332 ,  342 .  FIG. 11  shows the analyte responsive segment  311  of the first assembly  303  coupled to the non-analyte responsive segment  312  of the second fiber assembly  304 . Similarly,  FIG. 11  shows the non-analyte responsive segment  312  of the first assembly  303  coupled to the analyte responsive segment  311  of the second assembly  302 . The coupling of segments of the first assembly  306  and the second assembly  307  may be accomplished with coupling means  381  known in the art, such as fiber couplers, fiber bonding, etc. 
     FIG. 11  shows the analyte responsive segments  311 ,  321 ,  331 ,  341  spaced apart from each other by the non-analyte-responsive segments  312 ,  322 ,  332 ,  342 , but other embodiments may have all of the analyte responsive segments positioned next to each other or distributed arbitrarily in any position on the external structure  399 . Further, while  FIG. 11  shows the analyte responsive segments  311 ,  321 ,  331 ,  341  at different positions on the first assembly  306  and the second assembly  307 , other embodiments may have the analyte responsive segments  311 ,  321 ,  331 ,  341  at the same positions on each fiber assembly. The coupling means  381  would then provide the transition from an analyte responsive segment to a non-analyte responsive segment. Hence, an arbitrarily long ribbon carrier could be manufactured with both analyte responsive and non-analyte responsive fibers on it, which would then be cut into shorter fiber assembly segments (similar to that described in regard to  FIG. 6 ). The fiber assembly segments could then be coupled by coupling means to provide an embodiment of the present invention similar to that shown in  FIG. 11   
   An embodiment of the present invention may be fabricated by producing a long single harness (e.g. 5000 meters) of the type depicted in  FIG. 6 . That is, the embodiment comprises a braided harness with analyte sensing fibers on the outside of a fiber carrier and low attenuation lead fiber portions embedded within the carrier. The braided harness can then be cut into the appropriate sensing lengths. The length may be determined by the attenuation of the sensing fiber segments. Standard fiber optic connectors may then be used to connect together the segments. 
   As described above, fiber optic sensors organized in a segmented arrangement as shown in the figures described above may be used to detect multiple types of analytes. For example, a first set of fibers may be coated with a coating normally comprising a cladding including a colorimetric indicator for sensing hydrogen cyanide. A second set of fibers may then be coated with a coating including an indicator for sensing chlorine gas. A third set of fibers may detect hydrogen sulfide and a fourth set, a nerve agent. Hence, the combination of fibers could detect all of hydrogen cyanide and chlorine gas, hydrogen sulfide and a nerve agent in a linearly segmented system. Preferably, the coatings are curable using either ultra violet light or heat. The coatings may then be cured as they are applied to optical cores as the cores are drawn. Exemplary coatings include silicones and acrylates. According to embodiments of the present invention, the (segmented) sensors may be packaged as shown in  FIGS. 5 and 5A  or  6  and  7 . 
   The decreased attenuation provided by an embodiment of the present invention is shown by examining typical optical fibers known in the art. Typically, chemically sensitive optical fiber may have attenuation on the order of 1 dB/m, while low attenuation multimode optical fiber (not chemically sensitive) may have attenuation as low as 0.04 dB/m. Each optical fiber of the four optical fiber embodiment (described above) requires three additional optical connectors with each connector having a loss of about 0.1 dB. In the array shown in  FIG. 1 , a 1:4 optical splitter is used to launch the light into the four separate fibers and a 4:1 optical combiner is used to combine the light for application to the photodetector. The optical splitter and the optical combiner are each estimated to provide an additional 1 dB of loss. 
