Patent Abstract:
A fiber optic sensor comprises two independent fibers having Bragg gratings which are coupled to commutating broadband optical sources through splitters and wavelength discriminators. The ratio of detected optical energy in each of two detectors examining the wave intensity returned to a wavelength discriminator coupled with the characteristic of the wavelength discriminator determines the wavelength returned by the grating. In another embodiment, tunable filters are utilized to detect minimum returned wave energy to extract a sensor wavelength Reference to the original grating wavelength indicates the application of either temperature or strain to the grating. In another embodiment, a plurality of Bragg grating sensor elements is coupled to sources and controllers wherein a dimensional change in a fiber having a Bragg grating is detected using a measurement system comprising broad-band sources, optical power splitters, a high-sensitivity wavelength discriminator, optical detectors, and a controller.

Full Description:
This is a division of application Ser. No. 09/286,092 filed on Apr. 2, 1999 now U.S. Pat. No. 6,597,822. 
    
    
     This invention was made with U.S. Government support under grant NAS 1-20579 awarded by the National Aeronautics and Space Administration. The U.S. Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The current invention applies to the field of fiber-optic sensors, wherein a dimensional change in a fiber having a Bragg grating is detected using a measurement system comprising broad=band sources, optical power splitters, a high-sensitivity wavelength discriminator, optical detectors, and a controller. 
     BACKGROUND OF THE INVENTION 
     There are several modern methods for fabricating optical waveguides for the low-loss containment and delivery of optical waves. One such waveguide is optical fiber which slightly higher index of refraction than the surrounding cladding. Typical values for the core diameter are of order 10 μm for single-mode fiber operating at communications wavelengths of 1300-15500 nm, and 50 μm or 62.5 μm for highly multi-mode fiber. Whether single-mode or multi-mode, the cladding diameter ha s most commonly an overall diameter of 125 μm, and a plastic jacket diameter is typically 250 μm for standard telecommunications fiber. The glass core is generally doped with germanium to achieve a slightly higher index of refraction than the surrounding cladding by a factor of roughly 1.003. The jacket is generally plastic and is used to protect the core and cladding elements. It also presents an optically discontinuous interface to the cladding thereby preventing coupling modes in the cladding to other adjacent fibers, and usually plays no significant part in the optical behavior of the individual fiber other than the usually rapid attenuation of cladding modes in comparison with bound core modes. 
     As described in the book by Snyder and Love entitled “Optical Waveguide Theory” published by Chapman and Hall (London, 1983), under the assumptions of longitudinal invariance and small index differences for which the scalar wave equation is applicable, the modal field magnitudes may be written
 
Ψ( r, φ, z )=ψ( r , φ) exp{ i (β z−ωt )}
 
where
         β is the propagation constant   ω is the angular frequency   t is time   z is the axial distance   r, φ is the polar trans-axial position along the fiber.       

     Single-mode fibers support just one order of bound mode known as the fundamental-mode which we denote as ψ 01 , and which is often referred to in the literature as LP 01 . The transverse field dependence for the fundamental-mode in the vicinity of the core may be approximated by a gaussian function as
 
ψ 01 ( r , φ)=exp{−( r/r   01 ) 2 }
 
where r 01  is the fundamental-mode spot size.
 
     Optical fiber couplers, also known as power splitters, are well known in the art, and generally comprise two fibers as described above having their jackets removed and bonded together with claddings reduced so as to place the fiber cores in close axial proximity such that energy from the core of one fiber couples into the core of the adjacent fiber. One such coupler is a fused coupler, fabricated by placing two fibers in close proximity, and heating and drawing them. The finished fused coupler has the two cores in close proximity, enabling the coupling of wave energy from one fiber to the other. A further subclass of fused coupler involves a substantially longer coupling length, and is known as a wavelength discriminator. The characteristics of a wavelength discriminator include wavelength-selective coupling from an input port to a first output port, as well as a second output port. As the wavelength is changed over the operating range of the wavelength discriminator, more energy is coupled into the first output port, and less is coupled into the second output port. The operation of a wavelength discriminator is described in “All-fibre grating strain-sensor demodulation technique using a wavelength division coupler” by Davis and Kersey in Electronics Letters, Jan. 6, 1994, Vol. 30 No. 1. 
