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BACKGROUND 
       [0001]    Downhole oil field equipment sometimes operates under great pressures and temperatures. Reflective sensors, i.e., sensors that are interrogated by reflecting light from the sensors, are sometimes useful in such situations because they may not include temperature-sensitive electronics. Fiber optics are sometimes used to carry the light used to interrogate the reflective sensors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is a schematic diagram of a completed well. 
           [0003]      FIG. 2  is a schematic of a wireline logging system. 
           [0004]      FIG. 3  is a schematic diagram of a drilling rig site showing a logging tool that is suspended from a wireline and disposed internally of a bore hole. 
           [0005]      FIG. 4  illustrates a prior art reflective sensor interrogating system. 
           [0006]      FIG. 5  illustrates an optical coupler. 
           [0007]      FIG. 6  illustrates a prior art reflective sensor interrogating system. 
           [0008]      FIGS. 7 and 8  illustrate optical fiber reflective sensor interrogation devices. 
           [0009]      FIGS. 9-11  illustrate the interface between optical fiber reflective sensor interrogation devices and reflective sensors. 
           [0010]      FIG. 12  illustrates a remote real time operating center. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In one embodiment, illustrated in  FIG. 1 , sensors  105  and  110  are located in a completed well bore  115  between a casing  120  and a well bore wall  125 . In one embodiment (not shown), the completed well includes production tubing inside the casing  120  and the sensors  105  and  110  are between the casing  120  and the well tubing. In one embodiment, surface equipment  130  is provided to process information from the sensors  105  and  110 . In one embodiment, communications media  135  and  140  are used to interrogate the sensors  105  and  110  and to carry the resulting information to the surface equipment  130 . In one embodiment, communications media  135  and  140  are optical waveguides. In one embodiment, communications media  135  and  140  are optical fibers. In one embodiment, communications media  135  and  140  are a combination of wires and optical fibers, with the wires carrying information part of the distance from the sensors  105  and  110  to the surface equipment  130  and the optical fibers carrying the information part of the distance. In one embodiment, each fiber  105  and  110  is dedicated to carrying information from a single sensor  105  or  110 . In one embodiment, each fiber  105  and  110  carries information from a plurality of sensors. In one embodiment, each communications media  135  and  140  is a single optical fiber. In one embodiment, each communications media comprises a plurality of optical fibers. In one embodiment, the communications media  135  and  140  comprise a single-mode optical fiber. In one embodiment, the communications media  135  and  140  comprises a multi-mode optical fiber. 
         [0012]    In one embodiment, the sensors  105  and  110  are Fabry-Pérot sensors. In one embodiment, the sensors  105  and  110  are used to measure temperature, pressure, position, index of refraction of a medium, acceleration, vibration, seismic energy, or acoustic energy. 
         [0013]      FIG. 1  is a greatly simplified illustration of a completed well. Many features of typical completed wells, such as the well head equipment, have been omitted from the drawing for illustrative purposes. 
         [0014]    In one embodiment of a wireline well logging system  200  at a drilling rig site, as depicted in  FIG. 2 , a logging truck or skid  205  on the earth&#39;s surface  210  houses a data gathering computer  215  and a winch  220  from which a wireline cable  225  extends into a well bore  230  drilled into a hydrocarbon bearing formation  232 . In one embodiment, the wireline cable  225  suspends a logging toolstring  235  within the well bore  230  to measure formation data as the logging tool  235  is raised or lowered by the wireline  225 . In one embodiment, the logging toolstring  235  includes a z-axis accelerometer  237  and several devices A, B, C. In different embodiment, these devices are instruments, mechanical devices, explosive devices, and/or sensors of the type described above (e.g., Fabry-Pérot sensors). 
         [0015]    In one embodiment, the wireline cable  225  not only conveys the logging toolstring  235  into the well, it also provides a link for power and communications between the surface equipment and the logging toolstring. 
         [0016]    In one embodiment, as the logging tool  235  is raised or lowered within the well bore  230 , a depth encoder  240  provides a measured depth of the extended cable. In one embodiment, a tension load cell  245  measures tension in the wireline  225  at the surface  210 . 
         [0017]    In one embodiment, the wireline cable  225  includes one or more optical fibers for interrogating one or more of devices A, B or C. 
         [0018]      FIG. 2  is a greatly simplified illustration of a wireline operation. Many details of such an operation have been omitted from the drawing for illustrative purposes. 
