Abstract:
A fiber grating sensor system is used to measure key parameters that include pressure, strain and temperature at specific locations and at high speed. The system relies on spectral properties associated with the fiber grating sensors, the light source and the optical detection system to provide these capabilities. The system has been successfully applied to measurement of pressures up to 1,200,000 psi and by increasing the spectral width of the light source extensions of pressure measurements to 4,000,000 psi and higher are possible. Temperature change measurements have been made of 400 degrees C. over a period of 25 micro-seconds limited by the physical response of the fiber sensors and the output detector bandwidth both of which can be greatly improved by reducing fiber sizes and with improved detectors. Novel methods have been devised to lower cost and enable measurements with spatial location, speed and accuracy that have been very difficult or not yet achieved.

Description:
The United States has rights in this invention pursuant to Contract Numbers W31P4Q-10-C-1087 and W31P4Q-11-C-0209 awarded by the US Army. 
    
    
     This application claims the benefit of U.S. Provisional Patent Application No. 61/628,106 by Eric Udd, entitled “Fiber grating sensor system for measuring key parameters during high speed events”, which was filed on Oct. 24, 2011. 
     BACKGROUND OF THE INVENTION 
     This disclosure describes means to measure the location, velocity, pressure, strain and temperature associated with high speed events. It also describes embodiments that can be used to support these measurements by using optimized optical fiber grating sensors and employing 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 U.S. patent application Ser. No. 12/217,666 which were abandoned teach a fiber grating sensor system for detection, localization and characterization of high speed pressure waves. The teachings associated in these prior art patents and patent applications are deemed to be fully incorporated into this disclosure. The present invention extends the capabilities of the high speed system with methods based on filtering techniques and improvements to sensor configurations that lowers the cost of sensors, improves the spatial resolution of the system with respect to pressure and temperature measurements and allows extensions of performance over wider ranges of pressure and temperature. 
     BRIEF DESCRIPTION OF THE PRESENT INVENTION 
     In the present invention a high speed fiber grating sensor system is described that is capable of measuring the position, velocity, pressure and temperate of a high speed event. The invention is particularly directed toward the measurement of localized pressure and temperature. This can include very high pressures of 1,000,000 to 4,000,000 psi and temperature ranges from absolute zero to the melting temperature of the material used to support the fiber grating. For quartz based systems this can be in excess of 1000 C and for sapphire in excess of 1600 C. 
     The invention consists of a light source that illuminates one or more specialized fiber gratings, in one or more fiber lines, that are placed and oriented along a path associated with the high speed event that is to be measured. The light source is designed with specific spectral profiles with edges and or peaks to provide wavelength measurement points. The reflected signals from the fiber grating sensors encountering the high speed event are then directed toward one of more optical detectors that may be wavelength dependent and are used to localize and characterize pressure, strain and temperature. For some embodiments additional filters with wavelength markers may be placed in front of the output optical detectors. The fiber grating sensors may be designed with specific wavelength markers. As an example these may be regions of low spectral reflectivity in chirped fiber gratings used to identify a specific spatial location. 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. 
     Therefore it is an object of the invention to provide a very high speed system that is capable of measuring pressure at a specific spatial location. 
     Another objective of the invention is to measure strain at a specific spatial location. 
     Another objective of the invention is to measure temperature at a specific spatial location. 
     Another objective is to measure pressure at a specific time in a high speed event. 
     Another objective is to measure strain at a specific time in a high speed event. 
     Another objective is to measure temperature at a specific time in a high speed event. 
     Another objective is to measure the localized pressure and temperature simultaneously in a high speed event. 
     Another objective is to measure the localized strain and temperature simultaneously in a high speed event. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a diagram of a basic high speed fiber grating sensor system. 
         FIG. 2  is a diagram of the intensity of a broadband light source as a function of wavelength. 
         FIG. 2 a    is a graph of the intensity of a broad band fiber light source operating in the telecommunications C-band. 
         FIG. 2 b    is a diagram of wavelength multiplexed light sources. 
         FIG. 2 c    is a graph of the spectra of multiplexed broad band light sources. 
         FIG. 2 d    is a graph of the spectra of multiplexed narrow and broad band light sources. 
         FIG. 3  is a diagram of the reflection spectrum of a chirped fiber grating fabricated using masking techniques so that certain defined spectral bands have lower reflectivity. 
         FIG. 4  is a graph of the spectral reflectivity of a 135 mm long chirped fiber grating with low reflectivity bands to use to spatially locate pressure, strain and temperature measurements. 
