Abstract:
Fiber grating environmental measurement systems are comprised of sensors that are configured to respond to changes in moisture or chemical content of the surrounding medium through the action of coatings and plates inducing strain that is measured. These sensors can also be used to monitor the interior of bonds for degradation due to aging, cracking, or chemical attack. Means to multiplex these sensors at high speed and with high sensitivity can be accomplished by using spectral filters placed to correspond to each fiber grating environmental sensor. By forming networks of spectral elements and using wavelength division multiplexing arrays of fiber grating sensors may be processed in a single fiber line allowing distributed high sensitivity, high bandwidth fiber optic grating environmental sensor systems to be realized.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/173,359 by Whitten L. Schulz, John Seim and Eric Udd, entitled, “Fiber Grating Environmental Sensing System” which was filed on Dec. 27, 1999. 
    
    
     This invention was made with Government support from contract DE-FG03-99ER82753 awarded by DOE and by contracts N68335-98-C-0122 and N68335-99-C-0242 awarded by NAVAIR. The government has certain rights to this invention. 
    
    
     REFERENCE TO RELATED PATENTS 
     This disclosure describes means to enhance the speed and sensitivity of fiber grating sensors systems to measure environmental effects and means to multiplex these sensors while retaining high speed characteristics. The background of these types of fiber grating sensors may be found in U.S. Pat. Nos. 5,380,995, 5,402,231, 5,592,965, 5,841,131 and 6,144,026. The teachings in those patents are incorporated into this disclosure by reference as though fully set forth below. 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to fiber optic grating systems and more particularly, to the measurement of environmental effects using fiber optic grating sensors. Typical fiber optic grating sensor systems are described in detail in U.S. Pat. Nos. 5,380,995, 5,402,231, 5,592,965, 5,841,131 and 6,144,026. 
     The need for low cost, a high performance fiber optic grating environmental sensor system that is capable of long term environmental monitoring, virtually immune to electromagnetic interference and passive is critical for such applications as moisture sensing and monitoring of adhesive bonds. Another advantage of these system is that when they are appropriately configured the frequency response of the system can be very high. 
     The present invention includes multi-axis fiber grating sensors that may be used to sense axial strain and temperature, or axial and transverse strain simultaneously to detect chemical changes such as moisture by using appropriate transducers or changes to the structural integrity of coatings such as adhesive bonds. Means are also described to multiplex these fiber grating sensors allowing high sensitivity and high speed measurements to be made. 
     In U.S. Pat. Nos. 5,380,995 and 5,397,891 fiber grating demodulation systems are described that involve single element fiber gratings and using spectral filters to demodulate fiber gratings. The present invention includes means to extend the demodulation system to multiple fiber grating sensors operating at high speed on a single fiber line. In U.S. Pat. Nos. 5,591,965, 5,627,927 the usage of fiber gratings to detect more than one dimension of strain is described. These ideas are extended in U.S. Pat. Nos. 5,869,835, 5,828,059 and 5,841,131 to include fibers with different geometries that can be used to enhance sensitivity or simplify alignment procedures for enhanced sensitivity of multi-parameter fiber sensing. In U.S. patent application Ser. No. 09/176,515, “High Speed Demodulation Systems for Fiber Grating Sensors”, by Eric Udd and Andreas Weisshaar means are described to process the output from multi-axis fiber grating sensors for improved sensitivity. All of these patents teaching are background for the present invention which optimizes the fiber grating sensor for optimum response to strain changes induced by changes in the state of its coating or surrounding media to form water/chemical sensors and monitor the status of adhesive joints through measurements of strain interior to the bond. 
     The present invention consists of an optical fiber whose axial, transverse and or temperature sensitivity has been optimized through the construction of the optical fiber or mechanical mechanisms to enhance sensitivity. High speed demodulation is provided by wavelength division multiplexing of these fiber grating sensors using series of fiber grating filters. The spectral filters are arranged so that each fiber grating sensor has a corresponding filter to match it, allowing higher speeds and sensitivity than many current approaches. To sense transverse strain at high speeds in birefringent optical fiber, the two spectral peaks associated with the fiber gratings are tracked individually by locking onto its preferred polarization state. 
     Therefore, it is an object of the present invention to monitor changes in moisture or chemical content of an environment through measured strain changes. 
     Another object of the invention is to provide a means of monitoring bond lines for degradation. 
     Another object of the invention is to provide means to measure changes in several fiber grating sensors at high speed and with high sensitivity simultaneously in a single fiber. 
     Another object of the invention is to measure transverse strain as well as axial strain at high speed and with high sensitivity. 
     These and other objects and advantages of the present invention will become apparent to those skilled in the art after considering the following detailed specification including the drawings wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art illustration of a grating written onto sidehole fiber. 
     FIG. 2 is a diagram showing the splitting of a spectral peak with transverse loading on grating written onto ordinary single mode fiber. 
     FIG. 3 is a diagram showing the separation of spectral peaks with transverse loading of a grating written onto PM fiber. 
     FIG. 4 is an illustration showing the basis of a fiber grating chemical sensor with a chemically sensitive coating attached to plates which are constricted and strain the grating as the coating swells in the presence of the target chemical. 
     FIG. 5 is an illustration of a chemical sensor employing m and n stacks of a chemical sensitive coating to change sensitivity of sensor 
     FIG. 6 is an illustration of a fiber embedded into composite tow bonded to stiff plates. As the chemically sensitive coating expands or contracts, the strain state in the fiber grating sensor changes. 
     FIG. 7 is an illustration showing a fiber grating embedded into composite part. As the affinity coating changes, the strain on the sensor will change. 
     FIG. 8 is an illustration of a fiber grating sensor with a single v-groove plate to prevent fiber rotation. 
     FIG. 9 is an illustration of a fiber grating sensor with a double v-groove design to eliminate possible rocking of the top plate. 
     FIG. 10 is an illustration showing the use of channels to prevent the top plate from rocking on fiber. 
     FIG. 11 is an illustration showing multiple sensing points for extended sensing range or higher accuracy through averaging. 
     FIG. 12 is an illustration showing the use of beveled plate to increase surface area of exposed coating and/or increase wicking action of target chemical into coating. 
     FIG. 13 a , FIG. 13 b , and FIG. 13 c , show different methods to increase target chemical absorption through the transducer plates. 
     FIG. 14 is an illustration showing how a flexible plate may be used to account for inconsistent swelling of the chemically sensitive coating. 
     FIG. 15 is an illustration showing the placement of the coating directly on the fiber. 
     FIG. 16 is an illustration showing the placement layers of chemically reactive composite tow over the fiber which may load the fiber in transverse strain when exposed to the target chemical. For example, some composite tows may swell in the presence of moisture. The void may be used to ensure a clean transverse load. 
     FIG. 17 is an illustration showing the wing of an aircraft where transverse fiber grating strain sensors are used to monitor the adhesive joints. 
     FIG. 18 is an illustration of a transverse fiber grating strain sensor embedded directly into the adhesive of a bond to monitor the health of the bond. 