     Fiber optic filters are well known in the art, and may be constructed using a combination of optical fiber and gratings. Using fiber of the previously described type, there are several techniques for creating fiber optic gratings. The earliest type of fiber grating-based filters involved gratings external to the fiber core, which were placed in the vicinity of the cladding as described in the publication “A single mode fiber evanescent grating reflector” by Sorin and Shaw in the Journal of Lightwave Technology LT-3:1041-1045 (1985), and in the U.S. patents by Sorin U.S. Pat. No. 4,986,624, Schmadel U.S. Pat. No. 4,268,116, and Ishikawa U.S. Pat. No. 4,622,663, All of these disclose periodic gratings which operate in the evanescent cladding area proximal to the core of the fiber, yet maintain a separation from the core. A second class of filters involve internal gratings fabricated within the optical fiber itself. One technique involves the creation of an in-fiber grating through the introduction of modulations of core refractive index, wherein these modulations are placed along periodic spatial intervals for the duration of the filter. In-core fiber gratings were discovered by Hill et al and published as “Photosensitivity in optical fiber waveguides: Application to reflected filter fabrication” in Applied Physics Letters 32:647-649 (1978). These gratings were written internally by interfering two counter propagating electromagnetic waves within the fiber core, one of which was produced from reflection of the first from the fiber end face. However, in-core gratings remained a curiosity until the work of Meltz et al in the late 1980s, who showed how to write them externally by the split-interferometer method involving side-illumination of the fiber core by two interfering beams produced by a laser as described in the publication “Formation of Bragg gratings in optical fibers by a transverse holographic method” in Optics Letters 14:823-825 (1989). U.S. patents Digiovanni U.S. Pat. No. 5,237,576 and Glenn U.S. Pat. No. 5,048,913, also disclose Bragg gratings. a class of grating for which the grating structure comprises a periodic modulation of the index of refraction over the extent of the grating. Short-period gratings reflect the filtered wavelength into a counter-propagating mode, and, for silica based optical fibers, have refractive index modulations with periodicity on the order of a third of the wavelength being filtered. Long-period gratings have this modulation period much longer than the filtered wavelength, and convert the energy of one mode into another mode propagating in the same direction, i.e., a co-propagating mode, as described in the publication “Efficient mode conversion in telecommunication fibre using externally written gratings” by Hill et al in Electronics Letters 26:1270-1272 (1990). The grating comprises a periodic variation in the index of refraction in the principal axis of the core of the fiber, such variation comprising a modulation on the order of 0.1% of the refractive index of the core, and having a period associated with either short or long-period gratings, as will be described later. 
     The use of fiber-optics in temperature measurement is disclosed in U.S. Pat. No. 5,015,943 by Mako et al. A laser source is beam split into two fibers, one of which is a sensing fiber exposed to an elevated temperature, and one of which is a reference fiber in an ambient environment. The optical energy from the two fibers is summed together, and an interference pattern results. As the temperature changes, the physical length of the sensing fiber optic cable changes, which causes the interference pattern to modulate. Each modulation cycle represents one wavelength change in length. Counting these interference patterns over time enables the measurement of temperature change. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for the measurement of sensor grating pitch, wherein the change in grating pitch can originate from a strain applied to the sensor grating, or it may originate from a temperature change wherein the sensor grating expands or contracts due to the coefficient of thermal expansion of the optical fiber enclosing the sensor grating. A pair of fibers, each having a sensor grating, is illuminated by a pair of broadband sources coupled through a pair of optical power splitters, and this sensor grating reflects wave energy over a narrow optical bandwidth. Reflected wave energy from the narrow-band sensor grating is passed through a wavelength discriminator, comprising a long-drawn optical coupler. A normalized power ratio comprises the difference in first and second detector power levels divided by the sum of the first and second power level. This intensity ratio is compared to the wavelength discriminator characteristic stored in a controller to look up the wavelength from a normalized power ratio value, and hence the pitch of the sensor grating. As the characteristic of the wavelength discriminator is essentially temperature invariant, this very accurately yields the sensor grating pitch. Comparing this reflected wavelength to the known wavelength of the grating indicates a change in wavelength brought about by either a temperature change or by the presence of a strain. In the case where a second sensor is also monitored, one sensor may be used as a reference to monitor the temperature of the second sensor, which is used to measure applied strain. In this manner, the temperature effect of the strain gauge may be cancelled by using the measured result of the reference sensor. Commutating the two sources in separate non-overlapping intervals enables the independent measurement of temperature, or strain, or any combination of the two. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a prior art grating. 
         FIGS. 1   b, c, d  show the spectral behavior of the prior art grating of  FIG. 1   a.    
         FIG. 1   e  is a prior art coupler/wavelength discriminator 
         FIG. 1   f  is a section view of the fused area of  FIG. 1   e.    
         FIG. 2  is a block diagram of the fiber optic sensor system. 
         FIG. 3  is a block diagram of the controller of FIG.  2 . 
         FIG. 4  is a wavelength discriminator. 
