Abstract:
Methods and apparatuses that sense physical parameters, such as pressure and strain, using optical waveguide sensors are described. A light source emits light at a predetermined wavelength along an optical waveguide having a fiber Bragg grating optical sensing element. That sensing element reflects light in accord with a sloped shape function of reflected light amplitude verses wavelength. A receiver converts the reflected light into electrical signals and an analyzer then determines a physical parameter based on changes of amplitude of the reflected light. The analyzer also maintains the wavelength of the light such that the wavelength corresponds to a slope wavelength of the shape function.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention generally relate to optical waveguide sensors, and more particularly to a fiber Bragg grating optical waveguide sensors that dynamically senses strain induced by a stimuli acting upon a transduction mechanism. 
   2. Description of the Related Art 
   A fiber Bragg grating (FBG) is an optical element that is formed by a photo-induced periodic modulation of the refractive index of an optical waveguide&#39;s core. An FBG element is highly reflective to light having wavelengths within a narrow bandwidth that is centered at a wavelength that is referred to as the Bragg wavelength. Other wavelengths pass through the FBG without reflection. The Bragg wavelength itself is dependent on physical parameters, such as temperature and strain, that impact on the refractive index. Therefore, FBG elements can be used as sensors to measure such parameters. After proper calibration, the Bragg wavelength acts is an absolute measure of the physical parameters. 
   One way of using fiber Bragg grating elements as sensors is to apply strain from an elastic structure (e.g., a diaphragm, bellows, etc.) to a fiber Bragg grating element. For example, U.S. Pat. No. 6,016,702, issued Jan. 25, 2000, entitled “High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments” by inventor Robert J. Maron discloses an optical waveguide sensor in which a compressible bellows is attached to an optical waveguide at one location while a rigid structure is attached at another. A fiber Bragg grating (FBG) is embedded within the optical waveguide between the compressible bellows and the rigid structure. When an external pressure change compresses the bellows the tension on the fiber Bragg grating is changed, which changes the Bragg wavelength. 
   Another example of using fiber Bragg grating elements as pressure sensors is presented in U.S. Pat. No. 6,422,084, issued Jul. 23, 2002, entitled “Bragg Grating Pressure Sensor” by Fernald, et al. That patent discloses optical waveguide sensors in which external pressure longitudinally compresses an optical waveguide having one or more fiber Bragg grating. The optical waveguide can be formed into a “dog bone” shape that includes a fiber Bragg grating and that can be formed under tension or compression to tailor the pressure sensing characteristics of the fiber Bragg grating. Another fiber Bragg grating outside of the narrow portion of the dog bone can provide for temperature compensation. 
   While the foregoing pressure sensing techniques are beneficial, those techniques may not be suitable for all applications. Therefore, fiber Bragg grating techniques suitable for dynamically sensing varying parameters such as pressure and strain would be useful. Also useful would be fiber Bragg grating techniques that provide for both static and dynamic measurements of parameters. 
   SUMMARY OF THE INVENTION 
   Embodiment of the present invention generally provides for optical waveguide measurement techniques that are suitable for sensing dynamically varying physical parameters such as pressure and strain. Furthermore, embodiments of the present invention also provide for both static and dynamic measurements of physical parameters. 
   The foregoing and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, more particular descriptions of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  illustrates an optical waveguide sensor having a sequence of sensors disposed along the optical waveguide; 
       FIG. 2  illustrates a dog bone pressure sensor having both a fiber Bragg grating pressure sensor and a fiber Bragg grating temperature sensor; 
       FIG. 3  illustrates a swept frequency optical waveguide measurement system that can be used for both dynamic and static measurements; 
       FIG. 4  schematically illustrates parking a narrow line width laser on the slope of a fiber Bragg grating; and 
       FIG. 5  schematically illustrates an optical waveguide AC strain measurement system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention provides for optical waveguide measurement systems that are suitable for sensing dynamically varying physical parameters such as pressure and strain. Some embodiments of the present invention enable both static and dynamic measurements of physical parameters. Embodiments of the present invention are suitable for use in harsh environments as found in oil and/or gas wells, engines, combustion chambers, etc. 
     FIG. 1  illustrates an optical waveguide sensor system  100  having a sequence of sensors  102  disposed along an optical waveguide  104 . Each sensor  102  includes at least one fiber Bragg grating  106 . Depending on the application and the specific configuration, the sensor system  100  can be operated in various ways. For example, a tunable light source  108 , such as a tunable laser or a broadband light source mated with a tunable filter, can inject light that is swept over a bandwidth into a coupler  110 . The coupler  110  passes the light onto the optical waveguide  104 . Reflections at the Bragg wavelengths of the various fiber Brag gratings  106  occur. The coupler  110  passes those reflections into a receiver  112 . The fiber Bragg gratings  106  are disposed such that the Bragg wavelengths depend on a physical parameter of interest. The output of the receiver  112  is passed to an analyzer  114  that determines from the Bragg wavelengths a measurement of the physical parameter of interest sensed by the sensors  102 . Alternatively, if each sensor in a string has a different wavelength, then a broadband light source without a tunable filter can be used as a signal can still be received from each sensor at the receiver  112 . 