     FIG. 19 is an illustration of three different embedding locations of transverse strain sensors into an adhesive joint. 
     FIG. 20 a  is a diagram showing uniform loading with clean spectral peaks and FIG. 20 b  shows non-uniform loading with more complex spectral profiles of gratings written onto polarization maintaining fiber. 
     FIG. 21 a  shows [data taken from] a transverse fiber grating strain sensor embedded into an adhesive joint that was placed under load. FIG. 21 b  shows data taken from the transverse fiber grating strain sensor. 
     FIG. 22 is an illustration of a dual axis fiber grating sensor embedded into an adhesive joint with its transverse strain sensing axis aligned at −45 degrees. 
     FIG. 23 shows data taken from a transverse fiber grating strain sensor embedded into an adhesive joint undergoing plastic deformation. 
     FIG. 24 shows data of the displacement of the instrument adhesive joint from FIG.  23 . 
     FIG. 25 is an illustration of a non-round coating on fiber to prevent rolling and maintain desired orientation. 
     FIG. 26 is an illustration of forming a non-round coating using heat. 
     FIG. 27 is a diagram of a prior art high-speed demodulation system employing a grating filter to demodulate a grating sensor. 
     FIG. 28 a  and FIG. 28 b  are diagrams showing different full width half max spectra for grating filters allows for selection of sensitivity and dynamic range. 
     FIG. 29 a  and FIG. 29 b  are diagrams showing how a change in the periodic spacing of the perturbations of the index of refraction, or grating spacing, changes the spectral position of the grating. 
     FIG. 30 a  and FIG. 30 b  are diagrams showing the bending of a simply supported beam to induce tension or compression in an attached or embedded grating. 
     FIG. 31 a  and FIG. 31 b  are diagrams showing a cantilever configuration for inducing tension or compression in an attached or embedded grating. 
     FIG. 32 is a diagram showing the stretching or compressing of a beam with force (F) to induce tension or compression in grating. 
     FIG. 33 is an illustration of a tunable grating filter requiring only one direction of tuning as the initial filter wavelength is lower than that of the sensor allowing it to be tuned into the range of the sensor. 
     FIG. 34 is an illustration of a tunable grating filter employing a grating in a tube to control the amount of strain transferred to the grating for a given displacement and allowing for tuning in both directions if the fiber is pre-tensioned in the tube and the grating is stretched or relaxed. 
     FIG. 35 a  and FIG. 35 b  are diagrams showing the application of tension or compression to surface mounted or embedded fiber grating through a pressure (P) differential across the diaphragm. 
     FIG. 36 a  and FIG. 36 b  are diagrams showing the deflection of a beam using a threaded stud to induce strain (positive or negative) in a grating. 
     FIG. 37 is a photograph of the exterior of prototype with fiber optic connections and knob on top to turn a threaded stud and deflect a beam used to put tension and compression on the fiber grating. 
     FIG. 38 is a photograph of the interior of prototype showing threaded stud, beam, and beam supports. 
     FIG. 39 is a diagram showing a beam with multiple color grating filters to filter different color grating sensors. 
     FIG. 40 is a diagram showing an adjustable comb filter. 
     FIG. 41 is a diagram showing a series of beams with attached grating filters at different wavelengths to form an adjustable comb filter. 
     FIG. 42 is a diagram showing a configuration where adjusting each filter independently with a knob-beam configuration is possible. 
     FIG. 43 is a diagram of a tunable grating filter based on thermal tuning. 
     FIG. 44 is a diagram showing multiplexing of the high speed demodulation system by introducing a time delay. 
     FIG. 45 is a diagram showing splitting the dual peak structure of a dual axis grating to two individual peaks. 
     FIG. 46 is a diagram showing the use of polarization controllers to separate out the two polarization states associated with a dual axis(transverse) grating sensor. 
     FIG. 47 is a diagram of an alternative design where the polarization controllers and polarizing fiber are placed before the last beam splitters to reduce errors associated with inconsistent polarization states in the filtered and reference legs. 
     FIG. 48 is a diagram showing the use of polarization maintaining (PM) fiber and beam splitters in conjunction with polarizers to control polarization states. 
     FIG. 49 is a diagram showing multiplexing of the transverse gratings by combining two light sources and splitting each wavelength to separate demodulators. 
     FIG. 50 is a diagram showing a “Cascading” configuration where beam splitters are used to divide the reflected light from the sensors among the separate demodulators. 
     FIG. 51 is a diagram showing the alternate location of detectors. 
     FIG. 52 is a diagram showing another alternate location of detectors to eliminate background light levels compared to FIG.  50 . 
     FIG. 53 is a diagram showing another method to demodulate several in line fiber grating sensors. This system also provides the capability of an absolute measurement by providing a reference detector. 
     FIG. 54 is a diagram showing an alternate configuration with reference detectors on each leg. 
     FIG. 55 is a diagram showing how gratings written into beam splitters can be employed to efficiently multiplex a high speed fiber grating demodulation system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, environmental sensing systems based on fiber gratings are described. The environmental grating sensors may be written onto ordinary single mode or birefringent fiber. For the case where the environmental sensor is subjected to a transverse load, it will behave differently depending on if it is written onto ordinary single mode fiber or onto birefringent fiber. To further increase the sensor&#39;s response to a transverse load, voids such as sideholes may be introduced into the fiber. FIG. 1 shows a prior art transverse fiber grating sensor written onto optical sidehole fiber as described in U.S. Pat. Nos. 5,828,059 and 5,841,131. The sidehole transverse fiber grating sensor  1  consists of a length of sidehole fiber  3  that may have sideholes  5 . When a grating  7  is written onto the core  9  of the sidehole fiber  3 , a single distinct spectral peak results. The sideholes  5  in the fiber may increase the sensor&#39;s  1  response to transverse strain. 
     Gratings written onto some sidehole fiber or ordinary single mode fiber will reflect a single spectral peak in the unloaded case. As the grating on some sidhole or single mode fiber is transversely loaded, the spectral peak will begin to broaden and eventually split as birefringence is induced in the fiber from the external load. FIG. 2 shows a typical spectral response to transverse loading for the case of a single grating written onto non birefringent optical fiber, such as some sidehole fiber. In the unloaded case  51 , a single spectral peak results. As the transverse load on the fiber sensor increases, the spectral peak  53  begins to broaden. With further increasing load, the spectral peak begins to split into two distinct spectral peaks  55 . 
     For the case where a grating is written onto birefringent fiber, two spectral peaks are reflected in the unloaded case, one for each polarization state. As the grating written onto birefringent fiber is transversely loaded, the spacing between the two spectral peaks will change. 
     FIG. 3 shows a typical spectral response to transverse loading for the case of a single grating written onto birefringent optical fiber. In the unloaded case, two spectral peaks  101  result with a peak separation  103 . As the transverse load increases, the separation of the two peaks  105  will increase. With further increasing transverse loading, the spectral peak separation  107  will further increase. 