         FIG. 5  is a graph of the response of a wavelength discriminator including reflected grating power applied to this wavelength discriminator. 
         FIG. 6  is a graph of the output function of the wavelength discriminator normalized power ratio (P1−P2)/(P1+P2). 
         FIG. 7  is the dynamic state of various internal nodes of the fiber optic sensor system during operation. 
         FIG. 8  is a three-wavelength, temperature/strain sensor. 
         FIG. 9  shows the wavelength detection properties of FIG.  8 . 
         FIG. 10  is a multi-wavelength strain/temperature measurement system. 
         FIG. 11  is an alternate wavelength detector for FIG.  10 . 
         FIG. 12  is a multi-wavelength strain/temperature measurement system using tunable gratings. 
         FIG. 13  shows the voltage waveforms for FIG.  12 . 
         FIG. 14  shows a temperature/strain measurement system having an alternate wavelength discriminator comprising a broadband grating and a splitter. 
         FIG. 15  shows the block diagram of the measurement controller of FIG.  14 . 
         FIG. 16  shows the input to the first and second detectors versus wavelength for the measurement system of FIG.  14 . 
         FIG. 17  shows a temperature/strain measurement system using a wavelength discriminator comprising a coarse wavelength discriminator and a fine wavelength discriminator. 
         FIG. 18  shows the characteristic transfer function for the fine wavelength discriminator and the coarse wavelength discriminator of FIG.  17 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows a prior art internal grating filter, comprising an optical fiber having a core  1 , a cladding  2 , and a grating  3  fabricated within the extent of the core  1 . The grating  3  comprises a modulation of the index of refraction of core  1  having a regular pitch  4 , where the grating  3  is used to create short-period grating behavior. For reflection of waves through the grating at wavelength λ b , the short-period grating function is as follows:
 Λ b =λ b /2n 
where
         Λ b =pitch of the desired Bragg grating.   λ b =conversion wavelength: For short period gratings. λ b  is the wavelength for which incident fundamental mode wave energy is converted to counter-propagating (traveling in the opposite direction) wave energy.   n=effective index of refraction of the fiber, which is dependent on the mode of the propagated wave.       
     Examining now the transfer curves for a short-period grating  3 ,  FIG. 1   b  shows the input source spectrum  7  applied to port  5 , and  FIG. 1   c  shows the reflected spectrum  8  and grating peak  9  reflected back to port  5 .  FIG. 1   d  shows the remaining optical energy continuing to port  6 . Filter notch  11  represents wave energy reflected by the short period Bragg grating back to the input port  5 , and is represented as spectrum  8  having peak  9  corresponding to the Bragg wavelength. The use of reflected wave energy at peak  9  is generally not available without the use of an optical coupler or some other device sensitive to the propagating direction of this wave. 
       FIG. 1   e  shows a prior art optical coupler. First fiber having a core  12  and cladding  13  is placed in proximity with a second fiber having a core  15  and a cladding  14 . Together, these fibers are heated and drawn to fuse the two fibers into one having a coupling length  16 .  FIG. 1   f  shows a section view of this fused middle section. Coupling length  16  and separation  17  determine the coupling characteristics of the coupler. If the coupling length  16  is short, a broadband coupler having a coupling coefficient related to separation  17  is formed. This is the typical construction for power splitter configurations. If the length  16  is many wavelengths long, a narrowband coupler is formed, also known as a wavelength discriminator. The characteristics of a wavelength discriminator are similar to those of a coupler, with an additional wavelength dependence, as shown in  FIG. 5 , which is described later. 
       FIG. 2  shows the present fiber-optic sensor. Measurement system  20  is coupled to fibers  45  and  51 . Each of fibers  45  and  51  has a Bragg grating  46  and  52  respectively. Measurement system  20  further comprises a controller  22  having a first source enable output  24  coupled to first source  36 , which may be any source of optical energy having a spectrum which includes the wavelength of the grating  46  on fiber  45 . A broadband light-emitting diode (LED) would provide an inexpensive broadband source. Similarly, second source enable output  26  is coupled to second source  40 , which has the same requirement of including in its output spectrum the wavelengths of the grating  52  of fiber  51 . Broadband sources  36  and  40  respectively couple energy through standard power splitters  42  and  44 , which provide optical energy to gratings  46  and  52  respectively. The gratings  46  and  52  may be internal Bragg gratings or external short period gratings. The short-period grating has the property of reflecting optical energy at the grating wavelength back to couplers  42  and  44 , where it is split into optical energy provided to cables  41  and  43  to wavelength discriminator  38 , the operation of which will be discussed later in FIG.  4 . Output wave energy, from wavelength discriminator  38  is separated into a first output on fiber  31  travelling to first detector  30 , which provides a voltage  28  proportional to the input optical level delivered on fiber  31 . Similarly, optical wave energy from the second output  33  of wavelength discriminator  38  is delivered to the second detector  34 , which produces a voltage  32  to controller  22  proportional to the input optical level delivered on fiber  33 . 
