Patent Document

[0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/959,862 by Eric Udd and Jerry John Benterou, entitled “High Speed Diagnostic System to Support Measurements of Energetic Materials and Damage”, which was filed on Jul. 17, 2007. 
     
    
       [0002]    The United States has rights in this invention pursuant to Contract No DE-AC52-07NA27344 between the United States Department of Energy and Lawrence Livermore National Security, LLC. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    This disclosure describes means to measure the location and velocity of a blast wave after detonation. It also describes configurations that can be used to support the measurement of location pressure and temperature by appropriately configuring the optical fiber grating sensors and utilizing appropriately configured read out techniques. Fiber grating sensor systems are described in detail in U.S. Pat. Nos. 5,380,995, 5,402,231, 5,828,059, 5,841,131, 6,144,026, and 6,335,524. Also U.S. patent application Ser. No. 11/071,278 by Eric Udd and Sean Calvert filed on Mar. 3, 2005 and abandoned teaches a fiber grating sensor system for detection, localization and characterization of high speed pressure waves. The present invention extends the capabilities of the high speed system to measure pressure, temperature, velocity and position of a high speed event such as a blast wave. 
         [0004]    The system has been demonstrated by the inventors with an operational bandwidth of 250 MHz that has allowed the velocity and position of a blast wave to be accurately measured interior to an energetic material after detonation with the duration of the events being on the order of 5 to 10 micro-seconds. The present invention in addition to measuring velocity and position can be extended to support the measurement of pressure and temperature allowing a much more complete set of measurements that are necessary to perform accurate diagnostics. 
       BRIEF DESCRIPTION OF THE PRESENT INVENTION 
       [0005]    In the present invention a high speed fiber grating sensor system is described that is capable of measuring the position, velocity, pressure and temperature of a high speed event that may be a blast wave generated by detonation of a highly energetic material. The invention is particularly directed toward the measurement of blast waves that have sufficient energy to destroy the optical fiber as they pass although the system will operate and provide more limited information for less energetic events. 
         [0006]    The invention consists of a light source that illuminates one or more fiber gratings, in one or more fiber lines, that are placed and oriented along a path associated with the blast wave that is to be measured. The reflected signals from the fiber gratings encountering the blast wave are then directed toward one of more wavelength filters that are used to localize and characterize the high speed blast wave. In order to separate pressure from temperature multiple fiber gratings or multi-parameter fiber grating sensors may be used. This can involve multiple read out detection lines that can as an example be used to separate out polarization states or it can involve separate fiber grating lines designed for different environmental responses to the blast wave. Therefore it is an object of the invention to provide a very high speed system that is capable of characterizing a blast or pressure wave. 
         [0007]    Another objective is to measure the velocity of a blast or pressure wave. 
         [0008]    Another objective is to measure the localized pressure of the blast or pressure wave. 
         [0009]    Another objective is to measure the localized temperature of the blast or pressure wave. 
         [0010]    Another objective is to characterize the pressure distribution of the leading edge of a blast or pressure wave. 
         [0011]    Another objective is to characterize the temperature distribution of the leading edge of a blast or pressure wave. 
         [0012]    Another objective is to simultaneously measure two or more of the following parameters, pressure, temperature, velocity and position of a blast or pressure wave. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention will be readily understood by the following detailed description, taken in conjunction with accompanying drawings, illustrating by way of examples the principles of the invention. The drawings illustrate the design and utility of preferred embodiments of the present invention, in which like elements are referred to by like reference symbols or numerals. The objects and elements in the drawings are not necessarily drawn to scale, proportion or precise positional relationship; instead emphasis is focused on illustrating the principles of the invention. 
           [0014]      FIG. 1  is an illustration of a prior art fiber grating that has a uniform period along its length. 
           [0015]      FIG. 2  is an illustration of a prior art chirped fiber grating that has a nonuniform period along its length. 
           [0016]      FIG. 3  is a graph showing the reflection and transmission spectrum of a 50 mm chirped fiber grating. 
           [0017]      FIG. 4  shows the spectrum of a 50 mm chirped fiber grating and a fiber light source. 
           [0018]      FIG. 5  shows the spectrum of a 100 mm chirped fiber grating before and after it has been cut back by 49 mm. 
           [0019]      FIG. 6  is an overall block diagram of a high speed fiber grating sensor system that was used to monitor a blast event. 
           [0020]      FIG. 7  is a block diagram showing the position of the chirped fiber grating with respect to the cylinder of nitromethane to support a blast test. 
           [0021]      FIG. 8  shows test results of a blast test using a 37 mm chirped fiber grating inserted into a tube of nitromethane. 
           [0022]      FIG. 9  is a waterfall plot of a chirped fiber grating that was trimmed to successively shorter lengths. 
           [0023]      FIG. 10  shows reflection data obtained from a chirped fiber grating as it is being trimmed. 
           [0024]      FIG. 11  is a system block diagram showing a set up to measure the leading edge of a blast wave that destroyed a chirped fiber grating. 