     FIG. 4 shows a chemical sensor based on transverse loading of a strain sensor based on a single grating or multiple gratings written onto birefringent or non-birefringent fiber. The chemical sensor  151  consists of a chemical sensitive coating  153  that expands in the presence of the target chemical to be sensed, such as moisture. As the chemical sensitive coating  153  expands, it exerts a force onto some stiff plates  155 . The outward expansion is prevented by clamps  157  and  159 . This directs the force into the grating sensor  161 . The effective result is a transverse strain impending on the grating sensor  161  in the presence of the target chemical. The stiff plates  155  provide a more even loading on the fiber as the chemical sensitive coating  153  expands. The relatively large exposed area of the chemical sensitive coating  153  increases the sensitivity and response time of this chemical sensor. 
     FIG. 5 shows another variation of a chemical sensor where a series of chemical sensitive coatings are cascaded together to increase the amount of force directed into the fiber grating sensor to increase sensitivity. This variation of the chemical sensor  201  consists of multiple stacks of chemically sensitive coating  203  with stiff plates  205 . As these multiple sets of chemical sensitive coatings  203  expand in the presence of the target chemical, their combined force is directed into the fiber grating sensor  207 . By controlling the quantity n and m of the stacks, the sensitivity of the chemical sensor  201  can be controlled. 
     FIG. 6 shows another variation of a chemical sensor where the grating sensor is embedded into a piece of composite tow where the force on the fiber is transverse. The chemical sensor  251  consists of a fiber grating sensor  253  that is formed from one or two gratings written onto birefringent or non-birefringent optical fiber. The fiber grating sensor  253  is embedded into a piece of composite tow  255  which can have many functions such as isolating the fiber grating sensor  253  from chemicals that would be damaging to the optical fiber and keeping the orientation of the fiber grating sensor  253  correct for the case where birefringent fiber is used. The composite tow piece  255  is surrounded by stiff plates  257  and chemical sensitive coating  259  (or affinity coating.) As the chemical sensitive coating expands or shrinks in the presence of the target chemical or chemicals, the force exerted on the tow  255  changes and hence the strain on the fiber grating sensor  253  allowing a measurement to be made. 
     FIG. 7 shows another variation of a chemical sensor  301  where the fiber grating sensor  303  is embedded into a composite part  305  with some optimal geometry for the chemical sensitive coating  307  (or affinity coating) to maximize the strain on the fiber grating sensor  303  in the presence of the target chemical or chemicals. 
     FIG. 8 shows another variation of a chemical sensor based on a v-groove configuration. This chemical sensor  350  consists of a fiber grating sensor  353  that is formed from a single or multiple gratings on birefringent or non-birefringent fiber placed into a v-groove  355 . This plate keeps the fiber in place and can help maintain the proper orientation  357  of the fiber if a grating in birefringent fiber is used. As the chemical sensitive coating  359  expands in the presence of the target chemical or chemicals, it exerts a force on the top plate  361  which transfers the force to the fiber grating sensor  353 . 
     FIG. 9 shows another variation of a chemical sensor based on a double v-groove configuration. The chemical sensor  401  consists of a double v-groove plate  403  that holds both the fiber grating sensor  405  and a dummy fiber  407  of the same diameter as the fiber grating sensor but without a grating element. This configuration helps to reduce the rocking effect of the top plate  409  on top of the fiber grating sensor  405  to provide a more consistent loading as the chemical sensitive coating  411  expands in the presence of the target chemical or chemicals. The v-grooves in plate  403  help keep the fibers in place and keep the fiber grating sensor  405  oriented if a birefringent fiber is used. 
     FIG. 10 shows another variation of a chemical sensor based on a v-groove configuration. The chemical sensor  451  consists of a v-groove plate  453  and side channels  455 . The side channels can help keep the top plate level for more consistent loading on the fiber grating sensor  459  as the chemical sensitive coating  461  expands in the presence of the target chemical or chemicals. The v-groove plate  453  helps keep the fiber in place and keeps the fiber grating sensor  459  oriented if a birefringent fiber is used. 
     FIG. 11 shows another variation of a chemical sensor based on a multiple v-groove configuration to support multiple sensing points. The chemical sensor  501  consists of multiple v-groove plates  503  and side channels  505  that allow for multiple fiber grating sensors  507  to be loaded as the chemical sensitive coating  509  expands against the plates  511 . This configuration can extend the sensing range and provide better accuracy by comparing the multiple grating sensors  507  to each other. 
     FIG. 12 shows how a beveled plate  551  may be used to increase the surface area of the chemical sensitive coating  553  and increase the wicking action of the target chemical or chemicals into the coating. This could increase sensitivity and decrease response times of the chemical sensor. 
     FIG. 13 a , FIG. 13 b , and FIG. 13 c  show plates of differing permeability  601  and holes  603  or slots  605  can be used to increase the volume and rate of absorption of the target chemical into the chemical sensitive coating. 
     FIG. 14 shows another variation of a chemical sensor  651  based on a flexible plate  653  to transfer the load from the chemical sensitive coating  655  to the fiber grating sensor  657  which can consist of one or more gratings written onto birefringent or non-birefringent fiber. The multiple v-groove plate  659  can hold multiple dummy fibers  661  to provide different loading schemes for the flexible plate  653 . The flexible plate  653  allows for inconsistent swelling of the chemical sensitive coating  655 . 
     FIG. 15 shows another variation of a chemical sensor where the chemical sensitive coating is placed directly on the fiber. The chemical sensor  701  consists of a fiber grating sensor  703  that can consist of a single or multiple gratings on birefringent or non-birefringent fiber. A chemical sensitive coating  705  exerts a transverse force on the fiber grating sensor  703  as it swells in the presence of a target chemical or chemicals. 
     FIG. 16 shows another variation of a chemical sensor where composite tow that is reactive to a target chemical is used to transversely load the fiber grating sensor. The chemical sensor  751  consists of chemically reactive composite tow  753  which expands or shrinks in the presence of the target chemical or chemicals to transfer a load to the fiber grating sensor  755 . The fiber grating sensor  755  can consist of one or more gratings on birefringent or non-birefringent fiber. A void  757  can be used to provide clean transverse loads on the fiber grating sensor  755 . 
     The above descriptions describe a transverse strain applied to the grating sensor on the presence of a target chemical such as moisture. Another application to the transverse strain sensing capability of the fiber grating written onto either ordinary single mode fiber or birefringent fiber is the direct measurement of transverse strain and strain gradients when embedded into a structure such as an adhesive joint. 
     One key problem facing structural designers is the ability to be able to monitor the structural integrity of adhesive joints. While these joints are used in many types of construction there is very strong motivation to use these in aerospace applications to improve strength and reliability while lowering construction costs and overall weight. FIG. 17 is a diagram of a wing structure  2001  that may be made of lightweight composite material. The wing  2001  is made up of sections that may be adhesively bonded and strings of fiber grating sensors  2003 ,  2005  and  2007  can monitor these bonds. 
