Fiber with multiple overlapping gratings
The invention of application Ser. No. 07/963,687 was conceived under DARPA Contract MDA 972-90-C-0026. The Government of the United States of America has a paid up, royalty free license to practice that invention and have the invention practiced for U.S. Government purposes. 
    
    
     This is divisional application of application Ser. No. 08/056,444 filed on May 3, 1993 (now U.S. Pat. No. 5,397,891) which is a continuation-in-part of application Ser. No. 07/963,687 filed on Oct. 20, 1992, now U.S. Pat. No. 5,380,995. 
    
    
     BACKGROUND OF THE INVENTION 
     There is a need for highly accurate and responsive environmental sensor systems that are low in weight, non-obtrusive, economical to manufacture and able to withstand harsh environments. Typical applications in need of such sensor systems are smart skins for aircraft and smart supporting structures for space stations. Such structures must be able to provide continuous indications of their physical state in real time, be extremely hardy and long lasting, be able to detect small changes over a large area of structure and have a large dynamic range, without interfering with other functions within the structure. 
     Optical pass band filters such as intra-core fiber gratings, Fabry-Perot etalon based filters, and acousto-optic filters have been proposed for fiber optic sensors. Fiber Fabry-Perot etalon based filters are available from Micron Optics and tunable acousto-optic filters are available from New Focus, Inc. under license from Bell Communication Research. 
     The fiber Fabry-Perot etalon based filters consist of two mirrored surfaces that can be controllably separated by piezoelectric drivers. Light is transmitted most fully when there is an integral number of wavelengths between the two mirrors and most strongly reflected when there is an integral number of waves plus half a wave of separation. Fiber gratings are constructed by doping an optical fiber with material such as germania. The side of the fiber is then exposed to an interference pattern of radiation to produce multiple variations in the refractive index within the fiber that are very closely and accurately spaced. By adjusting the fringe spacing of the interference pattern, the periodic index of refraction produced can be varied as desired. 
     The reflecting center wavelength of the spectral envelope of a fiber grating changes linearly with temperature and strain. For a temperature change ΔT and a strain ε, the fractional Bragg wavelength shift is given by: ##EQU1## where ∝ is the thermal expansion coefficient of the fiber, ξ represents the thermal optic coefficient or dn/dT of the doped silica core material and p e  is the photo elastic constant. For temperature, the index change is the predominant effect, being about fifteen times greater than the expansion coefficient. As reported by W. W. Morey, Distributed Fiber Grating Sensors, Proceedings Of The Seventh Optical Fiber Sensors Conference, p. 285-288, Sydney, Australia, December, 1990, temperature responses of fiber gratings vary with fiber type, but they have found to have been linear up to 500° C. Typical temperature responses are 0.0043 nm./°C. at 833 nm. for Andrew PM fiber and 0.0074 nm./°C. for Corning FlexCore Fiber at 824 nm. When the fiber grating is strained, the Bragg wavelength changes to photoelastically induce a change in the refractive index. For silica, the photoelastic constant is 0.22. Bragg wavelength changes under tension have been measured to 45 kpsi stress, giving a 2.3 nm. shift for a slope of 5.2×10 -4  nm. per microstrain at 820 nm. The fiber gratings can be used both as multiplexed distributed grating sensors, and time and frequency division multiplexed sensors, as described in W. W. Morey. However, there has been a need to integrate such fiber gratings or other optical band pass filters into practical and economical sensor systems that can be manufactured using available components. 
     BRIEF DESCRIPTION OF THE PRESENT INVENTION 
     In the present invention, sensor systems are constructed using one or more fiber gratings as remote sensors of environmental effects, primarily strain or temperature, so that the sense, magnitude and position of an environmentally induced change may be measured. Such systems may be used to support health monitoring and structural assessment systems for aircraft, spacecraft, naval vessels, buildings, bridges and other structures. They can be used to augment process control for chemical and pharmaceutical plants. They can also be used to support other fiber optic sensor systems such as those needing a very accurate spectrometer for light source stabilization. 
     Each of the sensor systems disclosed herein uses light from a relatively broadband light source, a remote fiber grating as the sensor, a local optical band pass filter and a light intensity detector. The spectral envelope of the remote fiber grating is shifted by variations in the environmental effect while the spectral envelope of the local optical band pass filter is controlled. The grating and the filter are optically connected so that their instantaneous spectral envelopes are compared so variations in the output of the comparison indicate the environmental effect. The light from the light source can be applied to either the grating or the filter to establish a first spectral envelope for comparison. The comparison is accomplished by conducting the first spectral envelope to the other optical device. Although the light can be conducted to the local grating first, for this discussion, the light is conducted first to the remote grating where its instantaneous characteristic spectral signature is used to reflect either a small bandwidth of light or to transmit all but the small bandwidth of light. The light thereafter has a spectral envelope characteristic of the environmental condition of the remote grating. When the remote grating is stretched or compressed, or heated or cooled, the center frequency of the spectral envelope shifts in wavelength. In the simplest embodiment, the spectral envelope is then compared to the spectral envelope of one or more local optical filters, chosen or controlled so that there is always some overlap with the spectral envelope of the remote grating. The output of this comparison varies with the change in the environmental effect. 
     In some embodiments, the local optical filter is modulated, at frequencies much higher than the frequencies of the environmental effect of interest to reduce system noise in the same frequency region as the environmental effects. The local optical filter also can have a signal representative of the environmental effect fed back thereto or to the light source to close the sensor loop. In this latter case, the signal needed to null the modulation is representative of the environmental effect. By driving the two mirrors of a fiber Fabry-Perot etalon based filter piezoelectrically, the transmission function can be modulated in analogy to piezoelectrically stretching a fiber grating. By sinusoidally modulating the fiber etalon, the signal from the remote fiber grating is modulated, producing odd and even harmonics that can be minimized or maximized to operate the system closed loop by readjusting the separation between the fiber mirrors. A similar effect is possible by substituting a acousto-optic filter. Such filters cause polarization conversion of selected wavelengths by means of an acousto-optic wave directed along the length of an integrated optic waveguide. 