           [0025]      FIG. 12  is an illustration of the changes that occur to the spectrum of a uniform fiber grating which is subject to the leading edge of a highly energetic blast wave with a well defined leading pressure wave edge. 
           [0026]      FIG. 13  illustrates how an array of fiber gratings may be used to determine the position and amplitude of a pressure wave by varying spacing, length and reflectivity. 
           [0027]      FIG. 14  shows how a nonuniform pressure wave may induce a complex reflected spectrum from a fiber grating sensor. 
           [0028]      FIG. 15  shows a high speed system that is capable of measuring the entire spectral profile of a fiber grating assembly. 
           [0029]      FIG. 16  is a diagram illustrating the usage of a circulator to support a high speed read out system. 
           [0030]      FIG. 16   a,b,c  is an illustration of the spectral profile associated with a chirped fiber grating and difficulties associated with the spectral edges. 
           [0031]      FIG. 17  is a diagram that illustrates the usage of short marker fiber gratings that are superposed on a chirped fiber grating, to simplify and improve localization of a high speed event. 
           [0032]      FIG. 18  is a graphical illustration of the response of a high speed readout system to a fiber grating sensor assembly similar to that shown in association with  FIG. 17 . 
           [0033]      FIG. 19  is an diagram of a high speed fiber grating sensor system that incorporates short marker fiber gratings superposed on a chirped fiber grating each operating on separate spectral bands. 
           [0034]      FIG. 20  is an illustration of side hole optical fiber and its spectral performance when under pressure. 
           [0035]      FIG. 21  is an illustration of birefringent optical fiber and its spectral performance when under pressure. 
           [0036]      FIG. 22  is a diagram of a system using side hole or birefringent fiber to measure pressure at very high speed. 
           [0037]      FIG. 23  is an illustration of the difference between orienting a chirped fiber grating with its short or long wavelength edge toward an incoming high pressure wave. 
           [0038]      FIG. 24  is a diagram of a high speed fiber grating sensor system designed to support the measurement of two polarization state reflected from a fiber grating sensor assembly so that two environmental parameter that may be pressure and temperature may be measured. 
           [0039]      FIG. 25  is a diagram of a high speed system consisting of multiple fiber lines that contain fiber grating sensor assemblies that may be spatially separated to support measurement of a high speed event. 
           [0040]      FIG. 26  is a diagram of a high speed system consisting of multiple fiber lines that contain fiber grating sensor assemblies that may be spatially separated to support measurement of a high speed event that incorporates a series of spectrally shaped filters. 
           [0041]      FIG. 27  is a diagram of an n port high speed fiber grating sensor system that may be used to support n fiber grating sensor subassemblies that may be spatially separated. 
           [0042]      FIG. 28  is a diagram of an n port high speed fiber grating sensor system that may be used to support n fiber grating sensor subassemblies that may be spatially separated that incorporates spectral filters and optically efficient beam directors that may be 3 port optical circulators. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0043]      FIG. 1  is a diagram of an optical fiber  1  that contains a grating  3  that has a uniform period of index of refraction variation along a length of the core  5  of the optical fiber  1 . When the fiber grating is illuminated with a light source, that may be spectrally broadband or a tunable laser source, the overall spectrum of light reflected from the fiber grating  3  is a relatively narrow spectral band  7 . A much broader spectral reflection may be obtained as shown in  FIG. 2  by using a chirped fiber grating  51  that is written in the core  53  of an optical fiber  55 . In this case the period of index of refraction variations of the chirped fiber grating  51  vary along the length of the core  53  of the optical fiber  55 . When a spectrally broadband light source illuminates this type of chirped fiber grating the reflected spectral profile  57  is much wider. 
         [0044]    Because portions of the fiber grating may effectively spectrally “shade” successive regions the reflection and transmission of a chirped fiber grating may vary and can be designed to be “flat” over a specific spectral range.  FIG. 3  shows the spectrum of the reflection  101  associated with an actual chirped fiber grating of 50 mm length. Note also that the transmission  103  of the same 50 mm long chirped fiber grating has a linear slope. Thus it is important to understand the orientation of a chirped fiber grating in a fiber grating measurement system as this can dramatically alter its spectral properties. 
         [0045]      FIG. 4  illustrates the output spectrum  151  of a typical ASE light source that has been “gain flattened” using an external fiber grating. The spectrum  151  is relatively flat over the region centered about 1550 nm but exhibits significant variations in amplitude at wavelengths shorter than about 1540 nm. This is typical for this type of light source which has the advantage of being very low noise while outputting high optical power levels. The reflection spectrum  101  from the 50 mm chirped fiber grating that is illuminated by this ASE light source is shown for comparison. 