     FIG. 18 shows an adhesive bond  2051  that joins two parts  2053  and  2055 . When the parts  2053  and  2055  are pulled apart by the forces  2057  and  2059  a shear load is formed along the line  2061 . A multi-axis fiber grating sensor  2063  can be placed along the length of the adhesive bond  2051  with its traverse axes  2065  and  2067  aligned along the shear line  2061  and orthogonal to it so that shear strain induced in the bond may be measured. While the diagram of FIG. 18 shows the fiber grating sensor  2063  place interior to the adhesive joint  2051  other positions are possible. 
     FIG. 19 shows the placement of three fiber grating sensing fibers  2101 ,  2103  and  2105  in the adhesive bond  2107 , between the bonded materials  2109  and  2111 . Note that the fiber grating sensor  2101  is placed well into the adhesive bond  2107  while the fiber grating sensor  2103  is placed near the edge and the fiber grating sensor  2105  is placed in the exterior. When an adhesive bond starts to fail under shear load it usually starts on the edge. So the placement of the fiber grating  2105  just exterior to the adhesive bond  2107  is in the area where failure is likely to first occur. This arrangement is also useful for enabling a system that could be used as a failure warning mechanism for existing adhesive bonds as an exterior bead of adhesive could be added and oriented fibers placed at the edge of a bond to provide a health monitoring system as a retrofit to existing structures or to simplify fabrication of new structures. 
     FIG. 20 a  and FIG. 20 b  are diagrams that are used to illustrate the action of a multi-axis fiber grating sensors that is placed inside of a material that is subject to strains and eventual failure. In particular this would be the case of an adhesive bond that is strained until it fails. In FIG. 20 a  a multi-axis fiber grating sensor  2151  with transverse sensing axes  2153  and  2155  is subject to a uniform loading force  2157  along the axis  2153 . When this type of uniform transverse loading occurs two spectral output peaks  2159  and  2161  occur that are smooth curves whose central wavelengths shift so that the two peaks  2159  and  2161  move apart or together with wavelength difference. FIG. 20 b  illustrates the case of the fiber optic grating sensor  2151  when the transverse loading force  2171  is not uniform. This would be the case for example when an adhesive bond under load starts to break apart along the line of the axis  2153 . In this case the spectral peak  2161  in FIG. 20 b  will split into two wavelength peaks  2173  and  2175 . The spectral separation between the peaks  2173  and  2175  provides a quantitative means to measure the difference in load along the axis  2153  generated by the force  2171  that consists of the load regions  2177  and  2179 . The intensity of the peaks  2173  and  2175  provide a means to determine the lengths of the load regions  2177  and  2179 . In the case of FIG. 20 b  the regions are nearly equal in length resulting in the two peaks being nearly equal in intensity. 
     FIG. 21 b  is a diagram showing experimental results that were obtained by using a multi-axis fiber grating to monitor an adhesive bond. Additional experimental data on joints that were tested using this technology can be found in W. L. Schulz, E. Udd, M. Morrell, J. Seim, I. Perez, A. Trego, “Health Monitoring of an Adhesive Joint using a Multiaxis Fiber Grating Strain Sensor System”, SPIE Proceedings, Vol. 3586, p. 41, 1999. In FIG. 21 a  the multi-axis fiber grating sensor  2201  is oriented at 45 degrees relative to the adhesive bond  2203 , and the plates  2205 ,  2207 ,  2209 , and  2211 . The fiber grating sensor  2201  is placed at the edge of the adhesive bond  2203  so that it can be used to predict the onset of failure during loading. The graph shown in FIG. 21 b  illustrates the spectral reflective output of the multi-axis fiber grating sensor  2201  of FIG. 21 a  as a function of load being applied by an Instron machine to the plates  2205  and  2207  that are being pulled apart. Note that after a certain load level is applied of approximately 1800 pounds the two major spectral peaks start to move apart with continuing increases in load. At about 2400 pounds the major spectral peak  2251  splits into the two peaks  2253  and  2255 . The spectral separation  2257  between these two peaks  2253  and  2255  is approximately 0.2 nm. Since the response of the multi-axis fiber grating sensor in the transverse direction is approximately a factor of 3 lower than in the axial direction a change of 0.2 nm corresponds to a change of about 600 micro-strain. The intensity of the two split peaks  2253  and  2255  being nearly equal means that along the axis of shear strain (along which one of the transverse axes of the multi-axis fiber  2201  is aligned as shown in FIG. 21 a ) about half the fiber grating length has been unloaded by about  600  micro-strain while the other half of the grating remains at the higher load level. Since the fiber grating used in this case is about 5 mm in length this means that approximately 2 mm of the fiber grating sensor  2201  along the shear strain axis has been unloaded due to a change in the adhesive bond  2203 . As the adhesive bond  2203  is subject to increasing load additional peaks arise with greater intensity indicating additional breakage and the overall spectral profile  2257  moves toward longer wavelengths indicating axial loading is occurring. Thus FIG. 21 a  and FIG. 21 b  illustrate the ability of a multi-axis fiber grating sensor  2201  to measure transverse strain which because of its orientation at 45 degrees measures shear strain in the adhesive bond  2203 . This figure also illustrates the ability to measure changing transverse strain gradients indicative of break up of the adhesive bond  2203  and axial strain changes that occur in this example before failure of the bond  2203 . 
     In addition to monitoring break up of adhesive bonds and failure it is possible to use multi-axis fiber grating sensors to monitor plastic deformation of an adhesive bond under cycling. FIG. 22 shows the position of the multi-axis fiber grating sensor  2301  that is oriented at −45 degrees relative to the adhesive bond  2303  and the plates  2305 ,  2307 ,  2309  and  2311 . As the plates  2311  an  2309  and pulled apart with increasing force and then unloaded the multi-axis fiber grating sensor  2301  can be used to monitor the adhesive bond  2303  in the neighborhood of its placement. FIG. 23 is a graph showing the displacement of the major spectral peaks during a cycle of the adhesive bond  2303 . The spectral profile  2351  shows the original spectrum after the multi-axis fiber grating sensor  2301  after placement in the adhesive bond  2303  but before loading. In this particular case after the adhesive bond  2303  was cycled it did not fail but the unloaded spectra after the cycle  2353  reflects a change in the strain fields interior to the adhesive joint  2303 . FIG. 24 illustrates the displacement of the plates  2309  and  2311  by an Instron loading machine during testing. Note that the adhesive joint  2303  has been plastically deformed during this cycle as was expected as the cycle was beyond the load expected to fail the part. The spectral profiles of FIG. 23 illustrate this process. 
     In the above sensor configurations, one possible configuration is to use one or more fiber gratings written onto birefringent fiber. The polarization axes associated with the birefringent fiber require that the fiber grating sensor be placed in a known orientation in order to maximize the sensitivity of the sensor&#39;s response to a transverse load. FIGS. 25 and 26 describe one possible method of controlling the orientation of a fiber grating sensor written onto birefringent fiber. 