     The present systems can simultaneously and continuously monitor each of many individual sensors in a fiber optic grating sensor serial string, while providing the ability to very accurately measure both the sense and magnitude of the environmental effect on each sensor. Although in the basic systems, open loop sensing is employed, closed loop methods can be used to desensitize the system to variations in loss, such as commonly are due to connectors and other similar loss mechanisms. Since the fiber grating sensors for remote placement are constructed of optical fibers having about the diameter of a human hair, they can be embedded in composite structures or attached to the surface of metal and other types of structures without noticeable intrusion, reduction in strength or increase in weight and volume. 
     A normal fiber grating is sensitive to both temperature and strain. In an environment where both the temperature and the strain is changing, some means need to be employed to separate the effect of temperature from the effect strain. Usually it is possible to isolate strain in a separate sensor. Therefore, the separate sensor can determine temperature variations, which can then be mathematically eliminated from the output of the strain and temperature sensitive sensor. However, it is possible to provide a modified fiber grating sensor where two gratings are superimposed in the same length of fiber. This &#34;double&#34; grating is then exposed to light at different frequencies and the resultant two outputs used to determine strain and temperature variations. This &#34;double&#34; grating method has the advantage that the same temperature and strain always exists on both superimposed gratings, whereas there is always a chance that in the two grating method, the gratings are at different temperatures or are being subjected to slightly different strain, even though they are very close to each other. 
     Therefore, it is an object of the present invention to provide sensor systems for smart structures using optical fiber gratings as the sensor elements. 
     Another object is to provide optical fiber grating sensors that can be distributed through a structure, to provide phase, amplitude and position data of environmental effects in that structure. 
     Another object is to provide either an open loop or closed loop sensor system that uses one or more optical fiber gratings as the sensing elements. 
     Another object is to provide a sensor system for environmental effects, which can operate over a temperature range of more than 700° C. in extremely harsh environments. 
     Another object is to provide an optical sensor system with a large dynamic range. 
     Another object is to provide a fiber sensor that can separate the effects of strain and temperature. 
     Another object is to provide a fiber with multiple gratings in the same location there along. 
    
    
     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 and the accompanying sheets of drawing wherein: 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of an apparatus to determine the characteristic transmission of an optical fiber grating; 
     FIG. 2 is a graph of a typical output of the apparatus of FIG. 1; 
     FIG. 3 is a circuit diagram of another apparatus for determining the spectral envelope characteristics of a optical fiber grating; 
     FIG. 4 is a graph of a typical reflection spectral envelope as served by the apparatus of FIG. 3; 
     FIG. 5 is an optical circuit diagram of a sensor system constructed according to the present invention showing reflection-transmission and reflection -reflection embodiment; 
     FIG. 6A is a graph of the spectral envelopes of the optical fiber gratings of the reflection-transmission portion of FIG. 5; 
     FIG. 6B is a graph of the spectral envelopes of the optical fiber gratings of the reflection-reflection portion of FIG. 5; 
     FIG. 7 is an optical circuit diagram of a modified optical fiber grating sensor system similar to the reflection-reflection portion of FIG. 5 utilizing two local gratings for increased dynamic range and to eliminate ambiguities; 
     FIG. 8A is a graph of the spectral envelopes of the gratings of FIG. 7; 
     FIG. 8B is a graph if the spectral envelopes that the sensor of FIG. 7 would produce if it was configured as a reflection-transmission device with less frequency sensitive gratings; 
     FIG. 9 is a diagrammatic perspective view of a fiber grating modulator; 
     FIG. 10 is an optical circuit diagram for a modified reflection-reflection sensor system utilizing the optical fiber grating modulator of FIG. 9; 
     FIG. 11 is a graph of the spectral envelopes of the two fiber gratings of FIG. 10 showing the movement thereof; 
     FIG. 12 is a graph of the spectral envelopes of the gratings of FIG. 10 when they are in one-to-one correspondence; 
     FIG. 13 is an optical circuit diagram of a reflection-transmission sensor system similar utilizing the optical fiber grating modulator of FIG. 9 and employing feedback to its light source; 
     FIG. 14 is an optical circuit diagram of a modified sensor system employing feedback to the local fiber grating modulation modulator thereof; 
     FIG. 15 is an optical circuit diagram of a reflection--reflection sensor system having multiple remote optical fiber gratings which respond at different light frequencies in their quiescent condition; 
     FIG. 16 is an optical circuit diagram of a closed loop multiple remote sensor system employing fiber grating modulators in both their reflective and band pass modes; 
     FIG. 17 is a diagrammatic view of a multiple sensor system employing a switch to change between multiple strings of optical fiber sensors; 
     FIG. 18 is a idealized cross-sectional view of a fiber grating modulator constructed to have a wide dynamic range; 
     FIGS. 19A and 19B are graphs of intensity versus wavelength for optical fiber gratings having differing bandwidths; 
     FIGS. 20A and 20B are greatly enlarged side views of an optical fiber during the formation of a double grating; 
     FIG. 21 is an optical circuit diagram of a closed loop remote sensor system employing double fiber gratings to simultaneously sense strain and temperature; 
     FIG. 22 is a sensor system similar to that shown in FIG. 5 with the local grating reflecting a spectral envelope to a reflecting remote grating; 
     FIG. 23 is a sensor system where light is transmitted through a local grating for reflection off a remote grating; 
     FIG. 24 is a sensor system similar to that of FIG. 14 where light is reflected off a local fiber grating modulator and then reflected off a remote fiber grating; 
     FIG. 25 is a sensor system similar to that of FIG. 15 where light is reflected off remote fiber gratings and then reflected off local fiber grating modulators; 