         [0046]    Chirped fiber gratings can be written over long lengths of many cm and in some cases have been written to lengths of over 1 m. To demonstrate the principles of measuring velocity in a blast wave 50 mm and 100 mm chirped fiber gratings were obtained. By inserting the chirped fiber grating into an explosive material that is detonated from one end the blast wave may be monitored as it propagates across the sample destroying the chirped fiber grating and effectively reducing its spectral reflection.  FIG. 5  illustrates this principle. A 100 mm chirped fiber grating is physically cut back in 2 to 3 mm increments through a distance of approximately 49 mm. The spectral reflection band from the 100 mm chirped fiber grating  201  is effectively halved as shown in the spectral reflection band  203 . Thus by monitoring the changes in the spectral reflection band the position along the chirped fiber grating may be determined during a blast wave which in turn can be used with a time base to generate velocity. 
         [0047]    The overall layout of the test system is shown in  FIG. 6 . Here a broad band light source  253  that may be an ASE light source which is a gain flattened 1550 nm erbium fiber light source pumped by a 980 nm laser diode is used to inject the light beam  255  into a beamsplitter  257  that may be a 50/50 coupler. One of the beamsplitter  257  output fiber legs  259  is attached to a chirped fiber grating  261  that is placed in an area generating a high speed energetic pressure wave and the broadband light beam  263  propagates to the chirped fiber grating  261 . When the detonation begins, a portion of the chirped fiber grating  261  is destroyed and the spectral reflectance decreases and a corresponding light beam  265  is reflected. The light beam  265  from the chirped fiber grating  261  is directed back to the beamsplitter  257  and into a second beamsplitter  267  that may be a 50/50 coupler. One output leg  269  of this second beamsplitter  267  is attached to a reference detector  271  that monitors the changes in the spectral reflection via the light beam  273  directly. The second output fiber leg  275  contains a second filter  277  that may be a chirped fiber grating that has a spectral reflection that overlays that of the chirped fiber grating  261  that is placed in the high pressure wave area (that may be generated by a blast wave). The light beam  279  that reflects from the second chirped fiber grating  277  as the light beam  281  is then directed back into the second beamsplitter  267  and a portion of this reflection  281  is then directed as the light beam  283  to a second detector  285  used to monitor this reflected signal. Since the chirped fiber grating  261  used as the sensor and the second filter  277  that may be a chirped fiber grating used as the reflector overlay spectrally light that is associated with the energetic event, that may be a blast wave, that is not in the spectral band of the chirped fiber grating sensor is filtered from the output detector. By comparing the reference and reflected signals on the detectors light induced by the pressure wave that may be a blast wave in the detection band can be monitored. 
         [0048]    A blast test was conducted to verify the performance of the invention. The test involved placement of an optical fiber  301  with a chirped fiber grating  303  with a length of approximately 37 mm into a cylindrical container  305  of nitromethane  307  as shown in  FIG. 7 . One end of the chirped fiber grating was placed at the bottom of the cylinder adjacent to the igniter  309 . A positioning tube  311  held the fiber near the center of the cylinder. 
         [0049]    The test results from the test shot are shown in  FIG. 8 . Both the reference and reflected signal detectors associated with  FIG. 7  showed the chirped fiber grating being destroyed at the same rate. This would indicate that the light associated with the nitromethane blast is not affecting the output reference signal in a significant way. The manufacturer of the chirped fiber grating was originally targeting an overall physical length of 100 mm. However when cut back tests were performed on a second identical “100 mm” chirped fiber grating the spectrum did not change until it was cut back to approximately 14 mm. This indicates that the length associated with the spectral band of the cut back fiber grating spectrum associated chirped fiber grating # 509  was about 36 or 37 mm. This matches up very well with the velocity associated with the pin timing used to support the first blast test. 
         [0050]    During the course of performing these tests it became evident that the chirped fiber grating specified at 100 mm actually had a physical length of approximately 74 mm. This was determined by using two sets of physical “cut back” tests on the 100 mm chirped fiber gratings. This was done by laying out the 100 mm fiber gratings in a straight line and then physically cutting them back by increments of 1 to 2 mm until the spectral band of the chirped fiber grating changed in a measurable manner. The spectrometer used to support these tests was an Ibsen I-MON 400 that can be operated at 200 Hz over the 1520 to 1580 nm spectral band. This was very useful in providing real time feedback during the cut back tests. 
         [0051]    Each of the fiber gratings tested was cut back from the longer wavelength end until a clear transition in response was observed. This allowed an unambiguous starting point for the chirped fiber grating sensor position. Plotting the response via a cut back test also allowed the overall position and effective length of the chirped fiber grating to be established. 
         [0052]    The cut back method, via mechanical or laser trimming can be used to establish the exact position of the fiber grating ends in terms of significant spectral bandwidth change. This information in turn can be used in coordination with the fiber grating manufacturer to optimize the performance of the chirped fiber gratings which in turn will result in improved velocity and position information.  FIG. 9  shows a waterfall plot of the reflected spectrum from a chirped fiber grating that is laser trimmed in length.  FIG. 10  shows actual data obtained from the spectral reflections of the cut back chirped fiber grating to be in good agreement with the numerical integration of the chirped fiber grating spectral bandwidth. 