     FIG. 25 shows a method to control the orientation of a fiber grating sensor based on birefringent fiber. In this case, a non-symmetric coating  801  is placed over the fiber grating sensor  803 . The orientation of the polarization axes of the fiber grating sensor  805  can be controlled by placing the flats  807  of the coating in the desired orientation. 
     FIG. 26 shows how the non-symmetric coating of FIG. 25 can be manufactured. The process begins with a fiber of known orientation  851  with a round fiber coating  853  that will melt with enough heat placed between two plates  855 . As the plates are heated  857 , the coating  859  will begin to melt and flow and form flats  861  where the coating touches the plates  855 . 
     In many areas where environmental sensing is required, there is a desire for high sensitivity and multiple sensing points. For this reason, a demodulation system with high sensitivity and a large multiplexing potential is needed. In the figures below, several systems are described that enhance the capability of a fiber grating demodulation system using spectral filters described in U.S. Pat. Nos. 5,380,995 and 5,397,891. 
     FIG. 27 shows a prior art fiber grating demodulation system using spectral filters described in U.S. Pat. Nos. 5,380,995 and 5,397,891. The fiber grating demodulation system consists of a broadband light source  3001  that directs broadband light through a beam splitter  3003  and to a fiber grating sensor  3005 . The fiber grating sensor  3005  reflects a spectral peak based on the strain on the grating that travels back through beam splitter  3003  and is then directed to a second beam splitter  3007  where it is split between lines  3009  and  3011 . The spectral peak traveling along line  3009  travels through a fiber grating filter  3013  that converts the spectral information into an amplitude based signal. The spectral peak then travels from the grating filter  3013  to the detector  3015 . The spectral peak in line  3011  travels directly to the high-speed detector  3017  to provide a reference measurement. The detector then outputs two voltages  3019  and  3021  that can be acquired by a data acquisition system  3023 . 
     FIG. 28 a  and FIG. 28 b  show typical spectral profiles from a grating written onto non-birefringent fiber. This is one possibility for the fiber grating filter described in FIG.  25 . In order to adjust the sensitivity of the fiber grating filter, gratings of different widths may be used to control the slope of the spectral profile. If a narrower grating is used as a filter, its spectral profile  3051 , shown in FIG. 28 a , will give more sensitivity due to its steeper slope, but will give less dynamic range for the sensor to sweep across. If a wider grating is used as a filter, its spectral profile  3053  will give a shallower slope, for decreased sensitivity, but a wider dynamic range, shown in FIG. 28 b.    
     FIG. 29 a  and FIG. 29 b  each show a typical response of a fiber grating sensor to an axial load. The grating under no load  3101 , shown in FIG. 29 a , will have a grating spacing  3103  resulting in a spectral peak at a lower center wavelength  3105 . As the fiber grating sensor is axially strained  3107 , the grating spacing  3109  results in a spectral peak at a higher center wavelength  3111 , shown in FIG. 29 b . This shows how the grating sensor will sweep across the grating filter in the system described in FIG.  27 . 
     When fiber grating sensors are installed onto or embedded into structures, many times the initial strain state is different than it was for the uninstalled sensor due to such mechanisms as residual stress. This initial tensile or compressive force results in the fiber grating sensor&#39;s initial spectral peak center to be at a different wavelength than the unstrained sensor. Referring back to the demodulation system of FIG. 27, if the spectral filter does not match up spectrally with the fiber grating sensor, then there will be no measurable change in amplitude as the sensor is modulated. For this reason, a tunable grating filter may be needed to ensure that the spectral filter matches up with the initial state of the installed sensor. The following figures describe methods for straining a fiber grating and thus providing a tunable grating filter. 
     FIG. 30 a  and FIG. 30 b  show] a tunable filter concept where a fiber grating sensor is attached to or embedded into a simply supported  3131  flexing beam  3133  above the neutral axis of the beam. As the beam is bent up  3135 , FIG. 30 a , or down  3137 , FIG. 30 b , the grating on the beam will be subjected to tension or compression allowing for a filter that can be tuned to both higher and lower wavelengths. The beam can also be supported other ways, such as fixed, etc. 
     FIG. 31 a  and FIG. 31 b  show a tunable filter concept utilizing a bending beam with a grating attached onto or embedded into the beam above the neutral axis of the beam. As the beam is bent up  3151 , FIG. 31 a , or down  3153 , FIG. 31 b , the grating on the beam will be subjected to tension or compression. 
     FIG. 32 shows a tunable filter concept utilizing a beam  3171  with a grating  3173  attached onto or embedded into the beam. As the beam is stretched or compressed with a force  3175 , the fiber grating will be subjected to tension or compression and thus can be tuned to higher or lower wavelengths. 
     FIG. 33 shows a tunable filter concept where a fiber grating  3181  is fixed at a point along its length  3183 . A force  3185  pulls on the grating to induce tension and thus a spectral shift to a higher wavelength. The fiber grating  3181  is written at a lower wavelength than is expected for the installed fiber grating sensor. An example of this would be to use a fiber grating filter in this configuration at 1297 nanometers for demodulating a fiber grating sensor with nominal wavelength at 1300 nanometers. This would allow for the tunable filter to match up with the fiber grating sensor by only having to tune it in one direction. 
     FIG. 34 shows an extension to FIG. 33 where the fiber grating  3201  is placed into a tube  3203  and fixed at either end of the tube  3205 ,  3207 . The tube is also fixed  3209 . The length of the tube  3211  can be varied to control the length of the sensor that is being stretched by force F  3213  and thus control the amount of strain on the fiber for a given displacement controlled by a precision screw such as a micrometer or a picomotor such as the one available from New Focus. This configuration could be a tension only type of tunable filter similar to FIG. 33, or the fiber could be pre-strained in the tube to allow for a wavelength shift in both directions if the fiber was allowed to relax. 
     FIG. 35 a  and FIG. 35 b  show a tunable filter concept utilizing a diaphragm  3221  with a fiber grating attached onto or embedded into the diaphragm off of its neutral axis. With a pressure differential on the diaphragm  3223 ,  3225  the diaphragm will deflect up or down and put tension or compression on the fiber grating. 
     FIG. 36 a  and FIG. 36 b  show an extension to the tunable filter concept shown in FIG.  30 . In this case, a threaded stud  3241  is threaded through a tap  3243  in the beam  3245 . As the stud  3241  is turned the beam  3245  is flexed up or down based on the direction of the turn. 
     FIG. 37 shows a picture of a prototype based on the concepts described in FIGS. 30 and 36. Here the tunable grating filter is enclosed in a box with an external knob to turn the threaded stud inside. The optical ports  3247  allow access to both sides of the grating to allow the filter to operate in transmission. 
     FIG. 38 shows a picture of the inside of the filter box of FIG.  37 . Here the beam  3261  with the attached grating can be seen with the stud threaded  3263  through it. 
     FIG. 39 shows an extension of the tunable filter concept where multiple gratings  3281 , 3283  of different wavelengths  3285 , 3287  are attached to or embedded into a beam with tuning provided by bending or a push/pull force. This allows for the potential of a single tunable filter handling multiple fiber grating sensors at different wavelengths. 