     FIG. 26 is a differential sensor system where two gratings are exposed to environmental effects at different locations and the difference in environmental effects are sensed; 
     FIG. 27 is a sensor system similar to that of FIG. 10 where light is reflected off a remote fiber grating subjected to large environmental effects and then reflected off local fiber grating modulators which cover the large spectral shift of the remote fiber grating; 
     FIG. 28 is a graph of the spectral envelopes of the gratings of FIG. 27 during an instant of operation; 
     FIG. 29 is a sensor system similar to that of FIG. 27 employing integrators to close the sensor loop; 
     FIG. 30 is a sensor system using different remote gratings and a plurality of local modulators operating at different frequencies to sense environmental effects at different places with a minimum of components; 
     FIG. 31 is a diagrammatic depection of fiber Fabry-Perot optical filter illustrating its typical response to a broad band light input; 
     FIG. 32 is a diagrammatic depection of fiber Fabry-Perot etalon optical filter illustrating its typical response to a broad band light input; 
     FIG. 33 is a graph of transmission intensity (I) of a Fabry-Perot etalon as the input wavelength (λ) is changed; 
     FIG. 34 is an optical circuit diagram showing a Fabry-Perot optical filter being used as a demodulator in a fiber grating closed loop sensor system; 
     FIG. 35 is an optical circuit diagram showing Fabry-Perot optical filters used as demodulators in a multiple fiber grating open loop sensor system; and 
     FIG. 36 is an optical circuit diagram like that of FIG. 35 showing acousto-optic optical filters used as demodulators in a multiple fiber grating open loop sensor system; and 
     FIG. 37 is an optical circuit diagram of a three axis sensor system using a single fiber grating sensor having four gratings thereat and closed loop demodulation. 
    
    
     DETAILED DESCRIPTION OF THE SHOWN EMBODIMENTS 
     Referring to the drawings more particularly by reference numbers, number 20 in FIG. 1 refers to a system to measure the spectral transmission properties of a fiber grating 23. The system 20 includes a tunable light source 24, such as a dye laser whose output light frequency can be shifted to scan the spectral response of the fiber grating 23. The fiber grating 23 contains an internal grating pattern whose spacing produces a wavelength selective reflection while allowing other frequencies of light to pass there through unimpeded, to fall on a detector 26. The detector 26 produces electrical output 27 in response to the intensity of the light thereat which is preserved by a recorder 28. The frequency response of the fiber grating 23 is indicated by an absence of light at the detector 26. A graph of typical intensity versus wavelength for the system 22 is shown in FIG. 2. It should be noted that the response of the fiber grating 23 to varying wavelengths can be relatively narrow in bandwidth. 
     In FIG. 3, a system 30 is shown for measuring the reflective spectral characteristics of the fiber grating 23. In system 30, light from the tunable light source 24 is conducted by an optical fiber 32 to a fiber beamsplitter 34, where it is split into two beams. One of the beams, 35, travels on arm 36 of the splitter 34 and is absorbed at the end 38 thereof, which is terminated to avoid back reflections. Back reflections of beam 35 would degrade the accuracy of the measurements made by the system 30. A suitable termination includes crushing the end 38 and enclosing it in index matching cement 39 to form termination 40, as shown in U.S. Pat. No. 4,834,893. 
     The other beam 41 from the beamsplitter 34 is conducted to the fiber grating 23 on arm 42. Most wavelengths pass through the grating 23 and are absorbed at a second termination 44. The narrow band of wavelengths in the return beam 45 that is reflected by the fiber grating 23 when the tunable light source 24 is producing the correct frequencies, is reflected back along fiber 42, pass through the beamsplitter 34 and are conducted to the detector 26 by means of fiber arm 46. The detector 26 converts the intensity of beam 45 into electrical signals and provides them as the output 27 to the recorder 28, where they are stored. A typical spectral envelope of the reflection of the fiber grating 23 is shown in FIG. 4. The shapes of the spectral envelopes shown in FIGS. 2 and 4 are characteristic of the number of grating lines and the amount of periodic index of refraction forming them while the spacing of the grating lines within the fiber grating 23 determines the center wavelength thereof. If the fiber grating 23 is stretched or compressed, the spacing between the grating lines changes and the spectral envelope moves down or up in wavelength, the fiber grating 23, as well as the other fiber gratings described herein, forming a tunable optical filter that reflectively separates a narrow frequency band (spectral envelope) from light otherwise passing there through. In the present invention, this phenomenon is used for environmental sensing. 
     In many applications, such as fiber optic smart structures, it is desirable to have a sensor embedded in a part and still be able to demodulate the output thereof. The sensor system 50, shown in FIG. 5, is a modification of system 30, wherein the fiber grating 23 is used as a remote sensor. In system 50, a second fiber grating 52 is positioned locally between the splitter 34 and the detector 26. Light is generated by a broadband light source 54, having a bandwidth at least as wide spectrally as the spectral envelopes of the remote and local fiber gratings 23 and 52, and the spectral envelope coverage by excursions thereof due to environmental effects, shown by arrow 55. A light emitting diode is typical of those devices that can be used for light source 54. When the portion of the light output of the light source 54, characteristic of the instantaneous spacing of the grating lines of the fiber grating 23, reflects off the fiber grating 23, it passes back through the beamsplitter 34 to the grating 52. If the spacings of the gratings 23 and 52 are identical, then the amount of light reflected by grating 23 falling on the detector 26 is minimized. However, as shown in FIG. 6A, if the spectral envelope 58 of the grating 52 does not exactly coincide with the instantaneous spectral envelope 60 of the grating 23, then the intensity of the light that falls on the detector 26, shown by crosshatched area 62, is increased. If environmental effects 55 are causing the spacing of the lines of the grating 23 to vary, then those effects will cause the spectral envelope 60 to move relative to the envelope 58, shown by arrow 63, consistent with the environmental effect 55 at the grating 23, which is turn causes the output 64 of the detector 26 to vary. 