         [0019]    In one embodiment of a measurement while drilling (“MWD”) or logging while drilling (“LWD”) environment  300 , illustrated in  FIG. 3 , a derrick  305  suspends a drill string  310  in a borehole  312 . In one embodiment, the volume within the borehole  312  around the drill string  310  is called the annulus  314 . In one embodiment, the drill string includes a bit  315 , a variety of actuators and sensors, shown schematically by element  320 , an instrument  325  (such as, for example, a formation testing instrument, an acoustic sensor, a resistivity tool, or the like), and a telemetry section  330 , through which the downhole equipment communicates with a surface telemetry system  335 . In one embodiment, a computer  340 , which in one embodiment includes input/output devices, memory, storage, and network communication equipment, including equipment necessary to connect to the Internet, receives data from the downhole equipment and sends commands to the downhole equipment. 
         [0020]    In one embodiment, element  320  includes sensors of the type described above (e.g., Fabry-Pérot sensors). In one embodiment, communications media (not shown) extend from the element  320  to surface equipment (not shown) where the information from the sensors is processed. In one embodiment, the communications media includes an optical fiber that is used to interrogate element  320 . In one embodiment, an optical fiber extends from element  320  to another element in the drill string  310  where information from the optical fiber is incorporated into telemetry data that is sent to the surface telemetry section. In one embodiment, an optical slip ring (not shown) is included to accommodate the transition of the optical fiber from non-rotating parts of the system to rotating parts of the system. 
         [0021]      FIG. 3  is greatly simplified and for clarity does not show many of the elements that are used in the drilling process. 
         [0022]      FIG. 4  shows a prior art method to interrogate a reflective sensor through an optical fiber using a coupler. A light source  405  and an optical processor  410  are typically housed within a housing  415 . Fiber optic cables couple the light source  405  and the optical processor to respective ports on a coupler  420 . A third port on the coupler  420  is coupled to a fiber optic cable  425  which carries light from the light source  405  to a reflective sensor  430 . The same fiber optic cable  425  carries reflected light from the sensor  430  to the coupler and then to the optical processor  410 . 
         [0023]    An example coupler, illustrated in  FIG. 5 , has four ports. In the example system shown in  FIG. 4 , the first port  505  receives light from the light source  405 . That light is split with half exiting the second port  510  and half exiting the third port  515 . The half exiting the third port is delivered to a device that absorbs the light in order to minimize reflections back into the system. In the system illustrated in  FIG. 4 , the half exiting the second port is transmitted to the sensor  430  where it is reflected and returned to the second port  510 . The coupler splits the returned light, with half exiting the first port  505  and half exiting the fourth port  520 . Thus, ignoring all other losses, 25 percent of the light transmitted from the light source  405  to the coupler  420  is returned to the optical processor  410 . 
         [0024]    In some prior art systems using single mode optical fibers, a circulator is used instead of a coupler. Rather than the 6 to 7 dB loss exhibited by the coupler, the circulator will introduce approximately a 1 dB loss. 
         [0025]    For long lengths of fiber optic cable  425 , the approach illustrated in  FIG. 4  results in a large proportion of the light returned to the coupler  420  being contributed by the Rayleigh backscattering of the launched light, illustrated by the word “Rayleigh” on  FIG. 4 . This backscattering does not contain any information useful in the measurement and its presence decreases the signal-to-noise ratio at the optical processor  410 . An illustration of the Rayleigh backscatter effect is the effect of looking at a road while driving on a foggy night with the vehicle high beams on. The backscattered light from the fog overwhelms the view of everything except the closest objects. 
         [0026]      FIG. 6  shows a prior art approach that reduces the backscattering detected and therefore provides an improvement over the approach of  FIG. 4 . The difference is the location of the coupler  420 , which is close to the sensor in  FIG. 6 . Further, two optical fibers are used: a first optical fiber  505  carries the light from the light source  405  to the coupler  420  and a second optical fiber  510  carries the reflected light from the coupler  420  to the optical processor  410 . Only a very short length of fiber (between the coupler and the sensor) contributes backscattering in the system of  FIG. 5 . This reduction of backscattering allows longer fiber lengths to be used and therefore permits the reach for the sensor system to be extended. This is highly desirable for monitoring deep oil wells, for example. 
         [0027]    The use of the terms “input” and “output” with respect to the system depicted in  FIG. 6  is relative to the housing  415  containing the light source  405  and the optical processor  410 . That is, the output optical fiber  505  carries the output of the light source  405  and the input optical fiber  510  carries the input to the optical processor  410 . This convention will be followed in describing the remaining figures in this application. 