     FIG. 40 shows the spectral profile  3301  of a series of tunable gratings. If each spectral peak were tunable independently, then a comb filter could be formed. 
     FIG. 41 shows a concept for the fiber grating comb filter shown in FIG. 40. A series of multiple beams  3303  or other tuning mechanisms each with a fiber grating  3305  of different wavelength attached or embedded could be connected together to form the comb filter. 
     FIG. 42 shows how the fiber grating comb filter could be packaged and tuned. A series of knobs  3321  connected to the beams with gratings at different wavelengths  3323  could be used to tune each individual grating to a higher or lower wavelength to form the desired comb profile. Optical ports  3325  would provide access to both ends of the series of gratings. 
     FIG. 43 shows another concept for a tunable grating filter. As a fiber grating responds similarly to heat as it does to strain due to thermal expansion/contraction, a tunable filter based on heating/cooling the fiber grating is feasible. A heat input  3341  would shift the grating filter  3343  to a higher wavelength. A heat output  3345  or cooling would shift the grating filter to a lower wavelength. 
     In addition to a tunable grating filter to support higher sensitivity and multiplexing of the grating based sensor such as a chemical sensor, additional schemes are described below that further enhance the multiplexing potential of a fiber grating sensor system. 
     FIG. 44 shows a modification of the demodulation system described in FIG. 27 where multiplexing is enabled through the use of time division multiplexing. The demodulation system  3361  consists of a pulsed broadband light source  3363  that directs a spectral pulse  3365  into a beam splitter  3367  and is split into two pulses  3369  and  3371 . The pulse  3369  will arrive at the grating sensor  3373  first and a spectral peak  3375  will be reflected back. The spectral pulse  3371  will reach the grating sensor  3377  later due to a time delay  3379  that could consist of a coil of fiber. The grating sensor  3377  will then reflect a spectral peak  3381 . The spectral peak  3375  will reach the beam splitter  3367  first and be split into two spectral peaks  3383  and  3385 . Spectral peak  3383  will be directed back toward the light source  3363  and will have no effect. Spectral peak  3385  will be directed toward a second beam splitter  3387  that will split it into two spectral peaks  3389  and  3391 . The spectral peak  3381  will reach the beam splitter  3367  after peak  3375  and will be split into two peaks that will follow the same paths as spectral peaks  3383  and  3385 , only they will be delayed by the amount determined in the time delay  3379 . This configuration allows for multiple gratings sensors at the same wavelength to be demodulated by one demodulation system with a single spectral filter  3393 . 
     In some of the demodulation cases described above, only a single spectral peak being reflected from the grating sensor can be demodulated. The following figures describe methods for utilizing this same demodulation system for the case of gratings written onto birefringent fiber where there are multiple peaks per sensor, refer to U.S. Pat. Nos. 5,591,965 and 5,828,059. 
     FIG. 45 shows a typical spectral profile  3401  for a grating written onto birefringent fiber. The profile consists of two peaks  3403  and  3405  associated with the polarization states of the birefringent fiber onto which the grating is written. In order to utilize the above described high speed demodulation system, these polarization peaks  3403  and  3405  can be separated  3407  into two separate peaks  3409  and  3411  that are compatible with the high speed demodulation system. 
     FIG. 46 shows a demodulation system utilizing the concept of FIG. 45 to demodulate a grating written onto birefringent fiber with the demodulation system employing a spectral filter described previously. The broad band light source  3421  directs a broad band spectral profile  3423  into a beam splitter  3425  which splits the broad band profile  3423  into two broadband profiles  3427  and  3429 . The profile  3429  can be dumped (ensuring no back reflections) or directed toward another grating sensor. The profile  3427  is directed toward a fiber grating sensor  3431  written onto birefringent fiber where two spectral peaks  3433  and  3435  associated with the polarization axes of the birefringent fiber will be reflected. These peaks are then directed toward the beam splitter  3425  and directed toward a second beam splitter  3437  and split into legs  3439  and  3441 . The two peaks traveling along leg  3439  are directed into beam splitter  3443  and split into legs  3445  and  3447 . The two peaks in leg  3445  are directed into a polarization controller  3449 . A length of polarizing fiber  3453  is used to ensure that one of the polarization states is blocked. The peak single  3455  is then directed into a spectral filter  3457  and converted into an amplitude based measurement measurable by a detector  3459  as described in FIG.  27 . The leg  3447  provides the reference leg described in FIG.  27 . The leg  3441  directs the two peaks associated with the two polarization states into a beam splitter  3461  that splits into two legs  3463  and  3465 . Leg  3463  directs the two peaks into a polarization controller  3467 . A length of polarizing fiber  3471  is used to ensure that one of the peaks is blocked. The peak  3473  is then directed into a spectral filter  3475  and converted into an amplitude based measurement measurable by a detector  3477  as described in FIG.  27 . The leg  3465  provides the reference leg described in FIG.  27 . To ensure that the polarization controllers and polarizing fibers are blocking the correct polarization peaks, a simple calibration could be performed by loading the fiber grating in transverse and noting whether or not the signals on the respective detectors change as expected. 
     FIG. 47 shows another method to separate the polarization states of the grating written onto birefringent fiber. This method places the polarization controllers before the beam splitter that splits the spectral data between the filtered and reference leg reducing errors associated with inconsistent polarization states in the filtered and referenced legs. A broadband light source  3481  outputs a broadband profile  3483  to a beam splitter  3485  that splits the profile  3483  into two legs  3487  and  3489 . The leg  3489  is dumped or can be connected to another grating sensor. The leg  3487  guides the broadband light to a fiber grating sensor  3491  that consists of a grating written onto birefringent fiber that reflects two spectral peaks  3493  and  3495  each associated with a polarization state of the birefringent fiber. These peaks  3493  and  3495  are then directed to the beam splitter  3485  and directed  3497  into a beam splitter  3499  that splits into legs  3501  and  3503 . The two peaks in leg  3501  are directed into a polarization controller. Polarizing fiber  3509  ensures that one of the polarization states is dropped. The peak  3511  is then directed in to a beam splitter  3513  that splits into two legs  3515  and  3517 . Leg  3515  directs the single peak associated with one of the polarization states of the fiber grating sensor written onto birefringent fiber into a spectral filter  3519  that converts the spectral information into an amplitude based signal measurable by a detector  3521 . The leg  3517  provides the reference leg. The two peaks in leg  3503  are directed into a polarization controller  3523 . A length of polarizing fiber  3527  ensures that one of the polarizing states is dropped. The peak  3529  is then directed into a beam splitter  3531  that splits into two legs  3533  and  3535 . Leg  3533  directs the single peak associated with one of the polarization states of the fiber grating sensor written onto birefringent fiber into a spectral filter  3537  that converts the spectral information into an amplitude based signal measurable by a detector  3539 . The leg  3535  provides the reference leg. 