     FIG. 2  illustrates an exemplary sensor  102  that is suitable for measuring parameters such as pressure and strain. The optical waveguide  104  includes a narrow core  202  that passes through a relatively thick cladding layer  204 . That cladding layer is thinned around the fiber Bragg grating  106  to form a narrow section that includes the fiber Bragg grating  106 . Around the narrow section is a shell  206  that is integrally mated with the cladding layer  204 . To adjust the characteristics of the resulting sensor  102 , when the shell  206  is mated with the cladding layer  204  the optical waveguide  104  could be under tension, under a slight compression (a large compression would tend to buckle the narrow section), or, more typically, unbiased. The result is a fiber Bragg grating having a particular Bragg wavelength. When external pressure or strain is applied to the shell  206 , longitudinal tension or compression occurs and the Bragg wavelength changes. A second fiber Bragg grating  212  outside of the narrow section can be included to provide a reference inside of the shell  206  for temperature compensation. 
     FIG. 3  illustrates a tunable laser method of using optical sensors  102  to provide dynamic (AC) measurements. In that method, a tunable laser  302  produces a narrow line width laser pulse  304  that is coupled by a coupler  110  into an optical waveguide  104  having at least one optical sensor  102 . The wavelength of the narrow line width laser pulse  304  is swept through a wavelength band that includes the Bragg wavelength of the fiber Bragg grating  106  in the optical sensor  102 . The shape function  306  of the fiber Bragg grating  106 , that is, its amplitude (Y-axis) verses wavelength (X-axis) characteristics, is determined by a high frequency receiver  112  and an analyzer  114 . Referring now to  FIG. 4 , a particular power level, say the  3 dB point down from the peak  402 , is selected by the analyzer. Then, the analyzer sets the wavelength of the tunable laser  302  to the wavelength  404  that corresponds to the selected power level. Thus, the wavelength of the tunable laser  302  is set at a specific wavelength that is on the shape function  306 . Then the intensity of the reflected light is monitored. Variations in the intensity correspond to dynamic pressure changes impressed on the optical sensor  102 . The high frequency receiver  112  and the analyzer  114  can provide wavelength and amplitude information from the variations in intensity. 
   The foregoing method illustrated with the assistance of  FIGS. 3 and 4  can also provide static pressure measurements. Since the position of the shape function  306  with respect to wavelength (shown in X-axis) depends on static pressure, the analyzer  114  can determine static pressure based on the wavelength position  409  of the peak  410  fiber Bragg grating reflection. It should be understood that while  FIGS. 3 and 4  only illustrate one optical sensor  102  the optical waveguide  104  could have numerous optical sensors  102 . 
   In addition to providing dynamic pressure measurements, the principles of the present invention also provide for determining dynamic (AC) strain. One technique of doing this is illustrated in  FIG. 5 . As shown, a light source  500  launches light into port  1  of a  4  port circulator  502 . That light is emitted from port  2  of the circulator  502  into an optical waveguide  104 . That waveguide includes a sensor  503  that is comprised of two fiber Bragg gratings,  504  and  506 . The gratings  504  and  506 , which have different Bragg wavelengths λ 1  and λ 2 , respectively, are separated by a long period grating  508  that is in a strain sensing field. When the light reaches gratings  504  and  506  those gratings reflect the Bragg wavelengths λ 1  and λ 2 , respectively. However, there is a strain induced loss within the long period grating  508 . Since λ 1  is reflected by grating  504  it signal is not attenuated by the long period grating  508 , and thus the power of wavelength λ 1  can act as a reference power. However, the power of λ 2  depends on the loss within the long period grating  508 , which in turn depends on the applied strain. Thus the ratio of the powers of λ 1  and λ 2  is a measure of strain on the long period grating. The long period grating  508  can also be disposed to measure strain due to applied pressure or some other stimuli. 
   Still referring to  FIG. 5 , the reflected light λ 1  and λ 2  on the optical waveguide  104  enters the circulator  502 . Wavelength λ 2  passes through a wavelength filter  510 , but wavelength λ 1  is reflected. The passed wavelength λ 2  is received and amplified by a first receiver  514 . The output of receiver  514  is passed to an analyzer  516 . Meanwhile, λ 1  is output from port  4  of the circulator  502 . The wavelength λ 1  is received and amplified by a second receiver  518 . The output of the second receiver  518  is applied to the analyzer  516 . The analyzer  516  compares the ratio of the reflected wavelengths and determines the dynamic (AC) strain applied to the long period grating  508 . 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.