       FIG. 3  describes in detail the controller  22  of FIG.  2 . Controller  22  further comprises a microprocessor  78  which produces first source enable output  24  and second source enable output  26 . In addition, first detector input  28  and second detector input  32  are processed by buffer amplifiers  62  and  64  respectively, which isolate the detector element from the following electronics, and produce respectively outputs  82  and  84 . These are processed by a difference amplifier  66  to produce a difference output at  86 , which is converted from an analog signal to a digital signal by A/D converter  74 , delivering a digital representation  90  of this signal to microprocessor  78 . Amplifier  68  produces a detector sum output  88 , which is similarly converted to a digital signal  92  by A/D converter  76 , which is also input to microprocessor  78 . A keypad  72  for input and a display  70  are also coupled to the microprocessor  78 , as is an auxiliary interface  80 . Microprocessor  78  may be chosen from several available units, including the PIC16 C71 from Micro-Chip. Inc. of Chandler, Ariz, which has the A/D converters  74  and  76  incorporated internally. As is clear to one skilled in the art, many microprocessor choices are available for  78 , including devices with internal or external ROM, RAM, A/D converters, and the like, of which many candidates from the Micro-Chip PIC-16 family would be suitable. While a particular microprocessor is shown for illustrative purposes, it is clear to one skilled in the art that other units could be substituted for these devices without changing the operation of the sensor. The principal requirements of microprocessor  78  are the ability to control the first and second sources, and to process the values provided by the first and second detectors in a manner which determines the wavelength of the sensor grating. 
       FIG. 4  shows the wavelength discriminator  38 . The wavelength discriminator has a first splitter input port  41 , a second splitter input port  43 , a first detector output port  31 , and a second detector output port  33 .  FIG. 5  shows the normalized output of wavelength discriminator  38  for the case where a swept-wavelength input is applied to first splitter input  41 , and no input is provided to second splitter input  43 . Curve  100  shows the output level of first detector output  31 , while curve  104  shows the output level of second detector output  33 . As can be seen from the graph, as the wavelength is varied from 1300 nm to 1316 nm, the first detector and second detector outputs vary in a complimentary manner, such that the sum of the first detector output and second detector output is nearly constant. The wavelength discriminator is a symmetric device, so if no optical signal were applied to first input  41  and a swept wavelength optical signal were applied to second input  43 , curve  100  would show the level of second output port  33 , while curve  104  would show the level of first output port  31 . 
       FIG. 6  shows a plot for normalized power ratio derived from first output curve  100  and second output curve  104 . If these two complimentary curves  100  and  104  are plotted as (P1−P2)/(P1+P2), then the plot of  FIG. 6  results, and we may now determine wavelength over monotonic regions such as from 1304 nm to 1312 nm by simply looking up the wavelength given the (P1−P2)/(P1+P2) normalized power ratio. Curve  114  represents the response to first source  36 , and curve  112  represents the response to second source  40 . The advantage of performing this lookup in this ratiometric manner of  FIG. 6  as opposed to the absolute output level on the curve  100  of  FIG. 5  is that variations in source power are normalized out of the result. Specifically, changes in the output power of sources  36  and  40  would modulate the values shown in plots  100  and  104  of FIG.  5 . but not the normalized power ratio shown in the plot of FIG.  6 . 
     Further examining the operation of the measurement system of  FIG. 2 , the first measurement is performed with only first source  36  enabled. Optical energy travels through first coupler  42  to fiber  45 , and to grating  46 . Optical energy at the wavelength λ 1  of grating  46  is reflected through fiber  45  back to first coupler  42 , through fiber  41 , where it is presented to wavelength discriminator  38 . No input is present on fiber  43  because second source  40  is not enabled. Optical energy from grating  46  is reflected, for example, at λ 1 =1309 nm. as shown in curve  102  of  FIG. 5 , and 0.4 volts is generated at  28  by first detector  30 . The second output  33  of wavelength discriminator  38  is applied to the second detector  34 , producing 0.6 volts at  32  as shown in curve  103  of FIG.  5 . By now finding the normalized power ratio of (0.4−0.6)/(0.4+0.6)=−20, it can be seen that this corresponds to 1309 nm wavelength on curve  114  at point  109  in FIG.  6 . 