     The output 64 of the detector 26 is not linear with variations of the environmental effect. One way to sense a more representative output is to operate both gratings 22 and 52 in their reflective modes. This can be done by placing a beamsplitter 65 in the fiber arm 46 as shown so that light 66 reflected by grating 52 falls on detector 67. The intensity of the light 66, as shown in FIG. 6B, is in proportion to the overlap 68 between the instantaneous spectral envelope 60 of grating 23 and the spectral envelope 69 of grating 52. 
     One disadvantage of either way of configuring the sensor system 50 is that ambiguities can arise where it is difficult to resolve whether the output 64 of the detector 26, or the output 68 of detector 67, represents a positioning of the spectral envelope 60 at a higher or lower center wavelength than the spectral envelope 58 or 69. One way to avoid this ambiguity is to design the two gratings 23 and 52 so that their spacing is different and the expected range of environmental effects cannot move the two envelopes 58 or 69, and 60 beyond exact wavelength correspondence, nor move them far enough away that the overlap 62 or 68 no longer exists and the output 64 or 70 goes to zero. It is desirable to operate the system 50 with the envelopes 58 or 69, and 60 substantially overlapped, because its sensitivity is greatly reduced as the detector output 64 or 70 approaches zero. 
     Another way to handle ambiguity is with the system 80 shown in FIG. 7. The system 80 has a second beamsplitter 82 connected to the fiber arm 46 so that light reflected from the grating 23 is split thereby, a portion 83 going by way of fiber 84 to reflect off the grating 52, passing through beamsplitter 65 to the detector 67, and the other portion 85 going by way of fiber 86 to reflect off a second fiber grating 88, whose spacing is chosen so that its spectral envelope 90, as shown in FIG. 8, does not appreciably overlap the spectral envelope 58 of grating 52 nor is the spacing there between wider than the spectral envelope 60. The quiescent spacing of the lines of grating 23 is chosen so that its spectral envelope 60 generally sits between the two spectral envelopes 58 and 90. In such a case, the light 91 passing through the beamsplitter 92 and falling on a second detector 92, produces a second output 94, representative of the light intensity of the area 96, common to the spectral envelopes 60 and 90 in FIG. 8A. Therefore, movement 63 of the spectral envelope 60 due to stressing of the fiber grating 23, can be non-ambiguously determined from a center wavelength below the envelope 58, through a center wavelength between the envelopes 58 and 90 to a center wavelength above the envelope 90 by comparing the two outputs 64 and 94. As has been previously the case with the other systems 30 and 50, back reflections from the unused leg 98 of the beamsplitter 82 are prevented by a suitable termination 100. 
     FIG. 8B illustrates the overlap in waveforms when the system 80 is configured for transmission through the local fiber gratings 52 and 88 and the grating 23, 52 and 88 have wider frequency spectral envelopes. The spectral envelope 102 of grating 52 and the spectral envelope 104 of grating 88 are shown with their maximum spacing where they are spaced so that the outputs on the detectors 67 and 93 can go up to maximum simultaneously only at a single position of the reflection spectral envelope 60 between them. As shown the reduced area 106 crosshatched with solid lines represents the output to detector 67 while detector 93 receives the maximum output 108, shown in dashed crosshatch, where envelopes 104 and 60 are completely overlapped. 
     The systems 30, 50, and 80 rely on DC or very low frequency measurements. To avoid noise problems, it is preferable to translate the output signals 70 and 94 to a higher frequency regime where noise can be distinguished. One way to increase the frequency is to include a fiber spectral modulator 110 (FIG. 9) or other similar device that oscillates the spectral envelope of the sensor grating 23 for demodulation purposes. The modulator 110 includes a piezoelectric mandrel 112. When oscillating voltage is applied to the mandrel 112 from a signal generator 114, it expands and contracts, changing the circumference of its outer diameter 116. By wrapping a fiber grating 118 around the mandrel 112, the expansion and contraction of the mandrel 112 modulates the spectral envelope of the fiber grating 118. The construction of piezoelectric cylinders suitable for mandrel 112 is shown in Davies et al, U.S. Pat. No. 4,002,377. 
     System 130 of FIG. 10 shows the modulator 110 substituted for the fiber grating 52 of system 50 to operate in an open loop configuration. In system 130, the light 131 that is reflected back by the remote fiber grating sensor 23 is reflected back by the modulator 110 to the detector 26, only when the spectral envelopes of the sensor 23 and the modulator 110 overlap. Since the modulator 110 is driven by the signal generator 114 at the frequency ω, to shift the center wavelength of its spectral envelope 132 more than any expected environmental effect (FIG. 11), the spectral envelope 60 of the light 131 reflected from the remote fiber grating sensor 23 is usually offset from the spectral envelope 132 of the local fiber grating modulator 110 and moving with respect thereto. Since the fiber grating modulator spectral envelope 132 is modulated with respect to the envelope 60, the overlap region. 134 is also modulated, and the net result is an amplitude modulated reflected light signal that falls on the detector 26 and is converted into an electrical signal on line 136 thereby. In particular, when the two envelopes 60 and 132 are off-set in center wavelength, the resulting signal contains the first and odd harmonics of the modulating frequency ω. The amplitude of the first harmonic signal depends upon the degree of wavelength off-set. The phase of the first harmonic signal changes by 180°, dependent upon which side of the reflected signal spectrum envelope 60, the modulated spectral envelope 132 is off-set. The first harmonic signal goes to zero when the two envelopes 60 and 132 overlap, as shown in FIG. 12, and instead, second and higher order even harmonics of ω predominate. For optimal open-loop operation, the remote fiber grating 23 and the fiber grating 118 in the modulator 110 are nominally constructed to be identical so that when environmental effects 55 are not present, the first harmonic signal from the remote fiber grating 23 is zero. As the remote fiber grating 23 is stretched or compressed, the resultant direction is determinable by the phase of the output first harmonic signal and the degree of compression or stretch determined by the amplitude of the signal on line 136. This can be determined by monitoring the output of the detector 26 with a synchronous demodulator 138, to which is also fed a signal of frequency ω from the signal generator 114. The signal at the output 140 of the synchronous demodulator 138 is the open loop output of the system 130 indicative of the environmental changes occurring at the fiber grating 23. 