         [0053]    When a fiber grating is exposed to a pressure it will compress, the overall period of the fiber grating will shorten and the spectrum will shift toward short wavelengths. By setting up a system with a linear filter such as a linear chirped fiber grating filter this shift may be measured and the local pressure inferred. 
         [0054]    Consider the system block diagram shown in  FIG. 11 . In this case a linear filter  351  that may be chirped fiber grating filter is used as a reflector. For simplicity in this diagram the length of a fiber grating  353  with uniform index of refraction variations is assumed to have a short length relative to the leading edge pressure wave  355 . In general this would not be the case and this will be expanded upon in association with  FIG. 12 . Before detonation the fiber grating sensor  353  spectrum is at a nominal wavelength f o . When after detonation the pressure wave  355  passes, the fiber grating  353  is compressed and there is an overall spectral shift toward shorter wavelengths. Because of the slope of the reflective filter  351  this short wavelength shift results in an increase in the amplitude of the light reflected from the reflected filter  351  that in turn can be used to interpret the pressure wave  355  amplitude. When the blast wave passes, the fiber grating is destroyed. The overall reflected amplitude drops to zero when the pressure wave  355  passes if it is energetic enough to destroy the fiber grating  353 . 
         [0055]    A more typical case would involve a fiber grating with a uniform period whose length is long compared to the leading edge pressure segment associated with the pressure or blast wave. This situation is illustrated by  FIG. 12 . The top portion of  FIG. 12  shows a uniform fiber grating  401  in an optical fiber  403  before detonation with an associated reflective spectral profile  405 . After detonation a pressure wave  407  is initiated from the right and the leading edge pressure wave propagates over a portion  409  of the fiber grating  401 . If the assumption is made that the leading edge of the pressure wave  407  is very small and the pressure behind it is uniform then the region of the fiber grating that is under compression will have a smaller period and a portion  411  of the overall spectrum  405  from the fiber grating  401  will be shifted toward shorter wavelengths. If the total spectrum from the fiber grating  401  is measured then the portion  411  of the fiber grating  401  under pressure may be determined by the relative amplitude of the spectral peaks  411  and  413  where  413  has the same spectral position as the spectral peak  405  but lower amplitude. The very fast system associated with  FIG. 11  captures an average spectral shift so information conveyed by this system is a combination of the magnitude of the pressure wave and how much of the fiber grating  401  is under this increased pressure. However a very fast spectrometer could be used to monitor the spectral output of the fiber grating. In this case the detailed information of the position and amplitude of the pressure wave could be captured. 
         [0056]      FIG. 13  shows a pressure wave  451  that may be a blast wave traveling along the length of an optical fiber  453  that contains a series of fiber gratings  455 ,  457 ,  459 ,  461  and  463 . These fiber gratings  455 ,  457 ,  459 ,  461  and  463  may be arranged so that their reflective wavelength spectral bands do not overlap. If the pressure wave  451  is of sufficient strength to destroy the fiber gratings  455 ,  457 ,  459 ,  461  and  463  in sequences while they are all illuminated by a broadband source then the amplitude of the return signal will drop suddenly with their disappearance enabling position markers that may be used to determine the position and velocity of the pressure wave  451 . By varying the amplitude, spacing and wavelength of the fiber gratings  455 ,  457 ,  459 ,  461  and  463  as well as other supporting fiber grating in the optical fiber  453  it would be possible to develop a large set of “markers” to support velocity and position measurements. 
         [0057]    The fiber gratings  455 ,  457 ,  459 ,  461  and  463  however also have the potential to enable detailed measurements of the shape of the pressure wave  453  that may be a blast wave. 
         [0058]      FIG. 14  shows a fiber grating spectral profile  501  prior to detonation and the generation of a pressure or blast wave. Because the pressure or blast wave  503  may not be uniform when it passes over a fiber grating  505  there will be a region  507  of the fiber grating  505  where the fiber grating is compressed in a non-uniform manner. The result is that the original fiber grating spectral profile  501  will be split into two spectral profiles; the spectral profile  509  corresponding to the region where the fiber grating  505  remains uniform as the pressure wave has not reached it and the spectral profile  511  corresponding to the region  507  of the fiber grating  505  that is compressed. The details of the spectral profile  511  can in turn be used to aid in the interpretation of the pressure distribution behind the pressure or blast wave  503 . 