     An entirely separate measurement can be made by disabling first source  36  and enabling second source  40 . In this case, optical energy would leave second splitter  44  through fiber  51  to grating  52 . Optical energy at wavelength λ 2    52  would be returned to second splitter  44  through fiber-optic cable  51 , leave second splitter  44  through fiber-optic cable  43 , entering wavelength discriminator  38 . Analogous to the earlier described processing, first source  36  would be disabled, hence no optical energy would be present in fiber  41 . In the case of wave energy input to fiber  43  instead of fiber  41 , the output characteristic of  FIG. 5  would be reversed such that curve  100  would be the output energy on fiber  33 , and curve  104  would represent the output energy of fiber  31 . If the grating  52  were reflecting at λ 2 =1306 nm. then second detector  34  would produce 0.75 volts as shown in curve  108  of FIG.  5 . First detector  30  would produce 0.25 volts as shown in curve  106  of FIG.  5 . The normalized power ratio of  FIG. 6  would be (0.25−0.75)/(0.25+0.75)=−0.5, corresponding to 1306 nm on curve  112  of  FIG. 6  at point  107 . 
       FIG. 7  shows the sensor measurement system operating in the earlier-described case where the wavelength of first sensor  46  is λ 1 =1309 nm and the wavelength of second sensor  52  is λ 2 =1306 nm. First, the detector offsets are determined by turning both first source  36  and second source  34  off. This produces the detector offset values OS 1  and OS 2 , which will be necessary to subtract from the power difference and power sum before calculation of the normalized power ratio (P1−P2)/(P1+P2). Thereafter, first source  36  and second source  40  are alternately enabled as shown in FIG.  7 . First detector  30  and second detector  34  produce the P1 and P2 values shown, and the difference sum, and the normalized power ratio value of difference/sum are computed as shown, wherein the power difference (P1−P2) and the sum (P1+P2) represent power quantities after removal of offsets OS 1  and OS 2 , which thereafter form the normalized power ratio (P1−P2)/(P1+P2). If the plot of  FIG. 6  normalized power ratio were kept in the memory of the microprocessor, either as a series of interpolated points, or as a power series wherein only the coefficients f0, f1, f2, f3 . . . fn of a polynomial are stored, and the power 
         λ   ⁡     (     P1   ,   P2     )       =       f   0     +       f   1     ⁡     [       P1   -   P2       P1   +   P2       ]       +         f   2     ⁡     [       P1   -   P2       P1   +   P2       ]       2     +         f   3     ⁡     [       P1   -   P2       P1   +   P2       ]       3     +   …   +         f   n     ⁡     [       P1   -   P2       P1   +   P2       ]       n             
series is of the form
 
where
         λ(P1, P2)=wavelength as a function of detector power ratio (P1−P2)/(P1+P2).       
     It would be possible to convert the given normalized power ratio(P1−P2)/(P1+P2) back to a wavelength λ 1 =1309 nm for the first sensor, and λ 2 =1306 nm for the second sensor. This determination could be done using either a look-up table derived from the normalized power ratio, or by storing the coefficients of a power series based on the normalized power ratio, and thereafter calculating for wavelength based on this power series. 
     If the sensors were operating either as temperature sensors or strain sensors, the applied strain or temperature could be computed from the following relationship:
 
Δλ=α1 ΔT +α2 ΔS 
 
where
         Δλ=change in sensor wavelength   α1=coefficient of thermal change for sensor   ΔT=change in sensor temperature   α2=coefficient of strain change for sensor   ΔS=change in sensor strain       

     In this equation, the change in sensor wavelength is expressed as the sum of a temperature related change and a strain related change. The coefficients al and al would be stored in the controller along with initial condition values to solve for total strain and total temperature. In this manner, any combinations of strain and temperature could be determined given a change in sensor wavelength and the wavelength discriminator characteristic curve, and first and second detector, inputs. 
       FIG. 8  shows a strain/temperature measurement system having a 3-way wavelength discriminator  162 . This system is analogous to the system described in  FIG. 2 , however, for an n-way wavelength discriminator, the output port associated with the excited port has the response shown in plot  186 , while the remaining ports have the characteristic shown in plot  188 . For example, in the case of  FIG. 8 , first source  134  sends broadband excitation through first splitter  136 , and wave energy at the example grating wavelength λ 1 =1300 nm is reflected through splitter  136  to wavelength discriminator port  167 . For this case, the output at port  168  has the characteristic shown in plot  186 , while the second output  174  and third output  180  have the responses shown by curve  188 . For λ 1 =1300 nm, the response of the first detector is shown as point  192 , while the second the third detectors have the response shown by point  194 . As before, a normalized plot of the response of curves  186  and  188  is shown in plot  190 . For the case of an n-way wavelength discriminator, the output curve  190  would be 
         P   ⁡     (   normalized   )       =     [         Pdet   ⁡     (   a   )       -     {       Pdet   ⁡     (   b   )       +       Pdet   ⁡     (   c   )       ⁢           ⁢   …     +     Pdet   ⁡     (   n   )         }           Pdet   ⁡     (   a   )       +     {       Pdet   ⁡     (   b   )       +       Pdet   ⁡     (   c   )       ⁢           ⁢   …     +     Pdet   ⁡     (   n   )         }         ]           
where
         Pdet(a)=output power from excited channel   Pdet(b) through Pdet(n)=output power from non-excited channel.       