         [0028]    One embodiment of an optical fiber reflective sensor interrogation system, illustrated in  FIG. 7 , eliminates the coupler (or the circulator) by employing an output optical fiber  705  that spans the distance from a light source  710  to the reflective sensor  715  and an input optical fiber  720  that spans the distance from the sensor  715  to an optical processor  725 . In one embodiment, light from the light source  710  is brought directly to the sensor by the output optical fiber  705 . In one embodiment, the light source  710  is located downhole close to the location of the sensor. In one embodiment, the input optical fiber  720  is placed in close proximity to the output optical fiber  705  and is oriented relative to the output optical fiber and the sensor so that the light that is reflected by the reflective sensor  715 , which is encoded by a transduction mechanism of the reflective sensor, is reflected primarily into the input optical fiber  720 . The reflected light is returned by the input optical fiber  720  to the optical processor  725 . 
         [0029]    Note that a housing  730  that includes the light source  710  and the optical processor  725  may include one or more optical fibers that extend from the light source  710  to a connector accessible from the outside of the housing  730  and one or more optical fibers that extend from a connector accessible from the outside of the housing  730  to the optical processor  725 . In that case, the output optical fiber  705  and input optical fiber  720  are considered to span the distance between the light source  710  and the sensor  715  and between the sensor  715  and the optical processor  725  if they span the distance between the connectors accessible from the outside of the housing  730  to the sensor  715 . Further, an optical fiber is considered to span a distance even if the optical fiber is spliced in that distance. 
         [0030]    In one embodiment, the light source  710  is a source of broadband white light, i.e., light that covers a broad spectrum. In one embodiment, the light source  710  is a light bulb. In one embodiment, the light source  710  is a source of black-body emissions. In one embodiment, the light source  710  is a narrow band source of light. In one embodiment, the light source  710  is a laser. In one embodiment, the light source  710  is a Light Emitting Diode (“LED”). In one embodiment, the light source  710  is a supercontinuum light source. 
         [0031]    In one embodiment, the optical processor includes a wedge  730  and a charge-coupled device (“CCD”) array  735 . The wedge focuses the reflected light on a detectable position in the CCD array that is indicative of the property being measured by the reflective sensor  715 . In one embodiment, the system shown in  FIG. 7  acts as a Fizeau interferometer. In one embodiment, the system shown in  FIG. 7  acts as a Fabry-Pérot interferometer. 
         [0032]    In one embodiment, the output optical fiber  705  and the input optical fiber  720  are considered to be a “device” with two inputs (one from the light source  710  and one from the sensor  715 ) and two outputs (one to the sensor  715  and one to the optical processor  725 ). 
         [0033]    In another embodiment, illustrated in  FIG. 8 , a single optical processor  805 , which is similar to the optical processor  725  described above, processes signals from two different sensors  810  and  815 . In one embodiment, measurements from one of the sensors are used to compensate measurements from the other sensor. For example, in one embodiment, sensor  810  is a pressure sensor and sensor  815  is a temperature sensor co-located with the pressure sensor  810 . In that case, the measurements from the temperature sensor  815  may be used to compensate (i.e., temperature adjust) the measurements from the pressure sensor  810 . 
         [0034]    In the embodiment shown in  FIG. 8 , light from a first light source  820  is routed to a first reflective sensor  810  by a first output optical fiber  825 . Reflected light from the reflective sensor  810  is routed to the optical processor  805  by a first input optical fiber  830 . Light from a second light source  835  is routed to a second reflective sensor  815  by a first output optical fiber  840 . Reflected light from the reflective sensor  815  is routed to the optical processor  805  by a second input optical fiber  845 . A controller (not shown) selects which input the optical processor  805  processes at any given time. 
         [0035]    In one embodiment, the optical fibers  825 ,  830 ,  840 , and  845  are considered to be a “device” with four inputs (one from each of the light sources  820  and  835  and one from each of the sensors  810  and  815 ) and four outputs (one from each of the sensors  810  and  815  and two to the optical processor  805 ). 
         [0036]    In another embodiment shown in  FIG. 9 , two sensors  905  and  910  are daisy-chained together. In one embodiment the sensor  905  is remotely deployed (i.e. more than 1 meter) from the sensor  910 . A single source of light  915  transmits light over an output optical fiber  920  to a first sensor  905 . The reflected light from the first sensor is transmitted over a linking optical fiber  925  to a second sensor  910 . The reflected light from the second sensor  910  is transmitted over an input optical fiber  930  to an optical processor  935 . 
         [0037]    In one embodiment, the sensors  905  and  910  are adjusted so that the returns from the two devices can be distinguished. In particular, in one embodiment, the distance between the window and the mirror (see  FIGS. 10 and 11  below) in sensor  905  is different from the distance between the window and the mirror in sensor  910 . 