         FIG. 5  is a photo of the cross section of a test pipe used to demonstrate pressure measurements capabilities during burn, deflagration and detonation of highly energetic material placed within the pipe. 
         FIG. 6  is graph of results associated with the pipe of  FIG. 5  and the 135 mm chirped fiber grating associated with  FIG. 4  placed within it. 
         FIG. 7  is an end on view of an optical fiber with side holes along its length. 
         FIG. 8  is an illustration in the change of the spectrum of a fiber grating written into side hole optical fiber with pressure. 
         FIG. 9  is an illustration of an end view of birefringent optical fiber. 
         FIG. 10  is an illustration of the change in spectrum of a fiber grating written into birefringent optical fiber when pressure is applied to it. 
         FIG. 11  is a graph of the spectrum of a fiber grating written into side hole optical fiber at atmospheric pressure. 
         FIG. 12  is a photo of a cross sectioned test pipe that was loaded with highly energetic material to evaluate the pressure and temperature response of a fiber grating written into side hole optical. 
         FIG. 13  is the response of the spectrum of the side hole fiber grating to high speed pressure and temperature changes in the test pipe of  FIG. 12 . 
         FIG. 14  is a block diagram of a system to measure high speed events with an optical fiber grating filter with low reflectivity spectral regions placed in front of the output detector. 
         FIG. 15  is a block diagram of a system to measure high speed events with an optical fiber grating filter with low reflectivity spectral regions placed at the output of the broadband light source. 
         FIG. 16  is a block diagram of the output detection portion of the system with a wavelength division multiplexing element splitting out portions of the spectral signal to a series of high speed optical detectors. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a diagram of a fiber grating sensor system for measuring key parameters during high speed events. That may include velocity, position, pressure, strain and temperature associated with burn deflagration and detonation of highly energetic material. A light source  1  that may be a spectrally broad band fiber or super radiant light source couples light into the fiber end  3  and the light beam  5  propagates through the input leg  7  of an optical coupler  9  which may be a three port optical circulator or a fiber beamsplitter. The light beam  5  is then directed to the output fiber leg  11  of the optical coupler  9  and directed into one or more fiber grating sensors  13 . The fiber grating sensor or sensors  13  may be uniform or chirped fiber gratings. A portion of the light beam  5  is reflected back to the optical coupler  9  as the light beam  15 . The spectral content of the light beam  15  depends primarily upon the spectral content of the illuminating light source  1  and the fiber grating sensor or sensors  13 . Secondary effects may occur due to spectral attenuation associated with the optical fiber leads and components associated with the optical coupler  9 . The return light beam  15  is then directed by the optical coupler  9  to the output lead  17  which couples the light beam to an output coupler  19  that may be an optical switch or a fixed fiber coupler. One portion of the light beam  15  is directed as the light beam  21  through an optical fiber  23  to an optical spectrometer  25  that is used to measure the spectral content of the light beam  21  prior to the high speed event. The second portion of the light beam  15  is directed as the light beam  27  through the optical fiber  29  to a high speed detector  31 . The high speed detector  31  may consist of a single optical detector, an optical detector with a wavelength selective filter or a series of optical detectors with a series of optical filters associated with each detector. The action of a high speed environmental effect  33  is that it causes the fiber grating sensor or sensors  13  to change spectral characteristics modifying the properties of the reflected light beam  15  that in turn can result in changes to the optical signal  27  that is converted by the optical detector  31  into electrical signals  35  that are electrically connected via the conductive path  37  to a digital recording instrument  39  that can be a high speed digital storage oscilloscope. The variations that can occur due the environmental effect  33  include changes in the physical length of the fiber grating sensor or sensors  13  due to destruction of part of their length, changes in spectral reflection due to compression caused by pressure moving part of the fiber grating spectrum toward shorter wavelengths, changes in temperature moving part of the spectrum to longer (higher temperature case) or shorter (lower temperature case), or longitudinal or transverse strain. Details on these spectral shifts associated with fiber gratings can be found in E. Udd,  Fiber Grating Sensors , in E. Udd and William B. Spillman,  Fiber Optic Sensors: an Introduction for Engineers and Scientists,  2 nd  Edition, Wiley, 2011. 