         [0059]    In order to make these spectral profile measurements at high speed a system similar to that associated with  FIG. 15  may be employed. The major elements of this Figure are similar to those described in association with  FIGS. 5 and 8 . A broadband light source  555  is used to illuminate a fiber grating  557  that may be a chirped fiber grating or a series of discrete fiber gratings. When a pressure or blast wave  559  passes through the fiber grating  559  a complex spectral profile associated with compression regions associated with the fiber grating  559  results. The reflective spectrum  561  passes through a set of beamsplitters and is directed as the reflective light beam  563  toward a wavelength division multiplexing (WDM) element  565  that might be a bulk optic grating or series of bulk optic gratings. The WDM element spread the spectrum associated with the light beam  563  across a series of n discrete spatially displaced high speed detectors  567  whose outputs  569  are directed to a data acquisition unit  571  where they can be captured, processed and displayed. A second portion of the reflected light beam  561  is directed through a series of couplers as the light beam  573  which is in turn converted to an electrical signal by the detector  575 . The output from the detector  575  is then directed into a data acquisition unit  577  that may be an oscilloscope to display the time varying amplitude of the output signal associated with the pressure or blast wave  559 . The output  579  from the unit  577  may be used as a trigger or timing signal for the data acquisition unit  571 . 
         [0060]      FIG. 16  is an illustration of how a three port circulator  601  may be used in place of the first beamsplitter associated with  FIGS. 5 ,  8  and  14 . In this case a broadband light source  603  inputs a light beam  605  into the optical fiber  607  that directs it into the circulator  601 . The light beam  605  is then directed into the optical fiber  609  to the test fiber grating  610  that may be a chirped fiber grating or array of fiber gratings. The reflected spectral signature  611  is then directed back to the 3 port circulator  601  and into the optical fiber  613  to the fiber beamsplitter  615  where it is split into the first output light beam  617  that is directed by the optical fiber  619  to the reference detector  621 . The second output light beam  623  is directed into the optical fiber  625  and reflected off the reference reflector  627  that is spectrally matched to the test fiber grating  610  and may be a dielectric reflector or a chirped fiber grating. The reflected light beam  629  is directed back through the beamsplitter  615  and a portion of it is directed as the light beam  631  into the optical fiber  633  to the output detector  635 . The main advantage of using a 3 port circulator is that it increases the overall optical efficiency of the system increasing signal to noise ratio. 
         [0061]    For optimum performance it is highly desirable to be able to very accurately know the physical position of the chirped fiber grating in the optical fiber. This physical position in turn is very important in being able to fully characterize a pressure wave or blast wave passing through the chirped fiber grating sensor. In general it would be highly desirable to have a chirped fiber grating that has a spectral profile  651  with very sharp edges as shown in  FIG. 16   a . Most chirped fiber grating have spectral profiles that are more similar to the spectral profile  653  shown in  FIG. 16   b . Here the edges of the spectral profile  653  are much less well defined as illustrated by the transition zones  655  and  657  in  FIG. 16   b  and the zones  659  and  661  in  FIG. 16   c . When a pressure or blast wave passes these transition zone regions the measurements have more noise and the errors in these regions are larger. One solution is to physically trim the fiber grating to eliminate the transition zone on the edge of the chirped fiber grating facing the oncoming pressure or blast wave. This can be done by cutting or trimming the chirped fiber grating with mechanical cutters or laser trimming. This results in a much sharper spectral profile edge as shown in association with  FIGS. 5 and 9 . The cutting or laser trimming approach is most practical for the edge facing the blast wave. To cut or trim the other edge of the chirped fiber grating to attain a similar result it would then be necessary to cut or trim and then splice the other end. The splicing procedure could damage the spectral profile and produce a mechanically weak junction. A method to avoid this problem is shown in  FIG. 17 . Here a chirped fiber grating  701  is written into an optical fiber  703 . At each physical end of the chirped fiber grating  701  short length fiber gratings  705  and  707  are written with a period that is distinct from those associated with the chirped fiber grating  701 . The spectral profile  709  corresponding to the chirped fiber grating  701  is broad and has transition zones  711  and  713  that have higher noise. The profiles  715  and  717  of the shorter fiber gratings  705  and  707  respectively provide clear measurement points for the start and end of the chirped fiber grating. 
         [0062]      FIG. 18  shows the amplitude of the reflected signal as a pressure or blast wave propagates along the length of the optical fiber  703  destroying the fiber gratings  707 ,  701  and  705 . Before detonation the reflected signal level  751  has constant amplitude. When the short fiber grating  707  is destroyed the amplitude drops rapidly in the region  753 . As the pressure or blast wave propagates through the chirped fiber grating  701  destroying it as is propagates through the amplitude decreases with a fairly linear slope in region  755 . When it reaches the second short fiber grating  705  there again is a sharp drop in amplitude in the region  757 . After the event is over the amplitude of the output signal again is constant in the region  759 . 