     Open loop operation of the fiber grating sensor 23 has limited dynamic range and linearity and is affected by variations in the attenuation of the various components throughout the systems 50, 80 or 130. The system 150 of FIG. 13 includes closed loop demodulation to circumvent the disadvantages of open loop operation by adding an integrator 152 connected to receive the output of the synchronous demodulator 138 and controlling the light source spectral output through current modulation. The first or a higher order odd harmonic is used as an error signal, which the integrator 152 converts into an adjustment of the current to a light source 154, whose spectrum can be biased by the voltage output of the integrator 152, which is fed thereto on line 156. The integrator 152, is adjusted to control the light source 154 so that the first harmonic signal is nulled out. Besides output 140, the output of the system 150 also can be the output 158 of the integrator voltage or other perimeters controlling the output of the light source 154 or the frequency of the light source output itself. 
     An alternative closed loop system 160 is shown in FIG. 14. Here the loop is closed on a two input local fiber grating modulator 162. As before, the light source 54 is a broadband light source, such as a light emitting diode, with enough spectral width to cover the sensing region of interest. The reflected spectral envelope from the remote fiber grating 23 is directed through the local grating fiber modulator 162, where it is amplitude modulated by the signal generator 114. The signal generator 114 is also connected to drive synchronous demodulator 138 at the same frequency ω as it drives the local grating modulator 162. The output of the detector 26 is fed into the synchronous demodulator 138, and the first harmonic is demodulated as an error signal. The resultant output 164 is fed to an integrator 166, which adjusts the DC voltage applied to the local fiber grating modulator 162 so that the grating thereon 162 is stretched or compressed with the stretching or compression of the remote fiber grating 23. The output 168 of the system 160 is therefore the voltage on the piezoelectric mandrel of the demodulator 162 required to match the local and remote fiber gratings 162 and 23 and represents any environmental effect at the fiber grating 23. 
     The system 170 shown in FIG. 15, multiplexes a series of remote fiber gratings 172, 174 and 176, using the closed loop techniques of system 160 and frequency division multiplexing. Preferably, the fiber gratings 172, 174 and 176 have widely spaced spectral envelopes and at the minimum are arranged so that there is no frequency overlap. In system 170, a broadband light source 178 capable of covering the desired spectral width for all the fiber grating sensors 172, 174 and 176 is used. The light from the light source 178 is coupled into one arm 180 of an n by n coupler, n equaling the number of remote fiber grating sensors, and in FIG. 15, the 3 by 3 coupler 182. The coupler 182 splits the light 183 from the source 178 into three light beams. The resultant split light beams travel on arms 184, 185 and 186 of the coupler 182 and the portions 187, 188 and 189 thereof within the instantaneous spectral envelope of local fiber grating modulators 190, 191 and 192 respectively, reflect back through the coupler 182. The local grating fiber modulators 190, 191 and 192 are operated at characteristic frequencies ω 1 , ω 2  and ω 3  by signal generators 193, 194 and 195, respectively. The light within the reflecting envelopes of the modulators 190, 191 and 192 passes through an arm 196 of the coupler 182, another coupler 198 and is selectively reflected off the remote fiber grating sensors 172, 174 and 176, dependent upon the stressed or environmental conditions thereof. The light reflecting off the grating sensors 172, 174 and 176 then passes to arm 200 of the coupler 198 positioned so that the light thereon falls on detector 202. The detector 202 produces an output on line 203 whose frequency components contain the environmental information from each sensor 172, 174 and 176. The line 203 feeds this output to synchronous demodulator/integrators 204, 206 and 208 operated at ω 1 , ω 2  and ω 3 , by the signal generators 193, 194 and 195, respectively. The output from each of the synchronous demodulators/integrators 204, 206 and 208 is applied to the modulators 190, 191 and 192, respectively, to correct the voltage thereon, so that the spectral envelopes between a local fiber grating modulator and its remote fiber grating sensor overlap as in system 160 of FIG. 14. Since these voltages mimic the changes on the sensors 172, 174 and 176, the outputs on ports 210, 212 and 214 of the synchronous demodulators/integrators 204, 206 and 208 can be used as the output signals of the system 170. 