       FIG. 2  is a graph showing the spectral output  51  of a broad band light source as a function of relative intensity versus wavelength. The spectrum which is typical of a broadband fiber light source operated in the telecommunication C band has a “forward” slope  53  in the region of 1560 nm, a “backward” slope  55  at about 1525 nm, a sharp peak  57  at about 1528 nm and gradual slope  59  between 1530 to nearly 1560 nm. The features of the spectral shape of the light source  1  can be used to measure key parameters at specific spatial positions associated with the fiber grating sensor or sensors  13 . Pressure and temperature measurements at specific spatial locations are examples of key parameters for many high speed applications. By modifying or adjusting these spectral characteristics performance of the system described in association with  FIG. 1  can be optimized. 
       FIG. 2 a    is a graph of an actual fiber light source with features similar to those described in association with  FIG. 2 .  FIG. 2 b    shows a series of light sources  71 ,  73  and  75  that generate light beams  77 ,  79  and  81  that are coupled into the wavelength combiner  83  (that might be a series of fiber beamsplitters or wavelength division multiplexing elements) and used to couple the combined light beam  85  into the source output fiber  87 . This effective light source could be used to replace the light source  1  in  FIG. 1 . It can be used to enhance performance by extending the effective bandwidth of the light source  1  which in turn results in greater dynamic range for pressure, strain and temperature measurements. The approach can also be used to provide more spectral edges and peaks that allow more measurements to be made at each spectral location. Additional details and descriptions on these capabilities will be made in the following paragraphs.  FIG. 2 c    shows in graphical form a series of multiplexed broad band light source profiles that enhance the overall spectral with of the light source and provide more spectral edges and peaks.  FIG. 2 d    shows in graphical form the output spectrum of narrow band light sources  97 ,  98  and  99  that have been multiplexed in combination with a broad band light source  92 . The purpose  FIGS. 2 b  to 2 d    is to illustrate some of the methods that can be used to modify the spectral shape of the light source  1 . It is also possible to use filters placed in front of the light source to modify the spectral shape of the light source and this will be described later. 
       FIG. 3  shows the spectral content  101  of a chirped fiber grating sensor  13  plotted on a graph of wavelength versus reflected intensity. The spectral content  101  has been modified to optimize for measurements of key parameters. The chirped fiber grating  13  has a specific spatial length  103  and at spatial intervals  105 ,  107 ,  109 ,  111 ,  113  and  115  the reflectivity of the fiber grating has been lowered. These intervals  105 ,  107 ,  109 ,  111 ,  113  and  115  may be uniform or vary in length. The number of intervals chosen determines the number of spatial positions that can be used to measure key parameters. The spacing of the intervals determines the physical location where measurements are to be made. 
     As a specific experimental example consider the chirped fiber grating sensor  13  profile  151  of  FIG. 4 . The chirped fiber grating associated with  FIG. 4  has a length of 135 mm and spans the spectrum from approximately 1532 to 1562 nm for an effective chirped rate of 2.2 nm per cm. Two low reflectivity spectral regions are defined at 1552 and 1559 nm by placing a metal mask (that in this case is approximately 5 mm) in front of the laser beam illuminating a phase mask during the fabrication of the fiber grating sensor. The metal mask can be located at any position along the phase mask, defining a spatial position identified by a drop of the spectral reflectivity at that point. The 135 mm chirped fiber grating sensor  13  associated with  FIG. 4  was then placed into an aluminum tube  201  shown in  FIG. 5 . The pipe  201  which is shown in cross section after test in  FIG. 5  is approximately 6.35 cm in diameter with a center hole  203  of 6.3 mm. The chirped fiber grating sensor  13  was then inserted into the pipe  201  and a read out system similar to that associated with  FIG. 1  connected. The center hole  203  of the pipe  201  was then loaded with highly energetic material and detonated by igniting end  205  of the pipe. The region  207  of the pipe  201  is the area associated with burn of the energetic material in the pipe. The region  209  is associated with deflagration which is the transition region from burn to deflagration. The region  211  is associated with full detonation of the energetic material. The expansion of the pipe center hole  201  enables the pressure to be measured in each region. 