         [0063]    In general n “marker” fiber gratings could be used across a chirped fiber grating to determine distinct points at which a pressure or blast wave crossed. It is also not necessary for the “marker” or chirped fiber gratings to use the same light source or the same wavelength band.  FIG. 19  illustrates a configuration where the “marker” fiber gratings are supported by a separate light source and detection system. A light source  801  that may be a broadband light source at 1550 nm is used to launch a light beam  803  into the optical fiber  805 . A second light source  807  operates on a distinct wavelength band from light source  803  and may be broadband and operating at 1300 nm launches a light beam  809  into the optical fiber  811 . The light beam  803  cross couples across the wavelength division multiplexing (WDM) element  813  and combines with the light beam  811  to form a new light beam  815  that is directed into the optical fiber  817 . A portion of the light beam  815  passes through the beam directing element  815  that may be a broadband fiber beamsplitter or a broadband 3 port circulator and continues on as the light beam  821  into the optical fiber  823  that contains a chirped fiber grating  825  that may be centered in the 1550 nm band and a series of “marker” short fiber gratings  827 ,  829 ,  831  and  833 . The reflected light beam  835  then is directed back by the beam direction element  819  to the optical fiber  837  that is attached to the WDM element  839 . A portion  841  of the light beam  835  that may be at 1550 nm is then directed to the optical fiber  843  to the optical detector  845  that is used to monitor the chirped fiber grating  825 . A second light beam  847  that may be at 1300 nm is directed into the optical fiber  849  and the output optical detector  851  that is used to monitor the output of the “marker” fiber gratings  827 ,  829 ,  831  and  833 . In this way the “marker” and chirped fiber grating reflections may be monitored independently and in combination to improve localization and characterization of a pressure or blast wave. 
         [0064]    In addition to pressure, velocity and position another important parameter to be monitored during passage of a pressure or blast wave is the local temperature.  FIG. 20  illustrates the cross section of a sidehole optical fiber  901 . The core  903  of this type of optical fiber is surrounded by two air holes  905 . A fiber grating written onto this type of optical fiber generates a single peak spectral profile  907  when the fiber core  903  has low birefringence. If the section of sidehole optical fiber with the fiber grating is spliced to ordinary single mode optical fiber capping the air holes  905  and pressure is applied the birefringence of the core changes. The result is a dual spectral peak profile  909  where the peak to peak spectral split is proportional to pressure and the overall spectral position depends on temperature (and axial strain if it is present). By isolating the optical fiber  901  from axial strain pressure and temperature may be monitored simultaneously. An alternative approach to measuring pressure and temperature simultaneously with a fiber grating involves writing the fiber grating onto birefringent optical fiber  951  that may be commercially available polarization preserving optical fiber shown in  FIG. 21 . The core  953  of the birefringent optical fiber  951  may have stress inducing elements  955  that induce a differential stress across the core  953 . The result is that in the absence of pressure the spectral profile  957  of the fiber grating has a dual peak structure. When pressure is applied the relative birefringence will increase the separation of the peaks will change as in the spectral profile  959 . The peak to peak separation and overall spectral position of the spectral peaks may be used to measure pressure and temperature in the absence of axial strain. 
         [0065]    If a very fast spectral read out system may be used such as that illustrated by  FIG. 15  then the spectral profiles of the fiber gratings written into sidehole or birefringent optical fibers and described in association with  FIGS. 20 and 21  may be used to measure pressure and temperature. There may however be cases where the events are very fast and a single point detector system can be constructed to support measurements of these extremely fast events. In this case the system shown in  FIG. 22  may be used. A light source  1001  that may be broadband is used to couple a light beam  1003  into an optical fiber  1005 . The light beam  1003  is directed to a beam directing element  1007  that may be a 3 port optical circulator or a fiber beamsplitter and in turn is directed to the optical fiber  1009  that contains sections of sidehole optical fiber  1111  and  1113  that are capped with fusion splices to conventional single mode fiber on both ends and have fiber gratings  1115  and  1117  written onto them. The fiber gratings are written at the same wavelength, with the same spectral profile and designed to have high reflectivity that may be higher than 50%. Before pressure is applied to the optical fiber  1009  by a pressure or blast wave  1119  the first fiber grating  1115  blocks a portion of the light beam  1003  from reaching the fiber grating  1117 . When the pressure or blast wave  1119  reaches the fiber grating  1117  it causes the spectral profile of the fiber grating  1117  to split and shift so that it moves out the spectral profile “shadow” caused by the fiber grating  1115  and the net reflective signal  1121  from the light beam  1003  increases until the fiber grating  1117  is destroyed. If the spectral profile of the fiber grating  1115  is designed to completely “shadow” the spectral profile of the fiber grating  1117  then these reflective amplitude transitions may be sharply defined. The reflected light beam  1121  returns to the beam directing element  1007 , that may be a 3 port circulator, and is directed to the optical fiber  1123  and the output detector  1125 . 