     The closed loop system 220 of FIG. 16 has its detectors 222, 224 and 226 behind each of the local fiber grating modulators 190, 191 and 192, so that they pick up signals from the remote fiber gratings 172, 174 and 176, respectively, outside the wavelength filtering band of the in-line fiber grating modulators 190, 191 and 192. The outputs from the detectors 222, 224 and 226 are each fed to synchronous demodulators 228, 230 and 232, whose outputs are fed to integrators 234, 236 and 238, respectively. The output voltages from the integrators 234, 236 and 238 are fed to close the loop with other fiber grating modulators shown as 192, 190 and 191, respectively, and these voltages are taken at ports 240, 242 and 244 as outputs representative of the environmental effects on the sensors 172, 174 and 176. Another way of seeing how the system 220 operates is to consider how the environmental effects are recovered for one sensor, for example grating 172. When the local fiber grating modulator 190 reflects its spectral envelope, only a portion of the spectral envelope is going to be reflected by the sensor 172. Since the spectral envelope portion was originally created by reflection off of a local fiber grating 190, its included wavelengths are always going to be blocked thereby. The spectral envelope portion then passes through modulator 192 and falls on detector 226. The output of the detector 226 is demodulated by demodulator 232, integrated by integrator 238 and applied as nulling feedback to modulator 191 to prevent the spectral envelope portion reflected by grating 172 from passing through modulator 191. The output of the integrator 238 is available at port 244 and is representative of the environmental effects at sensor 172. Each sensor 172, 174 and 176 operates the same way, so that separate outputs are produced at each port 240, 242 and 244. 
     If a switch 250 is positioned in arm 196, as shown in FIG. 17, then the system 220 can be used to support other strings 252, 254 and 256 of fiber optic grating sensors, such as sensors 172, 174 and 176. It is also possible to configure the above systems for transmission rather than reflection. However, the reflective approach has the advantage of a single point of ingress and egress. 
     One of the issues associated with a fiber grating demodulator operating in closed loop form is that the local, controllable fiber grating must have sufficient dynamic range to match environmental effects that may be induced on the remote fiber grating sensor. For increased dynamic range, the localized fiber grating could be configured as shown in FIG. 18. Here a fiber grating 260 is embedded in an epoxy block 262, selected to match the conditions expected to be encountered by the remote fiber grating. The block 262 is then bonded to control plates 264 and 266 at least one of which, 264, is connected to the output 268 of a stepper motor 269. The output 268 is shown diagrammatically connected to the block 264 by springs 270 and 272, so that the cross members 274 and 276 can be piezoelectrically driven to produce fine control of the stressing of the grating 260, while the output 268 of the stepper motor 269 provides gross control. 
     The dynamic range of each of the open loop fiber grating modulators discussed above, is limited by the spectral width of its fiber grating as shown in FIG. 19A. As an example, the typical fiber grating has a spectral coefficient of about 5×10 -4  nanometers per microstrain, and a spectral width of about one nanometer. A grating modulator having this fiber grating would have to have a maximum dynamic range of 2,000 microstrain and the effective dynamic range could be substantially less as the upper limit is at the point at which the signal level falls off to zero. The dynamic range can be improved by using a fiber grating with a broader spectral range, as shown in FIG. 19B, but with a loss in sensitivity. 
     FIGS. 20A and 20B illustrate the construction method for producing a double grating 300 at a single location 302 on a sensitized optical fiber 304. The method is similar to the method of producing a single grating, in that the sensitized optical fiber 304 is exposed to the fringe pattern 306 of two identical frequency laser beams 308 and 310 striking the sensitized optical fiber 304 at a first angle, θ 1 , to form a grating 311 that reflects a first wavelength, λ 1 . Thereafter the same location 302 is exposed to the interference pattern of two identical frequency laser beams 312 and 314 arraigned to strike the sensitized optical fiber 304 at a second angle, θ 2 , to form a second grating 316 that reflects a second wavelength λ 2 . The &#34;double&#34; grating 300 so produced is useful in the sensor system 320 of FIG. 21, which can sense temperature and physical strain at the same location and in the same manner can provide the same result when used in the other sensor systems, described herein, modified to used light at two discrete frequency bands. Normally the beams 308, 310, 312, and 314 are all at the same frequency. However, instead of changing the angles between the beams, the frequency of beams 312 and 314 can be different from the frequency of beams 308 and 310, to produce the two different interference patterns. 
     The system 320 of FIG. 21 uses the double grating 300 as the remote sensor element. The system 320 has two discrete light sources 322 and 324. Light source 322 produces light having a wavelength band centered about λ 1  and light source 324 produces light having a wave length band centered about λ 2 . The light from the light sources 322 and 324 is coupled onto a fiber 326 by a wavelength division multiplexing element 328, which passes light at λ 1  straight through and cross-couples light at λ 2  into the fiber 326 to produce a combined light beam 330 thereon. The light beam 330 passes through a beamsplitter 332 and two spectral envelopes are reflected by the double grating 300 dependent upon the temperature and strain thereat as light beam 334. The light beam 334 is reflected off a second double grating 336 positioned on a piezoelectric modulator 338. The resultant two spectral portions forming portions of beam 340, are conducted by beamsplitter 342 to a second wavelength division multiplexing element 344, which allows the portion of the light at λ 1  to pass straight through onto a detector 346 while cross-coupling the portion of the light at λ 2  to detector 348. In response, the detectors 346 and 348 produce outputs 350 and 352 to a dual synchronous demodulator 354. As in the previous systems, the modulator 338 and the synchronous demodulator 354 are fed the same frequency signal by a signal generator 356 which impresses the signal onto the light beam 340 and is used by the synchronous demodulator 354 to demodulate the outputs 350 and 352 into outputs Z 1  and Z 2 , while producing a feed back signal 358 to a closed loop bias driver shown as integrator 360, whose output 362 is also fed to the modulator 338 as a biasing input thereto. The outputs Z 1  and Z 2  are related by: 
     
         Z.sub.1 =a.sub.1 (λ.sub.1)T+b.sub.1 (λ.sub.1)ε 
    
     
         Z.sub.2 =a.sub.2 (λ.sub.2)T+b.sub.2 (λ.sub.2)ε 
    
     where a 1  (λ 1 ), b 1  (λ 1 ), a 2  (λ 2 ), and b 2  (λ 2 ) are known coefficients dependent on λ 1  and λ 2 , T is temperature and ε is strain. Therefore ##EQU2## This mathematical calculation is accomplished in a numeric processor 364 to produce outputs representative of the instantaneous temperature, T and strain, ε of the grating 300. 