     FIG. 48 shows an alternative system where polarization maintaining fiber is used throughout most of the system along with polarization maintaining beam splitters so that the two polarization states are each directed to the appropriate demodulator filter set. A broad band light source  3561  directs broad band light  3563  into a polarization maintaining beam splitter  3565  that splits the broadband light  3563  into two parts  3567  and  3569 . Broadband light  3569  is dumped or can be connected to another fiber grating sensor. Broadband light  3567  is directed along the fiber that is placed into a tube  3571  that provides strain relief for the fiber going into a part  3572  to a fiber grating sensor  3573  written onto birefringent fiber that reflects two peaks  3575  and  3577  associated with each polarization state of the birefringent fiber. The peaks  3575  and  3577  are directed to beam splitter  3565  and then directed to polarization maintaining beam splitter  3581  that splits into two legs  3583  and  3585 . The leg  3583  directs both peaks associated with the polarization axes of the fiber grating written onto birefringent fiber to a length of polarizing fiber  3587  that is oriented to block one of the polarization states. The fiber and beam splitters after this length of polarizing fiber  3587  does not need to be polarization maintaining. The resulting single peak from  3587  then travels to a beam splitter  3589  and is split into legs  3591  and  3593 . Leg  3591  directs the single peak to a fiber grating filter  3595  that converts the spectral information into an amplitude based signal measurable by a detector  3597 . The  3593  leg forms the reference leg. The leg  3585  directs both peaks associated with the polarization axes of the fiber rating written onto birefringent fiber to a length of polarizing fiber  3599  that is oriented to block one of the polarization states different from that of  3587 . The fiber and beam splitters after this length of polarizing fiber  3599  does not need to be polarization maintaining. The resulting single peak from  3599  then travels to a beam splitter  3601  and is split into legs  3603  and  3605 . Leg  3603  directs the single peak to a fiber grating filter  3607  that converts the spectral information into an amplitude based signal measurable by a detector  3609 . The  3605  leg forms the reference leg. 
     FIG. 49 shows a method to add multiplexing capability to the system shown in FIG. 48 by employing two broadband light sources and two gratings written at different wavelengths. In this case, two broadband light sources  3621  and  3623  of different central wavelengths are combined using a wavelength division multiplexer  3625 . The resulting two broadband profiles are directed into leg  3627  and to a beam splitter  3629  that splits into two legs  3631  and  3630 . Leg  3630  is dumped or could be connected to a fiber grating sensor. Leg  3631  directs the two broadband profiles to a grating sensor  3635  written onto birefringent fiber and reflecting two peaks  3637  and  3639  each associated with the polarization axes of the birefringent fiber. The throughput of the grating sensor  3635  is directed to another grating sensor  3641  written onto birefringent fiber at a different wavelength than grating sensor  3635  and reflecting two peaks  3643  and  3645  each associated with the polarization axes of the birefringent fiber. The resulting four peaks  3637 ,  3639 ,  3643 , and  3645  are then directed to a beam splitter  3629  and directed to a wavelength division multiplexer (providing lower loss)or a beamsplitter  3647  that divides the four peaks into two pairs associated with the center wavelengths of the broadband light sources  3621  and  3623 . One pair of peaks travels along leg  3649  into a demodulation system  3653  similar to that described in FIG.  48 . The other pair of peaks travels along leg  3651  into a demodulation system  3655  similar to that described in FIG.  48 . The approach of FIG. 49 could be extended to large numbers of sensors by using the wavelength division multiplexing element  3647  to divide the spectrum into discrete packets for each fiber grating sensor, demodulation subsystem combination. 
     In order to multiplex a large number of fiber grating sensors using wavelength division multiplexing while retaining high speed characteristics and sensitivity it would be highly desirable to have the lowest possible loss system available. 
     FIG. 50 shows a system that may be used to multiplex fiber optic gratings at high speed using low cost 2 by 2 fiber couplers. There are different means to operate the system shown in FIG.  50 . As an example the light source  3801  could be a broadband light source such as a light emitting, superradiant laser diode or doped fiber light source (erbium doped light sources being currently most common), which could be used to illuminate a series of fiber grating sensors spaced in wavelength simultaneously. The light source  3801  could also be a tunable light source such as a tunable laser diode that could be used to spectrally scan the string of fiber grating sensors. Returning to FIG. 50, the light source  3801  emits a beam of light that is coupled into one end of the fiber coupler  3805  (bulk optic components or integrated optic beamsplitters could be used, currently the losses associated with these devices are higher and they are not as cost effective). The light beam  3803  is then split by the beamsplitter  3805  into a light beam  3807  that exits the system in FIG. 50 but it could also be used to illuminate another set of fiber grating sensors on a second fiber line. The second split portion of the light beam  3803  is the light beam  3809  that is directed toward the fiber grating sensor  3811  centered about the wavelength λ 1 . A portion  3819  of the light beam  3809  is reflected by the fiber grating sensor  3811 . The spectral change of the light beam  3819  is indicative of the environmental state of the fiber grating. The light beam  3819  then traverses the fiber beamsplitter  3805  a second time and a portion of it is directed to the beamsplitter  3823  where it is split again by the beamsplitters  3825  and  3827  eventually resulting in the light beam  3821  hitting the beamsplitter  3829 . The light beam  3809  then proceeds past  3811  to the fiber grating sensor  3813  that is centered about the wavelength λ 2 . A portion  3831  of the light beam  3809  is reflected off the fiber grating sensor  3813  and is split by the beamsplitters  3805 ,  3823 ,  3825  and  3827  to form the light beam  3835  that is directed toward the beamsplitter  3829 . In a similar manner portions of the light beam  3809  are reflected from the fiber grating sensors  3815  centered about λ 3  and  3817  centered about λ 8 . The net result is that at the beamsplitter  3829  there is a light beam consisting of reflections off the series of fiber grating sensors  3811 ,  3813 ,  3815  and  3817  divided by the action of the beamsplitters  3805 ,  3823 ,  3825  and  3827 . A similar light beam  3837  falls onto the beamsplitter  3839 . Analogous combination light beams  3841  and  3843  fall onto the beamsplitters  3845  and  3847  respectively. 
     When the light beam  3849  corresponding to reflections off all the fiber grating sensors  3811 ,  3813 ,  3815  . . .  3817  falls onto the beamsplitter  3829  it splits into the light beams  3851  and  3853 . The light beam  3851  falls onto the output detector  3855  whose output signal acts as reference. The light beam  3853  passes through the fiber grating filter  3857  that acts to modulate the spectral signal reflected from the fiber grating sensor  3811 . The light beam  3859  passing through the fiber grating filter  3857  then falls onto the output signal detector  3861 . Note that the output signal from detector  3861  contains a constant component associated with the reflections off all the other fiber grating sensors in the system in addition to that of  3811 . The result is an offset for the output signal that becomes increasingly large with additional fiber grating sensors. Similar considerations apply to the beamsplitter, fiber grating filter detector sets  3863 ,  3865 ,  3867 ,  3869 ,  3871 ,  3873  and  3875 . 