         [0038]    In one embodiment, the distance between the window and the mirror in sensor  905  is substantially the same as the distance between the window and the mirror in sensor  910 . 
         [0039]    In one embodiment, the optical fibers  920 ,  925 , and  930  are considered a “device” with three inputs (one from the light source  915  and one from each of the sensors  905  and  910 ) and three outputs (one to each of the sensors  905  and  910  and one to the optical processor  935 ). 
         [0040]    In one embodiment of the interface between the optical fibers and the reflective sensor, illustrated in  FIG. 10 , a reflective sensor  1005  includes a housing  1010 , a window  1015 , and a mirror  1022 . In one embodiment, the distance δ between the window  1015  and the mirror  1022  is predictably influenced by the property being measured. For example, variations in temperature and pressure can cause δ to vary. The round trip distance from the light source to the optical processor (see  FIGS. 7 and 8 ) is, therefore, related to a measure of the property (i.e., the temperature or pressure). 
         [0041]    In the embodiment shown in  FIG. 10 , the output optical fiber  1020  and the input optical fiber  1025  follow approximately parallel paths (i.e., in one embodiment, they are touching along their entire paths or they are within 0.25 of a fiber diameter over their entire paths) until they approach the sensor  1005 . At that point they deviate toward each other along paths at angles θ 1  and θ 2  relative to a center line between the two fibers. In one embodiment, θ 1  and θ 2  are between 0 and 45 degrees. In one embodiment (not shown) the fibers deviate away from each other before they deviate toward each other. In one embodiment, θ 1  and θ 2  are between 3 and 12 degrees. In one embodiment, the output optical fiber  1020  and the input optical fiber  1025  are arranged so that light traveling through output optical fiber  1020  reflects from the window  1015  and the mirror  1022 , sometimes after multiple reflections between the window  1015  and the mirror  1022 , to input optical fiber  1025 . 
         [0042]    In one embodiment, the window  1015  has two surfaces: a first surface  1030  closest to the output optical fiber  1020  and the input optical fiber  1025 , and a second surface  1035 . In one embodiment, the first surface  1030  is inclined relative to the second surface  1035  so that the reflection from the first surface  1030  does not reach the input optical fiber  1025 . The Fabry-Pérot sensor is therefore limited to the second surface  1035  and the mirror  1022  and is not affected by the first surface  1030 . 
         [0043]    In one embodiment, the output optical fiber  1020  and input optical fiber  1025  have ball lenses formed at their distal ends, i.e., at their ends closest to the window  1015 . In one embodiment, the ball lenses are formed by melting the ends of the fibers using the plasma discharge from an electric arc. 
         [0044]    In one embodiment, the ball lenses are located between 0.1 and 2.0 mm from the plate  1015 . 
         [0045]    In one embodiment, the ball lens at the end of the output optical fiber  1020  is approximately (i.e., within 10 percent) the same size as the ball lens at the end of the input optical fiber  1025 . In one embodiment, the diameter of the ball lens at the end of the input optical fiber  1025  is approximately (i.e., +/−10%) 0.5 mm. In one embodiment, the diameter of the ball lens at the end of the output optical fiber  1020  is approximately (i.e., +/−10%) 0.3 mm. In one embodiment, the ratio between the diameter of the ball lens at the end of the output optical fiber  1020  and the diameter of the ball lens at the end of the input optical fiber  1025  is between 0.5 and 1.0. The larger ball on the input side collects more light, which is useful because the light exiting the output side will diverge. 
         [0046]    In one embodiment, the numerical aperture of the ball lens at the end of the output optical fiber  1020  (i.e., the angular width of the beam that comes out of the lens) is approximately (i.e., within 10 percent) the same size as the numerical aperture of the ball lens at the end of the input optical fiber  1025  (i.e., the acceptance angle of the lens). In one embodiment, the ratio of the numerical aperture of the ball lens at the end of the output optical fiber  1020  and the numerical aperture of the ball lens at the end of the input optical fiber  1025  is between 0.5 and 1.0. 
         [0047]    In one embodiment (not shown), the ball lenses are replaced by traditional collimating lenses separate from the two fibers. 
         [0048]    In one embodiment, the lenses are graded index lenses. 