     FIG. 22 is a sensor system 380 similar to that shown in FIG. 5 with the positions of the remote grating 23 and local grating 52 reversed. Light from the light source 54 is conducted through the beamsplitter 34 and a spectral envelope is reflected off of grating 52 back through the beamsplitter 34 to the remote grating 23. The remote grating 23 reflects the portion of the spectral envelope of grating 52 that is common to its instantaneous spectral envelope back through beamsplitter 65 to the detector 67 to produce the output 70. 
     An even simpler system 390 is shown in FIG. 23. Light from the light source 54 is transmitted through the local grating 52 to produce a spectral envelope through beamsplitter 34 to the remote fiber grating 23. The grating 23 then reflects light in the frequencies common to the transmission spectral envelope of grating 52 and the reflection spectral envelope of grating 23 back through the beamsplitter 34 to the detector 26, which produces the same output 64 as is produced in system 50 of FIG. 5. 
     The sensor system 400 of FIG. 24 is similar to system 160 of FIG. 14, except that the light from the source 54 is first passed through the beamsplitter 34 to reflect a spectral envelope off of the modulator 162, which reflected spectrum then passes through beamsplitter 34 and another beamsplitter 402 to reflect a common spectral portion thereof off of the remote grating 23. The portion of the light signal that reflects from both modulator 162 and grating 23 then passes back through the beamsplitter 402 to the detector 26, which produces an output 404 to the synchronous demodulator 138. The signal generator 114, as before, is connected to drive the synchronous demodulator 138 at the same frequency, ω, that it drives the local grating modulator 162. The output 404 of the detector 26 is fed to the synchronous demodulator 138 and the first harmonic is demodulated as an error signal. The resultant output 406 is fed to the integrator 166, which adjusts the DC voltage applied to the local fiber grating modulator 162, so that the grating thereon is stretched or compressed with the stretching of compressing of the remote fiber grating 23. The output 408 of the system 400 therefore is the voltage of the piezoelectric mandrel of the demodulator 162, required to match the local and remote fiber gratings 162 and 23 and represents any environmental effect at the fiber grating 23. 
     The system 420 of FIG. 25 is similar to system 170 of FIG. 15, except that light from the light source 178 is first reflected off remote gratings 172, 174 and 176 to produce specific spectral envelopes through the beamsplitter 422 and the 3 by 3 beamsplitter 424 to the local fiber grating modulators 190, 191 and 192, wherein the overlapping portions are reflected back to the detector 202. Otherwise, system 420 operates essentially the same as system 170, producing outputs 426, 428 and 430 representative of the environmental effects at the remote gratings 172, 174 and 176 from the synchronous demodulators 204, 206 and 208, respectively. 
     The system 450 shown in FIG. 26 is a differential sensor system where both gratings 452 and 454 thereof are exposed to environmental effects. The gratings 452 and 454 are shown as double gratings for illustrative purposes and could be single gratings in applications where they are exposed to strain or temperature. The gratings 452 and 454 may be identical if only the knowledge of a difference in environmental effects is to be sensed, or they may be constructed with slight differences in spectral envelopes if the direction of any difference is to be detected. Like in system 320, light in two frequency bands is produced by light sources 456 and 458. The light is combined by a 3 by 3 beamsplitter 460 and transmitted to the double grating 452 which reflects two narrow spectral envelopes back through the beamsplitter 460 to a wavelength division multiplexing element 462, which separates the two spectral envelopes for detection by detectors 464 and 466. The light that is not reflected by grating 452 passes to grating 454. If the reflection spectral envelopes of grating 454 are different than those of grating 452, then additional light is reflected back to the detectors 464 and 466 so that although light always is reaching the detectors, its intensity is double when there is no overlap between spectral envelopes with respect to the intensity when the spectral envelopes are in perfect registration. The outputs 468 and 470 from the detectors can be processed as in system 320. 
     In some applications such as is shown with the sensor system 500 in FIG. 27, a remote fiber grating sensor 502 may be required to operate over a very wide dynamic range that is beyond the capability of a single, simple, cost effective, piezoelectric driven, fiber grating modulator 504 and signal generator 505, 506 and signal generator 507, or 508 and signal generator 509. One solution is to operate a multiple fiber grating demodulator 510 supported by the multiple fiber grating modulators 504, 506 and 508. The signal generators 505, 507, and 509 cause the fiber grating modulators 504, 506 and 508 to operate at different frequencies ω 1 , ω 2 , and ω 3 , but with slightly overlapping spectral envelopes as shown in FIG. 28. The reflective spectral envelope 512 produced by grating modulator 504 can vary as shown by arrow 513, the reflective spectral envelope 514 produced by grating modulator 506 can vary as shown by arrow 515, and the reflective spectral envelope 516 produced by modulator 508 can vary as shown by arrow 517. The frequencies ω 1 , ω 2 , and ω 3  are generally higher than any frequency of interest in the environmental effect 518 to be sensed. However, most large amplitude environmental effects are at relatively low frequencies. Typical effects having large amplitudes include the flexing of a freeway bridge in response to heavy trucks, the movement of a high rise building in response to wind or earthquakes, and the vibration of the structure tying modules of a space station together. 
     In system 500, the light source 519 produces light through a beamsplitter 520 to the remote grating 502 to produce the reflective spectral envelope 522 shown in FIG. 28, whose dynamic range is shown by the arrow 523. Eventually the spectral envelope 522 will overlap all of envelopes 512, 514 and 516. Therefore, when the spectral envelope 522 passes through beamsplitters 520 and 526, the light in the spectral overlap 524 reflects off modulator 504 and the light in the spectral overlap 525 reflects off modulator 506. The light in both overlaps 524 and 525 passes through beamsplitter 526 to a detector 528. The output 530 of the detector 528 is sent to the synchronous demodulators 510, where the first and second harmonics ω 1  and 2 ω 1 , ω 2  and 2 ω 2 , and ω 3  and 2 ω 3  to are used to produce an output 532 indicative of the environmental effect 518 at grating 502. 