     A lookup table constructed from the values of curve  190  would produce the value for λ 1 =1300 nm as shown at point  196 . Similarly, when second source  144  excites grating  150 , wave energy at the exemplar wavelength λ 2 =1305 nm would return through splitter  146 , fiber  173 , and now fiber  174  would contain the response shown in plot  186 . Fibers  168  and  180  would contain wave energy shown in plot  188 . corresponding to point  200 . The normalized power ratio for λ 2 =1305 nm is represented by point  204  of the plot  190 . The case where third source  154  excites grating  160  is shown in third detector response  186 , and first and second detector responses  188 . For the case where third grating wavelength is 1310 nm, the responses of the third detector. first and second detectors, and normalized power ratio are shown in points  206 ,  208 , and  210 . It is clear to one skilled in the art that this system is extendable to n ports of measurement, where each port has a source, a splitter, and each splitter port is connected to an input port of an n-way wavelength discriminator. Each output port of the n-way wavelength discriminator is coupled to a detector, and the response of each detector is measured, and the normalized power ratio is formed from the ratio of the difference between the response of an excited port and the responses of all of the non-excited ports, divided by the sum of all of the responses of excited and non-excited ports. 
       FIG. 10  shows a strain/temperature sensor system  211  attached to a fiber  220  comprising a plurality of gratings  224 ,  226 , and  230 . These sensors operate as earlier described, but are sequentially applied to various parts of a fiber  220 . Each sensor  224 ,  226 , and  230  reflects wave energy at respective unique wavelengths λ 1 , λ 2 , and λ n . Since gratings  224  and  226  have no effect on out-of-band waves at λ n , splitter  218  delivers to fiber  268  the superposition of reflected unique wavelengths λ 1  through λ n . Wavelength separator  236  has broadband outputs which respond only to the range of reflected wavelengths for that given output. For example, output  235  is responsive only to the range of λ 1 , and output  243  is only responsive to the range of λ 2 , and output  249  is only responsive to the range of λ n . This requires that the sensor wavelengths and wavelength separator characteristics be chosen such that isolated response of a given wavelength separator to a given sensor grating wavelength occur. In this manner, output  235  represents exclusively the range of wavelengths of sensor  224 , output  243  represents exclusively the range of wavelengths of sensor  226 , and output  249  represents exclusively the range of wavelengths of sensor  230 . The conversion of the outputs of separator  236  into a detected wavelength occurs as was earlier described in  FIGS. 4 ,  5 , and  6 . In this manner, multiple sensors can share a single fiber, as long as each produces a unique wavelength. 
     An alternate wavelength measurement apparatus  318  is shown in  FIG. 11 , which performs the same function as  270  of FIG.  10 . While the wavelength measurement apparatus  270  uses a wavelength separator  236  followed by narrowband wavelength discriminators  234 ,  242 , and  248 , the wavelength measurement apparatus  318  of  FIG. 11  utilizes a broadband wavelength discriminator  316  followed by wavelength separators  312  and  314 . These produce complimentary outputs  296  and  304  for λ 1 , complimentary outputs  298  and  306  for λ 2 , and complimentary outputs  300  and  308  for λ n . Detectors  232 ,  240 ,  246 ,  238 ,  244 , and  250  operate in a manner identical to those of FIG.  10 . 