         [0066]    When a pressure or blast wave encounters the end of a chirped fiber grating there may be a portion of the fiber grating subject to compression before it is destroyed by passage of the wave if it is of sufficiently high amplitude.  FIG. 23  illustrates that the spectral shape changes associated with the chirped fiber grating will be different depending on whether the short or long wavelength end is directed toward the incoming direction of the blast wave.  FIG. 23   a  shows the spectral profile  1051  with the long wavelength edge  1053  corresponding to the end nearest the blast wave  1055 . When the blast wave  1055  encounters the long wavelength edge  1053  of the chirped fiber grating it is compressed and shifts toward shorter wavelengths which if the chirped fiber grating is designed to have moderate reflectivity that may be between 20 and 80% then the spectral profile  1051  near the long wavelength edge will have a higher reflectivity region  1057  or bump whose amplitude will be proportional to the pressure rise. In  FIG. 23   b  the spectral profile  1059  of a chirped fiber grating has the short wavelength edge  1061  directed toward the pressure or blast wave  1055 . In this case when the blast wave encounters the short wavelength edge it compresses a region of the chirped fiber grating near the edge  1061  and drives the spectral profile away from the main profile resulting in a dip  1063  again providing a means to measure pressure. 
         [0067]    While the shifts of due to the pressure or blast wave are likely to be principally pressure they may also be due in part to temperature. By using polarization maintaining fiber throughout a system such as that shown on  FIG. 24  and a fast detector array to measure the entire spectral profile (see  FIG. 15 ) both parameters could be measured by extracting two distinct signatures corresponding to each of the polarization states. In  FIG. 24  a broadband light source  1101  is used to launch a light beam  1103  into a polarization preserving fiber lead  1105 . The light beam enters a polarization preserving beam guiding element  1107  that may be a polarization preserving beamsplitter or polarization preserving 3 port circulator. A portion of the light beam  1103  is directed into the polarization preserving fiber lead  1109  as the light beam  1111  and directed to the fiber grating sensor assembly  1113  that may be a chirped fiber grating, an array of short length fiber gratings or a combination of both. The reflected light beam  1115  from the fiber grating sensor assembly  1113  is directed back into the beam guiding element  1107  and a portion of the light beam  1115  is directed into the polarization preserving fiber leg  1117  as the light beam  1119 . The polarizing coupler  1121  then splits out the two orthogonal polarization states of the light beam  1119  into the light beam  1123  that is launched into the optical fiber lead  1125  and directed toward the first output detector  1127 , and the light beam  1129  that is coupled into the optical fiber  1131  and directed into the second output detector  1133 . 
         [0068]    For lowest possible cost and the highest degree of flexibility it is desirable to produce in large quantities fiber grating sensors that have similar characteristics. As an example a fiber grating fabrication system might be set up with a set of phase masks to produce 100 mm long chirped fiber gratings centered about 1550 nm with out of chirped fiber grating spectral band short length marker fiber gratings incorporated into the chirp fiber grating physical structure. This could be done with a single phase mask and a production line set up to write fiber gratings of this type onto 1 m lengths of optical fiber. A single fiber grating system of this type could be used to monitor a 100 mm length. But there are cases where it is desirable to be able to measure highly energetic pressure waves over longer lengths over distances of 200 mm, 400 mm or longer. It is possible to make longer chirped fiber gratings but the costs go up with longer lengths and there are costs associated with each new fabrication set up. Instead of this more costly procedure standard measurement systems can be set up to support 2, 4, 8 and higher numbers of the shorter fiber gratings using multiple fibers that offer the user flexibility in terms of the length of the region to be measured as well as the flexibility to make measurement of different regions of a material.  FIG. 25  shows a system configured to support four single fibers each of which may have an identical fiber grating sensor assembly. A broadband light source  1151  launches a beam of light  1153  into an optical fiber  1155  that is attached to a first fiber coupler  1157  that splits the light beam  1153  into the light beams  1159  and  1161  that are in turn directed via the optical fibers  1163  and  1165  to the second fiber coupler  1167  and the third fiber coupler  1169 . The fiber coupler  1167  splits the light beam  1159  into the light beams  1171  and  1173 . The light beam  1171  propagates along the optical fiber  1175  to a fourth coupler  1177  and a portion of the light beam  1171  continues onward as the light beam  1179  via the optical fiber  1181 . The light beam  1179  then passes through an optical fiber connection  1183  that may be a physical optical fiber connector or a fusion splice, and onto a fiber grating sensor assembly  1185 . The reflected optical light beam  1187  from the fiber grating sensor assembly is then directed back through the connection  1183  to the coupler  1177  and a portion of it becomes light beam  1189  that is directed via the optical fiber  1191  to the output detector  1193  where it is converted to an electrical signal. Similarly the light beam  1173  is directed though the coupler  1195  and a portion of it becomes light beam  1197  that propagates though the connection  1199  and onto the fiber grating sensor assembly  1201  and reflects back as the light beam  1203  past the connection  1199  to the coupler  1195  where a portion of it is split into the light beam  1205  that falls onto the output detector  1207 . The third fiber grating assembly  1209  and the fourth fiber grating assembly  1211  are monitored in a similar manner by light beams reflecting off them and a portion being directed toward the output detectors  1213  and  1215  respectively. 