   Hence, the total optical losses through a single fiber in the embodiment shown in  FIG. 1  are estimated as 20 m×1 dB/m (sensing segment loss)+60 m×0.04 dB/m (lead segment loss)+3×0.1 dB (connector loss)+2×1 dB (splitter/combiner loss)=24.7 dB. Hence, the total optical power attenuation from the source to the photodetector is estimated as 24.7 dB. On the other hand, if the entire optical fiber is chemically sensitive fiber as is known in the art, the total attenuation is estimated as 80 m×1 dB/m=80 dB. Therefore, the embodiment depicted in  FIG. 1  may provide 55.3 dB less attenuation than a sensing system using a single chemically sensitive optical fiber. 
   An embodiment of the present invention may be used to provide for continuous chemical surveillance over a desired distance. As shown in  FIG. 2A , a structure having multiple optical fibers may be deployed ( FIG. 2A  shows four fibers  10 ,  20 ,  30 ,  40 , but more or fewer fibers may be used in accordance with the present invention), where each optical fiber has a sensing segment. To provide for continuous chemical surveillance, the sensing segments for a given chemistry should be positioned such that the segments are offset which may include contiguous, or overlapping. Hence, the presence of an analyte at any position along the structure may be detected, and spatially resolved within the distance covered by one or more segments that respond to its presence, while maintaining the low loss characteristics described above. This type of arrangement is particularly useful to determine the presence of an analyte at a particular sensing segment in place at a known location over otherwise undifferentiated distances such as for perimeter surveillance. 
   Another embodiment of the present invention provides for chemical surveillance at selected spaced-apart positions along the length of the detecting structure (see  FIG. 9 ). As shown in  FIG. 2B , a structure having multiple optical fibers may be deployed ( FIG. 2B  shows four fibers  10 ,  20 ,  30 ,  40 , but more or fewer fibers may be used in accordance with the present invention), where each optical fiber has a sensing segment. However, unlike the structure depicted in  FIG. 2A , the sensing segments may not be contiguous or overlapping. Instead, the sensing segments are spaced-apart, positioned at those intervals at which the detection of an analyte is desired, while intervening positions use the low loss lead fiber portion discussed above. Hence, the sensing segment in one optical fiber may be spaced-apart from the sensing segments in the other optical fibers. The length of the sensing segments may differ. Hence, the presence of an analyte at selected positions can be detected, while maintaining the low loss characteristics described above. This is particularly useful in structures where sensing segments can be located at separate spaced-apart locations such as windows, doors, air intakes and the like openings in the structure where an attack may be targeted. 
     FIG. 9  depicts an embodiment of the present invention used for detecting analytes in separate spaced-apart physical locations. A detecting structure comprising four fibers  10 ,  20 ,  30 ,  40  is disposed through four separate areas  901 . The separate areas  901  may be separate rooms, portions of rooms, separate buildings, etc.  FIG. 9  shows the sensing segments  11 ,  21 ,  31  and  41  extending throughout an entire dimension of separate areas  901 , but the sensing segments may extend for only a portion of the areas  901  or may extend past the areas  901 , as illustrated by segment  13 . The fibers may be fabricated in structures such as those shown in  FIGS. 5 ,  5 A or  6  and may be coupled together with optical connectors  195 . A control element  190 , comprising a light source  110  and a photodetector  120  both coupled to a processor  191 , transmits light into the fibers  10 ,  20 ,  30 ,  40  and receives light from the fibers  10 ,  20 ,  30 ,  40 . The processor  191  may be a digital signal processor that both controls the light generated by the light source  110  and processes the electrical signals generated by the photodetector  120 . The control element  190  provides an external output  192 , which indicates whether an analyte has been sensed by any of the fibers  10 ,  20 ,  30 ,  40  and, therefore, indicates the location of the sensed analyte. As discussed above, an alternative embodiment may have the light source  110  and the photodetector  120  disposed at the same end of the fibers  10 ,  20 ,  30 ,  40  and a reflection element disposed at the distal end of the fibers. 
   What has been described is considered merely illustrative of the invention. Those skilled in the art are competent to make variations and modifications of the illustrations herein still within the spirit and scope of the invention as claimed hereinafter.