         [0049]    In one embodiment, the ends of the output optical fiber  1020  and input optical fiber  1025  are not melted to form balls. Instead, they are cleaved. In one embodiment, the fibers are cleaved or polished along a plane normal to the fiber axis or along a plane angled away from perpendicular to the fiber axis by 6-12 degrees. The latter cleaving arrangement is to avoid back reflection to the source. In one embodiment, the cleaving arrangement is used to orient the beam of light exiting the output optical fiber  1020  toward the sensor and to orient the reception sensitivity of the input optical fiber  1025  toward the sensor while keeping both fibers parallel but separated by a small distance for more compact packaging. In one embodiment, the cleaving arrangement is used with a single lens for both fibers. In one embodiment, the cleaving arrangement is used with a lens for each fiber. In one embodiment, the cleaving arrangement is used without lenses. 
         [0050]    In one embodiment, shown in  FIGS. 11 and 12 , the output optical fiber  1105  and the input optical fiber  1110  are substantially parallel (i.e., touching or within 0.25 fiber diameters) throughout their lengths and are jointly terminated at their distal ends by a single ball  1115  formed by melting the two fiber ends together. In one embodiment, the ball is formed by laying the two fibers side by side and then melting the two fiber ends with the plasma discharge from an electric arc. In particular, in one embodiment, the following process is followed to form the single ball  1115 :
       a. The coating is removed off the ends of the fibers for a distance of approximately 40 mm (i.e., enough to perform the remaining elements of the process).   b. The fibers are cleaned.   c. The end of the fibers are cleaved (removes approximately 15 mm of fiber).   d. The two fibers are mounted next to each other (i.e., with their lengths near the cleaned and cleaved ends approximately parallel), in a vertical position, with their cleaned and cleaved ends at approximately the same location.   e. The end of the fibers are melted simultaneously using a time sequence of plasma arcs at an arc location. The fibers are exposed to the plasma arcs for a sufficient time to form the ball, i.e., typically 0.1 to 2.0 seconds for each arc. In one embodiment, the fibers are fed into a ball-forming location near, typically above, the arc location as the fibers are melted so that the ball forms and hangs from the fibers at the ball-forming location.       
 
         [0056]    In one embodiment, the fiber ends are not melted together into a ball  1115  as shown in  FIGS. 11 and 12 . Instead, a single separate lens (not shown) is used. 
         [0057]    In one embodiment, a computer program for controlling the operation of one or the systems shown in  FIG. 1 ,  2 , or  3  is stored on a computer readable media  1305 , such as a CD or DVD, as shown in  FIG. 13 . In one embodiment a computer  1310 , which may be the same as computer in the surface equipment  130  ( FIG. 1 ), data gathering computer  215  ( FIG. 2 ), or the computer  340  ( FIG. 3 ), or a computer located below the earth&#39;s surface, reads the computer program from the computer readable media  1305  through an input/output device  1315  and stores it in a memory  1320  where it is prepared for execution through compiling and linking, if necessary, and then executed. In one embodiment, the system accepts inputs through an input/output device  1315 , such as a keyboard, and provides outputs through an input/output device  1315 , such as a monitor or printer. In one embodiment, the system stores the results of calculations in memory  1320  or modifies such calculations that already exist in memory  1320 . 
         [0058]    In one embodiment, the results of calculations that reside in memory  1320  are made available through a network  1325  to a remote real time operating center  1330 . In one embodiment, the remote real time operating center  1330  makes the results of calculations available through a network  1335  to help in the planning of oil wells  1340 , in the drilling of oil wells  1340 , or in production of oil from oil wells  1340 . Similarly, in one embodiment, the systems shown in  FIG. 1 ,  2 , or  3  can be controlled from the remote real time operating center  1330 . 
         [0059]    The word “couple” or “coupling” as used herein shall mean an electrical, electromagnetic, or mechanical connection and a direct or indirect connection. 
         [0060]    In addition to power being provided from the surface through wireline cable  225 , power may also be provided by a battery located in the wireline logging toolstring  235 . Similarly, the downhole equipment in the MWD/LWD system shown in  FIG. 3  may be powered by a downhole battery. 
         [0061]    The text above describes one or more specific embodiments of a broader invention. The invention also is carried out in a variety of alternate embodiments and thus is not limited to those described here. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. Further, the device and system described herein is not limited in use to oil and gas applications. It can be used in any application in which Fabry-Pérot or Fizeau interferometers have application or in any application in which optical fibers are used to carry interrogating signals. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Summary:
A system includes an optical source. The system further includes a reflective sensor remotely deployed from the optical source. The system further includes an optical processor. The system further includes a forward optical waveguide spanning the distance from, and transmitting light from, the optical source to the reflective sensor. The system further includes a return optical waveguide spanning the distance from, and transmitting light from, the reflective sensor to the optical processor. The forward optical waveguide follows substantially the same path as, but is completely separate from, the return optical waveguide.