Abstract:
An accelerometer is provided for a fiber optic laser. Strain applied to the fiber optic laser results in an emission wavelength shift. The fiber optic laser is joined to a transducer and extends laterally across said transducer. Acceleration of the transducer in a predefined direction causes strain in said fiber optic laser. The transducer can have many possible designs. There is further provided a system for sensing acceleration which includes a pumping laser and a distributor joined to the fiber optic laser. Return signals from the fiber optic laser are provided to an interferometer and analysis circuitry. In the absence of a transducer, the system can operate as a strain sensor.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefore. 

   CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
   None. 
   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates generally to a fiber optic sensors and more particularly to a fiber optic accelerometer. 
   (2) Description of the Prior Art 
   Linear fiber optic arrays are used in Navy tactical and surveillance towed array applications as well as commercial towed seismic streamer applications for oil and gas exploration. Linear arrays may also be used in ocean bottom applications or placed down oil wells for oil field monitoring. 
   Fiber optic sensor arrays use interferometers and lasers to interrogate a plurality of sensors formed in an optical fiber. Typically these sensors are made from a fiber optic sensing segment positioned between two reflective portions. The sensing segment is usually wound on a compliant mandrel. Acoustic pressure on the sensor results in strain in the fiber optic sensing segment. Strain in the fiber optic sensing segment is measured using the interferometer. Various schemes such as wavelength division multiplexing and time division multiplexing exist for increasing the number of sensors on a single optical fiber. Fiber optic sensor arrays are also known that use multiplexed fiber laser sensors to provide an acoustic pressure signal. 
   A newer type of electronic linear sonar array is a vector sensor array. A vector sensor array uses sensor elements that provide a vector reading of the acoustic field, rather than just the scalar pressure. A typical form of the sensor element involves a combination of 2 or 3 orthogonal accelerometers and a pressure sensor. 
   Such a vector sensor array has several potential advantages over linear arrays of pressure sensors. Because of the added directionality of the individual element, the gain is increased per unit array length. A single linear array can resolve the left-right bearing ambiguity seen in pressure sensor arrays. Finally, the array can increase effective gain against anisotropic background noise fields. Previous linear vector sensor arrays have used conventional (non-fiber optic) accelerometers and pressure sensors combined with electronic telemetry. They have shared the same intensive, complicated hand assembly techniques that have resulted in high cost and low reliability in pressure sensor arrays. 
   It is therefore the intent of this invention to provide a directional fiber optic accelerometer that can be used to form a linear array of vector sensors. 
   SUMMARY OF THE INVENTION 
   Accordingly, there is provided an accelerometer utilizing a fiber optic laser. Strain applied to the fiber optic laser results in an emission wavelength shift. The fiber optic laser is joined to a transducer and extends laterally across the transducer. Acceleration of the transducer in a predefined direction causes strain in said fiber optic laser. The transducer can have many possible designs. There is further provided a system for sensing acceleration which includes a pumping laser and a distributor joined to the fiber optic laser. Return signals from the fiber optic laser are provided to an interferometer and analysis circuitry. In the absence of a transducer, the system can operate as a strain sensor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing invention will become readily apparent by referring to the following detailed description and the appended drawings in which: 
       FIG. 1  is a diagram showing a fiber optic laser sensor accelerometer; 
       FIG. 2  is a diagram showing a fiber optic laser accelerometer having a transducer constructed according to a first embodiment; 
       FIG. 3  is a diagram showing a fiber optic laser accelerometer having a transducer constructed according to a second embodiment; 
       FIG. 4  is a diagram showing a fiber optic laser accelerometer having a transducer constructed according to a third embodiment; and 
       FIG. 5  is a diagram showing a fiber optic laser accelerometer having a transducer constructed according to a fourth embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  provides a first embodiment of a fiber laser accelerometer  10 . The fiber laser accelerometer  10  includes a fiber laser  12 . Fiber laser  12  can be either a Fabry-Perot type cavity fiber laser or a distributed feedback fiber laser. In a Fabry-Perot type fiber laser, the laser cavity is a length of erbium-doped optical fiber with a Bragg grating written in the fiber core at one end of the laser cavity, with another Bragg grating or other reflector at the other end of the laser cavity. In a distributed feedback fiber laser, the fiber laser cavity is a length of erbium-doped optical fiber having a grating written over the full length of the cavity. The distributed feedback fiber laser will have a phase shift written into the grating at the center of the cavity. A pump laser  14  is provided for transmitting coherent light through optical fiber  16 . Pump laser  14  can be any laser such as a diode laser operating at 980 nm or 1480 nm. Pump laser  14  is joined by fiber  16  to a distributor  18 . Distributor  18  can be a wavelength division multiplexer, circulator or the like. Wavelength division multiplexer operates by providing light at the pump laser wavelength to the fiber laser  12 . Returning light from the fiber laser  12  would be at a different frequency and would be guided along a different path. A circulator can carry out the same function by transferring light to the next port of the circulator. 
   After coherent light passes through distributor  18 , it is absorbed by the doping material in fiber laser  12 . Absorption of this light causes a gain inversion in the laser cavity that leads to laser emission. This causes fiber laser  12  to emit a narrow single mode of light. Fiber laser  12  is mounted to a transducer  20  such that acceleration of the transducer material is made to provide strain of the fiber laser cavity. Strain of the fiber laser cavity shifts the emission wavelength of the fiber laser  12 . Transducer  20  can have a wide variety of constructions that will be discussed hereinafter. 