         [0069]    The system shown in  FIG. 26  is an extension of that shown and described in association with  FIG. 25 . In addition to the elements that are common to both systems, the system of  FIG. 26  has the spectral filter element  1251  in front of the output detector  1193 , this could be a chirped fiber grating filter with a sloping spectral profile and a second detector  1253  to monitor the reflection of the light off the filter  1251 . In a similar manner the fiber grating sensor assembly  1201  is monitored by the output detector  1207  in combination with the filter  1255  and the second detector  1257 . Similar arrangements are made to support monitoring the fiber grating sensor assemblies  1209  and  1211 . The motivation for the filter combinations is to filter out any stray light associated with energetic events and to provide options for different shaped filters that can be used to provide details at high speed of how the spectral profiles of the fiber grating sensor assemblies are changing allowing such parameters as pressure changes to be inferred. In addition the three detector positions  1261 ,  1263  and  1265  can be used to monitor the amplitude of the return signals from the fiber grating sensor assemblies  1185 ,  1201 ,  1209  and  1211 , either in pairs in the case of  1261  and  1263  or all four in the case of  1265 . These detector positions  1261 ,  1263  and  1265  can be supported by single detector elements or pairs of detectors with filter assemblies similar to those described earlier. Which combination of detectors is most desirable depends on the specifics of the test parameters to be measured, the speed and accuracy of the electronic support equipment used to support the test and the physical displacement of the fiber grating sensor assemblies. Both the systems shown in  FIGS. 25 and 26  offer the end user considerable flexibility in supporting a wide variety of measurements using a “standard” fiber grating sensor assembly. 
         [0070]    The systems described in association with  FIGS. 25 and 26  can be extended to n fiber lines.  FIG. 27  shows a light source  1301  that couples a beam of light  1303  into an optical fiber end  1305  which directs the light beam  1303  to the n port coupler  1307 . The light beam  1303  is divided by the n port coupler into n light beams the first of which light beam  1309  is directed into the optical fiber  1311  where it is directed to the fiber coupler  1313 . A portion of the light beam  1309  is then split by the fiber coupler  1313  into the light beam  1315  that is propagates down the optical fiber  1317  to the fiber grating sensor assembly  1319 . A portion of the light beam  1315  is then directed back to the fiber coupler  1313  and a portion of it is directed by the fiber coupler  1313  into the optical fiber  1321  as the light beam  1323  which falls onto the output detector  1325 . In a similar manner the light beam  1327  is directed into the second fiber line  1329  to the coupler  1331  that directs a portion of the light beam  1327  to the fiber grating assembly  1333  where whose reflected signal is directed as a new beam to the output detector  1335  by the fiber coupler  1331 . Each of the n lines associated with this system have similar elements and operation. If the fiber grating assemblies associated with the system of  FIG. 27  are spaced sufficiently and the high speed event is sufficiently energetic so that the fiber grating assemblies are destroyed then a single optical fiber output  1337  from the n port coupler  1307  may be used to direct a light beam  1339  from the n fiber grating sensor assemblies to an output optical detector  1341  that can be used to support monitoring the high speed event. 
         [0071]      FIG. 28  shows another system that extends the capabilities of that associated with  FIG. 27 . Here the light source  1401  couples a beam of light  1403  into an optical fiber  1405  that serves as the input port to the n output port coupler  1407 . A portion of the light beam  1403  is split into the light beam  1409  that is coupled into the optical fiber  1411  and propagates past the fiber coupler  1413  and a portion of the light beam  1409  is split into the light beam  1415  that is coupled into the optical fiber  1417  that contains the fiber grating sensor assembly  1419 . A portion of the light beam  1415  reflects off the fiber grating sensor assembly  1419  as the light beam  1421  and enters the fiber coupler  1413 . A portion of light beam  1421  is then split as light beam  1423  to the beam directing element  1425  that may be a 3 port optical circulator. The light beam  1423  then passes the filter  1427  and a portion of it passes through to the output detector  1429 . Another portion reflects from the filter as light beam  1431  and directed by the beam directing element  1425  to the output detector  1433 . A second light beam  1435  enters a second optical fiber  1435  that acts as the second output port of the n port coupler  1407  and enters a fiber subassembly consisting of the optical fiber  1437 , the fiber coupler  1439 , fiber grating sensor assembly  1441 , beam directing element  1443 , filter  1445  and output detectors  1447  and  1449  that behave in a manner similar to that associated with the first fiber line. Similarly n fiber sensing and detection lines can be supported. The output fiber line  1451  of the n port coupler  1407  can be used to direct the light beam  1453  that captures signals from each of the n fiber grating assemblies which in turn in directed to the output detector  1455 . 
         [0072]    Thus there has been shown and described a novel system for measuring high intensity pressure or blast waves or other environmental parameters including those that destroy and optical fiber and fulfills all the objectives and advantages sought therefore. Many change, modifications, variations and applications of the subject invention will become apparent to those skilled in the art after consideration of the specification and 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 to the claims that follow:

Technology Category: g