     Another approach to the fiber grating sensor system shown in FIG. 50 is to have the light source  3801  be a tunable laser. In this case each fiber grating sensor  3811 ,  3813 ,  3815 , . . .  3817  is illuminated in sequence. The only variation in intensity as the light source is swept corresponds to the filter/detector pair corresponding to the illuminated grating. As an example when fiber grating sensor  3811  is swept the reflected light beam from  3811  is directed through the series of beamsplitters  3805 ,  3823 ,  3825 ,  3827  and  3829  to the fiber grating filter  3857  which in turn modulates the swept signal and by comparing the output of  3861  to  3855  the wavelength may be determined. Similarly the output of the fiber sensor grating  3813  can be read out by the optics/detector set  3857 , fiber sensor grating  3815  by the optics/detector set  3865  and  3817  by the optics/detector set  3875 . While one fiber sensor grating is being readout by the tunable laser  3801  the other optics/detector sets have a fixed ratio. 
     FIG. 50 illustrates the case where two by two couplers are used. As shown in FIG. 51 it is also possible to use 1 by n couplers to achieve similar results. In this case the same light source  3801  is used to illuminate the sequence of fiber grating sensors  3811 ,  3813 ,  3815  and  3817 . The reflected light beams from these fiber grating sensors are then directed to the 1 by n beamsplitter  3901  into n light beams each of which is directed through a fiber grating filter and onto the output detectors corresponding to each fiber grating sensor. In the simplest case the spectral signal would be modulated directly and not referenced. Reference detectors such as  3903  could be added with reference beamsplitters such as  3905  to compensate for system level fluctuations. An alternative configuration would be to place a reference detector  3909  at one of the output legs of the two by two beamsplitter  3907 . 
     FIG. 52 shows a configuration of a multiplexed fiber grating sensor system similar to that shown in FIG. 50 where instead of the output signal detectors monitoring the optical beams passing through the filters the light is reflected. This configuration eliminates cross talk between the fiber gratings. As an example the reflection from the fiber grating sensor  3811  is modulated only by the fiber grating filter  3955  which is designed to modulate light only about the center frequency of the fiber grating sensor  3811 . The modulated light is then reflected to the output detector  3951 . In a similar manner the fiber grating filter  3957  acts only to modulate the reflected light from the fiber grating sensor  3813  and in turn directs its modulated output light signal to the detector  3953 . The configuration in FIG. 49 could be modified to replace the two by two couplers with a 1 by n coupler in direct analogy to FIG.  51 . 
     FIG. 53 illustrates a system comprised of fiber gratings in a single fiber line with a series of fiber beamsplitters. This system can be operated in a number of different ways. In the first case consider the light source  4001  to be a broadband light source that might be a light emitting diode or a superradiant diode. The light source  4001  couples the light beam  4003  into the beamsplitter  4005 . A portion of the light beam  4007  is directed through a series of fiber gratings  4011 ,  4013 ,  4015  . . .  4017  in the optical fiber line  4119 . Another portion of the beam  4003  that is split by the beamsplitter  4005  is split off into the light beam  4009  that exits the system in FIG. 53 but alternatively could be used to support another line of fiber gratings. The reflected spectra from the fiber gratings  4011 ,  4013 ,  4015  . . .  4017  return to the beamsplitter  4005  and a portion of these spectra are directed along the output fiber  4021  as the light beam  4023 . The light beam  4023  passes to the first fiber beamsplitter  4025  and a portion of it  4027  is split off to the reference detector  4029  along the fiber  4031 . The signal from the detector  4029  is used to monitor the overall light level of the light source and components up to this point in the system. The second portion of the beam  4023 ,  4033  is directed to the fiber grating filter  4035  that has a wavelength designed to match that of fiber grating sensor  4011 . The reflected spectra from the fiber grating filter  4035  is then directed back to the beamsplitter  4025  and onto the detector  4036 . In a similar manner reflections from the fiber grating filters  4037 ,  4039  and  4041  are directed to the detectors  4043 ,  4045  and  4047 . Note that the first detector  4036  response includes signals that include reflections from all the filters  4035 ,  4037 ,  4039  and  4041 . These reflections are reduced in intensity through the action of the beamsplitters  4025 ,  4051 ,  4053  and  4055 . Since there are n signals from the n fiber grating spectra reflected by the filters  4035 ,  4037 ,  4039  and  4041  that are directed to the output detectors  4036 ,  4043 ,  4045  and  4047  a system of equations is established that can be used to separate the signals for each individual sensor  4011 ,  4013 ,  4015  and  4017 . The reference detector  4029  can be used to establish a baseline to compensate for light source  4001  and system level fluctuations before the string of fiber grating filters  4035 ,  4037 ,  4039  and  4041 . 
     A second means to operate the system of FIG. 53 is to have the light source  4001  be tunable over the range of the fiber grating sensors  4011 ,  4013 ,  4015  and  4017 . In this case as the light source is tuned over fiber grating  4011  a reflection off this grating reflects off the filter  4035 . A portion of the reflected signal is directed to the output detector  4029  that can be referenced against the output monitoring detector  4037 . In a similar manner fiber grating sensor  4013  can be monitored via fiber grating filter  4037  using the output detector  4043 . Fiber grating sensor  4015  can be monitored via fiber grating filter  4039  and output detector  4045 . Fiber grating sensor  4017  can be monitored via fiber grating filter  4041  and output detector  4047 . Since only one fiber grating is illuminated at a time the signals on the output detectors  4036 ,  4043 ,  4045  and  4047  are not mixed and it is not necessary to solve a series of equations. The limitations of this approach rather than the first one described in association with FIG. 53 involve the speed with which the light source may be tuned limiting the overall response of the system and the cost of the tunable light source relative to a broadband one such as a light emitting diode. 
     FIG. 54 is similar to FIG. 53 with the addition of the reference detectors  4061 ,  4063  and  4065  to aid in eliminating errors due to component induced intensity fluctuations in the system. 
     FIG. 55 shows a system that also is a single fiber output configuration. In this case the light source  4301  may be a broadband light source or a tunable laser diode. When the light source is a broadband light source that illuminates a series of fiber grating sensors  4303  . . .  4305  simultaneously, the light reflected off the fiber gratings  4303  and  4305  is split by the coupler  4307  into the light beam  4309 . A tap coupler  4311  is used to couple a small amount of light to the reference detector  4313  that monitors system level light fluctuations. A combination fiber grating filter/beamsplitter  4315  is used to modulate light reflected from the fiber grating sensor  4303  onto the output detector  4317 . A combination fiber grating filter/beamsplitter  4319  is used modulate light reflected from the fiber grating sensor  4305  onto the output detector  4321 . By taking the ratio of the outputs of detectors  4317  and  4313  the spectral fluctuations of fiber grating sensor  4303 , which is centered about λ 1 , can be tracked and environmental changes measured. Similarly by taking the ratio of the outputs of detectors  4321  and  4313  the spectral fluctuations of the fiber grating sensor  4305  which is centered about λ n  can be tracked and environmental changes measured. 
     Many changes, modifications, alterations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.