       FIG. 12  shows a measurement system  340  connected to fiber  350 , which has a series of sensors  352 ,  354 , and  358 , which operate the same as those described earlier in  FIG. 10. A  single broadband source excites fiber  350  through splitter  348 . Splitter  348  returns aggregate reflected waves from sensors  352 ,  354 , and  358  on fiber  356 . A series of tunable filters  362 ,  364 , and  368  is coupled to detector  360 . Each of these filters is tuned over a narrow range through the application of a control voltage  372 ,  374 , and  378 . In operation, filters  364  and  368  have a voltage applied which reflects wave energy out of the range reflected by the sensors  354  and  358 , enabling the passage of waves reflected by sensor  352  to pass through and on to tunable filter  362 . Tunable filter  362  is swept over its tuning range, and produces a minimum output at detector  360  at the point where the grating  352  matches the tuned filter  362 . Controller  380  has the characteristic of tunable filter  362  stored in memory such that the voltage  372  producing a minimum detected output  370  enables the extraction of corresponding wavelength for λ 1 . Next, tunable filters  362  and  368  are tuned out of the band of grating  352  and  358 , and tunable filter  364  is swept over its range until a detector minimum is found. As earlier, this minimum voltage corresponds to the wavelength λ 2 . This process continues for as many sensor gratings and tunable filters that are present in the system. In practice, there are many ways of fabricating tunable gratings, including the application of a material with an index of refraction which varies with an applied voltage, the application of a tensile force to a fiber having a grating, or the application of a magnetic field to a grating in close proximity to a material having an index of refraction which changes with an applied magnetic field. It should be clear to one skilled in the art that there are many different ways of practicing such tunable filters, wherein an applied control voltage changes the wavelength of reflection of the tunable filter. 
       FIG. 13  shows the waveforms for the system of FIG.  12 . Tunable filter control voltage points  390 ,  392 , and  394  correspond to the detector minima  396 ,  398 , and  400  shown, and therefore enable the recovery of sensor wavelengths λ 1  λ 2 , and λ n . 
     While the foregoing description is drawn to specific implementations, it is clear to one skilled in the art that other embodiments are available. For example, the earlier described functions SUM and DIFF, which relate to the normalized power ratio, could be implemented using operational amplifiers computing these measurements as analog values, or they could be implemented digitally, operating on digitized detector values. These converters could be either integral to the microprocessor, or external, and the sum and difference values could either be computed through direct reading of the values of the detectors, or through reading sum and difference voltages of alternate circuitry. While the multiple sensor system of  FIGS. 10 and 12  are drawn to a 3 sensor system, it is clear to one skilled in the art that these could be drawn to arbitrary numbers of channels operating as strain sensors, temperature sensors, or both. There are also many ways of extracting sensor wavelength from the systems described. For clarity, time division processing has been shown, wherein at a particular time, only a single channel of the system is active, and only one particular wavelength value is recovered. In addition to the explicitly described method of time division processing, there are many modulation schemes wherein each of the sensor values is modulated in frequency or amplitude, and later demodulated to recover the desired value. In this manner, all of the channels of the system could operate simultaneously, rather than sequentially. The use of specific examples for illustration and understanding of the operation of the system does not imply an exclusive manner in which these systems could be implemented. 
       FIG. 14  shows a strain/temperature measurement system  20  similar to that of  FIG. 2 , but with a different wavelength discriminator. In the alternate embodiment of  FIG. 14 , the elements having the same numbering as those of  FIG. 2  perform the same function as earlier described, but the wavelength discriminator now comprises third splitter  400  which has as inputs the previously described fibers  41  and  43 , and has a normalizing output  406  which is wavelength-invariant compared to wavelength determining output  405 . The wavelength-determining output  405  is formed from broad-bandwidth grating  404 , which has an output amplitude varying with wavelength over the tuning range of the sensor gratings, as will be described later. First detector  408  and second detector  410  accept optical inputs  405  and  406 , respectively, and produce electrical outputs  412  and  414  which are proportional to the respective optical inputs  405  and  406 . 
       FIG. 15  shows the controller  401  of  FIG. 14 , which is similar to the controller of  FIG. 3 , and has similarly-functioning elements numbered the same as those of  FIG. 3 , as was described earlier. First detector output  412  drives buffer  416  and produces output  420 , which is digitized by analog-digital converter  424  and is presented as a digital input  428  to microprocessor  78 . Second detector output  414  drives buffer  418  to produce signal  422  which is converted to a digital input  430  by analog-digital converter  426  and delivered to microprocessor  78 . 