     The synchronous demodulators 510, determine the amplitudes of the signals ω 1 , 2 ω 1 , ω 2 , 2 ω 2 , ω 3  and 2 ω 3 . By monitoring the amplitude and phase of the signals, ω 1  and 2 ω 1 , ω 2  and 2 ω 2 , and ω 3  and 2 ω 3 , the position of the remote fiber grating spectral envelope 522 relative to each fiber grating modulator spectral envelope 512, 514, and 516 can be determined. When, as shown in FIG. 28, the spectral envelope 522 primarily overlaps the spectral envelope 512 of the remote fiber grating 504 driven at ω 1 , the result is that the primary signals are at ω 1  and 2 ω 1 . The phase of ω 1  indicates what side of the spectral envelope 512, the spectral envelope 522 of the modulator 504 is on, and the ratio between the amplitudes of the signals at ω 1  and 2 ω 1  determines its position on that side of the spectral envelope 512. 
     It is also possible to run system 500 as a closed loop system, as shown in system 550 of FIG. 29, wherein integrators 552, 554 and 556 are used to null out the first harmonic. Again referring to FIG. 28, the first harmonic, ω 1  would be nulled out so that the spectral envelope of the fiber grating modulator 504 operating at ω 1  would be overlaid with the remote fiber grating spectral envelope 522. When the limit of dynamic range of the fiber grating modulator 504 is reached, it unlocks and a hand off occurs to the spectrally adjacent fiber modulator 506 operating at ω 2 , and so forth. 
     As shown in system 570 of FIG. 30, the technique can also be extended to cover multiplexed in-line remote fiber grating sensors 502 1 , 502 2 , . . . 502 n  which are designed to operate about center wavelengths λ 1 , λ 2 , . . . λ n , that are spaced so that environmentally induced spectral overlap does not occur. Enough bandwidth is included between ω 1  and ω n  so that all of the remote fiber gratings can produce outputs 532 1 , 532 2  through 532 n  representative of the environment effects thereat. 
     A Fabry-Perot optical filter 600 is shown in FIG. 31. It includes a pair of optical fibers 602 and 604 whose facing ends 606 and 608 are partially mirrored. The one of the fibers 602 is maintained stationary with respect to a physical reference 610 by a fixed support 612. The other fiber 604 is mounted in a piezoelectric driver 614 which can move the end 608 in the direction shown by arrow 616. This movement converts a broad spectrum input light beam 218 into an output light beam 620 whose frequencies are modulated as shown. An etalon type Fabry-Perot optical filter 622 can be created by gluing an optical fiber portion 624 between the partially mirrored ends 606 and 608 as shown in FIG. 32. The transmission characteristic when driven at a single frequency of the etalon type Fabry-Perot optical filter 622 is shown in FIG. 33. 
     As shown in FIG. 34, the Fabry-Perot optical filter 600 or 622 can be used as the demodulator element in a sensor system 628. The system 628 includes a broadband light source 630 whose light output is conducted by a first arm 632 of a fiber beamsplitter 634, which splits it into two beams. One of the beams, 635, travels on arm 636 of the splitter 634 and is absorbed at the end 638 thereof. 
     The other beam 640 from the beamsplitter 634 is conducted to a remote fiber grating 641 on arm 642. Most wavelengths pass through the grating 641 and are absorbed at a second termination 644. The narrow band of wavelengths, representative of environmental effects, in the return beam 645 that is reflected by the fiber grating 641 are transmitted back along arm 642, pass through the beamsplitter 634 and a portion 646 is conducted through the Fabry-Perot optical filter 622 to a detector 648 by means of fiber arm 650. The detector 648 converts the intensity of beam portion 646 into an electrical signal 648 conducted to the controller 650, which feeds it back as output 652 to the filter 622 to close the control loop and provide the system output. 
     Fabry-Perot and acousto-optical band pass filters can also be used in open loop applications by providing a stable drive signal to them. Such systems 660 and 662 are shown in FIGS. 35 and 36. Light from a broadband light source 664 is provided to an array of fiber gratings 666, which reflects back a frequency spectrum from each grating. The frequency spectrums are conducted through beamsplitter 668 to a wavelength division multiplexing beamsplitter 670 to separate the light colors, λ 1 , λ 2 , and λ 3  to the Fabry-Perot filters 222 (FIG. 35) or the acousto-optic filters 672 (FIG. 36). The filters 622 or 672 pass portions to detectors 648 whose electrical outputs represent the environmental effects at the sensor array 666. 
     In system 680 a four times overwritten remote grating sensor 682 is used to sense three strain axes (x, y, and z) and temperature (t) at the same time. The angles of at least two of the gratings (usually y and z) are shifted from 90° across the fiber so that they respond to strains in the other two directions. The outputs of the y and z gratings will have a portion of the x strain in them which can be removed arithmetically. The temperature grating is aligned with one of the gratings, usually the one that is oriented across the fiber, so that the temperature can be determined as aforesaid. The gratings of the sensor 682 produce four light signals having different center frequencies (colors), λ x , λ y , λ z  and λ t . The colors are split by a wavelength division multiplexing beamsplitter 684 to be demodulated by suitable filters, the demodulation system of FIG. 34 being shown. 
     Thus there has been shown and described novel fiber grating sensor systems which fulfill all the objects and advantages sought therefor. Many changes, modifications, variations, uses and applications of the subject invention will however will become apparent to those skilled in the art after considering this specification and the accompanying drawings. All such changes, modifications, alterations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.