       FIG. 6  shows the output of the 135 mm chirped fiber grating  13  associated with  FIG. 4  when illuminated by a light source  1  with spectral characteristics similar to those illustrated by  FIG. 2 . During burn pressure rises and the entire chirped fiber grating spectrum  151  shifts toward shorter wavelengths. Because this is the sloped region  59  of the light source  1  the result is an overall spectral shift during burn that corresponds to a specific pressure rise over the time interval  251  of about 270,000 psi. During the second time interval  253  deflagration occurs and the passage of the peaks that originally were at 1552 and 1559 nm move over the peak  57  of the light source  1  at pressures that correspond to approximately 920,000 and 1,190,000 psi respectively. The sharp dips in the amplitude of the output detector  31  are very clear as the spectral region corresponding to these spatial locations are forced by pressure over the 1528 inn spectral peak  57 . The measured pressures at these locations correlate closely with pressure calculations associated with the expansion of the center hole  203  of the aluminum pipe  201 , 
     As another illustration of using the special spectral shape of the light source  1  to perform measurements of key parameters, consider the side hole optical fiber  301  whose cross section is shown in  FIG. 7 . It consists of an optical fiber  302  with a light guiding core  303  and two or more air holes  305  and  307  placed about the core  303 . In the case of  FIG. 7  there are two side holes  303  and  307  placed on either side of the optical core  303 . When a fiber grating is written onto side hole optical fiber  301 ; a single uniform fiber grating spectra  351  shown in  FIG. 8  results. When sufficient pressure is applied the single peak spectrum  351  splits into the two spectral peaks  353  and  355 . The peak to peak separation enables a measurement of pressure only and the overall spectral shift provides a measure of temperature (see E. Udd,  Fiber Grating Sensors , in E. Udd and William B. Spillman,  Fiber Optic Sensors: an Introduction for Engineers and Scientists,  2 nd  Edition, Wiley, 2011). 
     Another type of optical fiber grating sensor  13  that may be used to produce dual spectral peaks is birefringent optical fiber. This type of optical fiber is available commercially in many forms as polarization maintaining optical fiber.  FIG. 9  illustrates a cross section of birefringent optical fiber  401 . That consists of an optical fiber  403  with an optical core  405  and two “side pits” of softer glass material  407  and  409 . This type of geometry for polarization preserving optical fiber is offered by Fibercore commercially and other types of polarization preserving fiber using stress rods and elliptically clad optical fibers are also offered. Their common feature is that across the optical core there are two distinct effective indices of refraction along transverse, orthogonal axes. When a fiber grating is written onto this type of optical fiber two spectral peaks  451  and  453  are created. 
     Pressure changes cause the peaks to shift further apart or together. This is shown in  FIG. 10 . 
       FIG. 11  is a graph of the spectra of a 6 mm uniform fiber grating sensor  13  written onto single mode side hole optical fiber with an overall diameter of 125 microns and side holes about the core of approximately 33 microns in diameter. The side hole fiber grating sensor  13  was then placed into a pipe shown in  FIG. 12  which was loaded with energetic material and cross sectioned after ignition. Unlike the pipe shown in  FIG. 5  detonation did not take place. Instead burn occurred and pressure and temperature fluctuated in the pipe during the test.  FIG. 13  shows the output on the optical detector  31  during the test. When ignition occurs; the single peak splits into two and the total power on the detector rapidly rises as overall reflectivity of the fiber grating sensor  13  rises. The peaks are forced by increasing pressure toward shorter wavelengths and fall over the spectral edge  55  of the light source  1  resulting in a drop of the optical signal to zero. As the pipe continues to heat up first one peak is forced back toward longer wavelengths and then the second peak is forced over and the total power is again maximized before pressure eventually again dominates and forces both peaks over the spectral edge  55 . In this way the effective change in temperature over very short time intervals can be measured. 
     To obtain still higher accuracy of the system one or more additional spectral filters could be used to shape the spectrum of the broad band light source  1  or modify the spectral characteristics of the optical detector  31 .  FIG. 14  shows the case where a spectral filter  601  that might be a fiber grating or a multi-channel wavelength division multiplexing device is used to provide one or more spectral filter edges.  FIG. 15  shows the case where an optical filter  651  is used in conjunction with the light source to provide additional spectral filter edges. Again a fiber grating that might be a chirped fiber grating with variable reflection such as that illustrated by  FIG. 3  might be used. 
     The optical detector  31  of  FIG. 1  can consist of wavelength division multiplexed detectors designed to measure changes in a particular spectral region at high speed.  FIG. 16  shows the input optical fiber  29  carrying the spectral signal  27  from the fiber grating sensors  13 . A wavelength division multiplexing device  501  that may consists of a series of dielectric filters, bulk grating or other dispersive elements separates portions of the optical spectrum associated with the light beam  27  into the light beams  503 ,  505 ,  507  and  509  that are directed via optical fibers or free space imaging to the high speed optical output detectors  511 ,  513 ,  515  and  517 . The electrical outputs from these detectors  519 ,  521 ,  523  and  525  are then directed into the output electrical cable  37  which in turn connects to the digital output device  39  that may be a multichannel digital oscilloscope. 
     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: