Patent Publication Number: US-7583371-B2

Title: Combined bragg grating wavelength interrogator and brillouin backscattering measuring instrument

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/696,766 filed Oct. 29, 2003, now U.S. Pat. No. 7,199,869, which is herein incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the invention generally relate to a method and apparatus for detecting changes in a reflective signal in a fiber optic sensing system. 
   2. Background of the Related Art 
   For fiber optic sensing systems, specifically Bragg grating-based systems, a dedicated opto-electronic instrument is required to measure environmentally-induced changes in peak wavelengths. If additional measurements, such as Brillouin-based temperature and/or strain measurements are needed, additional dedicated opto-electronic instrumentation is required. Systems having dedicated instruments for sensing both Bragg grating and Brillouin based measurements can be extremely costly and complex. 
   Therefore, there is a need for an improved fiber optic sensing system. 
   SUMMARY OF THE INVENTION 
   A method and apparatus for sensing using an optical fiber are provided. In one embodiment, a method for sensing an attribute (such as wavelength and/or frequency) of a reflected signal in an optical fiber sensing system comprising an interrogator coupled to a Bragg grating sensor by an optical cable includes the steps of producing a first optical signal, coupling the first optical signal to an optical cable, receiving a first reflected signal from a Bragg grating sensor within the optical cable, resolving a wavelength spectrum difference between the first optical signal and first reflected signal, producing a second optical signal, coupling the second optical signal to the optical cable, receiving a second reflected signal caused by Brillouin backscattering within the optical cable, and resolving a shift in wavelength spectrum between the second optical signal and second reflected signal. 
   In another embodiment, an apparatus for sensing an attribute in returning optical signals includes a Bragg grating sensor coupled by an optical fiber to a light source and signal detection circuit. The light source is suitable for producing optical signals tunable over a range of wavelengths and is adapted to generate a signal having sufficient intensity to produce Brillouin scattering of the signal while propagating in the optical fiber. The signal detection circuit includes a first sensing branch for detecting an attribute of a signal reflected from the Bragg grating, a second sensing branch for sensing an attribute of back-scattered signals and an optical switch for diverting signals returning from the optical fiber to the optical signal detection circuit selectively between the first and second branches. Embodiments of the method and apparatus are particularly useful for sensing temperature and strain in hazardous locations such as down hole gas and oil field applications and the like. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof that 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. So that the manner in which the above-recited embodiments of the invention are obtained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof 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  is a system-level view of a fiber optic sensing system suitable for use in oil or gas well applications; 
       FIG. 2  is one embodiment of a sensor of the system of  FIG. 1 ; 
       FIG. 3  is a schematic of one embodiment of an interrogator of  FIG. 1 ; and 
       FIGS. 4A-C  are a flow diagram of one embodiment of a method for sensing wavelength shifts in returning optical signals. 
     To facilitate understanding, identical reference numerals have been used, wherever possible, to designate identical elements that are common to the figures. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a simplified schematic of an oil or gas well  110  having an optical fiber sensing system  100  adapted to sense environmental conditions within the well  110  using a method and apparatus of the present invention. The well  110  includes a main bore  112  extending from a wellhead  114 . The sensing system  100  utilizes both Bragg grating reflections and non-linear induced back scatter signals to resolve environmental conditions along the sensing path. In one embodiment, wavelengths and/or frequency of reflected signals are indicative of temperature and strain information of the environmental conditions within the well  110 . 
   The sensing system  100  includes an interrogator  160  coupled by an optic cable  162  to at least one sensor  164 . The sensor  164  may be a single point sensor or other suitable Bragg grating sensor. One sensor  164  that may be utilized is available from Weatherford, Inc., located in Houston, Tex. Another example of a sensor  164  that may be utilized is described in U.S. Pat. No. 6,422,084, entitled “Bragg Grating Pressure Sensor”, issued Jul. 23, 2002 to Fernald et al.; and U.S. Pat. No. 6,452,667, entitled “Pressure Isolated Bragg Grating Temperature Sensor”, issued Sep. 17, 2002, to Fernald et al., all of which are hereby incorporated by reference in their entireties. 
     FIG. 2  depicts one embodiment of the sensor  164 . The sensor  164  includes a large diameter optical waveguide  210 , has at least one core  212  surrounded by a cladding  214 , similar to that disclosed in U.S. Pat. No. 6,363,089 entitled “Large Diameter Optical Waveguide, Grating, and Laser”, which is incorporated herein by reference. The waveguide  210  comprises silica glass (SiO 2 ) based material having the appropriate dopants, as is known, to allow light  215  to propagate in either direction along the core  212  and/or within the waveguide  210 . The core  212  has an outer dimension d 1  and the waveguide  210  has an outer dimension d 2 . Other materials for the optical waveguide  210  may be used if desired. For example, the waveguide  210  may be made of any glass, e.g., silica, phosphate glass, or other glasses; or solely plastic. 
   In one embodiment, the outer dimension d 2  of the cladding  214  is at least about 0.3 mm and outer dimension d 1  of the core  212  such that it propagates only a few spatial modes (e.g., less than about 6). For example for single spatial mode propagation, the core  212  has a substantially circular transverse cross-sectional shape with a diameter d 1  less than about 12.5 microns, depending on the wavelength of light. The invention will also work with larger or non-circular cores that propagate a few (less than about 6) spatial modes, in one or more transverse directions. The outer diameter d 2  of the cladding  214  and the length L have values that will resist buckling when the waveguide  210  is placed in axial compression as indicated by the arrows  218 . 
   The waveguide  210  may be ground or etched to provide tapered (or beveled or angled) outer corners or edges  224  (shown in phantom) to provide a seat for the waveguide  210  to mate with another part (not shown) and/or to adjust the force angles on the waveguide  210 , or for other reasons. The angle of the beveled corners  224  is set to achieve the desired function. Further, the waveguide may be etched or ground to provide nubs for an attachment of a pigtail assembly to the waveguide. Further, the size of the waveguide  210  has inherent mechanical rigidity that improves packaging options and reduces bend losses. 
   The waveguide has a Bragg grating  216  impressed (or embedded or imprinted) therein. The Bragg grating  216 , as is known, is a periodic or aperiodic variation in the effective refractive index and/or effective optical absorption coefficient of an optical waveguide. The grating  216  may be in the core  212  and/or in the cladding  214  (shown in the core  212  in  FIG. 2 ). Any wavelength-tunable grating or reflective element embedded, etched, imprinted, or otherwise formed in the waveguide  210  may be used if desired. The waveguide  210  may be photosensitive if a grating  216  are to be written into the waveguide  210 . As used herein, the term “grating” means any of such reflective elements. Further, the reflective element (or grating)  16  may be used in reflection and/or transmission of light. Light  215  incident on the grating  216  reflects a portion thereof as indicated by a line  236  having a predetermined wavelength band of light, and passes the remaining wavelengths of the incident light  215  (within a predetermined wavelength range), as indicated by a line  238  (as is known). 
   The grating  216  has a grating length Lg, which is determined based on the application, may be any desired length. A typical grating  216  has a grating length Lg in the range of about 3-40 mm. Other sizes or ranges may be used if desired. The length Lg of the grating  216  may be shorter than or substantially the same length as the length L of the waveguide  210 . Also, the core  212  need not be located in the center of the waveguide  210  but may be located anywhere in the waveguide  210 . 
   Accordingly, we have found that the present invention also reduces coupling between the core and cladding modes due to the increased end cross-sectional area between the core and cladding of the waveguide. Thus, a grating  216  written in the core  212  of the waveguide  210  exhibits less optical transmission loss and exhibits a cleaner optical profile than a conventional fiber grating because the large cladding region dissipates coupled cladding modes, thereby reducing the coupling of the core  212  to the cladding  214  modes. In general, the greater the difference in cross-sectional area between the core  212  and the cladding  214  the smaller the mode field overlap and the lower the coupling to the cladding modes. The thickness of the cladding  214  between the cladding outer diameter and the core outer diameter may be set to optimize this effect. Other diameters of the core  212  and waveguide  210  may be used if desired such that the cladding modes are reduced to the desired levels. 
   The waveguide  210  may have end cross-sectional shapes other than circular, such as square, rectangular, elliptical, clam-shell, octagonal, multi-sided, or any other desired shapes, discussed more hereinafter. Also, the waveguide may resemble a short “block” type or a longer “cane” type geometry, depending on the length of the waveguide and outer dimension of the waveguide. 
   The side cross-section of the outer surface of the waveguide  210  may have a varying geometry, depending on the application. For example, the waveguide  210  may have a “dogbone” shape having a narrow central section and larger outer sections. The dogbone shape may be used to provide increased sensitivity in converting axial force to length change ΔL and/or wavelength shift Δλ of the grating  216  and may be achieved by etching, grinding, machining, heating &amp; stretching, or other known techniques. 
   The dimensions and geometries for any of the embodiments described herein are merely for illustrative purposes and, as such, any other dimensions may be used if desired, depending on the application, size, performance, manufacturing requirements, or other factors, in view of the teachings herein. 
   The optical waveguide  210  may be formed by heating, collapsing and fusing a glass capillary tube to a fiber (not shown) by a laser, filament, flame, etc., as is described U.S. Pat. No. 6,519,388, entitled “Tube-Encased Fiber Grating,” which is incorporated herein by reference. Alternatively, other techniques may be used to fuse the fiber to the tube, such as using a high temperature glass solder, e.g., a silica solder (powder or solid), such that the fiber, the tube and the solder all become fused to each other, or using laser welding/fusing or other fusing techniques. 
   The Bragg grating may be written in the fiber before or after the capillary tube is encased around and fused to the fiber, such as is discussed in the above referenced U.S. Pat. No. 6,519,388. If the grating is written in the fiber after the tube is encased around the grating, the grating may be written through the tube into the fiber by any desired technique, such as is described in U.S. Pat. No. 6,298,184, entitled “Method and Apparatus For Forming A Tube-Encased Bragg Grating”, filed Dec. 4, 1998, which is incorporated herein by reference. 
   Returning to  FIG. 1 , the optic cable  162  generally includes one or more optical fibers suitable for transmitting optic signals between the interrogator  160  and the sensor  164 . Examples of suitable optic cables are described in U.S. Pat. No. 6,404,961, issued Jun. 11, 2002 to Bonja et al., and U.S. patent application Ser. No. 10/422,396, filed Apr. 24, 2003 by Dowd et al., both of which are hereby incorporated by reference in their entireties. Suitable cables are also available from Weatherford, Inc. 
   In the embodiment depicted in  FIG. 1 , the optic cable  162  includes one or more single-mode optical fibers  122  disposed in a protective sleeve  124  suitable to protect the optical fibers  122  in a down hole well environment. In some applications, the optical cable  162  may extend up to and exceed 12 kilometers through main bore  112  and/or at least one of the secondary bores (not shown) that may branch out from the main bore  112  within the well  110 . 
   In one embodiment, the sleeve  124  includes an inner tube  126  seam welded around the one or more optical fibers  122 , a spacer  128  and an outer metal tube  130 . The inner tube  126  may be filled with a material  132 , for example a getter gel, utilized to support the one or more optical fibers  122  in the inner tube  126 . The outer metal tube  130  is welded around the spacer  128  that is disposed between the inner and outer tubes  126 ,  130 . A barrier material (not shown) having low hydrogen permeability may be disposed on at least one of the tubes  126 ,  130 . 
   The interrogator  160  is configured to transmit and receive optical signals through the optic cable  162 . The interrogator  160  is suitable for interrogating both Bragg grating based sensors and non-linear induced backscatter signals to provide a metric indicative of the wavelength and/or frequency of reflected signals that are indicative environmental conditions within the well, for example, temperature and strain. In addition, other environmental conditions may be detected by the optical Bragg grating based sensor such as pressure, seismic disturbances, chemicals, etc., as is well known in the art. It is also contemplated within the scope of present invention that multiple optical Bragg grating based sensors positioned along the cable and multiplexed as is known in the art. 
     FIG. 3  depicts one embodiment of the interrogator  160 . The interrogator  160  includes a signal generator  302 , a pulse module  304 , a frequency detector  316  and an optical wavemeter  318 . A controller  340  is coupled to the interrogator  160  for processing information provided by the frequency detector  316  and the optical wavemeter  318 . Alternatively, the controller  340  may be an integral part of the interrogator  160 . The controller  340  may also manages signal generation, collection and interpretation of data, and the general operation of the sensing system  100 . 
   The controller  340  includes a central processing unit (CPU)  342 , support circuits  344  and memory  346 . The CPU  342  may be one of any form of general purpose computer processor that can be used in an industrial setting configured to interface with the interrogator  160 . The memory  346  is coupled to the CPU  342 . The memory  346 , or computer-readable medium, may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. The support circuits  344  are coupled to the CPU  342  for supporting the processor in a conventional manner. These circuits include cache, power supplies, clock circuits, input/output circuitry and subsystems, and the like. 
   The signal generator  302  is coupled to a first tap  322 . The first tap  322  selectively directs portions of an output of the signal generator  302  to the pulse module  304  and the wavemeter  318 . The percentage of the signal diverted by the first tap  322  may be selected based on the depth of the sensor  164  within the well, among other factors. 
   A second tap  324  is disposed between the first tap  322  and the wavemeter  318  to divert a portion of the signal passing therebetween to the frequency detector  316 . The pulse module  304  is coupled to the optic cable  162  such that an output signal (shown by arrow  306 ) from the interrogator  160  may be sent through the cable  162  to the sensor  164  (shown in  FIG. 1 ). 
   An optical switch  320  is disposed in the interrogator  160  for selectively diverting reflected return signals (shown by arrow  308 ) between a first return path  310  and a second return path  312 . The first return path  310  directs the return signals  306  reflected from the sensor  164  to the wavemeter  318 . The second return path  312  directs the Brillouin backscattered return signals  306  to the frequency detector  316 . 
   The signal generator  302  is configured to produce an optical signal into the optical fiber  122 . The signal generator  302  may produce a single polarized optical signal and may have an output adjustable in power and of intensity sufficient to produce Brillouin scattering of the signal as the signal propagates through the optical fiber  122  down the well  110 . In one embodiment, the intensity of the signal is at least about 100 microwatts. The signal generator  102  is configured to produce an output signal tunable at least between wavelengths reflected and transmissive to the sensor  164  (e.g., not in the sensor band). In one embodiment, the signal generator  302  is a semiconductor laser having an output signal tunable between at least about 3 to about 6 kilometers over the reflected band of the sensor  164 . Alternatively, the signal generator  302  may be a broadband light source coupled with a tunable filter. 
   The pulse module  304  may be set to a first state that allows the output signal of the signal generator to pass directly therethrough. The pulse module  304  may be set to a second state that pulses the output signal through the optical cable  162 . The pulse rate is generally selected to allow individual pulses to be reflected without interference from subsequently launched signals. 
   The second return path  312  may include one or more signal conditioning devices suitable for enhancing the performance of the frequency detector  316  in analyzing Brillouin backscattered return signals. In one embodiment, the conditioning device disposed on the second return path  312  between the switch  320  and the frequency detector  316  is a Rayleigh filter  314 . The Rayleigh filter  314  conditions the returning signals and improve system performance by removing extraneous portions of the reflected signal not required for the analysis of the conditions along the sensing path. 
   The wavemeter  318  is a high resolution wavelength detector and is configured to receive reflected signals returning through the optical fiber  122  of the cable  162  and launched signals tapped from the signal generator  302 . The wavemeter  318  is also configured to determine the amplitude of the optical signals. The wavemeter  318  may include one or more photodiodes for converting the optical signal to a digital signal. The wavemeter  318  provides the controller  340  with a metric indicative of the wavelength (and/or frequency) of the launched and reflected signals. 
   The frequency detector  316  is a high resolution frequency detector and is configured to receive backscattered signals returning through the optical fiber  122  of the cable  162  and launched signals tapped from the signal generator  302 . The frequency detector  316  resolves a difference in frequency between the launched and reflected signals. A metric indicative of the difference in frequency is provided to the controller  340  which is indicative of environmental conditions at the portion of the fiber from which the backscattered signal was reflected. 
   A sensing method  400 , described below with reference to the flow diagrams illustrated in  FIGS. 4A-C , is generally stored in the memory  346  of the controller  340 , typically as a software routine. The software routine may also be stored and/or executed by a second CPU (not shown) that is remotely located from the hardware being controlled by the CPU  342 . When the routine is executed by the CPU  342 , the controller  340  provides instructions to the signal generator  302  and receives data from the frequency detector  316  and wavemeter  318 , from which an attribute, such as wavelength and/or frequency of returning optical signals may be resolved. In one mode of operation, the resolved attributes may be indicative of temperature and/or strain at one or more predefined positions along the optical fiber  122  that corresponds depth and/or location within the well  110 . 
     FIGS. 4A-C  are flow diagrams of one embodiment of the method  400  for resolving a measure of one or more environmental conditions from attributes of returning optical signals. The method  400  begins by performing a point sensing step  420  and may be followed by a distributed sensing step  460 . The point sensing step  420  is mainly utilized to resolve a measure of one or more environmental conditions at the sensor  164 . 
   The point sensing step  420  begins at step  422  by producing a series of output signals  306  from the signal generator  302  through a band of wavelengths from the interrogator  160  to the sensor  164 . In one embodiment, the output signals  306  may be produced by scanning a laser through a predefined range of output wavelengths. 
   At step  424 , the tap  322  selectively diverts a portion of the signal  306  produced by the interrogator  160  to the wavemeter  318  through the tap  324 . At step  426 , the wavemeter  318  records and/or characterizes the amplitude versus time of the reflected signal  308  returning via the first return path  310 . At step  330 , a peak wavelength of the reflected signal is resolved by determining the time corresponding to the peak amplitude of the reflected signal, from which the wavelength may be derived using the time/wavelength relationship characterized from the launched signal. At step  432 , the wavelength information of the reflected signal  308  is correlated to environmental parameters (for example, strain and/or temperature) at the sensor  164 . 
   The distributed sensing step  460  begins at step  462  where the signal generator  302  produces a signal tuned to a wavelength not in the band of the grating (e.g., sensor  164 ). The signal generally has sufficient power to induce Brillouin scattering as the signal propagates through the optical cable  162 . At step  464 , the taps  322 ,  324  selectively diverts a portion of the produced signal  306  to the frequency detector  316 . At step  466 , the remainder of the signal (e.g., the portion not diverted at step  464 ) is pulsed by the pulse module  304 . The pulse module  304  is set to pulse the output signal  306  traveling down the optic cable  162  toward the sensor  164 . 
   At step  468 , the frequency detector  316  records and/or characterizes the frequency (and/or wavelength) of the output signal  306 . At step  470 , the frequency detector  3126  records and/or characterizes the frequency (and/or wavelength) of the backscattered (reflected) signal  308  returning to the frequency detector  316  via the second return path  320 . At step  372 , a shift in difference in the frequencies (and/or wavelengths) between the output signal  306  and the backscattered signal  308  is resolved which indicates a change in the environmental parameters along the distributed length of the optical cable  162 . The shift in the backscattered signal  308  is indicative of changes in environmental conditions along the sensing string (e.g., the length of the optical cable  162 ). Analysis of the change in wavelength of the backscattered signal  308  can be resolved, for example, by the controller  340 , to provide distributed strain and temperature information over the length of the optical cable  162 , which corresponds to distinct locations along the main bore  112  of the well  110 . 
   Thus, a method and apparatus has been presented for accurately sensing the attributes in both Bragg grating and Brillouin backscattered optical signals. The invention advantageously minimizes the amount of measurement equipment required to sense both types of reflected signals, thereby reducing the cost of and complexity of measurement equipment. The invention is particularly suitable for use in hazardous locations, such as oil and gas well applications, where changes in signal wavelengths are indicative of environmental changes within the well, such as changes in temperature and strain. 
   Although several embodiments which incorporate the teachings of the present invention have been shown and described in detail, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.