   Fiber laser emission returns along fiber  16  to wavelength division multiplexer  18  where it is separated onto analysis path  22 . An interferometer  24  such as a Mach-Zehnder interferometer is positioned on analysis path  22 . Interferometer  24  converts the shifted emission wavelength of fiber laser into a phase shift of the fiber laser light. Interferometer  24  typically includes a first beam splitter  24 A having a delay loop  24 B on one leg and a modulator  24 C on the second leg. Signals from the delay loop  24 B and modulator  24 C are combined in a second beam splitter  24 D to produce the phase shifted signal. Other designs can produce the same type of signal. A receiver  26  receives the phase shifted signal. Receiver  26  is capable of demodulating and detecting the signal from the fiber laser by various methods well known in the art. The signal from the fiber laser gives an indication of strain in the fiber laser. 
     FIG. 2  provides greater detail concerning the mounting of fiber laser  12  on transducer  20 . Transducer  20  has at least one flap  28 . Flap  28  is attached to a base  30  of the transducer  20  and extends toward fiber laser  12  at an angle. Equal numbers of flaps  28  can be provided on each side of a center  32  of the fiber laser  12 , and all flaps  28  angle toward the center  32 . Transducer  20  can be made from a polymer material that allows vertical movement of flaps  28 , as shown, but resists movement in transverse direction. Movement in the longitudinal direction cannot be controlled without affecting the vertical motion induced by acceleration. The affect of the longitudinal motion tends to cancel out because of the flap arrangement. If the transducer  20  is to the right, the left flaps move up and the right flaps move down. The net strain on fiber laser  12  is low. 
   The volume in between flaps  28  can be a vacuum, gas, liquid, or solid; however, it should allow movement of flaps  28 . A gas, liquid or solid could provide damping, if necessary. The combined top surfaces  34  of the flaps  28  forms an interrupted curved surface  36 . The fiber laser  12  or fiber  16  extending beyond fiber laser  12  is mounted under tension across the top surfaces  34  of the flaps  28  and fixed to the outer two flaps with an adhesive  38  such as ultra-violet cured epoxy. Adhesive  38  may interfere with fiber laser  12  if applied to the laser grating region. Therefore the adhesive is preferably applied to the fiber region outside of the laser grating. Mounting to the outermost flaps  28  reduces the constraint on the ends of the fiber laser  12  and yields greater sensitivity to acceleration. 
   Interrupted curved surface  36  allows fiber laser  12  contact with all of the flap upper surfaces  34 . Friction holds the fiber laser  12  in place laterally. The body of each flap  28  serves as an inertial mass. When transducer  20  is accelerated away from fiber laser  12 , flaps  28  move toward fiber laser  12  and outward from the center  32  due to the angle of the flaps  28 . This causes an unsupported portion  40  of the fiber laser  12  between the centermost two flaps  28  to be further tensioned, shifting the fiber laser emission frequency. The outer flaps  28  move with the inner flaps  28  and produce additional strain on the fiber portion at center  32 . The outer flaps  28  also add their mass to the response to the acceleration. 
   The fiber laser  12  strain sensitivity is weighted by the mode profile. In a feedback laser, this is a strong exponential function centered at the center  32  of the fiber laser  12 . Because of this, while the laser may be 50 mm long, the effective sensitive length may be only 8 mm. It is this effective length  40  which is positioned between the center two flaps  28 . The flaps  28  have sufficient width in the direction perpendicular to the direction being sensed to ensure that the flaps  28  are much more stiff against motion in that direction. This stiffness in the transverse direction ensures that the accelerometer  10  has good isolation against responding to accelerations in that direction, known as cross-axis isolation. Transducer  20  structure also provides good isolation against longitudinal accelerations in the direction parallel to the fiber laser. In this direction, the half of the flaps  28  on one side of the center  32  move upward and outward, while the half of the flaps  28  on the other side of the center  32  move downward and inward. These two motions tend to cancel, producing little net strain on the center  32  of the fiber laser  12 . 
   The unsupported fiber portion  40  between the center two flaps  28  can also vibrate in a string mode. This mode is undesirable because it is equally susceptible to acceleration in both directions transverse to the fiber laser  12 . Thus, it is desirable to minimize this mode and push its resonance to a frequency above that in the sensing range. The response of this mode is controlled by mounting the fiber laser  12  to the transducer  20  with sufficient tension. The embodiment shown in  FIG. 2  minimizes this tension. 
     FIG. 3  shows an embodiment of transducer  20  using only two opposing flaps  28  with the rest of the fiber laser attaching to solid material  42  on either side. In this embodiment the tension must be sufficient for the length of the unsupported span  40  between the flaps  28  and the solid material  42  to control the string mode of vibration on that fiber span  40 . Attachment to the rigid structure  42  at either end of the fiber laser  12  stiffens the transducer  20  by limiting the motion of the center two flaps  28 . 
     FIG. 4  shows an embodiment where the fiber optic  16  is attached to rigid structure  42  at the end of transducer  20  beyond the last flaps  28 . 
     FIG. 5  shows an embodiment where transducer  20  has a straight mounting surface  44  instead of the curved mounting surface  36  shown in the other embodiments. Top surfaces  34  of flaps  28  are in a plane. In this embodiment, fiber laser  12  may need to be mounted to transducer  20  on multiple flaps  28 . Mounting means other than direct adhesive such as a bracket can be used to avoid interference with fiber laser  12 . 
   It is possible to build transducer  20  with only one flap  28  instead of two opposing flaps  28 . However, this cuts the strain response roughly in half and does not provide the rejection against longitudinal acceleration. 
   It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the invention by those skilled in the art, without departing from the spirit and scope of this invention, which is therefore understood to be limited only by the scope of the appended claims.