       FIG. 16  shows the characteristic response of the wavelength discriminator having a normalizing input  406 , represented by response curve  464 , and wavelength-determining input  405 , represented by response curve  450 . As the reflected wave from grating  46  or grating  52  passes through third splitter  400 , equal amounts of energy are presented into grating  404 , and to normalizing input  406 , As the wavelength applied to third splitter  400  is varied, normalizing output  406  follows the response of curve  464 , while the wavelength-determining input  405  follows the response of curve  450 , in accordance with the characteristic response of broadly tuned grating  404 , whose characteristics are chosen to include a monotonic region from first discrimination wavelength  452  to final discrimination wavelength  454 . In the case where grating  46  is reflecting a wavelength of 1306 nm, curve  460  represents the spectral energy of reflected energy from grating  46 , which is applied to curve  460  to produce an output of approximately 1.0 units. This same reflected response  456  applied to grating  404  having the response of curve  450  and produces an output of approximately 0.25 units. As can be seen from  FIG. 16 , as long as the range of input wavelength is between first discrimination wavelength  452  and final discrimination wavelength  454 , it is possible to recover the wavelength from curve  450 . By using the ratio of response  450  to response  464 , the effect of intensity variations in first source  36  and second source  40  is removed, as was discussed for the system of FIG.  2 . By keeping a copy of the characteristic curve of this normalized function of curve  450  divided by curve  464  in the microprocessor  78 , it is possible to resolve any input wavelength in the range first discrimination wavelength  452  to final discrimination wavelength  454  when given the first detector output  412  and second detector output  414 . As described earlier, this determination can be made by storing the response of curves  450  and  452  in a look-up table, or by specifying the curve as the coefficients of a polynomial, or in many other ways, all of which form representations of the characteristic curves of  450  or the ratio of curve  450  divided by curve  452 . 
       FIG. 17  shows another embodiment  503  of a temperature/strain sensor comprising the old elements of  FIG. 2  with a new wavelength discriminator circuit. This new wavelength discriminator comprises third splitter  470 , fourth splitter  488 , a coarse wavelength discriminator  474 , and a fine wavelength discriminator  492 , coarse wavelength first and second detectors  478  and  484 , and fine wavelength discriminator first and second detectors  504  and  498 . The operation of the coarse wavelength discriminator comprising coarse wavelength discriminator  474 , first detector  478 , and second detector  484  is similar to that described in  FIGS. 4 ,  5 , and  6 , and has a usable wavelength range matched to that of the sensor grating operating range. However, in addition to the coarse wavelength discriminator, a fine wavelength discriminator comprising fine wavelength discriminator  492 , and first detector  504  and second detector  498  are used. Third splitter  470  and fourth splitter  488  produce the signals for simultaneous delivery to the coarse and fine wavelength discriminators, as all 4 detectors are used simultaneously, although as described earlier, the first source  36  and second source  40 , operate during different intervals, or have orthogonal modulation functions which enable the discrimination of the two detector outputs through the use of a modulation function applied to the sources and a demodulation function applied to the detectors. 
       FIG. 18  shows the details of the fine and coarse wavelength discriminators. Curves  516  and  510  represent the optical response of the wavelength discriminator, as measured at fibers  476  and  482 , as well as the detected electrical responses of  480  and  486  to changes in wavelength of sensor  46  or  52 , all of which function as earlier described in the system of FIG.  2 . For the case of sensor  46  reflecting optical energy at 1302 nm. fiber  472  carries optical wave energy which is provided to coarse wavelength discriminator  474 . First output optical fiber  476  carries the energy of curve  512 , while second output optical fiber  482  carries the energy of curve  514 . Fine wavelength discriminator  492  has many more cycles in the same monotonic range of coarse wavelength discriminator  474 , as is seen by the periodicity of curves  510  and  516  of the coarse wavelength discriminator, compared to curves  522  and  524  of the fine wavelength discriminator. The monotonic curve of  510  and  516  is necessary over the tuning range of the reflecting gratings  46  and  52  to ensure single-wavelength resolution. The multiple cycles of discriminator  522  and  524  enable the more precise measurement of wavelength when used in combination with the coarse wavelength discriminator  474 . Fine wavelength discriminator is fed by fiber  491 , and has a first output  502  which carries the energy of curve  522  and a second output  496  which carries the energy of curve  524  when excited by the signal of fiber  491 . When the input signal is provided by fiber  493 , the characteristic of the first and second outputs reverse, as was described earlier in  FIGS. 4 ,  5 , and  6 . In this manner, sensor  46  reflecting a 1302 nm wavelength produces a first coarse detector response of  512 , a second coarse detector response of  514 , a first fine detector response of  526 , and a second fine detector response of  528 . Sensor  52  reflecting a wavelength of 1311 nm produces a first coarse detector response of  518 , a second coarse detector response of  520 , a first fine detector response of  532 , and a second fine detector response of  530 . As is clear to one skilled in the art, any combination of curve storage methods for maintaining the characteristic curves of  510 ,  516 ,  522 , and  524  or the difference divided by the sum of curves  510  to  516 , or curves  522  and  524  could be stored using the previously described look-up tables, polynomial coefficients, or interpolated points for use by the microprocessor  78  of the controller  501  of FIG.  17 .

Technology Classification (CPC): 6